High Organic Carbon Accumulation Rates Research Articles

Evolution of nitrogen cycling and primary productivity in the tropics during the Late Ordovician mass extinction

The late Ordovician is characterized by significant environmental changes in global climate, oceanic redox conditions, as well as a major disaster for marine organisms—the Late Ordovician mass extinction (LOME) (ca. 445 Ma). During this event, planktonic and benthic faunas as well as microbial communities were substantially influenced, and changes in the microbial ecosystems could provide useful clues for the dynamics of marine nutrients and primary production. Here, we utilize new nitrogen isotope (δ15Nbulk) and abundance data, as well as organic carbon accumulation rates (OCARs) from an Ordovician-Silurian (OS) deep marine section, Tianba, deposited under consistently anoxic water columns in the Yangtze Shelf Sea (South China), to unravel the evolution of nitrogen cycling and primary productivity. The OCARs for Tianba section were the lowest during the sea level lowstands and Hirnantian glaciation maximum but were high at the eve and aftermath of the Hirnantian glaciation maximum, consistent with the OCARs dynamics for another anoxic deep-water section located in Laurentia Block in the tropical ocean. Low OCARs mainly demonstrate that the marine primary productivity was low, with N2-fixers populating the ecosystems during the Hirnantian glaciation maximum as evidenced by the spatial distribution of δ15Nbulk values in the paleotropical waters. However, the high OCARs mainly imply high marine primary productivity right before and after the Hirnantian glaciation maximum in the paleotropics, which were likely fueled by fixed nitrogen sourced from upwellings. The spatial heterogeneity in δ15Nbulk values for the top of Wufeng Formation and coeval strata in the paleotropics suggests that fixed nitrogen species (including NH4+ and NO3−) in surface waters may vary in space. The cooccurrence of zooplankton extinction with spatial variation of δ15Nbulk values suggests that the dynamics of surface waters redox conditions could have played a significant role in the extinction pattern of planktonic faunas during the LOME-1.

Evidence for high organic carbon export to the early Cambrian seafloor

Oxygenation of the early Cambrian ocean is commonly ascribed to high organic export to the sediment due to the rise of algae and filter-feeding animals, but direct evidence of elevated export fluxes has been lacking to date. Here, we report an integrated proxy dataset (U-Mo isotopes, Fe speciation, and major and trace elements) for lower Cambrian black shales at Yuanjia on the Yangtze Platform (South China). These shales are characterized by high iron speciation ratios and Mo and U enrichments, indicating persistently euxinic water conditions during their deposition, but a broad range of δ98Mo values (−0.10‰ to +1.94‰) implies strongly variable watermass sulfidity. High and variable nutrient element (Cu, Zn, Ni, Cd) enrichments at Yuanjia are consistent with elevated and variable organic productivity and export fluxes from the euphotic zone to the early Cambrian seafloor. The Yuanjia shales also exhibit super-heavy δ238U values (to +0.84‰) that cannot be explained by U reduction in sediment porewaters. We hypothesize that these values record a U reduction process within the water column, where U isotope fractionation can be much larger than in sediment porewaters. Such processes may have operated within a benthic organic flocculent layer at the sediment-water interface, a feature that is characteristic of high productivity systems with anoxic bottom waters. Furthermore, the coupling between δ238U, nutrient elements, and δ98Mo suggests that elevated productivity levels were driven likely by episodic upwelling in early Cambrian oceans, supplying abundant nutrients for productivity and influencing H2S concentrations in the water column. These observations, along with high organic carbon accumulation rates (5600 mg cm−2 kyr−1), are direct evidence of high organic export fluxes to the early Cambrian seafloor, thus providing key support for the hypothesis that enhanced organic export and burial triggered a major atmospheric-oceanic oxygenation event that stimulated the subsequent explosion of Cambrian life.

