ABSTRACT

Magnesian calcite represents about 90% of carbonate mud that has accumulated in a Holocene (< 1.5 ka) pond basin on Lee Stocking Island, Bahamas. Additional minerals include aragonite (< 10%), as well as trace amounts of dolomite, celestine, and silt- to sand-size reworked authigenic gypsum. The mud fraction constitutes 40-75 % of the sediment, and has a bimodal grain size. More than 80% of the particles are < 2 µm in size, made up of mostly stubby, elongate euhedral to ragged, anhedral crystals, as well as spheroidal and platy calcite bodies, 0.15-0.2 µm in diameter. Silt-size particles are mostly 20-40 µm in diameter, and consist of fragmented bioclasts (mostly mollusks) as well as angular to subrounded clasts of microporous magnesian calcite (2-4 wt % Mg). Intraparticle porosity arises from likely microborings (<< 1 µm), whereas larger circular pores, 1-2 µm, could be sites of once-present bacterial filaments around which calcite precipitated. The mud fraction of the pond sediment is variably Sr-rich: in the clay size fraction, bulk Sr concentrations are 4000-7000 ppm; microprobe analysis of microporous silt-size particles reveals Sr contents of up to 1.2 wt %. ¹⁸O_PDB (-1 to -3o/oo) and ¹³C_PDB (-3 to -6o/oo) values of clay-size sediment are distinct from surrounding Quaternary sediment and limestone, and suggest that the clay fraction is not mechanically weathered detritus.

I interpret the mud-size particles to have precipitated from warm (> 30°C) hypersaline pond waters, at times exceeding gypsum saturation. Elevated Sr content is likely a product of changing Sr/Ca_fluid ratios in pond water related to gypsum formation. By incorporating carbon and oxygen isotope stratigraphies of indigenous biota (the foraminifer Ammonia beccarii, and gastropod Cerithidea sp.) with that of pond mud, this sedimentary basin preserves a record of high-order environmental changes over the last 1500 years that likely influenced mud precipitation and its accumulation; these changes reflect variation in water residence time, temperature, likely bacterial productivity, and bicarbonate sources.