Terrestrial Organic Matter Fraction Research Articles

Source, distribution and degradation of sedimentary organic matter in the South Yellow Sea and East China Sea

Sedimentary organic matter (SOM) plays an important role in marine carbon and nitrogen cycling, but the sources, degradation states and fates of SOM are not well understood. In the present study, we determined the total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C), and total hydrolysable amino acids (THAA) to elucidate the sources and degradation states of SOM in the South Yellow Sea (SYS) and the East China Sea (ECS). THAA concentrations ranged from 4.90 to 29.13 μmol/g, with a mean of 11.03 ± 5.06 μmol/g. The distribution pattern of THAA was similar to that of TOC (r = 0.782, P < 0.01) and TN (r = 0.865, P < 0.01), with higher THAA concentrations in the coastal areas. Based on the two end-member mixing model, the fraction of terrestrial organic matter (Ft) ranged from 12.11% to 51.34% (average: 30.17 ± 9.61%), with much higher variability at the nearshore stations compared to the offshore stations. The C/N ratios and the δ13C values showed the different distribution trend, and there was a negative correlation between them (r = −0.479, P < 0.05). Based on the C/N ratios and δ13C values, the SOM in the study area was a mixture of marine organic matter (OM) and soil-derived OM. In addition, our study showed that SOM was dominated by diatomaceous OM compared to carbonate-derived OM and that terrestrial inputs and primary production were the dominant factors controlling the distribution of OM in the study area. Carbon and nitrogen-normalized yields of THAA (THAA-C% and THAA-N%) showed that relatively fresh OM tended to occur in the coastal areas off the Shandong Peninsula. However, the generally low degradation index (DI) values revealed that all SOM in the study area had undergone significant degradation. Glycine increased in relative abundance as the SOM was degraded, while aromatic amino acids (Tyr and Phe) and neutral amino acids (Leu, Ile and Val) had consistently decreasing trends following SOM degradation.

Distribution and sources of sedimentary organic matter in Chitrapuzha, a tropical tidal river, southwest coast of India

ABSTRACTSurface sediment samples were collected from five stations along the Chitrapuzha River, during the monsoon, post-monsoon and pre-monsoon seasons, and analyzed for biochemical composition, total organic carbon (TOC), total nitrogen (TN), and stable isotopic ratios of carbon (δ13C) and nitrogen (δ15N) to identify the major sources of organic matter. Sediment grain size was found to be the key factor influencing the organic matter accumulation in surface sediments. The δ13C values vary between −27.94‰ and −26.05‰ and δ15N values ranges from −1.66‰ to −4.75‰ in surface sediments. The δ13C values indicate terrestrial inputs from higher plants, while δ15N values suggest decomposition processes in the sediments. The fraction of terrestrial organic matter (TrOM) to the total organic matter (TOM) pool ranges from 41 to 84% in the surface sediments as estimated by δ13C-based two end members mixing model. Intermediate values of TOC/TN ratios signal a combined input of both autochthonous and terrestrial organic matter sources, while protein-to-carbohydrate ratios show freshly deposited detritus in the majority of the stations. The threshold levels of protein, biopolymeric carbon, and algal contribution to biopolymeric carbon (BPC) indicate eutrophic conditions prevailing in the study area.

Surface nitrate utilization in the Bering sea since 180 kA BP: Insight from sedimentary nitrogen isotopes

We present high-resolution records of sedimentary nitrogen (δ15Nbulk) and carbon isotope ratios (δ13Cbulk) from piston core SO201-2-85KL located in the western Bering Sea. The records reflect changes in surface nitrate utilization and terrestrial organic matter contribution in submillennial resolution that span the last 180kyr. The δ15Nbulk record is characterized by a minimum during the penultimate interglacial indicating low nitrate utilization (~62–80%) despite the relatively high export production inferred from opal concentrations along with a significant reduction in the terrestrial organic matter fraction (mterr). This suggests that the consumption of the nitrate pool at our site was incomplete and even more reduced than today (~84%). δ15Nbulk increases from Marine Isotope Stage (MIS) 5.4 and culminates during the Last Glacial Maximum, which indicates that nitrate utilization in the Bering Sea was raised during cold intervals (MIS 5.4, 5.2, 4) and almost complete during MIS 3 and 2 (~93–100%). This is in agreement with previous hypotheses suggesting that stronger glacial stratification reduced the nutrient supply from the subeuphotic zone, thereby increasing the iron-to-nutrient ratio and therefore the nitrate utilization in the mixed surface layer. Large variations in δ15Nbulk were also recorded from 180 to 130ka BP (MIS 6), indicating a potential link to insolation and sea-level forcing and its related feedbacks. Millennial-scale oscillations were observed in δ15Nbulk and δ13Cbulk that might be related to Greenland interstadials.

