Amino acid concentrations in interstitial waters from ODP Sites 113-689 and 113-690

Biogenic calcareous and siliceous sediments were drilled at ODP Sites 689 and 690 on the Maud Rise, Antarctic Ocean. We analyzed dissolved combined amino acids (DCAA) and dissolved free amino acids (DFAA) in interstitial waters in order to characterize the amino acids in dissolved organic matter. The DFAA was predominant over the DCAA in interstitial waters at Sites 689 and 690, which contradicted the previous results from interstitial water and seawater studies. The DCAA in the interstitial waters probably originated from calcareous biogenic debris with less amounts of siliceous debris. Although glutamic acid constituted 41% of the total concentration of DCAA, it accounted for only 1% of the total concentration of DFAA due to the adsorption and/or reaction with biogenic carbonate. Ornithine, a nonprotein amino acid, is a decomposed product of arginine and made up 17 mol% of the total DFAA and. The total hydrolyzable amino acids (=DCAA + DFAA) accounted for 5 to 28% of the dissolved organic carbon (DOC) concentration, which implied that high molecular weight organic matter was a major contributor for the DOM (dissolved organic matter) in interstitial waters. Fairly positive correlation between the dissolved manganese and the total DCAA values suggested that the redox condition plays a significant role in controlling the total DCAA content. A small decrease in the sulfate concentration in the interstitial waters from both sites suggested fairly low microbial activity by sulfate-reducing bacteria.

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.