North Atlantic deep water circulation collapse ... - GeoScienceWorld

North Atlantic deep water circulation collapse during Heinrich events Dan Seidov Earth System Science Center, Pennsylvania State University, University Park, Pennsylvania 16802-2711, USA Mark Maslin Environmental Change Research Centre, Department of Geography, University College London, 26 Bedford Way, London WC1H 0AP, United Kingdom ABSTRACT Paleoceanographic proxy data and ocean general circulation models have been combined to investigate the response of the North Atlantic Ocean to Heinrich-type meltwater episodes. Because of the uncertainties over the origin of the Heinrich events, three different scenarios have been modeled: (1) a Heinrich event caused by a dramatic increase in the iceberg discharge from the Laurentide ice sheet or Labrador ice shelf, (2) a Heinrich event driven by enhanced iceberg discharge from the Barents shelf, which was transported into the northern North Atlantic, and (3) a meltwater episode that was confined to the Nordic Seas. The ocean regional and global circulation models cannot distinguish between the Laurentide and Barents shelf Heinrich event scenarios because both led to a complete cessation of the deep-water thermohaline conveyor belt. The meltwater episode confined to the Nordic Seas only weakened the overturning and may represent circulation changes that may occur during the Dansgaard-Oeschger events. INTRODUCTION The deep-ocean thermohaline circulation is primarily controlled by the magnitude of the North Atlantic deep water (NADW) production (see review in Schmitz and McCarthey, 1993). At present, warm and salty subtropical water is carried to the high latitudes in the North Atlantic by the North Atlantic Current. As it moves northward, this water cools sufficiently to sink, beginning the global deep-ocean conveyor system. Both high-latitude salinity and temperature have, however, varied dramatically during the Pleistocene. It has been suggested that the thermohaline ocean circulation was reduced during glacial periods due to alterations in atmospheric circulation as well as the input of freshwater from melting icebergs (e.g., Duplessy et al., 1988, 1991; Sarnthein et al., 1994; Seidov et al., 1996) and collapsed during meltwater episodes (e.g., Maslin at al., 1995; Manabe and Stouffer, 1995; Rahmstorf, 1995; Zahn et al., 1997; Rosell-Melé et al., 1997). A combination of paleoceanographic proxy data and ocean general circulation models may help to assess further the circulation effects and origins of the quasi-periodic ice rafting pulses called Heinrich events (e.g., Heinrich, 1988; Andrews, 1998). Heinrich events occur every 7 to 13 k.y. and have a duration between 100 and 500 yr (Dowdeswell et al., 1995). The ice-rafted debris found in deep-sea sediment during the Heinrich events may have originated from either the Laurentide ice sheet (e.g., Grousset et al., 1993; Robinson et al., 1995; Gwiazda et al., 1996a) or the European ice sheet (e.g., Grousset et al., 1993; Gwiazda et al., 1996b; Rasmussen et al., 1997; see Fig. 1A). At present it is debated whether the Heinrich events are caused by internal ice-sheet dynamics (McAyeal, 1993) or external climate changes (Broecker, 1994; Hulbe, 1997). HEINRICH EVENT MODEL SCENARIOS To address the Heinrich event problem, we have combined the results of proxy studies with both regional and global circulation models. To obtain regular-grid coverage of sea-surface temperature we have used CLIMAP (1981) last glacial maximum reconstruction. This data set has received much criticism because it assumes the Nordic Seas to be constantly ice locked, and it does not reflect the full cooling of the tropics (e.g., Guilderson et al., 1994). To correct for these, we have altered the CLIMAP (1981) data set by: (1) using the Sarnthein et al. (1995) data to model the seasonally ice free Nordic Seas sea-surface temperature and (2) modeled an additional “cold tropic” data set by reducing the sea-surface temperature between 20°S to 20°N by 4 °C and decreasing the sea-surface temperature poleward expoGeology; January 1999; v. 27; no. 1; p. 23–26; 3 figures; 1 table.

