Late Triassic to Early Jurassic carbon isotope chemostratigraphy and organo-facies evolution in a distal to proximal transect of the North German Basin

Abstract

Throughout the Latest Triassic and the Early Jurassic, major changes in paleogeography, climate and eustatic sea-level impacted on the development of shelf depositional environments. Secular trends in environmental conditions were punctuated by transient perturbations that occurred in relation to large-scale volcanic events, such as the emplacement of the Central Atlantic Magmatic Province at the Triassic/Jurassic boundary and the Karoo–Ferrar Large Igneous Province in the early Toarcian. We here present bulk organic (HAWK programmed pyrolysis) and organic carbon isotope (δ¹³C_org) data for three drill cores recovering Latest Triassic and Early Jurassic strata (Rhaetian to Toarcian). Study sites are located in the northeastern part of the Central European Epicontinental Sea and were positioned along a distal–proximal transect of the North German Basin. This allows discussing the differential response of depositional settings and organo-facies toward secular and transient environmental change. Biostratigraphically anchored trends in δ¹³C_org values allow the precise correlation along the transect, as well as with distant sites. At all North German locations, diagnostic secular trends in δ¹³C_org are punctuated by transient negative carbon isotope excursions, reflecting perturbations of the global carbon cycle at the Triassic/Jurassic boundary and in the early Toarcian. Stratigraphic gaps occurred during sea-level lowstands and are most pronounced at shallow proximal sites. Programmed pyrolysis data indicate spatiotemporal organo-facies trends that on a temporal scale occurred in response to sea-level and climate trends, while spatial patterns were governed by basin morphology and paleobathymetry. Substantial marine organic matter accumulations occurred at high sea level during the Toarcian only, and were most continuous at distal sites.

Graphical Abstract

Introduction

The Early Jurassic recorded major changes in global environmental conditions, such as major climate shifts (e.g., Dera et al. 2011; Ruebsam and Schwark 2021) and high amplitude sea-level changes (e.g., Gély and Lorenz 2006; Zimmermann et al. 2015; Haq 2018). Environmental changes occurred in relation to paleogeographic reorganization, such as the breakup of the super-continent Pangaea that was accompanied by periods of intensified volcanism, rifting, and the flooding of continental areas (e.g., Gély and Lorenz 2006; Cortés et al. 2009; Dera et al. 2011). The emplacement of the Central Atlantic Magmatic Province (CAMP) at the Triassic/Jurassic boundary (TJB, c. 201 Ma; e.g., Ruhl et al. 2020) and the Karoo–Ferrar Large Igneous Province (K–F-LIP) in the early Toarcian (c. 183 Ma; e.g., Pálfy et al. 2002; Ware et al. 2023) represent major magmatic events that impacted global carbon cycle dynamics (e.g., van de Schootbrugge et al. 2018; Xu et al. 2018; Pieńkowski et al. 2020; Storm et al. 2020), climate conditions (e.g., Dera et al. 2011), as well as on marine and continental ecosystems (e.g., Hallam 1986, 1990, 1996; Harries and Little 1999; Caruthers et al. 2014; Reolid et al. 2022).

Early Jurassic climate and sea-level trends affected organic carbon burial and preservation in marine sediments (e.g., Silva and Duarte 2015; Silva et al. 2021). The early Toarcian Oceanic Anoxic Event (Toa-OAE; Early Jurassic, c. 183 Ma) hereby presents the most widespread organic carbon burial event that occurred during a time of profound environmental changes (e.g., Jenkyns and Clayton 1986; Jenkyns 1988; Jenkyns et al. 2002; for a review see Silva et al. 2021).

Numerous works address the TJB and the early Toarcian events, while only very few studies have been conducted on semicontinuous sedimentary archives spanning the entire Early Jurassic (e.g., Ikeda and Tada 2014; Ikeda and Hori 2014; Peti et al. 2017; van de Schootbrugge et al. 2018; Pieńkowski et al. 2020; Storm et al. 2020).

Here, we present a bulk organic and isotope geochemical study of three sedimentary archives (drill cores) spanning Rhaetian to Toarcian strata, situated along a distal–proximal transect at the northeastern margin of the European epicontinental sea (Fig. 1). Data allow discussing spatiotemporal trends in organo-facies that occur in response to local depositional conditions versus transregional to global environmental change. Results of this study provide insights into the development of a marginal marine depositional environment throughout the entire Early Jurassic.

Fig. 1

a Early Jurassic global paleogeography after Blakey (2016). AO Arctic Ocean, CAMP Central Atlantic Magmatic Province, CEES Central European Epicontinental Sea, F Fennoscandia, L Laurentia, LIP large igneous provinces, NG northern Gondwana, NWTS Northwest Tethys Shelf. b Early Jurassic (Toarcian) paleogeography of the northeastern part of the Central European Epicontinental Sea (modified from Ruebsam et al. 2014, 2020; Ansorge et al. 2023). Positions of the investigated drill cores are indicated (1: core Schandelah, 2: core Kb KSS 5/66; 3: core Kb Barth 10/65). The position of the extensively studied Dotternhausen section (D) in the South German Basin, to which we refer in the text, is indicated as well. Marine regions indicated as shallow sea may have been subject to subaerial exposure during sea-level lowstands. c Uppermost Triassic and Lower Jurassic spatial distribution of biofacies along an SW–NE transect in the North German Basin (modified from Zimmermann et al. 2015; Barth et al. 2018a, 2018b). Biofacies shifts occur in response to transgressive–regressive cycles (WNGB Western North German Basin, ENGB Eastern North German Basin, EBS Eastern Baltic Sea area, PB Polish Basin)

Paleogeography

The study sites (cores Kb KSS 5/66 and Kb Barth 10/65: Mecklenburg–Vorpommern; core Schandelah: Niedersachsen) are located in northeastern Germany. During Late Triassic–Early Jurassic times, this area was part of the Central European Epicontinental Sea (CEES), a shallow-marine setting that was intensively structured into serval basins and sub-basins, separated by shoals and islands of variable size (e.g., Ziegler 1990).

The CEES covered the peri-Tethys lowlands bordering the Northwest Tethys Shelf (NWTS) (Fig. 1a). During the Late Triassic to the Early Jurassic, the CEES developed in conjunction with rifting, related to the breakup of the Gondwana. Evolution of this epicontinental sea was closely linked to global- and regional-scale tectonic processes that controlled basin subsidence (Wienholz 1967; Ziegler 1982; Dadlez et al. 1995; Poprawa 1997; Scheck-Wenderoth et al. 2008) and eustatic sea-level changes (e.g., Gély and Lorenz 2006; Barth et al. 2018b), which affected paleo-water depths, position of the paleo-coastline, opening/closing of the epicontinental seaways and current systems (e.g., Bjerrum et al. 2001; Korte et al. 2015). Absolute water depths are poorly known, but probably vary from a few tenths of meters in shallow areas above shoals and swells to 100–200 m in central basin areas and depressions (Ziegler 1990). Water depths will have also changed in dependency of local/regional-scale tectonic settings (subsidence/uplift) and eustatic sea-level variation (e.g., Zimmermann et al. 2015; Barth et al. 2018b; Haq 2018). Throughout the Early Jurassic, eustatic sea-level variations of up to 100 m in magnitude have substantially impacted on water depths and thus on paleogeography and depositional conditions (Hallam 1997, 2001; Haq 2018; Krencker et al. 2019).

From the Late Triassic to the Early Jurassic, the CEES underwent a transformation from a mainly continental environment of the Late Triassic towards a shallow-marine epicontinental sea of the Early Jurassic (see Zimmermann et al. 2015 and Barth et al. 2018b and references therein).

The study area is located in the northern part of the epicontinental sea, in proximity to landmasses to the east and northeast (Figs. 1b, 2). Detrital input from sources to the North and South contributed to progradational coastal–deltaic systems stretching up to axial parts of the basin during sea-level lowstands (Barth et al. 2014; Zimmermann et al. 2015; 2018; Zimmermann 2016). Accordingly, sedimentation in the northeastern part of the basin was controlled by the interplay of sea-level variation and fluvial discharge regulating the filling of the accommodation space (Zimmermann et al. 2015, 2018; Barth et al. 2018a, 2018b) (Figs. 1c, 2).

