CONCLUSIONS

Variations in the mineralogy and grain size of the sediment on the leeward side of the Great Bahama Bank have proved to be valuable in determining the platform response to climate changes. Trends characteristic of different environmental regimes through both time and space have become evident:

  1. At the distal site (Hole 1006A), the 18O record and the aragonite record are well preserved and are in good agreement with each other. The proximal site, Hole 1003A, shows gaps in the sedimentary record, implying an erosional depositional regime on the platform slope. The 18O record here is also distorted because of diagenetic overprinting.
  2. The interglacials (sea-level highstands) are dominated by the fine fraction (<63 µm), which shows no spacial variation in concentration. These sediments are predominantly neritic, bank-top derived aragonite needles exported off the platform. The carbonate mineralogy is dominated by aragonite, which decreases in concentration with increasing distance from the platform top. Within the subordinate coarse fraction (>63 µm), the very fine to medium sand-size fractions dominate. Dolomite appears as a product of early diagenesis at the proximal site. The terrigenous input (quartz) is very low. With increasing distance from the platform top, HMC and dolomite decrease in concentration, whereas LMC and quartz increase. These interglacial sediments have a low diagenetic potential because of low permeability and restricted fluid flow.
  3. The glacials (sea-level lowstands) are clearly coarser grained but show a general decrease in coarse-sediment abundance with increasing distance from the platform top. The carbonate mineralogy for the fine fraction indicates that a mix of finer LMC-rich sediment was derived from pelagic input, and coarser HMC-rich sediment dominates these sediments. The HMC-rich sediment is derived from HMC-rich cements formed during early diagenesis and magnesium-calcite micrite cements from eroded upper slope material. Thus, with increasing distance from the platform, the LMC concentrations increase and the HMC concentrations decrease. Within the coarse fraction, the coarse to very coarse sand-size fractions dominate. Aragonite is in low concentrations because of relatively little neritic carbonate production and decreases in concentration with increasing distance from the platform top. Dolomite occurs in slightly higher concentrations, as does quartz. The dolomite shows the reverse spacial variation as seen during the interglacials, appearing in highest concentrations with quartz at the distal site, thus suggesting that glacial dolomite is of detrital origin. These glacial sediments have a high diagenetic potential because of high initial fluid flow and high permeability.
  4. Because diagenesis has been shown to occur preferentially within coarse-grained sediments, it shows discontinuities both vertically and laterally within the sediment column because it is also dependent on the grain size of the sediment. Thus, it occurs on a vertical scale preferentially during glacials. As the grain size of the sediment decreases with increasing distance from the platform top, the diagenetic potential also decreases.
  5. Grain size and mineralogy do not adequately describe coarse-grained, periplatform sediments. It is important to know the composition of the grains and whether they are pelagic or bank derived, skeletal or nonskeletal. Glacial sediments are characterized by cortoids, whereas planktonic and benthic foraminifers and pteropods dominate interglacial sediments. Further research is needed to see how these different types of grains are related to the observed grain size and mineralogical data.
  6. Kroon et al. (Chap. 2, this volume) observed a period of possible surface-water warming during the Late Pleistocene at Site 1006 recorded in planktonic foraminifers. In our study, a distinct change in the mineralogy and grain size of the sediments was also observed at the same level. Since isotope Stage 11, the input of aragonite and HMC increases, whereas LMC decreases. The grain-size distribution shows that coarser sediments are deposited. Paleoceanographic changes might have been the cause for this shift in the overall sedimentation pattern on the Great Bahama Bank during this period.

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