Terrestrial Organic Matter Sources Research Articles

Examining the source and composition of organic matter through saturated hydrocarbons, δ13C, and δ15N in a subtropical water reservoir

The sources of organic matter in the sediments of the Passaúna reservoir, an important water supply for the local population, were thoroughly investigated. The objective was to identify the origins of organic matter through the analysis of saturated hydrocarbons, elemental composition (total organic carbon and total nitrogen), and the content of δ13C and δ15N isotopes. This comprehensive approach allowed us to trace the sources of organic matter and discerns indicating heightened primary productivity within the reservoir. To achieve this, two sediment cores spanning a 140-year interval (1880–2020) were retrieved from the reservoir. Core 2 accumulates the majority of sediments, particularly near the dam area. In these parts, sediment deposits can reach up to 1 m above the pre-impoundment soil. Sediments near the area where core 1 was collected contain more sand, resulting in lower thickness compared to core 2. Sediment core 1 primarily reflects terrestrial sources of organic matter, as supported by stable isotope values of δ13C and δ15N. The δ13C values ranged from −23.0 ‰ to −25.7 ‰ in core 1 and from −28.4 ‰ to −29.2 ‰ in core 2. Meanwhile, the δ15N values ranged from 6.6 ‰ to 10.8 ‰ in core 1 and from 3.8 ‰ to 7.6 ‰ in core 2. The distribution of saturated hydrocarbons revealed that organic matter originates from both allochthonous and autochthonous sources. Periods of intense primary productivity were indicated by the presence of n-C16, n-C17, n-C18, and n-C19 alkanes. Additionally, we observed periods characterized by high primary productivity, indicative of elevated nutrient input likely resulting from increased urbanization and industrial activity in the area.

Evaluate climate change and anthropogenic activities influencing geochemical variations in sediment between and within the avulsion period in the Lower Yellow River avulsion channels

The geochemical data from sediments in avulsion channels provide historical evidence of climate change and human-induced alterations in river basin environments. The present study focused on the particle size of sediments in cores and the level of geochemical variation in avulsion channels of the Lower Yellow River Delta (YRD), China. The sediment samples were collected in a depth range of 20–400 cm in avulsion channels. The collected samples were analyzed for sediment particle size and geochemical composition using standard methods. The results demonstrated rapid increases in agriculture practices, rainfall pattern changes, and terrestrial sediment runoff reduction in river basins after the 1960s. The reduced sediment loads in the Sanmenxia Reservoir significantly changed the sediment grain size and geochemical levels in the avulsion channel from August 1960.8 to January −1961.1. In particular, TC, TN, and C/N levels decreased with increasing sediment depth. The C/N values of <12 denoted completely reduced terrestrial sources of organic matter in the channel during the August 1960–January 1961 period compared to the July 1953–August 1960 period. The two-way ANOVA p-values were (p <0.016-p<0.001) strong between the avulsion periods but had no significant variation within the avulsion periods. We emphasize that this study provides a close interplay of different historical periods of geochemical variation in avulsion channel sediments in the alluvial fan YRD, and we argue that the evolution of the middle upstream river basin was subjective by climate change and human developmental actions, which impacted the YRD. In particular, reservoir-interrupted water flow and sediment reduction impacts associated with geochemical fluctuations are documented in the YRD.

Multi-analytical Approach (FTIR, XRF, XRD, GC-MS) Characterization of Source Rocks from the Anza Basin, Kenya

Potential source rocks from three wells (Chalbi-3, Sirius-1, and Ndovu-1) in the Anza Basin of northeastern Kenya were analyzed using elemental carbon-hydrogen-nitrogen analysis, and mineralogical characterization by X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM-EDX).)&lt;strong&gt;. &lt;/strong&gt;A Gas Chromatography-Mass Spectrometry (GC-MS) analysis of the saturated aliphatic biomarkers was carried out to illuminate the organic matter source. The analytical results revealed that the source rocks have moderate to high total organic carbon (%TOC) content, suggesting conditions in the Basin favor organic production and preservation. The Hydrogen Indices (HI: 0.19–0.60 atomic ratios) typify a predominance of mixed type II/III (oil/gas-prone) with more type III (gas-prone) and less II (oil-prone) kerogens. The FTIR, XRF, and XRD results reveal that the studied source rock samples comprised mainly of quartz, followed by silicate-clay minerals and calcite minerals. The solvent-extractable organic matter investigation revealed biomarker distributions of n-alkanes and isoprenoids (pristane/ phytane ratios), suggesting that source rocks were derived from algae and bacteria deposited weakly anoxic and low toxic environmental conditions, with minimal contribution from terrestrial organic matter sources. Consequently, all three wells have hydrocarbon generation potential, particularly Ndovu-1, which displays good organic matter content to produce oil and gas. The hydrocarbon potential is excellent and capable of making expulsions of oil and/or gas from the wells at sufficient depths.

