Past extent of sea ice in the northern North Atlantic inferred from foraminiferal paleotemperature estimates

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1 PALEOCEANOGRAPHY, VOL. 18, NO. 2, 1047, doi: /2002pa000771, 2003 Past extent of sea ice in the northern North Atlantic inferred from foraminiferal paleotemperature estimates Michael Sarnthein, Uwe Pflaumann, and Mara Weinelt Institut für Geowissenschaften, University of Kiel, Kiel, Germany Received 11 February 2002; revised 17 January 2003; accepted 28 February 2003; published 11 June [1] Using 150 core top samples, we developed a conservative but fairly robust new measure to reconstruct past changes in (maximum) seasonal sea ice distribution in the northern North Atlantic, hitherto a major unknown. The proxy is based on Similarity Maximum Modern-Analog Technique (SIMMAX) estimates of threshold temperatures near the sea surface (SST). Today, almost 100% of all sites with SST >2.5 C during summer, >0.4 for winters , and >0.75 C for Little Ice Age winters lie seaward of the sea ice margin. When applied to >60 sediment records of the Last Glacial Maximum, this proxy shows that peak glacial sea ice was far more restricted than in the classic CLIMAP Project Members [1981] reconstruction. During glacial summer, sea ice only covered the Arctic Ocean and western Fram Strait. The northern North Atlantic and Nordic Seas were largely ice-free and thus formed a high-latitude moisture source for the continued buildup of continental ice sheets. In contrast, sea ice spread far south across the Iceland Faeroe Ridge during glacial winter, with an inferred patch of sea ice also in the central east Atlantic, near the center of the Azores High. A broad ice-free channel extended from 50 to 60 N, forming an ideal site for large-scale convection of glacial upper North Atlantic Deep Water. The extreme seasonality in glacial sea ice formation and melt in the Nordic Seas implies a major consumption of the regional energy income. INDEX TERMS: 1620 Global Change: Climate dynamics (3309); 4207 Oceanography: General: Arctic and Antarctic oceanography; 4267 Oceanography: General: Paleoceanography; KEYWORDS: sea ice proxy, glacial North Atlantic, glacial sea ice distribution, seasonal sea ice variations Citation: Sarnthein, M., U. Pflaumann, and M. Weinelt, Past extent of sea ice in the northern North Atlantic inferred from foraminiferal paleotemperature estimates, Paleoceanography, 18(2), 1047, doi: /2002pa000771, Introduction [2] The extent of sea ice represents one of the most variable and sensitive parameters of the global climate system and, in turn, strongly influences the amount of solar radiation and the exchanges of heat, mass, and momentum between the ocean and atmosphere in high latitudes. Accordingly, the systematic reconstructions of sea-surface temperature (SST) and sea ice by CLIMAP Project Members [1976, 1981] pioneered the modern understanding of the Glacial Earth. In particular, their estimates of past sea ice provided crucial evidence for assessing potential variations in (1) past deep-water formation, (2) evaporation in the northern high-latitude Atlantic, and (3) in the albedo which characterizes ice-covered sea regions. All three are important features for initiating and/or validating ocean and atmosphere general-circulation models (O- and A-GCMs) (summary in the work of Sarnthein et al. [2001] and Gildor and Tziperman [2000]). [3] In northern high latitudes, CLIMAP Project Members [1976, 1981] mainly inferred the extent of sea ice from negative evidence such as the absence of subpolar-water indicators. Perennial (August) sea ice was deduced from (1) the absence of coccolithophorids; (2) the very low accumulation rates ( numbers ) of left coiling Neogloboquadrina Copyright 2003 by the American Geophysical Union /03/2002PA pachyderma in the sediment; and (3) from much reduced bulk sedimentation rates. The presence of ice-rafted material (IRD) was also regarded as an indicator of seasonally open sea conditions. [4] In contrast, the sediment record of the Southern Ocean is much different. Here, abundant diatoms have allowed for development of direct proxies to reconstruct past variations in the