Comparison of isotopic and geochemical characteristics of sediments from a gas- and liquids-prone wells in Marcellus Shale from Appalachian Basin, West Virginia

The Middle Devonian age Marcellus Shale contains one of the largest shale gas plays in North America. Hydrocarbon production in the eastern part of the play is mostly “dry gas,” consisting of essentially pure methane. Production of natural gas liquids (condensate) increases toward the west, which is the area currently, being targeted by developers. Two Marcellus Shale cores from West Virginia were analyzed to compare the isotopic and geochemical characteristics of a liquids-prone well (WV-7) in Wetzel County with a gas-prone well (WV-6) in Monongalia County. The contrasts between the cores indicate that the conditions of the Marcellus Shale deposition were different between the two sites. The dominant organic matter preserved in each core is isotopically different; δ13Corg values are lighter on average in WV-6 compared with WV-7. A possible explanation is that a larger fraction of terrestrial organic matter was preserved in the WV-6 core, whereas WV-7 may contain a greater percentage of marine organic matter. Clastic-influx proxies (e.g. Ti/Al, Ca/Al and Mg/Al) also suggest that the WV-6 core site received a higher siliciclastic input compared to WV-7, consistent with a more proximal location to dry land and the delivery of greater amounts of terrestrial organic matter. Depleted δ13Ccarb values, low concentrations of redox sensitive elements (e.g. V, Cr, Ni and U), and high variability δ15N values in the WV-6 core all suggest the presence of higher dissolved oxygen concentration and short term shifts in an oxic/anoxic boundary near the sediment–water interface during deposition. These lines of evidence indicate that the depositional conditions were favorable for the accumulation of predominantly gas-prone Type III kerogen in the Marcellus Shale at the WV-6 site. In contrast, the Marcellus Shale at the WV-7 site was deposited in a more distal area that received a low terrestrial sediment supply, organic matter primarily derived from marine algae, and bottom water conditions that were dominantly anoxic. Such conditions were favorable for the accumulation of Type II kerogen that has a greater capacity to generate liquid hydrocarbons. Differences between the liquids-prone and gas-prone parts of the Marcellus Shale play have been largely ascribed to depth-of-burial and thermal maturation history; this study indicates that depositional environment and sedimentary facies may have played significant roles as well.

Distribution and sources of sedimentary organic matter in a tropical estuary, south west coast of India (Cochin estuary): A baseline study

Surface sediments samples were collected from 9 stations of the Cochin estuary during the monsoon, post-monsoon and pre-monsoon seasons and were analyzed for grain size, total organic carbon (OC), total nitrogen (TN) and stable isotopic ratios of carbon (δ13C) and nitrogen (δ15N) to identify major sources of organic matter in surface sediments. Sediment grain size is found to be the key factor influencing the organic matter accumulation in surface sediments. The δ13C values ranges from −27.5‰ to −21.7‰ in surface sediments with a gradual increase from inner part of the estuary to the seaward side that suggest an increasing contribution of marine autogenous organic matter towards the seaward side. The δ15N value varies between 3.1‰ and 6.7‰ and it exhibits complex spatial and seasonal distributions in the study area. It is found that the dynamic cycling of nitrogen through various biogeochemical and organic matter degradation processes modifies the OC/TN ratios and δ15N to a considerable degree. The fraction of terrestrial organic matter in the total organic matter pool ranges from 13% to 74% in the surface sediments as estimated by δ13C based two end member mixing model.

Productivity and sedimentary δ15N variability for the last 17,000 years along the northern Gulf of Alaska continental slope

