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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 187 Os/ 188 Os 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 187 Os/ 188 Os 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 187 Os/ 188 Os of seawater during the Cenozoic to first order mimics the marine 87 Sr/ 86 Sr 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 187 Os/ 188 Os to unradiogenic values without significantly affecting the marine strontium system. Second, organic-rich sediments are characterized by high 187 Re/ 188 Os; resulting 187 Os/ 188 Os ingrowth-trajectories are similar to the average slope of the Cenozoic 187 Os/ 188 Os seawater record. Trends towards more radiogenic 187 Os/ 188 Os 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 lowfrequency (tectonic) forcing.

Continental flood basalt (CFB) volcanism is hypothesized to have played a causative role in global climate change and mass extinctions. Uncertainties associated with radiometric dating preclude a clear chronological assessment of the environmental consequences of CFB volcanism. Our results document a 25% decline in the marine 187Os/188Os record that predates the Cretaceous-Tertiary boundary (KTB) and coincides with late Maastrichtian warming. We argue that this decline provides a chemostratigraphic marker of Deccan volcanism and thus constitutes compelling evidence that the main environmental consequence of Deccan volcanism was a transient global warming event of 3 degrees to 5 degrees C that is fully resolved from the KTB mass extinction.

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