THE DISTRIBUTION OF ORGANIC MATTER IN ARAGONITE SPHERULES: INSIGHTS INTO POTENTIAL BIOTIC MECHANISMS OF SPHERULE FORMATION

Small, mm-diameter aragonite spherules form in shallow, low-energy, hypersaline lakes on Kiritimati, Republic of Kiribati (2°N, 157°W), an atoll in the northern Line Islands. Each spherule forms as the result of the complex integration of physical, chemical and biological processes. This study aims to assess the original depositional as well as syn- and post-depositional processes that form these spherules. Spherules were collected from 48 and 23 cm long sediment cores from a shallow, hypersaline lake with an underlying microbial mat. Spherule diameters ranged from 20 mm to 2 mm, and size increased while relative abundance decreased (70% to 30%) with depth. Transmitted plane light (PL) and Scanning Electron Microscopy (SEM) show three types of spherules: radial spherules with with acicular aragonite crystals and no dark laminae/inclusions, laminated spherules with dark and light laminae and numerous dark inclusions, and cemented spherules, bound together with aragonite cement. Fine mm-scale layers of dark-colored organic matter (OM) encrusted by acicular aragonite enriched in C and low in Ca (Energy-Dispersive Spectrometry) form concentric lamentations within the laminated spherules. Microbial communities and functional genes related to photosynthesis and sulfate reduction are abundant in bulk sediment from depths of the core with a greater abundance of laminated spherules. This may imply that microbial activity promotes carbonate precipitation in these sediments. PL microscopy of cemented spherules shows dissolution (pores, etches) at the outermost margin of single spherules, as well as micritization of original carbonate crystals. In addition, EDS-SEM elemental maps show regions of high Mg (10 mol%) in cemented spherules, evidence that syn-depositional diagenesis alters both radial and laminated spherules. Based on the observation of greater OM abundance in spherule laminae, we hypothesize that OM acts as a nucleation site on the surfaces of spherules and microbially-induced alteration of alkalinity subsequently leads to aragonite laminae precipitation. Future measurements of OM-rich layers by Super-resolution Autofluorescence Microscopy and nano-scale secondary ion mass spectroscopy will be performed to determine OM distribution and elemental and isotope ratios of S, C and N.