distribution pattern of sea ice (recently summarized in the work of Crosta et al. [1998], Armand [2000], and Gersonde and Zielinski [2000]). Tentatively, Koç et al. [1993] applied this approach to the diatom record in sediment cores from the European Nordic Seas. However, these sediments are almost barren of siliceous fossils prior to the Bølling (>14,700 cal ka; Stabell [1986]) and thus prevent any diatom-based reconstruction of sea ice during the Last Glacial Maximum (LGM). [5] de Vernal and Hillaire- Marcel [2000] and Rochon et al. [1999] evaluated the distribution of dinoflagellate cysts as a potential proxy for the extent of seasonal sea ice in the North Atlantic. However, during times of low plankton production the dinoflagellate signal appears highly susceptible to lateral particle reworking and thus may lead to unrealistic reconstructions of the past, especially near the continental margins. [6] In summary, sediments of the northern North Atlantic have to date lacked a trustworthy quantitative proxy to reconstruct past distribution patterns of sea ice. This gap may now be overcome by a new quantitative technique 25-1

2 25-2 SARNTHEIN ET AL.: GLAMAP 2000 SEA ICE Figure 1. Modern limits (bold broken lines) of >50% sea ice concentration (satellite radiometric observations of Gloersen et al. [1992]) in the northern North Atlantic during summer (July to September) as compared to SIMMAX-based estimates of modern summer SST at 10 m water depth. Large numbers label isotherms ( C). See color version of this figure at back of this issue. using planktic foraminifera-based estimates of threshold paleotemperatures. [7] Hebbeln et al. [1994] first proposed use of the planktic foraminifera species Turborotalita quinqueloba as a potential indicator of ice-free conditions in the Fram Strait. Over the last few years, the Kiel group has established a new and enlarged set of planktic foraminifera counts in 947 carefully screened modern sediment samples from the Atlantic between 50 S and 85 N. This data set serves as a reference base for estimating sea surface temperatures (SST) by means of the SIMMAX modern analog technique [Pflaumann et al., 1996, 2003]. Although not then tested by modern analog sea ice patterns, Weinelt et al. [1996] (using an early SIMMAX version of only 506 core top samples) first employed SIMMAX SST estimates warmer than 3.5 C to infer the existence of ice-free Nordic Seas during the glacial summer, a pattern clearly different from the complete sea ice cover proposed by CLIMAP Project Members [1981]. [8] Based on nearly 150 improved SIMMAX SST estimates (for 10 m water depth), we now present a new conservative measure of maximum seasonal sea ice cover in the northern North Atlantic (Figures 1 and 2). We also apply this new sea ice proxy to some 60 sediment cores for reconstructing the sea ice cover of the northern North Atlantic during the LGM. In this reconstruction the age of the LGM is assumed as 21,500 18,000 years B.P., equal to the Last Isotope Maximum, following the definition of GLAMAP 2000 [Sarnthein et al., 2003]. The age definitions for each single core used in this study are documented by Vogelsang et al. [2001]. [9] Complementary results for the LGM sea ice distribution in the high-latitude South Atlantic and Southern Ocean have been deduced from diatom tracer species in 50 cores by the GLAMAP 2000 working group in Bremerhaven [Gersonde et al., 2003]. 2. Strategy to Deduce a Sea Ice Proxy [10] In general, the relationship between modern SST and sea ice distribution appears non-linear [de Vernal and Hillaire-Marcel, 2000] since local winds and currents play an important role for ice drift. Satellite-radiometric observations in the northern North Atlantic from [Gloersen et al., 1992] show that sea ice concentrations of >50% during summer (July to September) are found in regions where the SST of Levitus and Boyer [1994] reach up to >2 C. During winter (January to March) sea ice occurs where SST reach up to 1 C. Off South Greenland SST below winter sea ice may rise up to 3.5 C. At the sites used for deducing the new sea ice proxy (Figures 1 and 2) modern SST values reach up to 2.5 C during both summer and winter (Figure 3).

