Late Holocene variability of upper North Atlantic Deep Water temperature and salinity
T. Marchitto, P. deMenocal
Geochemistry, Geophysics, Geosystems, 4(12), 1100, doi: 10.1029/2003GC000598, 2003. Reprint (pdf)
Abstract. Magnesium/calcium ratios in benthic foraminifera (Cibicidoides pachyderma) from a sediment core on the Laurentian Slope (1854 m) exhibit strong millennial-scale fluctuations during the past 4000 yr. We convert these data to seawater paleotemperatures using a new monospecific linear equation. Results suggest that the temperature of upper North Atlantic Deep Water (dominated by Labrador Sea Water today) has varied by at least 2C during the late Holocene. Millennial scale coolings coincide with previously identified periods of increased drift ice and regional glacier advances, including the Little Ice Age. Paired oxygen isotope measurements indicate that salinity and perhaps density were reduced during the cold periods. We discuss possible mechanisms for transmitting this cold, fresh signal from surface waters to intermediate depths. Our reconstructed late Holocene ranges in upper North Atlantic Deep Water properties greatly exceed those of the instrumental record and imply that large changes may be yet to come.
Amplitudes and phasing of surface and deep NW Atlantic Ocean temperature changes during the late Holocene
P. deMenocal, T. Marchitto, T. Guilderson, C. Hillaire-Marcel
Talk presented at 2002 Fall AGU
Abstract. Within the Atlantic sector the NW Atlantic exhibits the largest range of interannual sea-surface temperature variability over the last 150 years of instrumental data. Most of the coherent SST variability can be attributed to large-scale reorganizations of atmosphere and ocean circulation fields associated with the North Atlantic Oscillation. At decadal and longer timescales, NAO-related surface ocean property changes regulate the composition and flux of Atlantic deep and intermediate waters. We examined the amplitude and phasing of NW Atlantic surface and deep ocean properties using a N-S transect of three well-dated late Holocene cores extending from South Greenland to Nova Scotia. In a core presently bathed by Labrador Sea Water (1850m), benthic foraminiferal Mg/Ca and stable isotopic measurements confirm that LSW was cooler and fresher during the Little Ice Age and prior cool periods associated with maxima in ice-rafted lithic grains. These excursions were several times greater than those observed over the last 80 years of instrumental data. Detailed Mg/Ca measurements on two species of planktonic foraminifera from two cores in the Labrador Sea document significantly warmer SSTs during the LIA and prior "cool" events over the last several thousand years. These results appear to confirm Keigwin and Pickart's (1999) initial results and extend the LIA warming to include the Labrador Sea where it is particularly well expressed. These results challenge the view that century-scale cooling events in the North Atlantic were zonally uniform and support recent modelling results by Shindell et al. (2001) which indicate that reduced solar irradiance during the LIA may have led to a dipolar SST pattern similar to a persistent negative phase of the NAO.
Holocene variations in upper North Atlantic Deep Water (Labrador Sea Water) properties
T. Marchitto, P. deMenocal
Project funded by NSF Earth Systems History program, 2002-2005
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