Non-lithifying microbial ecosystem dissolves peritidal lime sand

Abstract

Microbialites accrete where environmental conditions and microbial metabolisms promote lithification, commonly through carbonate cementation. On Little Ambergris Cay, Turks and Caicos Islands, microbial mats occur widely in peritidal environments above ooid sand but do not become lithified or preserved. Sediment cores and porewater geochemistry indicated that aerobic respiration and sulfide oxidation inhibit lithification and dissolve calcium carbonate sand despite widespread aragonite precipitation from platform surface waters. Here, we report that in tidally pumped environments, microbial metabolisms can negate the effects of taphonomically-favorable seawater chemistry on carbonate mineral saturation and microbialite development.

Fig. 1. Microbial mats and their environment on Little Ambergris Cay.

a Caicos platform and its location in the southeastern Bahamian archipelago (inset). Satellite imagery from Google Earth. b Little Ambergris Cay drone orthomosaic marked with core, pond, and tidal bay porewater sampling locations. White contours mark the maximum tide level derived from tide monitoring stations and a photogrammetric digital elevation model, simplified to remove local bias from mangrove canopies above intertidal surfaces. c Aerial view southeast over interior basin rimmed by eolian grainstone ridge at the eastern end of Little Ambergris Cay. d Shallowest subtidal smooth mats on the edge of a tidal creek. e Intertidal polygonal mats. f Supratidal and upper intertidal blister mats with cerithid gastropods. Scales in top panels of d–f are 15 cm.

Fig. 2. Sediments associated with microbial mats and crusts.

a Top of core VC-03 with polygonal microbial mat overlying ooid-skeletal sand with cerithid gastropods (black arrows) and bivalve fragments (white arrows). b VC-03 consists of ooid-skeletal sand throughout its 223 cm length. c Nodule in a partially indurated interval of core VC-03 at 178 cm preserving small burrows and ripple cross-lamination. d Plane-polarized light photomicrograph showing dissolution features in ooid-Halimeda sand in VC-03 at 150 cm. e Laminated organic material interpreted as a decaying mat in VC-05 at 60 cm, and a thin, mud-rich interval at 42 cm. f Calcifying mats (arrows) along the rim of the hypersaline pond. Researchers for scale. g Mineralization of mangrove leaves and filaments in the calcified mats rimming the pond.

Fig. 3. Sedimentology and geochemistry of vibracores.

Geochemical data and graphic logs show the effects of tidal pumping and biogeochemical processes on aragonite saturation. Additional logs from cores without porewater sampling are shown in Supplementary Fig. 2 and Supplementary Fig. 3. Evap. = evaporation, ALK = alkalinity, V-PDB = Vienna Pee Dee Belemnite. a Core VC-01, situated in low supratidal mats on the flank of the eolian grainstone berm. b Core VC-03, located in intertidal polygonal mats near a mangrove thicket. c Core VC-04, situated in a subtidal, muddy Batophora algae patch.

Fig. 4. Tidal bay porewater geochemistry.

Contours show the low and high tide structure of tidal bay porewater temperature (T), salinity (S), aragonite saturation (Ω), and sulfate anomaly at the interior basin inlet on the southwestern side of Little Ambergris Cay.

Fig. 5. Processes impacting porewater carbonate mineral saturation.

Symbol shapes indicate sample location, symbol colors show depth, and arrows mark the effect of biogeochemical processes on DIC and alkalinity (ALK). a DIC and alkalinity data show that dissolution of ooid-skeletal sand contributed to increasing porewater carbon content and maintained aragonite saturation. If respiration-driven dissolution occurred by completely oxidizing organic matter, then the highest DIC, deep porewater must have ~5 mmol/kg of respired carbon. b Geochemical reactions that may non-uniquely sum to produce the DIC and alkalinity relationship observed. Aerobic respiration (red) and sulfide oxidation to sulfate (yellow) lower Ω, while aragonite dissolution (dark blue), anaerobic respiration by sulfate reduction (light blue), incomplete anaerobic respiration, and fermentation of organic matter to dissolved organic matter by sulfate reduction (purple), and evaporation (orange) increase Ω. c DIC concentration and DIC δ¹³C data show that core tops (shallower than ca. 100 cm) are dominated by DIC from respired organic material and core bottoms contain increasing amounts of DIC from dissolved aragonite sand. d Ternary diagram of inorganic carbon sources in the highest DIC, deep porewater (10.8 mmol/kg), which has a δ¹³C value of −1.25‰. It cannot have more than ~30% contribution from seawater DIC without exceeding observed salinities (white breaks in the color scale), so isotope mass balance indicates that it cannot have more than about 30% (~3 mmol/kg) contribution from respired carbon. This discrepancy with the stoichiometry of respiration-driven dissolution in panel a implicates a leak of unoxidized organic matter such as from the production of dissolved organic matter by incomplete anaerobic respiration and fermentation.