Banktop responses to Quaternary fluctuations in sea level recorded in periplatform sediments

Periplatform sediment from a 12-m core recovered from Northwest Providence Channel, Bahamas, contains a geochemical and paleontological record of sea-surface temperatures, ice volumes, and the response of banktops to highstands of sea level. A comparison of these data suggests that fluctuations of carbonate mineralogy in periplatform sediments result from fluctuations of sea level and from patterns of banktop sedimentation.

Highstands of sea level that flood carbonate platforms are recorded in periplatfonn sediments as abrupt increases of exported Sr-rich aragonite derived from banktop orgiuiisms superposed on a background of Sr-poor aragonite (pteropods) and calcite (foraminifera and coccoliths) derived from planktonic sources. The pulses of banktop sediment coincide with increases of water temperature determined from foraminiferal assemblages and with decreases in ratios of oxygen isotopes, indicating decreased ice volume and rising sea level.

Following these abrupt changes is a more gradual decline in bank-derived sediment, although paleotemperatures from foraminiferal assemblages and oxygen isotopic data clearly show that warm conditions and a highstand of sea level persist. We suggest that this decrease of offbank transport is part of autocyclic sedimentation patterns of shallow-water carbonate environments. Offbank transport is restricted as reefs, sand shoals, and islands reach sea level. Also, green algal production may decline because progradation of tidal flats decreases lagoon area, and the hydrologic and ecologic conditions change as lagoons are filled.