Seawater pH at the dawn of animal life

The boron isotopic composition (δ11B) of bulk limestones provides a potentially powerful tool for reconstructing seawater pH deep into the geologic past (Kasemann et al., 2005; Paris et al., 2010). Here, we present δ11B of 35 calcitic limestones derived from a ca. 9 my interval of the terminal Proterozoic Nama Group of southern Namibia. These units immediately precede the so-called Cambrian Radiation—the greatest diversification of metazoans in Earth history. The Nama Group represents one of the best preserved (average [Sr] = 1805 ppm; Mn/Sr < 2; δ18O > -10‰) and most continuous terminal Proterozoic limestone sequences known in the world. The carbonate units investigated here were deposited between ca. 552 and 543 Ma in a semidivided foreland basin of the Kalahari Craton (Grotzinger and Miller, 2008). Depositional environments were shore-associated and ranged from upper shoreline/tidal flats to below-wave-base lower shoreface, and comprise calcisiltites, calcarenites, heterolithic interbeds, grainstones, and microbialites (Saylor et al., 1998; Grotzinger and Miller, 2008). The δ11B of the 35 sampled Nama Group carbonates were obtained via MC-ICP-MS. Samples were screened (Paris et al., 2010) for contamination of the δ11B signal by clays (using [Al] as a proxy for clay content) and by open-system meteoric diagenesis (δ11B-δ18O correlation). The δ11B values of the limestones ranged from 0.5 to 10.8‰ (avg. = 5.3‰), which is consistent with the observed increasing trend in carbonate δ11B (Paris et al., 2010) from the -6.2 to 2.7‰ values reported for Neoproterozoic cap carbonate dolostones (Kasemann et al., 2005) to the ca. 25‰ value reported for most modern marine carbonates. B/Ca ratios for the sampled limestones ranged from 3.4 to 24.0 ppm (avg. = 11.0 ppm). Assuming a seawater temperature of 25° C, a salinity of 35, a depth of 10 m, a seawater δ11B of 25‰ (based upon 380 Ma halites; Paris et al., 2010), and a boron isotope fractionation factor of 1.0272 (Klochko et al., 2006), the measured δ11B of the Nama Group carbonates suggest that seawater pH ranged from ca. 7.6 to 8.6 (avg. = 8.2) over the 9 My interval of Neoproterozoic time. Furthermore, seawater pH exhibited consistent oscillations of 0.6 to 1.0 units over relatively short timescales (200 ky to 1 my) between 552 and 545 Ma. Of potential import is the relative stabilization of seawater pH between ca. 545 and 543 Ma—the 2 my interval immediately preceding the Cambrian Radiation of animal life.