Fig. 2

Paleogeographic evolution (biofacies provinces and depositional environments) evolution of the northern CEES during the Early Jurassic. AS Altmark Swell, BM Bohemian Massif, C Calvörde Island, FH Fennoscandian High, FS Fallstein Swell, RM Rhenish Massif; modified from Zimmermann 2016; Barth et al. 2018a, bb; 2018a, b; Ansorge et al. 2023; including data from Schulze 1964; Ott 1967; Bauss 1976). Resulting from the general transgressive trend and pronounced sea-level fluctuations, the well sites situated along an NE–SW running transect experienced variable depositional conditions during the studied stratigraphic interval. Site Barth 10/65 was located in proximity to the paleo-shoreline during Hettangian to Sinemurian times and was subject to deepening during the Pliensbachian to Toarcian. Site Schandelah for the entire study interval was in a distal position. Site KSS 5/66 represents proximal conditions during the Hettangian to Sinemurian, but changed to a more distal setting during the Pliensbachian to Toarcian. Whether the Altmark High (Bauss 1976) and the Calvörde High (Schulze 1964) emerged as islands or remained as submarine swells is not exactly known. The Fallstein Swell was a submarine elevation with significant reduction in the thickness of Lower Jurassic strata (Ott 1967)

Materials and methods

Core description and stratigraphy

Core Schandelah

A detailed lithological and biostratigraphic description of core Schandelah is provided by van de Schootbrugge et al. (2018) and Visentin et al. (2021). In brief, cored section comprises about 338 m of Latest Triassic (Rhaetian) to Early Jurassic (Toarcian) strata (Fig. 3). Age determination is based on diagnostic ammonites, ostracods, palynomorphs and calcareous nannofossils. Moreover, characteristic trends in high-resolution stable organic carbon isotope values (δ¹³C_org) provide an additional stratigraphic guidance (van de Schootbrugge et al. 2018). The Rhaetian is represented by a succession of siltstones and sandstones (Exter Formation). The Hettangian is dominated by marine heterolithic mud–silt and sandstone alterations (Angulatenton Formation) deposited on a shoreface environment. Mudstones with intercalated argillaceous marlstones, deposited in a marine prodelta to offshore setting, dominate the Sinemurian, Pliensbachian, and Toarcian strata (Arietenton, Obtususton, Capricornumergel, Amaltheenton formations). Horizontal-laminated mud- and marlstones occur in the Toarcian and are assigned to the Posidonienschiefer Formation (Fig. 3). Hardgrounds indicating erosion and reworking during sea-level lowstand occur in the Rhaetian, the Hettangian, the upper Sinemurian and the upper Pliensbachian (van de Schootbrugge et al. 2018).

Fig. 3

Stratigraphy, lithology and geochemical data for the Schandelah core (abbreviations ammonite zones: Li.; Liassicus, Se.: Semicostatum, Ob.: Obtusum, Ox.: Oxynotum, Ra.: Raricostatum, J.: Jamesoni, Ib.: Ibex, D.: Davoei, T.: Tenuicostatum, F.: Falciferum, Bifr.: Bifrons; abbreviations for formations: Arnst.: Arnstadt; Ariet.: Arietenton, Capr.: Capricornumergel, PS: Posidonienschiefer). Lithological description, biostratigraphic data, as well as high-resolution δ¹³C_org data (in grey) are taken from van de Schootbrugge et al. (2018). To express major and minor changes in the geochemical proxies (carbonate, TOC, HI, OI), data are plotted on both linear and logarithmic scales

Core KSS 5/66

Core Kb KSS 5/1966, located in SW Mecklenburg–Vorpommern, recovers about 520 m of Upper Triassic (Norian–Rhaetian) to Early Jurassic (Toarcian) strata (Fig. 4; for detailed description see Zimmermann et al. 2015). The Norian–Rhaetian (c. 1870–1740 m; Arnstadt and Exter formation), Hettangian (c. 1740–1665 m; Angulatenton and lower Löcknitz formations) and Sinemurian (c. 1665–1613 m; upper Löcknitz and Obtususton formations) strata are mainly composed of siltstones with several sandstone intervals (Fig. 4). The Löcknitz Formation here refers to the sandy facies of the Hettangian–Sinemurian in northeastern Germany. Mud- and siltstones dominate in the Pliensbachian (c. 1613–1525 m; Capricornumergel and Amaltheenton formations). In the lowermost Toarcian, a dark-grey laminated mudstone is assigned to the Posidonienschiefer Formation (c. 1525–1518 m). The base of the organic-rich mudstones of the Posidonienschiefer is marked by a thin argillaceous sandstone with mudclasts, indicating some erosion (Fig. 4). The Posidonienschiefer Formation is overlain by the greenish-gray mudstones of the Grimmen Formation (Ansorge et al. 2023), formerly named Grüne Serie (1518–1450 m). Thick deltaic sandstone intervals occur in the upper Toarcian in the intervals from 1450–1420 m (lower Glashütte Formation) and from 1390–1350 m (upper Glashütte Formation). The sandstones of the upper Glashütte Formation are not precisely dated and might be of uppermost Toarcian and/or lower Aalenian age (Fig. S1 in the supplement). The intervening interval from 1420 to 1390 m is characterized by a dark laminated calcareous mudstone assigned to the Dörnten Member (Fig. 4). Throughout the Rhaetian to Toarcian, the depositional environment shifted between prodelta to delta front settings (Zimmermann et al. 2015). The Keuper-type pedogenic strata of the Norian Arnstadt Formation were mainly formed in terrestrial playa-like environments.

Fig. 4

Stratigraphy, lithology and geochemical data for core KSS 5/66 (abbreviations of ammonite zones: Turn.: Turneri, Obt.: Obtusum, D.: Davoei, M.: Margaritatus, T.: Tenuicostatum, V.: Variabilis, Th.: Thouarense; abbreviations for formations: Capr.: Capricornumergel, PS: Posidonienschiefer; l./u. Glashütte: lower/upper Glashütte). In order to express major and minor changes in the geochemical proxies (carbonate, TOC, HI, OI), data are plotted on linear and logarithmic scales. Lithological key as shown in Fig. 3

Ammonite finds allowed to identify most ammonite biozones in the Pliensbachian and Toarcian. No ammonites have been documented from sediments attributed to the Rhaetian, Hettangian and Sinemurian. For this interval, the age assignment is based on micropaleontological and palynological data, as well as on local–regional lithostratigraphic correlations (Zimmermann et al. 2015; Barth et al. 2018a; also see Fig. S1 in the supplement). Biostratigraphic uncertainties also exists in the Toarcian interval. In particular, mud- and siltstones of the upper Grimmen Formation lack diagnostic index fossils. Details on the biostratigraphy are summarized in figure S1 in the supplement.

Core Barth 10/65

Core Kb Barth 10/1965, located in northern Mecklenburg–Vorpommern, comprises about 500 m of Rhaetian to Toarcian strata (Fig. 5; for detailed description see Barth et al. 2018a, 2018b). The Rhaetian (c. 750–705 m; Exter Formation), Hettangian (c. 705–618 m; lower Löcknitz Formation) and Sinemurian (c. 618–463 m; upper Löcknitz and Obtususton formations) strata are composed of siltstones with several sandstone intervals, which grade into silty mudstones in the upper Sinemurian. Rhaetian to lower Sinemurian strata have been deposited in upper/lower deltaic plain to shoreface settings. Intervals showing evidence for reworking and erosion occur at the TJB (693 m) and in the Sinemurian (560 m) reflecting deposition in a very shallow water setting (Fig. 5). Upper Sinemurian strata have been deposited in offshore to prodelta settings, indicating a deepening of the depositional environment (Barth et al. 2018a, 2018b). The basal part of the Pliensbachian (463–450 m; Capricornumergel Formation) is composed of argillaceous marlstones. Mud- and siltstones dominate in the upper Pliensbachian (450–340 m; Amaltheenton Formation and Wolgast Formation). In the lowermost Toarcian a dark-gray laminated mudstone is assigned to the Posidonienschiefer Formation (340–338 m), which grades into greenish-gray mudstones of the Grimmen Formation (formerly Grüne Serie; 338–255 m) (Fig. 5). Pliensbachian and Toarcian strata have been deposited in an offshore to prodelta setting (Barth et al. 2018b). However, a reworked interval at the Pliensbachian/Toarcian boundary (340 m) indicates the marked shallowing of the depositional environment (Barth et al. 2018b).

Fig. 5

Stratigraphy, lithology and geochemical data for core Barth 10/65 (abbreviations of ammonite zones: J.: Jamesoni, I.: Ibex, D.: Davoei, Margar.: Margaritatus, T.: Tenuicostatum; abbreviations for formations: Cap.: Capricornumergel, Lö.: Löcknitz, PS: Posidonienschiefer). In order to express major and minor changes in the geochemical proxies (carbonate, TOC, HI, OI), data are plotted on both linear and logarithmic scales. The lithological key as shown in Fig. 3

Ammonite finds allowed to identify most ammonite biozones in Pliensbachian to and Toarcian strata. No ammonites have been documented from Rhaetian–Sinemurian strata. In these intervals, the age assignment is based on micropaleontological and palynological data, as well as on local–regional lithostratigraphic correlations (Barth et al. 2018a, 2018b). Biostratigraphic uncertainties also exists in the Toarcian interval. Mud- and siltstones of the upper Grimmen Formation lack diagnostic index fossils. The Posidonienschiefer Formation can be dated to the lower Toarcian Tenuicostatum to Falciferum ammonite zones. Details on the biostratigraphy are summarized in figure S1 in the supplement.

