Milankovitch paleoceanographic cycles in geophysical logs from ODP Leg 105, Labrador Sea and Baffin Bay

Ocean Drilling Program (ODP) Sites 645 and 646, in Baffin Bay and the Labrador Sea, respectively, were logged following drilling during Leg 105. Geophysical logs in ODP drill sites yield long, continuous records of sedimentary sequences and thus provide the opportunity for examining evidence of periodicity in the Milankovitch band. Such logging records are extremely valuable where core recovery is incomplete and/or where cyclicity is on a scale larger than the length of a core (< 1 cycle/9.5 m), making recognition of cyclicity difficult on the basis of core description or carbonate data. Because these logs are continuous, they can be treated statistically with spectral techniques to enhance understanding of cyclic processes. Because these logs also indicate lithology, they can be readily interpreted in light of available sedimentary and geochemical analyses on any cores. The interval 288-455 mbsf of Site 645 was logged with sonic, resistivity, and gamma-ray tools. This zone corresponds to an estimated time interval of 1.9-3.1 Ma. These logs exhibit strong cyclicity throughout, which is mainly controlled by variations in porosity. The high sedimentation rate, which permits a vertical resolution of about 5 k.y. for sonic and 17 k.y. for resistivity logs, allows detection of cycle frequencies that correspond to a dominant 100-k.y. period and subordinate ~40-k.y. periodicity in the upper part of the logged interval and perhaps to an increasingly important ~ 20-k.y. period in the lower interval. The cycles most likely correspond to variations in flux of ice-rafted debris and/or to variations in activity of bottom currents. The patterns suggest waning influence of precession and increasing importance of obliquity as glaciation progressed in the Baffin Bay region. At Site 646, the openhole interval of 210-737 mbsf was logged with neutron, spectral-gamma-ray, gamma-spectroscopy, sonic, and resistivity tools. This interval corresponds in time to -8.5-2.1 Ma. Because of variations in sedimentation rate downhole (average 90 m/m.y.), the time resolution of logs varies somewhat. In general, variance is strongest in the eccentricity (?100 k.y.) and obliquity ( ? 41 k.y.) bands, but vertical resolution limits our ability to resolve frequencies corresponding to precession, if present. Again, the variation is largely in porosity, which apparently is directly related to variations in clay content. The cycles, which are alternations between clay-rich and coarser-grained sediment, most likely reflect orbital control on changes in intensity of bottom currents. Log responses indicate that significant increases in the intensity of bottom-current activity on Eirik Ridge occurred at about 5.4-5.5 Ma and 4.1-4.2 Ma. A third sedimentary change occurred at about 3.4 Ma, causing a major shift in the porosity baseline and character of log cycles and corresponding to both the onset of biogenic silica sedimentation and a substantial increase in sedimentation rate.