1 Introduction

The composition of terrigenous sediments is largely influenced by the nature of parent rocks and sedimentary processes during transportation and within the depositional basin (Armstrong-Altrin 2015; Basu 2017). Hence, the spatial and temporal variations during sedimentation can be able to understand by the mineralogical and geochemical compositions of sediments, and numerous studies utilized these as a device to understand sediment provenance (Bhattacharjee et al. 2017; Craigie 2018; Hou et al. 2017; Mitra et al. 2017; Paikaray et al. 2008; Saha et al. 2010, 2018; Verma et al. 2016a; Verma and Armstrong-Altrin 2016; Wang et al. 2017a, 2017b). In sediments, unstable minerals and mobile elements can be affected easily by weathering and erosional processes during transportation. However, immobile elements like REE, Co, Cr, Th, and Sc in sediments are considered as good provenance indicators, because they are not fractionated during sedimentary processes like diagenesis and weathering (Fedo et al. 1995). The fine-grained detrital sediments are particularly helpful for provenance study, because, they provide new insights into the sedimentation processes through geological time (Ramos-Vázquez et al. 2017; Selvaraj et al. 2015).

Previous studies by Carranza-Edwards et al. (2001) reported the composition of coastal sediments from the northwestern part of the Gulf of Mexico and inferred their provenance. Recently, Armstrong-Altrin et al. (2015a) and Hernández-Hinojosa et al. (2018) investigated the origin of coastal sediments in the Veracruz and Tabasco states, Gulf of Mexico. Other studies documented the textural characteristics (Kasper-Zubillaga et al. 2013), heavy metal concentrations and geochemistry of estuary (Rosales-Hoz et al. 2015) and continental slope sediments (Armstrong-Altrin et al. 2015b; Ramos-Vázquez et al. 2017) from the western part of the Gulf of Mexico. Although numerous researchers investigated the provenance of coastal sediments along the Gulf of Mexico, studies concentrated on the textural and geochemical characteristics of fine-grained sediments in the southwestern Gulf of Mexico are little. In the present study, we document texture, mineralogy, geochemistry, and 14C age data of two fine-grained sediment cores (GM6 and GM7) retrieved in the southwestern Gulf of Mexico. The aims of this investigation are to investigate the chemical weathering, source rock composition, radiocarbon age, and the depositional condition.

2 Study area

Sampling of two sediment cores GM6 (Latitude 19°08′64.54″N and Longitude 93°59′81.90″W) and GM7 (Latitude 19°12′37.45″N and Longitude 94°06′13.15″W) (Fig. 1) was performed in the southwestern part of the Gulf of Mexico, during a cruise program by a research vessel “Justo Sierra”.

3 Geological backgrounds

A brief description about the rivers traversing along the southwestern Mexico and the geological setting of the study area is given below:

Fig. 1
figure 1

Map showing core locations and geology of the Gulf of Mexico coastal areas (Source: Carta Geológica; scale 1:1,000,000) (modified after Armstrong-Altrin 2015). MVB = Mexican Volcanic Belt; EMAP = Eastern Mexican Alkaline Province

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The Usumacinta and Grijalva Rivers, ~ 1100 km and ~ 640 km long, respectively, are the longest rivers in Mexico, that discharges annually ~ 2678 m3s− 1, with peak discharges from July to November and rank second in fresh water discharge to the Gulf of Mexico (Day et al. 2003; Salas-de-León et al. 2008). These rivers originate from the northwestern part of the Guatemala and Chiapas (Mexico), respectively, and join together just ~ 50 km upstream the coast of the Gulf of Mexico (Armstrong-Altrin et al. 2017).