Geochemical analyses

HAWK pyrolysis

Programmed pyrolysis afforded a HAWK (Wildcat Technologies, USA) instrument operated in classical pyrolysis mode. An aliquot of 70 mg of sediment was weighed into a crucible and inserted by an autoloader into the HAWK device. Determination of thermally released bitumen (S1 in mg HC/g rock) occurred isothermally at 300 °C. The amount of kerogen was determined via the hydrocarbon yield upon pyrolysis (S2 in mg HC/g rock) through heating in an inert environment using a ramp rate of 25 °C from 300 to 650 °C. The pyrolysis measurements of both CO and CO₂ (S3CO in mg CO₂/g rock and S3CO₂ in mg CO₂/g rock) gave the oxygen concentration of the kerogen. Thermal maturity (T_max in °C) was determined as the temperature of maximum pyrolysis yields recorded by the S2 peak.

The TOC was determined as the sum of reactive carbon (sum of bitumen and kerogen), determined in classical programmed pyrolysis mode in an inert environment, and the amount of non-generative carbon that was determined in oxygenated combustion mode. The carbonate content was determined via the release of CO₂ upon heating from 300 to 850 °C in oxidation mode and converted to calcite carbonate equivalents using a stochiometric factor of 8.33.

Organic carbon isotope analysis

Stable carbon isotope measurements of bulk organic carbon (δ¹³C_org) were carried out on decalcified bulk sediment sample material after treatment with HCl (10% and 25%). Stable isotope analysis was performed using a Thermo Scientific Elemental analyzer (Flash EA) coupled to a Delta V mass spectrometer with a Conflow III interface. Isotope ratios are expressed in conventional delta notation: δ_sample (‰) = [(R_sample—R_standard) / R_standard—1] × 1000, where R is the ratio of ¹³C/¹²C of the sample and the V-PDB standard. Reproducibility and accuracy were monitored by replicate standard and sample analysis and were better than 0.1‰ (1σ). Carbon isotope values were calibrated to a certified caffeine standard (IAEA-600, δ¹³C = −27.771‰ VPDB).

Cyclostratigraphic analysis

Cyclostratigraphic analysis was conducted on gamma ray (GR) data for Toarcian strata for cores KSS 5/66 and Barth 10/65. A detailed description on the analytical procedure is provided in the supplement.

Results

Core Schandelah

Stable organic carbon isotopes

For the Schandelah core, stable organic carbon isotope values have been previously reported by van de Schootbrugge et al. (2018). The δ¹³C_org values were independently measured for all samples analyzed in this study, as the sample material and the exact stratigraphic position were not identical to those from previous works. The δ¹³C_org values fall in the range −32.1 to −24.1‰ (Fig. 3). Heavy δ¹³C_org values of −24.7‰ on average are seen in samples from the Rhaetian (Upper Triassic). Only one sample at 326.45 m showed a lighter δ¹³C_org signature of −27.0‰. Organic carbon isotope values decline to −29.4‰ at the TJB that is marked by a negative excursion in the δ¹³C_org values. In the Hettangian strata, δ¹³C_org values range from −26.9 to −25.4‰ (−26.6‰ on average) (Fig. 3). More variable δ¹³C_org values in the range of −26.9 to −24.7‰ are seen in the samples from the Sinemurian (−25.6‰ on average). Samples from the Pliensbachian strata show δ¹³C_org values in the range of −27.2 to −24.6‰ (−26.0‰ on average) (Fig. 3). Lowest δ¹³C_org values of on average −28.9‰ are recorded in samples from the Toarcian. A marked decline associated with a negative excursion in δ¹³C_org to values as low as −32.2‰ is seen in the lower Toarcian strata and marks the basal Posidonienschiefer Formation (Fig. 3). The negative shift in the δ¹³C_org is followed by a return to heavier values of about −26.1‰. Subsequently, δ¹³C_org declines to values of about −29.5‰ in the middle part of the Posidonienschiefer Formation (upper Falciferum Zone) and remains low throughout the upper part of this formation (mainly Bifrons Zone).

Carbonate content

In the samples from the Schandelah core, carbonate contents are variable (1.0–87.8 wt.%), but mainly stay below 10 wt.% (Fig. 3). Slightly increased carbonate abundances of 11.3–21.5 wt.% (6.1 wt.% on average) occur in a few samples from the Rhaetian (Upper Triassic). Apart from a sample of lowermost Hettangian age that shows a carbonate content of 12.4 wt.%, overall low carbonate abundances (≤ 3.0 wt.%) were measured for the Hettangian (2.2 wt.% on average). Slightly increased and more variable carbonate contents in the range 1.8–15.7 wt.% (6.5 wt.% on average) are seen in samples from the Sinemurian. In particular, samples from the lowermost Sinemurian show increased carbonate contents that fall in the range 10.9–15.7 wt.% (Fig. 3). Variable and increased carbonate contents in the range 9.7–74.4 wt% are also seen in the lower Pliensbachian Capricornumergel Formation (Jamesoni to Davoei zones). Mainly low carbonate contents < 5 wt.% are seen in samples from the upper Pliensbachian (Margaritatus and Spinatum zones). Only two samples show increased contents of 9.8 and 21.2 wt.% (Fig. 3). Highest but also variable carbonate contents in the range of 1.6–87.8 wt.% (23.4 wt.% on average) were measured in samples from the Toarcian, whereby carbonate contents > 5 wt.% solely occur in samples from the Posidonienschiefer Formation (Fig. 3).

Total organic carbon content

Variable TOC contents in the range 0.3–20.7 wt.% were measured in samples from the Schandelah core (Fig. 3). Most samples from the Rhaetian to Pliensbachian are TOC-lean and abundances remain below 1.5 wt.%. Slightly increased TOC contents in the range of 1.5–2.5 wt.% are limited to a thin interval in the lower Sinemurian and a second in the Pliensbachian Margaritatus Zone (Fig. 3). A substantial increase in TOC abundances to values > 2.5 w% is solely seen in samples from the Posidonienschiefer Formation (Fig. 3).

Hydrogen and oxygen indices

Samples from the Rhaetian to the lowermost Toarcian show low HI values that do not exceed 200 mgHC/gTOC (range: 8–152 mgHC/gTOC; 39 mgHC/gTOC on average). OI values are variable and fall in the range 18–249 mgCO₂/gTOC (70 mgCO₂/gTOC on average). On the contrary, samples from the Toarcian Posidonienschiefer Formation show high HI values in the range 249–780 mgHC/gTOC (689 mgHC/gTOC on average) and low OI values in the range 10–31 mgCO₂/gTOC (13 mgCO₂/gTOC on average) (Fig. 3).

T_max values

The T_max values are variable and range from 406 to 434 °C (Fig. 3). Lowered T_max values that do not exceed 425 °C were measured in samples from the Toarcian Posidonienschiefer Formation (range: 406–425 °C; 416 °C on average). In contrast, samples from Rhaetian to lowermost Toarcian strata show increased and variable T_max values in the range 419–434 °C (428 °C on average).

Core KSS 5/66

Stable organic carbon isotopes

In samples from core KSS 5/66, δ¹³C_org values fall in the range of −33.0 to −23.7‰ (Fig. 4). Samples from the Norian–Rhaetian (upper Triassic) show slightly enriched δ¹³C_org values that vary between −26.1 and −24.3‰ (−25.3‰ on average). Samples from strata assigned to the Hettangian show slightly lowered δ¹³C_org values in the range of −26.5 to −24.8‰ (−25.8‰ on average). A minor increase in δ¹³C_org to values in the range of −25.7 to −24.3‰ (−24.8‰ on average) is seen in samples from the Sinemurian. Samples from the Pliensbachian show variable δ¹³C_org values in the range of 26.1 to −23.8‰ (−25.4‰ on average). Samples from the Toarcian exhibit a substantial decline in the δ¹³C_org to values that mainly range from −33.0 to −27.2‰ (Fig. 4). Heavier values in the range −27.5 to −25.6‰ solely occur in samples from the lowermost Toarcian (lower Tenuicostatum Zone). The Posidonienschiefer Formation is marked by a negative shift in δ¹³C_org and records the lowest δ¹³C_org values (Fig. 4). Slightly increased δ¹³C_org values in the range of −28.4 to −27.2‰ were measured in samples from the Toarcian Grimmen Formation. Samples from the Toarcian Dörnten Member show δ¹³C_org values that range from −29.6 to −27.9‰. A significant increase in the ¹³C_org values that fall in the range of −24.6 to −24.0‰ occur in uppermost Toarcian strata (Fig. 4).