Terpane Characterization of Crude Oils from Niger Delta, Nigeria: A Geochemical Appraisal

The geochemistry of crude oils from the Niger Delta, Nigeria, were evaluated using the characterization of C₁₉ to C₃₅ terpanes. Analyses of two representative crude oils from Western and Central Niger Delta showed abundances of C₁₉ to C₂₉ tricyclic terpanes constituted 12.09% and 29.74%, C₂₄ tetracyclic terpane 0.31% and 0.15% and C₂₇ to C₃₅ pentacyclic terpanes 87.61% and 70.12%, respectively. Diagnostic ratios of terpanes indicated relatively low abundances of C₂₃, C₂₈ and C₂₉ tricyclic terpanes, low abundances of homohopanes, a significantly high abundance of oleanane and that the Niger Delta crude oils were derived from terrestrial organic matter source rocks deposited in an oxic environment during the Tertiary period. Multivariate oil-oil correlation plot showed the Western and Central Niger Delta crude oils are not distinct, but moderately related (genetically). However, diagnostic ratios of C₂₄ tetracyclic terpane, which was high and moderate and gammacerane, which was low and high, revealed crude oils from Western Niger Delta were derived from predominantly terrestrial source and crude oils from Central Niger Delta were derived from terrestrial source with input from marine organic matter, respectively. Tricyclic terpanes/hopanes and isomerization ratios of C₃₂ homohopanes indicated the Niger Delta crude oils were generated at high maturity, at top of the oil generation window.

Geochemical distribution and forms of phosphorus in the surface sediment of Netravathi-Gurupur estuary, southwestern coast of India

Estuaries are the most productive transition ecosystem and phosphorus (P) plays an important role in these ecosystems. Therefore, in the present study, sequential extraction method was used to determine the abundance of five sediment P fractions (calcium (Ca-P), Iron (FeP), aluminum (AlP), exchangeable (Ex-P) and organic (OrgP) bound P) in Netravathi-Gurupur estuary, India. Total phosphorus (TP) content varied from 435-810 mg/kg (non-monsoon) and 258–699 mg/kg (monsoon). Inorganic P was dominant part. Different P fractions followed similar order (Fe-P > Ca-P > Al-P > Org-P > Ex-P) with respect to seasons. FeP was dominant fraction, indicating probable anthropogenic stress. Sediment may act as source of P as bioavailable P constituted 40–69.2 % of TP. Molar ratio of OC to Org-P in sediment indicated terrestrial sources of organic matter. However, the estimated phosphorus pollution index were lower than one except a few cases indicating less ecological risk with respect to sedimentary TP load.

The Characteristics of Lipid Biomarkers from the Abyssal and Hadal Sediments of the Yap Trench and the Influence of V-Shape Topography

Lipid biomarkers from deep-sea sediments have been observed in several studies, but little is known about their occurrence in trench system sediments. Here, we determined the concentrations of lipid biomarkers (fatty acids and neutral lipids) in sediments from the north Yap Trench. Our results showed that short-chain (C12–20) saturated fatty acids (SFAs) contributed more than (C12:1–23:1) monounsaturated fatty acids (MUFAs), and (C16:3–24:4) polyunsaturated fatty acids (PUFAs). Most fatty acids (FAs) suggest that bacteria and algae were the main contributors to marine organic matter. In contrast, terrestrial organic matter (OM) was a minor contributor to long carbon chain fatty acids greater than C20. On the other side, the observed neutral lipids such as alkanes (C14–C27), alkanols (C12–C20), alkenes (C17:1–C26:1), phytol and sterol (C27–29) indicate that phytoplankton and bacteria were the main contributors of organic materials in the sediments, and the carbon chain of neutral lipids C20–29 offering the sources of terrestrial organic matter. The extremely depleted δ13C values of fatty acids give the sources of organic carbon in the sediments from bacteria, algae, and methane-related microbes. This study is important for understanding the biogeochemical activities in deep-sea environments, particularly in the abyss and hadal zones. It will be helpful to understand the sources, transfer, and deposition of organic matter in marine trenches.

