Pacific 187 Os/188 Os isotope chemistry  and U–Pb geochronology: synchroneity of global Os isotope change across OAE 2

Studies of OAE 2 sections beyond the Atlantic Ocean, Western Interior Seaway (WIS) and European pelagic shelf are limited. Here, we present initial osmium isotope stratigraphy (187Os/188Os–OsiOsi) from two proto-Pacific sites that span the Cenomanian–Turonian boundary interval (CTBI): the Yezo Group (YG) section, Hokkaido, Japan, and the Great Valley Sequence (GVS), California, USA; to evaluate the 187Os/188Os seawater chemistry of the proto-Pacific. Additionally we combine new 206Pb/238U zircon CA-ID-TIMS geochronology from five volcanic tuff horizons of the Yezo Group section to test and facilitate inter-basinal integration with the WIS using radio-isotopically constrained age–depth models for both sections, and quantitatively constrain the absolute timing and duration of events across the CTBI.\ud
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The YG shows an almost identical OsiOsi profile to that of the WIS, and very similar to that of other sites of the proto-Atlantic and European pelagic oceans (Turgeon and Creaser, 2008 and Du Vivier et al., 2014). The characteristics of the OsiOsi profile are radiogenic and heterogeneous (∼0.55–0.85) prior to the OAE 2, and synchronous with the inferred OAE 2 onset the OsiOsi abruptly become unradiogenic and remain relatively homogeneous (∼0.20–0.30) before showing a gradual return to more radiogenic View the MathML sourceOsi(∼0.70) throughout the middle to late OAE 2. A206A206Pb/238U zircon age of an interbedded tuff (HK017) in the adjacent horizon to the first unradiogenic OsiOsi value constrains the age of the OsiOsi inflection at 94.44±0.14 Ma94.44±0.14 Ma. This age, including uncertainty, agrees with the interpolated age of the same point in the OsiOsi profile (94.28±0.25 Ma94.28±0.25 Ma) in the only other dated OAE 2 section, the WIS; indicating a coeval shift in seawater chemistry associated with volcanism at the OAE 2 onset at the levels of temporal resolution (ca. 0.1 Myr). Further, prior to the onset of OAE 2 an enhanced radiogenic inflection in the OsiOsi profile of the YG is correlative, within uncertainty, with a similar trend in the WIS based on the U–Pb age–depth model. The interpolated ages, 94.78±0.12 Ma94.78±0.12 Ma and 94.66±0.25 Ma94.66±0.25 Ma for this OsiOsi inflection in the YG and WIS, respectively, indicate that palaeocirculation was sufficient to simultaneously influence transbasinal seawater chemistry. In contrast, the pre-OAE 2 OsiOsi profile for the GVS is disparate to that of the YG and those of the proto-Atlantic and European pelagic shelf locations. We interpret the pre OAE 2 heterogeneous OsiOsi values (0.30–0.95) to record a palaeobasin that was regionally influenced interchangeably by both unradiogenic (hydrothermal flux) and radiogenic (continental flux) Os.\ud
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We conclude that the OsiOsi profiles from the proto-Pacific sections record both trends that are consistent globally (OAE 2 onset, syn and post OAE 2), but also show regional differences (pre OAE 2) between OAE 2 sections worldwide. As such the OsiOsi profiles coupled with U–Pb geochronology facilitate the correlation of OAE 2 stratigraphy, and demonstrate both regional and global ocean dynamics