Are there “black holes” in carbonate deposystems?

Abstract

The likelihood that extensive dissolution of aragonite (and high magnesian calcite) takes place during very early burial, even in relatively shallow tropical settings, has wide implications for interpreting and modelling ancient limestones. Some low energy environments may constitute net sinks (“black holes”) for carbonates. If this is the case attempts to model sediment budgets and develop depth-productivity profiles for carbonate systems require as yet unavailable quantitative data on the effects of these processes. We query whether the established trends of diminishing carbonate productivity with depth could, for some settings such as ramps, be in part an artifact of selective, offshore dissolution more than simply due to reduced benthic production. Many fossil assemblages from low energy settings may be limited to refractory (residual) skeletal elements, which limits our ability to make palaeoenvironmental determinations: some important carbonate grain producers such as the green algae may have been especially prone to early dissolution. The possibility exists that what is now mud-grade carbonate in low energy deposits may not represent original material deposited from suspension but could represent transformed aragonite from in situ production by the skeletal fauna. The selective removal of labile aragonitic (and high Mg calcite) components could produce sediments that appear to have had the composition of cool water types, and the reduced diagenetic potential of many Palaeozoic and cool water carbonates could be a consequence of the effects of early aragonite dissolution rather than due to differences in primary (depositional) composition.