Atlantic Meridional Overturning Circulation (AMOC) = thermohaline circulation in N Atlantic. Wikipedia

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1 Last time.

2 Atlantic Meridional Overturning Circulation (AMOC) = thermohaline circulation in N Atlantic Wikipedia

3 Dansgaard-Oeschger events HOLOCENE ice record smeared out here Last interglacial

4 Dansgaard-Oeschger events occurred throughout N hemisphere and into tropics

5 Monday s main points 1) Ice-age changes in ocean temperatures 2) Oceanic trigger points in global climate change 3) Western boundary currents, thermohaline circulation 4) Atlantic Meridional Overturning Circulation (AMOC) 5) Why the Atlantic Ocean is key in global climate changes

6 Millennial-Scale Climate Changes 6) The Greenland Ice Sheet as a climate archive 7) Dansgaard-Oeschger Events: quick, radical, and mysterious

7 Today s main points 1) two types of millennial-scale climate changes in N Atlantic 2) Heinrich Events: climate impacts 3) Hypotheses about causes of Heinrich Events 4) Possible hysteresis loop controlled by AMOC shutdown

8 1960s: Dansgaard and Oeschger discover D-O cycles in Gcamp Century ice core 1980s: Hartmut Heinrich discovers Heinrich Events in Atlantic deep-sea cores Heinrich, H. (March 1988). "Origin and consequences of cyclic ice rafting in the northeast Atlantic Ocean during the past 130,000 years". Quaternary Research. 29 (2): doi: / (88)

9 Heinrich Events Episodes of intense deposition of iceberg-rafted debris (IRD) in North Atlantic during the late Pleistocene First described by Hartmut Heinrich, oceanographer and physicist, Federal Maritime and Hydrographic Agency, Hamburg occurred roughly every ,000 years during late Pleistocene

10 iceberg-rafted debris (IRD)

11 Heinrich events: cold culminations of the cooling phases of (some) D-O events

12 Heinrich events record calving collapses of northern hemisphere ice shelves, and the consequent release of a prodigious volume of shelf ice, sea ice, and icebergs from outlet glaciers. H. events were rapid: they lasted around 750 years, and their abrupt onset occurred in just several years.

13 Start dates of Heinrich events (calendar years BP) H0 ~12,000 H1 16,800 H2 24,000 H3 ~31,000 H4 38,000 H5 45, H6 ~60, red numbers = years between H events in 1000s of years

14 Start dates of Heinrich events (calendar years BP) H0 ~12,000 H1 16,800 H2 24,000 H3 ~31,000 H4 38,000 H5 45, H6 ~60, red numbers = years between H events in 1000s of years

15 2 types of millennial-scale temperature changes in N Atlantic Dansgaard-Oeschger events: roughly every 1500 years Heinrich events: 2800 to 15,000 years between

16 2 types of millennial-scale temperature changes in N Atlantic

17 No evidence of Heinrich events during penultimate glaciation ratio of calcium versus strontium (Ca/Sr) from a marine North Atlantic drill site (IODP U1308), that serves as a proxy for the mineralogically-distinctive "detrital carbonate" that characterizes Heinrich events is shown in blue (Hodell et al., 2008). Shown in red are detrital carbonate over the last three glaciations (Bond et al., 1999; Obrochta et al., 2012, Obrochta et al., 2014) confirms that Ca/Sr is an indicator of Heinrich events.

18 Lithology suggests that at start of Heinrich Events, icebergs came mostly from nearby Iceland; later in event, they were mostly from Hudson Strait

19 During Heinrich Events: 1) SST in North Atlantic cooled dramatically 2) Polar forams increased in abundance 3) del 18 O in deep sea indicates influx of lighter 16 O (meltwater)

20 During Heinrich Events: 4) Decline N Atlantic salinity 5) Higher wind velocities at high latitude (increased flux of Asian dust to Greenland) 6) del 18 O of Chinese speleothems becomes less negative, suggesting low latitude cooling

21 Heinrich Events cool the North Atlantic, shut down the AMOC

22 What caused D-O Oscillations and Heinrich Events? ice-rafted quartz grain noc.soton.ac.uk

23 Clue #1: Heinrich events tended to occur immediately before the rapid warming of the warmest D-O Events So.at the cool ends of some D-O events

24 CLUE #2: Heinrich Events accompanied global climatic changes but with complex, regional responses warmer, wetter Global climate anomalies during Heinrich events and between Dansgaard-Oeschger events. Overpeck and Cole, 2006.

25 For example.monsoonal precip. in SE China tracked temperature changes in Greenland that record D-O and Heinrich events China YD Greenland

26 More complex, regional responses Florida was warmer and wetter when the N Atlantic was colder during H. Events. QSR 2006

27 CLUE #3: Strongest impacts of Heinrich Events on climate were in northern hemisphere ANT Greenland

28

29 most viable mechanism for abrupt climate changes in the North Atlantic region is still associated with reorganizations of the ocean circulation as originally proposed by Ruddiman and McIntyre [1981] to interpret the large shifts they found in planktonic foraminifera assemblages in North Atlantic marine sediments during the last deglaciation. Thanks for reading!

