DID THE EVOLUTION OF THE CENOZOIC INTERTIDAL DETRITIVORE COMMUNITY ELIMINATE MANGROVE (MARINE) PEAT FROM SILICICLASTIC ENVIRONMENTS?

Primary dolomite and magnesian calcite cements in Pennsylvanian coal balls indicate that coal-ball peat accumulated in marine mires, in siliciclastic depositional environments, despite the absence of thick peat in these environments today. In modern marine (mangrove) wetlands, the rate of siliciclastic sedimentation is too high for thick peat to form, although thick mangrove peat does accumulate in carbonate settings. Given the lack of thick marine peat in modern siliclastic depositional settings, its occurrence in the Pennsylvanian is perplexing. The productivity of land-plant communities has increased over geologic time and since modern mangrove communities are among the most productive, the expectation would be for more, not less marine peat in modern siliciclastic environments.

The taphonomy of Pennsylvanian and modern marine peat differs significantly. Modern marine peat consists mostly of roots (>90%). Pennsylvanian marine peat has significantly fewer roots (33 -67% in localities with Mg-rich, marine cement). These taphonomic differences suggest that the evolution of new detritivore groups may prevent the accumulation of thick marine peat in siliclastic settings. In modern marine wetlands, most above-ground, mangrove litter is consumed by groups that radiated after the Paleozoic: termites in the early to mid Eocene; sesaramid and ocypodid crabs in the Eocene-Oligocene. In the Indo-Pacific, sesarmid crabs are the major consumers of mangrove leaves and propagules, recycling over 71% of the surface litter in one Australian mangrove forest. Mixed ocypodid - sesarmid crab communities were less efficient, but still recycled 33% of mangrove surface litter. In the New World, ocypodid and gecarcinid crabs and snails apparently replace sesarmids as the major consumers of mangrove leaves and propagules. In all mangrove forests, termites, which build nests in living trees, recycle most of the aerial wood. Thick mangrove leaf mats can accumulate in modern mangrove peats, but only in areas without crabs; large pieces of wood seldom occur in mangrove peat. Modern intertidal detritivores and termites produce the distinctive taphonomic signature of modern mangrove peat on carbonate substrates, and may prevent the accumulation of thick mangrove peat in siliciclastic depositional settings.