Preservation Of Sedimentary Organic Matter Research Articles

Restoration and Evolution of the Paleogene (E1f2) Shale Sedimentary Environment in the Subei Basin, China.

Restoring the sedimentary environment of paleolakes is of great significance to the formation of laminated calcareous shale deposited in paleolakes and the prediction of shale oil reservoir distribution. This article focuses on the second section shale of the Paleogene in the Funing Formation in the Gaoyou Sag, Subei Basin, China, and uses X-ray fluorescence diffraction technology and core lithology analysis methods to obtain the content datum of major and trace elements such as Sr, Cu, Ba, Ga, V, and Ni in shale at different depths. Based on the empirical values of Sr/Cu, Sr/Ba, V/(V + Ni), and total organic carbon, paleoenvironmental evolution of the continental shale was determined and studied, including the changes in paleoclimate temperature, paleosalinity, paleowater depth, and strong or weak redox intensity. The research results indicate that the sedimentary environment of the paleolake in the Paleogene Funing Formation, second section, in the Gaoyou Sag is mainly characterized by a dry and hot climate; the salinity of paleolake water is that of stable brackish water, and the entire sedimentary period of the Funing Formation, second section, is dominated by a reduction environment, which is conducive to the preservation of sedimentary organic matter. The frequent changes in the depth of sedimentary water and the alternating dry and hot climate are the main reasons for the development of laminated calcareous shale in the second section of the Paleogene Funing Formation of the Gaoyou Sag and have also contributed to the abundant commercial resources of laminated calcareous shale oil in the second section of the Funing Formation.

Sedimentary Environments and Paleoclimate Control of the Middle Miocene Balikpapan Group, Lower Kutai Basin (Indonesia): Implications for Evaluation of the Hydrocarbon Potential

Sedimentary organic matter concentrated in source rocks forms the main source for the formation of hydrocarbons. Its deposition and preservation are strongly controlled by the depositional environment and paleoclimate. This study evaluates the paleoenvironment and the paleoclimatic controls of sediments in the Middle Miocene Balikpapan Group, Mahakam Delta of the Lower Kutai Basin, Indonesia. The sedimentary succession of the Mentawir Formation, encountered in three wells (MHK 1, MHK 3, and MHK 4), contains interbedded sandstones, siltstones, shales, and coal. Gamma ray log analysis has revealed four facies associations: (a) funnel-, (b) bell-, (c) cylindrical-, and (d) bow-shaped patterns, which, together with sedimentological and mineralogical analysis, suggest a fluvio-deltaic depositional environment during the Middle Miocene in the study area. Sedimentary successions from wells MHK 1 and MHK 3 comprise interbedded sandstone and siltstones and are interpreted to represent repeatedly occurring delta plain, delta front, and prodelta deposits. The succession encountered in well MHK 4 mostly consists of amalgamated sandstones and indicates a predominantly fluvial to upper delta plain environment with distributary channels and crevasse splays interbedded with only thin delta front deposits. X-ray diffraction–clay fraction analysis shows that the <2 μm clay-sized fraction consists of kaolinite (38%–67%), illite (14%–29%), chlorite (2%–17%), and mixed-layer illite/smectite (I/S) (14%–30%). Kaolinite formation and abundance indicates a hinterland climate classified as type Af (tropical rainforest) and intensive chemical weathering conditions in the source areas related to tropical to sub-tropical climates with high precipitation. Under such climatic conditions, kaolinite and I/S mixed-layer minerals are preferentially formed because the characteristic ions, K+, Na+, Ca2+, and Fe2+, are leached away. Thus, the production, transport, distribution, and preservation of sedimentary organic matter in the onshore Mentawir Formation of the Balikpapan Group are predominantly controlled by the humid tropical climate and fluvio-deltaic processes.

Isotopic and spectral signatures unravel the sources, preservation and degradation of sedimentary organic matter in the Dongzhai Harbor mangrove estuary, southern China

