The production and fate of fish-derived carbonate crystals in tropical shallow marine environments

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A major component of the marine-atmospheric carbon cycle is the precipitation and dissolution of calcium carbonate in seawater. Calcium carbonate is the mineral that makes up rocks such as limestone and chalk. Detailed knowledge of this component is important to our understanding of the global carbon cycle, and to the earth system as a whole. This proposal aims to explore recent findings that represent a fundamental and previously unexpected change to our understanding of the marine inorganic carbon budget. Large amounts of calcium carbonate are produced in the global oceans by marine plankton and, specifically, by microscopic organisms such as coccolithophores and foraminifera. However, recent research by part of the research team here has highlighted the significant additional contribution to oceanic carbonate production by marine bony fish. These all ingest seawater and are now known to also produce calcium carbonate within their guts and excrete these precipitates at very high rates as part of their normal functioning in seawater. This previously unrecognised source of marine carbonate is significant in its own right but, when combined with new estimates of global fish biomass, it is clear that it makes a major contribution to carbonate production in particular regions of the ocean, and a smaller but significant contribution globally. Following excretion, these carbonate precipitates sink in the open oceans (along with other pelagic carbonates) and, once they reach the chemical lysoclines for carbonate (the depth at which ocean waters become undersaturated with respect to calcium carbonate), much of this material will dissolve. Thus the likely fate of these crystals in the open oceans is to dissolve at depth. In contrast, the fate of fish-derived precipitates in shallow tropical marine waters, whilst unknown, may be very different. Because fish production of calcium carbonate is positively correlated with seawater temperature, higher marine temperatures in the tropics will result in highest rates of precipitate production by fish. In addition, the shallow warm waters of tropical platform and shelf environments are typically saturated with respect to calcium carbonate, thus allowing substantial carbonate preservation and sediment accumulation to occur. Thus in these settings, with both high production rates and less potential for crystal dissolution, we hypothesise that fish-derived carbonates are making an important, but previously unrecognised, contribution to shallow water tropical carbonate sediment budgets and, especially, to the fine-grained (carbonate mud) fractions that are often volumetrically important in tropical carbonate depositional environments. It is significant to note that where attempts have been made to quantify fine-grained carbonate sediment fraction sources a significant proportion of the mud fraction remains of unknown origin (e.g. between 10 and 40% in Bahamian sediments) but is morphologically distinct from that attributable to any previously considered biogenic or inorganic source. This project will test the hypothesis that fish carbonates make up a substantial portion of this unknown fraction of carbonate mud. We will undertake an assessment of the ultrastructural and elemental characteristics of crystals that are produced by four representative shallow water tropical fish species and determine whether such crystals are contributing to carbonate sediment accumulation in these environments. If this material is indeed accumulating in these environments this would represent a previously unrecognised (and potentially volumetrically important) source for tropical shallow water carbonate sediments.