Organic matter preserved in modern ooids from Shark Bay and the Bahamas

Abstract

Ooids - small, concentrically laminated carbonate grains - are an important component of the sedimentary rock record, yet many details of their formation are not well understood. In particular, the role of microorganisms and organic matter is controversial. To learn about the geochemical and biological environment in which ooids form, modern ooids from two localities (Carbla Beach, Shark Bay, Western Australia, and Highborne Cay, Bahamas) were dissolved incrementally. Analysis of radiocarbon by accelerator mass spectrometry of C02 liberated during incremental dissolution of ooid carbonate provided information about the timescale of ooid formation and the ages of these particular ooids: Highborne Cay ooids began forming at least 1,000 years ago, with a possible erosional event approximately 500 years ago, and Carbla Beach ooids began forming at least 1,250 years ago. Each dissolution step liberated lipids, which were analyzed by gas chromatography-mass spectrometry and gas chromatography-isotope ratio mass spectrometry. The organic component of the ooids, which is -3% by weight, is composed primarily of straight-chain, saturated fatty acids with chain lengths that vary between C=12 and C=30. Also present are branched, short-chain fatty acids, and long-chain fatty ketones. Most compounds have oD 13C values that fall between -12%o and -30%o, and oD values between -50%o and -150%o. The distribution of lipids and their stable isotope signatures, which is consistent through incremental dissolutions, provides a fingerprint of the complex microbial community that lives in close proximity to growing ooids. These results suggest it is possible that this consortium of microbes alters the microenvironment of growing ooids in such a way that stimulates carbonate precipitation and protects ooids from dissolution.