Sedimentary organic matter (SOM) production and transformation processes have a prominent role in regulating biogenic element cycling in mangrove ecosystems. However, aquaculture biodeposits can substantially affect source apportionments and biogeochemical processes of SOM within mangroves. In this study, natural stable isotopes (13C and 15N), spectral techniques, and hydrogeochemical analysis were used to elucidate the sources, preservation, and degradation of SOM across the Dongzhai Harbor (DZH) estuary, southern China, where aquaculture systems are extensive. Along the estuarine salinity gradient, sediment and bottom water samples were collected from the upper fluvial mangrove zones to the lower marine endmember. Results indicated that source contributions to SOM along the estuarine gradient were spatially heterogeneous. Aquaculture-derived organic matter was the largest contributor to the Sanjiang River (44.29 %) in the upper estuary, and terrestrial sources from C3 mangrove forests were highest in the Yanfeng East River (62.87 %) near the middle estuary. Farther seaward, marine plankton sources were elevated in the Dongzhai Harbor channel (64.15 %) and Yanfeng West River (48.83 %) of the lower estuary. The terrestrial/microbial humic-like component (C1), tryptophan-like component (C2), and soil-derived fulvic acid (C3) were identified from the samples using fluorescence excitation emission matrices and parallel factor analysis (EEM-PARAFAC). Results show that in the upper and middle estuary, SOM was preserved by sedimentation due to the abundance of fine-grained aquaculture-derived biodeposits, weak hydrodynamic conditions, and the formation of Fe/Mn-humus complexes. In contrast, in the lower estuary, SOM originated from marine plankton and was rapidly degraded via aerobic respiration. Moreover, correlations between δ13C and δ15N values vs C/N ratios further suggest progressive SOM degradation from initial diagenesis to post-depositional geochemical processes. This study provides new insights into aquacultural and environmental factors controlling SOM preservation and degradation within estuaries. The findings have implications for shoreline stability and the protection and restoration of estuarine mangrove wetlands.

Distribution Characteristics and Genesis of Marine Anoxic Conditions in the Southwest of the Upper Yangtze Basin During the Late Ordovician–Early Silurian, South China

The distribution characteristics and formation of marine anoxic conditions through the Late Ordovician–Early Silurian (O–S) remain poorly resolved despite their importance in the preservation of sedimentary organic matter and the formation of black shale. In this study, the major, trace, and pyrite δ34S (δ34Spy) contents of 36 shale samples at the edge of the southwest depocenter of the Upper Yangtze Basin (Tianlin and Changhebian sections) were analyzed to understand the redox conditions, terrigenous clastic inputs, and primary productivity changes. The iron speciation and enrichment factor of U and Mo show that the range of anoxic conditions gradually expanded from the sedimentation center to the edge during the late Katian stage, peaked in the early Hirnantian stage followed by a rapid decrease, and expanded again during the early Rhuddanian stage. Comprehensive index analysis showed that the increase of terrigenous clastic input and the relative decrease of primary productivity due to tectonism and sea level change controlled the transformation of the water column from anoxic, especially euxinic, to suboxic-oxic conditions. This is reflected in the correlation between paleo-salinity, δ34Spy, chemical index of alteration, and the organic carbon accumulation rate. This work emphasizes the control of terrigenous clastic input and sulfate availability on the transformation of marine redox conditions during the O-S period in semi-restricted basins.

Spatial distribution, compositional pattern and source apportionment of n-alkanes in surface sediments of the Bohai Sea, Yellow Sea, and East China Sea and implications of carbon sink

China's marginal seas (CMSs, including the Bohai Sea, Yellow Sea, East China Sea) are a significant sink for both terrestrial organic matter (OM) and marine OM, and they play an important role in the global biogeochemical carbon cycle. The spatial distribution and origin of organic matter based on n-alkanes in the surface sediments of CMSs and the implications of carbon sinks were comparatively analyzed. The n-alkane content in surface sediment from the Bohai Sea was higher than that of the Yellow Sea and the East China Sea. The spatial distribution trends of marine and terrestrial organic matter are obviously different in the surface sediments of China's marginal seas. The n-alkanes in the sediments of the Yellow Sea and the East China Sea were mainly derived from terrestrial higher plants, and land-based influence gradually decreased from the near shore to the open sea. Higher concentration of terrigenous OM are concentrated nearshore, especially near estuaries, such as the Yellow River Estuary, the Old Yellow River Estuary and the Yangtze River Estuary. The input of n-alkanes from woody plants in the Bohai Sea area was slightly higher than that of herbaceous plants, and the input of herbaceous plants in the Yellow Sea and East China Sea was slightly dominant. The distribution of marine organic matter is controlled by marine productivity and the sedimentary environment. Due to climate change, the decomposition and enrichment of organic matter also show the climate effect of carbon molecular combinations. As a semiclosed sea area, the Bohai Sea was beneficial to the growth and reproduction of marine phytoplankton. From the perspective of petroleum pollution, the Bohai Sea was the most serious, followed by the Yellow Sea, and the East China Sea was the lightest. The carbon burial amount of terrestrial organic matter accounts for approximately 7% of the terrestrial organic matter burial amount of global marginal sea sediments, indicating that China's marginal sea plays an important role in the global carbon cycle. The result provide a basis for further understanding the source pattern and burial preservation of sedimentary organic matter in this sea area.

