The role of pCO2 on climate and biogenic calcite production during the Aptian

The late Barremian - early Aptian time interval was marked by intense volcanic activity related to the Ontong Java Plateau (OJP) thought to have induced a global perturbation in the ocean-atmosphere system culminating in the early Aptian Oceanic Anoxic Event (OAE) 1a, an episode of widespread organic matter burial in oxygen-depleted oceans. This extreme event represents a “natural experiment” useful to decipher the ecosystem response to large injection of CO2, relevant for the understanding of future global climatic changes, the biota response and how pre-perturbation conditions can be restored. Several studies have been conducted on OAE 1a providing evidence for a progressive reduction in the production of biogenic carbonate accompanied by higher surface-water fertility and major warming interrupted by transient cooler interludes. In our study, we aimed at a better understanding of the role of pCO2 on the climatic variability during OAE 1a as well as on the production of calcite from calcareous nannoplankton. In addition, we intended to understand how, either the biota and the climate, recovered after OAE 1a and which conditions were imposed during the middle-late Aptian. We present a reconstruction of surface water paleotemperature, fertility and calcite paleofluxes through the Aptian (~121 to ~113 Ma) based on the calcareous nannofossil record from Italy (Cismon and Piobbico cores) and Pacific Ocean (DSDP Site 463). The nannofossil data were correlated with temperature proxies (oxygen isotopes, TEX86) and trace metal concentrations. In the studied sites we identified similar and coheval variations in nannofossil calcite paleofluxes as well as in the abundance of temperature and fertility indicators, which well correlate with the geochemical data, thus suggesting global drivers inducing these variations. We found evidence for a direct connection between OJP activity, CO2 emissions and global warming in the early phase of OAE 1a accompanied by a decrease in calcification rates. Cooling events occuring during OAE 1a, were probably promoted by temporary CO2 sequestration due to burial of organic matter although under persisting OJP volcanism. The end of OAE 1a coincides with the vanishing of OJP activity and a marked cooling accompanied by a relative recovery in carbonate fluxes. The late Aptian was probably marked by at least two main volcanic phases related to construction of the Hikurangi and Kerguelen Plateaus, affecting biogenic calcite production but promoting cooler temperatures. A stasis in volcanism allowed a recovery of paleofluxes during the Nannoconus truittii acme, although paleofluxes never reached pre-OAE 1a values. We conclude that, during the Aptian, abnormal volcanic activity directly affected the climate and the ocean at global scale. Coccolithophores were forced to face excess CO2, climatic changes and variations in surface water fertility and acidification, but survived and adapted to extreme paleoenvironmental conditions.