30 Ok, so what were these ocean reorganizations and what caused them? allearth.org/

31 the "binge purge glaciology hypothesis of Heirnrich Events based on internal ice dynamics MacAyeal, D. R. Binge/purge oscillations of the Laurentide ice sheet as a cause of the North Atlantic s Heinrich Events. Paleoceonography, Vol 8, No. 6, pgs , 1993.

32 Rate of glacier sliding (hence rate of mass transfer) depends on temperature and pressure at bed swisseduc.ch Canadian Rockies glacier movement = sliding at bed + internal deformation

33 Decaying radioactive isotopes in Earth generate heat flow = 1/10 W per m 2 on average over continental crust swisseduc.ch East Antarctic: heated from below and piling up

34 The thicker the ice, the greater its basal pressure, and the more heat it retains from geothermal heat flux swisseduc.ch regelation ice at base of Swiss glacier

35 Basal melt fraction Marshall and Clark (2002) Geophys Res Lett: Basal temperature evolution of North American ice sheets and implications for the 100-kyr cycle Ice sheet thickening

36 Laurentide Ice Sheet undergoes long periods of growth (binge) interrupted by short periods of rapid discharge (purge) During binge period (~8000 years) ice builds up over Hudson Bay while geothermal heat slowly warms glacial bed (basal shear stress also increasing as ice thickens)

37 Warming bed finally converts from frozen to thawed Liquid water on bed triggers surge of outlet glaciers Ice sheet then takes ca years to build up again Douglas MacAyeal, University of Chicago (center)

38 Paleoceanography 1993

39 binging (growth phase)

40 still binging

41 More icebergs released purge phase (glacial surge) + bed lubricated by meltwater

42 Purge phase = Heinrich event = swarms of ice bergs in N Atlantic Is this why Neanderthals never reached N America by steamship?

43

44 Is the binge & purge hypothesis plausible? Today, most discharge of ANT ice sheets via ice streams Antarctic ice stream velocities can be over 1 km/yr USGS

45 Is the binge & purge hypothesis plausible? If an ice stream s terminus begins to calve, retreat can be catastrophically fast. Ice sheet s core can be drawn down. Ice in the Northeast Greenland Ice Stream can travel more than 500 meters each year. Credit: NASA's Goddard Space Flight Center.

46 Binge & Purge H: internal to Laurentide ice sheet Other ideas: Gerard Bond proposed that ca yr oscillation in irradiance triggered both D-O events and Heinrich Events How? By affecting binge-purge process or something else?

47 Unanswered: 1) What triggered D-O events? 2) Were they periodic?

48 Unanswered: 1) What triggered D-O events? 2) Were they periodic? 3) What are atmospheric/ oceanographic mechanisms through which D-O events and Heinrich events affect global climate? 4) Why did Heinrich events occur at the cool, end of only some D-O events?

49 Regardless of the glaciological trigger..the AMOC s switch being flipped by glaciological oscillator? triangles are ice bergs circles are locations of core records TODAY episodically, 40,000 to 14,000 yr BP Zahn, R., J. Schonfeld, H.-R. Kudrass, M.-H. Park, H. Erlenkeuser, and P. Grootes, 1997: Thermohaline instability in the North Atlantic during meltwater events: Stable isotope and ice-rafted detritus records from core S075-26KL, Portugese margin. Paleoceanography, 12,

50 freshwater lid dispersed freshwater lid hysteresis loop controlled by changes in freshwater input from ice sheets Bard (2002), Climate shock PHYSICS TODAY

51 ice-sheet surge, iceberg flotilla, freshwater lid freshwater lid disperses Bard (2002), Climate shock PHYSICS TODAY

52 What other autogenic*, cyclic climate systems exist? *autogenic = self-caused, NOT driven by seasonal insolation cycles (monsoons), or diurnal insolation cycles (convection cells, sea breezes)

53 ENSO Pacific Decadal Oscillation

54 What other autogenic*, cyclic climate systems exist in the atmosphere-ocean??? ENSO Heinrich 100,000-yr Enigma cycle Dansgaard-Oeschger

55 Why are D-O and H events absent today? Ice-Age Earth Today s Earth atmos atmos ocean ocean cryosphere cryosphere

56 Important Points from this lecture 1.During ice ages, Earth experienced rapid, extreme climatic fluctuations at time scales much faster that Milankovitch forcing 2.Dansgaard-Oeschger events: 1,400-2,000 year periodicity?, warm spikes, originate in North Atlantic region, global effects

57 3. Heinrich events: triggered erratically by extreme D-O cooling, probably glaciological in origin, global climate effects 4. Both D-O events and Heinrich events probably involve changes in freshwater fluxes from ice sheets to North Atlantic Both remain poorly understood

58 5. The Atlantic Meriodional Overturning Circulation is a trigger point in the oceanatmosphere system, at least when the Laurentide Ice Sheet is present. cycled repeatedly during ice age PNAS 2012

59

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