S. E. Calvert

Nitrogen and carbon isotopic composition of marine and terrestrial organic matter in Arctic Ocean sediments:: implications for nutrient utilization and organic matter composition

Abstract Relationships between organic carbon, total nitrogen and organic nitrogen concentrations and variations in δ 13 C org and δ 15 N org are examined in surface sediments from the eastern central Arctic Ocean and the Yermak Plateau. Removing the organic matter from samples with KOBr/KOH and determining residual as well as total N shows that there is a significant amount of bound inorganic N in the samples, which causes TOC/N total ratios to be low (4–10 depending on the organic content). TOC/N org ratios are significantly higher (8–16). This correction of organic TOC/N ratios for the presence of soil-derived bound ammonium is especially important in samples with high illite concentrations, the clay mineral mainly responsible for ammonium adsorption. The isotopic composition of the organic N fraction was estimated by determining the isotopic composition of the total and inorganic nitrogen fractions and assuming mass-balance. A strong correlation between δ 15 N org values of the sediments and the nitrate concentration of surface waters indicates different relative nitrate utilization rates of the phytoplankton in various regions of the Arctic Ocean. On the Yermak Plateau, low δ 15 N org values correspond to high nitrate concentrations, whereas in the central Arctic Ocean high δ 15 N org values are found beneath low nitrate waters. Sediment δ 13 C org values are close to −23.0‰ in the Yermak Plateau region and approximately −21.4‰ in the central Arctic Ocean. Particulate organic matter collected from meltwater ponds and ice-cores are relatively enriched in 13 C ( δ 13 C org =−15.3 to −20.6‰) most likely due to low CO 2 (aq) concentrations in these environments. A maximum terrestrial contribution of 30% of the organic matter to sediments in the central Arctic Ocean is derived, based on the carbon isotope data and various assumptions about the isotopic composition of the potential endmembers.

Glacial‐interglacial variability in denitrification in the World's Oceans: Causes and consequences

The late Quaternary history of water-column denitrifcation of the eastern Pacific margins and the Arabian Sea is reconstructed using sedimentary δ15N measurements. The δ15N values in six piston cores raised from these regions show remarkably similar cyclic variations, being heavy (9–10.5‰) during the interglacials and 2–3‰ lighter during the glacials. This implies that denitrification in these regions decreased substantially during the glacial periods. The glacial decline in denitrification is attributed to reduced upwelling and flux of organic material through the oxygen minimum zone. Since water-column denitrification in these areas accounts for about half of the fixed-nitrogen loss in the modern ocean, the inferred decrease in denitrification should have increased the oceanic nitrate inventory during glacial periods. Because nitrate is a limiting nutrient, oceanic productivity and attendant changes in CO2 may therefore have been modulated on glacial-interglacial timescales by variations in the oceanic NO3 content.

Reduced nitrogen fixation in the glacial ocean inferred from changes in marine nitrogen and phosphorus inventories

To explain the lower atmospheric CO2 concentrations during glacial periods, it has been suggested that the productivity of marine phytoplankton was stimulated by an increased flux of iron-bearing dust to the oceans. One component of this theory is that iron—an essential element/nutrient for nitrogen-fixing organisms—will increase the rate of marine nitrogen fixation, fuelling the growth of other marine phytoplankton and increasing CO2 uptake. Here we present data that questions this hypothesis. From a sediment core off the northwestern continental margin of Mexico, we show that denitrification and phosphorite formation—processes that occur in oxygen-deficient upwelling regions, removing respectively nitrogen and phosphorus from the ocean—declined in glacial periods, thus increasing marine inventories of nitrogen and phosphorus. But increases in phosphorus were smaller and less rapid, leading to increased N/P ratios in the oceans. Acknowledging that phytoplankton require nitrogen and phosphorus in constant proportions, the Redfield ratio, and that N/P ratios greater than the Redfield ratio are likely to suppress nitrogen fixation, we suggest therefore that marine productivity did not increase in glacial periods in response to either increased nutrient inventories or greater iron supply.

