Middle and Upper Miocene cyclostratigraphy of downhole logs and short- to long-term astronomical cycles in carbonate production of the Great Bahama Bank

Ocean Drilling Program Site 1006 is located in the Santaren Channel off the Western slope of the Great Bahama Bank. Downhole logs of sediment physical and chemical properties from Hole 1006A contain repetitive alternations ('cycles'), which, using the nannofossil and planktic foraminifer biostratigraphy as a guide to the sedimentation rate, we show to be paced by the Earth's climatic precession. The formation microscanner resistivity image log shows a high-resolution record of the sedimentary cycles, and we use it to measure cycle thicknesses. The record of cycle thickness contains long-term cycles that repeat about every 400 kyr, which we correlate to the 400-kyr cycles in orbital eccentricity. This broad-scale tuning results in an age profile for the core that is largely consistent with previously astronomically tuned biostratigraphic dates. It has previously been established that the Bahama Bank produces and exports carbonate preferentially during sea-level highstands. Each highstand produces one sedimentary cycle, hence we infer that sea level also fluctuates with a mainly precessional periodicity (although obliquity is also found in the log records). We attribute the 400-kyr period in cycle thickness to eccentricity modulation of the amplitude of sea-level fluctuations: a high-amplitude sea-level rise will cause more carbonate to be generated and deposited than a low-amplitude sea-level rise. (C) 2002 Elsevier Science B.V. All rights reserved.