Exploring origin of oxygen-consuming organic matter in a newly developed quasi-hypoxic coastal ocean, the Bohai Sea (China): A stable carbon isotope perspective.

The Bohai Sea, adjacent to the Northwest Pacific, is a semi-enclosed shallow-water marginal sea that was considered on a critical path of eutrophication and environmental degradation. To better understand the Bohai Sea metabolism-induced summertime dissolved oxygen (DO) decline, five field surveys were conducted between July 2019 and July 2021 to investigate the seasonal/interannual and spatial variations in DO, dissolved inorganic carbon (DIC), and stable isotopic composition of DIC (δ13CDIC). Although the water-mixing scheme was subject to spatial variation, a uniform apparent ratio of δ13CDIC versus apparent oxygen utilization was estimated at -0.0122‰ per μmol O2 kg-1 in the Bohai Sea in summer. Based on a three-endmember water-mixing model and the mass balance of DIC and its stable isotopic composition, the assumed uniform δ13C values of oxygen-consuming organic matter in the Bohai Sea DO-deficient areas was estimated to be -19.47 ± 1.85‰ in 2020 and between -20.6‰ and - 18.1‰ in 2021. This isotopic composition is very similar to the δ13C value of organic matter from marine diatoms, but different to that of terrestrial organic matter sources surrounding the Bohai Sea. Our results indicate that nearly all the organic matter consumed by community respiration in the Bohai Sea is produced in situ by marine plankton. To mitigate the seasonal DO shortage in the Bohai Sea, reduction of allochthonous nutrients is crucial.

The applicability of commonly-used tracers in identifying eroded organic matter sources

Tracing decomposition of organic carbon sources can balance carbon budget and reduce soil erosion targetedly. However, tracers may provide inaccurate source information, possibly due to the instability of organic matter under different land use types. Little is known about which stable tracer to choose based on different erosion scenarios. We, therefore, evaluated the availability of three commonly-used tracers techniques (i.e., bulk stable isotopes, fluorescence proxies and stoichiometric ratios) under different mixed end-members (i.e., Woodland, arable land and grassland) via a laboratory-scale controlled experiment. Combining fluorescence excitation emission matrix (EEM)-parallel factor analysis (PARAFAC), Bayesian isotope mixing model and three evaluation criteria. Bulk stable isotopes (δ13C, δ15N) were considered to be the most suitable tracers for terrestrial organic matter sources tracing (Most of the R2 > 0.85, % difference < 10%), especially under Arable land-Woodland (A-W) mixed end-member, followed by traditional tracers (C/N, DOC). Unfortunately, the fluorescent indices were not suitable for terrestrial organic matter sources tracing, due to the similarities between different terrestrial organic matter sources, especially under Grassland-Woodland (G-W) mixed end-member (Most of the P-value > 0.05, % difference > 10%). Our results recommend that when selecting a tracer, not only the stability of the tracer should be considered, but also the difference among eroded organic matter sources. This study provides new insights into the validation of commonly-used tracers in identifying eroded organic matter sources accurately.

Isotopic niche of coastal fish and cephalopods off the Campos Basin, southeastern Brazil

