Introduction

The Late Ordovician mass extinction (LOME) is the second largest mass extinction of the Phanerozoic1,2, and has been linked to the Hirnantian glaciation2. The LOME is subdivided into two intervals (LOMEI-1 and LOMEI-2) which occurred at the onset and the end of the Hirnantian glaciation, respectively2,3,4,5. The LOME is marked by the disappearance of ~ 85% marine species2,6,7 or ~ 53% of marine genera8. Massive environmental changes were coeval with the LOME, such as rapid global cooling (Hirnantian glaciation) corresponding to expansion of ice sheets on the Gondwana supercontinent9,10,11,12, and expanded anoxia and/or euxinia in ocean4,13,14,15,16,17.

In the geologic record, mass extinction events and global climate changes are often associated to the emplacement of a large igneous province (LIP; e.g., Ref.18), but no LIP basalts have been found in the LOME interval, although a Suordakh LIP was postulated19 mainly based on the evidence of the poorly aged-constrained Suordakh dolerite eruption within the Katian20. Volcanic activity during the LOME has been indirectly inferred by coeval mercury (Hg) peaks found in sedimentary successions (e.g., middle Katian, upper Katian, LOMEI-1 at uppermost Katian and LOMEI-2 in upper Hirnantian) at Wangjiawan (in South China)21, Monitor Range (Laurentia)22, drillhole XY5 (South China) and Vinini Creek (Laurentia)23, and peri-Baltic region24.

Sedimentary Hg has been increasingly used as a tracer for volcanic activity during mass extinction events (e.g., ref.25). Volcanism is the primary sources of atmosphere Hg before the Anthropocene26 and, given its short residence time in the atmosphere, gaseous volcanic Hg is easily transported and deposited in different depositional environments globally25,27. Therefore, sedimentary Hg can be used as a proxy for volcanism25,28,29,30,31. However, increases of Hg deposition in different depositional setting can be also linked to other (local) factors, such as increase of riverine Hg transport to marine settings32, massive oxidation of terrestrial organic matter16,31, and development of euxinic conditions33. Hence, a peak of Hg concentration in the sedimentary record does not unequivocally indicate massive coeval volcanic activity.

In this study, we present new Hg concentration data across the Ordovician–Silurian boundary from three sections at Shuanghe, Qiliao and Tianba on the Yangtze Shelf Sea in South China4,14. Combined with previous data on redox conditions and climate changes from the same outcropped sections4, these Hg data are used to explore Hg deposition during the LOME in the study area, shedding new light on the origin of the Hg peaks coeval to the biological and environmental changes.

Geological setting

During the Ordovician–Silurian transition, the Yangtze Platform (South China Block) was located near the equator34 (Fig. 1). After the middle Katian, it gradually evolved into a siliciclastic-dominated shelf basin, called the Yangtze Shelf Sea4,35. The shale strata, which include the Late Ordovician Wufeng Formation and early Silurian Lungmachi Formation, or Wufeng-Lungmachi Shale, deposited on the shelf with deepening northwards to the Panthalassic Ocean4,36,37. The bottom black shale interval of Wufeng-Lungmachi Shale in South China corresponds to typical, organic-rich shales (hot shale, i.e., more radioactive shale)7,38. Owing to the glaciation, a rapid sea level drop occurred during the Hirnantian. Paleo-water-depth in the Yangtze Shelf Sea during this glaciation was likely about 40–100 meters37. The Kuanyinchiao Bed at the top of the Wufeng Formation was formed during this glaciation time (Fig. 2), and contains characteristic cold water Hirnantian fauna. The marine carbonate Kuanyinchiao Bed is widely distributed, and has a conformity contact with the underlying Wufeng Formation in the study area. High-resolution graptolite zones have been previously identified in South China39, providing a solid biostratigraphic framework and allowing correlation with other Ordovician–Silurian boundary sections around the world7.