3 SARNTHEIN ET AL.: GLAMAP 2000 SEA ICE 25-3 Figure 2. Modern limits (bold broken lines) of >50% sea ice concentration (satellite radiometric observations of Gloersen et al. [1992]) in the northern North Atlantic during winter (January to March) and as reconstructed for the Little Ice Age [Kellogg, 1980] in comparison to SIMMAX-based estimates of modern winter SST at 10 m water depth. Large numbers label isotherms ( C). See color version of this figure at back of this issue. [11] Use of these modern observations as a basis for past reconstructions is hampered by the fact that planktic foraminifera-based estimates of SST along and behind the sea ice frontal region may produce a more complicated pattern (Figure 3). This is because surface currents can lead to a local bias in these estimates near the cold end of the SIMMAX transfer function used in this study [Pflaumann et al., 1996, 2003]. Several reasons for such a bias can exist: (1) Foraminifera characteristic of warm, highly saline water masses drift far below the cold but meltwater-enriched, and hence less dense, polar water masses of the East Greenland Current, even well under the polar sea ice off east and northeast northern Greenland. (2) Cold polar specimens float in meltwater-enriched polar water masses across the polar front and over the underlying more saline but warm water masses of the Atlantic inflow. This occurs in the southeastern branches of the East Greenland and Jan Mayen Currents, especially during summer. Finally (3), samples from sites with extremely low sedimentation rates contain a long-term mixture of faunas and related climate signals because of intra-holocene current and SST changes. Accordingly, many foraminiferal assemblages either record Little Ice Age (LIA) conditions or even those of the Holocene climatic optimum. [12] Such caveats lead to inconsistencies in foraminiferal distributions that clearly work against the goal of developing a simple transfer function to deduce past sea ice cover from planktic foraminifera assemblages. Therefore we propose a different and minimalistic approach for the reconstruction of sea ice extent. We define new SST threshold estimates for the two seasonal ice extremes as a conservative measure to derive the largest-possible sea ice cover per analogy to the sea ice observed during summers and winters of [Gloersen et al., 1992], as well as to the extreme ice extent of the LIA winters [Kellogg, 1980]). When accepting the modern analog principle, this simple empirical approach leads us to conservative, fairly robust estimates of the minimum extent of ice-free regions during past summer and winter times. [13] Temperature estimates are based on the SIMMAX- 28 Modern Analog Technique [Pflaumann et al., 2003], an improved version of the SIMMAX-24 technique [Pflaumann et al., 1996]. Each estimate is based on 10 closest modern analogs out of 947 modern core top samples from the Atlantic Ocean between 86 N and 56 S, with specimens of planktic foraminifera counted in samples from regions with low species diversity. SIMMAX SST estimation includes a weighting by the reciprocal value of geographical distance. The modern and glacial SST data used in this paper (Figures 1, 2, 4, and 5 are available under

4 25-4 SARNTHEIN ET AL.: GLAMAP 2000 SEA ICE Figure 3. SIMMAX-based SST estimates and correlative measured SST values at 10 m water depth for summer and winter in the Nordic and Arctic Seas. Asterisks mark SST estimates for sediment samples from regions beneath sea ice (with >50% ice concentration) during summer and winter seasons as indicated. Horizontal lines show temperature limits of maximum sea ice cover during modern summer and winter ( ; Gloersen et al. [1992]), and during Little Ice Age winters [Kellogg, 1980]. Pflaumann et al. at query=glamap. 3. Derivation of the Modern Sea Ice Limit From SIMMAX-Based SST Estimates [14] During modern summer, eight sites with SIMMAX SST estimates less than 2.5 C lie beneath the modern sea ice cover (defined as >50% ice concentration; Figures 1 and 3). On the other hand, 100% of all modern SST estimates greater than 2.5 C lie seaward of the ice margin, in addition to 17 samples with low SST of C. A single exception from this threshold value is a SST estimate of 3.4 C at a site below the ice north of Svalbard. However, this outlier can be clearly rejected since sedimentation rates at this site are as low as cm/ka ( 14 C ages of 5.9 cal ka at 2.5 cm and of 9.96 cal ka at 6.5 cm core depth; Vogelsang et al. [2001]), which implies that faunal signals of different climate phases are mixed. [15] We therefore regard the threshold SST of 2.5 C asa robust proxy for the maximum extension of summer sea ice, or conversely, for the minimum extent of