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Subtropical Atlantic Salinity Variability and Atlantic Meridional Circulation During the Last Deglaciation

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Subtropical Atlantic Salinity Variability and Atlantic Meridional Circulation During the Last Deglaciation Subtropical Atlantic salinity variability and Atlantic meridional circulation during the last deglaciation Anders E. Carlson1*, Delia W. Oppo1, Rosemarie E. Came2†, Allegra N. LeGrande3, Lloyd D. Keigwin1, William B. Curry1 1Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 2Geology and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA 3National Aeronautics and Space Administration, Goddard Institute for Space Studies and Center for Climate Systems Research, Columbia University, New York, New York 10025, USA ABSTRACT During the last deglaciation (ca. 21–10 ka), freshening of the North Atlantic surface likely caused reductions in Atlantic meridional overturning circulation (AMOC); the mechanisms related to AMOC recovery remain poorly understood. Here we present three new deglacial δ18 δ18 surface temperature and Oseawater ( Osw) reconstructions from the western subtropical δ18 North and South Atlantic. Similarities to tropical Caribbean and western Atlantic Osw records suggest that a salty surface water mass accumulated in the western Atlantic from δ18 27°S to 33°N during periods of reduced AMOC. However, Osw decreases led deep AMOC resumption by hundreds of years. We suggest that the northward export of salt previously trapped in the western Atlantic resulted in the early establishment of a shallow overturning circulation that eventually culminated in deep AMOC resumption, implying that AMOC may constitute a self-limiting system. INTRODUCTION melting of icebergs during Heinrich Event 1 foraminifera Globorotalia infl ata (thermo cline The tropics and subtropics of the North (H1) (17.5–16.0 ka) (McManus et al., 2004). dwelling; Anand et al., 2003) for the core top, Atlantic are regions of net evaporation, which A smaller reduction occurred during the Holocene, and deglacial sections of the core increases the salinity of surface waters in Younger Dryas cold event (ca. 13–11.5 ka), (Fig. 2; GSA Data Repository Fig. DR11). the subtropical North Atlantic. Subduction probably due to the routing of North American For the Blake Outer Ridge and the Brazilian δ18 of salty northeastern subtropical waters into freshwater to the northern North Atlantic (e.g., margin, we measured Ocalcite and Mg/Ca of the thermocline also increases the salinity of Broecker et al., 1990; Carlson et al., 2007) the planktonic foraminifera Globigerinoides western Atlantic thermocline waters. These and increased iceberg discharge (McManus ruber (white) (surface dwelling; Anand et al., high-salinity waters are an important com- et al., 2004). Following each weakening, the 2003) for the core tops and deglacial sec- ponent of the upper limb of modern Atlantic AMOC eventually resumed (McManus et al., tions of the two cores (Fig. 2 and Fig. DR1). meridional overturning circulation (AMOC) 2004). However, the mechanisms behind these After converting Mg/Ca to CT, we corrected δ18 (Broecker et al., 1990; Schmitz, 1995), hence resumptions are unclear (Weaver et al., 2003; Ocalcite for changes in CT and continental ice δ18 changes in their salinity may have infl uenced Knorr and Lohmann, 2003; Schmidt et al., volume to reconstruct Osw (Fig. 3 and Fig. AMOC in the past (Schmidt et al., 2004, 2006; 2004; Weldeab et al., 2006; Leduc et al., 2007), DR1). In the case of the Bermuda Rise with Weldeab et al., 2006). Once these waters reach especially in the case of the Oldest Dryas, its deep water depth, we developed a weight- the subpolar North Atlantic and cool, convec- when the deep AMOC (>2500 m water depth) dependent Mg/Ca-CT calibration to account tion occurs, forming North Atlantic deepwater resumption lagged the end of H1 by ~1.3 k.y. for the effects of dissolution. Detailed methods and continuing the AMOC (Schmitz, 1995). (McManus et al., 2004; Robinson et al., 2005). and reservoir corrected-calibrated age model During the Last Glacial Maximum (LGM, Because the subtropical western Atlantic sur- construction are provided in the GSA Data δ18 ca. 21 ka), AMOC strength was apparently face hydrology exerts a strong control on mod- Repository. We compare our Osw results to reduced relative to present, and during the ern AMOC, we evaluate the possibility that the simulations using the National Aeronautics subsequent deglaciation underwent two oscil- subtropics played an important role in modu- and Space Administration Goddard Institute lations in strength with attendant effects on lating AMOC in the past. for Space Studies fully coupled atmosphere- climate (Boyle and Keigwin, 1987; McManus ocean general circulation model (GCM) et al., 2004; Robinson et al., 2005). The larger METHODS ModelE-R that tracks water isotopes (Schmidt of these reductions spanning the Oldest Dryas We reconstructed calcifi cation temperature et al., 2007). ModelE-R was forced with 0.1 cold event (ca. 18.5–14.7 ka) was likely in (CT) and sea-surface salinity (SSS)–dependent Sverdrups (1 Sverdrup [Sv] = 106 m3 s–1) of δ18 δ18 response to the input of freshwater from ini- Oseawater ( Osw) from three cores located fresh water to the North Atlantic for 100 yr. We tial retreat of Northern Hemisphere ice sheets along the northward return fl ow of surface compare the average of the past 20 yr of this ca. 19 ka (Clark et al., 2004), reinforced by the water associated with AMOC (Broecker et al., experiment to the control simulation. 1990; Schmitz, 1995). Core OCE326-GGC5 *Current address: Department of Geology and is located on the Bermuda Rise, KNR140– 1GSA Data Repository item 2008246, detailed Geophysics, University of Wisconsin, Madison, WI, 51GGC is on the Blake Outer Ridge, and methods and results description, core data, and addi- 53706, USA; [email protected]. tional tables and fi gures, is available online at www. †Current address: Department of Geological Sci- KNR159–5–36GGC is along the Brazilian geosociety.org/pubs/ft2008.htm, or on request from ences, Jackson School of Geoscience, University of margin (Fig. 1). For the Bermuda Rise, we [email protected] or Documents Secretary, δ18 Texas, Austin, TX, 78713, USA. measured Ocalcite and Mg/Ca of the planktonic GSA, P.O. Box 9140, Boulder, CO 80301, USA. © 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, December December 2008; 2008 v. 36; no. 12; p. 991–994; doi: 10.1130/G25080A.1; 3 fi gures; Data Repository item 2008246. 991 MD01-2334 (0.0/-0.8) 51GGC GGC5 (0.3/0.7) (0.5/0.4) VM28-122 (0.2/0.4) ITCZ July January ITCZ GeoB3129-3911 (0.3/0.6) 36GGC (0.3/0.6) –0.22 –0.18 –0.14–0.10 –0.06 –0.02 0.02 0.06 0.10 0.14 0.18 0.22 δ18 Osw (‰) Figure 1. Locations of cores GGC5 on Ber- muda Rise (33°42′N, 57°35′W, 4550 m water depth), 51GGC on Blake Outer Ridge (32°47′N, 76°17′W, 1790 m water depth), 36GGC on Bra- zilian margin (27°31′S, 46°28′W, 1268 m water depth), VM28–122 (11°34′N, 78°25′W, 3623 m water depth) in Caribbean (Schmidt et al., 2004), GeoB3129–3911 (4°37′S, 36°38′W, Figure 2. Subtropical calcifi cation tempera- 830 m water depth) on northeastern Brazil ture (CT) (gray dots with 3 point smoothed δ18 δ18 margin (Weldeab et al., 2006), and MD01– black line) and Ocalcite ( Oc) (gray line). 2334 (37°33′ N, 10°08′W, 2460 m water depth) A: Bermuda Rise (GGC5) Globorotalia infl ata δ18 on Iberian margin (Skinner and Shackleton, CT. B: GGC5 Oc. C: Blake Outer Ridge (51GGC) Globigerinoides ruber CT. D: 51GGC Figure 3. Subtropical-tropical sea-surface 2006). Green line is modern July Intertropical δ18 δ18 salinities (SSS) ( Osw), Atlantic meridi- Convergence Zone (ITCZ) location; blue line G. ruber Oc. E: Brazilian margin (36GGC) G. ruber CT. F: 36GGC G. ruber δ18O . Black onal overturning circulation (AMOC), and is January location. Also shown is National c North Atlantic climate. A: Summit Green- Aeronautics and Space Administration God- boxes denote radiocarbon age control. Dark gray bars denote Last Glacial Maximum land (GISP—Greenland Ice Sheet Project) dard Institute for Space Studies general δ18 (LGM), and Oldest (OD) and Younger Dryas O (Grootes et al., 1993). B: Bermuda Rise circulation ModelE-R (Schmidt et al., 2007) 231Pa/230Th record from subtropical North surface δ18O response to ~50% reduction (YD) cold events. Light gray bar denotes sw Atlantic (McManus et al., 2004). C: δ18O Bølling/Allerød Warm Period (B/A). sw in Atlantic meridional overturning circu- from Bermuda Rise (square symbols, 3 point lation (AMOC) strength. Changes in core smoothing). D: δ18O from Blake Outer δ18O during reduced AMOC are indicated in sw sw Ridge (circular symbols, 3 point smoothed). parentheses (fi rst value is for Younger Dryas, E: δ18O from Caribbean (diamond sym- δ18 sw second is for Oldest Dryas). Because Osw δ18 identical, millennial-scale variability (Figs. 3C, bols) (Schmidt et al., 2004). F: Osw from changes differ somewhat in timing between northeastern Brazil margin (open circular records, changes in core δ18O were calcu- 3D, and 3G). Bermuda Rise and Brazilian mar- sw symbols, 3 point smoothed) (Weldeab et al., lated using average of three points prior to δ18 gin Osw began to increase at 18.5–18.0 ka, 2006). G: δ18O from Brazilian margin (open δ18O change and average of three points sw sw reaching maxima ca. 16.5 ka, with subsequent diamond symbols, 3 point smoothed). Black just after change. δ18 decreases. In contrast, Blake Outer Ridge Osw boxes represent radio carbon age control. was highest during the LGM with a subsequent Records are smoothed to reduce sig- decrease. It began a gradual increase again nifi cance of noisy data while increasing signifi cance of the signal (Beving ton and RESULTS 18.5–18.0 ka, reaching a maximum ca.
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