Biological skeletal carbonate records changes in major-ion chemistry of paleo-oceans.

Abstract

The history of the chemical evolution of seawater is of first-order importance given its fundamental role in a broad spectrum of geologic, geochemical, and paleontologic phenomena. A growing body of evidence for a more dynamic evolution of seawater chemistry than previously considered has been building over the past two decades. Central to this body of evidence are oscillating global trends, on a 100- to 200-million-year time scale, in the mineralogy of marine carbonate cements (1), late-stage salts in marine evaporites (2), and calcifying organisms that are interpreted to record secular change in seawater chemistry (3, 4). The degree to which the basic seawater signal has varied over the past 550 million years is, however, an issue that has been strongly debated in the literature (e.g., refs. 5–9). This primarily reflects the arguably equivocal nature of the evidence used to infer secular changes in seawater chemistry and the mechanisms for driving that change. Reconstructions of seawater chemical composition based on marine inorganic precipitate and fossil proxy records may be compromised by the influence of various environmental factors, and by the geochemical insults of diagenesis on their mineralogy and chemical composition (5, 10, 11). Complimentary modeling efforts that focus on constraining the mechanism(s) for driving substantial chemical change in seawater are challenged by the uncertainties associated with defining model input parameters (2, 5, 9). The degree to which the basic seawater signal has varied over the past 550 million years is an issue that has been strongly debated. In a recent issue of PNAS, Stanley et al. (12) provide compelling, experimentally derived evidence that the skeletal carbonate of biologically simple marine organisms, analogous to inorganic marine carbonate, has the potential to record faithfully changes in the major-ion chemistry of paleo-oceans. Their results, in concert with recently published …