Seawater Os heterogeneity during the OAE 2 and across the Cenomanian-Turonian Boundary: Implications for global ocean paleocirculation

Alice D.C. Du Vivier1, David Selby1, Darren R. Gröcke1, Bradley B. Sageman2, Silke Voigt3 1Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK 2Department of Geological Sciences, Northwestern University, Evanston, Illinois 60208, USA 3Leibniz Institute of Marine Sciences (IFM-Geomar), Wischhofstrasse 1-3, 24184 Kiel, Germany Organic-rich sediments (ORS) from two OAE 2 sites (ODP Site 1260B and Furlo) show significant enrichment in Os (>1 ppb) and possess mantle-like initial 187Os/188Os (IOs, ~0.13) prior to the onset and during the lower part of the OAE 2 [1]. However, these sites show significantly different IOs profiles for the upper part of the OAE 2, suggesting Os heterogeneity within the global ocean. We use new IOs data from correlative sections based on chemostratigraphy and biostratigraphy (Rock Canyon, Pueblo USA and Wunstorf, Germany) to evaluate the evolution of global seawater IOs during OAE 2. Wunstorf and Rock Canyon OAE 2 sections are the European type and CTB GSSP sections, respectively. These sections have average Os abundances that are an order of magnitude lower than that from Furlo and Site 1260B (410 and 245 pg/g vs. 3630 and 2575 pg/g, respectively). Redox is important for Os capture in ORS, but not a controlling factor for Os uptake. The significant enrichment in Os in ORS from Furlo and Site 1260B may relate to the slow sedimentation rate of the ORS (~ 0.2 cm/kyrs vs ~ 1.8 cm/kyrs and ~ 0.5cm/kyrs at Wunstorf and Pueblo), thus permitting a greater contact time for Os uptake. Spikes in the ORS Os abundance coincide with unradiogenic IOs (0.13 to 0.30). Our high-resolution IOs coupled with that of site 1260B and Furlo [1] show distinct variations in IOs throughout the OAE 2. This suggests that the 187Os/188Os ratio in the global ocean was not homogeneous. During the peak of OAE 2 the IOs at Furlo and Site 1260B are more unradiogenic (0.13 and 0.15) than that recorded at Wunstorf, 0.18 and Pueblo, ~ 0.2. The IOs profile from Wunstorf and Pueblo show a similar gradual return to radiogenic IOs towards the CTB, ~300 kyrs after the onset of OAE 2, which is very similar to that from Furlo. However, Site 1260B remains unradiogenic (IOs = 0.13 to 0.30) and rapidly returns to more radiogenic values (~0.5) at the end of OAE 2. The unradiogenic IOs coincide with the onset of the Caribbean LIP. However, the extended duration of unradiogenic IOs at Site 1260B in comparison to Pueblo, Wunstorf and Furlo, may relate to its proximity to the LIP. Additionally, the variation in IOs between the sites at the onset of OAE 2 questions the homogenous mixing of the global ocean with respect to Os. We suggest that the heterogeneity of global seawater Os is the result of variations in global ocean circulation, as a result of latitudinal climate belts and paleogeography. Furthermore, if the LIP contemporaneously affected the global seawater Os isotope composition, which is suggested by the IOs, then the onset of magmatism was ~94.4 Ma, almost identical to the 40Ar/39Ar dating, which has a 1-2 myr uncertainty, of the Caribbean LIP. Moreover, the IOs data suggests that the effect of the Caribbean LIP on the global ocean was on the order of only 300 to 800 kyrs. [1] Turgeon & Creaser, 2009, Nature, 454, 323-326