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Phosphorite nodules recovered from the upper west Florida slope provide an analog for phosphogenesis under conditions of marginal upwelling that are significantly different from the regional, continental shelf-upwelling models applied to the majority of the southeastern United States. We believe that ferruginous (AI and All) and nonferruginous (BI and BII) phosphorite nodules record the episodic precipitation of francolite since the mid-Miocene (12–15 Ma) in response to positioning of the Gulf of Mexico Loop Current. It is likely that during sea level highstands, deflection of the Loop Current landward increases the C flux, establishing conditions suitable to the concentration of dissolved inorganic phosphorous (DIP) within redox stratified sediments. Francolite containing approximately 6.2% CO2 (a-values: 9.316-9.324 Å) apparently precipitates in response to the early diagenesis of organic matter and/or Fe-P shuttling under suboxic to anoxic conditions (δ13C:-6.55-0.47%o PDB, δ18O: -4.92-2.07%o PDB), forming nonferruginous nodules (<6% Fe2O3) and hardgrounds. Anhedral to subhedral francolite (ovoids, globules, and botryoids) encountered in BI and BII nodules is mineralogically consistent with primary nucleation, and appears to record a microbial component. Subsequent recrystallization of francolite, in response to submarine diagenesis, obliterates such fabrics, producing euhedral, hexagonal crystallites (AI and All) possessing a decrease in carbonate substitution to 4.9-5.5% CO2 (a-values: 9.322-9.328 Å).

During sea level lowstands, oxic conditions prevail, favoring Fe-enrichment of nodules (up to 22% Fe2O3) at the expense of carbonate minerals and glaucony. This is consistent with the evolution of primary, BII nodules into ferruginous lithotypes (AI and All). Aragonite dissolution and the dissolution and/or reprecipitation of both calcite and francolite (δ13C: -1.09-0.67%c PDB, δ18O: -0.61-1.38%® PDB) within ferruginous nodules occurs during such intervals in response to changes in pore water redox and pH. Both petrographic observations and major element trends illustrate Fe-enrichment processes associated with loss of fine micritic cements and biogenic carbonate as a result of dissolution and/or replacement by FeOOH precipitates, as well as the oxidation of glaucony.

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