Latitudinal variability of preserved sedimentary organic matter along the Peruvian continental margin as inferred from petrographic and geochemical properties

Processes controlling preservation and accumulation of organic matter (OM) in marine sediments are highly sensitive towards both natural and anthropogenic impacts. Thus, the sedimentary OM is considered a key proxy for the characterization of recent and past sedimentary environments. Along the Peruvian continental margin (PCM), high primary production and an intense oxygen minimum zone (OMZ) favor the accumulation and preservation of OM in sediments. Spatial variations in oceanographic conditions impact on both productivity and the redox conditions, in particular the OMZ intensity. These factors in turn lead to spatial differences in depositional conditions and OM accumulation and preservation. Therefore, this study aims at characterizing nature and types of recent sedimentary OM along the PCM covering a wide latitudinal and bathymetric range. With this purpose, a total of 29 surface sediment samples were collected from Tumbes (3°S) to San Juan (15°S) and analyzed by means of organic petrography (palynofacies analysis) and Rock-Eval pyrolysis. Results allowed characterizing the OM from recent sediments. Along the PCM, the sedimentary OM is predominantly amorphous, revealing its overwhelming marine origin. The highest abundances of gelified amorphous OM, total organic carbon, S1 and S2 Rock-Eval fractions occurred towards the southern part of the study area, especially off Callao (12°S) and Pisco (14°S), considered important upwelling and primary productivity centers. A principal component analysis performed allowed the description of main OM depositional environments. In the outer continental shelf of Callao (12°S) and OMZ of Pisco (14°S), accumulation of well-preserved OM occurs whereas in the inner shelf of Callao redox oscillations lead to less OM accumulation and pyrite formation in sediments. North of Callao, from Tumbes (3°S) to Huacho (11°S), a mixed signal of OM accumulation was observed. Finally, along the slope, transportation processes seem to play a key role in OM accumulation. This study also allowed the evaluation of certain proxies (e.g. Rock-Eval hydrogen index and oxygen index) routinely used in palaeoceanographic studies.

Paleoweathering, hydrothermal activity and organic matter enrichment during the formation of earliest Cambrian black strata in the northwest Tarim Basin, China

The earliest Cambrian black strata are widely deposited in many continents, and attracted increasing attention from scholars worldwide. Based on detailed investigations of strata from the Shiairike section in the Aksu area, northwest Tarim Basin, China, the effects of paleoweathering and hydrothermal activity on the organic matter enrichment of this black series was revealed. Combined organic and inorganic geochemistry evidence indicated that relatively strong weathering carried abundant nutrient elements and terrigenous detritus into the sea, which promoted a period of paleoproductivity and enhanced the sinking of dissolved organic matter. Moreover, the hydrothermal activity triggered the enrichment of abundant reducing gases (e.g. H2S) and minerals, whereby gases created euxinic benthic conditions that were favorable for the preservation of sedimentary organic matter, while minerals simultaneously diluted primary productivity. As a result, the low section of the black strata was characterized by dominant siliceous rocks with enrichments of P, Ba, Zn and Cu; whereas the upper section contained abundant organic matters that had the potential to generate and expel a remarkable volume of hydrocarbons. These results provide a theoretical basis for the international exploration of paleo-hydrothermal minerals within the early Cambrian black strata as well as that of the deep Cambrian oil and gas, particularly in the Tarim Basin, northwest China.