nentially from 4 °C to 0 °C between 20°S and 60°S and 20°N and 60°N. Three control runs were constructed: (i) a modern Holocene control run using the modern sea-surface climatology of Levitus and Boyer (1994) and Levitus et al. (1994); (ii) a CLIMAP last glacial maximum control run, and (iii) the “cold tropic” glacial control run. The wind stress data for all runs was from the Hamburg atmosphere general circulation model with either present-day (for modern Holocene), or CLIMAP (for all paleo scenarios) sea surface conditions (Lorenz et al., 1996). Three Heinrich event data sets were compiled based on the last glacial maximum data sets and on the known Heinrich event ice-rafting debris distribution in the North Atlantic (Grousset et al., 1993; Robinson et al., 1995). It should be emphasized that we do not model iceberg drift and melting; rather we simulate the consequences of the iceberg meltwater by prescribing its impact on sea-surface salinity. Three different possible Heinrich-type events were modeled based on a composition of the last glacial maximum data and a specially designed sea-surface salinity: (1) Heinrich event (H L ), with the low-salinity band at about 50°N caused by iceberg surges from the Laurentide ice sheet (Maslin et al., 1995) or from the Labrador Sea (Hulbe, 1997); (2) Heinrich-type event caused by the iceberg flotilla from Barents shelf (H B) that spread over the northern North Atlantic (Sarnthein et al., 1995; Seidov et al., 1996); and (3) meltwater event that would be constrained to the Nordic Seas only. The three scenarios are summarized by the scheme in Figure 1A. This scheme and quantified surface paleo-salinity anomalies are used to generate Heinrich event scenarios. For example, Heinrich event 1 is characterized by a lens of freshwater with the local minimum in the eastern Norwegian Sea (Sarnthein et al., 1995) of about 28–29 psu (during last glacial maximum it had 33–34 psu) and in the eastern North Atlantic (Maslin et al., 1995) of about 33 psu (35 psu at the glacial time). This information is reflected in our simplified sea-surface salinity forcing for the H B scenario (Fig. 1B). In total, three Heinrich event scenarios are formed by placing lenses or tongues of freshwater by reducing the last glacial maximum surface salinity as shown in Figure 1, B and C. The third meltwater scenario looks somewhat like those in the Nordic Seas in Figure 1 B, but with the low-salinity confided to the Nordic Seas and without any freshwater signal to the areas south of Iceland. NUMERICAL MODELS AND SETUP OF EXPERIMENTS Two different numerical general ocean circulation models were used: a regional North Atlantic planetary geostrophic ocean circulation model (Seidov, 1996) and the global Geophysical Fluid Dynamics Laboratory modular ocean model (Bryan, 1969; Cox, 1984; Pacanowski et al., 1993). In the North Atlantic, the two models gave practically similar results. However, because the planetary-geostrophic approach may lead to significant discrepancies in the equatorial region, our conclusions in this study are based on both the global ocean circulation simulated using the global model, and the regional modeling outlined in Seidov et al. (1996). The basic two control cases, the modern and the last glacial maximum that were first studied locally, have been recently remodeled globally (Seidov and Haupt, 1997). All global numerical experiments have been carried out in two steps. First, the ocean model has been run in the modern and last glacial maximum cases for about 10 000 model years from rest, using the appropriate boundary conditions. The Heinrich event states are the results of three separate integrations of over only 500 years each from the last glacial maximum steady state. This duration mediates the estimated stretch of Heinrich events ongoing from several hundreds to a thousand years (Sarnthein et al., 23

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1995; Dowdeswell et al., 1995). In every Heinrich event scenario, the last glacial maximum sea-surface condition was replaced by the Heinrich event surface condition. In this study, we use sea-surface salinity to represent the effective meltwater impact via the known local nature of the freshwater forcing. Proxy 100 100

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data provide estimates for the amplitude and location of strong drop of the sea-surface salinity due to fresh water capping (Fig. 1). Although the uncertainties of these data are rather high (up to ±1 psu [e.g., Duplessy et al., 1991; Maslin et al., 1995]), the signal is still very strong in some areas over 4 psu (e.g., Maslin et al., 1995). Hence, one may expect robust modeling results based on such a signal. RESULTS AND DISCUSSION The Atlantic meridional overturning indicates a strong NADW production and compares well with other simulations (e.g., Wright and Stocker, 1991; Rahmstorf, 1995; Manabe and Stouffer, 1995; Seidov et al., 1996). The modeled last glacial maximum convection is somewhat different from the modern one (Table 1), with the NADW production occurring in the middle of the northern North Atlantic, and with strongly reduced convection in the Nordic Seas (Seidov et al., 1996; Seidov and Haupt, 1997). In spite of this major location shift, the rate of glacial overturning is not as different as might be expected (Table 1). This leads us to the conclusion that convection in the northern North Atlantic is a key factor in maintaining the normal even though reduced conveyor operation during glacial periods. In contrast, at the two Heinrich events, H B and H L , the modern-type conveyor collapsed completely. The collapse is independent of the source of the meltwater if only it capped the convection in the central northern North Atlantic. This is supported by the fact that they have almost identical northward heat fluxes in Figure 2. The third scenario of a meltwater episode (the one with the low-salinity impact confined to the Nordic Seas), however, did not bring about the same total collapse, although the overturning was somewhat weakened relative to the last glacial maximum control (Table 1 and Fig. 2). This scenario may provide a model of the causation of the Dansgaard-Oeschger events, as the meltwater episode confined to the Nordic Seas would explain the strong cooling signal seen in the Nordic Seas and the Greenland ice cores (e.g., Dansgaard et al., 1993) coexistent with the muted response from the North Atlantic (Zahn et al., 1997; Bond et al., 1997; Rosell-Melé et al., 1997). It may also explain why the Dansgaard-Oeschger events may also occur during the Holocene (O’Brien et al., 1995; Bond et al., 1997) as the Nordic Seas continues to be surrounded by ice and prone to fresh water discharges during interglacial periods. We conclude, therefore, that a specific realization of each Heinrich event is not important; it is the spreading of meltwater over the northern North Atlantic that is of primary importance. The modeling results suggest that the amplitude of the meltwater disturbance is not as important as it is believed to be. It seems that the only important feature is whether there is sufficient meltwater transported to the key convection sites in the northern North Atlantic to cap deep-water formation. For example, in the Laurentide scenario, if icebergs are diverted southward, their meltwater only increases the stable stratification in subtropics, and their impact on the circulation is minimal. This also implies that ocean circulation models can not be used to

