MICROSTRUCTURAL PRE-CHARACTERISATION OF CALCAREOUS MICROFOSSILS APPLIED AS PALAEOENVIRONMENTAL PROXIES IN EARLY DANIAN

Calcareous microfossils provide an excellent matrix for isotopic analyses used in palaeoceanographic and palaeoclimatic reconstructions. However, factors such as diagenesis can limit these results: partial dissolution of calcareous microfossils, for example, can bias analytical results and consequently lead to inaccuracies in estimates of past oceanographic parameters. Here we address the importance of geochemical pre-characterisation through microstructural analysis of calcareous microfossils used as palaeoenvironmental proxies. Different instrumental techniques (µRaman, µXRD, and LA-ICP-MS) were applied to different carbonate matrices from the Danian (~65.5 Ma) in two different contexts of the South Atlantic Ocean. The main objective was to identify samples suitable for δ13C and δ18O isotopic studies. In the first context, the deep sea (ODP site 1262 Walvis Ridge), the occurrence of authigenic carbonates such as girvasite and siderite, which increased the Mg, Mn, Ba, and Fe contents of the samples analyzed by LA-ICP-MS, was identified in benthic foraminiferal samples of the species Nuttalides truempy. Our results show that the δ¹³C records of the benthic foraminifera and the planktonic ones show the expected surface-bottom gradient, suggesting the preservation of the original environmental signals. However, δ¹⁸O records of benthic and planktonic foraminifera show strongly overlapping and scattering values, suggesting an initial diagenetic change in the isotopic signals. The results support that thermodynamic changes occurred in conjunction with a decrease in bottom and pore water oxygenation to favor the precipitation of authigenic carbonate phases, which altered the isotopic fractionation of δ¹⁸O. However, the bulks clearly show the negative values of δ¹³C and δ¹⁸O. The negative excursions characteristic of the Dan-C2 hyperthermal event, with remarkably low values, characterize the recording of a shallow ocean environmental signal. These low δ¹³C and δ¹⁸O were mainly recorded by highly abundant calcisphers (calcareous dinocysts), suggesting that this group of microfossils has a high potential for palaeoceanographic reconstructions of the surface ocean. The second palaeoenvironment is the Cerro Azul section in the Neuquén Basin, Argentina, which provides a continuous record of the Cretaceous-Paleogene transition. Benthic foraminiferal assemblages were strongly affected by environmental changes during this event. On the other hand, the ostracods showed an excellent degree of preservation, allowing palaeoecological conclusions to be drawn for four species from the Danian: Paracypris bertelsae Ceolin and Whatley, 2015, Cytherella spp. Togoina argentinensis and Henryhowella (aWichmannella) meridionalis (Bertels, 1974). To assess the reliability of the results, the same previous instrumental pre-characterization techniques were used. The results show that the species Togoina argentinensis and Henryhowella (Wichmannella) meridionalis have calcite in their main compositions, with small differences in crystallinity, but without the identification of authigenic phases formed during diagenesis. The ^{δ13C and δ18O} values show clear interspecific differences between smooth and decorated samples. These differences in stable isotope values were probably controlled by microhabitat preferences, which would be comparable to the patterns described for benthic foraminifera.  The carbon cycle and the climate system did indeed change significantly at the beginning of the Paleogene. However, diagenesis is a factor that can affect the robustness of these paleoclimate reconstructions. Assessing the degree of diagenetic alteration of microfossil calcite is strongly recommended to properly interpret stable isotope and trace element analyses.