Geochemistry and sedimentology of the Upper Ordovician–lower Silurian black shale in the northern margin of the Upper Yangtze Platform, South China: implications for depositional controls on organic-matter accumulation

Organic-rich black shales from the Upper Yangtze Platform from the Upper Ordovician and lower Silurian are considered excellent source rocks and unconventional reservoirs of hydrocarbons in South China. This research combined geochemical analyses and detailed sedimentological observations to investigate the depositional controls on organic-matter abundance in the prolific black mudstone of the Wufeng Formation and Long-1 Member of the lower Silurian Longmaxi. Six primary lithofacies deposited at variable marine water depths and under different water column chemistries were identified from a ∼50 m-long profile based on microscopic observations of sediment texture and structure, and analyses of total organic carbon (TOC) content, major and trace-element abundances, and pyrite framboid size. Our results showed that TOC content is strongly correlated with terrigenous SiO2 content and the non-detrital components of V, U and Mo, suggesting that the accumulation of organic matter in the studied shale was controlled by terrigenous clast fluxes and anoxic water conditions. However, the weak covariance relationship between TOC content and productivity proxies, including P/Ti and Ba/Al, demonstrates that the accumulation of organic matter was not controlled by primary productivity. The three high-productivity lithofacies of the Wufeng Formation and Long-1 Member have low Co*Mn values, indicating active marine upwelling during sedimentary periods. The northern margin of the Upper Yangtze Platform was an open basin system influenced by the Kwangsian Orogeny and global sea-level changes, but oxygen-depleted bottom waters in the basin favoured the accumulation and preservation of sedimentary organic matter, resulting in the formation of organic-rich black shales.

Difference Analysis of Organic Matter Enrichment Mechanisms in Upper Ordovician-Lower Silurian Shale from the Yangtze Region of Southern China and Its Geological Significance in Shale Gas Exploration

The upper Ordovician-lower Silurian shale has always been the main target of marine shale gas exploration in southern China. However, the shale gas content varies greatly across different regions. The organic matter content is one of the most important factors in determining gas content; therefore, determining the enrichment mechanisms of organic matter is an important problem that needs to be solved urgently. In this paper, upper Ordovician-lower Silurian shale samples from the X-1 and Y-1 wells that are located in the southern Sichuan area of the upper Yangtze region and the northwestern Jiangxi area of the lower Yangtze region, respectively, are selected for analysis. Based on the core sample description, well logging data analysis, mineral and elemental composition analysis, silicon isotope analysis, and TOC (total organic carbon) content analysis, the upper Ordovician-lower Silurian shale is studied to quantitatively calculate its content of excess silicon. Subsequently, the results of elemental analysis and silicon isotope analysis are used to determine the origin of excess silicon. Finally, we used U/Th to determine the characteristics of the redox environment and the relationship between excess barium and TOC content to judge paleoproductivity and further studied the mechanism underlying sedimentary organic matter enrichment in the study area. The results show that the excess silicon from the upper Ordovician-lower Silurian shale in the upper Yangtze area is derived from biogenesis. The sedimentary water body is divided into an oxygen-rich upper water layer that has higher paleoproductivity and a strongly reducing lower water that is conducive to the preservation of sedimentary organic matter. Thus, for the upper Ordovician-lower Silurian shale in the upper Yangtze region, exploration should be conducted in the center of the blocks with high TOC contents and strongly reducing water body. However, the excess silicon in the upper Ordovician-lower Silurian shale of the lower Yangtze area originates from hydrothermal activity that can enhance the reducibility of the bottom water and carry nutrients from the crust to improve paleoproductivity and enrich sedimentary organic matter. Therefore, for the upper Ordovician-lower Silurian shale in the lower Yangtze region, exploration should be conducted in the blocks near the junction of the two plates where hydrothermal activity was active.

Mechanism analysis of organic matter enrichment in different sedimentary backgrounds: A case study of the Lower Cambrian and the Upper Ordovician-Lower Silurian, in Yangtze region

In order to meet the demand of shale oil and gas exploration, it is more and more important to study the controlling effect of sedimentary environment on the enrichment of organic matter. In this paper, the Lower Cambrian and Upper Ordovician-Lower Silurian shales in the Yangtze region are studied. Firstly, according to the content of Mo and TOC, the water closure property of the shale depositional period is judged. Then this study selected typical wells, and calculated quantitatively whether there was excess siliceous minerals in shale and the content of it. The origin of excess siliceous minerals is determined by Al, Fe and Mn element. The results show that the Lower Cambrian shale is deposited in a weak to moderate restricted water environment, and the Upper Ordovician and Lower Silurian shales are deposited in a strong restricted water environment in the Yangtze region. Excess siliceous minerals in the Lower Cambrian shale is of hydrothermal origin. On the one hand, hydrothermal activity can enhance the reductivity of the water bottom. On the other hand, it can improve the biological productivity, so that the sedimentary organic matter can be enriched. Excess siliceous minerals in Upper Ordovician and Lower Silurian shale is biogenic. The strong restriction of the water leads to stratification. The oxygen content in the upper layer makes the biological productivity higher and the lower layer more reductive, which is beneficial to the preservation of sedimentary organic matter.