The Coatzacoalcos River originates from the Veracruz and Oaxaca States and drains between 17°46′N–92°25′W and 18°10′N–94°31′W. The Coatzacoalcos River lithofacies type consists of metasedimentary and volcanic rocks of the Oaxacan Complex. The Oaxacan Complex is located in the north of the Oaxaca State, which represents the most extensive outcrop of ~ 1 Ga basement in southern Mexico (~ 10,000 km2). It is composed of anorthosite, garnet-bearing charnockite, gabbros (U–Pb age of ~ 1230 Ma), diorite (~ 1012 Ma), and syenite (~ 1130 Ma) (Solari et al. 2007). In general, the lithology of the Gulf of Mexico coastal area was dominated by alluvium (Quaternary), andesite and basalt (Cenozoic), sandstone and shale (Mesozoic), and schist and gneiss (Paleozoic) (Verma et al. 2016b) (Fig. 1).

4 Materials and methods

The 5.1 and 4.8-m-long sediment cores (GM6 and GM7) were recovered at 856 and 989 m water depth, respectively, at the southwestern Gulf of Mexico (Fig. 1). Forty samples at several intervals were selected for various analyses to deduce sediment provenance (Table 1).

Table 1 Grain size and textural parameters (Folk 1980) for the deep-sea sediments of the southwestern Gulf of Mexico
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Petrographic analysis was performed on 20 thin-sections to realize sediment composition. The grain-size and mineralogy were determined by Beckman Coulter particle size analyzer and Siemens D5000 X-ray Diffractometer (XRD), respectively, located at the Sedimentology Laboratory, Universidad Nacional Autónoma de México (UNAM). In addition, the chemistry of selected minerals was determined by a PHILLIPS XL-30 scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) at UNAM, Mexico.

For geochemical analysis, 20 g of 40 sediment samples were powdered (<62 μm) by an agate mortar. Bulk-sediment major-element concentrations were determined by X-ray fluorescence analysis (XRF) on fused glass beads at the geochemistry laboratory at UNAM, Mexico. The analytical accuracy is better than 5%. A detail about the analytical procedure and standards was described in Lozano and Bernal (2005). The trace and rare earth element concentrations were quantified using an inductively coupled plasma mass spectrometer (ICP-MS), following a methodology provided by Jarvis (1988). The accuracy, measured by an international standard BCR-2 was generally within 10%.

Samples for foraminiferal analysis were wet sieved through a 60 ASTM mesh (0.25 mm). Aliquots of the washed residues containing 300–500 foraminiferal individuals were analyzed through the microscope, and the specimens were taxonomically determined and counted. Separate fractions were used for benthic and planktic foraminifers. Benthic foraminifers were picked from the ≥63 μm fraction and planktonic from the ≥150 μm. Planktonic foraminifers were used to construct a biostratigraphic framework and benthics were analyzed for hypoxic characteristics.

Chronostratigraphic framework was achieved by 14C dating on 20 micrograms of foraminiferal species Globigerinoides ruber, sent to Beta Analytics for accelerator mass spectrometry (AMS) determination. The data were converted to calendar years with the CALIB program (Stuiver and Reimer 1993, version 502; http://calib.qub.ac.uk) (Table 2).

Table 2 14C AMS ages for mixed planktic foraminifers
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5 Results

5.1 Time framework

The time framework was elaborated based on seven 14C dates, which was followed by the biostratigraphical scheme of Kennett and Huddlestun (1972) and Kennett et al. (1985). The bottom section of core GM6 reaches 23,615 cal yrs. BP (subzone Y2), with the limit of Pleistocene/Holocene at around 195 cm interval. Core GM7 bottom section revealed a 14C age of about 19,007 cal yrs. BP, also fall within the subzone Y2. The Pleistocene/Holocene boundary is identified at 130 cm interval (Table 2, Fig. 2).

Fig. 2
figure 2

Biostratigraphic correlation and 14C ages (cal yrs. BP) for the sediment cores. The zone (Y) and sub-zones (Y1, and Y2) are based on the biostratigraphic classification of Kennett and Huddlestun (1972) and Kennett et al. (1985)

