Miocene seawater 187Os/188Os ratios inferred from metalliferous carbonates

Abstract

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 Ž 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 Ž 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 Ž 18 O and bulk carbonate Ž 13 C variations that reflect glacioeustatic events.

Neogene marine isotopic evolution and the erosion of Lesser Himalayan strata: Implications for Cenozoic tectonic history

“…Based on discussions The strontium isotope data can similarly be fit with two linear segments, from 22.2 to 15.8 (dR/dt = 6.86 × 10 −5 Ma −1 ) and from 15.5 to 9.2 Ma (dR/dt = 2.67 × 10 −5 Ma −1 ). Data sources: Ravizza (1993), Oslick et al (1994), Peucker-Ehrenbrink et al (1995, and Reusch et al (1998). above, we postulate that initial uplift and exhumation of upper Lesser Himalayan strata (possibly the oLH) took place at ∼16 Ma, and that it is recorded in marked inflections in the marine Os and Sr records at that time. Decreased erosion of the GH and increased erosion of younger oLH strata could have plausibly driven the observed changes in the Os and Sr paleoseawater curves beginning near 16 Ma, because radiogenic Os is concentrated in organic rich shale that accumulated in proximal parts of the Neoproterozoic and Cambrian margin (e.g., Singh et al, 1999), and radiogenic Sr is concentrated in the highgrade rocks of the GH (France-Lanord et al, 1993;Ahmad et al, 2000).…”

“…Here, we present a combined tectonic-geochemical model that links earlier uplift of the LH to the Neogene paleoseawater record of 187 Os/ 188 Os and 87 Sr/ 86 Sr (Ravizza, 1993;Oslick et al, 1994;Peucker-Ehrenbrink et al, 1995;and Reusch et al, 1998). The 187 Os/ 188 Os record shows a period of stability from about 29 Ma to 16 Ma (Fig.…”

Precise Re–Os ages of organic-rich mudrocks and the Os isotope composition of Jurassic seawater

“…It is now well-established that the Os isotope composition of seawater, expressed as 187 Os/ 188 Os, has varied during the Cenozoic in a manner analogous to that for Sr [5][6][7][8][9]. These variations have been attributed to processes similar to those responsible for Cohen et al, accepted EPSL 20 January 1999 4 the increase in the 87 Sr/ 86 Sr ratio of seawater over the same period, and reflect the changing balance between the major inputs to the oceans from continental weathering, carbonate dissolution, and hydrothermal activity at mid-ocean ridges.…”

“…The Os isotope record for Cenozoic seawater has been recovered from a number of marine materials, including deep-sea pelagic sediments [5], metalliferous coatings on carbonate-rich sediments [6,7,9], and hydrogenous Fe-Mn crusts [8]. However, significant discrepancies exist between Os isotope records obtained from different materials (e.g.…”

Re-Os dating of the Kuroko ores from the Wanibuchi Mine, Shimane Prefecture, southwestern Japan.

“…It is now well established that the Os isotopic compositions of seawater has changed considerably through the last 60 Ma (Peucker-Ehrenbrink et al, 1995;Reusch et al, 1998). Miocene metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) have 187 Os/ 188 Os that range from about 0.73 to 0.78 (Reusch et al, 1998), and the 187 Os/ 188 Os ratios for the middle Miocene metalliferous sediments from the Pacific are 0.732 and 0.847 (PeuckerEhrenbrink et al, 1995). These studies suggest that the 187 Os/…”