High Resolution Record of Seawater Osmium Isotopes Over the Last 100,000 Years

We present a high-resolution record of the osmium isotopic composition of oceans over the past 100,000 years. The main intent of this study is to investigate the claims that the Os isotopic composition of the deep oceans varies on a glacial-interglacial timescale and consequently the mean residence time of Os in the oceans is between 8 and 10 ka (Oxburgh, 1998; 2001), about a factor of four to five less than that estimated from mass balance calculations (Sharma et al., 1997, 1999; Levasseur et al., 1999; Peucker-Ehrenbrink, 2002). At present, it is commonly believed that Os in seawater is derived from continents (rivers), submarine alteration (abyssal peridotites?), and micrometeorites. A shorter residence time would require the need to search for a yet another substantial source of Os to the deep ocean. We have measured Os isotopes in Fe-Mn fraction of core TNO57-21, which is a piston core recovered from ODP site #1089, located in the Cape Basin on the Agullhas drift (41° S, 7° E, depth = 4825 m). This site is unique in that it experienced extremely high sedimentation rates with little input of continental dust or volcanic ash. Therefore, the sediment is composed entirely of authigenic clays and carbonates. Osmium sequestered in the Fe-Mn portion of the core should yield the ¹⁸⁷Os/¹⁸⁸Os ratio of ambient seawater. If Os has a residence time of 8-10 ka, we would expect to observe glacial-interglacial variations in our record reflective of variations in the intensity of continental weathering. If, on the other hand, the residence time is ~40 ka, as predicted by mass-balance equations, then the Os isotopic composition should remain essentially constant throughout the past glacial-interglacial cycle. Our data for the TNO57-21 show substantial variation during the past 100 Ka extending into the Holocene. Intriguingly, during MIS 4 and MIS 2 the isotopic composition of the oceans was more radiogenic than during much of MIS 3. Our record is relatively stable with ¹⁸⁷Os/¹⁸⁸Os ratio of ~1.04 during MIS 5, 4 and 2, but shows significant variability during MIS 3: at 56 ka the ¹⁸⁷Os/¹⁸⁸Os rises from 1.04 to the modern seawater composition of 1.06. It drops to a ratio of 1.0 at 40 ka and then rises to 1.04 at 36 ka. We also observe a rise in the Os isotopic composition from ~1.04 in MIS2 to ~1.06 in the most recent Holocene samples, a magnitude of change consistent with that observed by Oxburgh (1998). However, our record shows a much more recent transition to the modern seawater composition, a transition continuing into the present. It further suggests that Os is not in steady state and may be subjected to climate forcing. Indeed, Lomb-Scargle Spectrum analysis of the record shows the presence of 27 ka and 18 ka wavelengths, which have normalized spectral powers of 4 and 3.5, respectively. Further analyses are underway to understand the cause of this variation and to obtain a more complete record for the Holocene.