Depth coenoclines and environmental considerations of western pacific larger foraminifera

摘要

Symbiont-bearing benthic foraminifera are restricted to the euphotic zone of tropical and warm-temperate seas. Species distribution is correlated with depth, and the continuous alteration of community structures represents a coenocline. Since depth is a composite environmental gradient, the coenocline of larger foraminifera is not stable but alters with changes in primary limiting factors: temperature, light, water movement, substrate, and nutrients.Temperature determines geographic distribution and affects the depth distribution of larger foraminifera by the development of a shallow thermocline that truncates the distribution of shallower species and excludes species adapted to the deepest euphotic zone. Within these constraints, light is the most important primary factor because larger foraminifera are at least partly dependent upon photosynthesis by their algal endosymbionts for growth and calcification. The microalgae show distinct intervals along the light gradient and the foraminiferal host develops various strategies for regulating light intensity. First, well-structured environments in shallow waters allow shelter against irradiation by protecting in shadow areas. Second, wall and test structures enable regulation of light penetration. A range of mechanisms allows species to resist the highest energies in the breaker zone of the reef edge and crest, where foraminifera attach to inorganic or organic hard substrates. Concentrations of dissolved and particulate organic matter in the water column, as well as sediments or other inorganic particles, influence depth distributions by changing water transparency and, therefore, photosynthesis. Permanent or episodic elevations of concentrations therefore compress the coenocline upward. Species adapted to hard substrates must compete for the reduced space, while species living in the deepest euphotic zone are at a disadvantage because they are insufficiently motile to surmount large depth differences. Changing light transparencies due to nutrient input and different hydrodynamic conditions alter relations between the light coenocline and water depth. Thus, paleodepth interpretations based on larger foraminiferal assemblages should be based not only on foraminiferal taxa and ecology, but also on environmental evidence for climate, terrigeneous influence, water transparency, and hydrodynamic conditions based on sedimentology, geochemistry, and associated fossil biota.