DID CHANGES IN SEAWATER CHEMISTRY PLAY A ROLE IN THE CAMBRIAN EXPLOSION?

Analyses of primary fluid inclusions contained in terminal Proterozoic (~544 Ma, Ara Group, Oman) and Early Cambrian (~515 Ma, Angarskaya Formation, Siberia) marine chevron halites are indicative of a three-fold increase of seawater Ca²⁺ concentrations during the Early Cambrian. The fluid inclusion analyses, and accompanying geochemical modeling, indicate that the increase in Ca²⁺ concentration was accompanied by a decrease in other major ions during the Early Cambrian. Over the same period, SO₄^2- decreased by up to 85 percent whereas Mg²⁺, K⁺, and Na⁺ decreased 18, 15, and 10 percent, respectively. This increase in the Ca²⁺ concentration of seawater may have created a chemical environment that was favorable for the onset of the development of calcium carbonate and calcium phosphate skeletons among metazoans. Metazoan diversification began ~1000 Ma, but early metazoans were small. Widespread evidence of trace fossils beginning ~550 Ma suggests that metazoans became larger and more abundant in the Neoproterozoic. Although fluctuations in the major-ion chemistry of seawater likely occurred many times in the Precambrian, the Early Cambrian was probably the first time metazoans experienced such high concentrations of Ca²⁺ in seawater. Therefore, we propose that this major pulse in the concentration of Ca²⁺ in seawater, following the rise of the metazoans, triggered metabolic changes that led to the pervasive biocalcification of the Cambrian Explosion.