The marine osmium isotope record

Over the past decade the marine osmium isotope record has been developed into a new tracer in palaeoceanographic research. Several analytical developments, particularly in the past few years, have significantly increased our ability to study the behaviour of osmium in the surficial environment. The Os-187/Os-188 and osmium concentration of seawater, river water, rain, and hydrothermal vent fluids have been measured directly. Recently, the behaviour of osmium in estuaries-critical for estimating the marine residence time of osmium-has been studied. Our knowledge of the surficial osmium cycle has thus significantly improved. In addition, reconstructions of past variations in the marine Os-187/Os-188 recently have been extended back into the Mesozoic. This review attempts to summarize our current understanding of the marine osmium system-present and past. The Os-187/Os-188 of seawater during the Cenozoic to first order mimics the marine Sr-87/Sr-86 record. It is therefore tempting to interpret both records as reflecting increased input of radiogenic osmium and strontium resulting from enhanced continental weathering regulated by climatic/tectonic processes. However, the marine osmium isotope system differs fundamentally from the marine strontium isotope system. This review emphasizes three important differences. First, large impacts are capable of resetting the Os-187/Os-188 to unradiogenic values without significantly affecting the marine strontium system. Second, organic-rich sediments are characterized by high Re-137/Os-188; resulting Os-187/Os-188 ingrowth-trajectories are similar to the average slope of the Cenozoic Os-187/Os-188 seawater record. Trends towards more radiogenic Os-187/Os-188 seawater therefore can be caused by weathering of organic-rich sediments at a constant rate. Third, the marine residence time of osmium is sufficiently short to capture short-periodic (glacial-interglacial) fluctuations that are inaccessible to the buffered marine strontium isotope system. This offers the opportunity to discriminate between high-frequency (climatic) and low-frequency (tectonic) forcing.