Terrigenous Organic Carbon Research Articles

Distribution of terrigenous lipids in marine sediments off northeastern Taiwan

Surface sediments on the continental margin off northeastern Taiwan have been analyzed for terrigenous lipids including n-alkanes, n-fatty alcohols, and sterols. Marine input to the sediments is particularly low based on the average n-C17/n-C29 alkane and n-C16/n-C28 fatty alcohol ratios, 0.15±0.13 and 0.13±0.06, respectively; this may be due to the fact that marine lipids are more prone to degradation than terrestrial ones. The study area has the highest plant wax n-alkane contribution (average carbon preference index 3.9±1.2) among the coastal marine areas surrounding Taiwan; lateral particle transport from the southern East China Sea shelf and river runoff from the east Taiwan coast are considered to be the major contributors. The distributions of plant wax n-alkane and n-alkanol concentrations normalized to total organic carbon (TOC) in the study area generally show maximum values on the upper slope of the southernmost Okinawa Trough, but not for phytosterols. Linear regression of TOC versus plant wax n-alkane concentrations show a weak relationship (r=0.64, p=0.001), and an even weaker relationship (r=0.42, p=0.05) between TOC and plant wax n-fatty alcohol concentrations is found. This could be attributed to several factors: (1) a complex input (not a point source) of terrigenous organic matter to the study area, (2) TOC also including marine organic matter, (3) temporal variations in river flow due to flooding, and (4) different rates of degradation for TOC and individual biomarkers. However, in spite of those factors, TOC and phytosterol concentrations are positively linearly correlated (r=0.85, p<0.001), implying that the dilution of phytosterols in terrigenous organic carbon with marine organic carbon with or without the phytosterols follows a nearly constant ratio, which is remarkable. In addition, the predominant source of diploptene in the sediments does not appear to be of higher plant origin.

Assessment of terrigenous organic carbon input to the total organic carbon in sediments from Scottish transitional waters (sea lochs): methodology and preliminary results

Abstract. This paper addresses the assessment of terrestrially derived organic carbon in sediments from two Scottish sea lochs. The results illustrate a smooth decrease in area-specific sediment oxygen uptake rates along a transect of six stations from the head of Loch Creran to the sea, from 18.7 mmol O2m-2d-1 to 6.6 mmol O2m-2d-1. Measurement of the losses on ignition at two temperatures (250°C and 500°C) of the sediment fraction from 1–2 cm depth at the same stations enabled the proportion of weight loss that occurred over the high temperature range to be calculated. These show a smooth increase from 0.33 to 0.62. These observations indicate that (a) the amount of easily biodegradable organic material in the sediment decreases by two-thirds along the transect and (b) the proportion of refractory organic material in the sediment increases along the same transect. This suggests strongly that terrigenous organic material, brought down by the River Creran is a very important fuel for sediment diagenetic processes in this system. Preliminary analyses of the lignin composition of the same sediments indicate the predominance of non-woody gymnosperm tissue. Lignin is used as a proxy for terrigenous allochthonous material. Comparative data for Loch Etive are also presented. Keywords: sedimentary organic carbon, Loch Creran, Loch Etive, oxygen uptake, lignin analysis

Accumulation of particulate organic carbon at the Eurasian continental margin during late Quaternary times: controlling mechanisms and paleoenvironmental significance

