Volcanically Initiated Shoaling of the Marine Calcite Compensation Depth during Oceanic Anoxic Event 2 ( 94 Ma)

Anomalous large igneous province (LIP) volcanism at the onset of Cretaceous Oceanic Anoxic Event 2 (~94 Ma; OAE2) triggered a well-studied episode of widespread marine anoxia and increased turnover of marine faunas. Following recent Ca isotope evidence¹, it is hypothesized that the CO₂ degassing from LIP sources may have altered seawater carbonate chemistry, creating knock-on effects to marine ecosystems, sedimentation, and geochemical cycling.

Here using a global compilation of carbonate content records from deep-sea sediment cores and epicontinental basins, we investigate carbonate sedimentation trends through OAE2. The carbonate content compilation resolves a shoaling of the calcite compensation depth (CCD) at pelagic sites. Based on the globally recognized unradiogenic initial osmium isotope (Os_i) excursion preceding OAE2 being LIP forced, a simple box model estimates that the volcanic Os flux increased by orders of magnitude above background Cretaceous levels within thousands of years. This finding is consistent with a shoaling of the CCD driven by rapid CO₂ addition to the ocean-atmosphere system.

New cores from IODP Expedition 369 in the Mentelle Basin of Australia (Site U1516) further exhibit clear evidence for suppressed carbonate sedimentation through a relatively conformable pelagic succession of OAE2. Chemostratigraphy based on Os_i, cyclostratigraphy, and inorganic geochemistry from Site U1516 constrain the timing and biogeochemical feedbacks associated with the volcanically initiated CCD shoaling. This decrease in marine calcite saturation, though apparently not severe enough to cause sustained dissolution in surface waters, likely adversely impacted calcifying taxa in an ocean also experiencing lower dissolved oxygen levels and thermal stress. Ocean acidification, expressed via a shoaling CCD, was a significant component of the paleoceanographic response to LIP volcanism at the onset of OAE2, along with the well-documented net expansion of bottom water anoxia, and should be considered a primary stress on the marine realm at that time.

¹Du Vivier et al., EPSL, v. 416 (2015) 121