Origin of Sr-rich magnesian calcite mud in a Holocene pond basin (Lee Stocking Island, Bahamas)

Magnesian calcite represents about 90% of carbonate mud that has accumulate d in a Holocene (< 1.5 ka) pond basin on Lee Stocking Island, Bahamas, Addi tional minerals include aragonite (< 10%), as well as trace amounts of dolo mite, 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 <mu>m in size, made up of mostly s tubby, elongate euhedral to ragged, anhedral crystals, as well as spheroida l and platy calcite bodies, 0.15-0.2 mum in diameter, Silt-size particles a re mostly 20-40 mum in diameter, and consist of fragmented bioclasts (mostl y mollusks) as well as angular to subrounded clasts of microporous magnesia n calcite (2-4 wt % Mg). Intraparticle porosity arises from likely microbor ings (much less than 1 mum), whereas larger circular pores, 1-2 Gem, could be sites of once-present bacterial filaments around which calcite precipita ted. The mud fraction of the pond sediment is variably Sr-rich: in the clay size fraction, bulk Sr concentrations are 4000-7000 ppm; microprobe analys is of microporous silt-size particles reveals Sr contents of up to 1.2 wt % , delta O-18(PDB) (-1 to -3 parts per thousand) and delta C-13(PDB) (-3 to -6 parts per thousand) values of clay-size sediment are distinct from surro unding Quaternary sediment and limestone, and suggest that the clay fractio n is not mechanically weathered detritus, I interpret the mud-size particles to have precipitated from warm (> 30 deg reesC) hypersaline pond waters, at times exceeding gypsum saturation, Eleva ted Sr content is likely a product of changing Sr/Ca-fluid ratios in pond w ater related to gypsum formation. By incorporating carbon and oxygen isotop e stratigraphies of indigenous biota (the foraminifer Ammonia beccarii, and gastropod Cerithidea sp,) with that of pond mud, this sedimentary basin pr eserves a record of high-order environmental changes over the last 1500 yea rs that likely influenced mud precipitation and its accumulation; these cha nges reflect variation in water residence time, temperature, likely bacteri al productivity, and bicarbonate sources.