Late Holocene Changes in Northwest Atlantic Ocean Temperatures Peter deMenocal Tom Marchitto (Lamont-Doherty Earth Obs

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 ~3.2°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.

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 => 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 duringIRD events LSW cold duringglacial 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 T:S 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 6.6±0.7°C • 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 (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.

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 Variabilityand Climate • Long history of proposed linkages (Blanford, 1891!) • Cosmogenic isotopes: 10Be, 14C • Contains decadal- to millennial-scale variability • 0.25% solar constantvariation = 0.50°C ∆T. From Stuiver et al. (1998)

31. Labrador Sea Water at 1800m (Pot. Vorticity minimum) (from R. Curry, WHOI)