OCEAN PH ON THE EVE OF THE CAMBRIAN RADIATION (Invited Presentation)

The boron isotopic composition (δ¹¹B) of limestones provides a potentially powerful tool for reconstructing seawater pH deep into the geologic past. Here, we present δ¹¹B of 35 limestones spanning a ca. 9 m.y. interval of the terminal Proterozoic Nama Group (Namibia), which immediately precedes the Cambrian Radiation—the greatest diversification of metazoans in Earth history.

The Nama Group represents one of the best preserved (avg. [Sr] = 1805 ppm; Mn/Sr < 2; δ¹⁸O > -10‰) and most continuous terminal Proterozoic limestone sequences known. The carbonate units investigated here were deposited between ca. 552 and 543 Ma in a semidivided foreland basin of the Kalahari Craton. 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 (Grotzinger and Miller, 2008).

The δ¹¹B of the 35 sampled Nama Group carbonates were obtained via MC-ICP-MS. Contamination of the δ¹¹B signal by clays and meteoric diagenesis was screened using [Al] and δ¹¹B-δ¹⁸O correlation, respectively. δ¹¹B ranged from 0.5 to 10.8‰, consistent with the increasing trend in carbonate δ¹¹B from -6.2 to 2.7‰ for Neoproterozoic cap carbonate dolostones to ca. 25‰ for modern carbonates (Paris et al., 2010).

Assuming seawater temp. = 25° C, sal. = 35, depth = 10 m, seawater δ¹¹B = 25‰ (380 Ma halites; Paris et al., 2010), and B-isotope fractionation = 1.0272 (Klochko et al., 2006), the δ¹¹B of the Nama Group carbonates suggest that ocean pH ranged from 7.6 to 8.6 over the 9 m.y. interval. Furthermore, seawater pH consistently oscillated by 0.6 to 1.0 units over relatively short timescales (200 k.y. to 1 m.y.) between 552 to 545 Ma. Of potential import is the relative stabilization of ocean pH between 545 and 543 Ma—the 2 m.y. immediately preceding the Cambrian Radiation of animal life.

δ¹¹B and δ¹³C are uncorrelated in these limestones, suggesting that the < 1 m.y. pH cycles were not driven by carbon cycle perturbations. It is also unlikely that these cycles are an artifact of changes in seawater δ¹¹B, as the residence time of boron is 11-17 m.y. We are investigating causes of these dramatic fluctuations in ocean pH that preceded the Early Cambrian diversification and calcareous mineralization of animals.