2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 10
Presentation Time: 10:30 AM


BRENNAN, Sean T., U.S. Geol Survey, MS 956, National Center, 12201 Sunrise Valley Drive, Reston, VA 20192, LOWENSTEIN, Tim K., SUNY at Binghamton, Dept Geological Sciences, Binghamton, NY 13902 and HORITA, Juske, Oak Ridge National Lab, PO Box 2008, Oak Ridge, TN 37831-2008, sbrennan@usgs.gov

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.