Carbonate content

Samples from core KSS 5/66 show variable, but mainly low carbonate contents that vary between 0.7 and 34.4 wt.% (Fig. 4). Norian–Rhaetian (Upper Triassic), Hettangian and Sinemurian strata is mainly carbonate-lean (range: 1.1–32.9 wt.%; 3.7 wt.% on average), with only a very few samples showing contents in the range 11.4–32.9 wt.% (Fig. 4). An increase in the carbonate content up to 15.5 wt.% (7.4 wt.% on average) is seen in samples from the lower Pliensbachian Capricornumergel Formation. Low carbonate abundances in the range 0.7–3.0 wt.% (2.0 wt.% on average) were measured in samples from the upper Pliensbachian (mainly Spinatum Zone) and the lower Toarcian (lower Tenuicostatum Zone). An increase in the carbonate contents to values in the range of 2.3–34.4 wt.% (12.0 wt.% on average) is seen in the samples from the Posidonienschiefer Formation (Fig. 4). Samples from the Grimmen Formation show variable, but mainly low carbonate contents in the range 1.4–8.9 wt.% (3.7 wt.% on average). Increased carbonate contents that fall in the range 3.3–22.3 wt.% (13.9 wt.% on average) occur in the Dörnten Member. Samples from the Toarcian upper Glashütte Formation are lean in carbonate (range: 1.5–2 wt.%; 1.8 wt.% on average).

Total organic carbon content

Samples from core KSS 5/66 show TOC contents in the range of 0.4–16.9 wt.% (Fig. 4). Samples from the Norian–Rhaetian (Upper Triassic) reveal TOC contents that vary between 0.4 and 1.7 wt.% (0.9 wt.% on average). Relatively invariant TOC abundances in the range 0.6–1.9 wt.% (1.3 wt.% on average) are seen in samples from the Hettangian to Pliensbachian. TOC contents remain low in the lowermost Toarcian sediments (1.3 wt.% on average). A substantial increase in the TOC contents to values in the range of 1.0–16.9 wt.% (9.4 wt.% on average) is seen in samples from the lower Toarcian Posidonienschiefer Formation (Fig. 4). Decreased TOC contents in the lower part of the Posidonienschiefer Formation are restricted to heterolithic intercalations. Samples representing the Grimmen Formation show lower TOC contents that fall in the range of 0.4–1.4 wt.% (0.6 wt.% on average). Increased TOC abundances covering the range of 2.8–8.5 wt.% are seen in samples from the Dörnten Member (Fig. 4). Upper Toarcian samples are mainly lean in TOC (range: 0.8–1.8 wt.%; 1.2 wt.% on average).

Hydrogen and oxygen indices

The HI values vary between 3 and 689 mgHC/gTOC (Fig. 4). High HI values in the range of 191–689 mgHC/gTOC (534 mgHC/gTOC on average) and 170–584 mgHC/gTOC (450 mgHC/gTOC on average) are only seen in samples from the Toarcian Posidonienschiefer Formation and the Dörnten Member, respectively. Samples from other stratigraphic intervals show overall low HI values that do not exceed 100 mgHC/gTOC (Fig. 4).

The OI values are highly variable and cover a range from 21 to 401 mgCO₂/gTOC (Fig. 4). Decreased OI values in the range of 20–62 mgCO₂/gTOC (31 mgCO₂/gTOC on average) and 28–56 mgCO₂/gTOC (39 mgCO₂/gTOC on average) are seen in samples from the Posidonienschiefer Formation and the Dörnten Member, respectively. Higher OI values in the range of 41–401 mgCO₂/gTOC (91 mgCO₂/gTOC on average) occur in samples from strata older than the Toarcian Posidonienschiefer Formation (Norian–Rhaetian to lowermost Toarcian). Higher OI values in the range of 45–285 mgCO₂/gTOC (149 mgCO₂/gTOC on average) and in the range of 64–81 mgCO₂/gTOC (75 mgCO₂/gTOC) on average also occur in the Toarcian Grimmen Formation and in uppermost Toarcian strata, respectively (Fig. 4).

T_max values

The T_max values measured in samples from core KSS 5/66 mainly fall in the range 410–433 °C. A few samples from Triassic strata show exceptionally low T_max values < 400 °C (Fig. 4). Such low values are considered being unreliable and result from a very low S2 peak intensities. Low T_max values in the range of 410–420 °C mainly occur in samples from the Posidonienschiefer Formation (Fig. 4). The T_max values from the remaining stratigraphic intervals mainly vary between 420 and 430 °C.

Core Barth 10/65

Stable organic carbon isotopes

The δ¹³C_org values of samples from core Barth 10/65 fall in the range of −33.2 to −23.5‰ (Fig. 5). Variable, but mainly heavy δ¹³C_org values in the range of −29.5 to −23.7‰ (−24.8‰ on average) were measured in samples from the Rhaetian (Upper Triassic). The TJB is marked by an abrupt decline in δ¹³C_org values of about −2‰ (Fig. 5). Heavier δ¹³C_org values that range from −26.7 to −24.5‰ are seen in samples from the Hettangian. In Sinemurian strata, δ¹³C_org values are highly variable and range from −27.1 to −23.8‰. The highest variability in the δ¹³C_org is seen in strata of the upper Sinemurian, while more stable values occur in lower Sinemurian sediments (Fig. 5). Variable δ¹³C_org values in the range of −26.2 to −23.6‰ occur in the Pliensbachian strata. Low δ¹³C_org values that fall in the range −26.2 to −25.2‰ have been measured in samples from the lower Pliensbachian (Jamesoni? to Davoei? zones). Slightly heavier and less variable δ¹³C_org values of −23.8 to −23.5‰ are seen in samples from the Margaritatus Zone. In sediments assigned to the Spinatum Zone, δ¹³C_org values range from −25.8 to −23.7‰. A declining trend is seen in the lower part of the Spinatum Zone, while lower values characterize the upper part of this biozone. A marked decline to lighter δ¹³C_org values in the range −33.2 to −27.2‰ is seen in the Toarcian Posidonienschiefer Formation. In the Grimmen Formation the δ¹³C_org values range from −29.5 to −28.5‰ (Fig. 5).

Carbonate content

Variable, but mainly low carbonate contents in the range 0.1–74.6 wt.% are seen in samples from core Barth 10/65 (Fig. 5). Lowest values that mainly fall in the range of 0.1 to 7.2 wt.% (2.4 wt.% on average) are seen in samples from the Rhaetian (Upper Triassic) and in samples from the Hettangian and lower Sinemurian (Bucklandi? and Turneri? zones). A minor increase in carbonate abundances is recorded in samples from the upper Sinemurian and the Pliensbachian (range: 0.2–39.8 wt.%; 8.8 wt.% on average). A notable increase in carbonate contents to values in range of 27.6–47.2 wt.% (36.5 wt.% on average) was determined for samples from the lower Pliensbachian Capriconumergel Formation. Samples from the Toarcian show variable carbonate contents in the range of 0.5–8.3 wt.% (2.3 wt.% on average). Slightly increased carbonate contents in the range of 1.1–8.3 wt.% (3.7 wt.% on average) occur in the Posidonienschiefer Formation, while lower carbonate contents in the range of 0.5–2.8 wt.% (1.2 wt.% on average) characterize samples from the Grimmen Formation (Fig. 5).

Total organic carbon content

Samples analyzed from core Barth 10/65 show TOC contents in the range of 0.2–14.9 wt.% (Fig. 5). Samples from the Rhaetian (Upper Triassic) and the Hettangian (Lower Jurassic) exhibit variable, but mainly low TOC contents in the range of 0.5–5.4 wt.% (1.8 wt.% on average). A minor increase occurs throughout this interval that reaches its maximum around the Hettangian/Sinemurian boundary (Fig. 5). In the Sinemurian, TOC contents fall in the range of 0.4–2.1 wt.% (1.2 wt.% on average). A minor TOC decline is seen throughout the Sinemurian, with a minimum being reached in the upper part of this stage. In samples from the Pliensbachian, TOC contents range from 0.5 to 2.1 wt.% (1.3 wt.% on average). A substantial increase of the TOC values that range from 0.5–14.9 wt.% (8.7 wt.% on average) was measured in samples from the lower Toarcian Posidonienschiefer Formation (Fig. 5). In contrast, samples from the Toarcian Grimmen Formation show low TOC contents in the range of 0.5–1.3 wt.% (0.7 wt.% on average).

