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An Early Cambrian Greenhouse Climate

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An Early Cambrian Greenhouse Climate

Thomas W Hearing et al. Sci Adv.

Abstract

The oceans of the early Cambrian (~541 to 509 million years ago) were the setting for a marked diversification of animal life. However, sea temperatures-a key component of the early Cambrian marine environment-remain unconstrained, in part because of a substantial time gap in the stable oxygen isotope (δ18O) record before the evolution of euconodonts. We show that previously overlooked sources of fossil biogenic phosphate have the potential to fill this gap. Pristine phosphatic microfossils from the Comley Limestones, UK, yield a robust δ18O signature, suggesting sea surface temperatures of 20° to 25°C at high southern paleolatitudes (~65°S to 70°S) between ~514 and 509 million years ago. These sea temperatures are consistent with the distribution of coeval evaporite and calcrete deposits, peak continental weathering rates, and also our climate model simulations for this interval. Our results support an early Cambrian greenhouse climate comparable to those of the late Mesozoic and early Cenozoic, offering a framework for exploring the interplay between biotic and environmental controls on Cambrian animal diversification.

Figures

Fig. 1
Fig. 1. Preservation of linguliformean brachiopods and Torellella from the Comley Limestones.
(A) SEM and EDX analyses of pristine brachiopods preserve alternating compact and porous phosphatic laminae [(1) and (2)]; compact laminae comprise densely packed phosphatic spherules (3). Diagenetically sensitive elements, particularly Fe and Mg, are restricted to porous laminae (4). BSE, backscattered electron image. (B) SEM and EDX analyses of altered brachiopods may preserve laminar microstructure, but compact laminae phosphate has recrystallized to micrometer-scale prismatic crystals (3). Diagenetically sensitive elements indicative of alteration are found throughout altered specimens (4). (C) Pristine Torellella specimens comprise densely packed phosphatic spherules a few tens of nanometers in diameter (3).
Fig. 2
Fig. 2. Early Cambrian δ18Ophos data from SSFs from the Comley Limestones.
(A) In bulk analyses, altered subsets are isotopically lighter than the pristine counterparts from the same sample. Sample labels follow Table 1; error bars of 1 SD; δ18Ophos indicates trisilver phosphate analysis in which only phosphate oxygen isotopes were measured. (B) In situ (SIMS) data from linguliformean brachiopod specimens (sample Ad) show that porous laminae are consistently isotopically lighter than compact laminae. See data S1 and fig. S3. Box plots display the median and first and third quartiles, with the whiskers extending up to 1.5 times the interquartile range. All isotope data are reported relative to VSMOW.
Fig. 3
Fig. 3. The Paleozoic phosphate δ18O record.
Conodonts, blue squares; well-preserved linguliformean brachiopods, dark blue triangles; well-preserved Torellella samples, yellow circles; phosphate hardground data, purple hexagons). This trend is most pronounced in the earlier part of the Paleozoic. Data span a range of paleolatitudes and water depths. Our Cambrian data are essentially contemporaneous at approximately 513 Ma ago; for clarity, Torellella and phosphate hardground points have been shifted slightly on the age axis. All values are relative to VSMOW. See data S3.
Fig. 4
Fig. 4. Cambrian isotopic SSTs in the context of Mesozoic and early Cenozoic greenhouse climate states.
Cambrian data (black diamonds) from the δ18Ophos values of pristine SSF samples from the Comley Limestones. Data are plotted in 10° bins of the modulus of paleolatitude to illustrate latitudinal temperature variation irrespective of paleocontinental configuration. Plotted point data summarized in box plots displaying the median and first and third quartiles, with the whiskers extending up to 1.5 times the interquartile range. Cretaceous data, Cenomanian to Turonian; Paleogene data, Paleocene to Eocene. For references and literature data, see data S4. The modern latitudinal mean (black line) and range (gray envelope) data from the 2013 World Ocean Atlas 1° resolution data set (60).
Fig. 5
Fig. 5. Early Cambrian mean annual SSTs, modeled by the FOAM GCM.
The simulation was run under present-day orbital configurations with a CO2-equivalent greenhouse gas forcing of 32 PAL (see Materials and Methods and fig. S4). Black spot marks the position of our δ18Ophos data on Avalonia.

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