Abstract
Seasonal variability in sea surface temperatures plays a fundamental role in climate dynamics and species distribution. Seasonal bias can also severely compromise the accuracy of mean annual temperature reconstructions. It is therefore essential to better understand seasonal variability in climates of the past. Many reconstructions of climate in deep time neglect this issue and rely on controversial assumptions, such as estimates of sea water oxygen isotope composition. Here we present absolute seasonal temperature reconstructions based on clumped isotope measurements in bivalve shells which, critically, do not rely on these assumptions. We reconstruct highly precise monthly sea surface temperatures at around 50 °N latitude from individual oyster and rudist shells of the Campanian greenhouse period about 78 million years ago, when the seasonal range at 50 °N comprised 15 to 27 °C. In agreement with fully coupled climate model simulations, we find that greenhouse climates outside the tropics were warmer and more seasonal than previously thought. We conclude that seasonal bias and assumptions about seawater composition can distort temperature reconstructions and our understanding of past greenhouse climates.
Highlights
Seasonal variability in sea surface temperatures plays a fundamental role in climate dynamics and species distribution
All specimens showed clear seasonal δ18Oc fluctuations of −2.0–0.0‰ in R. diluvianum, −2.0–0.0‰ in A. incurva and −2.7‰ to −1.0‰ in B. suecicus on which shell chronologies were based
Seasonal δ18Oc patterns show that the specimens record 3 (A. incurva and B. suecicus) to 6 (R. diluvianum) full years of growth
Summary
Seasonal variability in sea surface temperatures plays a fundamental role in climate dynamics and species distribution. We conclude that seasonal bias and assumptions about seawater composition can distort temperature reconstructions and our understanding of past greenhouse climates. The reliability of many past seasonal reconstructions is undermined by seasonal bias and poorly constrained assumptions of seawater composition (δ18Osw bias[17]) This hampers our understanding of past warm climates and hinders accurate evaluation of climate models[16,18,19]. The clumped isotope thermometer yields accurate SST reconstructions independent of δ18Osw assumptions[30] It allows the reconstruction of δ18Osw, yielding information about the (local) hydrological cycle, an important aspect of climate rarely constrained in deep time, rectifying bias in the popular carbonate δ18Oc temperature proxy. The large sample sizes required for individual Δ47-based temperature estimates (>2 mg) have complicated paleoseasonality reconstructions using this accurate method[33], but a recently developed statistical approach enables its use for seasonality reconstructions[17]
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