Late Holocene Changes in

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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)
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Holocene 1-2 kyr cooling cycles
 Century-scale cooling events  Widespread  1-2
C amplitude  Transitions within decades  Deep
circulation responses Bond et al., 1997,
2001 deMenocal et al., 2000 Marchitto and
deMenocal, subm. Keigwin, 1996 Dahl-Jenssen et
al., 1998 Lamb, 1995 Bianchi et al., 1999.
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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 d18O composition of N.
  pachyderma (right) to monitor Late Holocene
  changes in NW Atlantic SSTs.
• Core site situated at southeastern edge of the
  subpolar gyre
• Well dated, detailed record

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Talk Overview

• Part 1 Core locations, setting, and hydrography
• Part 2 Mg/Ca calibration and foram ecology
• Part 3 The records Did the NW Atlantic warm
  during late Holocene cool events?
• Part 4 Implications and Conclusions

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Part 1 Orphan Knoll MC23, GGC024 (3530 m)
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NW Atlantic Foram Productivity

• Spring and fall blooms
• Most of N. pachy. flux occurs in late Spring
  (May-June)
• Due to lengthening day, warming surface waters,
  and nutrient availability.

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Orphan Knoll Hydrography
N. pachyderma (right) records May-June conditions
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Part 2 Mg/Ca vs. SST calibration

• NW Atlantic coretops
• Seven cores lt3500m
• 45-62N, 28-55W
• N. pachy (r) Mg/Ca consistent with annual avg.
  SST
• G. bulloides reflects calcification in warmer
  seasonal waters, and/or heterog. shell Mg.
• Consistent with Elderfield and Gansen (2000)
  results.

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Cores and Methods

• Orphan Knoll
• Multicore-gravity core pair MC-23/GGC24 (3530m)
• Bomb 14C coretop, 15 AMS radiocarbon dates (G.
  bulloides)
• 14 cm/ka sed. rate 20-80 year resolution
• Planktonic Mg/Ca - N. pachy. (right)
• 75 specimens, 150-212µm size, cleaned
• Analysis by ICP-AES
• 40 replicates 0.13 mmol/mol error
• Elderfield and Gansen (2000) temperature equation

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Part 3 MC23 Mg/Ca and d18O
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NW Atlantic SSTs warmed during Holocene cool
events

• NW Atlantic was consistently warmer during
  Holocene lithic events.
• Warming was 3-5C
• Negative NAO-like signature (i.e cool NE
  Atlantic N. Europe, warm Labrador Sea)

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Implications for Surface Water property changes

• Modern Surface Waters
• SST 7.5C
• Salinity 34.5 psu

• Little Ice Age Surface Waters
• SST 12C
• Salinity 35.7 psu (calcd from Mg/Ca and d18O)
• LIA properties similar to modern N. Atlantic
  Drift water

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Is this really a record of surface conditions?

• Changes in seasonality?
• No evidence for isotopic shifts between
  bulloides, N. pachy (r,l) (Hillaire-Marcel,
  deVernal, Sandrine data).

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Is this really a record of surface conditions?

• Changes in seasonality?
• No evidence for isotopic shifts between
  bulloides, N. pachy (r,l) (Sandrine,
  Hillaire-Marcel, deVernal, unpub).
• Dissolution?
• No apparent correlation between shell weight
  Mg/Ca.

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Is this really a record of surface conditions?

• Changes in seasonality?
• No evidence for isotopic shifts between
  bulloides, N. pachy (r,l) (Sandrine,
  Hillaire-Marcel, deVernal, unpub).
• Dissolution?
• No apparent correlation between shell weight
  Mg/Ca.
• Changes in salinity?
• No. (Nürnberg et al., 1996)

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Part 4 Annular-Mode pattern for Holocene events?
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Solar Variability Century-scale pulsing of
Solar luminosity
Holocene event pacing linked to changes in solar
irradiance (Bond et al., 2001)
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NAO variability during the latest Holocene

• Persistent negative NAO during Late Maunder
  Minimum (AD 1650-1710)
• Cook et al., 2002 (Europe, N. America)
• Luterbacher et al., 2001 (Europe)
• Appenzeller et al., 1998 (Greenland)
• Mann et al., 1998 (Circum - North Atlantic)
• Nesje et al., 2002 (antiphase European glaciers)

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Correlation Map Surface Temps vs. Solar
irradiance (AD 1650-1850)
Shindell et al., 2001
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Modeled surface temperature changesduring the
Maunder Minimum (ca. 1680 AD)
Persistent negative NAO pattern
Annual Temperature change (C Shindell et al.,
2002)
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Summary
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Age Control

• 15 AMS 14C dates to 9.5 ka BP (G. bulloides)
• Bomb 14C coretop
• 14 cm/ka average sed. rate.
• 1.2-2.4 yr BP Low sed. rate interval (6 cm/ka)