open sea conditions during the geologic past. A 2.5 threshold temperature is considered conservative because SIMMAX-based SST estimates in the central Nordic Seas tend to underestimate the actual SST, by C near the Arctic front along the eastern margin of the Arctic water mass. On the other hand, near the cold end of the SIMMAX SST regression summer SST estimates below 3 C tend to overestimate the actual temperatures by up to 3.5 C [Pflaumann et al., 2003]. Hence this potential bias does not affect the validity of the outlined threshold temperature since estimates of 2.5 C may correspond to real SST as low as 1.6 to 1.0 C below the ice (Figure 3). [16] The obvious lack of estimated (and measured) SST near 5 7 C for summer and 1 2 C for winter in Figure 3 corresponds to a zone close to the Arctic front, where cold polar and warm Atlantic water masses mix. This narrow band is strongly underrepresented in our matrix of SST values because the density in core top sampling is insufficient. [17] During modern winter, the measured sea ice cover between [Gloersen et al., 1992] parallels SIM- MAX SST of less than 0.4 C (Figures 2 and 3). Except for two single ice-covered sites at 0.5 and 0.75 C, all winter SST estimates higher than the threshold value of 0.4 C lie seaward of the winter sea ice limit. In addition, some 55 sites with winter SST lower than 0.4 C lie outward of the sea ice (Figure 3). We used the threshold value of 0.4 C to reconstruct a hypothetical outer margin of modern winter sea ice cover on the basis of our core top samples (Figure 2). Based on this criterion, the inferred ice cover would spread almost to the limits of the greatly advanced sea ice limit reported for extreme winters during the Little Ice Age (LIA) AD [Kellogg, 1980], little different from the ice margin reconstructed by the LIA threshold value of 0.75 C. This apparent overestimate of sea ice extent may partly result from the SIMMAX technique, which in many cases overestimates the real winter SST by up to C (Figure 3). However, the discrepancy between the observed and reconstructed sea ice cover may also simply result from the age of the core top samples. Since sedimentation rates in large parts of the central Nordic Seas rarely exceed 1 2 cm/1000 years, a 1-cm thick core top sample typically integrates some years, thus incorporating the low-temperature signals of the LIA. [18] In summary, SIMMAX-based winter SST values of 0.4 and 0.75 C, respectively, form a conservative and apparently robust method for reconstructing past variations in the minimum extent of open sea conditions seaward of the winter sea ice cover per analogy to today and to the LIA. This method of seasonal sea ice reconstruction stands

5 SARNTHEIN ET AL.: GLAMAP 2000 SEA ICE 25-5 Figure 4. Maximum extent of sea ice (hatched) and SIMMAX SST in the North Atlantic during LGM summer. Sea ice limit deduced from 2.5 C isotherm as defined in Figure 3. Solid and open arrows mark assumed cold and warm surface water currents. Large numbers label isotherms ( C). See color version of this figure at back of this issue. only so long as the modern analog principle is accepted, an assumption, however, which applies to all paleoceanographic proxies based on faunal data. As recently shown by Malmgren et al. [2001] and Weinelt et al. [2003], the trends and quality of SIMMAX-based SST estimates come close to the SST estimates independently deduced by the artificial neural-network (ANN) technique. Thus any reservations that potential no-analog cases may affect the quality of our SST estimates, and thus the sea ice reconstruction for the LGM, are considered to have little significance. Reservations may also arise due to the potential biases introduced by the differential depth habitats of the planktic species used for the SIMMAX transfer function. However, the most abundant planktic species in northern high latitudes (N. pachyderma s and T. quinqueloba) do not show significant changes in their habitat depths across the central ice-free and the western ice-covered Nordic Seas [Simstich et al., 2003; Weinelt et al., 2001]. 