WERE FORAMINIFERA THE BEGINNING OF THE END FOR STROMATOLITES?

Exuma Sound of the Bahama Archipelago supports modern, living stromatolites and thrombolites, thus, providing an excellent opportunity to study modern processes in microbialites that dominated the ecosystem for much of Earth’s history. The oldest demonstrably biogenic fossil stromatolites are ~3.45 billion years old and are the most visible manifestations of pervasive life on early Earth. Changes in stromatolite abundance and morphology over time document complex interplays between biological, geochemical and geological processes. We focus on one of the greatest geological enigmas: the possible connection between stromatolite decline and the rise of complex life. While much is known about stromatolite autotrophs, little is known about their heterotrophic eukaryotes. Details of stromatolite formation and preservation are poorly understood, and a drastic decline in their occurrence and diversity in the late Precambrian has long been a conundrum. A popular hypothesis to explain this decline beginning at ~1BYA is that eukaryotic organisms evolved to become predators on stromatolites. To date, the most commonly proposed predatory culprit is an unidentified metazoan, although evidence of such an organism is lacking from the fossil record. Thecate foraminifera, which are not expected to leave an obvious fossil record, are additional possible stromatolitic predators, but they have been largely ignored in this context. We tested the hypotheses that (1) thecate foraminiferal activity caused the textural change from stromatolites (layered sediment fabric) to thrombolites (clotted sediment fabric) and (2) foraminifera caused the decimation of Neoproterozoic stromatolites. It is impossible to recreate the Neoproterozoic, so experimental, ecological, microscopic, molecular, and chemical studies of modern analogs from Highborne Cay (Exumas) served to indirectly test our hypotheses. A suite of results from experiments and in situ data will be presented. For example, to determine the impact of thecate foraminiferal pseudopodial activity on stromatolite sediment fabric, a long-term experiment was conducted where cultured foraminifera were seeded onto stromatolite core surfaces; after many months, cores were CT scanned to assess sediment-fabric changes. Supported by NSF OCE-0926421 & -0926372.