Christina E. Keller

A volcanic CO2 pulse triggered the Cretaceous Oceanic Anoxic Event 1a and a biocalcification crisis

The Aptian Oceanic Anoxic Event 1a (OAE1a, ca.120 Ma ago) is one of the most prominent of a series of geologically brief intervals in the Cretaceous characterized by the deposition of organic carbon–rich sediments. OAEs reflect major perturbations in the global carbon cycle evidenced by sedimentary carbon isotope records. However, the triggering mechanisms for OAEs remain controversial. Here we present a bulk-rock and molecular (marine and terrestrial bio-markers) C isotope record at unprecedented time resolution, from the Cismon section of northern Italy, that shows that OAE1a conditions were reached over a period of several thousands of years through a stepwise perturbation of the carbon cycle. The documented sequence of events is most compatible with a trigger associated with increased CO 2 emissions, possibly leading to a doubling of p CO 2 , which in turn caused larger C isotope fractionation in marine and terrestrial organisms and a major biotic crisis in the calcareous nannoplankton. Our data also show that a release of isotopically light carbon from partial methane hydrate dissociation probably played a minor role in the OAE1a carbon cycle perturbation.

Calcareous Nannoplankton Response to Surface-Water Acidification Around Oceanic Anoxic Event 1a

Acidification of the Ancient Oceans Ocean acidification fueled by rising levels of atmospheric CO2 is likely to become a major challenge for ocean ecosystems. Understanding how marine biota responded to similar events in Earths history may provide clues as to what to expect—and what to prevent—in the future. To this end, Erba et al. (p. 428) present a detailed stratigraphic and geochemical characterization of 120-million-year-old marine sediments from a time when the oceans acidified because of a massive outgassing of volcanic CO2. Microscopic fossils in the sediments, such as calcareous nannoplankton, show evidence of having responded to this major disruption through species-specific adaptations like deforming and shrinking their cells. These changes allowed these abundant and diverse organisms to avoid extinction, even through a subsequent global depletion of ocean oxygen levels. Rather than going extinct, calcareous plankton adapted to ocean acidification ~120 million years ago. Ocean acidification induced by atmospheric CO2 may be a major threat to marine ecosystems, particularly to calcareous nannoplankton. We show that, during the Aptian (~120 million years ago) Oceanic Anoxic Event 1a, which resulted from a massive addition of volcanic CO2, the morphological features of calcareous nannofossils traced the biological response to acidified surface waters. We observe the demise of heavily calcified nannoconids and reduced calcite paleofluxes at the beginning of a pre-anoxia calcification crisis. Ephemeral coccolith dwarfism and malformation represent species-specific adjustments to survive lower pH, whereas later, abundance peaks indicate intermittent alkalinity recovery. Deepwater acidification occurred with a delay of 25,000 to 30,000 years. After the dissolution climax, nannoplankton and carbonate recovery developed over ~160,000 years under persisting global dysoxia-anoxia.

Orbital control on carbon cycle and oceanography in the mid‐Cretaceous greenhouse

We established a new high-resolution carbonate carbon isotope record of the Albian interval of the Marne a Fucoidi Formation (Central Apennines, Italy), which was deposited on the southern margin of the western Tethys Ocean. Bulk carbonate sampled with 10–15 cm spacing was used for the construction of a continuous carbon isotope curve through the Albian stage. Spectral analyses reveal prominent 400 kyr cyclicity in the δ13C curve, which correlates with Milankovitch long eccentricity changes. Cycles occurring in our record resemble those observed in several Cenozoic δ13C records, suggesting that a link between orbital forcing and carbon cycling existed also under mid-Cretaceous greenhouse conditions. Based on comparisons with Cenozoic eccentricity-carbon cycle links we hypothesize that 400 kyr cycles in the mid-Cretaceous were related to a fluctuating monsoonal regime, coupled with an unstable oceanic structure, which made the oceanic carbon reservoir sensitive to orbital variations. In the Tethys these oceanographic conditions lasted until the Late Albian, and then were replaced by a more stable circulation mode, less sensitive to orbital forcing.