Data on the amount and composition of organic carbon were determined in sediment cores from the Kara and Laptev Sea continental margin, representing oxygen isotope stages 1–6. The characterization of organic matter is based on hydrogen index (HI) values, n-alkanes and maceral composition, indicating the predominance of terrigenous organic matter through space and time. The variations in the amount and composition of organic carbon are mainly influenced by changes in fluvial sediment supply, Atlantic water inflow, and continental ice sheets. During oxygen isotope stage (OIS) 6, high organic carbon contents in sediments from the Laptev Sea and western East Siberian Sea continental margin were probably caused by the increased glacial erosion and further transport in the eastward-flowing boundary current along the continental margin. During OIS 5 and early OIS 3, some increased amounts of marine organic matter were preserved in sediments east of the Lomonosov Ridge, suggesting an influence of nutrient-rich Pacific waters. During OIS 2, terrigenous organic carbon supply was increased along the Barents and western Kara Sea continental margin caused by extended continental ice sheets in the Barents Sea (Svalbard to Franz Josef Land) area and increased glacial erosion. Along the Laptev Sea continental margin, on the other hand, the supply of terrigenous (organic) matter was significantly reduced due to the lack of major ice sheets and reduced river discharge. Towards the Holocene, the amount of total organic carbon (TOC) increased along the Kara and Laptev Sea continental margin, reaching average values of up to 0.5 g C cm −2 ky −1. Between about 8 and 10 ka (9 and 11 Cal ka), i.e., during times when the inner shallow Kara and Laptev seas became largely flooded for the first time after the Last Glacial Maximum, maximum supply of terrigenous organic carbon occurred, which is related to an increase in coastal erosion and Siberian river discharge. During the last 8000 years, the increased amount of marine organic carbon preserved in the sediments from the Kara and Laptev Sea continental margin is interpreted as a result of the intensification of Atlantic water inflow along the Eurasian continental margin.

Biomarkers as organic-carbon-source and environmental indicators in the Late Quaternary Arctic Ocean: problems and perspectives

In order to examine the variation in marine and terrigenous sources of the organic matter during the last 15,000 Cal. yr BP, hydrogen index values, C/N ratios, and specific biomolecules (short- and long-chain n -alkanes, short-chain fatty acids, sterols, alkenones , and pigments) were determined in three sediment cores from the Laptev Sea continental margin . The results show that the interpretation of the biomarker data is much more complicated and less definitive in comparison to similar data sets from low-latitude open-ocean environments. This is mainly caused by the complexity of the Arctic Ocean system, which is characterized by a high seasonality of sea-ice cover and primary productivity, sea-ice sediment transport and a high fluvial supply of freshwater (aquatic) organic matter. A combination of organic geochemical, organic petrographic and micropaleontological data may yield to a more precise identification of organic-carbon sources for these complex systems. On the Laptev Sea shelf (cores KD9502-14 and PS2725-5), terrigenous organic-carbon input controlled by river discharge seems to be predominant during the last about 10,000 Cal. yr BP. Maximum supply of terrigenous organic carbon was reached in the Early Holocene (i.e., about 9000–10,000 Cal. yr BP). Further offshore at the upper continental slope (core PS2458-4), a major change in organic-composition occurred near 10,000 Cal. yr BP. During this period hydrogen indices as well as the amounts of short-chain n -alkanes, short-chain fatty acids, dinosterol and brassicasterol significantly increased, indicating increased relative proportions of marine organic matter preserved in the later time interval.

Biogenic sediments in the West Caroline Basin, the western equatorial Pacific during the last 330,000 years

Primary productivity as well as new production have played an important role in the global carbon cycle during the late Quaternary. The mean organic carbon/nitrogen atomic ratio was 8.3 from the C4402 sediments in the West Caroline Basin, which indicates a limited contribution of terrigenous organic carbon to the organic matter in the sediments. The fluctuation of carbonate preservation shows that the accumulation rate of organic carbon has reflected primary productivity. The results from sediment trap experiments which were carried out at Site C4402 indicate that plankton bloom was characterized by a large influx of siliceous plankton, when organic carbon was effectively removed. Palaeoproductivity estimates calculated by using the formula of Sarnthein et al. (1988)would be plausible because the fluctuation pattern of such estimates correlates closely with changes in the accumulation rate of biogenic opal in the sediments. The primary productivity in the West Caroline Basin increased during Stage 2, Stage 3, late Stage 6, Stage 6/7 boundary and Stage 8. At a glance, the results are similar to other studies that have reported higher glacial productivity in low-latitude regions. However, some leads and lags are found between the West Caroline Basin and other areas of the equatorial Pacific and Atlantic. The spectral analysis of MAR Organic indicates that the western equatorial Pacific reaches a maximum in MAR Organic approximately 7000 years before the maxima in the eastern Pacific. A regional variation of the response of MAR Organic to the orbital forcing climatic condition suggests that the efficiency of the organic carbon pump may vary with longitude in the equatorial Pacific.

