Mathematics – Logic
Scientific paper
Jul 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998e%26psl.160..163r&link_type=abstract
Earth and Planetary Science Letters, vol. 160, Issue 1-2, pp.163-178
Mathematics
Logic
13
Osmium, Sea Water, Weathering, Global Change, Miocene
Scientific paper
Seawater 187 Os/ 188 Os ratios for the Middle Miocene were reconstructed by measuring the 187 Os/ 188 Os ratios of metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific 187 Os/ 188 Os measurements are nearly indistinguishable and are consistent with previously published Os isotope records from Pacific cores. The Atlantic data reported here provide the first direct evidence that the long-term sedimentary 187 Os/ 188 Os record reflects whole-ocean changes in the Os isotopic composition of seawater. The Pacific and the Atlantic Os measurements confirm a long-term 0.01/Myr increase in marine 187 Os/ 188 Os ratios that began no later than 16 Ma. The beginning of the Os isotopic increase coincided with a decrease in the rate of increase of marine 87 Sr/ 86 Sr ratios at 16 Ma. A large increase of 1 in benthic foraminiferal 18 O values, interpreted to reflect global cooling and ice sheet growth, began approximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate 13 C values began more than 2 Myr later around 13.6 Ma. The post-16 Ma increase in marine 187 Os/ 188 Os ratios was most likely forced by weathering of radiogenic materials, either old sediments or sialic crust with a sedimentary protolith. We consider two possible Miocene-specific geologic events that can account for both this increase in marine 187 Os/ 188 Os ratios and also nearly constant 87 Sr/ 86 Sr ratios: (1) the first glacial erosion of sediment-covered cratons in the Northern Hemisphere; (2) the exhumation of the Australian passive margin-New Guinea arc system. The latter event offers a mechanism, via enhanced availability of soluble Ca and Mg silicates in the arc terrane, for the maintenance of assumed low CO 2 levels after 15 Ma. The temporal resolution (three samples/Myr) of the 187 Os/ 188 Os record from Site 598, for which a stable isotope stratigraphy was also constructed, is significantly higher than that of previously published records. These high resolution data suggest oscillations with amplitudes of 0.01 to 0.02 and periods of around 1 Myr. Although variations in the 187 Os/ 188 Os record of this magnitude can be easily resolved analytically, this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent. However, the short-term 187 Os/ 188 Os variations may correlate inversely with short-term benthic foraminiferal 18 O and bulk carbonate 13 C variations that reflect glacioeustatic events.
Maasch Kirk A.
Ravizza Greg
Reusch Douglas N.
Wright Douglas J.
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