Biogenic opal, organic carbon, organic matter stable isotope, and trace metal data from a well‐dated, high‐resolution jumbo piston core (EW0408–85JC; 59° 33.3′N, 144° 9.21′W, 682 m water depth) recovered from the northern Gulf of Alaska continental slope reveal changes in productivity and nutrient utilization over the last 17,000 years. Maximum values of opal concentration (∼10%) occur during the deglacial Bølling‐Allerød (B‐A) interval and earliest Holocene (11.2 to 10.8 cal ka BP), moderate values (∼6%) occur during the Younger Dryas (13.0 to 11.2 cal ka BP) and Holocene, and minimum values (∼3.5%) occur during the Late Glacial Interval (LGI). When converted to opal mass accumulation rates, the highest values (∼5000 g cm−2 kyr−1) occur during the LGI prior to 16.7 cal ka BP, which points to a strong influence by LGI glacimarine sedimentation regimes. Similar patterns are also observed in total organic carbon and cadmium paleoproductivity proxies. Mid‐Holocene peaks in the terrestrial organic matter fraction at 5.5, 4.7, 3.5, and 1.2 cal ka BP indicate periods of enhanced delivery of glaciomarine sediments by the Alaska Coastal Current. The B‐A and earliest Holocene intervals are laminated, and enrichments of redox‐sensitive elements suggest dysoxic‐to‐anoxic conditions in the water column. The laminations are also associated with mildly enriched sedimentary δ15N ratios, indicating a link between productivity, nitrogen cycle dynamics, and sedimentary anoxia. After applying a correction for terrestrial δ15N contributions based on end‐member mixing models of terrestrial and marine organic matter, the resulting B‐A marine δ15N (6.3 ± 0.4 ‰) ratios are consistent with either mild denitrification, or increased nitrate utilization. These findings can be explained by increased micronutrient (Fe) availability during episodes of rapid rising sea level that released iron from the previously subaerial coastal plain; iron input from enhanced terrestrial runoff; and/or the intermittent presence of seasonal sea ice resulting from altered ocean/atmospheric circulation during the B‐A in the Gulf of Alaska.

The sequestration of terrestrial organic carbon in Arctic Ocean sediments: A comparison of methods and implications for regional carbon budgets

A variety of approaches have previously been developed to estimate the fraction of terrestrial or marine organic carbon present in aquatic sediments. The task of quantifying each component is especially important for the Arctic due to the regions’ sensitivity to global climate change and the potential for enhanced terrestrial organic carbon inputs with continued Arctic warming to alter carbon sequestration. Yet it is unclear how each approach compares in defining organic carbon sources in sediments as well as their impact on regional or pan-Arctic carbon budgets. Here, we investigated multiple methods: (1) two end-member mixing models utilizing bulk stable carbon isotopes; (2) the relationship between long-chain n-alkanes and organic carbon (ALKOC); (3) principal components analysis (PCA) combined with scaling of a large suite of lipid biomarkers; and (4) ratios of branched and isoprenoid glycerol dialkyl glycerol tetraether lipids (the BIT index) to calculate the fraction of terrestrial organic matter components preserved in Arctic marine sediments. Estimated terrestrial organic carbon content among approaches showed considerable variation for identical sediment samples. For a majority of the samples, the BIT index resulted in the lowest estimates for terrestrial organic carbon, corroborating recent suggestions that this proxy may represent a distinct fraction of terrestrial organic matter; i.e., peat or soil organic matter, as opposed to markers such as n-alkanes or long-chain fatty acids which measure higher plant wax inputs. Because of the patchy inputs of n-alkanes to this region from coastal erosion in the western Arctic, the ALKOC approach was not as effective as when applied to river-dominated margins found in the eastern Arctic. The difficulties in constraining a marine δ 13C end-member limit the applicability of stable isotope mixing models in polar regions. Estimates of terrestrial organic carbon using the lipid-based PCA method and the bulk δ 13C mixing model approach varied drastically at each site, suggesting that organic matter fractions such as amino acids or carbohydrates may affect bulk organic matter composition in a manner that is not captured in the lipid-based analysis. Overall, terrestrial organic matter inputs to the Chukchi and western Beaufort Seas using the average of the methods at each site ranged from 11% to 44%, indicating that land-derived organic matter plays a substantial role in carbon dynamics in the western Arctic Ocean.

Distribution and sources of organic matter in surficial sediments on the shelf and slope off Tokachi, western North Pacific, inferred from C and N stable isotopes and C / N ratios

