RECONSTRUCTING CAMBRIAN CLIMATES USING OXYGEN ISOTOPE DATA FROM SMALL SHELLY FOSSILS

Stable isotope-based climate reconstructions of the Cambrian are controversial. Low δ¹⁸O_calcite values in well-preserved carbonate shells and limestones suggest Cambrian temperatures could have been 30-40°C warmer than today, presupposing seawater isotopic composition was similar to today. Alternatively, temperatures could have been similar to today if seawater was many ‰ lighter in oxygen isotope composition compared with today. The “hot” Cambrian interpretation is difficult to reconcile with ocean climate models and inferred physiological constraints from early metazoan life. The “low δ¹⁸O oceans” interpretation is difficult to reconcile with models on the control of seawater isotopic composition. It has therefore been suggested that the oxygen isotope compositions of carbonates have been increasingly altered through time, and are unreliable as a proxy for ancient environments.

We have targeted marine authigenic and biogenic phosphate phases for oxygen isotope analysis, because of the much greater resistance of phosphate to isotopic resetting during burial and diagenesis. We have measured δ¹⁸O values on phosphatic fossils from the Cambrian known as small shelly fossils (SSFs). SSFs from several regions were sampled and analyzed: (1) the Pioche Formation, Nevada, United States; (2) the Dengying Formation, Shaanxi Province, China; (3) the Bayan Gol Formation, Orolgiin Gorge, Mongolia; and (4) the Kulpara Formation, Yorke Peninsula, Australia. These measurements, combined with published isotopic compositions of conodonts, chert-associated apatite, shark teeth, and phosphorite deposits, show an overall trend towards isotopically lighter marine phosphate δ¹⁸O in the Cambrian. The shift through time is similar in magnitude to the record from best preserved carbonates, though lingering uncertainties in the phosphate oxygen isotope paleothermometer complicate this comparison. These data suggest that the trend in δ¹⁸O values is a preserved primary signal which most likely requires a combination of warmer conditions and isotopically lighter ocean water in the Cambrian.