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5.2 Granulometric analysis

The textural characteristics were studied at five intervals for the GM6 and GM7 cores (Table 1). The sediments are enriched in silt and clay contents, and poor in sand content in the cores GM6 (50.5 ± 1.67, 49.5 ± 1.67, and 0.04 ± 0.05, respectively) and GM7 (49.7 ± 2.93, 50.3 ± 3.05, and 0.06 ± 0.13, respectively) (Table 1). The mean grain size (Mz) is similar for the GM6 and GM7 cores, which ranges from 7.99 ϕ to 8.13 ϕ and 7.80 ϕ to 8.17 ϕ, respectively, indicating that the sediments were dominated by silt and clay. The standard deviation values vary between 1.53–1.60 and 1.54–1.74 ϕ in the GM6 and GM7 cores, respectively, which reveal that the sediments are poorly sorted. The skewness (Sk1) values vary between 0.02–0.05 and 0.02–0.07 ϕ for the GM6 and GM7 cores, respectively, indicating that the energy was constant during the deposition of sediments. The kurtosis (KG) values of the GM6 and GM7 cores (0.92–0.94 and 0.93–0.95, respectively) indicate that the sediments are mesokurtic type (Table 1).

5.3 Mineralogy

The X-ray diffraction (XRD) data reveal that quartz and calcite are the major minerals, and muscovite, anorthite, halite, diopside, fayalite, and pyrite are the minor minerals in core GM6 (Fig. 3a; 300–301 cm interval; Last glacial maximum, subzone Y2). On the other hand, in core GM7, quartz and calcite are identified as major, and halite, albite, magnetite, aragonite, and manganosite as minor minerals (Fig. 3c; 290–291 cm interval). Smectite and kaolinite are the dominant clay minerals in both cores (Fig. 3b and d).

Fig. 3
figure 3

X-ray diffraction patterns for the cores GM6 (a and b) and GM7 (c and d)

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The SEM energy dispersive spectrometer (SEM-EDS) analysis of sediments on selected intervals of the cores GM6 and GM7 reveals the concentration of minerals such as, plagioclase, halite, pyrite, and biogenic residues (Fig. 4). The minerals identified by SEM-EDS are consistent with the minerals identified by XRD method.

Fig. 4
figure 4

SEM-EDS spectrum. The yellow rectangles in a, c, and e show the analysis positions. a, b Shell with calcite and plagioclase (GM6, 256–257 cm interval); c, d Foraminifer with calcite and halite (GM7, 300–301 cm interval); e, f Pyrite with sulfate and iron (GM6, 316–317 cm interval)

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5.4 Major element concentrations

The variations in major element concentrations between the GM6 and GM7 cores, as well as its vertical distributions, are not statistically significant (Table 3). The upper continental crust (UCC; Taylor and McLennan 1985) normalized major element patterns in the two cores are similar, except MnO content, which is slightly higher in few samples at different intervals of cores GM6 and GM7 (Fig. 5). The enrichment of MnO is probably due to the mineral manganosite. The Na2O and K2O contents in the cores GM6 and GM7 are depleted relative to UCC, indicating that the sediments are low in feldspar content (Nagarajan et al. 2017). The enrichment of Fe2O3 and TiO2 contents is probably due to the presence of accessary minerals like magnetite and ilmenite, respectively (Papadopoulos et al. 2016). Geochemically, the sediments are classified mostly as shale (Fig. 6; Herron 1988).

Table 3 Major element concentrations (weight %) for the deep sea sediments of the southwestern Gulf of Mexico
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Fig. 5
figure 5

Upper continental crust (UCC) normalized major element patterns (Taylor and McLennan 1985)

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Fig. 6
figure 6

Geochemical classification diagram for the core sediments, southwestern Gulf of Mexico (Herron 1988)

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5.5 Trace element concentrations

The trace element data for the GM6 and GM7 cores are listed in Table 4 and the UCC normalized patterns are illustrated in Fig. 7. The trace element contents between the GM6 and GM7 cores are almost similar, except Y and Zn. The Sc, Cr, Ni, and Cu contents in sediments are low relative to those in UCC. The low concentrations of Rb, Ba, and U in the core sediments reveal its mobility during sedimentary processes (Feng and Kerrich 1992). The Zr and Hf contents, which represent detrital input, are depleted in core sediments relative to those in UCC. The correlations between Al2O3 and V, Cr, and Sc for the GM6 (r = 0.38, 0.38, and − 0.19, respectively; n = 20. r is correlation coefficient; the same follows) and GM7 (r = 0.39, 0.34, and 0.09, respectively; n = 20) cores are not statistically significant, indicating that these elements are not associated with aluminosilicates (Tzifas et al. 2017; Zaid 2015; Zaid et al. 2017).

