THE FATE OF WOOD IN MARINE SILICICLASTIC ENVIRONMENTS: A COMPARISON OF PETROLEUM SEEP AND NON-SEEP ENVIRONMENTS

Wood is an important resource for marine communities on the continental shelf and slope. In order to better understand the rates and processes of wood decomposition in this depositional system, the Shelf-Slope Experimental Taphonomy Initiative (SSETI) deployed six different kinds of conifer and angiosperm wood on the Gulf Coast continental shelf and slope, in petroleum seep (547-570 mbsl) and 'normal' siliclastic (183-360 mbsl) environments. We collected samples after 2 years (all depths) and either 6 years (183 mbsl) or 8 years (547-570 mbsl). For each block, we measured the amount of wood remaining and identified the organisms responsible for wood decomposition. A diverse community of organisms colonized wood at both petroleum seep and non-seep sites. Teredinids (Bankia and Teredothyra), pholads and probably xylophagids were the predominent destroyers of wood in both settings. All 2-year samples showed evidence of wood-eating molluscs. Although limnorid isopods contributed to the destruction of wood at deep carbonate-dominated sites in the Caribbean (183-267 mbsl), we found almost no limnorids and few limnorid burrows on wood samples recovered from deep siliciclastic sites in the Gulf of Mexico. Wood-dwelling (and perhaps wood-eating) sipunculids colonized blocks from petroleum seep sites after 8 years. Because petroleum seep sites were significantly deeper than non-seep sites, the presence of wood-dwelling sipunculids may be related to site depth and not the presence of petroleum. After 2 years, seep and non-seep sites had equivalent amounts of wood remaining, (av. 30-40% of each block). After six years, non-seep sites had almost no remaining wood, and half of the original blocks were destroyed completely. After eight years, all seep sites had some wood remaining (av. 20-30% of each block), and only one fifth of the original blocks were destroyed completely. Two factors may contribute to the persistence of wood at petroleum seep sites: 1) wood at petroleum seeps may become buried more often than wood at non-seep sites. Burial protects wood from consumption by molluscs and limnorids; however exhumed wood is quickly re-colonized by wood-eating invertebrates; 2) colonization by wood-dwelling sipunculids may inhibit re-infestation of previously-buried wood blocks by wood-eating molluscs.