Temporal and Spatial Dynamics of Particulate Organic Carbon in the Lake Pontchartrain Estuary, Southeast Louisiana, U.S.A

The spatial and temporal distribution of particulate organic carbon (POC) was examined in the Lake Pontchartrain estuary, along with changes in freshwater and nutrient inputs. Increased turbidity due to increases in suspended particulate matter (SPM) concentrations during high freshwater discharge resulted in low inputs from autochthonous POC. For example, annual averages of chlorophylla(2·3±0·9μg l−1) and POC (0·9±0·6mg l−1) were found to be considerably lower than in other shallow turbid systems. A low N/P ratio was maintained throughout the study (from 2·3 to 8·3), primarily due to low inputs of dissolved inorganic nitrogen (DIN). Thus, phytoplankton communities in the Lake Pontchartrain estuary were primarily limited by light and nitrogen. Stations in the upper estuary were generally more light-limited as opposed to nitrogen-limited stations in the lower estuary. Based on chlorophylla/carotenoid ratios, the most dominant phytoplankton classes were cyanobacteria, cryptophytes, chrysophytes and chlorophytes—with episodic cyanobacterial blooms occurring in the summer months. High carbon/chlorophyllaratios suggested that much of the POC contained low concentrations of chlorophyllathat had degraded during its transport from adjacent wetlands. High C/N ratios (7·4 to 30·6) further suggested that allochtonous inputs of organic carbon from terrigenous sources were likely to be important. Lignin-phenol concentrations in sediments from the upper region of the estuary indicated that the Lake Pontchartrain estuary had received higher inputs of terrigenous organic carbon compared to other estuaries in the Gulf region.

Sources and contribution of terrigenous organic carbon to surface sediments in the Gulf of Mexico

The sources and burial processes of organic matter in marine sediments are not well understood, yet they are important if we are to have a better understanding of the global carbon cycle1. In particular, the nature and fraction of the terrestrial organic carbon preserved in marine sediments is poorly constrained. Here we use the chemical and stable carbon isotope signatures of oxidation products from a macromolecular component (lignin)2 of the terrigenous organic matter preserved in offshore surface sediments in the Gulf of Mexico to complement similar data from an existing onshore transect3 in this region. The complete onshore–offshore data set, along with radiocarbon dates of the bulk organic material at the same sites, allows the differentiation of material originating from plants that photosynthesize using the C4 mechanism from those that undergo C3 photosynthesis. We conclude that the offshore lignins derive from erosion of the extensive grassland (C4) soils of the Mississippi River drainage basin, and that the nearshore lignins originate largely from C3 plant detritus from coastal forests and swamps. This distribution is probably due to the hydrodynamic sorting of the different source materials4 during their seaward transport. These results suggest that previous studies3,5 have significantly underestimated the terrigenous fraction of organic matter in offshore sediments by not recognizing the contribution of C4 vegetation to the carbon-isotope composition. Such an underestimate may force revisions in the assessment of past marine primary productivity and associated organic carbon fluxes6, and of organic matter preservation/remineralization7 and nutrient cycling8 in marine sediments.

Marine and terrigenous origin of organic matter in modern sediments of the equatorial East Atlantic: the σ 13C and molecular record

