Sampling the oxygen minimum zone off Pakistan: glacial-interglacial variations of anoxia and productivity (preliminary results, sonne 90 cruise)

Abstract

During cruise sonne 90 the well developed oxygen minimum zone (OMZ) in the northeastern Arabian Sea was sampled in great detail to study surface ocean productivity and organic carbon accumulation/preservation during the last hundred to hundred thousands of years. Nearly anoxic conditions on and below the seafloor are indicated by the lack of macrobenthos and nekton, absence of bioturbation, high marine organic carbon accumulation and preservation of laminated sediments between about 300 and 1000 m water depth on the steep continental slope. This pattern approximately coincides with the zone of extremely low O 2 concentrations presently observed in the water column. Accumulation of organic material on the deeper slope near the lower boundary of the OMZ is enhanced by downslope transport. Downcore we observe a complex alternation of dark-colored, laminated, and light-colored, homogeneous facies. In general, laminated, organic carbon-rich intervals were deposited under suboxic bottom water and enhanced productivity conditions, mainly during warm stages (e.g., Bölling/Alleröd and Holocene from 8000 yrs B.P. to Recent). The homogeneous, bioturbated intervals indicate normal bottom water oxygen levels and reduced productivity during or just after cold stages (e.g. Terminations IA and B, and Younger Dryas). The striking facies similarities of the sedimentary record in cores collected from different areas and water depths suggest a unique pattern of late Pleistocene C org accumulation in the northeastern Arabian Sea which depends mainly on regional surface ocean productivity and intermediate water oxygenation. However, the sedimentary record is also influenced by variations in sediment supply from the Makran and Karachi shelf and from Indus River discharge during rapid sea-level changes.