Growth History of Stromatolites in a Holocene Fringing Reef, Stocking Island, Bahamas

Abstract

ABSTRACT A stromatolite and algal ridge reef complex 2.1 m thick fringes the east coast of Stocking Island, Exuma Cays, Bahamas. This reef was established on a Pleistocene calcarenite terrace about 4500 yr B.P. Stromatolites, which occur in back-reef and reef-flat zones, are up to 1 m thick and were constructed by cyanobacterial-dominated communities. Study of the growth history of these stromatolites, ranging in scope from facies analyses to details of microfabric construction, presents new perspectives on stromatolite formation. Lithologies identified in eight cores from across the Stocking Island reef complex, together with plots of 13 radiocarbon dates in relation to a Bahamian sea-level curve, indicate that this reef began as an intertidal vermetid gastropod buildup. Subsequent flooding of the Pleistocene terrace allowed the branching coralline alga Neogoniolithon strictum to overgrow the vermetids and eventually form an emergent algal ridge about 1500 years ago. Shifting sands accumulated in the lee of this ridge and excluded most benthic communities and herbivores, thereby promoting growth of cyanobacterial mats that formed stromatolite buildups. With a decrease in wave energy over the last 500 years, possibly due to the growth of offshore patch reefs, the urchin Echinometra lucunter coloni ed the algal ridge. Resultant bioerosion by this urchin destroyed the emergent part of the ridge and is now undercutting the seaward edge of the stromatolite buildups. Lamination in the Stocking Island stromatolites results from early lithificafion processes in cyanobacterial mats, possibly in response to biogeochemical changes in the mats during hiatuses in sediment accretion. These processes, which create partially indurated laminae with a distinct microstructure, involve precipitation of thin micrite crusts, intense microboring along a surface below this crust, micritization of sediment grains, and precipitation of point-contact cement between micritized grains. Introduction of turf algae to the cyanobacterial mat community disrupts formation of the lithified laminae, thereby inhibiting stromatolite development.