Abstract

AbstractEarly Jurassic marine palaeotemperatures have been typically quantified by oxygen‐isotope palaeothermometry of benthic and nektonic carbonate and phosphatic macrofossils. However, records of Early Jurassic sea‐surface temperatures that can be directly compared with general circulation model simulations of past climates are currently unavailable. The TEX86 sea‐surface temperature proxy is based upon the relative abundance of glycerol dialkyl glycerol tetraethers preserved in organic‐carbon‐bearing sediments. This proxy has been used extensively on Cretaceous and Cenozoic materials and, in one study, on Middle and Upper Jurassic sediments. Here, TEX86 is applied, for the first time, to Lower Jurassic (Sinemurian–Pliensbachian) sediments cored at Deep Sea Drilling Project Site 547 in the North Atlantic. The abundance of glycerol dialkyl glycerol tetraethers in these sediments is very low, despite biomarker and Rock‐Eval data suggesting that thermal maturity is, generally, low. Sea floor oxygenation and a high input of reworked terrestrially sourced organic matter may explain the low concentrations. For samples from which it was possible to quantify the relative abundance of glycerol dialkyl glycerol tetraethers, TEX86 values range from 0·78 to 0·88, equating to sea‐surface temperatures in excess of >28°C. These temperatures are broadly comparable with new general circulation model simulations of the Sinemurian and Pliensbachian stages and support the general view of a predominantly warm climate. The new proxy data suggest that, under favourable geological conditions, it is possible to extend the record of TEX86‐based sea‐surface temperatures back into the Early Jurassic.

Highlights

  • There is a consensus that the Mesozoic Era was characterized predominantly by warm ‘greenhouse climates’ (e.g. Fischer, 1981; Frakes & Francis, 1988; Frakes et al, 1992), the spatial and temporal variability in climate during the Mesozoic is, generally, uncertain

  • Three samples between ca 847 to 846 mbsf have exceptionally high Tmax values >470°C, suggesting that they are extremely mature. Because these samples occur at the shallowest burial depths of the sample set, it could suggest that reworked, more mature, organic matter is present in these samples

  • The C31 homohopane bb/(bb + ba + ab) ratios of >0Á5 in most GDGTbearing samples support the interpretation of low-maturity organic matter and suggest that the TEX86 values reported here have not been compromised by catagenesis (Schouten et al, 2004)

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Summary

Introduction

There is a consensus that the Mesozoic Era was characterized predominantly by warm ‘greenhouse climates’ (e.g. Fischer, 1981; Frakes & Francis, 1988; Frakes et al, 1992), the spatial and temporal variability in climate during the Mesozoic is, generally, uncertain. The abundance of available records and the variety of proxies tends to decrease with increasing age, such that there are relatively more data available for the Cretaceous than the Jurassic Period. These factors limit the understanding of past climate dynamics and sensitivity, during intervals of major environmental change, and hinder the ability to compare the geological record with General Circulation Model (GCM) simulations of past climates. To reconstruct SSTs prior to the mid-Cretaceous, other proxies must be sought and tested

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