Assessing the contribution of the La Luna Sea to the global sink of organic carbon during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2)

Abstract

Epicontinental seas were important features of the paleogeographic landscape during the Cretaceous; however, the role they played as sinks of organic carbon is still poorly understood. The La Luna Formation (Albian-Coniacian) is a series of organic-rich limestones deposited in northwestern South America on an epicontinental sea (the La Luna Sea). This formation offers a forty-million-year continuous record of environmental change characterized by periods of oceanic anoxia in an epicontinental sea. The La Luna Sea, may have played an important role –although so far unexplored– in carbon cycling through the ocean during the Cretaceous, specifically during short-term, global-scale disruptions in the carbon cycle known as oceanic anoxic events (OAEs). To evaluate the role of the La Luna Sea in global carbon cycle perturbations, we conducted a detailed lithological and chemostratigraphic analysis of two stratigraphic sections from the Upper Magdalena Basin of Colombia, both of which encompass the Oceanic Anoxic Event 2 (OAE2) at the Cenomanian–Turonian boundary. Compared to deposits in the modern ocean, the La Luna Formation has high total organic carbon (TOC) before, during, and after OAE2. Foraminifera and nannoplankton assemblages also imply a stressed upper water column during OAE2. Geochemical and paleontological evidence suggests that the sediment-water interface was anoxic across the late Cenomanian and early Turonian. Strata deposited just after OAE2, however, contain inoceramid bivalves, consistent with short-lived re‑oxygenation of the benthic layer. Estimates of primary productivity, the covariation of Mo and U enrichment factors, and relations between Cd, Mo, Co, and Mn also reveal that the La Luna Sea was biogeochemically similar to the modern Cariaco Basin. Despite high concentrations of organic carbon found in the La Luna Formation, mass-accumulation rates of organic carbon are low, a finding that can be explained by a reduction in the accumulation rate of sediments caused by the peak of sea-level transgression that took place at the Cenomanian–Turonian transition. Based on the areal extent of the La Luna Sea and mass-accumulation rates of organic carbon, 1.7 Eg of C were removed from the ocean over 500 ky and deposited in the La Luna Sea. Interestingly, although the La Luna Sea was one-third the size of the Western Interior Seaway (WIS), the amount of organic carbon buried in the WIS during OAE2 was similar (1.4 Eg of C). In these two epicontinental seas, 3.1 Eg of C were removed from the ocean during OAE2, accounting only for 3.4% of the total C needed to cause a perturbation of the carbon cycle similar to that observed during OAE2. The low amount of organic carbon buried in the La Luna Sea and the WIS suggests that neither of these inland seas were responsible for the efficient removal of organic carbon from the ocean during OAE2. This conclusion challenges the explanation that epicontinental seas were major sinks of organic carbon—and therefore they did not play a significant role in the carbon cycle during the Mesozoic OAEs and other disruptions of the carbon cycle in Earth's history.