Figure 1
figure 1

Geological setting. (a) Late Ordovician (ca. 445~444 Ma) paleogeography. Adapted from Ron Blakey, © 2016 Colorado Plateau Geosystems Inc.; (b) Simplified paleogeographic map of the Yangtze Shelf Sea during the Late Ordovician4, showing section localities analysed during the present study, including the Shuanghe (SH) inner shelf outcrop section, Qiliao (QL) mid-shelf outcrop section and the Tianba (TB) outer shelf-slope drill core section. Scale bar = 100 km. Also included in (a) and (b) are additional sites with Hg anomalies > 100 ppb. These sites comprise: 1. Vinini Creek, United States23; 2. Monitor Range, United States22; 3. Dob's Linn, Scotland53; 4. Zbrza PIG-1, Poland24; 5. Shuanghe (SH), South China; 6. Qiliao (QL), South China; 7. Tianba (TB), South China; 8. Yanzhi, South China48; 9. XY5, South China23; 10. Jiaoye, South China48; 11. Wangjiawan, South China21,22; 12. Dingjiapo, South China21.

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Figure 2
figure 2

Chemostratigraphy of organic-carbon isotope, Hg/TOC, TOC, Hg/TS, TS, Hg concentration and Fe species across the O–S boundary from the Shuanghe (a), Qiliao (b) and Tianba (c) in South China. Organic-carbon isotope and Fe species data are from Ref.4. TOC data are mainly from Ref.4,14. Intensity of volcanic activity was estimated by distribution and thickness of volcanic ash layers deposited in South China across the Ordovician and Silurian transition. Graptolite zones: D. cn. Dicellograptus complanatus; D. cx. Dicellograptus complexus; P. pacificus Parakidograptus pacificus; M. e. Metabolograptus extraodinarius; M. P. Metabolograptus persculptus; A. a. Akidograptus ascensus; P. a. Parakidograptus acuminatus; C. v. Cystograptus vesiculosus; C. c. Coronograptus cyphus.

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From the Late Ordovician to early Silurian, volcanic ash layers deposited are extensively reported, especially in North America40 and South China41,42,43. In North America, over 100 volcanic ash layers dominantly occurred in pre-late Katian stage40. There were two pulses of volcanic ash layers deposited in South China in the Late Ordovician and early Silurian42. The first pulse occurred in late Katian stage41,42,44 and the second pulse erupted around the boundary between the Rhuddanian stage and the Aeronian stage (ca. 440.8 Ma)42,45.

Materials and methods

Fresh rock samples were collected from three sections (Shuanghe, Qiliao and Tianba sections) that were deposited from proximal to distal areas on the Yangtze Shelf Sea, South China (Fig. 1). For each section, high-resolution graptolite zones have been previously defined4. Previous studies have reported, from the same rock samples, TOC contents, C-isotopes, Fe-speciation, major elements and trace elements concentrations4,12,46. In this study, we measured Hg concentration and TS content of all samples, and the TOC content of 6 new samples.

Hg concentration was measured using a Lumex RA-915 M mercury analyzer with pyrolyzer PYRO-915 + at State Key Laboratory of Biogeology and Envionmental Geology, China University of Geosciences. An aliquot of ~ 50 mg of powdered sample was weighed in a glass boat and was heated in the pyrolyzer at 700 °C. Volatilized Hg concentration was quantified via atomic absorption spectrometry. A soil standard (GSD-17a; Hg = 120 ± 10) was used to calibrate the instrument. Repeated measurements of the standard at the start of each run and throughout the analysis sequence indicate reproducibility was generally better than 10% for Hg concentrations.

For the measurement of TOC content, sample powders (2 g) were decarbonated with HCl (10% vol/vol) prior to TOC analyses on a LECO CS-230 analyzer. TS was measured directly by bulk sample. Analytical precision was generally better than 5% and 8% for TOC, and TS contents, respectively. Hg concentrations have been normalized for TOC and TS contents25,33.

