Bulk and amino acid parameters of core SO188-342KL from the Northern Bay of Bengal for 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 18 ka. Bulk organic proxies (TOC, TIC, TN, d13CTOC, d15NTN) 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 15 ka 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 7 ka 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 13 ka 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 (18-17 ka 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.

Amino acids and carbohydrates in sediments and interstitial waters of ODP Site 112-681

Total organic carbon (TOC), dissolved organic carbon (DOC), total hydrolyzable amino acids (THAA), amino sugars (THAS), and carbohydrates (THCHO) were measured in sediments and interstitial waters from Site 681 (ODP Leg 112). TOC concentrations vary between 0.75% and 8.2% by weight of dry sediment and exhibit a general decrease with depth. DOC concentrations range from 6.1 to 49.5 mg/L, but do not correlate with TOC concentrations in the sediment. Amino compounds (AA and AS) and sugars account for 0.5% to 8% and 0.5% to 3% of TOC, respectively, while amino compounds make up between 2% and 27% of total nitrogen. Dissolved hydrolyzable amino acids (free and combined) and amino sugars were found in concentrations from 3.7 to 150 µM and from 0.1 to 3.7 µM, respectively, and together account for an average of 8.5% of DOC. Dissolved hydrolyzable carbohydrates are in the range of 6 to 49 µM. Amino acid spectra are dominated by glycine, alanine, leucine, and phenylalanine; nonproteinaceous amino acids (gamma-amino butyric acid, beta-alanine, and ornithine) are enriched in the deeper part of the section, gamma-amino butyric acid and beta-alanine are thought to be indicators of continued microbial degradation of TOC. Glycine, serine, glutamic acid, alanine, aspartic acid, and ornithine are the dominating amino compounds in the pore waters. Spectra of carbohydrates in sediments are dominated by glucose, galactose, and mannose, while dissolved sugars are dominated by glucose and fructose. In contrast to the lack of correlation between abundances of bulk TOC and DOC in corresponding interstitial waters, amino compounds and sugars do show some correlation between sediments and pore waters: A depth increase of aspartic acid, serine, glycine, and glutamic acid in the pore waters is reflected in a decrease in the sediment, while an enrichment in valine, iso-leucine, leucine, and phenylalanine in the sediment is mirrored by a decrease in the interstitial waters. The distribution of individual hexoseamines appears to be related to zones of bacterial decomposition of organic matter. Low glucoseamine to galactoseamine ratios coincide with zones of sulfate depletion in the interstitial waters.

Amino acids from ODP Holes 117-722B and 177-724C

Sedimentary d15N records are valuable archives of ocean history but they are often modified during early diagenesis. Here we quantify the effect of early diagenetic enrichment on sedimentary N-isotope composition in order to obtain the pristine signal of reactive N assimilated in the euphotic zone. This is possible by using paired data of d15N and amino acid composition of sediment samples, which can be applied to estimate the degree of organic matter degradation. We determined d15N and amino acid composition in coeval sediments from Ocean Drilling Program (ODP) Hole 772 B in the central Arabian Sea and from Hole 724 C situated on the Oman Margin in the western Arabian Sea coastal upwelling area. The records span the last 130 kyr and include two glacial-interglacial cycles. These new data are used in conjunction with data available for surface sediments that cover a wide range of organic matter degradation states, and with other cores from the northern and eastern Arabian Sea to explore spatial variations in the isotopic signal. In order to reconstruct pristine N values we apply the relationship between organic matter degradation and 15N enrichment in surface sediments to correct the core records for early diagenetic enrichment. Reconstructed d15N values suggest a significant role of N2-fixation during glacial stages. An evaluation of two preservation indices based on amino acid composition (Reactivity Index, RI; Jennerjahn and Ittekkot, 1997; and the Degradation Index, DI; Dauwe et al., 1999) in both recent sediments and core samples suggests that the RI is more suitable than the DI in correcting Arabian Sea d15N records for early diagenetic enrichment.

Molecular dissolved organic matter composition and environmental parameters from a laboratory incubation study

Dissolved organic matter (DOM) in the oceans constitutes a major carbon pool involved in global biogeochemical cycles. More than 96% of the marine DOM resists microbial degradation for thousands of years. The composition of this refractory DOM (RDOM) exhibits a molecular signature which is ubiquitously detected in the deep oceans. Surprisingly efficient microbial transformation of labile into RDOM was shown experimentally, implying that microorganisms produce far more RDOM than needed to sustain the global pool. By assessing the microbial formation and transformation of DOM in unprecedented molecular detail for 3 years, we show that most of the newly formed RDOM is molecularly different from deep sea RDOM. Only <0.4% of the net community production was channeled into RDOM molecularly undistinguishable from deep sea DOM. Our study provides novel experimentally derived molecular evidence and data for global models on the production, turnover and accumulation of marine DOM.

