Climatic influences on sediment deposition and turbidite frequency in the Nitinat Fan, British Columbia

Continental margins in regions influenced by temperate ice sheets experience climate-dependent changes in sedimentary processes. A new stratigraphy is presented for Ocean Drilling Program Hole 88813, from the Nitinat Fan, on the Vancouver Margin, in which we examine the relationship between fluctuations in glacial-sourced sediment delivered to the continental shelf and turbidite character and frequency. Glacial/interglacial episodes are determined based on: (1) core lithology, where sandy/coarse-grained sediment and fine-grained clay/silty clay represents glacial and interglacial intervals, respectively; (2) delta O-18 of planktonic foraminifera G. bulloides, in which higher values indicate warmer sea surface temperatures and lower global ice volume; (3) magnetic susceptibility, in which higher values indicate coarser grain sizes deposited during glacials: and (4) ratios of sinistral N. pachyderma to dextral N. incompta, in which higher proportions of N. pachyderma are a sign of cooler sea surface temperatures. Finally, using radiocarbon C-14 dates, specific Marine Isotope Stages (MIS) within the top 240 m of core are assigned: MIS 2-4 (2-118 mbsf), MIS 5 (118-157 mbsf), and MIS 6 (213-240 mbsf). Using this chronology, sedimentation rates for the Nitinat Fan were greater (187 cm/kyr) during glacial MIS 2-4 than interglacial MIS 5 (69 cm/kyr). Additionally, during glacial MIS 2-4 thicker turbidites (upwards of similar to 150 cm) were deposited relatively frequently (similar to 75 year periodicity) compared with those from MIS 5, which are relatively thin (up to 27 cm) and deposited more rarely (similar to 130 year periodicity). These results indicate that turbidite frequency and thickness are linked to climate through ice sheet extent, since transport of glacimarine sediment to the continental slope promotes turbidity currents that are responsible for the most significant amount of offshore sedimentation. This study shows that geological processes influencing turbidite deposition can fluctuate over time and may not necessarily be represented by ongoing processes within the modern depositional setting. (C) 2009 Elsevier B.V. All rights reserved.