Results

At Shuanghe section, Hg concentrations range from 5 to 161 ppb (average is 61 ppb) (Table 1). Background Hg concentration is ~ 50 ppb. Higher Hg concentrations are found at the base of the lower Katian Wufeng Formation, in the upper Katian, LOMEI-1 (end-Katian), LOMEI-2 (Hirnantian), and in the upper Rhuddanian (Fig. 2). At Qiliao section, Hg concentrations range from 27 to 600 ppb (average is 245 ppb) (Table 1). Background Hg concentration is ~ 80 ppb. Higher Hg concentrations occur at LOMEI-1, LOMEI-2, and in the upper Rhuddanian (Fig. 2). At Tianba section, Hg concentrations range from 8 to 498 ppb (average is 130 ppb) (Table 1). Background Hg concentration is ~ 45 ppb. Hg spikes occur in the upper Katian, LOMEI-1, LOMEI-2, and upper Rhuddanian (Fig. 2).

Table 1 Hg concentration, TOC and TS contents, Hg/TOC, Hg/TS ratios at Shuanghe (SH), Qiliao (QL) and Tianba (TB) sections, South China. 4,14
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At Shuanghe, TOC contents range from 0.1 to 8.8% (average is 3.8%)4,14 (Table 1). Higher TOC values occur at base of the early Katian Wufeng Formation, LOMEI-1, and LOMEI-2 (Fig. 2). At Qiliao, TOC contents range from 2.6 to 14% (average is 6.0%)4 (Table 1). Higher TOC values occur at LOMEI-1 and LOMEI-2 (Fig. 2). At Tianba, TOC contents range from 0.1 to 16% (average is 3.8%) (Table 1). TOC increases at base of the lower Katian Wufeng Formation, LOMEI-1, and LOMEI-2, respectively (Fig. 2).

At Shuanghe, TS contents range from 0.1 to 4.0% (average is 1.3%) (Table 1). TS increases at base of the low-Katian Wufeng Formation, in the upper Katian, and at LOMEI-1 and LOMEI-2 (Fig. 2). At Qiliao, TS contents range from 0.1 to 5.6% (average is 1.3%) (Table 1). Higher TS occurs at LOME-1 and LOME-2 (Fig. 2). At Tianba, TS contents range from 0.1 to 0.5% (average is 0.16%) (Table 1). TS is higher in lower Katian Linxiang Formation, and at LOMEI-1 and LOMEI-2 (Fig. 2).

At Shuanghe, peaks of Hg/TOC occur in the middle Katian at the base of the Wufeng Formation (from ~ 20 to ~ 80 ppb/wt.%), end-Katian LOMEI-1 (from ~ 10 to 30 ppb/wt.%) and late-Hirnantian LOMEI-2 horizons (from ~ 18 to ~ 60 ppb/wt.%) (Fig. 2). At Qiliao, peaks of Hg/TOC occur at LOMEI-1 (~ 20 to ~ 50 ppb/wt.%), LOMEI-2 (from ~ 40 to ~ 65 ppb/wt.%), and in the upper Rhuddanian (from ~ 50 to ~ 100 ppb/wt.%) (Fig. 2). At Tianba, peaks of Hg/TOC occur in the middle-Katian Linxiang Formation (from ~ 30 to ~ 280 ppb/wt.%), in the upper Katian (from ~ 30 to ~ 140 ppb/wt.%), and at LOMEI-1 (from ~ 40 to ~ 220 ppb/wt.%) (Fig. 2).

At Shuanghe, peaks of Hg/TS occur at the middle-Katian Linxiang Formation (from ~ 40 to ~ 150 ppb/wt.%), upper Katian, LOMEI-1 (from ~ 40 to ~ 120 ppb/wt.%) and LOMEI-2 horizons (from ~ 40 to ~ 150 ppb/wt.%) (Fig. 2). At Qiliao, peaks of Hg/TS occur in upper Katian (from ~ 1000 to ~ 3000 ppb/wt.%), at the LOMEI-1 (from ~ 250 to ~ 1200 ppb/wt.%), and in the upper Rhuddanian (from ~ 200 to ~ 3200 ppb/wt.%). At Tianba, peaks of Hg/TS occur in the upper Katian (from ~ 70 to ~ 2400 ppb/wt.%), LOMEI-1 horizon (from ~ 700 to ~ 5200 ppb/wt.%), LOMEI-2 horizon (from ~ 1700 to ~ 3500 ppb/wt.%), and in the upper Rhuddanian (from ~ 1800 to ~ 7000 ppb/wt.%) (Fig. 2).