Hydrogen and oxygen indices

Samples from core Barth 10/65 show HI values that range from 4 to 644 mgHC/gTOC (Fig. 5). Variable, but mainly low HI values that vary from 4 to 320 mgHC/gTOC (44 mgHC/gTOC on average) were measured in samples from the Rhaetian (Upper Triassic). Samples from the Lower Jurassic (Hettangian, Sinemurian and Pliensbachian) show overall low HI values < 80 mgHC/gTOC (27 mgHC/gTOC on average). Increased HI values in the range of 62–644 mgHC/gTOC (527 mgHC/gTOC on average) were measured in samples from the lower Toarcian Posidonienschiefer Formation (Fig. 5). Samples from the Toarcian Grimmen Formation show HI values that fall in the range of 24–114 mgHC/gTOC (41 mgHC/gTOC on average).

The OI values vary between 14 and 209 mgCO₂/gTOC (Fig. 5). Apart of samples from the lower Toarcian Posidonienschiefer Formation that yielded OI values mainly < 50 mgCO₂/gTOC, higher values mainly > 50 mgCO₂/gTOC were determined for samples from the other stratigraphic intervals (Fig. 5).

T_max values

Samples from core Barth 10/65 show T_max values in the range of 408–436 °C. Two samples show exceptionally low T_max values < 400 °C (Fig. 5). These values are considered being unreliable and result from a very low S2 peak intensities. Low T_max values < 420 are seen in samples from the Posidonienschiefer Formation, but are not restricted to this lithological unit.

Discussion

Chemostratigraphy

Lower Jurassic chemostratigraphy

Throughout the Latest Triassic and the Early Jurassic, comparable and characteristic trends in stable organic and inorganic carbon isotope values (δ¹³C_org, δ¹³C_carb) have been documented from coeval sediment archives worldwide and have been proven for their chemostratigraphic correlation potential (e.g., Peti et al. 2017; van de Schootbrugge et al. 2018; De Lena et al. 2019; Mercuzot et al. 2019; Storm et al. 2020; Ruebsam and Al-Husseini 2021; Ullmann et al. 2021; Caruthers et al. 2022; Bodin et al. 2023). Therefore, chemostratigraphy can provide additional age constrains for sedimentary successions with partly limited biostratigraphic control. Thereby, it does not only contribute more robust dating but in certain cases allows for a significant increase in temporal resolution. Isotope trends or excursions often last shorter than the average duration of ammonite zones, which is approximately 1.5 Myrs for the Early Jurassic (e.g., Storm et al. 2020).

Throughout the Uppermost Triassic and Lower Jurassic strata, comparable trends in the δ¹³C_org values are seen in all three sediment archives investigated (Fig. 6). For core Schandelah, a robust biostratigraphic framework is based on age-diagnostic ammonites, calcareous nannoplankton, palynomorphs and ostracodes (van de Schootbrugge et al. 2018; Visentin et al. 2021). High ammonite occurrences and the enhanced preservation of calcareous nannoplankton in Jurassic strata can be linked to an offshore marine environments. Lower abundances of age-diagnostic marine index fossils in the cores KSS 5/66 and Barth 10/65 are explained by predominantly coastal–deltaic settings and temporal freshwater input during lowstands (Fig. 2). Moreover, biostratigraphic and chemostratigraphic (δ¹³C_org) data of core Schandelah can be correlated with data from the Mochras core (Cardigan Bay Basin, UK; Xu et al. 2018; Storm et al. 2020) (see Fig. S2 in the supplement). This proves that the trends in the δ¹³C_org values from the Schandelah core represent a robust chemostratigraphic framework for the Lower Jurassic of the North German Basin. Accordingly, we use the bio- and chemostratigraphic framework of core Schandelah as stratigraphic guidance and compare it with data from cores KSS 5/66 and Barth 10/65. Generic segments visually evident in δ¹³C_org data are assigned to δ¹³C_org rising limbs with steady increase in δ¹³C_org, δ¹³C_org falling limbs with steady decline in δ¹³C_org, δ¹³C_org valley intervals with low δ¹³C_org values and δ¹³C_org plateau intervals with heavy δ¹³C_org values, as previously applied by Ruebsam and Al-Husseini (2020, 2021).

Fig. 6

Suggested correlation of cores Schandelah, KSS 5/66 and Barth 10/65 based on biostratigraphic and chemostratigraphic (δ¹³C_org) data. For lithological key see Fig. 3. In the three cores, the TJB is not constrained by ammonites, but is evident in characteristic palynomorph assemblages (Fig. S1 in the supplement). In all sediment archives investigated, the early Toarcian carbon isotope excursion (Toa-CIE) is highly prominent. The Triassic/Jurassic carbon isotope excursion (T/J-CIEs) is observed in strata from cores Schandelah and Barth 10/65. In core KSS 5/66 the sample resolution is insufficient to record the T/J-CIE (see text for discussion)

In core Schandelah, δ¹³C_org values show an abrupt decline, superimposed by a sharp negative δ¹³C_org shift at the TJB that is preceded by two transient negative δ¹³C_org shifts in uppermost Triassic strata. These recurrent and transient negative carbon isotope excursions (T/J-CIEs) are reported from coeval strata worldwide and are interpreted to reflect global carbon cycle perturbations linked to the emplacement of the CAMP (e.g., Ruhl et al. 2009, 2020; Caruthers et al. 2022). A comparable pattern of negative δ¹³C_org shifts is also evident in the TJB interval of core Barth 10/65 (Fig. 6). In core KSS 5/66, the T/J-CIEs are not evident, which can be attributed to an incised deltaic channel fill, dated to the Deltoidospora–Concavisporites Zone, which cuts down to the lower Rhaetipollis–Limbosporites Zone (Barth et al. 2018a; see Fig. S1 in the supplement).

In the Schandelah core, sediments of the Hettangian and the lower Sinemurian (Bucklandi? and Semicostatum zones) record a δ¹³C_org valley. Sediments of the Sinemurian Turneri, Obtusum and Oxynotum zones record a δ¹³C_org plateau, while a δ¹³C_org decline (falling limb) is seen in the Raricostatum Zone. The Sinemurian δ¹³C_org plateau is further characterized by transient negative δ¹³C_org shifts that are also noted in sediment archives from adjacent basins (Obtusum and Oxynotum negative CIEs; Peti et al. 2017; Storm et al. 2020). Subsequent to the δ¹³C_org falling limb in the uppermost Sinemurian Raricostatum Zone, a δ¹³C_org valley is evident in the lower Pliensbachian Jamesoni to mid-Margaritatus zones. The upper Margaritatus Zone records a δ¹³C plateau, followed by a δ¹³C_org falling limb in the uppermost part of this zone. The Spinatum Zone corresponds to a δ¹³C_org valley (Fig. 6).

Throughout Hettangian to Pliensbachian, secular trends seen in the δ¹³C_org values in core Schandelah are also evident in data from cores KSS 5/66 and Barth 10/65. In addition, a comparable magnitude of isotopic change is observed in all sediment archives, with values mainly ranging from −27‰ to −24‰ (Fig. 6). In all three cores investigated, no significant organo-facies changes are observed in Rhaetian to Pliensbachian strata (see section: organo-facies characterization). Accordingly, comparable trends in the δ¹³C_org values can be assumed to reflect changes in the regional to transregional (potentially global) carbon cycle and can thus be used for chemostratigraphic correlation of the different sediment archives (Figs. 6, S2).

In all three sedimentary archives, most significant changes in the δ¹³C_org values occur in Toarcian strata dated to the upper Tenuicostatum and lower Falciferum zones. Here, a sharp decline in the ¹³C_org values marks the early Toarcian negative carbon isotope excursion (Toa-CIE), a prominent chemostratigraphic marker, that has been documented from coeval sedimentary archives worldwide (e.g., Hesselbo et al. 2000, 2007; Kemp et al. 2005; Bodin et al. 2010; Them et al. 2017; Izumi et al. 2018; for a review see Ruebsam and Al-Husseini 2020). The Toa-CIE has been interpreted to reflect a global carbon cycle perturbation, linked to the emission of huge quantities of ¹²C-enriched greenhouse gases (e.g., Hesselbo et al. 2000; Kemp et al. 2005; Beerling and Brentnall 2007; Them et al. 2017; Ruebsam et al. 2019). The Toa-CIE occurred in close temporal relationship with the emplacement of the K–F-LIP that may have triggered a cascade of environmental perturbations (e.g., Pálfy and Smith 2000). In core Schandelah and at the Hondelage clay pit, the Toa-CIE is immediately followed by a transient positive δ¹³C excursion (van de Schootbrugge et al. 2018; Mutterlose et al. 2022). This is followed by a δ¹³C_org falling limb and a δ¹³C_org valley is recorded in the mid–upper Toarcian strata dated to the upper Falciferum to Variabilis zones (Fig. 6). In cores KSS 5/66 and Barth 10/65, a transient positive excursion in δ¹³C_org values subsequent to the Toa-CIE is less pronounced, as δ¹³C_org values in post Toa-CIE strata remain fairly low (about −30 to −28‰). An increase in the δ¹³C_org to values of about −24‰ was determined in sediments of core KSS 5/66 in the uppermost Toarcian Glashütte Formation that is not older than the Thouarsense Zone (Fig. 6).