Comparative Analysis of the Siliceous Source and Organic Matter Enrichment Mechanism of the Upper Ordovician–Lower Silurian Shale in the Upper-Lower Yangtze Area

Organic matter is the material basis of hydrocarbon generation and the abundance of organic matter is a main factor of regional selection and evaluation in shale gas. Also the enrichment is influenced by sedimentary environments. Thus, it is important for the study on the geological factors controlling organic matter enrichment and further to provide scientific basis of regional selection and evaluation by organic matter enrichment area with analysis of the factors. In this paper, the Upper Ordovician–Lower Silurian shale from representative wells in the Upper-Lower Yangtze area is selected as the research object. The goal of this study is to quantitatively calculate the excess siliceous mineral content in shale siliceous minerals and determine the origin of excess silicon based on Al, Fe, and Mn elements; as well as to analyze the sedimentary organic matter enrichment mechanism based on the water body redox environment and bio-productivity. The results show that excess silicon from the Upper Ordovician–Lower Silurian shale in the Upper Yangtze area is biogenic and deposited in closed water bodies. On the one hand, the upper water body contains oxygen, which leads to higher bio-productivity. On the other hand, the lower water body has strong reducibility, which is conducive to sedimentary organic matter preservation. However, the excess silicon in the Upper Ordovician–Lower Silurian shale of the Lower Yangtze area is derived from hydrothermal solution. Hydrothermal activity can enhance the bottom water reducibility, and its nutrient elements can improve bio-productivity and enrich sedimentary organic matter. Therefore, the organic matter enrichment, which depends on the biological productivity and redox conditions, is controlled by the water closure in the Upper Yangtze and hydrothermal activities in the Lower Yangtze respectively. It led to a conclusion that in the process of regional selection and evaluation of shale gas in the Late Ordovician–Early Silurian, it is favorable in the area of relatively strong closure, which is the center of cratonic depression, in the Upper Yangtze and in the hydrotherm-active area, which is the plate connection of the Lower Yangtze and the Cathaysian, in the Lower Yangtze.

Paleoenvironmental reconstruction by means of palynofacies and lithofacies analyses: An example from the Upper Triassic subsurface succession of the Hyblean Plateau Petroleum System (SE Sicily, Italy)

A combined palynofacies and lithofacies analysis was carried out on two borehole successions (Streppenosa 1 and Bimmisca 1) from the Hyblean Plateau Petroleum System (SE Sicily, Italy). It was found that both the wells penetrated the most important source and seal rocks of the Sicilian region (Noto and Streppenosa formations), previously assigned to the Late Triassic-Early Jurassic, deposited within a carbonate platform-basin system. Based on new palynological data, the organic rich succession (Noto Formation and Upper Streppenosa Member) can now be entirely assigned to the Rhaetian, thus constraining its deposition to a time interval characterized by increasing global humidity and seasonality. The integrated palynofacies and lithofacies data enabled characterization of the timing of the drowning phases of the carbonate platform-basin system as being controlled by relative sea level changes mostly triggered by the Triassic extensional tectonic activity. During the first phase of the relative sea-level rise, clayey and organic-rich sediments were deposited only in the deepest portion of the basin. As the sea level continued to rise, the entire system drowned completely and suboxic-anoxic basinal sediments were deposited across the whole Hyblean region, onlapping the shallow-water facies. In the meantime increasing global humidity contributed to an increased freshwater input in the marine depositional system as documented by the presence of fern spores and clay. It caused water stratification and subsequent anoxia at marine basins, favoring the preservation of sedimentary organic matter. This atmospheric change could be related to the degassing of the Central Atlantic Magmatic Province.

The Fate of Carbon in Sediments of the Xingu and Tapajós Clearwater Rivers, Eastern Amazon

