186-8 Insights on Ocean-Climate Response to LIP Volcanism during OAE1a and OAE2 Through the Lens of Ca and Os Isotopes

Abstract:

Extensive Large Igneous Province (LIP) volcanism during the Cretaceous ‘greenhouse’ caused elevated atmospheric CO₂ levels, warm climates, weakened ocean circulation, and widespread ocean deoxygenation. LIP-triggered short-term global carbon (C) cycle perturbations termed Oceanic Anoxic Events (OAEs) show characteristic carbon isotope (δ¹³C) excursions and have been identified by widespread deposition of black shales enriched in organic carbon (>1 wt.%) in the rock record. The Early Aptian OAE1a (Selli Event, ~119.5Ma) and Cenomanian-Turonian OAE2 (Bonarelli Event, ~94Ma) are the two most significant and extensive OAEs. In this study, we synthesize new δ¹³C_bulk, calcium isotope (δ^44/40Ca), and osmium isotope (Os_i) records for OAE1a from the Les Sauzeries core (Vocontian Basin, SE France) and for OAE2 from south-central Mexico with existing δ¹³C, δ^44/40Ca, and Os_i records for these events from other locations. Collectively, the new dataset represents multiple globally distributed sites (e.g., Western Interior Seaway, Tethys, and proto-Atlantic), varied depositional settings, and different carbonate archives. 

The sites are chemostratigraphically correlated, with confirmation from published biostratigraphy where available. The Os isotope records support assertions that CO₂ emissions during LIP eruptions caused ocean acidification (OA), which is expected to reduce carbonate production and increase δ^44/40Ca values. Accordingly, the δ^44/40Ca records for OAE1a and OAE2 resolve distinctive negative-to-positive shifts that begin before and peak after abrupt excursions to unradiogenic Os_i values, which precede the δ¹³C-defined event onset. Several features of the δ^44/40Ca records are consistent with the high-fidelity preservation of primary kinetic isotope effects. Observations include: 1) different δ^44/40Ca records for OAE1a and OAE2 show similar stratigraphic signals that oscillate around different absolute values, despite large separations in space and time; 2) δ^44/40Ca and Os_i records for OAE2 exhibit synchronous precursor shifts, even though t_Ca = 10²-10³´t_Os; and 3) δ^44/40Ca values for R. cushmani increase as tests become smaller and malformed leading into OAE2. We interpret these δ^44/40Ca records to reflect similar responses of the carbonate factory to changing ocean chemistry driven by LIP volcanism during both OAE1a and OAE2. 

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