Discussion

Hg host and digenesis

Hg concentrations in sediments are controlled mainly by local depositional environment, primary volcanic loading, and post depositional diagenesis47. In general, Hg is mainly associated with organic matter under “normal” conditions and with sulfide only under strong euxinic conditions28,47,48. Hg/TOC or Hg/TS ratios are then used to assess the excess input of Hg besides of the sources from Hg-TOC complexes or HgS33,48,49.

Cross-plots between TOC and Hg (Fig. 3) show that Hg concentrations have no correlation with TOC contents under euxinic environments but have positive correlation (R2 is 0.5 to 0.73, Qiaoliao and Tianba sections) or weak correlation (R2 = 0.18, Shuanghe section) with TOC contents under non-euxinic environments. It suggests that organic matter is an important host of Hg in these marine sedimentary rocks owing to the association between Hg and organic matter and the reactive Hg organic complexes50. Cross-plots between TS and Hg (Fig. 3) show strong correlation (R2 = 0.81, Qiaoliao section) to no correlation (R2 is less than 0.31, Shuanghe and Tianba sections) under euxinic environments, but show no correlation under non-euxinic environments. This suggests that sulfide is not a host of Hg under non-euxinic conditions but may be a host of Hg during euxinic intervals48.

Figure 3
figure 3

Crossplots of TOC vs. Hg, TS vs. Hg of samples from euxinic and non-euxinic conditions across the Ordovician–Silurian boundary at Shuanghe (a,b), Qiliao (c,d), and Tianba (e,f). The euxinic and non-euxinic conditions are determined by Fe species and trace elements from ref.4.

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Normalization of Hg concentrations to TOC and TS can reflect Hg anomalies. Hg/TOC ratios could be inflated because of diagenetic degradation of organic matter, which lowers TOC content22, or due to analytical uncertainty for TOC < 0.2%51. Therefore, high Hg/TOC ratios in the upper Linxiang Formation of the middle Katian at Shuanghe and Tianba (Table 1) do not indicate true positive Hg anomalies, but are linked to very low TOC values (< 0.1%), and are not plotted in Fig. 2. However, the high Hg/TOC ratios in the lowermost Wufeng Formation are associated with TOC contents higher than 0.2%, suggesting true Hg positive anomalies in the middle Katian at Shaunghe and Tianba. In addition, all other recorded high Hg/TOC ratios are associated with TOC > 2.0%22.

A diagenetic effect is excluded because of the well-preserved conditions of primary laminated shale4,14. In addition, organic-carbon isotope data record the global positive excursion of the glacial interval in the Hirnantian stage, also suggesting a primary chemostratigraphic signal preserved in the sample containing matured organic matter52. These analyses point to no or weak diagenetic effect on our geochemical data across the O–S boundary.

Hg enrichment pattern across the Ordovician–Silurian transition

We correlate the Hg/TOC and Hg/TS curves across the Ordovician–Silurian boundary in the three study sections, with the published Hg curves elsewhere (Fig. 4). These Hg anomalies all correspond to Hg concentration peak and peaks of Hg/TOC and/or Hg/TS ratios, indicating increase of Hg input. Two Hg positive anomalies occur in the middle and late Katian, three anomalies occur at LOMEI-1, LOMEI-2 and Late Rhuddanian, respectively (Fig. 4). Elsewhere, one Hg positive anomaly had also been found in late Katian from a drill hole XY5 in South China23,52 and Monitor Range in North America22. Two Hg positive anomalies at LOMEI-1 and LOMEI-2 had also been found in many locations such as Wangjiawan, Dingjiapo21, XY5 drill hole52 in South China and Monitor Range section in North America22. In other locations in South China, Hg positive anomalies were found in the early Rhuddanian48. Hg positive anomalies in Poland in the middle Katian, LOMEI-1, and LOMEI-2 were reported, respectively24. Besides, Hg positive anomalies at LOMEI-2 and in the Rhuddanian were reported in Scotland53. Overall, the Hg anomaly in the middle Katian in our study sections can be correlated with those in other location in South China52, Monitor Range in U.S.22, and in Poland24. The Hg anomalies in the upper Katian and at LOMEI-1 in our study section can be correlated with those in other locations in South China21,52, Monitor Range in U.S.22, and Poland24. The Hg positive anomaly at LOMEI-2 in our study sections can be correlated with those in other locations in South China21,52, Monitor Range in U.S.22, Poland24, and Scotland53. The Hg positive anomaly in the upper Rhuddanian in our study sections can be correlated with those in Scotland53 and other location in South China48.

