Assessing mineralogical and geochemical heterogeneity in the sub 63 micron size fraction of Holocene lime muds

Silt- and clay-size material (<= 63 mu m) of Holocene lime muds from south Florida and south Australia were subdivided into five fractions (<= 10 mu m, >10-20 mu m, >20-30 mu m, >30-45 mu m, and >45-63 mu m) by wet sieving. Carbonate mineralogy, elemental chemistry (Mg, Sr, Fe, and Mn), and stable carbon and oxygen 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 <= 10 mu m fraction, by up to 10% relative to other size classes. This aragonite enrichment causes increase in Sr concentration and increase in delta ¹³ C and delta ¹⁸ O values (on average by up to 1.0% and 0.8% respectively, relative to means of other size classes). In south Florida, this very fine-grained, high Sr, isotopically enriched aragonite probably has a calcareous algal source. Overall, within-sample mineralogical and geochemical variation is probably not large enough to invalidate use of the <= 63 mu m fraction as a comparator for ancient micrites. However, local and regional-scale facies variation does cause significant change in mineralogy and geochemistry. Carbon isotope compositions, in particular, show marked variation that might survive burial diagenetic stabilization.