Climatic and ecological changes during the Aptian traced by calcareous nannofossils and oxygen isotopes

The Lower Cretaceous was marked by the emplacement of large igneous provinces (LIPs) that formed gigantic oceanic plateaus, affecting ecosystems at global scale with biota forced to face excess CO2, climate and global perturbations in the ocean-atmosphere system. In particular, the Ontong Java Plateau (OJP) volcanism was coeval with an episode of oxygen depletion in the oceans with consequent burial of massive amounts of organic matter coinciding with the Early Aptian Oceanic Anoxic Event (OAE)1a. Several studies have been conducted to reconstruct temperature variations across the Aptian and especially during OAE 1a. There is a general consensus about a major warming characterizing OAE 1a although some authors have provided evidence for transient cooling interludes. The climatic conditions for the Late Aptian are, instead, less constrained, and a complete record is not available yet. Here we present a reconstruction of surface water paleotemperature and fertility based on calcareous nannofossil record from the Tethys and Pacific Ocean. The data, integrated with oxygen-isotope records and dataset from other latitudes, provide a detailed picture of climatic and ecological changes during the Aptian, showing variations never highlighted before. The dataset collected indicate that a maximum warming characterized the early phase of anoxia under intense volcanic activity of OJP. A short lived (~35ky) cooling, following a rapid increase of weathering rates, interrupted the major warming. A generally warm and humid climate was probably the promoter of continental run off and consequent nutrient supply to the oceans. Peaks in trace metal concentrations suggest that biolimiting metals might have additionally fertilized the oceans. Nannofossils are suggestive of meso- to eutrophic conditions reached under the highest temperature and the maximum of OJP volcanism. The rest of OAE 1a was characterized by subsequent cooling events, probably promoted by CO2 sequestration during burial of organic matter. In this phase, high productivity was probably maintained by N2 –fixing cyanobacteria while nannofossil indicators of high fertility were rare. The end of anoxia coincides with the vanishing of volcanism and a pronounced cooling. The mid-Aptian was characterized by high surface water fertility and progressively decreasing temperatures, probably suggestive of intense continental weathering, supplying nutrients to the oceans and maintaining low pCO2. The lowest temperatures combined with low fertility, were reached in the Late Aptian across the interval characterized by blooming of Nannoconus truittii. A stasis in volcanism and less humid climate are proposed for this interval. The data presented indicate that OJP activity played a direct role in inducing global warming during the Early Aptian, whereas other mechanisms acted as feedback processes, favoring temporary cold interludes, although under persisting volcanism and eventually leading to the end of anoxia.