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Figure 1. A: Reconstruction of last glacial maximum circulation based on paleoceanographic proxy data (e.g., CLIMAP, 1981; Duplessy et al., 1991; Grousset et al., 1993; Robinson et al., 1995; Sarnthein et al., 1995). Two possible source regions of icebergs, Laurentide ice sheet (HL ) and the Barents shelf (HB), are indicated. These represent two possible causes of Heinrich events; HMcA represents MacAyeal (1993) ice instability model and HD represents global climate change Denton model (Broecker, 1994). Resultant meltwater effects of both scenarios on North Atlantic are modeled along with scenario whereby meltwater is trapped in Nordic seas (MWENS = meltwater event in Nordic Seas) and does not affect central Atlantic Ocean. B: Low-salinity source in HB scenario. Idealized sea-surface salinity anomalies (in psu) of Heinrich event relative to last glacial maximum (LGM) are depicted. All salinity anomalies are negative and shown as shaded area. C: As in B for HL scenario. GEOLOGY, January 1999

determine the origin or cause of the Heinrich events because the key factor is the capping of the very same glacial convection sites. We observe no difference in the deep-ocean circulation patterns when the scenarios were run with either the CLIMAP tropical surface temperatures or our adapted “cold tropic” scenarios. Our interpretation is that cooling of the tropics does not affect the stability of the thermohaline circulation nearly as strong as high-latitude cooling can do, and that the overall conveyor operation indeed is sensitive mostly to the North Atlantic deep water production rather than to tropical heating or cooling. The overturning in the tropical-subtropical region is more influenced by the wind stress than by isopycnal outcrop, which is of more importance in the high latitudes. Given the same wind pattern, there is the same Ekman pumping, which would lead to a comparable isopycnal structure, even though buoyancy was somewhat changed due to cooling in the tropics. Nevertheless, these conclusions may be biased in a stand-alone ocean modeling, and the use of a coupled ocean-atmospheric model may be more appropriate for a better judgement on the cool tropics problem. Figure 3 summarizes the above-mentioned simulations for the modern Holocene, last glacial maximum, and Heinrich event circulation, using both global and regional models, and taking into account other authors simulations of the freshwater impact. It outlines the major features of a continuum of glacial-to-interglacial modes of the global conveyor as revealed by simulations and supported by paleoreconstructions of Heinrich events (e.g., Broecker, 1994; Maslin at al., 1995; Robinson et al., 1995; Sarnthein et al., 1995; Curry and Oppo, 1997; Zahn et al., 1997; Bond et al., 1997; RosellMelé et al., 1997; Vidal et al., 1998).

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Figure 2. Atlantic northward heat flux in two control runs (modern Holocene and last glacial maximum [LGM]) and three Heinrich event scenarios (Barents shelf, Laurentide ice sheet, and meltwater event in Nordic Seas [MWENS] scenarios). Heat fluxes are given in PW (1 PW = 1015 W). MWENS scenario gives northward heat transport only somewhat lower than for LGM; other two scenarios indicate dramatic drop of transport. HB and HL scenarios have practically same heat transport pattern, i.e., southward heat transport to south of 35°N and very low northward transport to north of this latitude. Present-day circulation gives northward cross-equatorial heat transport of more than 0.1 PW in Atlantic; last glacial maximum has cross-equatorial heat transport close to zero. GEOLOGY, January 1999

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Figure 3. North Atlantic conveyor during interglacial (modern Holocene), glacial, and Heinrich event scenarios. Typical overturning patterns are shown on left panel (see also Table 1), and corresponding three-dimensional circulation patterns are sketched in right (EQ is equator). Key role of North Atlantic Current zonality and deep convection are emphasized. Black circles show areas of deep convection in modern and last glacial maximum diagrams. There is no deep convection during Heinrich events HB and HL. MWENS circulation pattern closely resembles LGM pattern (see also Fig. 2). 25

