Changes and impacts of the Atlantic meridional overturning circulation: lessons from the past for a changing future. Cristiano M. Chiessi et al.

Transcription

1 Changes and impacts of the Atlantic meridional overturning circulation: lessons from the past for a changing future Cristiano M. Chiessi et al.

2 Alexander M. Piotrowski, Aline Govin, Gesine Mollenhauer, Jacob N.W. Howe, Jeroen Groeneveld, Maria Helena B.M. Hollanda, Matthias Prange, Matthias Zabel, Ralf Tiedemann, Ricardo I.F. da Trindade, Stefan Mulitza, Yancheng Zhang Abdelfettah Sifddine, Ana Luisa S. Albuquerque, Andreas Mackensen, André O. Sawakuchi, Enno Schefuß, Francisco W. da Cruz Jr., Ines Voigt, Jörg Lippold, Pedro L.S. Dias, Tercio Ambrizzi, et al.

3 Outline The Atlantic meridional overturning circulation Case studies Thermal evolution of the South Atlantic and South America Origin of increased terrigenous supply to the NE South American margin Glacial CO 2 and the Atlantic meridional overturning circulation Final remarks

4 Depth [m] The Atlantic meridional overturning circulation 0 Salinity [psu] AAIW NADW AABW 40 o S 20 o S EQ 20 o N 40 o N 60 o N Atlantic meridional overturning circulation (AMOC) Antonov et al. (2010)

5 The Atlantic meridional overturning circulation 60 o N Shallow Deep Bottom Upwelling Downwelling EQ Sv = 10 6 m 3 s o S 60 o W 0 o Ganachaud and Wunsch (2000)

6 The Atlantic meridional overturning circulation 60 o N 1.3 PW 1 PW = W EQ 60 o S 0.3 PW Itaipu produces PW (14 GW) Strong impact in the distribution of energy in the Atlantic realm 60 o W 0 o Ganachaud and Wunsch (2000)

7 The Atlantic meridional overturning circulation IPCC AR5 numerical models suggest a decrease in strength of the AMOC Between 20 30% (RCP4.5) e 36 44% (RCP8.5) in 2100 Changes in AMOC may impact global climate Weaver et al. (2012), Collins et al. (2013)

8 The Atlantic meridional overturning circulation

9 The Atlantic meridional overturning circulation Instrumental records are too short to appropriately investigate changes in AMOC Srokosz and Bryden (2015)

10 Outline The Atlantic meridional overturning circulation Case studies Thermal evolution of the South Atlantic and South America Origin of increased terrigenous supply to the NE South American margin Glacial CO 2 and the Atlantic meridional overturning circulation Final remarks

11 The context Most of the AMOC northward heat transport should happen through the NBC Figure courtesy of Andre L. Belem based on Peterson and Stramma (1991) and Locarnini et al. (2010)

12 The context During times of AMOC slowdown it has been suggested that the BC might redirect the excess heat to the South Atlantic Maier-Reimer et al. (1990), Crowley (2011)

13 The context Heinrich Stadial 1 is probably the best example of an AMOC slowdown event Available records lack the necessary temporal resolution Is there a direct relationship between the strength of the AMOC and SST of the BC on millennial time-scale?

14 Material and methods GeoB Influence of the BC Age model based on 9 AMS 14 C Sedimentation rates cm kyr -1 Temporal resolution yr Chiessi et al. (2008), Razik et al. (2013), Chiessi et al. (2015)

15 Material and methods SSTs reconstructed via Mg/Ca ratios in planktonic foraminifera Sediment trap calibration 75 µm Globigerinoides ruber Anand et al. (2003)

16 Material and methods MATs reconstructed via branched glycerol dialkyl glycerol tetraethers (GDGTs) analyses Global soil calibration Branched GDGTs Peterse et al. (2012)

17 Thermal evolution SST: high during HS1 and low during the YD MAT: two-step increase (second half of HS1 and during the YD) MAT lags SST! Chiessi et al. (2015)

18 In-phase thermal evolution of the BC and the NBC during HS1 and the BA, but an opposite behavior during the YD Similar changes in SST in the eastern South Atlantic Bard et al. (2000), Barker et al. (2009), Chiessi et al. (2015), McManus et al. (2004), Shakun et al. (2012), Weldeab et al. (2006)

19 Changes in MAT are remarkably synchronous with atmospheric CO 2 Bush et al. (2004), Chiessi et al. (2015), EPICA Com. Mem. (2006), Monnin et al. (2004), Shakun et al. (2012)

20 Partial conclusions BC was an important conduit and storage volume for part of the heat not transported to the North Atlantic under a sluggish AMOC BC and NBC in-phase (for SST) during HS1 and the BA, contradicting previous assumptions of a BC NBC anti-phase Similar behavior in the eastern South Atlantic suggest a South Atlantic-wide pattern in SST evolution during most of Termination 1 The lag of MAT rise relative to SST rise suggest that the long duration of HS1 was fundamental to drive the Earth out of the last glacial

