Abstract

Holocene sediments from the central Santa Barbara Basin (SBB) contain invaluable paleoceanographic information and document paleoclimatic supra-regional teleconnections. The well-constrained dating and stratigraphy of the SBB can serve as a model for the paleoclimatic exploitation of laminated sediments elsewhere. Here we present an improved 6,700-yr master chronology that is based on sequential downcore counting of annual varves and 14C-AMS dating, starting from the June 2004 sediment surface. Diagnostic lamination patterns and the presence of prominent marker layers in an ~ 9 m long sequence of X-radiographic images from various cores (including ODP Hole 893A and piston core 6P) offer convenient dating for other central SBB sediment cores via direct X-radiographic and/or visual correlation against the master chronology presented here. The new sequence of X-radiographs of piston core 6P spans the coring hiatus at 7.04 mbsf in ODP Hole 893A. Varve-count ages agree well with an independent chronology based on calibrated 14C-ages of planktic foraminifers. In contrast, 14C ages of benthic foraminifers and of kerogen from flood layers are more strongly affected by variable 14C reservoir effects, and are therefore of limited value for absolute dating of SBB sediment layers. A late Holocene shift towards larger 14C age differences between planktic and benthic foraminifers after ~ 2,000 yr. B.P. suggests influx of a more 14C-depleted (thus older and oxygen-depleted) source of sub-sill bottom water into the SBB, consistent with the absence of bioturbating benthos and the enhanced preservation of varves observed in the cores. The independent varve chronology allowed us to derive a record of the regional radiocarbon reservoir age (ΔR), by comparing with the 14C dates for the last 6,700 years.

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