REFINING GLOBAL CHRONOSTRATIGRAPHY OF OCEAN ANOXIC EVENT 1A (OAE1A): CHEMOSTRATIGRAPHIC CORRELATION OF HIGH-PRECISION RADIOISOTOPIC AGES ANCHOR AN ASTROCHRONOLOGIC RECORD

A range of estimates for the age, duration, and mechanistic drivers of Ocean Anoxic Event 1a (OAE1a) have been proposed, largely reflecting scarcity of in-situ radioisotopic dates within the event, as well as debate concerning the relative age of Ontong Java Plateau (OJP) volcanism. Differences approaching 5 myrs exist among proposed age-models, which has led to discrepancies in the interpretation of driving mechanisms. Emplacement of the OJP is commonly cited as the most likely causal mechanism, a hypothesis supported by marked global shifts to unradiogenic osmium and negative carbon isotope values that define OAE1a onset. However, recent ⁴⁰Ar/³⁹Ar age data has challenged the OJP hypothesis.

To address the geochronologic disparities associated with OAE1a, we focus on two objectives: 1) developing a new high-precision Aptian geochronologic framework by dating rhyolitic tuffs from the mudstone and ash-rich Aptian succession of Hokkaido, Japan. Both C and Os isotope profiles, supported by biostratigraphy, define the event across which 19 new high-precision CA-IDTIMS U-Pb zircon ages have been determined; and 2) employing δ¹³C and Os chemostratigraphy to export our new radioisotopic age model to a core record of OAE1a from the Vocontian Basin, France (Les Sauzeries core or “LS”), where hemipelagic deposits preserve sedimentary oscillations that prior studies have shown to be modulated by astronomically-controlled climate cycles.

Here, we summarize the new geochronology, which has yielded age assignments for the global δ¹³C segment boundary chemostratigraphy developed for OAE1a in prior studies. We utilize both δ¹³C and Os records to identify selected tie-points in the coeval LS record. Astrochronologic testing and calibration of selected elemental and stratigraphic data (such as Ca and Mn analyzed with both XRF and ICP methods), was conducted using the Astrochron software package, resulting in the construction of a “floating” astrochronology for the core. Age-calibrated chemostratigraphic tie-points allow the floating age model to be anchored in geologic