STRATEGIES FOR ENERGY CAPTURE INDICATE ENVIRONMENTS OF CARBONATE PRODUCTION (Invited Presentation)

The distinction between Photozoan and Heterozoan Associations is a primary consideration when interpreting carbonate depositional environments. Because biogenic carbonate production requires energy, recognizing whether sources are primary or secondary production of organic matter is critical. Photozoan carbonates are produced in waters sufficiently shallow and clear to support photosynthesis as the direct energy source, and the morphologies of major producers reflect whether irradiance supported three-dimensional structures, two-dimensional surfaces, or something in between. Hydrodynamics is reflected in associated sediment textures and by robustness of the calcifiers. Evidence for rapid production rates (i.e., hypercalcification) also indicates mechanisms such as evaporation or warming that increased carbonate supersaturation of surrounding waters. Environments of production and deposition of Heterozoan carbonates are more diverse and challenging. Coralline red algae in Heterozoan carbonates indicate production within the photic zone. Larger benthic foraminifers can thrive in cooler waters and at lower carbonate saturation states than aragonite-producing corals and calcareous algae, so their presence, along with coralline algae, can indicate transitional conditions, especially on shelf and ramp settings. In Heterozoan carbonates where benthic photosynthesizers are absent, conditions that can be inferred are that food supply, generally plankton, is sufficient to support thriving populations of the major carbonate producers. In modern shallow coastal systems, bivalves, gastropods and barnacles thrive in phytoplankton-rich waters, though their preservation potential may be limited by dysoxic pore waters. On outer shelf and slope environments, physical processes such as contour currents and internal waves can deliver plankton to azooxanthellate corals, bryozoans, and crinoids, promoting accumulation of aphotic bioherms. Environmental conditions supporting oligophotic Photozoan assemblages can transition to conditions supporting aphotic Heterozoan plankton feeders with relatively small changes in depth or overlying water transparency. Fortunately, functional morphologies of these two energetic strategies are very different.