Early Danian paleotemperature records from Neuquén Basin-Argentina: Applications of the ostracod Sr/Ca paleothermometer

The early Danian is marked by a recovery in marine and terrestrial ecosystems after the Cretaceous-Paleogene mass extinction. Both sea surface temperature (SST) and bottom water temperature (BWT) reconstructions are essential for a better understanding of paleoclimatic developments occurring at this critical time interval. Paleotemperature reconstructions based on ostracods are an excellent alternative for studies in regions where benthic foraminifera are scarce or absent. In this context, we present a paleotemperature reconstruction of the Cerro Azul Section, Neuquén Basin, Argentina, based on the ostracod Henryhowella (Wichmannella) meridionalis (Bertels, 1974). Trace element/calcium ratios (El/Ca) were determined on well preserved specimens using two different analytical techniques: (i) oxidative cleaning followed by acid digestion (measured with ICP OES), and (ii) laser ablation (measured with LA-ICP-MS). Our results show that El/Ca ratios of ornamented species are more effectively quantified using acid digestion. Although some intervals show high values of the Al/Ca ratio, which could suggest the presence of remaining clay and/or organic contaminants. Low values of both Fe/Ca and Mn/Ca ratios indicate efficient cleaning regarding oxyhydroxides. As the incorporation of Mg and Sr into inorganic calcite is a thermodynamic process, we mapped the distribution of both trace elements on ostracod valves in nanoscale using Synchrotron-based X-ray fluorescence (nannoXRF). Our results show that Sr/Ca has a negligible correlation with Al/Ca, and Sr appears to be more homogeneously distributed compared to Mg in Henryhowella (Wichmannella) meridionalis valves. Therefore, we opted to apply the Sr/Ca ratio as a paleothermometer in the studied Danian ostracods, consisting in the first ostracod-based BWT record for the Paleogene. Our results indicate stable BWT values throughout the studied interval at the Cerro Azul Section, ranging between 5.1 and 8.8 °C (oxidative cleaning), and 7 to 13 °C (laser ablation). BWTs show the expected offset (colder temperatures) to TEX H 86 and stable oxygen isotope (δ 18 O)-derived SSTs, which ranged from 28.3 to 31.8 °C, with TEX H 86 depicting a 2.5 °C warming between calcareous nannofossil zones AP0 and AP1a. The present study demonstrates the potential of using the elemental composition of ostracod valves as a new tool for reconstructing past deep ocean temperature variations.