Organic carbon (C) and total nitrogen (N) contents and corresponding isotope ratios were determined in surficial sediment (0–3 cm) at 94 stations ranging from 21 to 1995 m water depth off Tokachi, Hokkaido, Japan, to elucidate the distribution and source of sedimentary organic matter. Suspended particulate organic matter (POM) in the seawater and suspended POM and sediment in the Tokachi River were also examined. δ 13C, δ 15N and C / N ratios of the samples in the Tokachi River suggest that the spring snowmelt is an important process for the transport of terrestrial organic matter to the coastal waters. δ 13C values of suspended POM in the surface seawater were higher in May and November than in August, while δ 15N values of the POM were higher in May and August than in November. These changes are attributed to seasonal changes in phytoplankton growth rate and nitrate availability. δ 13C and δ 15N values in the sediments off Tokachi were lowest near the Tokachi River mouth, and increased offshore to constant values that persisted from 134 to 1995 m water depth. The spatial variation in C / N ratios in the sediment mirrored those of δ 13C and δ 15N. Comparison of δ 13C, δ 15N and C / N ratios in the sediments off Tokachi with those in the Tokachi River and seawater indicates that about half of the organic matter in the sediment was of terrestrial origin near the Tokachi River mouth, and the sedimentary organic matter from 134 to 1995 m water depth was of marine origin. The organic C content in the sediment was high near the Tokachi River mouth, and also around 1000 m water depth. The C content was significantly correlated with silt plus clay content, with different regression lines for those stations shallower and deeper than 134 m, owing to several stations of higher C content with the elevated C / N ratio on the inner shelf. These results suggest that transport and deposition of organic-rich fine sediment particles by hydrodynamic processes were major factors controlling C content off Tokachi. In addition, the supply of a fraction of terrestrial organic matter with high C / N probably also affected C content on the inner shelf.

Influence of water column anoxia and sediment supply on the burial and preservation of organic carbon in marine shales

Previous work has suggested that the laminated, organic-rich and bioturbated, organic-poor shales of the Camp Run Member of the Late Devonian-Early Mississippian New Albany Shale formed under anoxic and oxygenated bottomwater conditions, respectively, and that the interbedding of the two faciès was due to the vertical oscillation of a water-column anoxic/oxic boundary where it impinged on the basin margin. We have extended this analysis by examining the chemical and mineralogical differences between the two shale facies in a single borehole core, by seeking evidence for deposition of the laminated shales under bottom-water oxia or anoxia, and by determining whether the laminated shales formed when the carbon supply to the sea floor was higher. The results of this study show that the laminated and bioturbated shales are mineralogically and chemically distinct; relative to Al, an index of the aluminosilicate content, Si, Ti, Fe, P, Na, Ba, Co, Cr, Cu, Mo, Ni, V, Zn, and Zr are all higher, whereas Mn, Ca, Mg, and Sr are lower in the laminated compared with the bioturbated shales. The differences are due to a higher quartz, feldspar, titanite/ilmenite, and zircon content in the laminated shales, probably indicating a coarser grain-size, and the greater abundance of manganoan calcite in the bioturbated shales. Dissolved oxygen was present in bottom waters during the deposition of some of the laminated shale intervals because of the presence of manganoan calcite, a phase that can only form in sediments with an oxic surface. In addition, the organic matter preserved in the two shale types is isotopically different; δ 13C organic values are 1.9z.permil; lighter on average in the laminated compared with bioturbated intervals, possibly indicating a larger fraction of terrestrial organic matter in the latter. δ 15N values are 1.9z.permil; lighter on average in laminated compared with bioturbated intervals, possibly indicating a larger fraction of terrestrial organic matter in the latter. δ 15N values are 1.9z.permil; lighter on average in laminated compared with bioturbated intervals, suggesting that nutrient drawdown was less during the deposition of the organic-rich, laminated shales. The chemical, mineralogical, and isotopic contrasts between the two shale facies of the Camp Run Member indicate that the conditions of sedimentation were different during their deposition. The difference was possibly related to variations in sea level, which would have caused the Camp Run shoreline to move closer to and farther from the core site, causing, in turn, the deposition of coarser and finer grained sediments that contained different mixtures of marine and terrestrial organic matter. Bottomwater conditions were anoxic during deposition of most laminated intervals. Bottom-water anoxia or dysoxia led to decreased burial and preservation of the essential nutrient phosphorus in the laminated, organic-rich shales relative to the rocks deposited beneath better oxygenated bottomwaters. Increased availability of phosphorus in the water column on long timescales leads to increased productivity and a higher settling flux of organic matter, causing bottom-water oxygen levels to fall. This is consistent with the nitrogen isotope evidence suggesting that production was probably higher during the deposition of the organic-rich shales. Thus, production variations coupled with enhanced sedimentary regeneration of phosphorus from sediments related to low oxygen bottom-water concentrations provide a general mechanism for the formation of the alternating facies of this member of the New Albany Shale.