The contributions of marine and terrigenous organic carbon in modern organic sediments from the equatorial East Atlantic was quantified, based on the stable carbon isotope composition standardized for sea surface temperature and water depth. In our binary σ 13C mixing model, the marine end member −26 ∞ is redefined at 23°C and 0 m water depth and the terrigenous end member, independent of SST and water depth, at − 26 ∞. Terrigenous carbon fractions account for more than 60% of total organic carbon (TOC) on the shelf off East Liberia and the Ivory Coast and off Gabon. On the upper slope the land-derived fraction decreases in general to less than 20% of the high TOC concentrations, which can reach 3.5 wt%. The distribution of plant wax n-alkanes (C 27, C 29, C 31) and C:N ratios do not parallel those of land-derived organic carbon, but may be controlled largely by carbon degradation and aeolian/aquatic sorting. The ratio of n-alkanols vs n-alkanes (HPA index) varies with water depth in a nonlinear mode. Since both groups of compounds stem from the same source, plant waxes, it is proposed that the HPA index is controlled mainly by degradation and to a lesser extent by sorting prior to degradation. Enhanced n-alkane concentrations (up to 580 μg/gTOC) in the Gambia Basin and in the central Guinea Basin clearly reflect the influx of aeolian organic matter from northeasterly trades near and below the Inter Tropical Convergence Zone (ITCZ). The particle flux from marine plankton is traced by high concentrations of both dinosterol, long-chain unsaturated methyl and ethyl ketones (C 37-C 39), and alkandiols in marine organic matter. Whereas alkenones, synthetized by prymnesiophyte algae generally, reflect upwelling-related productivity off-shore, dinosterol, synthetized by dinoflagellates, is enriched in near-shore areas of high marine productivity linked to fluvial fertilization. All marine biomarker groups show a surprisingly low concentration below the equatorial high productivity belt.

The relationship between sedimentary organic carbon isotopic composition and organic biomarker compound concentration

A study of organic biomarker compounds which could serve as tracers of terrigenous and marine sedimentary organic matter sources was performed on samples from a 208.7 m hydraulic piston core hole (DSDP Hole 619) from the hemipelagic Pigmy Basin in the northern Gulf of Mexico. Organic carbon-normalized concentrations of total long chain (C 37–C 39) alkenones and some individual C 27–C 29 desmethyl sterols were determined to be useful proportional indicators (tracers) of preserved marine and terrigenous organic carbon, respectively. The alkenones, whose only known source is marine phytoplankton of the class Prymnesiophyceae, generally occurred in higher concentrations in interglacial isotope stages 1 and 5a-b than in the intervening glacial stages. Sterols (C 27-C 29), apparently of a dominantly terrigenous origin, occurred in lower concentrations during interglacial stages than in glacial stages. Tracers of both terrigenous and marine organic matter appear to be affected by the differential diagenetic alteration of the biomarker/C org ratios, as indicated by a simple, first-order kinetic model. The lack of any desmethylor 4a-methylsterol which is linearly related to the proportion of marine sedimentary organic matter (as scaled by δ 13C org) indicates that either 1. (1) sedimentary diagenesis has obscured the biomarker/C org vs. δ 13C org record 2. (2) phytoplanktonic assemblage changes caused variations in the biomarker/C org ratio of the primary input. Preferential preservation of terrigenous sterols may result in a biased sedimentary record of sterol input which could be misinterpreted as indicating solely terrigenous sterol sources. A simple model which characterizes the effects of sedimentary diagenesis on the relationship between C org-normalized biomarker ratios and δ 13C org demonstrates the potential problems of long-term, differential-diagenetic skewing on those tracer records.

The sources and deposition of organic matter in the Late Quaternary Pigmy Basin, Gulf of Mexico

The concentration and carbon isotopic composition (δ 13C) of sedimentary organic carbon (C org), N/C ratios, and terrigenous and marine δ 13C org endmembers form a basis from which to address problems of Late Quaternary glacial-interglacial climatic variability in a 208.7 m hydraulic piston core (DSDP 619) from the Pigmy Basin in the northern Gulf of Mexico. While interpretations of sedimentary δ 13C org time series records are often not unique, paired analyses of δ 13C org and N/C are consistent with the hypothesis that the C org in the Pigmy Basin is a climatically determined mixture of C 3-photosynthetic terrigenous and marine organic matter, confirming the earlier δ 13C org model of Sackett (1964). A high resolution (∼ 1.4–2.9 Ka/sample) δ 13C org record shows that sedimentary organic carbon in interglacial oxygen isotope (sub)stages 1 and 5a-b are enriched in 13C (average ±1 σ values are −24.2 ± 1.2%o and −22.9 ± 0.7%o relative to PDB, respectively) while glacial isotope stage values 2 are relatively depleted (-25.6 ± 0.5%). Concentrations of terrigenous and marine sedimentary organic carbon are calculated for the first time using δ 13C org and C org measurements, and empirically determined terrigenous and marine δ 13C org endmembers. The net accumulation rate of terrigenous organic carbon is 4.3 ± 2.6 times higher in isotope stages 2–4 than in (sub)stages 1 and 5a-b, recording higher erosion rates of terrigenous organic material in glacial times. The concentration and net accumulation rates of marine and terrigenous C org suggest that the nutrient-bearing plume of the Mississippi River may have advanced and retreated across the Pigmy Basin as sea level fell and rose in response to glacial-interglacial sea level change.