Table 4 Trace element concentrations (ppm) for the deep sea sediments of the southwestern Gulf of Mexico
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Fig. 7
figure 7

Upper continental crust (UCC) normalized trace element patterns (Taylor and McLennan 1985)

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5.6 Rare earth element concentrations

The variation in ΣREE contents in the cores GM6 (136–157 ppm) and GM7 (110–165 ppm) is statistically not significant (Table 5). The correlation between ∑REE and Zr contents is positive for the GM6 (r = 0.57, n = 20) and GM7 (r = 0.45, n = 20) cores. On the other hand, the correlation between ∑REE and Al2O3 is negative for the GM6 (r = − 0.14, n = 20) and GM7 (r = − 0.15, n = 20) cores, which indicates that the REE contents are housed in accessory mineral like zircon (Armstrong-Altrin et al. 2012; Periasamy and Venkateshwarlu 2017; Wang et al. 2017c, 2017d). The North American shale composite (NASC) normalized REE patterns for the GM6 and GM7 cores are characterized by flat light REE (LREE) and enriched heavy REE (HREE) with negative and positive Eu anomalies (Fig. 8). However, the Eu anomalies in cores GM6 (Eu/Eu* = 0.892 to 1.03) and GM7 (Eu/Eu* = 0.887 to 1.14) are similar (Table 5).

Table 5 Rare earth element concentrations (ppm) for the deep sea sediments of the Southwestern Gulf of Mexico
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Fig. 8
figure 8

North American shale composite (NASC) normalized REE patterns for the deep-sea sediments recovered from the southwestern Gulf of Mexico (Gromet et al. 1984)

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6 Discussion

6.1 Palaeoweathering indices and sediment recycling

The chemical composition of detrital sediments is highly useful to interpret the intensity of weathering as well as sediment recycling (Tawfik et al. 2017). Although huge number of weathering indices was proposed by various authors, in this study, we preferred the most commonly used weathering indices like CIA (Nesbitt and Young 1982), CIW (Harnois 1988), and PIA (Fedo et al. 1995). These weathering indices are represented by the following equations: CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] × 100, CIW = [Al2O3/Al2O3 + CaO* + Na2O] × 100, and PIA = [Al2O3 – K2O/(Al2O3 + CaO* + Na2O)] × 100, in which CaO* represents CaO only incorporated in the silicate fraction. The Ca content in the silicate fraction was calculated by the equation: Ca = CaOt – CaOtrg and CaOtrg = (Al2O3)t × (CaO/Al2O3)UCC, where “t” = the total abundance in the sample and “trg” = terrigenous.

The CIA, CIW, and PIA values increase if weathering increases and values of ≤50 indicate low weathering (Fedo et al. 1995; Harnois 1988; Nesbitt and Young 1982). The CIA and CIW values for the GM6 core vary from 56 to 69 and from 60 to 75, respectively, and for GM7, which vary from 54 to 71 and from 57 to 77, respectively (Table 3). Similarly, the PIA values for the core GM6 vary from 57 to 73 and for core GM7, which vary between 55 and 74 (Table 3). The CIA, CIW, and PIA values of the core sediments are indicating a moderate to intense weathering condition (Table 3). On the other hand, index of compositional variability (ICV = [(CaO + K2O + Na2O + Fe2O3(t) + MgO + MnO + TiO2)/Al2O3]; Cox et al. 1995), where Fe2O3(t) represents total iron, is another method commonly used to infer sediment recycling, which decreases when weathering increases (Armstrong-Altrin et al. 2014, 2017; Madhavaraju et al. 2017; Nagarajan et al. 2017). According to Cox et al. (1995), the ICV value is high (>1) in little-weathered detrital minerals like pyroxene and feldspar and it is low (<1) in highly-weathered fine-grained sediments like clay. The ICV values for the core sediments GM6 (1.36–1.76) and GM7 (1.27–1.82) reveal a moderate weathering intensity in the source area (Table 3).

