Early Cretaceous sea surface temperature evolution in subtropical shallow seas.

Late Cretaceous sea surface temperatures (SST) are, amongst others, traditionally reconstructed by compiling oxygen isotope records of planktonic foraminifera obtained from globally distributed pelagic IODP drill cores. In contrast, the evolution of Early Cretaceous SSTs is essentially based on the organic TEX palaeothermometer, as oxygen-isotope data derived from well-preserved 'glassy' foraminifer calcite are currently lacking. In order to evaluate the extraordinary warm TEX-derived SSTs of the Barremian to Aptian (130-123 Ma) subtropics, we present highly resolved sclerochemical profiles of pristine rudist bivalve shells from Tethyan and proto-North Atlantic shallow water carbonate platforms. An inverse correlation of seasonal ontogenetic variations in δO and Mg/Ca ratios demonstrates the fidelity of oxygen isotopes as palaeotemperature proxy. The new data shows moderate mean annual SSTs (22-26 °C) for large parts of the Barremian and Aptian and transient warm pulses for the so-called Mid-Barremian Event and Oceanic Anoxic Event 1a (reaching mean annual SSTs of 28 to 30 °C). A positive shift in mean annual oxygen-isotope values (δO: ≤ - 0.3‰) coupled with invariant Mg/Ca ratios at the Barremian-Aptian boundary points to a significant net loss of O in Tethyan shallow-marine settings. As the positive oxygen-isotope rudist shell values are recorded immediately beneath a major superregional hiatal surface, they are interpreted to be related to a major cooling phase and potential glacio-eustatic sea-level lowering. Our new sclerochemical findings are in clear contrast to open ocean SST records based on TEX, which indicate exceptionally warm Barremian to earliest Aptian subtropical oceans and weak meridional SST gradients.