Organic compounds in sediments and pore waters of ODP Sites 117-723 and 117-724

Total organic carbon, amino compounds, and carbohydrates were measured in pore waters and sediments of Pliocene to Pleistocene age from Sites 723 and 724 (ODP Leg 117) to evaluate (1) relationships between organic matter in the sediment and in the pore water, (2) the imprint of lithological variations on the abundance and contribution of organic substances, (3) degradation of amino compounds and carbohydrates with time and/or depth, and (4) the dependence of the ammonia concentration in the pore water on the degradation of amino compounds in the sediment. Total organic carbon concentrations (TOC) of the investigated sediment samples range from 0.9% to 8.7%, and total nitrogen concentrations (TN) from 0.1% to 0.5%. Up to 4.9% of the TOC is contributed by hydrolyzable amino acids (THAA) which are present in amounts between 1.1 and 21.3 µmol/g dry sediment and decrease strongly downhole. Hydrolyzable carbohydrates (THCHO) were found in concentrations from 1.3 to 6.6 ?mol/g sediment constituting between 0.1% and 2.0% of the TOC. Differences between the distribution patterns of monomers in Sites 723 and 724 indicate higher terrigenous influence for Site 724 and, furthermore, enhanced input of organic matter that is relatively resistant to microbial degradation. Lithologically distinct facies close to the Pliocene/Pleistocene boundary yield different organic matter compositions. Laminated horizons seem to correspond with enhanced amounts of biogenic siliceous material and minor microbiological degradation. Total amounts of dissolved organic carbon (DOC) in pore waters vary between 11 and 131 mg/L. Concentrations of DOC as well as of dissolved amino compounds and carbohydrates appear to be related to microbial activity and/or associated redox zones and not so much to the abundance of organic matter in the sediments. Distributions of amino acids and monosaccharides in pore waters show a general enrichment in relatively stable components in comparison to those of the sediments. Nevertheless, the same trend appears between amino acids present in the sediments from Sites 723 and 724 as well as between amino acids in pore waters from these two sites, indicating a direct relation between the dissolved and the sedimentary organic fractions. Different ammonia concentrations in the pore waters of Sites 723 and 724 seem to be related to enhanced release of ammonia from degradation of amino compounds in Site 723.

Geochemistry on Holocene sediments from the Mediterranean

The enhanced accumulation of organic matter in Eastern Mediterranean sapropels and their unusually low d15N values have been attributed to either enhanced nutrient availability which led to elevated primary production and carbon sequestration or to enhanced organic matter preservation under anoxic conditions. In order to evaluate these two hypothesis we have determined Ba/Al ratios, amino acid composition, N and organic C concentrations and d15N in sinking particles, surface sediments, eight spatially distributed core records of the youngest sapropel S1 (10-6 ka) and older sapropels (S5, S6) from two locations. These data suggest that (i) temporal and spatial variations in d15N of sedimentary N are driven by different degrees of diagenesis at different sites rather than by changes in N-sources or primary productivity and (ii) present day TOC export production would suffice to create a sapropel like S1 under conditions of deep-water anoxia. This implies that both enhanced TOC accumulation and d15N depletion in sapropels were due to the absence of oxygen in deep waters. Thus preservation plays a major role for the accumulation of organic-rich sediments casting doubt on the need of enhanced primary production for sapropel formation.

Biogeochemical measurements of sediments collected in the Black Sea during the MSM15/1 cruise in 2010

Biogeochemical measurements in sediment cores collected with a TV-MUC in the Black Sea during MSM15/1, Northwest Crimea (HYPOX Project), at water depths between 105-207 m. Sampling was performed along gradient of oxygen bottom water concentrations between oxic (150 µmol L-1), variable hypoxic (3-60 µmol L-1 O2) and anoxic, sulfidic conditions. concentrations of organic carbon (Corg) and nitrogen (N) were measured on finely powdered, freeze-dried subsamples of sediment using a using a Fisons NA-1500 elemental analyzer. For organic carbon determination samples were pre-treated with 12.5% HCl to remove carbonates. Chlorophyll a (chl a), phaeopigments (PHAEO) and chloroplastic pigment equivalents (CPE) was measured according to Schubert et al., (2005) and total hydrolyzable amino acids (THAA) and single amino acid: ASP, GLU, SER, HIS, GLY, THR, ARG, ALA, TYR, MET, VAL, PHE, ILE, LEU, LYS following Dauwe et al., 1998.