Journal of Conference Abstracts

The Guild of Neritic Calcifiers: Evolutionary, Ecological, and Biogeochemical Consequences of Habitat Variation Over Time

Robert W. Buddemeier (buddrw@kgs.ukans.edu)¹ & Bradley N. Opdyke (bno@basins.anu.edu.au)²

¹ Kansas Geological Survey, 1930 Constant Avenue, Lawrence, KS 66047, USA

² The Australian National University, Canberra, Australia

Variability of calcium carbonate deposition in space and time is obvious; the causes of differing depositional patterns and biota are not. We address the problem by defining two guilds of neritic calcifiers that are both pertinent today and useful in considering global-scale changes in carbonate deposition over geologic time.

"Guild" is a widely used ecological term (Simberloff and Dayan, 1991) originally coined by Root (1967) to describe "A group of species that exploit the same class of environmental resources in a similar way. This term groups together species, without regard to taxonomic positions, that overlap significantly in their niche requirements." We apply the concept to an exploration of interactions and relationships within an overarching category of shallow-water benthic marine calcifying organisms (a 'superguild' containing two component guilds).

The environmental resource exploited is the dissolved Ca⁺⁺ and CO₃^-- concentrations of near-surface seawater. Exploitation is accomplished, at the superguild level, by deposition of a calcium carbonate exoskeleton by the plants and invertebrate animals that make up the guild(s). However, the crux of the discussion is distinction of two modes that distinguish the guilds: a. Guild I is largely tropical and is dominant only within a rather narrow range of environmental conditions, where it may develop reefs or extensive deposits (commonly of the less-stable minerals, aragonite or high-magnesium calcite). Production and accumulation rates for this guild are on the order of m/ka for the Holocene. Guild I is characterized by photosynthetically-coupled calcification, by plants or by animals with symbiotic algae. This mode of calcification appears to be limited to water significantly supersaturated with respect to calcium carbonate.b. Guild II is found globally, and typically does not produce reefs. Although production (typically of low-magnesium calcite) can be high, accumulation and preservation potential is relatively low (typically cm/ka), both during the Holocene and in the geologic record. This guild depends on strictly animal calcification fueled by heterotrophic feeding, and can calcify in water undersaturated with respect to CaCO₃.

The habitat considered consists of those parts of the world ocean where the bottom is within the actual or potential photic zone. This also corresponds roughly to the oceanographic 'mixed layer' (atmosphere-equilibrated), and under present conditions, to shelf regions. The time scale considered is the Cambrian to present. This is a major upscaling of the guild concept from the traditional community and ecosystem applications to global and evolutionary scales. We consider this consistent both with the fundamental definition and the spirit of its applications to the study of ecological and coevolutionary dynamics.

The Holocene is a warm stage in the generally cold Pleistocene. In a broader geologic context, the biota we see today represent relatively "cold" water, "high" carbonate saturation state conditions. This is especially relevant to the distribution of Guild I; although it is often considered a group of warm-water organisms, its dominance is critically dependent on saturation state and light availability as well as temperature. The geologic record indicates that once environmental conditions shift beyond its collective 'comfort zone,' Guild I communities collapse and the component organisms retreat to their respective refugia. Guild II then becomes generally dominant until environmental conditions change and create a sufficiently extensive 'superniche' so that representatives of Guild I can reassemble into locally dominant communities and ecosystems. Consideration of the requirements and characteristics of these two guilds provides new insight into the interactions of evolution, paleoclimate, and paleobiogeography.

Root RB, Ecological Monographs, 37, 317-350, (1967).

Simberloff D & Dayan T, Annual Review of Ecology and Systematics, 22, 115-143, (1991).

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