4. North Atlantic Sea Ice During the Last Glacial Maximum [19] During LGM summer, our sea ice estimates reveal a northern North Atlantic which was largely ice-free up to approximately 80 N (Figure 4), much different from the CLIMAP Project Members [1981] reconstruction which concluded that the Nordic Seas were totally ice-covered. There appears to have been an open water channel which extended along the eastern ocean margin up to northern Svalbard during summer, likely the result of a weak glacial West Spitsbergen Current (Figure 4). Weinelt et al. [2003] document the centennial-scale variability of glacial sea ice margins in their separate study. Rare extremely cold glacial summers (<10% of all cases in the time series of Weinelt et al. [2003]) may have led to a possible advance of sea ice along the western and eastern continental margins down to 60/65 N. In this case, the southern central Nordic Seas remained the only ice-free region during LGM summer. [20] These LGM sea ice limits essentially differ from the pattern displayed by CLIMAP Project Members [1981] and Kellogg [1980], where the Nordic Seas have a perennial ice cover down to latitudes south of Iceland and west of Scotland similar to the modern Arctic Sea. Various new evidence may help to explain this discrepancy. Especially, CLIMAP had no access to any sediment reference record from the modern ice-covered Arctic Sea for calibrating sea ice proxies. In contrast to the original assumptions, modern high-resolution stratigraphy has revealed high to very high hemipelagic sedimentation rates for many sites in the LGM Nordic Seas [Vogelsang et al., 2001]. CLIMAP considered low planktic foraminiferal and coccolith abundances for the glacial Nordic Seas as key evidence for perennial sea ice

6 25-6 SARNTHEIN ET AL.: GLAMAP 2000 SEA ICE Figure 5. Maximum extent of sea ice and SIMMAX SST in the North Atlantic during LGM winter. Sea ice cover per analogy to (up to 0.4 C isotherm) is hatched in blue; additional sea ice per analogy to the Little Ice Age (up to 0.75 C isotherm) is hatched in gray, as defined in Figure 3. Large numbers label isotherms ( C). See color version of this figure at back of this issue. cover. Today these abundances are known to be much higher than in the Arctic Sea and can now be ascribed to a sea ice-free scenario during summer like in the Arctic Domain. Finally, IRD numbers that are generally regarded as tracer of seasonally open sea conditions turned out to be fairly high, even in the glacial northern Nordic and southern Arctic Seas [Nørgaard-Pedersen et al., 2003]. [21] The results of our new sea ice threshold proxy largely support earlier findings of Hebbeln et al. [1994] and Weinelt et al. [1996], as well as those of Rochon et al. [1999]. The incursion of relatively warm water masses into the Nordic ice-free sea regions may have resulted in small-scale, but more or less continual, formation of deep water during the shorter glacial summers, as traced by the extremely heavy d 18 O and d 13 C signals of benthic foraminifera, reported by Jung [1996], Voelker [1999], and Sarnthein et al. [2001]. The ice-free areas would also have also led to seasonally enhanced evaporation in the Nordic Seas, and thus contributed to the continued syn- to late-glacial buildup of large ice sheets in Scandinavia and, in particular, on the Barents Shelf [Vorren and Laberg, 1996]. [22] When using the LIA ice extent as the reference standard to reconstruct the LGM winter scenario, the SIMMAX-based SST estimates suggest widespread sea ice (Figure 5). In this scenario, sea ice extends far south into the Northern Atlantic, in the west almost up to Newfoundland, in the center up to the southern coast of Iceland, and in the east up to the Rockall Plateau. When based instead on the analog of modern remote sensing data [Gloersen et al., 1992], the sea ice extent during glacial winter would have been somewhat smaller, but still reached up to a line south of Faroe-Iceland. In the northwest Atlantic, the inferred sea ice margin clearly indicates glacial warm water advection up to western Iceland along a track that is similar to the modern Irminger Current. The two winter reconstructions both suggest an isolated patch of sea ice in the eastern central North Atlantic, possibly located near the center of a long-term stable Azores High during glacial winter. Differences to the tentative pattern published by CLIMAP Project Members [1981] are less significant than for LGM summer. [23] The large seasonal variability of the reconstructed sea ice extent implies a similar pronounced seasonality of climate on the circum-atlantic continents, especially in Europe. Moreover, the annual sea ice melt would have involved a major local consumption of both incoming solar and laterally derived heat energy. These postulates are corroborated by model experiments that test the impact of strong seasonal sea ice variation on changes in European climate and ocean circulation, and on carbon transfer between the ocean and the atmosphere [Pinot et al., 1999; Ganopolski and Rahmstorf, 2001; Gildor and Tziperman, 2000; Schulz and Paul, 2003]. [24] The broad sea ice-free channel inferred to extend south-north near N and 30 W during glacial winter may have formed an ideal site for large-scale convection of Upper North Atlantic Deep Water, the benthic isotope signals of which are observed at numerous sites across the North Atlantic shallower than 2400 m water depth [Duplessy et al., 1988, 1991; Sarnthein et al., 1994, 2001]. 5. Conclusions [25] Sea ice is an important boundary condition for ocean and climate modeling. However, in contrast to the Southern Ocean, the glacial extent of sea ice has remained highly controversial in the North Atlantic, where pre-holocene sediments lack the main sea ice tracer of the Southern Ocean, siliceous plankton. [26] A new quantitative proxy for the maximum extent of sea ice (defined as >50% ice concentration) has been derived from the maximum (seasonal) SST estimated by means of the SIMMAX technique [Pflaumann et al., 2003]