Origin and fate of sedimentary organic matter in the northern Bay of Bengal during the last 18 ka

The Northern Bay of Bengal (NBoB) is a globally important region for deep-sea organic matter (OM) deposition due to massive fluvial discharge from the Ganges-Brahmaputra-Meghna (G-B-M) rivers and moderate to high surface productivity. Previous studies have focused on carbon burial in turbiditic sediments of the Bengal Fan. However, little is known about the storage of carbon in pelagic and hemipelagic sediments of the Bay of Bengal over millennial time scales. This study presents a comprehensive history of OM origin and fate as well as a quantification of carbon sediment storage in the Eastern Bengal Slope (EBS) during the last 18ka. Bulk organic proxies (TOC, TIC, TN, δ13CTOC, δ15NTN) and content and composition of total hydrolysable amino acids (THAA) in a sediment core (SO188-342KL) from the EBS were analyzed. Three periods of high OM accumulation were identified: the Late Glacial (LG), the Bölling/Alleröd (B/A), and the Early Holocene Climatic Optimum (EHCO). Lower eustatic sea level before 15ka BP allowed a closer connection between the EBS and the fluvial debouch, favoring high terrestrial OM input to the core site. This connection was progressively lost between 15 and 7ka BP as sea level rose to its present height and terrestrial OM input decreased considerably. Export and preservation of marine OM was stimulated during periods of summer monsoon intensification (B/A and EHCO) as a consequence of higher surface productivity enhanced by cyclonic-eddy nutrient pumping and fluvial nutrient delivery into the photic zone. Changes in the THAA composition indicate that the marine plankton community structure shifted from calcareous-dominated before 13ka BP to siliceous-dominated afterwards. They also indicate that the relative proportion of marine versus terrestrial OM deposited at site 342KL was primarily driven by relative sea level and enlarged during the Holocene. The ballasting effect of lithogenic particles during periods of high coastal proximity and/or enhanced fluvial discharge promoted the export and preservation of OM. The high organic carbon accumulation rates in the EBS during the LG (18–17ka BP) were 5-fold higher than at present and comparable to those of glacial upwelling areas. Despite the differences in sediment and OM transport and storage among the Western and Eastern sectors of the NBoB, this region remains important for global carbon sequestration during sea level low-stands. In addition, the summer monsoon was a key promotor of terrestrial and marine OM export to the deep-ocean, highlighting its relevance as regulator of the global carbon budget.

Seasonal Dynamics of Soil Labile Organic Carbon and Enzyme Activities in Relation to Vegetation Types in Hangzhou Bay Tidal Flat Wetland.

Soil labile organic carbon and soil enzymes play important roles in the carbon cycle of coastal wetlands that have high organic carbon accumulation rates. Soils under three vegetations (Phragmites australis, Spartina alterniflora, and Scirpusm mariqueter) as well as bare mudflat in Hangzhou Bay wetland of China were collected seasonally. Seasonal dynamics and correlations of soil labile organic carbon fractions and soil enzyme activities were analyzed. The results showed that there were significant differences among vegetation types in the contents of soil organic carbon (SOC) and dissolved organic carbon (DOC), excepting for that of microbial biomass carbon (MBC). The P. australis soil was with the highest content of both SOC (7.86 g kg-1) and DOC (306 mg kg-1), while the S. mariqueter soil was with the lowest content of SOC (6.83 g kg-1), and the bare mudflat was with the lowest content of DOC (270 mg kg-1). Soil enzyme activities were significantly different among vegetation types except for urease. The P. australis had the highest annual average activity of alkaline phosphomonoesterase (21.4 mg kg-1 h-1), and the S. alterniflora had the highest annual average activities of β-glycosidase (4.10 mg kg-1 h-1) and invertase (9.81mg g-1 24h-1); however, the bare mudflat had the lowest activities of alkaline phosphomonoesterase (16.2 mg kg-1 h-1), β-glycosidase (2.87 mg kg-1 h-1), and invertase (8.02 mg g-1 24h-1). Analysis also showed that the soil labile organic carbon fractions and soil enzyme activities had distinct seasonal dynamics. In addition, the soil MBC content was significantly correlated with the activities of urease and β-glucosidase. The DOC content was significantly correlated with the activities of urease, alkaline phosphomonoesterase, and invertase. The results indicated that vegetation type is an important factor influencing the spatial-temporal variation of soil enzyme activities and labile organic carbon in coastal wetlands.

