Temperature and hydrological variations during the Late-Glacial in the central Mediterranean: application of the novel ostracod-clumped thermometer

In this study we show, for the first time, the absence of a vital effect in the clumped isotope carbonate (Δ47) fossil ostracod signal, as well as the ability of the novel ostracod-Δ47 thermometer to reconstruct past hydrological conditions in complex lacustrine systems. Furthermore, through the application of Δ47 analyses on the ostracod species Candona angulata and Cyprideis torosa from Lake Trasimeno record (central Italy), which today precipitate their shells during the cold and the warm season respectively, we provide evidence that by combining biological (i.e., ostracod shell precipitation timing), paleontological (i.e., identification of ostracod species) and geochemical (i.e., Δ47) approaches, the ostracod-Δ47 thermometer can be used to accurately reconstruct past seasonality. This also implies that, despite the absence of a vital effect, not all species can be combined for Δ47 analyses in environments with seasonal temperature variations; rather, only those that precipitate their shells during the same season should be considered. The application of the ostracod-Δ47 thermometer on the Trasimeno lacustrine record gives rise to the first continental warm season paleotemperature reconstruction of the last 43 ky in central Mediterranean area. The combination of Δ47 and classic stable isotope (d18Oost) measured on ostracod shells provides the isotopic composition of the water from which the carbonate precipitated (d18Ow) and thereby, changes in the evaporation/precipitation balance in this area. Before the Last Glacial Maximum (LGM), equivalent to the Marine Isotopic Stage 3 (MIS3, from 43 to 29ky), warm season temperatures ranged from 15 +/- 1.6 °C to 22 +/- 2.3 °C, being from 2 to 6 °C colder than today. Hydrological conditions during this period were similar to the present-day ones, characterized by a permanent lake and a high evaporation/precipitation ratio (E/P). The drastic decrease of the warm season temperatures (ranging from 10 +/- 2.9 °C to 17 +/- 3.1 °C) and of the E/P ratio during LGM and Lateglacial (MIS2, from 29 to 11.6 ky) corresponded well to the global climate cooling and low summer insolation, suggesting an amplifying role, of this last one, in the effects of the millennial scale climatic variations. At the Pleistocene/Holocene transition, both warm season temperature (25 +/- 2 ºC) and the E/P ratio increased in conjunction with the summer insolation. During the early Holocene, warm season temperature (23 +/- 2 ºC) closely resemble present-day values. However, cold season temperatures (12 +/- 2 ºC) were approximately 4 ºC warmer than today. Notably, no hydrological differences were identified between the warm and the cold season underlying a lower seasonality contrast compared to the present, along with enhanced warm season precipitation. The good agreement between the Δ47 temperatures reconstructed for the last 1 ky and the temperatures presently recorded at Lake Trasimeno (8 ºC cold and 22 ºC for warm season), confirms the accuracy of the analyses and the applicability of the ostracod-Δ47 thermometer to reconstruct seasonal temperature changes.