Computer Science
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998gecoa..62.3689m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 62, Issue 23-24, pp.3689-3694
Computer Science
Scientific paper
Comparison of 34 S values obtained using an oxic and anoxic gravimetric method on Miocene phosphorite from Florida indicates that oxidation of Fe-sulfides can significantly lower S isotope values. The lowest S isotope values are obtained from phosphorite grains sampled from cores recovered from northeastern Florida (NEF). 34 S values of NEF phosphorite grains range from 11.9 to 19.2 CDT when prepared under oxic conditions, and 17.4 to 22.8 CDT when prepared under anoxic conditions. Phosphorites from NEF have a mean terrigenous insoluble residue content of 29 wt% and Fe-sulfide contents of 0.5 to 3 wt%. The chemical composition of the carbonate fluorapatite from NEF phosphorite indicates minimal alteration by postdepositional weathering. Most of the Fe-sulfide oxidation is believed to occur rapidly during drying of bulk powdered samples after washing to remove soluble sulfate, and possibly during filtration and acid digestion. Phosphorites sampled from shallow open pit mines of the Central Florida Phosphate District (CFPD) have low terrigenous insoluble residue contents (mean of 7 wt%) and pyrite contents less than 0.5 wt%. 34 S values of CFPD phosphorite grains range from 22.4 to 25.5 CDT and show no difference for samples prepared under oxic and anoxic conditions. 34 S values of CFPD phosphorites are slightly enriched relative to Miocene seawater and indicate formation immediately below the suboxic/anoxic sediment boundary. Regional differences in pyrite content and S isotope composition of the phosphorite are related to proximity to terrigenous sediment sources and postdepositional weathering. Lower than Miocene seawater 34 S values of NEF phosphorites obtained using the anoxic gravimetric method may reflect microbial oxidation of sulfide at the suboxic/anoxic sediment interface or extremely rapid oxidation of pyrite during sample filtration in the lab.
Compton John S.
Mallinson David J.
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