Towards an improved organic carbon budget for the western Barents Sea shelf

Abstract. There is generally a lack of knowledge on how marine organic carbon accumulation is linked to vertical export and primary productivity patterns in the Arctic Ocean. Despite the fact that annual primary production in the Arctic has increased as a consequence of shrinking sea ice, its effect on flux, preservation, and accumulation of organic carbon is still not well understood. In this study, a multi-proxy geochemical and organic-sedimentological approach is coupled with organic facies modelling, focusing on regional calculations of carbon cycling and carbon burial on the western Barents Shelf between northern Scandinavia and Svalbard. OF-Mod 3-D, an organic facies modelling software tool, is used to reconstruct and quantify the marine and terrestrial organic carbon fractions and to make inferences about marine primary productivity changes across the marginal ice zone (MIZ). By calibrating the model against an extensive set of sediment surface samples, we improve the Holocene organic carbon budget for ice-free and seasonally ice-covered areas in the western Barents Sea. The results show that higher organic carbon accumulation rates in the MIZ are best explained by enhanced surface water productivity compared to ice-free regions, implying that shrinking sea ice may reveal a significant effect on the overall organic carbon storage capacity of the western Barents Sea shelf.

Soil microbiological variability under different successional stages of the Chongming Dongtan wetland and its effect on soil organic carbon storage

Estuarine wetlands undergoing rapid succession have high organic carbon accumulation rates due to their high primary productivity. A previous study indicated that organic carbon storage in soil differed among successional stages of the Chongming Dongtan tidal wetland due to differences in soil microbial respiration (SMR) and plant biomass. To investigate the mechanisms underlying the differences in SMR, variability in microbial and physicochemical characteristics of soil from different successional stages was studied. Bacterial community composition varied among the successional stages. Some dominant and site-specific bacteria inhabiting the tidal flat may have influenced SMR and soil carbon storage capability. Uncultured members of the Cytophagales and Pseudomonas genera, which include efficient degraders of complex organic compounds, were found primarily in the high tidal flat where the highest SMR and relatively low soil organic carbon storage were observed. Soil environment factors such as particle size, salinity, and total nitrogen (TN) may have affected microbial community composition, in particular the dominant bacterial species, and hence, SMR. The relatively small particle size, low salinity, and high TN in the high tidal flat provided suitable conditions for chemoorganotrophic bacteria such as Cytophagales and Pseudomonas species, which are specialized in the degradation of organic matter.

The western North Atlantic record of MIS 13 to 10: Changes in primary productivity, organic carbon accumulation and benthic foraminiferal assemblages in sediments from the Blake Outer Ridge (ODP Site 1058)

We used multiple paleoceanographic proxies to examine sediments from the Blake Outer Ridge (ODP Site 1058, 2996 m water depth) that span the MIS 13–10 time interval. Benthic foraminiferal δ 13C, organic δ 13C, and total δ 15N values are lower in MIS 12 and 10 than in MS 11, and indicate glacial–interglacial changes in both thermohaline circulation and ocean-wide biogeochemical processes that affect deep and surface waters. Three time intervals of higher productivity, as evidenced by higher organic carbon accumulation rates (1.6–4.5 g m − 2 yr − 1 ), occur at the end of MIS 13 and during the early and late parts of MIS 11, in association with the transition from obliquity minima to obliquity maxima. Benthic faunas in these intervals show a distinct increase in opportunistic species that feed on phytodetritus ( Epistominella exigua) or are adapted to elevated but intermittent organic carbon fluxes ( Bulimina aculeata and Bulimina marginata) and that imply episodes of recurrent phytoplankton blooms at the surface and consequent export of labile organic matter to the seafloor. Late interglacial MIS 11.3 is a time of low organic carbon fluxes (< 0.5 g m − 2 yr − 1 ) and is characterized by higher abundances of epifaunal benthic foraminiferal taxa ( Cibicidoides wuellerstorfi, Gyroidina spp., Gyroidinoides spp. and Nuttallides umbonifera) that are typical of modern oligotrophic environments. In contrast, during glacial MIS 12 and 10 higher organic carbon fluxes (> 0.5 g m − 2 yr − 1 ) and benthic assemblages dominated by infaunal taxa ( Uvigerina spp . and Bulimina alazanensis) indicate sustained organic carbon delivery and diminished pore water oxygenation.

