Why did ancient chemosynthetic seep and vent assemblages occur in shallower water than they do today?

Abstract

Cold-seep communities have relatively low diversity, are dominated by one or two taxa present in high density and high biomass in comparison with the surrounding fauna, and are restricted to aphotic habitats. Their associated heterotrophic fauna are usually distinctive from the fauna of their surroundings. In contrast, a more commonplace chemoautotrophically based community occurs in shallow photic habitats. The associated heterotrophic fauna includes many of the species typical of the surrounding communities and typically dominates abundance, whereas the species with chemoautotrophic symbionts dominate biomass. All modern seep assemblages are deeper than 550 m. Many fossil seep assemblages occurred in water as shallow as the mid-shelf (~200 m). In contrast, communities where species with chemoautotrophic symbionts are biomass dominants, but not numerical dominants, are common in shallow waters at present but rarely reported in the geological record. We suggest that the absence of cold-seep communities on the continental shelf presently is due to a combination of predation and competitive exclusion by primary consumers limiting the presence of species dependent on chemoautotrophic symbionts. We suggest that coldseep assemblages are more common at shelf depths in the fossil record for two reasons: (a) The biases of preservation have accentuated their distribution by transforming communities where species with chemoautotrophic symbionts dominate by biomass, but not numerically, into cold-seep-appearing assemblages. (b) The importance of predation pressure and oligotrophy has varied, with decreased predation pressure accompanying increased oligotrophy favoring cold-seep communities. We suggest that the paucity of shallowwater assemblages with species harboring chemoautotrophic symbionts as biomass dominants in the fossil record is based on the reliance of paleoecological analysis on numerical abundance data when energy flow analyses are required to identify these assemblages. The distinctiveness of the fossil seep assemblage is intensified by taphonomic processes that bias the assemblage against small individuals and epifaunal species, so that diversity declines, the small heterotrophic component of the assemblage is significantly reduced, and the epifaunal component is minimized. The final assemblage is usually dominated by the better-preserved large infaunal clams which perchance are also the species with chemoautotrophic symbionts. In contrast, preservation does not enhance the distinctiveness of these chemoautotroph-harboring species in shallow water.

Application of trophic transfer efficiency and age structure in the trophic analysis of fossil assemblages

“…Clear exceptions occur in all three petroleum seep assemblages and, less obviously, in the chemoautotrophic assemblages from Copano Bay. This bias towards the importance of predator refuges in chemoautotrophic assemblages supports the importance of predation in determining the distribution of chemoautotrophic communities (Callender & Powell 1999).…”

16S rDNA-based phylogeny of sulphur-oxidising bacterial endosymbionts in marine bivalves from cold-seep habitats

“…It is also well documented in the fossil cold-seep record (Callender & Powell 1999). Several observations suggest that the nutrition of C. disjuncta could be mixotrophic.…”

A unique Valanginian paleoenvironment at an iron ore deposit near Zengõvárkony (Mecsek Mts, South Hungary), and a possible genetic model

“…Van Dover (2000) reported rich decapod faunas at hydrothermal vents, which may explain the rich crustacean remnants (coprolites and crabantennae) at Zengõvárkony. On the other hand, Callender and Powell (1999) and Little et al (2002) demonstrated that vent/seep communities were continuously ubiquitous in space and time from neritic to bathyal environments. This leaves the door open for the challenging possibility that the decapods that left the rich coprolite deposits were shallow marine and vent-restricted animals.…”