The stable isotopes of carbon (δ13C) and nitrogen (δ15N) of three demersal guilds (benthic feeders, benthic/nekton feeders and nekton feeders) were assessed to understand their trophic relationships on the southeastern Brazilian coast. The organic matter (OM) pool that fuels the demersal food web is composed of a mixture of autochthonous (marine) and allochthonous (terrigenous) sources, highlighting the Paraíba do Sul River (PSR) influence in the shallow coastal waters, regardless of seasonality. Of the studied guilds, the nekton feeder guild (Doryteuthis plei, Lophius gastrophysus, Percophis brasiliensis and Saurida caribbaea) in the demersal food web had the highest δ15N’ values in the dry and rainy seasons. The benthic feeder guild (Atlantoraja platana, Dactylopterus volitans, Dules auriga, Etropus longimanus, Prionotus nudigula and Zapteryx brevirostris) presented the widest isotopic niche breadth (highest total niche area and trophic diversity) due to the assimilation of a variety of benthic crustaceans, with different isotope signatures. The assimilation of benthic and pelagic crustaceans by the benthic feeder and benthic/nekton feeder (Atlantoraja cyclophora, Octopus vulgaris and Priacanthus arenatus) guilds resulted in isotopic overlap (28.1–50.6%). Seasonal trends from benthic and inshore to pelagic and offshore associated food webs were observed for the benthic feeder and benthic/nekton feeder guilds. Our results highlight the importance of terrestrial OM sources from PSR to shallow coastal demersal food webs off inner and medium continental shelf in southeastern Brazil. In addition, the trophic relationships of megafauna from benthic to nekton feeders were discussed.

Mercury concentrations and associations with dissolved organic matter are modified by water residence time in eastern Canadian lakes along a 30° latitudinal gradient

AbstractSurface water mercury (Hg) and dissolved organic carbon (DOC) concentrations and their ratios, which play a critical role in food chain bioaccumulation of Hg, were examined in lakes from southern boreal, sub‐Arctic taiga, Arctic tundra and polar desert landscapes of eastern and northern Canada. The study sites investigated span a 30° latitudinal gradient representing differences in climate, ecosystem productivity, and atmospheric mercury deposition. Lakes were selected to obtain a range of simple morphometrics such as area, depth, volume and catchment area, with corresponding differences in water residence times (WRT), ranging from 0.1 to 7.5 years. Total mercury (THg) and mono‐methylmercury (MMHg) concentrations correlated positively but weakly with DOC in lake surface waters along the climate gradient, consistent with lower ecosystem and organic matter productivity at higher latitudes. Specific UV absorbance, an indicator of terrestrial organic matter sources, was found to explain some residual variability in THg not explained by DOC. Concentrations of THg and MMHg and their ratios with DOC, particularly the MMHg : DOC ratio as well as %MMHg, were best explained by inverse associations with WRT. These relationships were apparent both within and between regions along the latitudinal gradient, suggesting a net‐negative effect of in‐lake processing on THg and MMHg concentrations associated with longer WRTs. Since the water MMHg : DOC ratio was previously shown to explain foodweb MMHg in the same study lakes, our results suggest that smaller lakes with shorter residence times are more susceptible to MMHg exposure even at low levels of inorganic Hg loading or MMHg production.

Mineralogical and geochemical imprints to determine the provenance, depositional history and tectonic settings of Triassic turbidites in the Semanggol and Semantan Basins, Peninsular Malaysia

• Semanggol and Semantan formations are part of Triassic successions in SE Asia. • Triassic turbidites exemplify inner to outer fan in a deep-marine fan system. • Semanggol turbidites were sourced from felsic rocks and represent passive margin. • Semantan rocks have felsic-intermediate origin and show active-continental margin. • Based on redox-sensitive metals, depositional conditions fluctuate from anoxic-oxic. • Type-III kerogen, mature-overmature proposing terrestrial input of organic matter. The present study provides systematic mineralogical, inorganic geochemical and Rock-Eval analyses of turbidite deposits of the Semanggol and Semantan formations to investigate their provenance, depositional environment and tectonic setting as well as type, origin and maturation of organic matter in black shale. These deposits are extensively distributed in Peninsular Malaysia and are observed in the field as conglomerate, pebbly sandstone, thick-bedded sandstone, interbedded sandstone and shale, laminated black shale, and tuffaceous sandstone. These sedimentary sequences are predominantly deep-marine and occasional shallow-marine deposits characterizing a deep-marine fan system. Based on Al 2 O 3 /TiO 2 ratios, more felsic rocks for the Semanggol Formation and felsic-intermediate rocks for the Semantan Formation are proposed as sources for their detrital fractions and this interpretation is also substantiated by the binary plot of Zr vs Ti. V/(V + Ni), V/Ni and Ni/Co analyses indicate anoxic/oxic conditions. Ternary diagram of immobile- trace-elements inferred passive margin for Semanggol turbidites and an active-continental margin for Semantan turbidites. The dissimilarity between geochemically and geologically derived tectonic settings, mainly for the Semanggol turbidites, is because of changing tectonic setting and recycled nature of sediment sources that were deposited in diverse tectonic settings. The black shale clay fractions from both formations dominantly contain illite and kaolininte with abundant traces of quartz. The illite content varies from 43 to 56% and illite crystallinity index (IC) from 0.2° to 0.4° Δ2θ (average: 0.28° Δ2θ), showing studied black shale fall in anchizone. Rock-Eval pyrolysis reveals Type-III kerogens, suggesting a terrestrial source of organic matter in black shale of both formations.

