HR: 11:20h
AN: PP12B-05    [Abstracts]
TI: Ventilation of the Deep Western North Atlantic During the Little Ice Age
AU: * Keigwin, L
EM: lkeigwin@whoi.edu
AF: WHOI, 360 Woods Hole Rd., Woods Hole, MA 02543
AU: Guilderson, T P
EM: guilderson1@llnl.gov
AF: CAMS Lawrence Livermore Nat. Lab., 7000 East Ave. L-397, Livermore, CA 94551
AB: It is commonly assumed that millennial scale changes in Holocene climate are a continuation of processes that occurred during glacial climate, with reduced amplitude. Implicit in this assumption is the belief that Holocene cold events are associated with reduced strength of the meridional overturning circulation (MOC). However, the paleo data for Holocene MOC change are weak and often contradictory. On the Bermuda Rise, for example, d13C of benthic foraminifera decreases suddenly during the Little Ice Age (LIA, beginning around 500 years ago), but Cd/Ca does not change. To bring another proxy for deep ocean ventilation to bear on the problem, we measured D14C on benthic foraminifera. We found that core top (zero age) benthic foraminifera have a D14C of -129 permil, whereas D14C during the beginning of the LIA was -86 permil, after 'undecaying' the data based on the planktonic foram calendar age. The core top result is consistent with WOCE 14C data for the waters bathing the Bermuda Rise (~4.6 km water depth). If we consider deep waters in the western North Atlantic to be a mixture of northern and southern source end members with D14C of -70 permil and -160 permil, respectively, we estimate bottom waters today at are a 50/50 mixture, whereas during the LIA the mixture was greater than 90 % northern source water. This result is inconsistent with both the d13C and Cd/Ca data, and suggests that during the most recent of the millennial cold episodes of the Holocene, North Atlantic Deep Water was enhanced rather than suppressed. This outcome is consistent with more extreme negative phases of the North Atlantic Oscillation during the LIA, if greater convection in the Greenland Sea led to greater overflow at Denmark Strait. If this is correct, then at least for the LIA, climate change caused MOC change.
DE: 1620 Climate dynamics (0429, 3309)
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005