Ichthyolith strontium isotope stratigraphy of a Neogene red clay sequence: calibrating eolian dust accumulation rates in the central North Pacific

Cenozoic pelagic (‘red’) clays of predominantly eolian and hydrogenous origin blanket much of the central North Pacific ocean basin. The eolian component is a key indicator of past paleoclimatic conditions; thus, Cenozoic atmospheric circulation can potentially be reconstructed through provenance studies of Pacific red clays, provided there are precise age controls. Methods commonly employed in the past to date red clay cores have included cobalt accumulation rates, ichthyolith biostratigraphy, magnetostratigraphy, and ichthyolith strontium isotope stratigraphy. The first two dating methods yield ages with large uncertainties, while magnetostratigraphy is only relevant to cores with accumulation rates in excess of 1 mm/kyr. Ichthyolith strontium isotope stratigraphy has shown promise as a chronological tool in marine studies, but has been only sparingly employed in the dating of marine red clay sequences. In this study, we present a complete age–depth profile for a large diameter piston core from the central North Pacific Ocean (EW9709 PC-01, 32.5°N, 141.2°W), consisting of 11 m of primarily wind-deposited dust. To generate this age–depth profile, strontium isotopic compositions were determined on fish teeth ichthyoliths previously cleaned of contaminants using a newly modified reductive cleaning procedure. Ages were determined by reference to the recently refined Sr isotope curve for Neogene seawater. Comparison with nearby giant piston core LL44-GPC3 reinforces the accuracy of our methods. The data for EW9709 PC-01 indicate a fairly constant sediment accumulation rate of ∼0.45 mm/kyr over most of the 24 Myr time period represented by this core.