Late Holocene Changes in Northwest Atlantic Ocean Temperatures Peter deMenocal Tom Marchitto...

Late Holocene Changes in Northwest Atlantic Ocean Temperatures

Peter deMenocalTom Marchitto (Lamont-Doherty Earth Obs)

Tom Guilderson (CAMS, Lawrence Livermore Nat. Lab)

Claude Hillaire-Marcel (GEOTOP, Univ. Montréal, Canada)

Holocene 1-2 kyr ice rafting cycles(Bond et al., 2001)

N. Atlantic Holocene climate records

Surface cooling was widespread... synchronous everywhere?

The Plan ...

• Measure Mg/Ca and 18O 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

Orphan Knoll: MC23, GGC024

Labrador Sea Bloom: May-June

Nova Scotia

Newfoundland

Orphan Knoll: Hydrographic Setting

Labrador Sea Water

• LSW spans 600-2000m; T ~3.2°C, 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.

LSW Shutdown (1968-1973)

GSA

(Lazier, 1980)

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.

10MC09GGC

Foraminiferal Mg/Ca vs. temperature

C. pachyderma

data from Lear et al. (2002)

Foraminiferal 18O as a temperature/salinity proxy

calcite 18O decreases with temperature

seawater 18O increases with salinity

Mg/Ca + 18Oforam => f(T, 18Osw, S)

Mg/Ca = f(T)

18Oforam = f(18Osw, T)

18Osw = f(S)

Lynch-Stieglitz et al. (1999)

Laurentian Slope core 10MC/09GGC Mg/Ca and 18O data (1854 m)

LSW cold during IRD events LSW cold during glacial advances

10MC/09GGC results vs. time

Estimating “paleo-LSW” properties

Late Holocene “paleo-LSW” properties

LSW instrumental (Yashayaev et al., in press)

LSW past 4000 yr

• much greater T:S variability than instrumental record• reduced density during cold, fresh periods

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).

23MC24GGC

90-013-017

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 6.6±0.7°C

• Consistent with sediment trap evidence for late spring bloom.

Labrador SSTs WARM during “cool events”!

Summary of results

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.

Pacemaker of Holocene climate variability appears to have been solar luminosity ...

Bond et al., 2001

Solar Variability: Century-scale “pulsing” of Solar luminosity

Only ~0.25% variability of incoming radiation (visible)

Regional climate responses to solar variability

• Shindell et al. (2001) simulated climate during the Maunder Minimum (1680’s) 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 -0.4°C).

• Modeled surface temperature changes resembled a negative NAO pattern, with cooling over northern Eurasia and warming over the Labrador Sea region.

Modeled surface temperature changesduring the Maunder Minimum (ca. 1680 AD)

Annual Temperature change (°C; Shindell et al., 2002)

Persistent negative NAO pattern

Longest European climate records also suggest “persistent negative NAO” during the LIA

(Luterbacher et al., 2002)

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).

Labrador Sea Water at 1800m (Pot. Vorticity minimum)

MC10GGC09

(from R. Curry, WHOI)

Labrador Sea Water convection(TTO & WOCE data)

deepconvection

shallowconvection

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 = 0.50°C ∆T.

From Stuiver et al. (1998)

Labrador Sea Water at 1800m (Pot. Vorticity minimum)

(from R. Curry, WHOI)