Abstract: Paleoecologic Investigations of Late Holocene Carbonate Sediments, Harrington Sound, Bermuda

G. R. Clark, II, A. M. Cowie

Sediment cores from Harrington Sound, Bermuda, show an abrupt transition, dated about 300-400 years ago, from a well-established Argopecten fauna to one dominated by the small bivalve Gouldia. As the environmental requirements of these two genera are very different, it was theorized that a major change in environmental conditions, such as the opening of Flatts Inlet (Harrington Sound's only surface connection with the sea), might have caused the transition. If so, other characteristics of the sediments might vary with the fauna.

One 80-cm core, representing approximately 1,000 years of deposition, was analyzed for variations in gross composition. The CaCO₃ content dropped gradually from about 90 percent at the surface to 85 percent at the base of the core; the organic content remained constant at about 5 percent; the insolubles (mostly radiolarian, diatom, and sponge debris) remained about 3 percent; and the dissolved and water-soluble substances (presumably organic, as other workers report relatively constant levels of dissolved salts in pore waters) increased from about 2 to 7 percent. Water content remained constant at about 50 percent. No significant variations were present at the faunal transition zone.

The same core also was subjected to a more elaborate analysis of the distribution of organic phosphates, with the idea that phosphates still bound to large molecules such as lipids, nucleic acids, and proteins (and their degradation products) probably would be in place and should reflect the general level of biologic productivity. Local variations were noted, but no significant differences were seen between levels above, below, and near the faunal transition.

These data do not support the hypothesis of a major environmental change causing the faunal transition; the dating of the transition does, however, focus speculation on a contemporary event: the first human settlement of the Bermudas in the early 17th century.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA