Fluorine mobility during early diagenesis of carbonate sediment: An indicator of mineral transformations

Abstract

The abundant occurrence of calcium carbonate minerals in marine sediment, their well-documented dissolution or precipitation during diagenesis, and their high F content suggest that carbonate mineral diagenesis may be an important influence on F behavior in marine sediment. To test this hypothesis, we have examined the geochemistry of F in shallow carbonate mud bank sediment of Florida Bay, USA.The F content of biogenic calcium carbonate in Florida Bay varies with mineralogy, positively correlates with the Mg content of calcite, and occurs in similar abundance to Sr (high-Mg calcite: FSr = 2.4 mol/mol; aragonite: 0.53–0.79 mol/mol; low-Mg calcite: 0.22 mol/mol). Models of porewater distributions, direct solute flux measurements, and reaction rate estimates over the upper 0–16 cm of sediment from Bob Allen Key Bank predict net fluxes out of the sediment of 170 μmol F/m2/d, 230 μmol Sr/m2/d, and 5.6 mmol Ca/m2/d. The net solute flux ratio of FSr (0.71 mol/mol) is consistent with dissolution of aragonite or high-Mg calcite, but FCa and SrCa ratios (28 mmol/mol and 40 mmol/mol, respectively) are 3–10 × that of biogenic carbonate sources. Selective dissolution of a high F and Sr content phase or, more likely (based on dissolution experiments), concurrent dissolution and reprecipitation of phases with different F and Sr contents account for this discrepancy. The loss of F, Sr, and Ca to fluorapatite precipitation, as predicted from a stoichiometric model of phosphate release, can be added to the transport-reaction model predicted net fluxes. A resulting, more complete mass balance model incorporating both carbonate mineral reactions and fluorapatite formation yields total release estimates of F, Sr, and Ca of 770 μmol, 332 μmol, and 53 mmol/m2/d, respectively. Calcium carbonate minerals apparently undergo transformation whereby 34 mmol high-Mg calcite/m2/d and 19 mmol aragonite/m2/d dissolve, and 46 mmol low-Mg calcite/m2/d precipitates (ignoring other cations besides Ca). If no fluorapatite formation occurs, the required fluxes are that 23 mmol aragonite/m2/d dissolves and 3.4 mmol high-Mg calcite/ m2/d and 14 mmol low-Mg calcite/m2/d precipitate. Net loss of CaCO3 from the deposit by dissolution is ~6–7 mmol/m2/d (3% of the accumulation flux). The magnitude of these fluxes could cause significant mineralogical and chemical changes on rapid time scales in nearshore carbonate sediment. Fluorine is probably mobile in other carbonate deposits undergoing diagenetic alteration on short and longer time scales and is a powerful additional constraint on the rates and nature of carbonate mineral diagenesis.