T75 Developing Perspectives on the Ecological Context of Biological Evolution Across the Neoproterozoic-Cambrian Transition

Monday, 28 October 2002
8:00 AM - 12:00 PM

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

Paper number: T75-11
Program: Technical Sessions
Time: 10:30 AM – 10:45 AM

Authors

Abstract

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, SO42- 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.