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

Abstract

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–Osi) 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.The YG shows an almost identical Osi 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; Du Vivier et al., 2014). The characteristics of the Osi profile are radiogenic and heterogeneous (∼0.55–0.85) prior to the OAE 2, and synchronous with the inferred OAE 2 onset the Osi abruptly become unradiogenic and remain relatively homogeneous (∼0.20–0.30) before showing a gradual return to more radiogenic Osi(∼0.70) throughout the middle to late OAE 2. A206Pb/238U zircon age of an interbedded tuff (HK017) in the adjacent horizon to the first unradiogenic Osi value constrains the age of the Osi inflection at 94.44±0.14 Ma. This age, including uncertainty, agrees with the interpolated age of the same point in the Osi profile (94.28±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 Osi 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 Ma and 94.66±0.25 Ma for this Osi inflection in the YG and WIS, respectively, indicate that palaeocirculation was sufficient to simultaneously influence transbasinal seawater chemistry. In contrast, the pre-OAE 2 Osi 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 Osi values (0.30–0.95) to record a palaeobasin that was regionally influenced interchangeably by both unradiogenic (hydrothermal flux) and radiogenic (continental flux) Os.We conclude that the Osi 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 Osi profiles coupled with U–Pb geochronology facilitate the correlation of OAE 2 stratigraphy, and demonstrate both regional and global ocean dynamics.