The elemental ratios such as SiO2/Al2O3, Al2O3/Na2O, and K2O/Na2O have been used extensively by various authors to interpret sediment recycling and are considered as a proxy to infer detrital input (Armstrong-Altrin 2009; Madhavaraju et al. 2016; Zaid 2013; Zaid et al. 2015). These ratios are >6, >5, and >1, respectively, if the sediment recycling is high (Tapia-Fernandez et al. 2017). The SiO2/Al2O3, Al2O3/Na2O, and K2O/Na2O ratios of the cores GM6 (2.82–3.27, 4–7, and 0.354–0.888, respectively) and GM7 (2.8–3.4, 5–7, and 0.512–0.949, respectively) are indicating low to moderate sediment recycling.

6.2 Sediment provenance

The provenance discrimination diagram of Roser and Korsch (1988) is widely preferred by various researchers to infer sediment provenance (Armstrong-Altrin 2009; Hernández-Hinojosa et al. 2018; Nagarajan et al. 2017; Tapia-Fernandez et al. 2017). The discrimination diagram of Roser and Korsch (1988) reveals that the sediments from the cores GM6 and GM7 were derived by the weathering of intermediate source rocks like basaltic andesite (Fig. 9). Similarly, the bivariate diagram based on La/Sc and Co/Th ratios also suggests that the sediments were most probably derived from an intermediate source rock (Fig. 10).

Fig. 9
figure 9

Provenance discriminant function diagram of Roser and Korsch (1988). The discriminant functions are: Discriminant Function 1 = (− 1.773•TiO2) + (0.607•Al2O3) + (0.760•Fe2O3) + (− 1.500•MgO) + (0.616•CaO) + (0.509•Na2O) + (− 1.224•K2O) + (− 9.090); Discriminant Function 2 = (0.445•TiO2) + (0.070•Al2O3) + (− 0.250•Fe2O3) + (− 1.142•MgO) + (0.438•CaO) + (1.475•Na2O) + (1.426•K2O)v+ (− 6.861)

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Fig. 10
figure 10

La/Sc versus Co/Th bivariate plot for the deep-sea sediments recovered from the southwestern Gulf of Mexico. n = number of samples; 1 this study; 2 Verma (2001), Nixon (1988); 3 Orozco-Esquivel et al. (2007); 4 Rodríguez et al. (2010), Schaaf et al. (2005); 5 Taylor and McLennan (1985)

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The characteristics of the REE pattern and the Eu anomaly can be used as a tool to infer sediment provenance (Cullers et al. 1997). The REE patterns of the GM6 and GM7 core sediments are enriched in LREE and depleted in HREE with low negative to positive Eu anomalies, suggesting the derivation of sediments from an intermediate igneous rock. Furthermore, we compared the chondrite-normalized REE patterns of the core sediments with the potential source rocks from the southern Gulf of Mexico (Fig. 11). This comparison again demonstrates that the intermediate rocks as the probable source rocks for the two sediment cores, which is consistent with the geology of the Gulf of Mexico (Fig. 1).

Fig. 11
figure 11

Average chondrite-normalized rare earth element patterns for the deep-sea sediments. Normalization values are from Taylor and McLennan (1985), n = number of samples; UCC = upper continental crust; 1 this study; 2 Verma (2001), Nixon (1988); 3 Orozco-Esquivel et al. (2007); 4 Rodríguez et al. (2010), Schaaf et al. (2005); 5 Taylor and McLennan (1985)

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6.3 Redox-sensitive trace element concentrations

Among other REEs, cerium (Ce) is sensitive to changes in redox conditions, so its depletion or enrichment can report changes in the oxygenation conditions of a depositional medium (Elderfield and Greaves 1982). A positive Ce anomaly (Ce/Ce* ≥ 1) in sediments suggests its deposition under an anoxic condition, whereas a negative Ce anomaly (Ce/Ce* < 1) represents an oxic condition (Tostevin et al. 2016). In this study, negative Ce anomaly (Ce/Ce* < 1) is observed, which varies from 0.845 to 0.907 and from 0.84 to 0.986 for the GM6 and GM7 cores, respectively, indicating an oxic depositional condition (Table 5).

