Late Holocene changes in Atlantic Surface and Deep Water Circulation

Transcription

1 Late Holocene changes in Atlantic Surface and Deep Water Circulation Peter B. demenocal Lamont-Doherty Earth Observatory of Columbia University Gerard C. Bond Lamont-Doherty Earth Obs. Tom Marchitto Univ. Colorado, Boulder Brad Linsley SUNY at Albany Tom Guilderson CAMS, Lawrence Livermore Nat. Lab)

2 Deep-sea sediment coring R/V Ewing (Columbia s research ship) Multicoring & Gravity corer

3 Ocean sediment tracers of Holocene climate change

4 Holocene Climate North Atlantic Bond et al., 1997

5 Holocene cooling events Century-scale cooling events Multicentury-scale cycles Widespread - N. Atlantic 1-2 C amplitude apparently paced by (small) solar irradiance changes From Bond et al., 2001

6 Pacemaker of Holocene climate variability appears to have been solar variability... Bond et al., 2001

7 Questions 1. Late Holocene changes in N. Atlantic surface and deep ocean circulation? 2. Late Holocene amplitudes and rates of change relative to historical record? 3. Links to solar variability? Links to the subtropics and ITCZ?

8 Coring with Gerard in the North Atlantic (1998) MC11 GGC13 MC10 GGC09

9 A focus on Intermediate Waters Historically very sensitive to surface climate changes. Responsive to NAO-related surface climate changes (LSW is cooler, fresher, thicker during NAO+) Large property changes over recent decades ( T ~0.8 C, S ~0.1 psu) Large, multidecadal trends. It s possible to resolve changes of this magnitude. Labrador Sea Water LSW at 1800m (courtesy Ruth Curry, WHOI)

10 Historical LSW property trends 3.8 GSA Temp ( C) Trend: 1 C/cent THET SALT Trend: 0.1 ppt/cent Year (AD) Salinity (psu) (OWS Bravo, J. Lazier)

11 Decadal property trends in Nordic Seas Freshening of Nordic Seas over last years To ~1.5 km depth Salinity Associated with amplifying NAO. Density Tracked down to 8 N OWS Mike: Norwegian Sea t = 28.0

12 Approach Coring transect along Laurentian Slope Cores within modern LSW Multicores assure modern sediments. Bottom water T & S: Benthic foraminiferal Mg/Ca and 18 O measurements. Surface water Temperature: Planktonic foraminiferal Mg/Ca, 18 O, % N. pachy (s.).

13 Core Locations: Laurentian Slope Two Intermediate-water sediment cores m (MC11, GGC13) m (MC10, GGC09) MC23 Approx yr sampling MC11 GGC13 MC10 GGC09 LSW at 1800m (courtesy Ruth Curry, WHOI)

14 Local Hydrography LSW GGC13 GGC09

15 Reconstructing Bottom Water Temperatures Benthic foraminiferal Mg/Ca data from Lear et al., 2002 data from Hall & Chan, 2004 This Study r 2 = 0.91 C. pachyderma Marchitto and demenocal, 2003 (G-cubed)

16 Age control MC11/GGC13 ~42 cm/ka MC10/GGC09 ~20 cm/ka AMS Dates: G. bulloides R = 90±25 yrs 2-sigma errors 1030m 1850m

17 Bottom Water Temperatures Marchitto and demenocal, 2003 (G-cubed) Reproducibility ±0.2 C

18 Estimating bottom water salinity changes calcite 18 O decreases with temperature seawater 18 O increases with salinity Lynch-Stieglitz et al. (1999) Assume Mg/Ca monitors bottom water temperature (1 ±0.1 C replicate precision) Calculate 18 O seawater from Mg/Ca and 18 O calcite Calculate bottom water salinity from 18 O seawater (1 ±0.1 ppt joint error)

19 Estimating paleo-lsw properties

20 Theta - Salinity plot: Historical vs. Paleo LSW historical data LSW past 4000 yr 2x greater temperature variability 6x greater salinity variability Lower density during cold (fresh) periods

21 Probable Mechanisms Deep water cooling events: Most likely deep convection changes (UNADW..). Colder periods also fresher, consistent with instrumental record (but larger) Greater variability at deep site (and stability of shallow site) may indicate changes in UNADW rather than LSW. Higher bottom current speeds during cool periods at Björn Drift (1630m; Hall et al., 2004).

22 Estimating bottom water temperature rates of change

23 Bottom Water Temperature: Rates of Change Late Holocene Historical record OWS Bravo (J. Lazier) 2 ±2.4 C/cent. LSW cooled by 0.6 C between Multidecadal Trend: 1 C/century cooling Past rates greater than historical trend. Late Holocene rates ~2x greater than historical trend.

24 Bottom Water Salinity: Rates of change Late Holocene Historical record Very rough estimates due to large errors (±0.1 ppt) and low resolution. ±0.6 ppt / century (2 ) LSW trend Multidecadal trend: -0.1 ppt / century Late Holocene rates (much?) greater than historical trend.

25 Surface Water records: Core Locations MC11 GGC13 MC10 GGC09

26 Lab. Sea 55 N Surface Water records: Laur. Slope 45 N 2-3 C centennial-scale cooling (Interannual range: ±1 C) Consistent long-term trends Regionally Synchronous Some correspondence with Bond s lithic events

27 N. Atlantic Surface and Deep waters

28 Links to the subtropics? Florida Straits surface ocean Venezuela Humidity/runoff

29 Some Observations North Atlantic intermediate waters cooled by 1-2 C and freshened during late Holocene cool events. North Atlantic Surface and Deep water changes were similar. Amplitudes were much larger than historical. In the subtropical Atlantic, surface waters became saltier during cool events and ITCZ shifted southward Analogies to LGM? (e.g. Chiang et al., 2003; Haug et al., 2001; Schmidt et al., 2004).

30 Were solar minima associated with cooler/expanded subpolar gyre, a more smaller/constrained subtropical gyre, and southward displacement of ITCZ?

31

32 Intermediate water summary This system appears to have been very responsive. Upper NADW cooled by 1-2 C during late Holocene cool events. Amplitudes and rates of change were many times greater than historical observations. Most probably due to changes in deep convection linked to surface climate changes