Orbital forcing of bottom‐current enhanced sedimentation on Feni Drift, NE Atlantic, during the mid‐Pleistocene

Abstract

Feni Ridge is a plastered sediment drift, deposited along the northwestern flank of Rockall Trough, NE Atlantic, under the influence of deep, geostrophic currents formed by intermittent overflows of Arctic Intermediate Water from the Norwegian Sea across the Iceland‐Scotland Ridge (NSOW). Lower Deep Water (LDW), a derivative of Antarctic Bottom Water from the South Atlantic mixed with old North Atlantic Deep Water (NADW), also bathes the lower flanks of the drift, below 2500 m. Deep Sea Drilling Project site 610 was drilled in a sediment wave field on the surface of Feni Drift (2417 m), though the sediments recovered proved to be typically pelagic, preserving a high‐resolution (>5 cm kyr−1) Quaternary record of orbitally modulated ice‐rafting and biogenic productivity cycles but showing little evidence of current affected deposition. From particle size and compositional analyses of the biogenic and lithogenic < 63 µm fractions of samples taken at ∼2.5 kyr intervals between 0.5 and 1.2 Ma, we derived indices of bottom current enrichment and flux of noncohesive (> 10 µm) silt, which we then compared with records of the percentage and flux of ice‐rafted detrital (IRD) sand, foraminifera, and coccoliths for the same interval. Cross‐spectral comparisons were then made between these records and variations in a model of northern hemisphere ice volume (ice sheet growth and decay in response to orbitally forced insolation changes at 65°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 sedimentation which is controlled by bottom currents, and this shows excellent phase and coherency agreement with the ice volume model at all orbital frequencies. The relative enrichment, coarseness, and flux of the contourite silt fraction of Feni Drift sediments all increase during glacial or cold stages of the mid‐Pleistocene, relative to warm stages, suggesting that currents were stronger during these periods. Possible explanations for such changes in current activity include: (1) an increased flux of NSOW via the Wyville‐Thompson Ridge into the Rockall Trough during glacials, but not due to increased production of NSOW; (2) LDW, 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 to Labrador Sea Deep Water formation today).