21 Outline The Atlantic meridional overturning circulation Case studies Thermal evolution of the South Atlantic and South America Origin of increased terrigenous supply to the NE South American margin Glacial CO 2 and the Atlantic meridional overturning circulation Final remarks

22 The context During times of AMOC slowdown it has been suggested that the NBC might have decreased its strength / reversed, raising the possibility of Amazon sediments being deposited off NE Brazil Chang et al. (2008), Schmidt et al. (2012)

23 The context Heinrich Stadial 1 is probably the best example of an AMOC slowdown event Available records show a major increase in sedimentation rates off NE Brazil! But where is the material deposited off NE Brazil coming from? Zhang et al. (2015)

24 The context Heinrich Stadial 1 is probably the best example of an AMOC slowdown event Available records show a major increase in sedimentation rates off NE Brazil! But where is the material deposited off NE Brazil coming from? Zhang et al. (2015)

25 Material and methods GeoB GeoB Influence of the NBC (upstream and Downstream the Amazon River mouth) Age models based on 27 AMS 14 C Sedimentation rates cm kyr -1 Zhang et al. (2015)

26 Material and methods Source of sediment reconstructed via end in bulk sediment Suspended sediment load and core top calibration Basu et al. (1990), Allègre et al. (1996), Viers et al. (2008), Bouchez et al. (2011), Horbe et al. (2014), Parra et al. (1997)

27 Material and methods Terrigenous input reconstructed via Fe/Ca ratios in bulk sediment Core top calibration Govin et al. (2012)

28 Origin of increased terrigenous supply Basu et al. (1990), Allègre et al. (1996), Viers et al. (2008), Bouchez et al. (2011), Horbe et al. (2014), Parra et al. (1997), Zhang et al. (2015) Upstream the Amazon mouth: material from NE Brazil Downstream the Amazon mouth: material from the Andes

29 Extremely high sedimentation rates triggered by terrigenous material from NE Brazil Zhang et al. (2015)

30 Partial conclusions First compilation of HS1 sedimentation rates for the western South Atlantic shows highest values off the semi-arid NE Brazil Neodymium isotopic values do not support a southeastward transport of Amazon sediment (as suggested by model experiments) Dramatic increase in precipitation over NE Brazil probably responsible for the extreme sedimentation rates Weak NBC favored deposition of sediments off NE Brazil

31 Outline The Atlantic meridional overturning circulation Case studies Thermal evolution of the South Atlantic and South America Origin of increased terrigenous supply to the NE South American margin Glacial CO 2 and the Atlantic meridional overturning circulation Final remarks

32 The context CO 2 concentration during the LGM was 90 ppm lower than PI and changes in deep Atlantic ventilation are commonly invoked as the primary cause of such difference Broecker (1982), Sigman et al. (2010)

33 The context Nutrient-based reconstructions of the LGM deep Atlantic suggest the replacement of NADW by AABW PI phosphate [mmol liter -1 ] LGM d 13 C [, VPDB] However, AABW is an inefficient piece in the biological pump, contradicting the role of the Atlantic in drawing down glacial CO 2! But was there indeed a LGM replacement of NADW by AABW? Bickert and Mackensen (2004), Curry and Oppo (2005), Lynch-Stieglitz et al. (2007)

34 Material and methods 36 late Holocene and LGM time slices 46 o S-40 o N, m water depth Age models based mainly on AMS 14 C Howe et al. (2016)

35 Material and methods Antarctic Intermediate Water: -8.3 North Atlantic Deep Water: Jeandel (1993), Stichel et al. (2012), Lambelet et al. (2016)

36 Glacial CO 2 Antarctic Intermediate Water: -8.3 North Atlantic Deep Water: LGM: presence of AABW in the deep Atlantic is not that extensive Jeandel (1993), Stichel et al. (2012), Howe et al. (2016)

37 Glacial CO 2 LGM: as much as 80% of NADW in the deep Atlantic Howe et al. (2016)

38 Glacial CO 2 Glacial: the deeper overturning cell was very sluggish Howe et al. (2016)

39 Partial conclusions Sustained production of NADW during the LGM The depleted glacial d 13 C values in the deep Atlantic cannot be explained solely by a change in water mass source A greater amount of respired carbon was stored in the abyssal Atlantic during the LGM A sluggish deep overturning cell, comprised of well-mixed northernand southern-sourced waters

40 Outline The Atlantic meridional overturning circulation Case studies Thermal evolution of the South Atlantic and South America Origin of increased terrigenous supply to the NE South American margin Glacial CO 2 and the Atlantic meridional overturning circulation Final remarks

41 Final remarks The AMOC plays a major role in global climate Cutting-edge numerical models suggest a significant decrease in its strength until the end of the century Past decreases in strength of the AMOC had major impacts Sea surface temperatures of the South Atlantic Precipitation over northeastern Brazil Concentration of atmospheric CO 2 The presence of thresholds in AMOC's behavior rises uncertainties on our present ability to appropriately project its future development The past may give us important hints to improve model performance

42 Many thanks for your attention! Prof. Dr. Cristiano M. Chiessi