The Xingu and Tapajos rivers in the eastern Amazon are the largest clearwater systems of the Amazon basin. Both rivers have “fluvial rias” (i.e., lake-like channels) in their downstream reaches as they are naturally impounded by the Amazon mainstem. Fluvial rias are widespread in the Amazon landscape and most of the sedimentary load from the major clearwater and blackwater rivers is deposited in these channels. So far, little is known about the role of Amazon rias as a trap and reactor for organic sediments. In this study, we used organic and inorganic geochemistry, magnetic susceptibility, diatom, and pollen analyses in sediments (suspended, riverbed, and downcore) of the Xingu and Tapajos rias to investigate the effects of hydrologic variations on the carbon budget in these clearwater rivers over the Holocene. Ages of sediment deposition (~100 to 5,500 years) were constrained by optically stimulated luminescence and radiocarbon. Major elements geochemistry and concentration of total organic carbon (TOC) indicate that seasonal hydrologic variations exert a strong influence on riverine productivity and on the input and preservation of organic matter in sediments. Stable carbon isotope data (δ13C from -31.04 to -27.49‰) and pollen analysis indicate that most of the carbon buried in rias is derived from forests. In the Xingu River, diatom analysis in bottom sediments revealed 65 infrageneric taxa that are mostly well-adapted to slack oligotrophic and acidic waters. TOC values in sediment cores are similar to values measured in riverbed sediments and indicate suitable conditions for organic matter preservation in sediments of the Xingu and Tapajos rias at least since the mid-Holocene, with carbon burial rates varying from about 84 g m-2 yr-1 to 169 g m-2 yr-1. However, redox-sensitive elements in sediment core indicate alternation between anoxic/dysoxic and oxic conditions in the water-sediment interface that may be linked to abrupt changes in precipitation. The variation between anoxic/dysoxic and oxic conditions in the water-sediment interface controls organic matter mineralization and methanogenesis. Thus, such changes promoted by hydrological variations significantly affect the capacity of Amazon rias to act either as sources or sinks of carbon.

Quantifying organic carbon storage and sources in sediments of Dong Rui mangrove forests, Tien Yen district, Quang Ninh province using carbon stable isotope

Quantifying organic carbon storage and sources in sediments of Dong Rui mangrove forests, Tien Yen district, Quang Ninh province using carbon stable isotope

The geochemical characteristics and factors controlling the organic matter accumulation of the Late Ordovician-Early Silurian black shale in the Upper Yangtze Basin, South China

Organic-rich black shale of the Upper Yangtze Basin from the Late Ordovician and Early Silurian is considered an excellent source rock in South China. The formation and preservation conditions of this resource are revealed by its geochemical characteristics in this study. Geochemical indices, including redox indices (V/(V + Ni), V/Cr, V/Sc, and Ni/Co) and primary productivity indices (P/Ti and Ba/Al), and paleoclimate, clastic flux and sedimentary rate analyses are presented to investigate the accumulation mechanism of organic matter. Redox indices suggest that a stagnant, anoxic environment predominated in the Upper Yangtze Basin during accumulation of Wufeng and Longmaxi formations. In contrast, ventilated and oxygenated marine conditions pervaded the Upper Yangtze Basin during deposition of Linxiang and Guanyinqiao formations. The concentrations of V and U demonstrate that accumulation of organic matter was mainly controlled by redox conditions. Besides, such factors as clastic fluxes, fresh water inflows or a mixed deposition with a rapid sedimentary rate cannot be ignored due to their influences on organic matter enrichment and preservation. However, weak co-variance relationship of TOC content and productivity proxies, including P/Ti and Ba/Al, demonstrates that the accumulation of organic matter was not controlled by primary productivity. Results of the present study suggest a depositional model that stresses the importance of tectonic movements and glacial events on the accumulation and preservation of organic matter. The model shows that the Upper Yangtze Basin was a semi-restricted basin system influenced by the isolation of Xuefeng, but also it implies that oxygen-depleted bottom water of the basin favored the accumulation and preservation of sedimentary organic matter, resulting in the formation of organic-rich black shale.

Sources of sedimentary biomarkers and proxies with potential paleoenvironmental significance for the Baltic Sea