The V/Cr ratio is considered as a good indicator of a redox condition, if this ratio is >4.5, which represents an anoxic condition, whereas <2 indicates an oxic condition in the depositional environment (Jones and Manning 1994). The V/Cr ratio in the GM6 (1.27–1.75) and GM7 (1.24 to 1.98, except one sample with value 2.13) cores are <2 (Table 4), indicating an oxic condition. In addition, Ni/Co ratio <5 and >5 indicate oxic and anoxic conditions, respectively (Jones and Manning 1994). The Ni/Co ratios for the GM6 and GM7 cores range from 0.70 to 1.47 and from 0.65 to 1.73, respectively, indicating an oxic condition. Similarly, the Cu/Zn ratio is also a good paleo-redox indicator, which is always high (>1) in the redox depositional condition (Goldberg and Humayun 2016). The Cu/Zn ratios in the GM6 and GM7 core sediments are <1 (0.57 ± 0.32 and 0.65 ± 0.30, respectively), indicating an oxic condition. Wignall and Myers (1988) documented that the authigenic uranium (Uau = Total U – Th/3) values between 5 and 12 suggest an oxic depositional condition, whereas values >12 are indicative of suboxic and anoxic conditions. The authigenic uranium values in GM6 core range from − 1.38 to 0.620 and for the GM7 core, they vary from − 1.57 to 0.231, suggesting an oxic depositional condition.

6.4 Distribution of benthic foraminifera

Benthic foraminifers were analyzed throughout the two cores to look for evidence of oxygenation in sediments. Benthic foraminifers in low oxygen environments are characterized by small size and low diversity (typically 2 or 3 species make up to 80% of the population; Sen Gupta and Machain-Castillo 1993), since only a few species can adapt to such conditions and they proliferate in great numbers due to lack of competition in a food-rich environment. Therefore, its populations are very abundant (9,000–147,000 individual species per gram of sediment - Ind/g; Perez-Cruz and Machain-Castillo 1990). The foraminiferal populations identified in both cores show moderate to high diversity (number of species S = 36 (GM6) and S = 33 (GM7)) and abundances from 316 to 7840 Ind/g. Moreover, Alabaminella turgida, a species found to be sensitive to hypoxic conditions (Pflum and Frerichs 1976) is abundant throughout these cores, particularly during the Pleistocene. Another characteristic of severe hypoxia caused by enrichment of organic matter is the acidity of the water. Planktonic foraminifers dissolve easily in such environments (as the Mexican Pacific, Arellano-Torres et al. 2013) and are not well preserved or absent in the sediments beneath those environments. In the studied sediments, abundant and well preserved benthic and planktic foraminifers are found throughout the cores (Fig. 12). Similarly, our observations based on the distribution of benthic foraminifers are in agreement with the findings of Machain-Castillo et al. (1998) that Pleistocene bathyal and abyssal waters were more oxygenated than present.

Fig. 12
figure 12

Thin-section photomicrographs taken under plane-polarized light, showing biogenic components (planktonic foraminifers, globigerinids). a GM6 (31–32 cm interval); b GM6 (151–152 cm interval); c GM7 (31–32 cm interval); d GM7 (151–152 cm interval)

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6.5 Mineralogical characterization

The low K2O and Na2O contents and K2O/Na2O ratio suggests the low proportion of K-feldspar and plagioclase in sediments (Dey et al. 2009; Nagarajan et al. 2007), which is also confirmed by XRD method in both sediment cores. The correlation between SiO2 versus K2O and Na2O in the cores GM6 (r = − 0.18 and 0.39, respectively, n = 20) and GM7 (r = 0.27 and 0.19, respectively, n = 20) are low, suggesting high mobility of K and Na at the time of weathering (Armstrong-Altrin and Machain-Castillo 2016; Tawfik et al. 2017). The K2O/Al2O3 ratio in clastic sediments may indicate the abundance of alkali feldspars and aluminosilicates, which are >5 and <5, respectively (Cox et al. 1995). The low K2O/Al2O3 ratios in the GM6 (0.08–0.14) and GM7 (0.08–0.17) sediment cores indicate the higher abundance of aluminosilicates than alkali feldspars (Table 3).

