Carbon-sulfur-iron systematics of the uppermost deep-water sediments of the Black Sea

Abstract

Box cores recovered during Leg 4 of the 1988 R/V “Knorr” Black Sea Oceanographic Expedition from deep-water regions of the basin were dominated by coccolith-rich, microlaminated (Unit 1) sediment and muddy, gray turbidite layers. Both organic carbon (OC) and pyrite sulfur values for Unit 1 display narrow ranges, with mean concentrations of 5.3 ± 1.1 (1σ) wt% and 1.3 ± 0.3 wt%, respectively. Unit 1 is not enriched in pyrite-S relative to sediments deposited under oxygenated bottom waters (normal marine sediments) with comparable OC concentrations. Carbon-sulfur relationships (evaluated on a calcium carbonate-free basis to avoid spurious correlations resulting from dilution effects) demonstrate that OC and pyrite-S are essentially decoupled. These observations, combined with the persistence of elevated pore-water sulfide to depth and a strong correlation between pyrite-S and the detrital Fe component argue strongly for limitation of pyrite formation in Unit 1 by the availability of reactive Fe. Unit- 1 Fe limitation is further indicated by degree-of-pyritization (DOP) studies (a measure of the extent to which the original potentially reactive Fe has been transformed to pyrite). These studies show sulfidation of reactive Fe ranging from 57% to 78%, with DOP values independent of OC concentration. Unit- 1 DOP profiles suggest that the majority of the pyrite is formed in the sulfidic water column and/or very close to the sediment-water interface. Pyrite-S concentrations of Unit 1, when compared with the particulate reduced sulfur fluxes measured in time-series sediment traps, are compatible with predominantly water-column pyrite formation. Because of the limitations in the supply of reactive Fe associated with the comparatively high supply of OC, the microlaminated sediment is characterized by C/S ratios greater than those typical of Holocene oxically deposited sediments. The turbidite muds of the deep basin display high reduced S values (relative to Holocene normal marine sediments) in samples with low OC concentrations (low C/S ratios). This reflects pyrite formation under anoxic-sulfidic bottom-water conditions in a probable upper-slope source region, as well as during transport and final deposition. Intermediate DOP values for the turbidites, in part a product of their rapid rate of deposition, reveal that Fe limitation is ultimately not a factor and that further pyrite should form during burial. However, the very rapid rate at which Unit- 1 pyrite forms suggests fundamental differences between the overall reactivities of the Fe phases associated with the microlaminated and turbiditic sediments. The signature of water-column anoxia with regard to sedimentary pyrite formation is clearly indicated by the high DOP values of Unit 1 and the comparatively high levels of S associated with the low concentrations of OC of the turbidite muds. This agrees with similar conclusions based on studies of ancient sedimentary rocks.