The burial of organic carbon as affected by the evolution of land plants

It is hypothesized that, over Phanerozoic time, the terrigenous organic carbon (orgC) pool became increasingly susceptible to biological decay through (a) reduction in the ratio of phenolic‐rich periderm to total biomass and (b) decline in the extent of lignification in foliage and other plant organs. Fungal evolution, meanwhile, resulted in greater abundance and higher activity levels of lignin‐degrading organisms, and faster turnover of refractory orgC. The result was reduced burial of orgC, which, in turn, checked the accumulation of O2 in the atmosphere and buffered the global redox balance against variation in biomass production by land plants. Feedback from O2 level to fungal metabolism of lignin further stabilized the system. Thus, the relatively small Paleozoic land biota could have caused much greater perturbations of redox balance than were caused by the much larger and more productive land biotas of the Tertiary.

Changes in paleoenvironments in the Atlantic Ocean during Cretaceous times: results from black shales studies

Large parts of the Cretaceous Atlantic Ocean are characterized by the wide-spread deposition of organic-carbon-rich sediments (»black shales«), with a maximum occurrence during Aptian/Albian and Cenomanian/Turonian times. In order to explain the origin of these black shales, changes in the amount and type of organic matter in relation to changes in paleogeography, paleoceanic circulation, and paleoclimate have been investigated in detail. The study is mainly based on calculation of accumulation rates of marine and terrigenous organic carbon, organic carbon/sedimentation rate relationships, and estimates of paleoproductivity.

Provenance, concentrations and nature of sedimentary organic nitrogen in the gulf of Maine

Factors controlling the provenance, concentrations, and nature of sedimentary organic matter (SOM), particularly the nitrogenous fraction, were examined for sites throughout the Gulf of Maine and two of its estuaries. Stable nitrogen isotope data corroborate previously reported Br: C ratios in indicating that macroalgae are important contributors to organic nitrogen pools in estuarine sediments. Significant contributions of terrigenous organic carbon in both the estuaries and the deep basins of the open Gulf are not accompanied by significant contributions of terrigenous organic nitrogen. Variations in concentrations of organic carbon, nitrogen, chlorophyll, and enzymatically hydrolyzable protein throughout the Gulf and its estuaries can be explained largely by variations in the delivery rate of organic matter to the benthos and the specific surface area of minerals in the sediments. Chlorophyll and protein concentrations represent relatively fresh organic matter and exhibit greater sensitivity to organic delivery variations than do total organic carbon or nitrogen. A strong, linear relationship between organic carbon and surface area is consistent with monolayer adsorption of SOM on mineral surfaces, which appears to inhibit degradation of the SOM. Amino acid compositions are remarkably constant throughout the Gulf, with large differences seen only with elevated tyrosine in estuaries and elevated basic amino acids and methionine in the open Gulf. Leucine and glutamic acid correlate with chlorophyll concentrations in open Gulf sites. Enzymatically hydrolyzable protein comprises small to major fractions of total hydrolyzable amino acids. Amino acid-nitrogen: total nitrogen correlates inversely with total nitrogen.

