Late Holocene Changes in

1
Late Holocene Changes in Northwest Atlantic
Ocean Temperatures Peter deMenocal Tom Marchitto
(Lamont-Doherty Earth Obs) Tom Guilderson (CAMS,
Lawrence Livermore Nat. Lab) Claude
Hillaire-Marcel (GEOTOP, Univ. Montréal,
Canada)
2
Holocene 1-2 kyr ice rafting cycles(Bond et al.,
2001)
3
N. Atlantic Holocene climate records
Surface cooling was widespread... synchronous
everywhere?
4
The Plan ...

• Measure Mg/Ca and d18O composition of N.
  pachyderma (right) to monitor Late Holocene
  changes in NW Atlantic SSTs
• Core site situated near the subpolar gyre - N.
  Atlantic Drift boundary
• Is N. pachy (right) a faithful, surface-dwelling
  species?
• How well does NPR Mg/Ca composition track SSTs?
• How large were past SST changes in this region?
• How do these changes compare with lithic indices?
• Implications conclusions

5
Orphan Knoll MC23, GGC024
6
Labrador Sea Bloom May-June
Nova Scotia
Newfoundland
7
Orphan Knoll Hydrographic Setting
8
Labrador Sea Water

• LSW spans 600-2000m T 3.2C, S 34.85 psu
• LSW historically very sensitive to surface
  climate changes.
• Responds to NAO forcing of surface climate and
  fluxes
• During high NAO state
• Cooling of Lab. Sea SSTs
• LSW formed is cooler, fresher, and thicker.
• Very rapid response (LSW vintages) Sy et al.,
  1997.
• Upper NADW (LSW) ventilation 4 Sv.

9
LSW Shutdown (1968-1973)
GSA
(Lazier, 1980)
10
Reconstruct Holocene changes in upper NADW

• Multicore (10MC) and Gravity core (09GGC) taken
  in 1998.
• Sedimented spur on Laurentian Slope.
• 1850 m water depth.
• Within the modern core of LSW (upper NADW).
• 16 cm/kyr sed. rate.

10MC 09GGC
11
Foraminiferal Mg/Ca vs. temperature
data from Lear et al. (2002)
C. pachyderma
12
Foraminiferal d18O as a temperature/salinity proxy
calcite d18O decreases with temperature
seawater d18O increases with salinity
Lynch-Stieglitz et al. (1999)
Mg/Ca d18Oforam gt f(T, d18Osw, S) Mg/Ca
f(T) d18Oforam f(d18Osw, T) d18Osw f(S)
13
Laurentian Slope core 10MC/09GGC Mg/Ca and d18O
data (1854 m)
14
10MC/09GGC results vs. time
LSW cold during IRD events
LSW cold during glacial advances
15
Estimating paleo-LSW properties
16
Late Holocene paleo-LSW properties
LSW instrumental (Yashayaev et al., in press)
LSW past 4000 yr

• much greater TS variability than instrumental
  record
• reduced density during cold, fresh periods

17
Part 2 Labrador SSTs during the late Holocene

• Two cores from the Labrador Sea
• Orphan Knoll - Multicore (23MC) and Gravity core
  (24GGC) taken in 1998.
• S. Greenland - Box core 90-013-017 taken by C.
  Hillaire-Marcel (Univ. Quebec).

90-013-017
23MC 24GGC
18
Mg/Ca and modern Labrador SSTs

• Southern Labrador Sea core site (23MC)
• Mg/Ca on N. pachyderma (right)
• Coretop Mg/Ca value indicates modern SST of
  6.60.7C
• Consistent with sediment trap evidence for late
  spring bloom.

19
Labrador SSTs WARM during cool events!
20
Summary of results
21
Part 3 Implications

• Cooling and freshening of upper NADW during late
  Holocene cool events.
• Changes were many times larger than historical.
• During cool events, LSW (upper NADW) may have
  formed elsewhere because ...
• Labrador Sea was warm during the LIA and latest
  Holocene cool events.
• Supports initial findings by Keigwin and Pickart
  (1999).
• Suggests that the Holocene events may have a
  NAO-like signature - regionally assymetric.

22
Pacemaker of Holocene climate variability appears
to have been solar luminosity ...
Bond et al., 2001
23
Solar Variability Century-scale pulsing of
Solar luminosity
Only 0.25 variability of incoming radiation
(visible)
24
Regional climate responses to solar variability

• Shindell et al. (2001) simulated climate during
  the Maunder Minimum (1680s) using a GCM with
  full stratosphere representation.
• Reduced irradiance during the Maunder minimum led
  to strat. ozone redistributions which amplified
  the cooling (global cooling of -0.4C).
• Modeled surface temperature changes resembled a
  negative NAO pattern, with cooling over northern
  Eurasia and warming over the Labrador Sea region.

25
Modeled surface temperature changesduring the
Maunder Minimum (ca. 1680 AD)
Persistent negative NAO pattern
Annual Temperature change (C Shindell et al.,
2002)
26
Longest European climate records also suggest
persistent negative NAO during the LIA
(Luterbacher et al., 2002)
27
Negative NAO climate signatures during the LIA?

• Northern Eurasia, N. Atlantic cool? YES
• Labrador Sea warms? YES
• Reduced Labrador Sea Water formation?
• Perhaps. LSW may have shoaled above core depth
• Cooler tropical ocean SSTs? (Hoerling et al.,
  2001)
• Perhaps. Cooler and drier western tropical
  Atlantic during LIA (Black et al., 1999 Haug et
  al., 2001 deMenocal et al., 2000).

28
Labrador Sea Water at 1800m (Pot. Vorticity
minimum)
MC10 GGC09
(from R. Curry, WHOI)
29
Labrador Sea Water convection(TTO WOCE data)
shallow convection
deep convection
30
Solar Variability and Climate

• Long history of proposed linkages (Blanford,
  1891!)
• Cosmogenic isotopes 10Be, 14C
• Contains decadal- to millennial-scale variability
• 0.25 solar constant variation 0.50C ?T.

From Stuiver et al. (1998)
31
Labrador Sea Water at 1800m (Pot. Vorticity
minimum)
(from R. Curry, WHOI)