Isotopic evidence for temperate oceans during the Cambrian Explosion

Thomas Wotte,Christian B Skovsted,Martin J Whitehouse,Artem Kouchinsky

doi:10.1038/s41598-019-42719-4

Thomas Wotte, Christian B Skovsted + Show 2 more

Open Access

https://doi.org/10.1038/s41598-019-42719-4

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Abstract

The Cambrian Explosion was a key event in the evolution of life on Earth. This event took place at a time when sea surface temperatures have been proposed to reach about 60 °C. Such high temperatures are clearly above the upper thermal limit of 38 °C for modern marine invertebrates and preclude a major biological revolution. To address this dichotomy, we performed in situ δ18O analyses of Cambrian phosphatic brachiopods via secondary ion mass spectrometry (SIMS). The δ18Ophosphate data, which are considered to represent the most primary δ18Oseawater signature, were identified by evaluating the diagenetic alteration of the analyzed shells. Assuming ice-free conditions for the Cambrian ocean and no change in δ18Oseawater (-1.4‰ to -1‰; V-SMOW) through time, our temperatures vary between 35 °C ± 12 °C and 41 °C ± 12 °C. They are thus clearly above (1) recent subequatorial sea surface temperatures of 27 °C–35 °C and (2) the upper lethal limit of 38 °C of marine organisms. Our new data can therefore be used to infer a minimal depletion in early Cambrian δ18Oseawater relative to today of about -3‰. With this presumption, our most pristine δ18Ophosphate values translate into sea surface temperatures of about 30 °C indicating habitable temperatures for subequatorial oceans during the Cambrian Explosion.

Highlights

The Cambrian Explosion was one of the most important events in the history of life on Earth
In Paleozoic successions, conodonts and phosphatic brachiopods have been used in addition to calcite-shelled brachiopods to provide information about the seawater temperatures via their oxygen isotope composition with high stratigraphic resolution[27,28,29]
We present a study of in situ oxygen isotope analyses of Cambrian phosphatic brachiopods using the secondary ion mass spectrometry (SIMS) technique (CAMECA ims1280 at the Nordsim laboratory, Swedish Museum of Natural History, Stockholm), in order to shed new light on the unexpected high seawater temperatures estimated for the Cambrian Period and their aforementioned implications

Summary

Introduction

During this fundamental biotic event, the metazoans invaded all kinds of marine habitats and evolved complex ecosystems and food webs for the first time[6,7] Insights into this often odd-looking Cambrian world are provided by a variety of global fossil Lagerstätten such as the Burgess Shale (Canada)[8], Chengjiang (South China)[9], or Sirius Passet (Greenland)[10] (Fig. 1). In Paleozoic successions, conodonts and phosphatic brachiopods have been used in addition to calcite-shelled brachiopods to provide information about the seawater temperatures via their oxygen isotope composition with high stratigraphic resolution[27,28,29]. Assuming ice-free conditions and no significant variation in seawater δ18O over time, the corresponding values of Ordovician δ18Oconodont in such analyses range from 15.3‰ to 19.6‰ (V-SMOW)[21] Temperatures calculated from these values vary from 25 °C to 44 °C21. Samples cover the Tommotian–Amgan interval (according to the Siberian nomenclature41,42) respectively the Terreneuvian (Cambrian Stage 2)–Miaolingian (Wuliuan Stage) interval (according to the international nomenclature[38,43]; Figs 1 and 2), and offer a unique window into the time of the Cambrian Explosion

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