Bio- and chemostratigraphic data indicate that the three wells investigated are valuable sedimentary archives recording the depositional history along a proximal–distal transect in the North German Basin from the Rhaetian to the Toarcian. However, bio- and chemostratigraphic data also indincate that the sedimentary archives are fragmentary and contain several hiatuses and condensed intervals. This becomes apparent from the relations between the three δ¹³C_org records, but also when correlating the sections from the North German Basin with the near-complete archive of the Mochras core (Fig. S2). In the three cores investigated, Rhaetian to Sinemurian strata were depositied in nearshore and coastal–deltaic environments. Accordingly, sea-level fluctuations contributed to surfaces of marine erosion and channel incisions during lowstands as well as shoreface erosions during transgressions (Figs. 1c, 6; Zimmermann et al. 2015, 2018). These hiatuses are evident in the sedimentological records, but are not well expressed in the δ¹³C_org record, due to the lack of diagnostic transient δ¹³C_org trends (see Storm et al. 2020). In the Pliensbachian to early Toarcian, the successively rising sea-level contributed to geographically widespread marine environments (Barth et al. 2018b). As a result of retrogradational shoreline shifts, lowstand hiatuses are practically absent. On the contrary,  limited detrital input during transgressions and sea-level high stands  led to strongly reduced sedimentation rates (Fig. 6). This is most pronounced in core Barth 10/65, in which the entire lower Pliensbachian (Jamesoni to lower Margaritatus zones, duration ca. 5.5 Myr; Storm et al. 2020), corresponding to a long-lasting δ¹³C_org valley, is represented by a sedimentary succession of only 13 m in thickness (Fig. 6). The same refers to the early Toarcian, of which the interval of the Semicelatum to Elegantulum subzones is represented by the ca. 1.5 m thick Posidonienschiefer in cores KSS 5/66 and Barth 10/65. The late Pliensbachian to early Toarcian record high-amplitude sea-level cycles with major stratigraphic gaps occurring upon sea-level lowstands (e.g., Morard et al. 2003; Suan et al. 2011; Peti et al. 2017; Krencker et al. 2019; Bodin et al. 2023). In the herein investigated cores KSS 5/66 and Barth 10/65, a hiatus preceding the Toa-CIE is recognized at the base of thin heterolithic sandstones below the Posidonienschiefer. The hiatus is associated to shoreface erosion during the transgressive phase of sequence Toa 1 sensu Zimmermann et al. (2015) and corresponds to the disconformable contact of upper Pliensbachian and lower Toarcian strata at Grimmen and Hondelage (Ernst 1967; Mutterlose et al. 2022). Although geographically widespread, the amount of erosion seems to be limited as the hiatus occurs within lower Toarcian strata in core KSS 5/65. Correlations with complete successions in the western North German Basin described by Hoffmann and Martin (1960) point to erosion of less than 2 m.

Stratigraphic constraints on the Toarcian Posidonienschiefer and Grimmen formations

In cores KSS 5/66 and Barth 10/65, the organic-rich shales of the Posidonienschiefer Formation are ca. 1.5 m thick and stratigraphically restricted to the uppermost Tenuicostatum and the lower Falciferum zones (Figs. 4, 5). In contrast, in core Schandelah the about 37 m-thick organic-rich shales of the Posidonienschiefer Formation span the uppermost Tenuicostatum Zone to the Bifrons Zone, and potentially extends into even younger ammonite zones (Fig. 3). This corresponds to Hoffmann (1960), who already noted that deposition of TOC-rich sediments lasted much longer in the western NGB.

In core Schandelah and the neighboring Hondelage site, the Toa-CIE occurs in the basal part of the Posidonienschiefer Formation (van de Schootbrugge et al. 2018; Visentin et al. 2021; Mutterlose et al. 2022) (Figs. 3, 6). In cores KSS 5/66 and Barth 10/65, the condensed Posidonienschiefer Formation approximately corresponds to the Toa-CIE onset interval and parts of the Toa-CIE core interval (δ¹³C_org falling limb and valley; see Ruebsam and Al-Husseini 2020). The upper part of the Toa-CIE core interval and the recovery interval (δ¹³C_org valley and rising limb) are recorded by sediments of the basal Grimmen Formation (Figs. 4, 5). This major change in sedimentation is constrained to the Exaratum Subzone in core KSS 5/65 and neighboring wells (Bauss 1976). In core KSS 5/65, the upper Grimmen Formation is constrained to the lower Bifrons Zone and the Dörnten Member (upper Posidonienschiefer) above is constrained to the interval of the upper Bifrons to Thouarsense zones (Bauss 1976).

Cyclostratigraphic analysis of natural gamma ray (GR) data can provide additional information on the time represented by a sediment succession. In cores KSS 5/66 and Barth 10/65, a marked cyclicity is seen in the GR data of the (silty) claystones of the Grimmen Formation (Fig. 7; see supplement for additional information). Cyclostratigraphic data combined with chemostratigraphic and biostratigraphic data allow to propose a correlation scheme for the cores KSS 5/66 and Barth 10/65 with the Dotternhausen Section in southern Germany, for which detailed biostratigraphic, chemostratigraphic and cyclostratigraphic data are available (e.g., Riegraf 1985; Röhl et al. 2001; Ruebsam et al. 2023).

Fig. 7

Correlation of δ¹³C_org trends and gamma ray (GR) cycles of the Toarcian Grimmen Formation in cores KSS 5/66 and Barth 10/65 with ¹³C_org trends and major carbonate cycles detected in the Dotternhausen Section in southern Germany (e: 100 kyr short eccentricity in black, E: 405 kyr long eccentricity in blue; for details on the cyclostratigraphic analysis and methodology see supplementary information). Data for the Dotternhausen Section are shown in the time domain (see Ruebsam et al. 2023). Chemostratigraphic and cyclostratigraphic correlation suggest that the Grimmen Formation encompasses most of the Falciferum Zone, as well as the Bifrons, Variabilis (V.) and Thouarsense (Th.) zones. Note: In core KSS 5/66, the upper Grimmen Formation is represented by the Glashütte and the Dörnten Members. Both members are absent in core Barth 10/65, reflecting spatial differences in depositional conditions at the northeastern margin of the North German Basin

The proposed correlation scheme shown in Fig. 7 points to a hiatus at the base of the Toarcian in cores KSS 5/66 and Barth 10/65, which is consistent with other localities in southern Germany and southern Lower Saxony (e.g., Prauss 1996; Arp and Gropengießer 2015; Arp et al. 2021, 2023). Another hiatus seems to be present at the base of thin heterolithic sandstones below organic-rich shales of the Posidonienschiefer in both cores. Moreover, the onset and core interval of the Toa-CIE appear to be extremely condensed. Similar observations have been made in coeval sections from other paleogeographic areas, were the occurrence and stratigraphic position of hiatuses and condensed intervals can be linked to eustatic sea-level variation (e.g., Krencker et al. 2019, 2022; Ruebsam et al. 2019, 2023; Ruebsam and Al-Husseini 2020; Bodin et al. 2023). Combined chemo- and cyclo-stratigraphic correlations are in agreement with biostratigraphic data for core KSS 5/65 and neighboring cores (Bauss 1986). Accordingly, the Grimmen Formation in core KSS 5/65 extents up into the lower Bifrons Zone, and the lower Glashütte Formation is contemporaneous to the Grimmen Formation in core Barth 10/65, corresponding to previous litho- and sequence-stratigraphic correlations (Zimmermann et al. 2015; Barth et al. 2018b). It is further interesting to note that cyclostratigraphic analysis provides timelines for the facies transitions between the cores KSS 5/66 and Barth 10/65 (see discussion in section: Spatiotemporal pattern in geochemical proxies). During the Toarcian, the site of core KSS 5/66 was situated in proximity to a delta front (distal mouth bar) and thus received substantial amounts of river-transported coarse clastics that are expressed in sandstone of the Glashütte Member (Figs. 2, 4, 7) (Zimmermann et al. 2015, 2018). Deposition of the TOC-rich Dörnten Member might be related to the position in the more distal and deeper part of the basin, situated below the wave/storm base (Fig. 2). On the contrary, Toarcian strata of core Barth 10/65 represents a setting north of the delta, at which river-transported coarse clastics were bypassed (Zimmermann et al. 2015, 2018). The lack of TOC-rich sediment in the in the Grimmen Formation most likely reflects the enhanced degradation of biomass in shallow marine oxic setting above the fair weather wave and storm base.