The clay fraction of the GM6 and GM7 cores were dominated by kaolinite and smectite, which are related to the derivation of terrigenous materials from tropical regions (Abdullayev and Leroy 2016). Similarly, enrichment of Cr, Ga, Ni, Rb, Y, and Zn concentrations in sediments is associated to the weathering of andesite (Saha et al. 2018). The correlations between Al2O3 versus Nb, Y, and Zr in the cores GM6 (r = 0.09, − 0.47, and − 0.69, respectively, n = 20) and GM7 (r = 0.26, 0.04, and − 0.36, respectively, n = 20) are not statistically significant, suggesting that Nb, Y, and Zr are linked to the source rocks of intermediate composition and accessory detrital phases such as xenotime and zircon, respectively (Varghese et al. 2018). On the other hand, the enrichment of MnO content identified in the core sediments is most likely due to the concentration of manganosite mineral (Figs. 3c and 5). Numerous studies addressed the occurrence of manganosite in the Upper Jurassic carbonate platform shelf of the Molango Mn deposit, located near to the coastal region of the southwestern Gulf of Mexico (Johnson et al. 2016; Okita 1992; Okita and Shanks III 1992). Hence, we believe that manganosite in the core sediments was transported by the rivers to the deep-sea area of the Gulf of Mexico.

We consider that the enrichment of CaO in sediments is due to the distribution of sand-size benthic foraminifers, which is also identified at intervals of 256–257 cm (GM6) (Fig. 4a, b) and 300–301 cm (GM7) (Fig. 4c, d). In addition, the presence of calcite is related to the ability of smectite to observe cations such as Ca2+ to the surface of its crystalline structure (Abdullayev and Leroy 2016). Sulfate content identified in both cores by SEM-EDS represents the pyrite mineral (Fig. 4e, f). According to Berner (1984), pyrite is a widespread mineral and most marine sediments contain at least traces of it, since it forms during shallow burial by the reaction of detrital iron minerals with H2S produced by bacterial sulfate reduction. The environment where this occurs is few centimeters below the water-surface interface. Most foraminifers live at the sediment water interface or just below it. Therefore, the foraminifers in the studied sediments were living under oxic conditions as revealed by their abundance, diversity and degree of preservation. Berner (1984) further indicated that in anoxic marine sediments, the high supply of organic matter and H2S produce high concentrations of pyrite, which is not the case of the sediments studied, as seen in Figs. 3 and 4.

7 Conclusions

The compositional variations of two sediment cores recovered at the southwestern Gulf of Mexico were analyzed to infer their age and provenance. Texturally, the sediments are classified as silt and clay, and chemically as shale, indicating the domination of fine-grained sediments in both cores. Based on the radiocarbon ages the Pleistocene/Holocene boundary is identified at 130 cm interval. The weathering indices (CIA, CIW, and PIA) indicated a moderate to high intensity of weathering.

The concentrations of major element, transition trace elements, and the similarity between the chondrite- normalized REE patterns of the deep-sea sediments and the source rocks revealed that the depositional basin received sediments from the intermediate igneous rocks like andesite and basaltic andesite, at least during the last 23,615 cal yrs. BP.

The enrichment of Zn content in two intervals reveals an influence of anthropogenic input such as pesticides and agricultural waste in modern times. The authigenic uranium content, Ce anomaly, V/Cr and Ni/Co ratios and the distribution of benthic foraminifers suggested that the sediments were deposited in an oxic depositional condition. The similarity in geochemical composition between the two sediment cores reveals that the depositional condition in the deep-sea area of the southwestern Gulf of Mexico was similar at least in the last 23,615 cal yrs.