The Baltic Sea is a shallow, semi-enclosed and intra-continental shelf sea characterized by anoxic bottom waters in the deepest basins, allowing for the preservation of sedimentary organic matter. In the present study, the most abundant, naturally-occurring lipids in surface sediments from the entire Baltic Sea and the Skagerrak area were identified and their potential sources were assigned. Together with long-chain n-alkanes derived from land plant leaf waxes, diploptene and branched glycerol dialkyl glycerol tetraethers (GDGTs) are of allochthonous origin, while isoprenoid GDGTs, hydroxylated isoprenoid GDGTs (OH-GDGTs), n-C25:1, n-C27:1 and n-C29:1 alkenes are autochthonous lipids. The isoprenoid and OH-GDGTs are probably derived from Thaumarchaeota and the long-chain n-alkenes from phototrophic organisms. Significant correlations were found between indexes based on isoprenoid and OH-GDGTs and Baltic Sea surface and bottom temperatures. The calibrations obtained for surface temperature have statistically similar slopes, but different intercepts than calibrations established for the Nordic Seas. The branched and isoprenoid tetraether index can be used to estimate the percentage of soil (terrestrial) organic matter in the sediments of the Baltic Sea. High values of the Paq’ ratio (defined here as the ratio of odd numbered n-C23 and n-C25 over n-C23 to n-C29 alkanes) in the northern Baltic Sea originate from the presence of both Sphagnum mosses in the drainage basin and submerged macrophytes, such as Potamogeton sp. and Myriophyllum sp., in the freshwater to brackish water of the coastal areas. The Paq’ ratio may thus reflect fluctuations in the regional expansion of freshwater to brackish coastal environments in the Baltic Sea.

Assessing the potential of amino acid <sup>13</sup>C patterns as a carbon source tracer in marine sediments: effects of algal growth conditions and sedimentary diagenesis

Abstract. Burial of organic carbon in marine sediments has a profound influence in marine biogeochemical cycles and provides a sink for greenhouse gases such as CO2 and CH4. However, tracing organic carbon from primary production sources as well as its transformations in the sediment record remains challenging. Here we examine a novel but growing tool for tracing the biosynthetic origin of amino acid carbon skeletons, based on naturally occurring stable carbon isotope patterns in individual amino acids (δ13CAA). We focus on two important aspects for δ13CAA utility in sedimentary paleoarchives: first, the fidelity of source diagnostic of algal δ13CAA patterns across different oceanographic growth conditions, and second, the ability of δ13CAA patterns to record the degree of subsequent microbial amino acid synthesis after sedimentary burial. Using the marine diatom Thalassiosira weissflogii, we tested under controlled conditions how δ13CAA patterns respond to changing environmental conditions, including light, salinity, temperature, and pH. Our findings show that while differing oceanic growth conditions can change macromolecular cellular composition, δ13CAA isotopic patterns remain largely invariant. These results emphasize that δ13CAA patterns should accurately record biosynthetic sources across widely disparate oceanographic conditions. We also explored how δ13CAA patterns change as a function of age, total nitrogen and organic carbon content after burial, in a marine sediment core from a coastal upwelling area off Peru. Based on the four most informative amino acids for distinguishing between diatom and bacterial sources (i.e., isoleucine, lysine, leucine and tyrosine), bacterially derived amino acids ranged from 10 to 15 % in the sediment layers from the last 5000 years, and up to 35 % during the last glacial period. The greater bacterial contributions in older sediments indicate that bacterial activity and amino acid resynthesis progressed, approximately as a function of sediment age, to a substantially larger degree than suggested by changes in total organic nitrogen and carbon content. It is uncertain whether archaea may have contributed to sedimentary δ13CAA patterns we observe, and controlled culturing studies will be needed to investigate whether δ13CAA patterns can differentiate bacterial from archeal sources. Further research efforts are also needed to understand how closely δ13CAA patterns derived from hydrolyzable amino acids represent total sedimentary proteineincous material, and more broadly sedimentary organic nitrogen. Overall, however, both our culturing and sediment studies suggest that δ13CAA patterns in sediments will represent a novel proxy for understanding both primary production sources, and the direct bacterial role in the ultimate preservation of sedimentary organic matter.

Compound specific amino acid δ15N in marine sediments: A new approach for studies of the marine nitrogen cycle

