SEAFOOD THROUGH LIME: SECULAR CHANGES IN FOOD AVAILABILITY IN CARBONATE ENVIRONMENTS DURING THE PALEOZOIC ERA

In his classic “Seafood Through Time” essay, Richard Bambach discussed the diversification of bivalve molluscs in the context of increasing food availability through the Paleozoic Era. He argued that, during the Early Paleozoic, bivalve diversification was impeded, compared to articulated brachiopods and other taxa that dominated most Paleozoic seafloors, because insufficient food was available to meet the energy requirements of a diverse and abundant bivalve biota. With the mid-Paleozoic diversification of land plants, Bambach hypothesized that food availability in marine settings increased significantly, thereby fueling an appreciable diversification of bivalves, beginning in the late Paleozoic.

Several years ago, I conducted an analysis of Paleozoic bivalves from the paleocontinent of Laurentia, in which I documented a siliciclastic/carbonate dichotomy among Ordovician faunas: bivalve diversity and abundance were significantly greater in Ordovician siliciclastic settings than in carbonates. However, during the Carboniferous and Permian, this dichotomy began to break down, and bivalves diversified appreciably in carbonates. Bambach suggested that this pattern might be explained by a particularly significant late Paleozoic rise in primary productivity in carbonate settings, where food limitations may have been especially acute prior to the advent of land plants.

Here, I consider anew two aspects of the aforementioned scenario, based on data and models that have become available since the earlier studies: 1) the global veracity of siliciclastic/carbonate distributional patterns among Paleozoic bivalves, which had been documented previously only for Laurentia; and 2) the possibility of assessing directly, with geochemical data, the evolution of food availability in Paleozoic carbonate environments. With respect to the latter, the increase in weathering associated with the rise of land plants may have increased dramatically the mobility of iron and other elements viewed as crucial fertilizer components that stimulate marine primary productivity, thereby increasing their abundances in marine environments, including carbonate settings, that were far removed from areas of direct siliciclastic input.