Carbon 13 in Pacific Deep and Intermediate Waters, 0‐370 ka: Implications for Ocean Circulation and Pleistocene CO 2

Stable isotopes in benthic foraminifera from Pacific sediments are used to assess hypotheses of systematic shifts in the depth distribution of oceanic nutrients and carbon during the ice ages. The carbon isotope differences between ∼1400 and ∼3200 m depth in the eastern Pacific are consistently greater in glacial than interglacial maxima over the last ∼370 kyr. This phenomenon of “bottom heavy” glacial nutrient distributions, which Boyle proposed as a cause of Pleistocene CO 2 change, occurs primarily in the 1/100 and 1/41 kyr −1 “Milankovitch” orbital frequency bands but appears to lack a coherent 1/23 kyr −1 band related to orbital precession. Averaged over oxygen‐isotope stages, glacial δ 13 C gradients from ∼1400 to ∼3200 m depth are 0.1‰ greater than interglacial gradients. The range of extreme shifts is somewhat larger, 0.2 to 0.5‰. In both cases, these changes in Pacific δ 13 C distributions are much smaller than observed in shorter records from the North Atlantic. This may be too small to be a dominant cause of atmospheric pCO 2 change, unless current models underestimate the sensitivity of pCO 2 to nutrient redistributions. This dampening of Pacific relative to Atlantic δ 13 C depth gradient favors a North Atlantic origin of the phenomenon, although local variations of Pacific intermediate water masses can not be excluded at present.