Reconstruction of Cenomanian shoal-water temperatures using integrated rudist shell sclerochemistry (oxygen isotopes, Mg/Ca, clumped isotopes)

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Reconstructions of Cretaceous sea surface temperature (SST) are predominantly based either on oxygen isotope analyses of pristine foraminifera calcite or on crenarchaeotal membrane lipid distributions (TEX86) in pelagic deposits. Both types of proxies provide at best estimates of mean annual SSTs of open ocean settings. In order to better understand the dynamics of Cretaceous global warmth and the impact of exceptionally high SSTs on carbonate platform production, information on shoal-water SST evolution and associated seasonality patterns are required. Well-preserved low-Mg calcite shells of rudist bivalves hold a strong potential to act as archives for the reconstruction of Cretaceous palaeoclimatic and palaeoenvironmental conditions. Sclerochemical analyses of ontogenetic isotopic and trace element variability in rudist shells also resolve sub-annual (seasonal) temperature fluctuations. The proposed project aims at reconstructing the temporal and spatial evolution of shallow-marine subtropical SSTs from a stratigraphically well-constrained Cenomanian-Turonian carbonate platform setting in the southern Apennines (Italy) based on rudist bivalve sclerochemistry. In order to cover a broad climatic range, the project will both focus on hothouse conditions during the Cenomanian-Turonian boundary event (CTBE) as well as transient cool episodes such as the Mid-Cenomanian Event (MCE) and the Plenus Cold Event. In contrast to single-proxy approaches, the integration of established palaeotemperature proxies such as oxygen isotopes and Mg/Ca ratios with the novel carbonate clumped isotope (CCI) thermometer will help to identify potential discrepancies between different palaeotemperature reconstructions and to critically assess the extremely warm open-ocean temperature estimates derived from TEX86. High-resolution sclerochemical analysis of rudist shells for oxygen isotopes, Mg/Ca and CCI, moreover, will enable to thoroughly constrain Cenomanian-Turonian SST changes and associated variations in SST seasonality. By means of a combined oxygen-isotope and CCI approach, stratigraphic changes in the oxygen isotopic composition of Cretaceous seawater are going to be reconstructed. This allows testing the hypothesis of transient build-up of polar ice during cool episodes such as the Plenus Cold Event. The salinity component of oxygen-isotope signatures stored in rudist shells will be as well extracted by the means of multi-proxy sclerochemistry (oxygen isotopes, CCI, Mg/Ca). The reconstruction of salinity signals across a proximal-distal carbonate platform transect will in turn help to decipher the potential role of highly saline platform top water masses on organic matter preservation in deeper basins during times of peaking SSTs.