Osmium isotope variations in the oceans recorded by Fe-Mn crusts

This study presents osmium (Os) isotope data for recent growth surfaces of hydrogenetic ferromanganese (Fe-Mn) crusts from the Pacific, Atlantic and I ndian Oceans. In general, these data indicate a relatively uniform Os isoto pic composition for modem seawater, but suggest that North Atlantic seawate r is slightly more radiogenic than that of the Pacific and Indian Oceans. T he systematic difference in the Os isotopic composition between the major o ceans probably reflects a greater input of old continental material with a high Re/Os ratio in the North Atlantic Ocean, consistent with the distribut ion of Nd and Pb isotopes. This spatial variation in the Os isotope composi tion in seawater is consistent with a residence One for Os of between 2 and 60 kyr. Indian Ocean samples show no evidence of a local source of radioge nic Os, which suggests that the present-day riverine input from the Himalay a-Tibet region is not a major source for Os. Recently formed Fe-Mn crusts f rom the TAG hydrothermal field in the North Atlantic yield an Os isotopic c omposition close to that of modem seawater, which indicates that, in this a rea, the input of unradiogenic Os from the hydrothermal alteration of ocean ic crust is small. However, some samples from the deep Pacific (greater tha n or equal to 4 km) possess a remarkably unradiogenic Os isotope compositio n (Os-187/Os-186 ratios as low as 4.3). The compositional control of Os inc orporation into the crusts and mixing relationships suggest that this unrad iogenic composition is most likely due to the direct incorporation of micro meteoritic or abyssal peridotite particles, rather than indicating the pres ence of an unradiogenic deep-water mass. Moreover, this unradiogenic signal appears to be temporary, and local, and has had little apparent effect on the overall evolution of seawater. These results confirm that input of cont inental material through erosion is the dominant source of Os in seawater, but it is not clear whether global Os variations are due to the input of ma ntle or meteoritic material, or simply indicate that the continental source itself is not uniform. (C) 1999 Elsevier Science B.V. All rights reserved.