PRESERVATION VERSUS CHEMICAL AND BIOLOGICAL DISSOLUTION OF CARBONATE AT THE EAST FLOWER GARDEN BRINE SEEP, GULF OF MEXICO

Fossilization in deep disphotic marine environments can be reduced by chemical dissolution and micro- and macro-bioerosion, leaving no fossil record; unusual environments in the disphotic zone can also be places where preservation is enhanced. We investigated carbonate preservation on the East Flower Garden Bank (EFG) (27º54’N, 93º34’W) where brine seeps are created by a salt diapir that rose (creating the high bank), then intersected the seafloor creating brine seeps. The seep we studied is on the edge of the bank where brine from a pool eventually runs off the bank in an outlet stream. Mesh bags filled with fresh bivalve shells (Arctica islandica, Codakia orbicularis, and Argopecten purpuratus) were placed in the brine pool and stream. The brine pool salinity is ~200 ppt, it is anoxic, and it has H₂S levels of 2,200µM. Where brine flows off the bank in the brine stream, the sulfur-rich brine mixes with normal seawater and gradually decreases in salinity. To document carbonate preservation and loss, we examined the bivalves after 12 years on site. Shells were examined in cross-section using a Scanning Electron Microscope. A ~1cm section of shell was impregnated with epoxy. Ten random photographs of the shell’s interior surface were point counted for density of microboring and we documented physical dissolution as present or absent.

No, or few borings or dissolution was present where the salinity was highest (in the brine pool), representing an excellent opportunity for Laggerstätte preservation. In the brine stream, the density of microbial borings in shells increased with distance from the brine pool as salinity and sulfide was increasingly diluted. In the brine stream closest to the brine pool, the mixing of sulfidic brine produced sulfuric acid and led to purely chemical dissolution of shell. Typical microbial borers such as fungi and certain photosynthetic algae seem unable to live in the anoxic sulfidic brine or the mixing zones, but they become the primary bioeroders as salinity drops closer to normal seawater levels (~35 ppt). Therefore, we find a potentially recognizable signature for ancient brine seep environments where excellent preservation is surrounded by absence of carbonate. Progressing away from the Laggerstätte we would find increasing presence of carbonate that would be micro-bioeroded.