Size-specific opal-bound nitrogen isotope measurements in North Pacific sediments

Contamination of N by non-diatomaceous opal should not normally compromise total-diatom-bound δ15N (0-150 μm) because the non-diatom opal typically contributes less than ∼5% to the total opal. However, the early deglacial (Heinrich Stadial 1-correlative) period in the subarctic North Pacific is an important possible exception: a substantial fraction of its low concentration of opal appears to be sponge spicules and radiolaria, such that the reconstructed total-diatom δ15Ndb decrease at this time may be an artifact. While the glacial-age sediments are also vulnerable to non-diatom contamination, this should work to lower δ15Ndb, such that the observed high glacial δ15Ndb in North Pacific sediments cannot be explained by contamination. Thus, the previous interpretation of enhanced nutrient consumption in the North Pacific regions during the last ice age remains valid.