CONCLUSION In conclusion, our modeling results suggest the following: The proxy data interpretation that some Heinrich events could cause the total collapse of the deep-water thermohaline conveyor belt is strongly supported by numerical simulations. However, a warning is to be issued that not all meltwater events may inevitably cause a complete cessation of the conveyor. On the basis of our simulations, we argue that (1) the collapse of the glacial conveyor belt is relatively independent of the origin and magnitude of the meltwater produced during a Heinrich event so long as it is transported to the northern North Atlantic convection sites; and (2) meltwater events that are contained within the Nordic Seas reduce the North Atlantic deep water production but do not cause the conveyor to collapse, and may provide an insight about how the Dansgaard-Oeschger events occur. ACKNOWLEDGMENTS We thank Bernd J. Haupt, Catherine Pyke, and Nick Mann for their help. We are grateful to the GPI and SFB313 group at Kiel University whose work on proxy data gave strong insight for this study. REFERENCES CITED Andrews, J. T., 1998, Abrupt changes (Heinrich events) in late Quaternary North Atlantic marine environments: Journal of Quaternary Science, v. 13, p. 3–16. Bond, G., Heinrich, H., Broecker, W., Labeyrie, L., McManus, J., Andrews, J., Huon, S., Jantschik, R., Clasen, S., Simet, C., Tedesco, K., Klas, M., and Bonani, G., 1992, Evidence for massive discharges of icebergs into the North Atlantic Ocean during the last glacial period: Nature, v. 360, p. 245–249. Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I., and Bonani, G., 1997, A pervasive millennial-scale cycle in North Atlantic and glacial climates: Science, v. 278, p. 1257–1266. Broecker, W. S., 1994, Massive iceberg discharges as triggers for global climate change: Nature, v. 372, p. 421–424. Bryan, K., 1969, A numerical method for the study of the circulation of the world ocean: Journal of Computational Physics, v. 4, p. 347–376. Climate Long-Range Investigation Mapping and Prediction (CLIMAP) Project Members, 1981, Seasonal reconstructions of the Earth’s surface at the last glacial maximum: Geological Society of America Map and Chart Series MC-36, p. 1–18. Cox, M. D., 1984, A primitive equation, 3-dimensional model of the ocean: Princeton, University, Ocean Group, Geophysical Fluid Dynamics Laboratory, Technical Report 1, 250 p. Curry, W. B., and Oppo, D. W., 1997, Synchronous, high-frequency oscillations in tropical sea surface temperatures and North Atlantic Deep Water production during the last glacial cycle: Paleoceanography, v. 12, p. 1–14. Dansgaard, W., Johnson, S. J., Clausen, H. B., Dahl-Jensen, D., Gundestrup, N. S., Hammer, C. U., Hvidberg, C. S., Steffensen, J. P., Sveinbjörnsdottir, A. E., Jouzel, J., and Bond, G., 1993, Evidence for general instability of past climate from a 250-kyr ice-core record: Nature, v. 364, p. 218–220. Dowdeswell, J. A., Maslin, M. A., Andrews, J. T., and McCave, I. N., 1995, Iceberg production, debris rafting, and the extent and thickness of Heinrich layers (H1, H2) in North Atlantic sediments: Geology, v. 23, p. 301–304. Duplessy, J.-C., Shackleton, N. J., Fairbanks, R. G., Labeyrie, L., Oppo, D., and Kallel, N., 1988, Deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation: Paleoceanography, v. 3, p. 343–360. Duplessy, J.-C., Labeyrie, L., Julliet-Lerclerc, A., Duprat, J., and Sarnthein, M., 1991, Surface salinity reconstruction of the North Atlantic Ocean during the last glacial maximum: Oceanolographic Acta, v. 14, p. 311–324. Grousset, F. E., Labeyrie, L., Sinko, J. A., Cremer, M., Bond, G., Duprat, J., Cortijo, E., and Huon, S., 1993, Patterns of ice-rafted detritus in the North Atlantic (40–55°N): Paleoceanography, v. 8, p. 175–192. Guilderson, T. P., Fairbanks, R. G., and Rubenstone, J. L., 1994, Tropical temperature variations since 20,000 years ago: Modulating interhemispheric climate change: Science, v. 263, p. 663–664. Gwiazda, R. H., Hemming, S. H., and Broecker, W. S., 1996a, Tracking the sources of icebergs with lead isotopes: The provenance of ice rafted debris in Heinrich event 2: Paleoceanography, v. 8, p. 77–93. Gwiazda, R. H., Hemming, S. R., and Broecker, W. S., 1996b, Provenance of icebergs during Heinrich event 3 and the contrast to their sources during other Heinrich events: Paleoceanography, v. 11, p. 371–378. Heinrich, H., 1988, Origin and consequences of cyclic ice rafting in the northeast Atlantic Ocean during the past 130,000 years: Quaternary Research, v. 29, p. 142–152.

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