Figure 4
figure 4

Chemostratigraphic correlation of Hg anomalies across the Ordovician–Silurian boundary. The abbreviation for graptolite zone can be seen in Fig. 2. ① to ⑤: Hg anomalies; Data of Wangjiawan and Dingjiapo in South China from Ref.21; Data of XY5 in South China from Ref.23; Data of Monitor Range in United States from Ref.22; Data of Zbrza PIG-1in Poland from Ref.24; Data of Dob's Linn in Scotland from Ref.53.

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Origin of the Hg anomalies in the study sections

The positive Hg anomalies (i.e., peaks of Hg/TOC and/or Hg/TS) in the middle and late Katian are associated with relative high TOC and TS contents at Shuanghe and Qiliao sections, indicating that this upper Katian Hg enrichment was not related to higher TOC or sulfide fluxes and reflect an enhanced Hg loading into the basins, supporting their interpretation as genuine Hg positive anomalies21,22,24,52. Abundant ash beds in the middle and upper Katian in South China42,43,44,54 (Fig. 2) and several ash beds in middle Katian in Poland24 have been found. Considering the regional/global signal of Hg anomaly in the middle and late Katian mentioned above, the inferred volcanism in South China or a ‘LIP’ in the middle-late Katian20,40,55 may contribute to the two positive Hg anomalies in the middle and late Katian in this study sections and other locations21,22,23, and large mass-independent sulfur isotope anomalies52. In other words, the Hg positive anomalies in the middle and late Katian suggest a volcanic increasing Hg loading during this time.

High Hg/TOC or Hg/TS ratios at the LOMEI-1 at the end-Katian in the three study sections are associated with relatively high TOC or TS contents, and high Hg concentrations, also suggesting higher Hg loading into the basins. The Hg spike at the LOMEI-2 at Shuanghe was associated with high TOC contents, invariable Hg/TOC ratios & TS contents, and high Hg/TS ratios, indicating that this Hg enrichment at Shuanghe was linked to higher TOC fluxes. Meanwhile, the Hg spike at the LOMEI-2 at Qiliao was associated with high TS contents, invariable Hg/TS ratios, relatively high TOC contents and Hg/TOC ratios (Fig. 2), suggesting that Hg enrichment at Qiliao might be related to local sulfide deposition. Sulfide-carrier Hg anomaly was also reported by previous study with coarse resolution across the LOME interval48. Hg profile at Tianba does not show any positive anomaly at the LOMEI-2, but the good covariation of Hg-TOC (Fig. 3e) suggests organic matter carrier of Hg during this time.

Overall, Hg positive anomaly at LOMEI-1 record higher loading of Hg into the basin, while an increase of Hg drawdown at LOMEI-2 due to more reducing conditions4 or increase of organic matter deposition in the late Hirnantian (Fig. 2). Higher Hg fluxes in the basins at LOMEI-1 could be related to different factors, as an increase of volcanic activity or enhanced continental weathering56,57. Considering the relatively low continental weathering indicated by chemical index alteration (CIA) values4 and the sporadical occurrence of ash beds in South China42,46 (Fig. 2), the abrupt Hg (and Hg/TOC) anomalies at LOMEI-1 would be of volcanic origin as suggested by previous published works in South China and U.S.21,22,23,24. However, intermittent or weak euxinia also developed during the LOMEI-14 with high Hg/TOC or Hg/TS ratios, relatively high TOC or TS contents in the three study sections, suggesting Hg enrichment during this time would be partially related to euxinic condition in water column.

