Early Cretaceous sea surface temperature evolution in subtropical shallow seas

Area of Science:

• Paleoceanography
• Marine Geochemistry
• Paleoclimatology

Background:

• Reconstructing Late Cretaceous sea surface temperatures (SST) traditionally relies on planktonic foraminifera oxygen isotope records.
• Early Cretaceous SST evolution is primarily based on the TEX86 paleothermometer due to a lack of well-preserved foraminifera calcite for oxygen isotope analysis.
• Extraordinarily warm TEX86-derived SSTs for the Barremian to Aptian subtropics require validation.

Purpose of the Study:

• To evaluate the high TEX86-derived SSTs during the Barremian-Aptian (130-123 Ma) subtropics.
• To reconstruct Early Cretaceous sea surface temperatures using a novel proxy.
• To investigate the fidelity of oxygen isotopes in rudist bivalve shells as a paleotemperature proxy.

Main Methods:

• Analysis of highly resolved sclerochemical profiles from pristine rudist bivalve shells.
• Utilized Tethyan and proto-North Atlantic shallow water carbonate platforms for sample collection.
• Examined oxygen isotope (δ18O) and Mg/Ca ratios to determine paleotemperatures and assess proxy fidelity.

Main Results:

• Demonstrated an inverse correlation between seasonal variations in δ18O and Mg/Ca ratios in rudist shells, confirming oxygen isotopes as a reliable paleotemperature proxy.
• Revealed moderate mean annual SSTs (22-26°C) for most of the Barremian and Aptian, with transient warm pulses (28-30°C) during the Mid-Barremian Event and Oceanic Anoxic Event 1a.
• Identified a significant net loss of 16O in Tethyan shallow-marine settings at the Barremian-Aptian boundary, indicated by a positive shift in δ18O values and invariant Mg/Ca ratios.

Conclusions:

• The new sclerochemical data indicate moderate Early Cretaceous subtropical SSTs, contrasting sharply with previous TEX86-based reconstructions suggesting exceptionally warm conditions.
• The positive oxygen-isotope shift at the Barremian-Aptian boundary, beneath a major hiatal surface, suggests a significant cooling phase and potential glacio-eustatic sea-level lowering.
• These findings highlight the importance of using multiple proxies and sample locations (shallow vs. open ocean) for robust paleoclimate reconstructions.