Incremental Trace Element and Stable Isotopic Analysis of Modern Bahamian Ooids

Ooids are ubiquitous carbonate particles in the geological record, but their specific mechanism of formation continues to be controversial. To delineate the processes responsible for modern ooid formation, the outer and inner cortices and nuclei from six Bahamian localities representing a spectrum of environments were incrementally dissolved. At each step, ooid surfaces were examined using SEM, evolved gas was analyzed for carbon and oxygen stable isotopes, and trace element compositions were determined using continuous leach techniques. SEM observations reveal that the outer cortex is punctuated by unfilled microborings while the inner cortex contains microborings filled with aragonite cement such as that associated with the microbe Solentia sp. Outer and inner cortices have a remarkably similar covarying d¹⁸O and d¹³C trend having a slope near 1, with low values in the outermost cortex, and high values nearest the nucleus. Continuous leaching reveals a corresponding change in composition, with the outer cortex having a high Mg/Ca ratio and a variable Sr/Ca ratio whereas the inner cortex has a low Mg/Ca ratio but constant Sr/Ca ratio indicating Sr contents near 5000ppm. This change in cortex chemistry is interpreted to reflect rapid precipitation of the outer cortex but diagenetic alteration by microbes and associated cements in the inner cortex. High d¹³C values near 5‰ in the inner cortex further indicate that ¹²C fixation occurs in the microenvironment of the borings in the ooid cortex, perhaps associated with the microborers themselves. Thus, Bahamian ooids are grains that have been altered by microbial processes that can be distinguished through incremental analysis.