The relatively high Hg concentration in the upper Rhuddanian is associated with non-euxinic conditions, relatively low TOC or TS contents, but relatively high Hg/TOC and Hg/TS ratios in this study. The abrupt increasing of Hg/TOC, or Hg/TS ratios coincides with invariable or small decreasing TOC or TS, excluding an inflation of too low TS and TOC contents. These probably suggest a genuine Hg positive anomaly and extra environmental loading related to volcanism or weathering56,57. This anomaly is associated with abrupt occurrence of frequent ash beds42 in Fig. 2 and relatively low weathering of CIA evidence10, pointing to a volcanic origin of increasing Hg loading. Previous study53 has also suggested that this anomaly in the Scotland derived from increased Hg flux rather than sequestration by anoxia/euxinia. These may indicate global Hg loading by volcanism during the late Rhuddanian.

Implications for the mass extinction

The interpretation of two different controls on the recorded Hg anomalies, i.e., a mixed origin of volcanism and euxinia for the Hg anomaly at the LOMEI-1 and a redox control on the Hg anomaly at the LOMEI-2, shed new lights on the Late Ordovician extinction mechanism. The middle-Katian Hg positive anomaly was temporally coincided with the middle-Katian genus richness drop58. The inferred volcanic-origin (although lack of solid volcanic lithology evidences) of Hg positive anomaly in the middle Katian in this study may suggest significant climatic effect triggered by volcanism on the initial of long-term mass extinction from middle Katian to late Hirnantian58,59.

The most distinct Hg anomaly and volcanism reported by previous studies are in the Late Katian and at LOMEI-1 (at end-Katian) (Fig. 4). As shown in Fig. 2, intensity of volcanic activity estimated by distribution and thickness of volcanic ash layers gradually decreased from the middle Katian to end-Katian. However, long-term weathering of volcanic deposits would enhance the nutrient input to ocean and thus result in large amount of organic matter burial and the consumption of CO2 in the atmosphere via biological pump59,60,61. Increasing burial rates of organic matter would gradually contribute to expansion of euxinia in bottom water4, finally driving the LOMEI-1. Additionally, Hg toxic effect could also have contributed to this extinction process57.

As shown by our data, the development of euxinic conditions at the LOMEI-2 was not related to volcanism (Fig. 2), but to the large amount of organic matter burial resulted from increased availability of nutrients either input from exposed continental shelves or recycled from organic matter degradation4. However, at the LOMEI-2, high organic carbon burial mentioned above created euxinic conditions in bottom water, accumulating Hg and killing the survivors such as conodonts, Hirnantia fauna (cool water brachiopod fauna)5 after LOMEI-1.

Conclusions

We present herein five Hg anomaly enrichments across the Ordovician–Silurian boundary, i.e., two anomalies in the middle and late Katian, three anomalies at LOMEI-1 (end-Katian), LOMEI-2 (late Hirnantian), and one spike in the late Rhuddanian of South China, respectively. All these Hg anomalies in the Late Ordovician and early Silurian were global or at least regional based on the global Hg chemostratigraphy correlation. The Hg positive anomalies in the middle–late Katian and in the late Rhuddanian were probably caused by primary volcanic loading. Our data suggest that during the mass extinction interval, the Hg positive anomalies at LOMEI-1 and LOMEI-2 have been controlled by different factors: i.e., volcanism probably caused the Hg anomaly at the LOMEI-1, and the development of strong euxinic conditions increased Hg drawdown at the LOMEI-2. Volcanism during the Middle and late Katian would probably enhance the expansion of euxinia at the end of Katian by long-term weathering of volcanic deposits, and was finally responsible for the LOMEI-1. Furthermore, there was no or weak volcanic loading of Hg during the LOMEI-2. Hg enrichment during this time was related to euxinic condition in water column, suggesting that the LOMEI-2 was linked to euxinia.