late holocene changes in northwest atlantic ocean temperatures peter demenocal tom marchitto...
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![Page 1: Late Holocene Changes in Northwest Atlantic Ocean Temperatures Peter deMenocal Tom Marchitto (Lamont-Doherty Earth Obs) Tom Guilderson (CAMS, Lawrence](https://reader030.vdocuments.mx/reader030/viewer/2022032521/56649d5d5503460f94a3b65d/html5/thumbnails/1.jpg)
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)
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Holocene 1-2 kyr ice rafting cycles(Bond et al., 2001)
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N. Atlantic Holocene climate records
Surface cooling was widespread... synchronous everywhere?
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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
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Orphan Knoll: MC23, GGC024
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Labrador Sea Bloom: May-June
Nova Scotia
Newfoundland
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Orphan Knoll: Hydrographic Setting
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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.
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LSW Shutdown (1968-1973)
GSA
(Lazier, 1980)
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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
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Foraminiferal Mg/Ca vs. temperature
C. pachyderma
data from Lear et al. (2002)
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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)
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Laurentian Slope core 10MC/09GGC Mg/Ca and 18O data (1854 m)
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LSW cold during IRD events LSW cold during glacial advances
10MC/09GGC results vs. time
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Estimating “paleo-LSW” properties
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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
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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
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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.
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Labrador SSTs WARM during “cool events”!
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Summary of results
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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.
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Pacemaker of Holocene climate variability appears to have been solar luminosity ...
Bond et al., 2001
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Solar Variability: Century-scale “pulsing” of Solar luminosity
Only ~0.25% variability of incoming radiation (visible)
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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.
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Modeled surface temperature changesduring the Maunder Minimum (ca. 1680 AD)
Annual Temperature change (°C; Shindell et al., 2002)
Persistent negative NAO pattern
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Longest European climate records also suggest “persistent negative NAO” during the LIA
(Luterbacher et al., 2002)
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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).
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Labrador Sea Water at 1800m (Pot. Vorticity minimum)
MC10GGC09
(from R. Curry, WHOI)
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Labrador Sea Water convection(TTO & WOCE data)
deepconvection
shallowconvection
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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)
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Labrador Sea Water at 1800m (Pot. Vorticity minimum)
(from R. Curry, WHOI)