T166 Advances and New Voices in Marine and Coastal Geoscience (Posters)

Tuesday, 11 October 2022
9:00 AM - 1:00 PM
Exhibit Hall F

STABLE ISOTOPE STUDY OF ENCRUSTERS ON CORALS FROM THE COCKBURN TOWN MEMBER (GROTTO BEACH FORMATION; EEMIAN, MIS 5E) FROM SAN SALVADOR AND GREAT INAGUA ISLANDS, BAHAMAS

Paper number: T166-14
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

Abstract

A 34 m-deep, 54 mm-diameter core through the Cockburn Town Member of the Grotto Beach Formation from The Gulf (TG) site on the south coast of San Salvador Island, Bahamas, contains coral reef deposits from the last Pleistocene interglacial highstand (Eemian, MIS 5e). Results of petrographic, stable isotope and XRD analyses of these deposits are compared with outcrops of this interval from the Cockburn Town Fossil Reef on the west coast of San Salvador and Matthew Town Marina and Devil’s Point sites on Great Inagua Island to better understand the conditions that resulted in heavy encrustation of corals, especially Acropora cervicornis, with microbialites.

Stable isotope analyses (δ13C and δ18O in ‰ VPDB) included various carbonate components throughout the entire TG core and focused on intervals with encrusters up to 14 cm thick at -9.2 to -9.7, -23.8 and -31.1 m. There is positive covariation (R2 = 0.7) between data for coral (lower values; δ13C = -1.8 to 1.9‰; δ18O = -3.6 to -1.1‰) and laminated microbialites (higher values; δ13C = 1.2 to 4.4‰; δ18O = -3.1 to -0.2‰). Outcrop data have similar trends, with a sharp increase in δ13C and δ18O values at the transition between corals and microbial encrusters. These differences can in part be attributed to isotopic fractionation between aragonitic corals and mixed aragonitic and calcitic microbialites, and to vital effect and greater degree of meteoric diagenetic modification of corals.

Microbialites incorporate fine carbonate sand and mud, and their composition overlaps with skeletal-peloidal-ooid sand and micritic sediment from TG core (δ13C = -1.9 to 3.3‰; δ18O = -2.6 to 0‰). Rhizoliths from -19.4 to -24.4 m in TG core have δ13C values as low as -5.4‰, and outcrop caliche has the most negative δ13C value of -8.2‰, representing formation from meteoric water. Conversely, the most positive δ13C values of microbialites may indicate elevated organic productivity due to enhanced nutrient input to shallow marine settings, possibly during post-storm run-off. The transitions from corals to thick encrusters may also reflect changes in water depth, with elevated δ13C and δ18O values of microbialites indicating shallower and more restricted conditions. These results complement other sedimentological and paleontological studies to provide insights into reef microbialite formation.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379301

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