• Bahamas;
  • environmental change;
  • Holocene;
  • platform-margin stratigraphy

Characteristics of environmental change along the Holocene (<6 ka BP) bank margin of south-west Exuma Sound, Bahamas, are defined by comparing mineral, isotope and lithologic stratigraphic patterns in deep-water (1081 m) slope strata in ODP Hole 631A (Exuma Sound) with proximal (≈6 km) shoal-water and insular geology of the Lee Stocking Island (LSI) region. After platform submergence, a bank-top transition from non-skeletal- to skeletal-dominated deposition occurred in the LSI region by ≈3·8 ka, and is recorded in the adjacent slope sediments by a contemporaneous change in the abundance of transported allochems. The transition at Site 631 is punctuated by an anomalous shift in foraminiferal δ13C (–1‰) and δ18O (–3‰), which may represent a period of climate change that established a brackish coastal zone at least 6 km wide, coincident (≈3·8–4 ka) with bank-top erosion and pedogenesis. Non-skeletal allochems reappeared in Hole 631A sediments as the dominant transported allochem type by ≈2·4 ka, then again over the last several hundred years, and are contemporaneous with periods of increased platform-margin circulation in the LSI region. Such rejuvenation of platform circulation may have resulted from rapid relative rises in sea level, superimposed on the net gradual post-glacial rise. At Site 631, a prominent (25%) decline in aragonite abundance occurred during the period from 2·4 ka to the last few hundred years, and coincided with an increase in magnesian calcite and skeletal debris, and a positive shift in foraminiferal δ18O values. These patterns lend support to the hypothesis that, during this period, the LSI margin was barred by skeletal shoals, allowing bank-top trapping of aragonite and hypersaline water, with increased coastal salinity (resulting from reflux of hypersaline waters through the shoals) and/or cooled coastal waters (caused by reduced exchange of warmed bank-derived water relative to wind-generated upwelling). Along the Exuma slope, a diagenetic overprint may further enhance the recent decline in aragonite accumulation. Integration of Holocene deep-water slope and bank-top stratigraphies along the bank margin of south-west Exuma Sound reveals a pattern of punctuated environmental change superimposed on longer term development of interglacial conditions.