Lower-altitude of the Himalayas before the mid-Pliocene as constrained by hydrological and thermal conditions

Contradictory paleoaltitudinal histories of the Himalayas arrived at using isotopic paleoaltimeters and ecological evidences have constrained our further exploration of the geodynamic mechanisms driving the India-Eurasia collision and the resultant interaction between tectonic uplift and climate change. In this study, based on evidence derived from glycerol dialkyl glycerol tetraethers (GDGTs), the hydrogen isotopes of leaf wax n-alkanes (δ2Halk), we estimated the paleoaltitude of the Gyirong Basin (GB), central Himalayas, for the 7.0∼3.2 Ma period. The paleoaltitudinal reconstruction based on δ2Halk values (∼6.3 km above sea level (asl)) was consistent with that of the revised δ18Oc values (∼5.6 km asl), and higher than the present-day altitude (∼4 km asl). The reconstructed paleotemperatures based on brGDGTs (14.2 ± 4.5°C for the lake surface; 7.5 ± 3.3°C for terrestrial organic matter source areas) were consistent with the ecological evidences, indicating a low-altitude paleoenvironment. This proves that a systemic bias exists between the isotopic paleoaltimeters and the ecological evidence, rather than a problem with specific indicators. The reconstructed paleoaltitudinal differences between the δ2Halk and δ18Oc values are similar to those of brGDGT reconstructions, seemingly representing the paleoaltitudinal differences between the source areas of terrestrial organic matter inputs and the lake surface, and suggesting that the isotopic palaeoaltimeters do not specifically record the higher-altitude parts of the basinal topography. Further paleoenvironmental analysis indicated that the warmer and wetter environment present during the Late Miocene to Mid-Pliocene would, on the face of it, be better suited to the application of brGDGT paleothermometers, and this in turn would potentially lead to a systemic overestimation by isotopic paleoaltimeters. We therefore adopted the paleoaltitudinal reconstructions which used brGDGTs and concluded that the lake surface of the GB during the Late Miocene to Mid-Pliocene was 2.5 ± 0.8 km asl, and that the surrounding mountains exceeded 3.6 km asl in height, implying that the central Himalayas underwent a rapid uplift of ∼1.5 km after the Mid-Pliocene.

Stable source of Holocene spring precipitation recorded in leaf wax hydrogen-isotope ratios from two New York lakes

Changes in synoptic atmospheric circulation patterns are thought to play a role in establishing millennial scale climate periods during the end of the late glacial and the Holocene. In the northeastern United States, multi-proxy evidence documents fluctuations in effective moisture and temperatures for this time period, but constraining the relationship between atmospheric processes and these climate regimes is not straightforward. Because the hydrogen-isotope ratios of sedimentary terrestrial leaf waxes can reflect precipitation δD, these long-chain hydrocarbon compounds are an excellent tool to investigate moisture sourcing. Here we present lake sediment and leaf wax carbon- and hydrogen-isotope records that span the past ∼14.0 thousand years from Heart Lake and Moose Pond in the Adirondack Mountains (ADK), New York. High initial lake productivity after basin inception is reflected in low C:N ratios (<15), and higher relative short chain n-alkane (n-C17, n-C19, and n-C21) to long chain n-alkane (n-C27, n-C29 and n-C31) concentration ratios. The Holocene record is characterized by low bulk and n-alkane δ13C (∼ −28‰, ∼ −31‰, respectively), high ACL25-35 (∼28), and high relative concentrations of long chain n-alkane homologues, indicating a dominantly terrestrial source of organic matter for this time period. Hydrogen-isotope ratios of n-C29n-alkane from both lake basins range only ∼20‰ through the Holocene and reconstructed precipitation δD (δDprecip) from both basins is in good agreement with that of modern modeled spring precipitation. This suggests there may have been no major changes in the sourcing of spring precipitation for the ADK throughout the time of record.

