Exxon USA, United States

Ocean Drilling Project (ODP) Hole 625B from 890 m water on the West Florida carbonate ramp-slope provides a continuous 235 m long record of cyclic sedimentation responding to global climatic change and regional tectonics over the past 5.4 Ma. Relatively rapid sedimentation rates (> 5 cm/1000 yr) and a 30 cm sampling interval (6-10,000 yr) permit a high-resolution investigation of Milankovitch-influenced carbonate sedimentation on a continental margin. Total carbonate, percent coarse (> 63 mu m), and oxygen isotope data exhibit good correlation with established mid- and late-Pleistocene composite records, indicating that the climate signal recorded on the West Florida slope closely parallels the global climate signal obtained from deep-sea sediments. A high amplitude, low frequency (100K yr) eccentricity signal of the past 800K yr is unique within the 5.4 Ma record of total carbonate and percent coarse. Prior to this, the data are characterized by lower amplitude variations and less regular periodicities. The interval from 0.8 to 2.8 Ma consists of low amplitude fluctuations with periodicities widely distributed in the obliquity band (30-60K yr). From 2.8 to 5.4 Ma, the data show larger amplitude, aperiodic variations. Sedimentation from the present to 2.8 Ma was controlled primarily by sea-level fluctuations associated with changing global ice volumes. During periods of lowered sea level, fine-grained terrigenous sediments from the Mississippi and other rivers diluted the indigenous pelagic carbonate sediments of the West Florida slope. The interval from 0.8 to 2.8 Ma differs from the late Quaternary in that smaller sea-level oscillations and less extensive North American ice sheets resulted in smaller-amplitude sediment cycles. Sediments older than 2.8 Ma record aperiodic fluctuations caused perhaps by Loop Current circulation responding to the final closing of the Isthmus of Panama between 3.5 and 4.0 Ma. Intensified circulation may have transported and redeposited terrigenous sediments from the Mississippi delta-fan to the West Florida carbonate ramp, creating high-amplitude, but aperiodic, sediment cycles. This study demonstrates that Milankovitch orbital variations can create well developed, primary pelagic sedimentary cycles on continental margins and that such cycles are not restricted to the very deep sea. The data from ODP 625B, in conjunction with other contemporaneous cores from contrasting settings, provide the opportunity to obtain a quantitative understanding of the interactions between external orbital forcing, climate, and continental margin sedimentation.