Disentangling environmental and diagenetic δ18O and δ13C signals from marine carbonates deposited under warm climate conditions during the early Danian

Abstract

Paleoceanographic reconstructions are usually based on the chemical composition of foraminiferal tests, and pristine calcite preservation is crucial to support reliable interpretations. Therefore, prior knowledge about the structure and chemical composition of microfossils calcite is important to assess whether geochemical results reflect environmental and/or post-burial signals. Here, we present the first microstructural study of early Danian benthic foraminiferal calcite, and evaluate environmental signals and post depositional biases in benthic and planktonic foraminiferal and bulk carbon (δ13C) and oxygen (δ18O) isotope records at Ocean Drilling Program (ODP) Site 1262 (South Atlantic Ocean). We focused our study on the first ∼350 kyr that followed the Cretaceous-Paleogene (K-Pg) boundary, encompassing the Dan-C2 carbon cycle perturbation. Benthic and planktonic δ13C records show the expected surface-to-bottom gradient, suggesting the preservation of original environmental signals. However, benthic and planktonic δ18O records show strongly overlapping and scattered values, suggesting diagenetic alteration of isotopic signals. We identified the occurrence of authigenic carbonates, such as girvasite and siderite, in benthic foraminiferal tests, which increased Mg, Mn, Ba and Fe contents of tests calcite. Our results support that thermodynamic changes occurred in tandem with the decrease of bottom and pore water oxygenation to favor precipitation of authigenic carbonate phases, which altered isotopic fractionation of δ18O. Nevertheless, bulk sediments clearly depict the negative δ13C and δ18O excursions characteristic of the DanC2, with remarkably low values, characterizing the record of a surface ocean environmental signal. This low δ13C and δ18O were mostly recorded by highly-abundant calcispheres (calcareous dinocysts), suggesting that this group of microfossils has a high potential for surface ocean paleoceanographic reconstructions.