OSMIUM ISOTOPE STRATIGRAPHY OF PROTO-PACIFIC AND SOUTH ATLANTIC SECTIONS FROM THE CENOMANIAN-TURONIAN BOUNDARY INTERVAL REVEAL THE GLOBAL EXTENT OF THE OAE 2

The oceanic anoxic event (OAE) 2 is considered to be a global phenomenon. However, hitherto multiple studies have focussed only on sections from the North Atlantic, Western Tethys and Western Interior Seaway (WIS). The mechanism(s) responsible for anoxia are being continually debated: global warming, volcanism, productivity and rate of ocean circulation. Here we investigate two Pacific sections; Hokkaido, Japan and Great Valley, California; and South Atlantic IODP 530A, utilizing high-resolution ¹⁸⁷Os/¹⁸⁸Os initial isotope stratigraphy (IOs) to assess global seawater composition prior to, during and post OAE2. We demonstrate that IOs profiles for Hokkaido and 530A are comparable to those of previously analysed OAE2 sections. In contrast to increasing radiogenic IOs in WIS prior to the onset of OAE2, values from Hokkaido are very consistent (IOs ~0.7), and at 530A IOs values are less radiogenic and show moderate variability, similar to ODP Site 1260B. The lower radiogenic values from 530A would imply that the South Atlantic Ocean was influenced by the mixing of weathered continental and juvenile oceanic crust simultaneously, which is likely to be associated with basaltic volcanism associated with CAMP. The IOs profile before and during OAE2 for Great Valley, California is considerably more variable and despite recording unradiogenic values in the syn OAE2 interval, there are continued fluctuations to radiogenic IOs.

The nominal variability in individual IOs values infers that regional factors, such as basin connectivity and/ or predominantly continental run-off, are controlling the IOs values. All sites show a time correlative shift to unradiogenic IOs values of ~0.2 from ~50 Kyrs prior to OAE2 and return to radiogenic values (>0.4) ~700 Kyrs after the start of OAE2. The homogeneity of the unradiogenic IOs is associated with a large volcanic pulse from the Caribbean Large Igneous Province (CLIP). Therefore analyses of these sites would suggest that: a) the radiogenic IOs associated with the influx of nutrients derived from continental weathering to the ocean was driving productivity prior to the onset of OAE2; b) circulation was efficient to record globally contemporaneous unradiogenic Os within the residence time of Os (≤10 Kyrs); and c) volcanism was of sufficient magnitude to influence global ocean IOs values.