Accumulation of organic-carbon-rich sediments in the Late Jurassic and Cretaceous Atlantic Ocean — A synthesis

Organic-carbon-rich sediments (“black shales”) were deposited at some 40 Deep Sea Drilling Project (DSDP) sites in the Atlantic Ocean during Late Jurassic and Cretaceous times. In order to reconstruct the depositional environments of black shale formation, we have studied in detail changes in the occurrence of black shales as well as the type and amount of organic matter in relation to changes in paleogeography, paleooceanic circulation and paleoclimate. We have calculated accumulation rates of marine and terrigenous organic carbon and combined these with organic carbon content—sedimentation rate relationships to obtain information about paleoenvironments. Main factors causing the preservation and accumulation of organic carbon are: (1) the supply of terrestrial organic matter; (2) the supply of marine organic matter; (3) the oxygenation of the deep water; and (4) the bulk sedimentation rate. Changes in the importance of these different factors and changes in the interrelationships among them determine the complex system of black shale formation and their cyclic variations. Increased supply of terrestrial organic matter due to increased river discharge and humid climate in Northeast America, Southwest Europe and South Africa were important in the Hauterivian—Albian Northwest and northern Northeast Atlantic and in the Late Jurassic—Aptian South Atlantic. In the latter, anoxic deep-water conditions due to restricted circulation in the nascent largely isolated basin as well as higher oceanic productivity, may have reinforced the high preservation rates of marine organic matter during that time. Off Northwest Africa, higher flux rates of marine organic matter were recorded in areas of higher productivity which may have been caused by fluvial nutrient supply during the Hauterivian and by intensified upwelling during Albian/Turonian times. In general, the depositional environment was oxygenated and the content of marine organic matter was a function of the sedimentation rate. Furthermore, at some places redeposition by turbidity currents or slumps may have been important for the accumulation of organic matter in the deep-sea basins. The only episode of widespread anoxia in the western and eastern basins of the North Atlantic may have occurred in the Cenomanian—Turonian when high amounts of marine organic matter were also preserved in areas of low productivity and low sedimentation rates.

Widespread, Synchronously Deposited, Burrow-Mottled Limestone Beds in Greenhorn Limestone (Upper Cretaceous) of Kansas and Southeastern Colorado

In central Kansas the Hartland Shale and Jetmore Chalk Members of the Greenhorn Limestone contain 16 burrow-mottled, ledge-forming, chalky limestone beds that can be traced across the entire outcrop. The same beds are recognized in equivalent strata of the Bridge Creek Limestone Member of westernmost Kansas, and all but one or two can be recognized in a Bridge Creek exposure in southwestern Pueblo County, Colorado. For a distance of nearly 450 mi some of these beds lie on, below, or close to equally persistent bentonite seams. The parallelism of bentonite seams and adjacent or nearly adjacent beds of limestone, and the uniform relative spacing of all the limestone beds prove that some of the latter are time parallel and suggest most convincingly that each of these widespr ad beds is time parallel. In contrast to intervening beds of laminated shaly chalk, the limestone beds lack internal stratification because of activity of a highly mobile infauna. The shaly chalk beds contain higher percentages of terrigenous detritus, organic carbon, and pyrite than the limestone beds, and are interpreted as reflecting greater rates of terrigenous detrital influx and an interstitial reducing environment that was inimical to development of a burrowing infauna. Smaller organic carbon and pyrite content of the limestones is believed to reflect a lower clay content of original sediments, and at least a partly oxidizing interstitial environment, as suggested by the high degree of bioturbation. Despite substrate differences, the aqueous environment directly above the sediment-water interface did not change significantly as sediment type alternated from detritus-rich to detritus-poor, because the shelly epifauna is essentially the same in adjacent be s of shaly chalk and chalky limestone. The widespread, apparently time-parallel limestone beds are believed to be the result of regionally manifested changes in volume of terrigenous detrital influx, coming principally from the west, that were superimposed on a more or less continuous accumulation of carbonate sediments. The noncrushed condition of macrofossils and fecal pellets suggests that limestone beds suffered early lithification; this process was probably influenced by initially greater purity of limestone-forming muds, slower sedimentation, and interstitial circulation resulting from bioturbation.