Stable phytoplankton community structure in the Arabian Sea over the past 200,000 years

Glacial to interglacial climate changes have been related to organic carbon cycling in oceanic surface waters, and this possible link has led to the development of sedimentary tracers of past marine biological production. For example, sediment records of organic carbon, opal and biogenic barium have been used to reconstruct past variations in production in different oceanic regimes, but these tracers cannot be used to discriminate between the relative contributions of different phytoplankton groups. Such a discrimination would provide greater insight into the operation of the biological ‘pump’ transporting material down out of surface waters, and into the possible influence of the structure of oceanic food chains on carbon fluxes. Several organic biomarker compounds have now been established for tracing the contribution of different planktonic groups to organic carbon in sediments. Here we show that four such biomarkers—dinosterol, alkenones, brassicasterol and chlorins, which represent dinoflagellates, prymnesiophytes, diatoms and chlorophyll-producers, respectively—have concordant concentration maxima that coincide with organic carbon maxima over the past 200,000 years in a sediment core from the northeastern Arabian Sea. Not only do these organic tracers track changes in ocean production in this region, but the similar distributions of dinosterol and brassicasterol indicate that the relative contributions of the dominant members of the phytoplankton community (diatoms and dinoflagellates) to production were roughly uniform on timescales greater than 3,000–4,000 years over the past 200,000 years.

On the late Pleistocene-Holocene sapropel record of climatic and oceanographic variability in the eastern Mediterranean

Sapropels and intercalated marls in a piston core from the eastern Mediterranean are chemically and mineralogical distinct: kaolinite, smectite, and total S contents, Fe/A1, Ba/A1, Co/A1, Mo/A1, and V/A1 are higher, whereas quartz, Mg calcite, illite contents, Si/A1, Ti/A1, K/A1, Rb/A1, and Zr/A1 are lower in the sapropels. Missing and “ghost” sapropels are identified by mineralogical and chemical properties that are not prone to diagenesis. Primary production was higher (Ba/A1) and bottom water and/or interstitial oxygenation was lower (Co/A1, Mo/A1, Ni/A1, and V/A1) during sapropel formation. Wind speeds (quartz/clay, Si/A1, and Zr/A1) and bottom water salinities (Mg calcite/calcite) were higher during periods of marl formation. Sapropels represent a fundamentally different sedimentary facies whose formation is linked to changes in the hydrological balance in the basin, driven by precessionally modulated changes in monsoon strength and subtropical precipitation changes, which altered circulation, production, and sediment source areas.

Carbon accumulation rates and the origin of the Holocene sapropel in the Black Sea

A detailed radiocarbon chronology obtained by accelerator mass spectrometry together with organic carbon and carbonate measurements on two Black Sea cores has been used to compare and contrast the burial fluxes of organic carbon in the Holocene sapropel and the modern sediment. At both deep-water and eastern-slope sites, the sapropel is separated from the modern facies by a variable thickness of compositionally homogeneous sediment with low levels of organic carbon and anomalously old radiocarbon ages. This homogeneous unit probably represents deposition by slumping or mudflow. The age limits of the sapropel are 1600-6600 B.P. at the deep-water site and 4000-6000 B.P. (radiocarbon years before 1950 A.D.) at the shallow-water site. The carbon accumulation rate in the deep-water sapropel is higher than that in the modern deep-water facies by a factor of 2 and is approximately the same as that in the modern sediment in shallow water. The revised chronology of sapropel formation and the differences in the carbon accumulation rates probably indicate that the sapropel was formed by increased production during the transition from the premodern lake to the modern marine phases of the Black Sea. This conclusion is consistent with the clear marine carbon-isotope signal in the organic mattermorexa0» in the sapropel in both cores (results to be reported elsewhere), in contrast to the mixed source of carbon in the other facies.«xa0less

Geochemical and isotopic evidence for post-glacial palaeoceanographic changes in Saanich Inlet, British Columbia

