Abstract:
Several recent studies [1-5] have shown a correlation between osmium isotopic variations in marine sediments and glacial-interglacial cycling. These correlated variations have been used to argue for direct climatic control on weathering intensity and/or on the composition of erosional products. However these rapid fluctuations in the marine Os isotopic records suggest a residence time for Os in seawater (3 to 10 ka) that is much shorter than that inferred from mass balance ( approximately 25 to 40 ka, [6-8]). Furthermore, potential problems linked to sampling site or sediment type, such as basin isolation, detrital contributions, and low sedimentation rate, could bias many of the existing Os records. To obtain a reliable, high resolution record of marine Os isotopic variations in Quaternary times, we are analyzing sediments from ODP Leg 175, Site 1084, drilled off the coast of Namibia beneath the Benguela Upwelling System. This is an open ocean site containing rapidly deposited ( approximately 18 cm/kyr) organic rich sediments, which should be unaffected by the possible problems that may have plagued earlier studies. We currently have results for 9 samples spanning the most recent 35 ka of this record. These samples have extremely high Re and Os concentrations (58-157 ppb and 0.19-0.34 ppb, respectively). Their (super 187) Os/ (super 188) Os ratios are quite constant (1.044 + or -0.013, 2sigma ), and show only a tiny hint of the nearly 7% decrease in (super 187) Os/ (super 188) Os during the last glacial maximum observed in several previous studies. Instead, the uniformity of the Benguela Os record is similar to that reported for an early Pleistocene record from the equatorial Pacific Ocean [9]. The discrepancies between the Os records from different sediment types and localities must be better understood before these records can be used to constrain relationships between climate and weathering.