Early Danian carbon cycle perturbation: calcareous nannofossil and geochemical response at Blake Nose region (ODP Site 1049C), North Atlantic

Abstract

Short-lived (104-105 years) carbon isotope excursions (CIEs), many of which are associated with some degree of ocean warming, are a feature of the warm climate state of the early Paleogene. The first of these Paleogene carbon cycle perturbations (DAN-C2 event: 65.80 – 65.70 Ma) has been recognized in deep-ocean sediment cores recovered from a number of Atlantic Ocean and Thethys locations. Although many studies have reported benthic foraminifera assemblage changes across this interval, studies of the calcareous nannofossil response across this event are scarce. Here we analyse the distribution of calcareous nannofossils between 65.98 and 65.70 Ma from deep-sea sediments recovered from Blake Nose Plateau and combine these results with geochemical data (X-ray fluorescence, calcium carbonate, total organic carbon, and mercury content) to understand the main palaeoecological changes across this event. From 65.98 to 65.80 Ma surface ocean productivity was high, mainly dominated by Futyania petalosa species. Fe/K ratios indicate arid climate conditions. Increased Hg/TOC (ppb/%) and Hg/Al (ppb/cps) ratios recorded at 65.90 Ma strongly suggest that volcanic activity, likely related to the Deccan Traps, preceded the DAN-C2 event. This finding is associated with an increase in calcium carbonate content (CaCO3 %) and biogenic production (Ca/Fe), leading to an improvement in calcareous nanoplankton preservation. At the onset of DAN-C2 (65.80 Ma), Shannon diversity (H) index shows increased nannofossil species diversity, with greater abundances of eutrophic and high fertility species. More intense weathering and nutrient runoff most likely drove this during the event. Additionally, we provide new evidence that calcareous nannofossils (Coccolithus pelagicus and Cruciplacolithus primus) reduced size across DAN-C2 event, in an interval of intense dissolution and carbon dioxide sequestration. Finally, we suggest that surface ocean currents dynamics are an important mechanism to explain the strongest δ13C negative excursion observed in Blake Nose. Eccentricity maxima cycle likely amplified the effects of similar-to-today CO2 levels, leading to local changes in paleo circulation combined with sediments remobilizations from Gulf of Mexico, and enhance CO2 drawdown to deep ocean sediments.