Marine Anoxia Promoted Carbonate Production During the Oae1a

Understanding the mechanisms driving carbonate platform evolution during major oceanic anoxic events is critical for reconstructing Cretaceous carbon cycle dynamics. During the Cretaceous, the oceans experienced multiple episodes of anoxia, often accompanied by large-scale drowning of shallow-water carbonate platforms. The Oceanic Anoxic Event 1a (OAE1a), which occurred in the mid-Cretaceous, represents one of the most prominent global paleoceanographic perturbations. Unlike other episodes, carbonate platform drowning during OAE1a was mainly restricted to parts of the northern Tethyan margin. Recent studies indicate that drowning in these regions predated OAE1a, suggesting that carbonate production on most platforms was not significantly disrupted during the event. Intriguingly, stable strontium isotope data from OAE1a carbonate deposits indicate elevated global carbonate productivity during the event, followed by a pronounced decline afterwards. To investigate the influence of marine anoxia on carbonate production, we compiled 54 carbonate-dominated successions to estimate carbonate accumulation rates during OAE1a. We also conducted uranium isotope analyses of OAE1a strata from isolated carbonate platforms in southern Iran and the Pacific to quantitatively assess global ocean redox changes. Our results show that (1) carbonate accumulation rates increased in most sections during OAE1a, and (2) the extent of oceanic anoxia increased by approximately 16-fold during the event, coinciding with a marked acceleration in carbonate accumulation. We propose that intensified anoxic conditions during OAE1a enhanced anaerobic oxidation of organic matter, thereby increasing seawater alkalinity and sustaining high rates of carbonate deposition. In contrast, post-event improvement in redox conditions reduced alkalinity, lowering carbonate productivity and leading to platform drowning in certain regions.