Surface water productivity and paleoceanographic implications in the Cenozoic Arctic Ocean

Study of bulk nitrogen contents and isotopic composition in Arctic Ocean sequences (Integrated Ocean Drilling Program Expedition 302) over the past 60 Ma revealed changes in the export flux and sources of sedimentary nitrogen. The paleoproductivity calculated from the fraction of organic nitrogen to total nitrogen is distinctly lower (&lt;20 g C m−2 a−1) during the ice‐covered Neogene compared to the ice‐free, warm, and biologically active early Paleogene (∼50–100 g C m−2 a−1). Nitrogen isotope measurements from late Paleocene to early Eocene sediments provide evidence for a stepwise stratification and nutrient depletion in surface water masses. Cyanobacterial nitrogen fixation appeared to be the main source of nutrient N in the Arctic Ocean during the early‐middle Eocene characterized by conditions of strong oxygen depletion, high nutrient N losses, and high organic carbon accumulation rates. We speculate that biological CO2 sequestration in the Arctic Ocean and enhanced organic carbon burial rates may have contributed to lower atmospheric CO2 subsequent to the early Eocene climate optimum.

Trans-Mediterranean comparison of geochemical paleoproductivity proxies in a mid-Pleistocene interrupted sapropel

Sapropel layers in the sediment record of the Mediterranean Sea reflect periods of climate-induced elevated marine production associated with precessional minima. Some of these layers feature interruptions that represent centennial-to-millennial returns to the predominantly low-productivity conditions of this sea. We have measured multiple geochemical paleoproductivity proxies in the interrupted sapropel layer that corresponds to insolation cycle 90 (955 ka) at 1-cm intervals in a four-site transect from the Balearic Basin to the Levantine Basin to assess similarities and differences in its expression at the four locations. The interrupted sapropels are similarly divided by an organic-carbon-poor interval into an upper layer in which organic carbon concentrations reach 2–7 wt.% and a lower layer in which they peak between 1 and 3 wt.%. The interruption was essentially synchronous in the Balearic Basin, the Tyrrhenian Basin, and the Ionian Basin, but it was delayed and shorter in the Levantine Basin. Lighter nitrogen isotope compositions consistently accompany higher organic carbon accumulation rates in the sapropel layers, which implies a shift from coccolith-dominated organic matter production towards a mode of primary production in which cyanobacteria were important during times of sapropel deposition. Organic carbon isotopic compositions become heavier as organic carbon accumulation increases at three of the locations, which is consistent with elevated productivity. However, they become lighter at the Levantine Basin site, which suggests greater delivery of isotopically light fluvial waters to this part of the Mediterranean Sea. The local paleoclimate conditions conducive to sapropel formation evidently were not completely uniform across the Mediterranean region but their occurrences were synchronous. Moreover, the co-existence of an essentially same-age interruption of the sapropels at the four locations shows that climate could change rapidly and concordantly across the region at much less than precessional intervals.

Climatically forced organic carbon burial in equatorial Atlantic and Pacific Oceans

Over the past ten years, sedimentary geochemists have noted that climate changes associated with Pleistocene glaciations have had a pronounced effect on the rate of organic carbon burial found in the tropical oceans1–4. In particular, two to five times higher organic carbon accumulation rates have been observed for the 18 kyr BP glacial maximum than for the Holocene in several short sediment cores from both oceans (see Fig. 1). Here I present 300-kyr time series for organic carbon mass accumulation rate from four sediment cores, two each from the equatorial Pacific and Atlantic, and show that orbital forcing of global climate has strongly affected the rate of organic carbon burial in the equatorial oceans.

