The role of sedimentological and geochemical indicators in the recognition of redox conditions in the southern part of the South Caspian Basin

Sediment constituents, total organic carbon contents, redox-sensitive trace-metals concentrations, and organic matter (OM) types have been analyzed on fifteen sediment cores (73cm-165cm long) taken from the southern part of the South Caspian Basin (SPSCB). 257 samples for sedimentological studies (particles size analysis, mineralogy and biota contents) and 120 samples for organic-inorganic geochemistry studies (TOC, S2 and RSTM concentrations) were analyzed. Sediments on the distal parts of shelf are mud dominated, while slope and proximal parts of the basin plain comprise interbedded debrite, and pelagic-hemipelagic sediments. Several factors (e.g. bottom dissolved oxygen (BDO), TOC, biota content, faecal pellets, and RSTM) suggest that these sediments were deposited under markedly different redox conditions, namely: (1) Shelf and slope settings (oxic sediments), and (2) intraslope basins and basin plain settings (dysoxic sediments). Observations suggest that basin plain and intraslope basins sediments were deposited under oxygen-depleted (oxygen minimum zone) conditions. The dysoxic sediments are enriched in RSTM (V, Zn, Cr, Cu, Ni), with high TOC values, with mud peloids, and contain type II OM, while the sediments of the oxic sediments are enriched in infaunal organisms, faecal pellets, and contain types III and mixture II/III OM.