Organic content and preservational controls in surficial shelf and slope sediments from the Arabian Sea (Pakistan margin)

Abstract

In the Arabian Sea, organic-rich deposits occur on the continental slope at depths roughly coincident with the impingement of an intense water-column oxygen minimum zone (OMZ). This relationship has often been taken as causal, and such deposits are often proposed as modern analogues of organic-rich facies from the palaeorecord. However, there are numerous other potential preservational controls on continental margins, and their relative importance remains unclear and much debated. We report the results of organic, inorganic and stable C isotope analyses on surficial sediments from three transects of the Pakistan margin, ranging in depth from well above to well below the OMZ. Organic carbon (C org) and total nitrogen (N total) contents show expected mid-slope maxima, although these do not mirror bottom water dissolved O 2 profiles. Maximal values are observed near the lower boundary of the OMZ, or even in homogenous sediments below it. Together, lignin yields and δ 13 C org values indicate a general paucity of terrigenous organic matter (OM). This information, combined with C org:N total relationships, suggests that the OM contents of all sediments are dictated by differing accumulation of a predominant marine fraction. While there are no obvious redox-related differences in C org:N total or OM hydrogen richness across the margin, there are clear minima in δ 13 C org within the OMZ, in apparent contradiction with maximal accumulation of marine OM. The latter phenomenon is ascribed either to local source effects, such as variable algal isotopic signatures, or to differences in OM preservation state, possibly linked to bottom water redox conditions. This remains to be clarified, but, along with the rough correspondence between mid-slope sedimentary C org maxima and bottom water O 2 minima, may indicate that O 2 availability is an important factor in determining sedimentary OM content on this margin. However, evidence from redox-sensitive indicators (I/C org, Mn/Al) for persistent accumulation of OM-rich deposits under oxidizing conditions in homogeneous sediments below the OMZ, clearly indicates that O 2 availability is not the sole or over-riding control on OM distributions.