The nitrogen (N) isotopic composition (δ15N) of bulk sedimentary N (δ15Nbulk) is a common tool for studying past biogeochemical cycling in the paleoceanographic record. Empirical evidence suggests that natural fluctuations in the δ15N of surface nutrient N are reflected in the δ15N of exported planktonic biomass and in sedimentary δ15Nbulk. However, δ15Nbulk is an analysis of total combustible sedimentary N, and therefore also includes mixtures of N sources and/or selective removal or preservation of N-containing compounds. Compound-specific nitrogen isotope analyses of individual amino acids (δ15NAA) are novel measurements with the potential to decouple δ15N changes in nutrient N from trophic effects, two main processes that can influence δ15Nbulk records. As a proof of concept study to examine how δ15NAA can be applied in marine sedimentary systems, we compare the δ15NAA signatures of surface and sinking POM sources with shallow surface sediments from the Santa Barbara Basin, a sub-oxic depositional environmental that exhibits excellent preservation of sedimentary organic matter. Our results demonstrate that δ15NAA signatures of both planktonic biomass and sinking POM are well preserved in such surface sediments. However, we also observed an unexpected inverse correlation between δ15N value of phenylalanine (δ15NPhe; the best AA proxy for N isotopic value at the base of the food web) and calculated trophic position. We used a simple N isotope mass balance model to confirm that over long time scales, δ15NPhe values should in fact be directly dependent on shifts in ecosystem trophic position. While this result may appear incongruent with current applications of δ15NAA in food webs, it is consistent with expectations that paleoarchives will integrate N dynamics over much longer timescales. We therefore propose that for paleoceanographic applications, key δ15NAA parameters are ecosystem trophic position, which determines relative partitioning of 15N into source AA versus trophic AA pools, and the integrated δ15NAA of all common protein AA (δ15NTHAA), which serves as a proxy for the δ15N of nutrient N. Together, we suggest that these can provide a coupled picture of regime shifts in planktonic ecosystem structure, δ15N at the base of food webs, and possibly additional information about nutrient dynamics.

Sources and preservation of sedimentary organic matter in the Southern Bohai Sea and the Yellow Sea: Evidence from lipid biomarkers

The region of Southern Bohai Sea (SBS) and Yellow Sea (YS) represents one of the best examples of large river-dominated ocean margins. The transport and accumulation of sediments played a primary role in distribution of the lipids in the region and could lead to a preferential accumulation of these more refractory land-based lipids. The spatial distribution of total organic carbon (TOC) matched with the lipid distributions over the shelf, and high TOC presented in the central YS, but TOC burial fluxes decreased from 110ton/km2/yr of SBS to 25–32ton/km2/yr of YS. Good correlations were observed between terrestrial n-alkanes and TOC in the sediments of SBS and northern YS, but they were poor for southern YS, meanwhile higher ratios of nC27+nC29+nC31/nC15+nC17+nC19 (12±6) were observed in SBS, and lower in YS (2.2±1.0), suggesting the Yellow River discharge was an important source of terrestrial organic matter to the shelf.

Carbon sinks in small Sahelian lakes as an unexpected effect of land use changes since the 1960s (Saga Gorou and Dallol Bosso, SW Niger)

For several decades, global change has resulted in an increase in runoff in the Sahelian belt, provoking major changes in the quality and quantity of sediments transported by drainage networks. One of the astonishing consequences is the establishment of numerous permanent lakes. The origins of particulate organic matter (OM) preserved within lacustrine sediments of three lakes were investigated by coupling optical observations (palynofacies) and bulk geochemistry (Rock-Eval 6 pyrolysis). An initial estimate of particulate organic carbon (OC) stored in these lacustrine sediments was assessed. Soil organic matter (SOM) was sampled from the surface (0-10cm) of various land-use and land-cover areas and was characterized and compared with sedimentary organic matter. Our results reveal that Lake Tankalawal is subjected to high autochthonous organic sedimentation (TOC ranges between 3.0 and 10.0wt.%), while lakes Bangou Kirey and Bi are characterized by weak sedimentation of non-indigenous OM originating from the soil erosion and shore vegetation (TOC<3.0wt.%). In sediments, the effects of early diagenesis on the OM induce not only a loss of labile and aquatic OM but also a significant loss of terrestrial OM, which is supposed to be more resistant than its aquatic counterpart. Both the preservation of OM in top sediments and the relative preservation of terrestrial OM impact the OC storage in lakes. Indeed, OC storage in lacustrine sediments (Lake Bangou Kirey) was two to seven times higher than the OC storage in Sahelian soils, where greater contributions of terrestrial OM to sedimentary OM are associated with lower OC storage in lacustrine sediments. For lakes Bangou Kirey and Bi, OC accumulation rates were also assessed; due to the identification of a sedimentary limit corresponding to the establishment of permanent lakes dated earlier 1960s. High OC fluxes were estimated and ranged between 104 and 213gOCm−2yr−1. Compared with other OC accumulation rates for various African lakes, these high values are similar to those calculated for reservoirs and are related to anthropogenic pressure, soil textures favoring erosion, and proper physical and chemical conditions for OM preservation in sediments. Accordingly, in these Sahelian environments that are generally viewed as non-efficient in storing OC, we claim that global change could promote a new OC sink. If other similar studies reinforce our assertion, then regional C budgets should be revisited.