Distribution of Geochemical Fractions of Phosphorus in Surface Sediment in Daya Bay, China.

Surface sediment samples were collected from 19 sites throughout Daya Bay, China, to study the concentrations, and spatial distributions of different fractions of phosphorus through sequential extraction methods. Like many coastal and marine areas, De-P was the dominant form of P, contributing 47.5% of TP, followed by O-P, contributing 25.5% of TP. Ex-P and Fe-P contribute the lowest to TP. The concentration of sedimentary TP ranged from 290.3~525.1 µg/g, with the average of 395.3 µg/g, which was a similar range to other estuaries and coastal areas. Based on the spatial distribution, Pearson correlation and Principal component analysis, different fractions of phosphorus showed different spatial distributions due to different sources. The molar ratio of organic carbon to phosphorus (TOC/O-P) ranged from 199 to 609, with the average of 413, which was much higher than the Redfield ratio, suggesting terrestrial sources of organic matter in Daya Bay surface sediment. The average bioavailable phosphorus was 149.6 µg/g and contributed 37.8% (24.6~56.0%) of TP, indicating that the surface sediments of Day Bay act as an important internal source of P.

Sub-annual fluorescence measurements of coral skeleton: relationship between skeletal luminescence and terrestrial humic-like substances

Some massive coral core slices reveal luminescent bands under ultraviolet light, which have been attributed to terrestrial humic acids in the skeleton. Coral luminescence has therefore been used to reconstruct past climate and hydrological variability. However, it has remained unresolved how closely coral luminescence at sub-annual resolution is related to terrestrial humic acid concentrations. This study presents a solution-based fluorescence method to quantify terrestrial humic substances in less than 4 mg of coral powder. The results show that in corals from Malaysia and Singapore, the luminescence green-to-blue ratio is correlated with skeletal concentrations of terrestrial humic substances (R2 > 0.40, p < 0.001) at two sites that are exposed to terrestrial dissolved organic matter from peatlands on Sumatra. In contrast, coral cores from two other sites located far from major terrestrial organic matter sources show lower green-to-blue values and no convincing correlation with fluorescence intensity of terrestrial humic substances in the skeleton. Abiogenic aragonite precipitation experiments with both terrestrial and marine organic matter sources confirmed that terrestrial humic substances are readily incorporated into aragonite, but not fluorescent organic matter from marine sources. The results of this study suggest that in coral cores with high luminescence green-to-blue ratios (> 0.6) and large downcore variability (range of ≥ 0.05), the green-to-blue ratio is strongly linked to variation in terrestrial humic substances. Coral cores therefore have the potential to reconstruct past variation in terrigenous dissolved organic carbon fluxes.

Carbon and nutrient accumulation in mangrove sediments affected by multiple environmental changes

In order to assess the impact of anthropogenic activities on carbon and nutrient accumulation, total organic carbon (TOC), nitrogen (TN), and phosphorus (TP) accumulation rates were examined in a 210Pb-dated mangrove sediment core from Sepetiba Bay, Brazil, a coastal region impacted by multiple environmental changes during the previous century. A 50-cm length sediment core was collected from a mangrove forest in Sepetiba Bay. Sediment subsamples were analyzed to measure TOC, TN, δ13C, and δ15N using an elemental analyzer attached to an isotope ratio mass spectrometer Thermo Finnigan Model Delta Plus XP, whereas colorimetric analysis were used to measure TP. For 210Pbex analyses, gamma-ray measurements were performed in a semiplanar intrinsic germanium high purity coaxial detector, coupled to a multichannel analyzer, whereas the sediment accumulation rate (SAR) was calculated according to the constant initial concentration (CIC) method. Also, carbon and nutrient fluxes were calculated using SAR and TOC, TN, and TP contents, whereas statistical differences were evaluated by ANOVA + Tukey HSD analysis with previous data normalization. The calculated sedimentation rate (~ 8.1 mm year−1) since the early 1900s was up to threefold higher than the global mean determined for mangrove forests (~ 2.8 mm year−1) and the regional sea level rise (~ 3.2 mm year−1). Significantly higher TOC, TN, and TP fluxes, up to nearly 1000, 90, and 15 g m−2 year−1, respectively, were observed after the water diversion from a nearby drainage basin in the 1950s and an increase in sewage effluent input, which increased in the early 1990s. After this period, lighter δ13C values (~ − 25‰) indicate an increased importance of the terrestrial organic matter source, while lower TOC:TN ratios (~ 11) and heavier δ15N values (~ + 9‰) suggest an increased influence of anthropogenic fertilization on inorganic nitrogen accumulation. The significantly higher accumulation rates during the last decades evidenced the role of mangrove sediments as sinks for anthropogenically enhanced inputs of carbon and nutrients. Also, studies on carbon and nutrient accumulation evidenced the need for further research in eutrophic coastal areas.

