Early Cretaceous sea surface temperature evolution in subtropical shallow seas

Area of Science:

• Paleoclimatology
• Marine Geology
• Geochemistry

Background:

• Reconstructing Early Cretaceous sea surface temperatures (SST) relies heavily on the organic TEX86 palaeothermometer due to a lack of oxygen isotope data from planktonic foraminifera.
• This study aims to evaluate the exceptionally warm subtropical SSTs indicated by TEX86 during the Barremian to Aptian (130-123 Ma).

Purpose of the Study:

• To reconstruct Early Cretaceous shallow-water SSTs using sclerochemical analysis of rudist bivalve shells.
• To compare these new shallow-water temperature records with existing open-ocean TEX86-based SST reconstructions.

Main Methods:

• Sclerochemical analysis of pristine rudist bivalve shells from Tethyan and proto-North Atlantic carbonate platforms.
• High-resolution analysis of oxygen isotopes (δ18O) and Mg/Ca ratios to establish temperature fidelity.

Main Results:

• Rudist δ18O and Mg/Ca ratios show an inverse correlation, confirming oxygen isotopes as a reliable palaeotemperature proxy.
• Mean annual SSTs were moderate (22-26°C) in Barremian-Aptian subtropics, with transient warm pulses (28-30°C) during the Mid-Barremian Event and Oceanic Anoxic Event 1a.
• A significant cooling phase and potential glacio-eustatic sea-level lowering occurred at the Barremian-Aptian boundary, indicated by a positive shift in δ18O values.

Conclusions:

• Early Cretaceous subtropical SSTs were cooler than previously estimated by TEX86, challenging existing models of weak meridional SST gradients.
• Rudist bivalve shells provide a valuable proxy for reconstructing shallow-marine palaeotemperatures.
• The findings highlight a significant cooling event at the Barremian-Aptian boundary, potentially linked to glacio-eustatic changes.