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Marine Aspects of Abrupt Climate Change
William Curry
Woods Hole Oceanographic Institution
NSF ACGEO
April 28, 2004
Outline
Geological Evidence for Abrupt Climate Change
Role of Ocean Circulation
Changes in Tropical Salinity/Hydrology
Recent Changes in Ocean Salinity
How do you reconstruct past deep water circulation?
Using the chemistry of fossil organisms and the
nutrient chemistry of the deep water
Reconstructing the past gradients in sea water
density – geostrophy
Measuring the export of radionuclides by deep
water currents
NADW
GEOSECS
NADW
Kroopnick (1985)
Benthic Foraminifera
Cibicidoides spp.
Cibicides spp.
Planulina spp.
d13C of SCO2
~ d13C of CaCO3
Data sources:
Duplessy et al., 1984
Curry and Lohmann, 1990
Mackensen et al., 1993
McCorkle and Keigwin, 1994
Glacial Period
Reduced Density Shear in
Florida Current during
glacial period
Lynch-Stieglitz et al. (1999)
Holocene
Glacial
Transport
Millennial-scale Variability
GISP2
H1
YD
NADW on
NADW off
Grootes and Stuiver, 1997
Bermuda Rise
Stuiver, et al., 1995
Boyle and Keigwin, 1987
231
Pa /
230
Th Method
Atlantic Ocean
235U
234U
231Pa
230Th
Burial
231Pa/230Th
= 0.093
Production
231Pa/230Th
= 0.093
231
Pa /
230
Th
GISP2
H1
YD
NADW on
NADW off
0.093
Bermuda Rise
McManus et al. (2004)
How do you reconstruct past salinity?
Using the oxygen isotopic and minor element
chemistry of fossil organisms
T = 16.5 – 4.3(dCaCO3- dwater) + 0.14(dCaCO3- dwater)2
T = ln( [Mg/Ca] / 0.38 ) / 0.09
dwater ~ [ dCaCO3, Mg/Ca ]
d18Ow and Salinity
GISP2
H1
YD
Maximum in tropical
salinity when North
Atlantic is cold
Atlantic
Pacific
Schmidt et al. (2004)
Stott et al. (2002)
Summary of Geological Evidence
Three lines of evidence imply that North Atlantic
circulation changes were linked to abrupt climate
changes – tracers, geostrophy, radionuclides
Reduced volume and mass flux of NADW, and most
likely a change in poleward heat transport
Freshening of subpolar North Atlantic linked to changes
in circulation – generally believed to be catastrophic
freshening
Tropical Atlantic and Pacific Oceans (a lot) saltier while
North Atlantic fresher – changes in hydrological cycle
must have been involved
Global SSS E-P distributions
Source: HydroBase2
Source: R.Schmitt,WHOI
Sea surface salinity distributions are determined
in large part by E-P patterns
High Latitude Freshening
North Atlantic deep waters have
become progressively fresher
since the 1960s
Dickson et al. (2002)
Reduced salinity in
Greenland Sea
throughout the water
column
Curry and Mauritzan, in preparation
Largest effects in the
upper 1000 m
Curry and Mauritzan, in preparation
Reduced N-S gradient
in density across the
Denmark Strait – 1990s
gradient is about 90% of
1960s gradient
Curry and Mauritzan, in preparation
Subtropical North Atlantic
Integrated across 24°N –
the latitude
of the Northern Hemisphere
salinity maximum -- the
changes translate to a 510% increase in net
evaporation rates over the
last
four decades
Curry et al. (2003) Nature
Pacific
Hawaii Ocean Time Series (HOTS) have increased since the late 1980s –
continuing a trend first noted by Wong et al. (2001)
Atlantic Ocean
Comparing observations from
1955-69 with 1985-99, we find
recent freshening at both ends
of a long meridional section
through the deep western basins
contrasted with salinification of
the surface waters in the
intervening tropics and subtropics
of both hemispheres. Salinity
increases at mid depths reflect
the trans-ocean spreading of
Mediterranean Outflow Waters
toward the western boundary and
into the South Atlantic.
Curry, Dickson and Yashayaev, Nature in press
Atlantic Salinity Change
Salinity Difference
1985-99 minus 1955-69
Salinity increases range 0.1 to 0.4 psu in all Atlantic
waters ventilated in the high evaporation regions of the
tropics and subtropics
Curry et al. (2003) Nature
Indo – Pacific
Comparing the 1960s to 1985-94, Wong et al (2001)
found systematic salinity decreases in the intermediate waters of both hemispheres
opposing salinity increases in the surface waters of the tropics and subtropics.
Indian
SAMW
Figure courtesy of A. Wong
Pacific Sections
Summary of Ocean Changes
Tropical Atlantic increased in salinity by 0.3 psu in last 40 years;
evidence - also observed in tropical Pacific (Wong et al., 2001)
Subpolar regions of North Atlantic freshening throughout water
column – 0.05 psu in 40 years. Subpolar freshening also in
South Atlantic, as well as both hemispheres of the Indo-Pacific
(Wong et al., 2001)
Freshening can be traced in Atlantic western boundary deep
waters to the equator
Greenland Sea surface water freshening has increased density
stratification and buoyancy – a fresh water lid?
The same salinity changes seen in geological examples of
abrupt climate change