L. E. Lisiecki & M. E. Raymo
PP42B-07 Talk: Thursday 11:50 in MW 2002
Many studies have shown that times of increased ice volume, particularly the last glacial maximum (LGM), correspond to decreases in North Atlantic deepwater (NADW) at depths below approximately 2000 m. Here we investigate the different effects of obliquity and precession forcing on NADW using 30 benthic d13C records spanning the last 425 kyr. We find that obliquity-forced minima in high latitude summer insolation correlate with lower benthic d13C values at Atlantic sites below 2500 m, consistent with reduced production of lower NADW as at the LGM. In contrast, precession-forced minima in northern hemisphere summer insolation correspond with greater benthic d13C values at Atlantic sites from 2500 - 4000 m water depth, suggesting increased production of lower NADW. Because obliquity and precession produce different responses to the same change in high-latitude northern insolation at the summer solstice, lower NADW production is likely more sensitive to insolation at a different latitude or season, perhaps due to sea ice formation, freshwater flux, or meridional heat transport.
L. E. Lisiecki & M. E. Raymo
PP51B-0582 Poster Friday AM
Because a large fraction of benthic d18O change is due to global ice volume change, benthic d18O is often used as stratigraphic tool to place marine records on a common age model and as a proxy for the timing of ice volume/sea level change. These applications require the assumptions that d18O change is rapidly transmitted throughout the deep ocean and that the effects of hydrographic changes are in phase with ice volume. Recently, Skinner and Shackleton [2005] found that the timing of benthic d18O change at the last termination differed by 4000 years between two sites in the Atlantic and Pacific. Based on Mg/Ca paleothermometry, they argued that these age discrepancies resulted from a late temperature increase in the Pacific and millennial-scale circulation changes in the Atlantic. Do these results imply that benthic d18O change may not accurately record the timing of terminations? We compare benthicd18O records from 34 sites in the Atlantic and Pacific to evaluate the impact of ocean mixing rates and deep water changes on the relative timing of terminations recorded in benthic d18O. Statistical analysis of sedimentation rates derived from the alignment of benthic d18O suggests an Atlantic lead over Pacific benthic d18O change during the last 6 terminations. We estimate an average termination age difference of 1300 years between the Atlantic and Pacific, approximately consistent with the delay expected due to ocean mixing rates, given that most glacial meltwater probably enters the North Atlantic. However, we also find evidence of brief 4000 yr lags during the middle of several terminations, suggesting that termination mid-points may make poor d18O tie points.