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Structural, Tectonic and Sedimentary Issues |
Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK
Variations in pelagic input rates over abyssal hill areas cannot be inferred easily from sedimentation rate variations between cores because of redistribution by downslope gravity processes and bottom currents. Some spatial averaging or adjustment for sediment transport is required. Relatively accurate mean sedimentation rates may be obtained, however, by regressing sediment thickness with distance from a mid-ocean ridge spreading centre, where the regression averages out the variations due to local redistribution. This is shown using sediment profiler records from the Galapagos spreading centre (SC) at 1° N in the eastern Pacific. Physical property data from Deep Sea Drilling Project (DSDP) Leg 70 are used to correct sediment thicknesses for compaction and to convert sedimentation rates to mass accumulation rates (MARs), which are c. 2 g cm–2 ka–1 in this region. These high MARs are due to enhanced equatorial productivity of pelagic organisms, which is also reflected in a c. 6% higher MAR for the ridge flank closest to the equator, corresponding to a rate of change of MAR with latitude of c. 16% per degree. The equatorial high productivity zone in the Panama Basin lies further south than in the central Pacific; peak sedimentation along the Ocean Drilling Program (ODP) Leg 138 transect (110° W) occurs near 1° N so has nearly zero gradient at this latitude, but has comparable gradient to the Galapagos SC north of 2° N. The zone of peak enhancement at 86° W in the Panama Basin may therefore be 1° or more further south than at 110° W. Some further sedimentation characteristics of the Galapagos spreading centre are also described, such as a scaling of thickness variability and the possibility of dating sea floor using sediment thickness.
Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK
