SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION DOI: 1.138/NGEO168 "Strength and geometry of the glacial Atlantic Meridional Overturning Circulation" S2 Map of core locations Core locations of the Holocene and LGM 231 / 23 Th compilation identifiable by core number in Supplementary Tables S1 (first column). Colour code indicates data available for both time periods (black), the Holocene only (red), or the LGM only (blue). Open squares indicate cores influenced by opal (preserved opal flux >.2 g/cm 2 kyr) and open triangles indicate cores affected by boundary scavenging (see text for details). NATURE GEOSCIENCE 1

2 S3 % biogenic opal versus /Th: (a) Holocene, (b) LGM. There is no significant correlation between 231 / 23 Th and %opal when preserved %opal is below 9 %. a b 2

1 S4 Overturning schemes Colour-coded fluxes [Sverdrup] and velocity vectors S4-1: Holocene Overturning scheme used for the model-data comparison shown in Fig. 3a, NADW: 2. Sv, AABW: 8 Sv, AAIW: 1 Sv.

3 1 S4 Overturning schemes Colour-coded fluxes [Sverdrup] and velocity vectors S4-1: Holocene Overturning scheme used for the model-data comparison shown in Fig. 3a, NADW: 2. Sv, AABW: 8 Sv, AAIW: 1 Sv. S4-2 LGM overturning circulation scheme (GNAIW: 2 Sv, AABW 4 Sv) showing the highest agreement to observational data (Fig. 3b) as derived from the sensitivity tests (Fig. 4) Atlantic Flux and Velocity Vector Depth S S 4S 3S 2S 1S 1N 2N 3N 4N N 6N Latitude -1 3

4 S4-3: Overturning circulation scheme used to test a very shallow 11 GNAIW overturning cell (S6-1). 4

5 S Model input parameters S-1: List of abbreviations and values for the Holocene and LGM parameters 19. Higher K 1 were used south of 4 S for the LGM and between 2. and 6 N for the Holocene to represent the higher opal concentrations of particles settling in the Southern Ocean and in the Northern North Atlantic. The glacial expansion of the southern opal belt is represented by a northward shift of the opal belt value and a gradual change over to the main basin K 1. Lower K Th 1 at S- S for the LGM accounts for the high percentage of opal in the settling material of this region. Variables Symbol Holocene LGM unit 231 production rate P.246 same dpm/(m 3 a) 23 Th production rate P Th.267 same dpm/(m 3 a) rticle sinking rate S same m/a 23 Th adsorption rate (7 N- S) -2 m Th K 1 1. same 1/a 23 Th adsorption rate ( S- S) -2 m Th K /a 23 Th adsorption rate ( S-7 S).6 same 1/a -2 m K 1 Th 23 Th adsorption rate 2- m K 1 Th 23 Th adsorption rate > m K 1 Th 23 Th desorption rate (7 N-7 S) all depths Th K adsorption rate (7 N-6 N) -2 m K adsorption rate (6 N-2. N) -2 m K adsorption rate (2. N-4 S) -2 m K adsorption rate (4 S-42. S) -2 m K adsorption rate (42. S-4 S) -2 m K adsorption rate (4 S-47. S) -2 m K adsorption rate (47. S- S) -2 m K adsorption rate ( S-7 S) -2 m K adsorption rate 2- m K adsorption rate > m K desorption rate (7 N-7 S) all depths K -1 7% of -2 m value % of -2 m value same 1/a same 1/a 1.6 same 1/a.8 same 1/a /a.8 same 1/a /a.8.2 1/a.8.4 1/a /a.44 same 1/a 7% of -2 m same 1/a value % of -2 m same 1/a value 1 same 1/a

6 S-2: Latitudinal variations of the equilibrium Fractionation Factors (FF) used in the model. FF is the fractionation factor that would be measured if particles were in equilibrium with surrounding seawater and is calculated from the 231 and 23 Th adsorption and desorption rate constant 3 : FF = ( 231 / 23 Th) diss / ( 231 / 23 Th) part. Latitude FF Holocene FF LGM 7 N 6 N 7.8 same 6 N 2. N N 4 S 7.8 same 4 S 42. S S 4 S S 47. S S S S S S 7 S.9 same Differences between Holocene and LGM account for shifts in the position of high biogenic opal flux regions. For the LGM, the southern opal belt was shifted ~ to the north 49, and the northern opal region was removed to account for the lower preserved opal fluxes due to sea ice cover. Fractionation factors were kept constant over the rest of the Atlantic Ocean (a reasonable assumption considering that the mean concentrations of opal in the Holocene and LGM sediment reported in Table S1 are similar: Holocene mean: 2.6 %, n=37, 1 SD=1.6 %; LGM mean=3.7 %, n=32, 1 SD=1.9 %). The position of the zone of Northern deep water formation was also shifted to the south by 1 degrees during the LGM 1 (S4). 6

7 S6 Tests on the geometry of the glacial AMOC S6-1: Very shallow northern overturning cell In addition to a shallow GNAIW overturning cell (S4-2) and the deep Holocene overturning scheme (S4-1) shown in Fig. 4a very shallow scheme (S4-3), based on a recent modelling study using the benthic δ 13 C database 11 has been tested. In this model geometry the penetration of GNAIW is limited to the upper 2 m and the core of the overturning cell is around 7 m. The fit between this model's results and observations is shown by stars and compared to the fit obtained with our optimal LGM geometry in the figure below. The very shallow geometry shows much weaker agreements with the LGM data base, primarily because of its inability to generate the low 231 / 23 Th values observed in the North Atlantic below 2 m (S7). The figure also shows the fit obtained between the LGM database and the model output generated with the Holocene geometry (black square). The grey lines indicate the result of a Monte-Carlo test in which a randomly generated data set of / 23 Th values (=number of LGM observations) uniformly distributed between.3 and.14 (~ the range of Atlantic 231 / 23 Th) was used in lieu of the 2d-model output. In one out of 1 million random data sets the mswd reached the upper grey line, in 1 out of 1 runs the lower grey line was crossed. 7

8 8

9 S6-2: Fit between model and observations above and below 3m Comparisons between model and observations above and below 3 m indicate that the fit is slightly poorer with stronger AABW. However, these differences are too small to be conclusive. This is because our database is skewed to the north where the influence of GNAIW is strongest. Increasing the South Atlantic database should provide a means to better constrain the strength of the glacial AABW 38. a: Cores deeper than 3 m: 9

10 b: Cores shallower than 3 m: 1

11 S6-3: Sensitivity test on the scavenging parameters used in the 2D model We tested the sensitivity of model output to scavenging parameters (fractionation factors, particle sinking rates) using the linear correlation coefficient r (a) and mswd (b) between the sediment 231 / 23 Th database and model output using the LGM circulation scheme (S4-2) and excluding samples with high opal or affected by boundary scavenging. rticle sinking rate (to test changes in particle flux) has been varied throughout the box model. Fractionation factor FF (to test changes in opal concentration) has been varied for the range between 6 N 4 S only, for other latitudes the values reported in S-2 have been kept. The best correlation is obtained with sinking rates of m/a and FF = 7.8. a 1. sensitivity test of model scavenging parameters linear correlation coefficient r FF=3.4 FF=.1 FF=7.8 FF=1. FF= particle sinking rate [m/year] b sensitivity test of model scavenging parameters particle sinking rate [m/year] mswd FF=3.4 FF=.1 FF=7.8 FF=1. FF=1.6 11

12 S6-4: Can the Holocene circulation scheme explain the LGM observations by varying particle parameters? Linear correlation coefficient r (a) and mswd (b) obtained between LGM sediment 231 / 23 Th and model output using the Holocene circulation scheme with varying particle sinking rate and fractionation factors. Poor correlations compared to the optimal LGM geometry (S4-2; 2 Sv GNAIW and 4 Sv AABW) and scavenging parameters (FF = 7.8; sinking rate = m/a) indicate that the LGM observations cannot be reproduced by increasing opal flux while keeping the Holocene circulation scheme. a.8 FF=3.4 FF=.1 linear correlation coefficient r FF=7.8 FF=1. FF=1.6 LGM FF= b.2 particle sinking rate [m/year] particle sinking rate [m/year] mswd FF=3.4 FF=.1 FF=7.8 FF=1. FF=1.6 LGM FF=7.8 12

13 S7 Sediment 231 / 23 Th database superimposed to the sediment 231 / 23 Th section generated by the 2-D scavenging model. Modelled sediment 231 / 23 Th sections generated with three different geometries of overturning cells compared to observational data. Numbers indicate core locations given in Supplementary Table S1 or in map S2. (a) Very shallow GNAIW cell (geometry following S4-3) compared to LGM observations. (b) Shallow GNAIW cell (geometry following S4-2) compared to LGM observations. (c) Holocene overturning scheme (S4-1) compared to Holocene and LGM observations. Core locations marked by a white borderline are from the African margin (#1, 2, 21) or the southern opal belt (#49). 13

very shallow LGM model run with 1 Sv GNAIW and 8 Sv AABW.

very shallow LGM model run with 3 Sv GNAIW and 2 Sv AABW.

14 231 / 23 Th a LGM observations superimposed on the 231 / 23 Th section generated by the very shallow LGM model run with 1 Sv GNAIW and 8 Sv AABW. b LGM observations superimposed on the 231 / 23 Th section generated by the very shallow LGM model run with 3 Sv GNAIW and 2 Sv AABW. LGM observations superimposed on the 231 / 23 Th section generated by the shallow LGM model run with 1 Sv GNAIW and 8 Sv AABW. c LGM observations superimposed on the 231 / 23 Th section generated by the best fitting shallow LGM model run (S4-2) with 2 Sv GNAIW and 4 Sv AABW. LGM observations superimposed on the 231 / 23 Th section generated by the Holocene model run (S4-1). Holocene observations superimposed on the 231 / 23 Th section generated by the Holocene model run (S4-1). 14

15 S8 Additional References 29. Gutjahr, M., M. Frank, C. Stirling, L. Keigwin & Halliday, A. Tracing the Nd isotope evolution of North Atlantic Deep and Intermediate Waters in the western North Atlantic since the LGM from Blake Ridge sediments. Earth and Planetary Science Letters 266, (28). 3. Francois, R., M. Bacon & Suman, D. Th-23 profiling in deep-sea sediments: Highresolution records of flux and dissolution of carbonate in the equatorial Atlantic during the last 24, years. leoceanography, (199). 31. Schlünz, B., R. Schneider, P. Müller & Wefer, G. Late Quaternary organic carbon accumulation south of Barbados: influence of the Orinoco and Amazon rivers?. Deep- Sea Research rt I-Oceanographic Research per 47 (2). 32. Frederichs, T., F. Schmieder, C. Hübscher, A. Figueiredo & Costa, E. Physical properties measured on 4 sediment cores from METEOR cruise M34/4.. doi:1.194/pangaea (1996). 33. Rühlemann, C. et al. Late Quaternary productivity changes in the western equatorial Atlantic: Evidence from 23 Th-normalized carbonate and organic carbon accumulation rates. Marine Geology 13, (1996). 34. Vidal, L., Schneider, R. R., Marchal, O. & Bickert, T. Link between the North and South Atlantic during the Heinrich events of the last glacial period. Climate Dynamics 1, (1999). 3. Heil, G. Abrupt climate shifts in the western tropical to subtropical Atlantic region during the last glacial. PhD Thesis University of Bremen, Germany (26). 36. Mulitza, S. et al. Sahel megadroughts triggered by glacial slowdowns of Atlantic meridional overturning. leoceanography 23, PA426 (28). 37. Vogelsang, E. & Sarnthein, M. Age control of sediment core M33-4. doi:1.194/pangaea (24). 38. Negre, C. et al. Reversed flow of Atlantic deepwater during the Last Glacial Maximum. Nature 468, (21). 39. Stoner, J., J. Channell, D. Hodell & Charles, C. A ~8 kyr paleomagnetic record from the sub-antarctic South Atlantic (Ocean Drilling Program Site 189). Journal of Geophysical Research 18 (23). 4. Channell, J., D. Hodell & B. Lehman. Relative geomagnetic paleointensity and 18 O at ODP Site 983 (Gardar Drift, North Atlantic) since 2 ka. Earth and Planetary Science Letters 13, (1997). 41. Grützner, J. et al. Astronomical age models for Pleistocene drift sediments from the western North Atlantic (ODP Sites 1-163). Marine Geology 189 (22). 42. Gherardi, J. et al. Evidence from NE Atlantic basin for variability in the rate of meridional overturning circulation through the last deglaciation. Earth and Planetary Science Letters 24 (2). 43. Chapman, M., N. Shackleton & J.-C., D. Sea surface temperature variability during the last glacial-interglacial cycle: assessing the magnitude and pattern of climate change in the North Atlantic. laeogeography, laeoclimatology, laeoecology 17, 1-2, (2). 44. Lehman, B. et al. Relative changes of the geomagnetic field intensity during the last 28 kyears from piston cores in the Acores area. Physics of The Earth and Planetary Interiors 93, (1996). 4. Schiebel, R. Planktic foraminferal sedimentation and the marine calcite budget Global Biochem. Cycles 16 (4) (22). 46. Heinrich, H. Origin and Consequences of Cyclic Ice Rafting in the Northeast Atlantic Ocean during the st 13, Years. Quaternary Research 29, (1988). 1

16 47. Grousset, F. et al. tterns of ice-rafted detritus in the glacial North Atlantic. leoceanography 8, (1993). 48. Labeyrie, L. et al. Hydrographic changes of the Southern Ocean (southeast Indian sector) over the last 23 kyr. leoceanography 11, 7-76 (1996). 49. Sarnthein, M., U. Pflaumann & Weinelt, M. st extent of sea ice in the northern North Atlantic inferred from foraminiferal paleotemperature estimates. leoceanography 18 (23).. Asmus, T. et al. Variations of biogenic particle flux in the southern Atlantic section of the Subantarctic zone during the late Quaternary: Evidence from sedimentary 231 ex and 23 Th ex. Marine Geology 19, (1999). 1. Labeyrie, L. et al. Changes in the vertical structure of the North Atlantic Ocean. Quaternary Science Reviews 11, (1992). 16

Supplementary Figure 1. New downcore data from this study. Triangles represent the depth of radiocarbon dates. Error bars represent 2 standard error

Supplementary Figure 1. New downcore data from this study. Triangles represent the depth of radiocarbon dates. Error bars represent 2 standard error of measurement (s.e.m.). 1 Supplementary Figure 2. Particle