Coral Reefs and Shoreline Sedimentation: Bathtub Rings around the Sea

Abstract

Deep sea and ice cores show that sea level and thus global temperature fluctuated often during the past 1.8 million years. Fossil coral reefs, tidal flats, and beaches are precise indicators of former sea level. Although large fluctuations are indicated by the geologic record, this paper focuses on the younger well-documented fluctuations recorded by coral reefs and shoreline deposits that accumulated during the past 140,000 years. During this relatively short period, fossil coral species and depositional processes have remained unchanged and diagenetic alteration of fossils and sediments was minimal. Coral reefs and shoreline accumulations were selected as sea-level indicators for two reasons: (1) they serve as bathtub rings and/or dipsticks for determining former sea levels, and (2) human activity had no influence on the sea in which they grew or accumulated. Emergent coral reefs worldwide suggest that global sea level was at least 6 m above present during isotope stage 5e ∼120 ka. Drowned coral reefs and oolitic beaches indicate sea level was ∼100 m below present during stage 2 as little as 12 ka. As many as eight sea-level fluctuations occurred between stages 2 and 5e, each of which was greater than those projected to result from burning fossil fuels. Ice core data suggest even more fluctuations between stages 5a and 5e. Because the record is written in stone, geologists, especially sedimentologists, should be well qualified to make future predictions. Geologists have for the most part been excluded from official decision making. Direct measurements of air:sea CO2 gas fluxes and carbonate sediment production rates were measured in whitings located on the Great Bahama Bank and in laboratory cultures of calcifying cyanobacteria and unicellular green algae. In situ gas flux measurements showed a reduction in atmospheric CO2 relative to adjacent waters outside of whitings. Similar results were also observed in laboratory cultures. Calcification rates in whitings and laboratory cultures ranged from ∼0.06 to 34.5 g CaCO3m−3hr−1. These results suggest that production of microbial carbonates may serve as a sink for inorganic carbon. Laboratory cultures of calcifying microbes were subjected to biological buffers to examine the role of photosynthetic uptake of inorganic carbon species in calcification. Results from these experiments indicate that microbial calcification mechanisms depend upon the species of inorganic carbon available to cells for photosynthesis and, thus, atmospheric CO2 concentrations. These results suggest fluctuations in Phanerozoic dominance trends for calcareous cyanobacteria and algae may be linked to fluctuations in atmospheric CO2.