Organo-facies characterization

Organic matter maturation

Samples from the cores investigated show variable T_max values that all fall in the range of 406–436 °C. The few samples with T_max values < 400 °C from cores KSS 5/66 and Barth 10/65 were excluded here as such low T_max values are considered to be unreliable and result from a very low intensity of the S2 peak and an unprecise T_max determination. Due to the broad range in the T_max values of about 30 °C, an exact thermal maturity evaluation is complicated (e.g., Espitalié et al. 1977; Bordenave et al. 1993). A depth relationship of the T_max values is not observed (Figs. 3–5). Thus, differences in the thermal maturity of the organic matter fail in explaining the variable T_max values. The widespread in the T_max values may therefore be explained by variations in the organo-facies and kerogen composition, which is accompanied by differences in the stability of the kerogen toward thermal breakdown during pyrolysis (Espitalié et al. 1977; Bordenave et al. 1993). Data attest to a heterogenous kerogen composition, comprising variable relative abundances of marine/aquatic, land plant and refractory organic matter. The latter can comprise strongly degraded kerogens as well as fossil re-deposited kerogen (Espitalié et al. 1977; Bordenave et al. 1993), which has been documented for Toarcian strata of the Polish Basin (Ruebsam et al. 2020). Despite the variability of the T_max values, samples from all cores investigated plot in the field for immature organic matter that has not reached the oil window (Fig. 8), confirming that the organic matter has not experienced a high degree of thermal alteration during burial. Accordingly, HI and OI values were not substantially affected by thermal alteration and primary vary in dependency of organic matter sources and early diagenetic preservation (Espitalié et al. 1977; Bordenave et al. 1993).

Fig. 8

Cross-plots of OI versus HI values and of T_max versus HI values for the Schandelah core (a, b), core KSS 5/66 (c, d) and core Barth 10/65 (e, f). Toarcian* defines the entire Toacian, excluding samples from the Toa-CIE interval. Kerogen classification after Espitalié et al. (1977) and Bordenave et al. (1993). At all sites, the Posidonienschiefer Formation (PS Fm) varies in its stratigraphic extent. In core Schandelah the PS Fm covers most of the Toarcian, while in cores KSS 5/66 and Barth 10/65, only the lower part of the Toa-CIE corresponds to the PS Fm. In core KSS 5/66 the Dörnten Member (D Mb) corresponds to the upper part of the PS Fm in core Schandelah

Organic matter composition

The composition of the sedimentary organic matter (kerogen type), in terms of its sources and state of preservation, can be inferred from HI and OI values (Espitalié et al. 1977; Bordenave et al. 1993). Moreover, changes in the T_max values will also occur in response to compositional changes of the kerogen, as a significant impact of thermal maturity on the T_max values can be excluded (see section: organic matter maturation).

In the cores investigated, most of the samples from the Rhaetian, Hettangian, Sinemurian and Pliensbachian plot in the field of kerogen types III-IV, indicating that the sedimentary organic matter is composed of land plant remains and strongly degraded marine/aquatic organic matter (Figs. 8a, c, e), which is consistent with the existing facies model (Zimmermann et al. 2015; Barth et al. 2018b). Sediments may also contain re-deposited refractory organic matter. The heterogeneity of the kerogen composition is expressed by highly variable T_max values (406–436 °C) (Figs. 8b, d, f). In cores Schandelah and KSS 5/66, slightly increased HI and decreased OI values are seen in samples from the Pliensbachian, indicating slightly increased abundances of marine organic matter. In core Barth 10, the HI and OI values from the Pliensbachian cannot be distinguished from those seen in Rhaetian, Hettangian and Sinemurian strata (Fig. 8e).

In all three cores, well-preserved marine/aquatic organic matter of kerogen types I-II solely occurs in samples from the TOC-rich Toarcian Posidonienschiefer Formation. In cores KSS 5/66 and Barth 10/65, this formation only covers the lower Toa-CIE interval, Toa-CIE onset, and core intervals, while in core Schandelah it spans a wider stratigraphic range (see section: chemostratigraphy). Highest HI values > 600 mgHC/gTOC occur in the Posidonienschiefer Formation of the Schandelah core (Fig. 8a). Here, the sedimentary organic matter is of predominantly marine origin. The T_max values stay below 425 °C, indicating the presence of marine, organic matter-derived kerogen that is less resistant towards thermal breakdown. Due to the narrow range of the HI values, the variability in the T_max values is best explained by variations in organic matter preservation and earliest diagenesis (e.g., Baskin and Peters 1992; Lewan 1998; Lewan and Ruble 2002). Lower and more variable HI values in the range of 170–700 mgHC/gTOC and 300–650 mgHC/gTOC occur in the Posidonienschiefer Formation of cores KSS 5/66 and Barth 10/65, respectively. Data indicate that the sedimentary organic matter is of predominantly marine origin with subordinate, but variable contributions from land plants (Figs. 8c, e). In addition, the marine organic material might have been increasingly exposed to aerobic degradation processes, causing the moderate to minor HI-decline (Pratt 1984). The variability of the T_max values support this interpretation (Figs. 8d, f).

Organic matter of kerogen type II is also confirmed for samples from the TOC-rich Dörnten Member of the Toarcian Grimmen Formation (Fig. 8c). Compared to samples from the lower Posidonienschiefer Formation, samples from the Dörnten Member show lowered HI and slightly increased T_max values, which point to the presence of marine organic matter, partially exposed to early aerobic degradation (Fig. 8d).

In cores KSS 5/66 and Barth 10/65, sedimentary organic matter in TOC-lean samples from Grimmen Formation can be attributed to kerogen types III/IV, corresponding to strongly degraded marine and land plant-derived organic matter, as well as substantial amounts of refractory organic matter.

Organo-facies and isotopic composition of the organic matter

In all three sediment archives investigated, the sedimentary organic matter in samples from Rhaetian to Pliensbachian strata can be attributed to kerogen types III-IV. Substantial variations in the kerogen type are not evident (see section: organic matter composition). Accordingly, trends in δ¹³C_org values can be interpreted to reflect transregional to global carbon cycles dynamics, while the impact of organo-facies on the δ¹³C_org values can be assumed of being minor (e.g., Tyson 1995). This interpretation is supported by the parallel evolution of trends in the δ¹³C_org values in all three sediment archives investigated, as well as its correlatability with trends documented from coeval strata in distant sections (Figs. 6, 7, S2; see section chemostratigraphy).

Major organo-facies changes, however, occur in Toarcian strata and are associated with the deposition of the Posidonienschiefer Formation (kerogen type I/II) and the Dörnten Member (kerogen type II). These changes towards a more marine kerogen are accompanied by a decline of the δ¹³C_org values, as previously documented from coeval sediment archives (van de Schootbrugge et al. 2013; Suan et al. 2015; Fantasia et al. 2019).

At all location investigated in this work, lower Toarcian strata record a major shift to lighter δ¹³C_org values. A decline in δ¹³C_org values from about −26 to −32‰ is seen in the lowermost Posidonienschiefer Formation in the Schandelah core. In cores KSS 5/66 and Barth 10/65, δ¹³C_org values decline from about −26‰ to about −33‰. In both cores, lowest δ¹³C_org values (< −32‰) occur in the thin interval of the Semicelatum to Elegantulum subzones. However, very low δ¹³C_org values in the range −32 to −30‰ also occur in samples from the lower Grimmen Formation above. This major shift in the δ¹³C_org values is a common feature of all stratigraphically complete lower Toarcian sediment archives and represents the Toa-CIE. The magnitude of isotopic change associated with the Toa-CIE reflects both: i) organo-facies (shift from type III/IV to type I/II kerogen) and ii) global changes in the carbon cycle (Suan et al. 2015; Fantasia et al. 2019; Remírez and Algeo 2020).

In the cross-plot of HI versus δ¹³C_org values, most samples analyzed plot along the Mesozoic organo-facies trend established by Tyson (1995). This includes all samples from the Rhaetian to Pliensbachian, samples from the post Toa-CIE Posidonienschiefer Formation in core Schandelah, as well as samples from the Dörnten Member and the upper Glashütte Formation in core KSS 5/66 (Fig. 9). Samples differing from this organo-facies trend include samples from the Toa-CIE, but also samples from post-Toa-CIE strata of the Grimmen Formation (Fig. 9). In cores KSS 5/66 and Barth 10/65, TOC-lean samples from the post-Toa-CIE strata of the Grimmen Formation show very low δ¹³C_org values (−30 to −28‰), although the sedimentary organic matter is of kerogen type II or IV. It is therefore unlikely that low δ¹³C_org values are explained by organo-facies. However, explaining the low δ¹³C_org values in samples from the Grimmen Formation is beyond the scope of this study and requires additional data.

