Variations in the osmium (Os) isotope ratio of seawater through time potentially provide a very sensitive record of continental weathering. However, the application of this system has been limited because the available record lacks sufficient temporal resolution either to relate to known tectonic or climatic events, or for use as a stratigraphic dating technique. Moreover, thus far, the Os isotopic composition of present-day seawater has not been measured directly, and remains poorly constrained, relative to present-day measurement precision. This study presents high-precison Os data for hydrogeneous ferromanganese (Fe-Mn) crusts from the Pacific, Atlantic and Indian Oceans. The development of reproducibly low-blank chemistry (0.13±0.01 pg/g) combined with the high concentration of Os in Fe-Mn crusts (1-3 ppb) allows the routine analysis of small samples necessary for high resolution study.
Recent surface layers of crusts, dated using U-Th disequilibria, from the Pacific and Indian oceans give indistinguishable ratios of 8.61±0.08 and 8.63±0.03, respectively. In contrast, North Atlantic surfaces give higher values of 8.804±0.07. Thus, these data suggest that for the present-day there is a small but systematic difference in the Os isotopic composition of seawater between the major oceans.
Crust BM1969.05 (39°0' N; 60°57' W; ca. 1850 m water depth) dated using 87Sr ages, grew over a period of ca. 20 Ma. Os isotope ratios, in general, become more radiogenic towards the present-day. However, high-resolution sampling reveals several finer scale features, in particular,
(i) for the period 20 to 16 Ma, there is a marked increase towards more radiogenic Os; (ii) from 2.5 Ma a period of constant 187Os/186Os was followed by a steep increase from 0.9 Ma to the present-day.
Crust D11-1 (111°38' N; 161°41' E; ca. 1770 m water depth) dated using 10Be/9Be, has been analysed for Os back to ca. 15 Ma. Os isotope ratios show a systematic increase to more radiogenic values for the present day, consistent with published data for the Pacific ocean.
These results suggest that a significant part of the radiogenic Os in present-day seawater is sourced in continental material that weathers into the North Atlantic ocean. The systematic differences in the isotopic composition between oceans, like Pb and Nd, probably reflect greater input of old continental material into the North Atlantic, and support a maximum residence time of 10-20 Ka. The marked shifts observed in the North Atlantic record coincide with known phases of increased glacial or tectonic activity. These features have not been observed in the Pacific data, but at present it not clear whether this reflects local variations in the Atlantic, real differences between the oceans or, for the period 20 to 16 Ma, simply that this time interval has not yet been adequately sampled in the Pacific.