The worm turned, and the ocean followed

Unless you hang drywall or travel in certain biogeochemical circles, the calcium sulfate mineral gypsum is probably not on your radar—nor is the amount of sulfate (SO42−) dissolved in seawater, which determines how much gypsum is left behind when that seawater evaporates. All the same, sulfate is the second most abundant negatively charged ion dissolved in seawater today, and tracking its changing concentration in the ocean over the earth's 4.5-billion-year history is one of a geochemist's best windows to the chemical and biological evolution of the early ocean and atmosphere. Now, in a novel slant on the sulfate tracer, Canfield and Farquhar in this issue of PNAS (1) link dramatic increases in seawater sulfate in the early Paleozoic Era, 540–460 million years ago, to a major biological innovation: the invasion of marine sediments by burrowing, mud-churning organisms (Fig. 1). And the net result, the authors argue, was the world's first massive deposits of gypsum. X-ray images of two sediment cores from the modern Black Sea. Sediments from the oxygen-free deep basin (Left) show undisturbed annual, submillimeter-scale layers (varves) preserved in the complete absence of animals on or below the seafloor. Those from the oxic shallow shelf (Right), by contrast, reveal nearly complete homogenization, with an active burrow layer extending at least 10 cm below the seafloor. Discrete burrows are visible in the X-ray. Only shell layers formed by storm reworking have been spared complete disruption. These extreme end members in degrees of bioturbation are much like the transition in earth history from no to pervasive sediment mixing described by Canfield and Farquhar (1). Shallow burrowers incapable of intense sediment mixing and sulfur oxidation characterized the time interval bridging these extremes. The first rigorous mixing of sediments by animals, in the early to mid Paleozoic, was a consequence in …