ASSESSING MINERALOGICAL AND GEOCHEMICAL HETEROGENEITY IN THE SUB-63-MICRON-SIZE FRACTION OF HOLOCENE LIME MUDS

Silt- and clay-size material (less than or equal to 63 mu m) of Holoce ne lime muds from south Florida and south Australia were subdivided in to five fractions (less than or equal to 10 mu m, >10-20 mu m, >20-30 mu m, >30-45 mu m, and >45-63 mu m) by wet sieving, Carbonate mineralo gy, elemental chemistry (Mg, Sr, Fe, and Mn), and stable carbon and ox ygen isotopic compositions were determined for each size fraction. The only statistically significant variation in carbonate mineralogy with size was enrichment in aragonite in the south Florida less than or eq ual to 10 mu m fraction, by up to 10% relative to other size classes. This aragonite enrichment causes increase in Sr concentration and incr ease in delta(13)C and delta(18)O values (on average by up to 1.0 part s per thousand and 0.8 parts per thousand respectively, relative to me ans of other size classes). In south Florida, this very fine-grained, high Sr, isotopically enriched aragonite probably has a calcareous alg al source. Overall, within-sample mineralogical and geochemical variat ion is probably not large enough to invalidate use of the less than or equal to 63 mu m fraction as a comparator for ancient micrites. Howev er, local and regional-scale facies variation does cause significant c hange in mineralogy and geochemistry. Carbon isotope compositions, in particular, show marked variation that might survive burial diagenetic stabilization.