Fig. 9

a Cross-plot of HI versus δ¹³C_org values for different stratigraphic intervals in cores Schandelah, KSS 5/66 and Barth 10/65. Organo-facies trends for the Mesozoic (here only represented by the Jurassic and Cretaceous) and the Carboniferous are from Tyson (1995) but are based on a limited number of samples only. b Cross-plot of HI values with δ¹³C_org values illustrating different parameters known to impact on HI and/or δ.¹³C_org values (see Tyson 1995).

Spatio-temporal pattern in geochemical proxies

Secular trends

Comparable trends in the δ¹³C_org values, TOC content and organic matter composition are seen at all investigated sites, confirming that these parameters were controlled by the interactions of global processes (e.g., climate, eustatic sea-level changes) and regional factors (e.g., basin evolution, surface runoff, paleogeography).

A decline in the median δ¹³C_org values occurred in the Hettangian and was followed by a return to slightly heavier median values in the Sinemurian and Pliensbachian strata. The lowermost Toarcian records a marked drop in median δ¹³C_org values by about 5‰, marking the Toa-CIE. Upwards, median δ¹³C_org values slightly increase in strata of the middle–upper Toarcian (Fig. 10). Trends in the δ¹³C_org values indicate that most profound changes in the carbon cycles occurred across the TJB and in the early Toarcian. Both, reconcile intensified volcanism during the emplacement of the CAMP and the K–F-LIP, respectively (e.g., Pálfy et al. 2002; Ruhl et al. 2020; Ware et al. 2023).

Fig. 10

Box and whisker plot showing generalized patterns in the geochemical data for upper Rhaetian to Toarcian stages in strata from cores Schandelah, KSS 5/66 and Barth 10/65 (¹includes data from Amaltheenton that extent into the lowermost Toarcian; ²Toa-CIE interval only, ³post-Toa-CIE interval). Red numbers and dashed lines are stage median values

Low abundances of sedimentary organic matter attributed to kerogen types III/IV are seen in Rhaetian to Pliensbachian strata (median TOC values 1.0–1.3 wt.%). This indicates that throughout the Latest Triassic and most of the Early Jurassic shallow and oxic depositional conditions at this part of the shelf prevented the preservation of large quantities of marine and land plant organic matter (Fig. 10). A minor increase in median HI values is seen in the Pliensbachian, but is not accompanied by a marked increase in median TOC values (Fig. 10). A major change in organic matter burial occurred in the lower Toarcian and is indicated by a substantial increase in the TOC contents (median TOC content 7.8 wt.%) in all sediment archives investigated. This marked increase in sedimentary TOC abundances resulted from the enhanced preservation of marine organic matter upon a sea-level highstand as confirmed by high HI and low OI values (Fig. 10). Median TOC abundances decline in middle–upper Toarcian strata, but also show a more location-specific pattern. The data confirm that the early Toarcian represents a major organic carbon burial event associated with exceptional environmental and depositional conditions not encountered in this paleogeographic region throughout the Uppermost Triassic and the following Lower Jurassic.

The minor increase in median HI values in the Pliensbachian and the major increase in median TOC and HI values in the lower Toarcian reconcile major sea-level rise events in the northern part of the CEES (Fig. 1c; Zimmermann et al. 2015; Barth et al. 2018b), and potentially also in other parts of the world (second order transgression JPl8SB-JTo5MFS; Haq 2018). In combination with expanding oxygen-deficient conditions, eustatic sea-level changes can be considered as a major factor for preservation of marine organic matter, during the Toa-OAE (e.g., Jenkyns 1988). A high sea-level has also been reconstructed for the early Pliensbachian (JSi5SB-JPl4MFS; Zimmermann et al. 2015; Barth et al. 2018b; Haq 2018) and was accompanied by an organic carbon burial event of rather regional extent (Hoffmann 1960; Silva et al. 2021).

Spatial trends

Spatial patterns in the geochemical proxies can be linked to local depositional conditions that changed for distinct time intervals along the distal–proximal transect (Fig. 2). For example, an increase in the thickness of Lower Jurassic (Hettangian-Toarcian) strata towards the northeastern margin of the North German Basin is seen (Schandelah: ~ 310 m; KSS 5/66: ~ 340 m; Barth 10/65: ~ 450). A higher sediment thickness at proximal sites can be explained by a short distance to the clastic sediment source. However, such a consistent pattern is not evident at stage level (Fig. S6 in the supplement). This highlights the role of local depositional processes that controlled accommodation space and sediment accumulation.

A consistent pattern is seen in the carbonate content that within each stage declined along the distal–proximal transect (Fig. 10). Highest carbonate contents in sediments from core Schandelah reflect a stronger influence of marine conditions and a higher abundances of calcareous nannoplankton at this more distal site (van de Schootbrugge et al. 2018; Visentin et al. 2021). On the contrary, lowest carbonate contents seen in core Barth 10/65 reflect a proximal depositional setting that was stronger affected by the influx of siliciclastic sediment from the nearby landmass (Figs. 2, 10). In core Barth 10/65 high carbonate values, however, occur in the lower Pliensbachian (Jamesoni to Davoei zones) that correspond to a sea-level highstand (Fig. 1c).

No consistent trends are seen in TOC, HI and OI values for the Rhaetian, Hettangian and Sinemurian. In the Rhaetian and Hettangian, TOC abundances increase along the distal–proximal transect. However, this trend is not accompanied by a systematic change in HI and OI values. This indicates that amount and composition of the sedimentary organic matter varied in dependency of local factors, such as depositional conditions, preservation of labile versus degraded marine organic matter and land plant organic matter supply. From the Pliensbachian on, a decrease in TOC and HI values that correlates with an increase in OI values is seen along the distal–proximal transect. This pattern confirms that higher abundances of better preserved marine organic matter occur in more distal settings that are characterized by higher water depths associated with low-energy depositional environments. Spatial differences in TOC and HI values seem to be most significant upon sea-level highstand, when differences in the depositional setting along the distal–proximal transect were most pronounced.

A systematic trend in the δ¹³C_org values along the distal–proximal transect is not apparent. For the Pliensbachian, an increase in the δ¹³C_org values along the distal–proximal transect that coincides with a decline in HI values most likely reflects increased abundances of degraded marine or land plant-derived organic matter in the proximal setting of core Barth 10/65 (Figs. 2, 10). Such a pattern of δ¹³C_org and HI values is also indicated for the Sinemurian, but absent in the other stratigraphic intervals. This indicates that the sedimentary δ¹³C_org signature is affected by several factors and processes, such as organic matter sources and preservation, marine primary productivity, CO₂ recycling in stratified basins, or precipitation and humidity (e.g., Küspert 1982; Tyson 1995).

Conclusions

Bulk organic (HAWK programmed pyrolysis) and δ¹³C_org data for sediments from cores Schandelah, KSS 5/66 and Barth 10/65 allow discussion of spatiotemporal trends in depositional conditions and organo-facies throughout the Uppermost Triassic (Norian–Rhaetian) and the Lower Jurassic (Hettangian–Toarcian). The three study sites are located along a distal–proximal transect, implying a distinct sensitivity of each depositional setting towards changes in environmental parameters (e.g., sea level, climate, sediment supply).

Biostratigraphically anchored trends in δ¹³C_org values allow the correlation of strata exposed in the study sites along the transect, as well as correlation with successions of distant sites. At all sites, secular trends in δ¹³C_org are punctuated by transient negative carbon isotope excursions, reflecting perturbations of the global carbon cycle at the TJB and in the early Toarcian. Spatial differences in the thickness of presumable chronostratigraphic units (biozones, δ¹³C_org segments) reflect location-specific depositional processes that controlled sediment accumulation. Stratigraphic gaps occurred during sea-level lowstands and are most pronounced at more shallow proximal sites. Reduced detrital input and stratigraphic condensation appears to be most significant during sea-level highstands.

Programmed pyrolysis data indicate spatiotemporal organo-facies trends that on a temporal scale occurred in response to sea-level and climate trends, while spatial pattern reflect basin morphology, paleobathymetry, and proximity to terrigenous organic matter sources. Substantial marine organic matter accumulations only occurred in Toarcian strata during high sea level and were most continuous at distal sites. At proximal sites, TOC-rich strata are of limited thickness. The spatiotemporal variability along the transect studied here confirms that reconstruction of paleoenvironmental conditions of sedimentation, including the evolution of environmental perturbations, should preferably not be based on a single site but be conducted in a regional context to apprehend the intrinsic heterogeneity of geological systems.