Abstract Coring at site ODP 1033B in Saanich Inlet recovered 59.4xa0m of mainly laminated olive-grey diatom ooze and an underlying 55.15xa0m of massive grey to olive-grey silty clay. Based on AMS radiocarbon dating, the boundary between the two units is between 11,000 and 13,800 calibrated years BP, and represents the Holocene-Pleistocene boundary. The lower unit represents glaciomarine deposition, whereas deposition of the upper unit began when the modern semi-restricted physiography of the fjord was established following glacial rebound and highly productive marine conditions were established. The glaciomarine clay is almost entirely terrigenous, whereas the diatom ooze contains 2–3xa0wt.% organic C and 20–40xa0wt.% biogenous silica; CaCO 3 contributions are minor, but there are several peaks in carbonate abundance in the upper unit. The isotopic composition of organic C and total N suggests that organic matter in the glaciomarine clay is dominantly terrestrial ( δ 13 C organic δ 15 N total =ca. 3‰) and in the diatom oozes it is mainly marine ( δ 13 C organic >−22‰ and δ 15 N total =ca. 10‰). The heavy δ 15 N total values probably record a contribution of isotopically heavy nitrate to the surface waters of the inlet that is transported to British Columbia (BC) coastal waters from the eastern tropical Pacific by the California Undercurrent. Major and minor elemental data suggest that the composition of the terrigenous material and its grain-size has changed over the last 15xa0kyr, and there are marked enrichments in several redox-sensitive elements in the diatom oozes. Thus, Cu, Mn, Mo, Ni, Pb, V and Zn have higher concentrations in the upper unit; Br and I are also enriched because of their association with organic matter. Mn is enriched in the anoxic diatom oozes due to the presence of manganoan carbonate (Mn peaks generally corresponding with carbonate peaks) formed in the sediment when deep water renewal caused precipitation of Mn oxyhydroxides, which dissolved in the anoxic sediment and was precipitated as a diagenetic phase. The remaining metals are enriched because of their removal to the sediment as sulphides (Cu, Mo, Ni, Pb and Zn) or as particle-reactive reduced species (V). Cr enrichment is obscured by the presence of Fe-rich chlorite. The lag in the enrichment of Mo with respect to organic C in the sediments indicates that anoxia developed some time after marine production increased following the semi-isolation of the fjord.

Transformations of biogenic particulates from the pelagic to the deep ocean realm

This overview compares and contrasts trends in the magnitude of the downward Particulate Organic Carbon (POC) #ux with observations on the vertical proles of biogeochemical parameters in the NE subarctic Pacic. Samples were collected at Ocean Station Papa (OSP, 503N, 1453W), between 18}22 May 1996, on pelagic stocks/rate processes, biogenic particle #uxes (drifting sediment traps, 100}1000 m), and vertical proles of biogeochemical parameters from MULVFS (Multiple Unit Large Volume Filtration System) pumps (0}1000 m). Evidence from thorium disequilibria, along with observations on the relative partitioning of particles between the 1}53 lm and ’53 lm classes in the 50 m mixed layer, indicate that there was little particle aggregation within the mixed layer, in contrast to the 50}100 m depth stratum where particle aggregation predominated. Vertical proles of thorium/uranium also provided

Organic-rich transitional facies in silled basins: Response to sea-level change

High-resolution profiles of organic carbon, {delta}{sup 13}C{sub org}, sulfur, {delta}{sup 38}S, and some trace elements in cores from two silled basins, Kau Bay (Indonesia) and the Black Sea, allow division of the sedimentary record into three distinct units, representing a Pleistocene fresh-water-brackish water facies (unit 3), a Holocene transitional facies (unit 2), and a modern fully marine facies (unit 1). The geochemical characteristics of these units are strikingly similar for both basins. The sediments form unit 3 are characterized by the predominance of terrestrial organic matter, Mo and U concentrations at crustal abundance, and positive {delta}{sup 34}S values. The transitional sediments (unit 2) are strongly enriched in marine organic matter and Mo, V, and U and have intermediate {delta} {sup 34}S values. Sediments from the modern marine facies (unit 1) are moderately organic-carbon rich and slightly enriched in Mo and U and have negative {delta}{sup 34}S values. The organic-carbon-rich sediments from unit 2 were formed by increased production during the transition from the Pleistocene isolated fresh-water-brackish water environment to the modern open-marine facies. This temporarily higher productivity was caused by displacement of nutrients from the deep water into the euphotic zone, owing to the gradual infilling by marine waters.

