ORBITAL FORCING OF BOTTOM-CURRENT ENHANCED SEDIMENTATION ON FENI DRIFT, NE ATLANTIC, DURING THE MIDPLEISTOCENE

Feni Ridge is a plastered sediment drift, deposited along the northwes tern flank of Rockall Trough, NE Atlantic, under the influence of deep , geostrophic currents formed by intermittent overflows of Arctic Inte rmediate Water from the Norwegian Sea across the Iceland-Scotland Ridg e (NSOW). Lower Deep Water (LDW), a derivative of Antarctic Bottom Wat er from the South Atlantic mixed with old North Atlantic Deep Water (N ADW), also bathes the lower flanks of the drift, below 2500 m. Deep Se a Drilling Project site 610 was drilled in a sediment wave field on th e surface of Feni Drift (2417 m), though the sediments recovered prove d to be typically pelagic, preserving a high-resolution (>5 cm kyr(-1) ) Quaternary record of orbitally modulated ice-rafting and biogenic pr oductivity cycles but showing little evidence of current affected depo sition. From particle size and compositional analyses of the biogenic and lithogenic < 63 mu m fractions of samples taken at similar to 2.5 kyr intervals between 0.5 and 1.2 Ma, we derived indices of bottom cur rent enrichment and flux of noncohesive (> 10 mu m) silt, which we the n compared with records of the percentage and flux of ice-rafted detri tal ORD) sand, foraminifera, and coccoliths for the same interval. Cro ss-spectral comparisons were then made between these records and varia tions in a model of northern hemisphere ice volume (ice sheet growth a nd decay in response to orbitally forced insolation changes at 65 degr ees N). Variations in the relative proportions and fluxes of IRD sand, foraminifera, and coccoliths all covary closely with orbitally forced ice volume fluctuations, but with differing amplitude, coherency and phase relationships at each Milankovitch frequency. However, there is a significant component of variance in pelagic and ice-rafted sediment ation which is controlled by bottom currents, and this shows excellent phase and coherency agreement with the ice volume model at all orbita l frequencies. The relative enrichment, coarseness, and flux of the co ntourite silt fraction of Feni Drift sediments all increase during gla cial or cold stages of the mid-Pleistocene, relative to warm stages, s uggesting that currents were stronger during these periods. Possible e xplanations for such changes in current activity include: (1) an incre ased flux of NSOW via the Wyville-Thompson Ridge into the Rockall Trou gh during glacials, but not due to increased production of NSOW; (2) L DW, presently lying below 2500 m, rises during glacials by at least 50 -60 m, thereby flooding over the surface of Feni Ridge at site 610; (3 ) the path of a major recirculation gyre in the lower-NADW/LDW system, presently situated marginally south of site 610, encroaches upon the southern tip of Feni Ridge during glacials; and (4) the Rockall Trough itself is a site of deep water formation during glacials (analogous t o Labrador Sea Deep Water formation today).