Microbial Precipitation and Diagenesis in Salt Ponds from Little Darby Island, Exumas, Bahamas.

Abstract:

Microbial activity is responsible for the majority of carbonate precipitation and early diagenesis in restricted hypersaline ponds on Little Darby Island, Bahamas. The four ponds on Little Darby exhibit a range of salinities (10-69) and sedimentary deposits that record the evolution of ponds from restricted shallow marine embayments to isolated hypersaline ponds. Only the largest and most hypersaline pond, Anaconda, was covered by a classically defined multi-layered microbial mat (3 cm thick) with calcium carbonate precipitates. Microbial laminations and organosedimentary layers were preserved throughout the 90cm sediment core. The brackish ponds had thinner (< 0.5cm) microbial mats and appeared to be non-carbonate precipitating, with no preserved laminated mats in the subsurface. Despite differences in observed precipitation, molecular and geochemical evidence for microbially induced carbonate precipitation and diageneiss was observed in all four ponds. High-throughput 454 pyrosequencing and phylogenetic analysis of 16S rRNA genes identified the same dominant bacterial groups in all four ponds, such as Actinobacteria, Alphaproteobacteria, Bacteroidetes, Chloroflexi, Cyanobacteria, Deltaproteobacteria, Gammaproteobacteria and Planctomycetes, with metabolisms previously linked to carbonate producing mat systems. Sulfate reduction and heterotrophic degradation of exopolymeric substances were identified as the primary mechanism for microbial carbonate precipitation. The δ¹³C values of carbonate sediments ranged from -5.5‰ to 3‰, with the more negative values representing the heterotrophic involvement in carbonate precipitation. The more positive values (0-3‰) were associated with the deeper sediments deposited in a marine environment before the ponds were isolated. Pore fluid chemical ratios of Ca²⁺/Cl^-, Mg²⁺/Cl^-, and Sr²⁺/Cl^- ratios also suggest that precipitation and recrystallization of carbonate minerals is occurring in the buried sediments and indicates that the microbial influence on buried sediments is not limited to surface mats. The results reported here demonstrate that the absence of microbially induced sedimentary structures, such as laminations in hypersaline ponds, does not imply the absence of microbially mediated carbonate precipitation.