Carbon and nitrogen isotopic composition of sedimenting particulate material at Station Papa in the subarctic northeast Pacific

Abstract The δ 13 C and δ 15 N of sedimenting particulate organic matter (POM) collected biweekly by sediment trap at 3800xa0m at Ocean Station Papa (OSP: 50°N, 135°W) in the NE subarctic Pacific from 1982 to 1990 changed significantly on seasonal and annual time-scales. δ 13 C POM ranged from −25.3 to −22.0‰, and δ 15 N POM ranged from 0.24 to 7.75‰, over the entire period, isotopically depleted values occurring mainly in summer and heavier values occurring in winter. Extreme depletion in δ 13 C POM values also occurred in the winters of 1982–83 and 1988–89 and in the late summer of 1985; these were not matched by significant changes in δ 15 N POM . The changes in isotope ratios are related to the annual changes in settling particulate fluxes only in a general way; flux maxima in particulate organic carbon (POC) and particulate organic nitrogen (PON) occurred in mid- and late summer in most years, but there is not a one-to-one correlation between these changes and the isotope variations. In some years, the isotope ratios begin to changed before the summer increase in settling fluxes. A one-year record of sedimenting δ 13 C POM and δ 15 N POM (1989–90) at three depths (200, 1000 and 3800xa0m) at the same location showed a marked carbon isotope enrichment with depth, consistent with decomposition or/and food-chain enrichment during particle settling, but very small changes in the nitrogen isotope composition of the same particles, suggesting that C and N are released from sedimenting particles via different pathways. Suspended δ 13 C POM and δ 15 N POM values increased significantly with depth and were higher than those of sedimenting POM below the euphotic zone, suggesting that the two types of POM do not interact in deep water. Seasonal variations in surface temperature, irradiance, phytoplankton growth rate, species composition and carbon fixation pathways do not appear to be important controls of the observed changes in δ 13 C POM ; the increases in temperature, irradiance and phytoplankton growth rate in the summer months should lead to enriched δ 13 C POM values rather than the observed isotopically light values in this season. Likewise, the summer increase in the growth of diatoms should produce isotopically heavy particles in view of the reported 13 C -enrichment of this group, and this should be augmented by the decrease in [CO2](aq) due to the summer increase in surface temperature. Opposing these possible controls, the planktonic food chain becomes shorter in the spring/early summer months, probably due to the migration to the surface of copepodites and their direct grazing on phytoplankton, and this results in 13 C enrichment relative to the winter months when the food chain is more complex. The relationship between sedimenting δ 15 N POM and the macro-nutrients at OSP is complicated by the fact that ammonium and urea are important phytoplankton substrates, and generally more so than nitrate. The change from isotopically heavy winter values to lighter summer values begins before the major drawdown of the surface nitrate, and the observed change is opposite to that observed in other ocean regimes, probably because nitrate never reaches limiting levels in this High Nutrient Low Chlorophyll regime. The change of phytoplankton composition from winter to summer also should lead to 15 N -enrichment, the opposite of the trends observed. Finally, the changes in sedimenting δ 15 N POM at OSP do not appear to be related to phytoplankton growth rate. We conclude that the nitrogen isotopic signal in settling POM at OSP reflects seasonal changes in food-web structure, the simpler (shorter) spring/summer plankton community causing a smaller isotopic fractionation from the nutrient substrate to sedimenting POM.