Organic geochemistry on surface sediments from the Laptev Sea

Sediment samples from the Laptev Sea, taken during the 1993 RV Polarstern expedition ARK IX/4 and the RV Ivan Kireyev expedition TRANSDRIFT I, were investigated for the amount and composition of their organic carbon fractions. Of major interest was the identification of different processes controlling organic carbon deposition (i.e. terrigenous supply vs. surface water productivity). Long-chain unsaturated alkenones derived from prymnesiophytes, and fatty acids derived from diatoms and dinoflagellates, were analysed by means of gas chromatography and mass spectrometry. First results on the distribution of these biomarkers in surface sediments indicate that the surface water productivity signal is well preserved in the sediment data. This is shown by the distribution of the 16:1(n-7) and 20:5(n-3) fatty acids indicative for diatoms, and the excellent correlation with the chlorophyll a concentrations in the surface water masses and the biogenic-opal content and increased hydrogen indices of the sediments. The high concentration of these unsaturated fatty acids in shallow water sediments shows the recent deposition of the organic material. In deep-sea sediments, on the other hand, the concentrations are low. This decreased content is typical for phytoplankton material which has been degraded by microorganisms or autoxidation. In general, the alkenone concentrations are very low, suggesting low production rates by prymnesiophytes. Only at one station from the lower continental margin influenced by the inflow of Atlantic water masses, were some higher amounts of alkenones determined. Long-chain n-alkanes as well as high C/N ratios and low hydrogen indices indicate the importance of (fluvial) supply of terrigenous organic matter.

(Table 1) Organic acid analyses from Leg 135 interstitial waters

For the first time, short-chain organic acids are described in interstitial waters from sediments and lithified materials in a backarc setting. Organic acids in interstitial waters from the Tonga forearc region were also analyzed and compared with previous organic acid analyses from the Mariana and Bonin forearc interstitial waters. In the Tonga backarc setting, propionate typically dominates the organic acid assemblage, and organic acids are a consistent feature of these interstitial waters. The persistent presence of ammonia and the dominance of propionate over formate in the backarc interstitial waters suggest that the organic acids in this setting have their origin in reductive deamination of amino acids derived from sedimentary proteinaceous material. The organic acid assemblage revealed in the samples from Hole 841B in the Tonga forearc are similar to the organic acid assemblage detected in the Mariana forearc, that is, formate dominates the assemblage over acetate or propionate. These forearc organic acid assemblages may both have formed by a similar mechanism.