Sediment accumulation in the central western Black Sea over the past 5100 years

The postdepositional particle flux (accumulation rate) over the last 5100 years was studied in the anoxic sediment units I and II from the central western Black Sea. Deposition of unit II (sapropel) in the deep Black Sea basin was divided further into two subunits. The beginning of the deposition of the sapropel in the Black Sea (5100–3100 years B.P., subunit IIb) was characterized by higher organic carbon accumulation rates due largely to higher primary production. Accumulation rates of lithogenic matter were at a minimum compared to younger units. During the subsequent 2100 years (3100–1000 years B.P., subunit IIa) organic carbon accumulation rates decreased considerably while accumulation rates of lithogenic matter increased to the present level. At 1000 years B.P. (unit I), total mass accumulation rates increased by a factor of 3. This increase was largely a result of an increase in carbonate accumulation rates by a factor of 11, which was caused by the rapid appearance of Emiliania huxleyi in the Black Sea. Higher carbonate accumulation rates in unit I are mainly the cause for the considerably lower organic carbon content compared to unit II, since the organic carbon accumulation rate in unit I did not decrease. General correspondence of accumulation rates of lithogenic matter in different cores through time suggests overregional long‐term (100‐year scale) climatic effects on the terrigenous particle supply to the Black Sea.

Upwelling in Cretaceous Western Interior Seaway: Sharon Springs Member, Pierre Shale: ABSTRACT

Upwelling zones are characterized by the following features: (1) facies distribution, which is related to oxygenation of bottom waters, of laminated, organic-rich sediment, phosphate, and glauconite; (2) abundant organic-rich fecal pellets; (3) abundant marine biota at all trophic levels; (4) high organic carbon accumulation rates, with predominance of marine organic matter; and (5) a distribution of the anoxic facies that is not controlled by bathymetry. By contrast, stratified stagnant basins characteristically (1) lack significant phosphate; (2) show little or no evidence of high biologic productivity; (3) have high organic carbon accumulation rates with significant influx of terrestrial organic matter; and (4) have an anoxic facies that is confined to the deepest part of the basin. The Sharon Springs Member of the Pierre Shale, usually described as a stagnant-basin deposit, shares more features in common with upwelling zones than with stratified stagnant basins. Specifically, the overall facies distribution of the Sharon Springs includes a basinward zone of laminated, highly organic-rich strata bounded on the west by phosphate, which is, in turn, bounded at the western shoreline by glauconitic sandstone. The same facies distribution might be expected on the eastern shoreline, but the Sharon Springs is truncated there by erosion. The organicmore » matter in the Sharon springs is almost entirely contained in fecal pellets and is predominantly marine in origin, organic carbon accumulation rates are comparable to those of upwelling zones. Finally, the Sharon Springs is noted for high concentrations of bones of marine reptiles. These bones occur in a band that may mark the zone of highest biologic productivity.« less

Patterns and Rates of Organic Carbon Accumulation in Recent Pelagic and Hemipelagic Marine Environments: ABSTRACT

Maps were constructed for the global distribution of percent organic carbon, sedimentation rates, bulk sediment accumulation rates, and organic carbon accumulation rates (OCARs), exclusive of the continental shelves, on the basis of both published and some new data from sediment cores. The OCAR data base uses only cores in which stratigraphic resolution is sufficient to define a mean sedimentation rate for the last 12,000 years and from which bulk density measurements are available or can be calculated using established relationships to carbonate content. Data coverage was adequate for much of the Pacific Ocean basin but was somewhat sparse for the Atlantic, Indian, and Southern oceans. Nonetheless, basic patterns of organic carbon accumulation can be observed from the maps. As expected, the OCARs are highest in the equatorial Pacific, where surface-water productivity is highest, and low under the central gyre regions where surface organic carbon flux and total sediment flux are both low. Higher OCARs also occur in more rapidly accumulating sediments near the continental margins, partly as a function of the enhanced preservation of labile carbon that results from higher sedimentation rates and, in some cases, from higher productivity in surface water masses or increased flux of terrigenous organic matter.more » The patterns of OCAR are similar, in general, to patterns of accumulation of biogenic opal, thereby reinforcing the link between biological productivity and OCARs. An analysis of the global impact of pelagic-hemipelagic OCARs, using this data base, suggests that no more than 0.21 x 10/sup 14/ gOC/year accumulate over the deep-sea floor at present - only about 16% of the estimated global annual organic carbon burial. However, the deep-sea organic carbon sink may have been more important in the past.« less