Comparing the organic matter source input of some Nigerian oils using steranes

Molecular geochemical method using Gas Chromatography Mass Spectrometry (GC-MS) was used to access the organic matter source input of the Agbada, Afowo, Yolde and Bima Formation oils. Steranes data obtained reveals that the studied Agbada Formation oil is dominated by terrestrial organic matter source inputs/origin with varying marine and lacustrine influences, thereby receiving a major contribution from terrestrial higher plants. The Afowo, Yolde and Bima Formation oils are dominated by marine organic matter source inputs with varying terrestrial and lacustrine influences, thereby receiving a major contribution from Planktonic and benthic Algae. Ternary plot of the relative amounts of C27, C28, and C29 regular steranes in the studied oils show marked resemblance for oils occurring in the Afowo Formation suggesting the two oils originated from the same petroleum source rock. Steranes of oils occurring in the studied Bima Formation does not match the oils occurring in the studied Yolde Formation. This suggests the possible existence of multiple petroleum systems within the northern Benue Trough. Inferred source rocks for the studied Bima oils is the marine source rocks associated with Bima and Fika Formations. Inferred source rock for the studied Yolde oils is the Marine source rocks associated with Yolde Formation. Inferred source rocks for the studied Afowo oils is the Organic-rich marine shale associated with Afowo and Araromi Formations. Inferred source rocks for the studied Agbada oils is the Organic-rich shales associated with Agbada Formation.

A comparative study of total organic carbon-δ13C signatures in the Triassic–Jurassic transitional beds of the Central European Basin and western Tethys shelf seas

Stratigraphic studies are an integral component in understanding the chronology of events that led to the end-Triassic mass extinction, by resolving causal relationships between environmental upheavals and biotic response. Successful correlation of Triassic–Jurassic (Tr–J) successions is complicated by the disappearance of macro-fossils that are otherwise central components in stratigraphic studies. This problem is exacerbated in multiple Tr–J sections situated in Europe, where the so-called “Event Beds” – assumed to demarcate the extinction interval – are virtually devoid of fossils. An alternative stratigraphic approach entails the reconstruction of carbon isotope records, where stratigraphic fluctuations in carbon isotope composition are considered to track changes in the global biogeochemical carbon cycle. The predominance of carbonate-lean sections has prompted the reconstruction of total organic carbon (TOC)-based carbon isotope records. However, bulk rock derived TOC is the diagenetically stabilized remnant of organic components that accumulated on the sea floor, and which can originate from multiple sources. In this study, we assess long-term TOC-based carbon isotope trends at two sites: Bonenburg (Central European Basin) and Kuhjoch (the Tr–J Global Stratotype Section and Point; western Tethys shelf seas). We focus on the TOC 13C-enrichment of the Event Beds with the aim of deciphering stratigraphic fluctuations in relation to their main driver (the exogenic carbon pool versus organic matter source changes). By studying the systematic co-variance of several sedimentary parameters (TOC, total nitrogen [TN], and the palynomorph composition), we infer that the TOC composition is possibly characterized by insignificant organic matter source changes in terms of the marine and terrestrial organic carbon contributions. By contrast, a clay mineralogical shift to more K-depleted minerals as well as the elevated occurrence of wood fragments in the Event Beds suggest a terrestrial organic matter source shift from immature substrates to substrates predominated by “pre-aged” or “fossil organic matter” under a changing continental weathering regime. This outcome urges for reservations when interpreting TOC-based carbon isotope records in terms of global C-cycle perturbations, especially when coinciding with lithological and mineralogical changes. On a more positive note, the shift towards positive carbon isotope values appears to be a recurring feature, possibly testifying to a globally significant climate-controlled weathering regime shift.

