186-13 From the Pacific to the Tethys: Refining Global Chronostratigraphy of Ocean Anoxic Event 1a (OAE1a) with a Radioisotopically Anchored Astrochronology

Abstract:

Various investigations have produced competing proposals for the age, duration, and supposed drivers of Ocean Anoxic Event 1a (OAE1a). The variance amongst these works are in large part driven by the scarcity of in-situ radioisotopic dates for the event, as well as debate concerning the relative age of Ontong Java Plateau (OJP) volcanism. Million-year differences among age models has led to discrepancies in the interpretation of driving mechanisms, although emplacement of the OJP is commonly cited as the most likely causal mechanism. This hypothesis is supported by marked global shifts to unradiogenic osmium and negative carbon isotope values that define OAE1a onset.

            In an effort to address the geochronologic disparities associated with OAE1a, we have developed a new astrochronologic model from a core record of OAE1a recovered 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. Numerous detailed astrochronologies for this time period exists, but our methodology leverages the robust radioisotope geochronology developed by Li et al. (2024), which measured 19 U-Pb dates in a series of rhyolitic tuffs intercalated within mudstones of the Soashibetsu-gawa Formation in Hokkaido, Japan.

            Here, we (1) summarize the geochronology constructed from Hokkaido which has yielded age assignments for the global d¹³C segment boundary chemostratigraphy developed for OAE1a in prior studies; (2) Utilize both d¹³C and Os records to identify selected tie-points in the coeval LS record and present the new “floating” astrochronology for the LS core (constructed using the Astrochron software package); and (3) Discuss the geospatial variability among d¹³C and Os records during this interval, which preserves a distinct shift when comparing Hokkaido to the various Tethys records. Our age-calibrated chemostratigraphic tie-points allow the floating age model to be anchored in geologic time. This work, which refines the time scale for OAE1a, supports the OJP hypothesis for OAE1a initiation by placing the age of onset at 119.53 +0.05/-0.06 Ma.

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