No abstract

No abstract

No abstract

Ocean Drilling Program Leg 103 occupied five sites on the Galicia margin, northwest of the Iberian Peninsula. Two holes (Holes 637A and 638B) yielded significant Cenozoic sedimentary sections ranging from late Miocene to late Pleis tocene in age. From the nannofossil stratigraphy, one hiatus is recognized in Hole 637A (2.35-2.4 Ma), whereas two hia tuses (one at 1.9-2.6 Ma and another at 3.5-3.7 Ma) are recognized in Hole 638B. Sediment-accumulation rates for the Cenozoic portions of these two holes have been calculated based on the nannofossil datums. The abundance ratios of Coccolithus pelagicus to Discoaster brouweri for Hole 637A show relatively low values and small fluctuations from 2.5 to 6.5 Ma but sharply increase and then widely fluctuate beginning at about 2.5 Ma. This may indicate relatively warmer, more stable surface-water temperatures from 2.5 to 6.5 Ma and cooler, variable surface-water temperatures af ter 2.5 Ma at Site 637. C. pelagicus/D. brouweri ratios from Hole 638B also show a trend of increasing values with time from late Miocene to late Pliocene, but with more fluctuations and a different pattern from that of Hole 637A.

Carbonate mineral cycles, controlled by Pleistocene climatic rhythms, have been discovered in a 11.6 m‐long‐piston core from the west Florida carbonate ramp slope at 890 m of water depth in the eastern Gulf of Mexico. Relatively high concentrations of aragonite, magnesian calcite, and dolomite, as well as insoluble residues, total organics, and mud‐sized sediment, all correlate with glacial periods defined by oxygen isotope stratigraphy. The aragonite is a low‐strontium variety derived from pteropods, and there is no evidence that concentration variations are controlled by dissolution. We propose that these mineral and textural cycles are the product of a combination of increased biological productivity and dilution by fine‐grained terrigenous mud during glacial episodes. Aragonite cycles in Bahamian periplatform slope sediment have also been correlated with climatic change. There the aragonite is a high‐strontium variety derived from shallow‐water calcareous algae, with relatively high concentrations correlated with high sea levels during interglacial periods. The carbonate mineral cycles along the west Florida ramp slope are 180° out of phase with Bahamian examples and also lack their marked asymmetry. It is now apparent that for deepwater, continental margin carbonates there are two types of carbonate mineral cyclicity: one that is “pelagic driven” during glacial periods, and one that is “platform driven” during interglacials. These contrasts emphasize the differences in deep water cyclic sedimentation on carbonate ramps versus rimmed carbonate platforms that are controlled by physiography, sediment source, oceanographic setting and proximity to terrigenous input. The productivity‐dilution model proposed here may be a valuable modern analog for ancient carbonate sequences deposited in epicontinental seas or on ramps attached to continental margins.