Distributions of geohopanoids in peat: Implications for the use of hopanoid-based proxies in natural archives

Hopanoids are pentacyclic triterpenoids produced by a wide range of bacteria. Within modern settings, hopanoids mostly occur in the biological 17β,21β(H) configuration. However, in some modern peatlands, the C31 hopane is present as the ‘thermally-mature’ 17α,21β(H) stereoisomer. This has traditionally been ascribed to isomerisation at the C-17 position catalysed by the acidic environment. However, recent work has argued that temperature and/or hydrology also exert a control upon hopane isomerisation. Such findings complicate the application of geohopanoids as palaeoenvironmental proxies. However, due to the small number of peats that have been studied, as well as the lack of peatland diversity sampled, the environmental controls regulating geohopanoid isomerisation remain poorly constrained. Here, we undertake a global approach to investigate the occurrence, distribution and diagenesis of geohopanoids within peat, combining previously published and newly generated data (n = 395) from peatlands with a wide temperature (−1 to 27 °C) and pH (3–8) range. Our results indicate that peats are characterised by a wide range of geohopanoids. However, the C31 hopane and C32 hopanoic acid (and occasionally the C32 hopanol) typically dominate. C32 hopanoic acids occur as αβ- and ββ-stereoisomers, with the ββ-isomer typically dominating. In contrast, C31 hopanes occur predominantly as the αβ-stereoisomer. These two observations collectively suggest that isomerisation is not inherited from an original biological precursor (i.e. biohopanoids). Using geohopanoid ββ/(αβ + ββ) indices, we demonstrate that the abundance of αβ-hopanoids is strongly influenced by the acidic environment, and we observe a significant positive correlation between C31 hopane isomerisation and pH (n = 94, r2 = 0.64, p < 0.001). Crucially, there is no correlation between C31 hopane isomerisation and temperature. We therefore conclude that within peats, αβ-hopanoids are acid-catalysed diagenetic products and their occurrence at shallow depths indicates that this isomerisation is rapid. This shows that geohopanoid ββ/(αβ + ββ) indices can be used to reconstruct pH within modern and ancient peat-forming environments. However, we only recommend using ββ/(αβ + ββ) indices to interrogate large amplitude (>1 pH unit) and longer-term (>1 kyr) variation. Overall, our findings demonstrate the potential of geohopanoids to provide unique new insights into understanding depositional environments and interpreting terrestrial organic matter sources in the geological record.

Sources and distribution of allochthonous organic matter in surface sediment from the Seomjin River to the southern inner shelf of Korea

The spatial distributions of δ13C, δ15N, and n-alkanes were investigated to determine the source and transportation of allochthonous organic matter from the mouth of the Seomjin River to the southern inner shelf break of Korea. Total organic carbon (%) ranged from 0.3% to 1.6% (average = 0.80%, n = 81), and the C/N ratio varied from 2.4 to 12.4 (average = 6.76, n = 81). The δ13C values ranged from −25.86 to −20.26‰ (average = −21.47‰, n = 81), and δ15N values ranged from 4.37‰ to 8.57‰ (average = 6.72‰, n = 81). The contribution of the terrestrial fraction of organic matter to the total ranged from 4.4% to 97.7% (average = 24.4%, n = 81), suggesting higher amounts around the catchment area and lower amounts in the offshore area. The concentration of total n-alkanes (nC25 − nC35) was higher at the boundary between the outer bay and inner shelf break (BOBIS). Average chain length and the carbon preference index both indicated that major leaf wax n-alkanes accounted for the observed distribution of terrestrial organic matter, and were dominant in the inner shelf break (around BOBIS) and outer shelf break. Based on the spatial distribution of the total n-alkanes and the sum of nC27, nC29, and nC31, the terrestrial organic matter distribution was considered to be controlled by local oceanographic conditions, especially at the center of the BOBIS. In addition to enabling the distribution and source of terrestrial organic matter to be identified, the n-alkanes indicated that minor anthropogenic allochthonous organic materials were superimposed on the total organic materials in the central part of Yeosu Bay and the catchment area. The n-alkane indices revealed weathered petroleum contamination, with contamination levels being relatively low at the present time.