Controls on organic matter distribution in oxygen minimum zone sediments from the continental slope off western India
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The causes of organic-rich sediments in contact with the perennial oxygen minimum zone (OMZ) on the continental slope off western India have been debated extensively, largely in relation to two main controlling factors: high productivity in the water column and enhanced preservation due to oxygen deficiency in bottom waters. In order to better understand the causes for organic-richness in sediments, three sediment cores recovered from the upper OMZ (UOMZ) at 500 m, middle OMZ (MOMZ) at 812 m and lower OMZ (LOMZ) at 1168 m water depths from the continental slope off Ratnagiri (west coast of India) were investigated for their organic matter (OM) and their stable isotopic (delta13C and delta15N), geochemical and rock magnetic parameters. The bottom water dissolved oxygen (DO) concentrations were 0.4 mu M, 5.8 mu M and 25 mu M at UOMZ, MOMZ and LOMZ core locations respectively. Maximum age of the sediments recorded in these cores was 4500 yrs BP. Sedimentation rates in the upper sections of the UOMZ and MOMZ cores were lower (5 to 7 cm/kyr) than that in the LOMZ core (20 cm/kyr). Organic carbon (Corg) content was highest (upto 7.9%) in the UOMZ core than in other cores (upto 4.9%). Enriched delta13Corg values (-18.6 to -16.8‰)) in the MOMZ core were indicative of reworked organic matter in the lower section. The delta15N values ranged from 4 to 7.4‰), however, with lower values for the reworked sediments at MOMZ site. The parameters representing productivity and redox conditions increased towards core top in all the cores, whereas terrigenous parameters decreased towards core top in the MOMZ and LOMZ cores. Despite the MOMZ core was always in OMZ in the lower section, very low Corg content coinciding with high terrigenous and reworked OM were deposited in the sediments prior to 1700 years BP. Rock magnetic properties indicate that the degree of diagenetic conditions varied in different sections of the cores and also among the cores within OMZ. Authigenic magnetite and authigenic uranium occurred in the lower sections of UOMZ and LOMZ cores and, authigenic magnetite in the upper sections of MOMZ core. These results suggest that productivity played an important role in organic-richness of the sediments and bottom water oxygen concentration has little effect on the preservation of OM. Besides, sedimentation rates, reworked organic matter, and topography at the core site indeed contribute on the distribution of organic matter in the Late Holocene sediments.
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