7 SARNTHEIN ET AL.: GLAMAP 2000 SEA ICE 25-7 from planktic-foraminifera counts in core top sediments below the modern sea ice cover. Today ( ), almost 100% of all SIMMAX-based SST estimates do not exceed 2.5 C for summer and 0.4 C for winter in areas that are more than 50% ice covered. When compared to sea ice conditions of the Little Ice Age winters, the threshold winter SST estimates increase to 0.75 C. These new threshold values provide a conservative estimate for the maximum extent of sea ice, or conversely for the minimum range of ice-free water in the past. [27] Applications of these threshold values to a set of more than 60 well-dated sediment cores shows that the glacial North Atlantic was characterized by an extreme seasonal variability of climate. During glacial summers, sea ice retreated up to the Arctic ocean and the western Fram Strait, in contrast to the CLIMAP Project Members [1981] reconstruction that postulated a complete perennial sea ice cover. Accordingly, most of the Nordic Seas remained ice free, leading to conditions that could support at least modest deepwater convection. During glacial winters, sea ice extended to the south of the Iceland Faroe Ridge. A small patch of ice may also have lain near the (modern and glacial) center of the Azores High. During the winter season, a broad meridional ice-free channel 50 to 60 N may have formed the site of intensive upper North Atlantic Deep Water formation. These new sea ice data sets will contribute to the initiation and/or testing of paleoceanographic ocean circulation models [cf. Paul and Schäfer-Neth, 2003]. [28] Acknowledgments. Discussions with J.-C. Duplessy and A. Mix helped focus the results of this paper. We received generous support from the German Science Foundation (DFG). Many data used in this study were generated in the framework of the DFG Special Research Project 313 in Kiel. 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8 25-8 SARNTHEIN ET AL.: GLAMAP 2000 SEA ICE Vogelsang, E., M. Sarnthein, and U. Pflaumann, d 18 O stratigraphy, chronology, and sea surface temperatures of Atlantic sediment records (GLAMAP 2000 Kiel), Ber. Rep. Inst. Geowiss., Univ. Kiel, 13, 244 pp., Vorren, T. O., and J. S. Laberg, Late glacial air temperature, oceanographic and ice sheet interactions in the southern Barents Sea region, Geol. Soc. Spec. Publ., 111, , Weinelt, M., M. Sarnthein, U. Pflaumann, H. Schulz, and S. Jung, Ice-free Nordic Seas during the Last Glacial Maximum?, Potential sites of deepwater formation, Paleoclim. Data Modell., 1, , Weinelt, M., et al., Paleoceanographic proxies in the northern North Atlantic, in The Northern North Atlantic: A Changing Environment, edited by P. Schäfer et al., pp , Springer- Verlag, New York, Weinelt, M., E. Vogelsang, M. Kucera, U. Pflaumann, M. Sarnthein, A. Völker, and H. Erlenkeuser, Variability of North Atlantic heat transfer during MIS 2, Paleoceanography, 18, doi: /2002pa000772, in press, U. Pflaumann, M. Sarnthein, and M. Weinelt, Institut für Geowissenschaften, University of Kiel, D Kiel, Germany. (ms@gpi.uni-kiel. de; up@gpi.uni-kiel.de; mw@gpi.uni-kiel.de)

9 SARNTHEIN ET AL.: GLAMAP 2000 SEA ICE Figure 1. Modern limits (bold broken lines) of >50% sea ice concentration (satellite radiometric observations of Gloersen et al. [1992]) in the northern North Atlantic during summer (July to September) as compared to SIMMAX-based estimates of modern summer SST at 10 m water depth. Large numbers label isotherms ( C). 25-2

10 SARNTHEIN ET AL.: GLAMAP 2000 SEA ICE Figure 2. Modern limits (bold broken lines) of >50% sea ice concentration (satellite radiometric observations of Gloersen et al. [1992]) in the northern North Atlantic during winter (January to March) and as reconstructed for the Little Ice Age [Kellogg, 1980] in comparison to SIMMAX-based estimates of modern winter SST at 10 m water depth. Large numbers label isotherms ( C). 25-3

11 SARNTHEIN ET AL.: GLAMAP 2000 SEA ICE Figure 4. Maximum extent of sea ice (hatched) and SIMMAX SST in the North Atlantic during LGM summer. Sea ice limit deduced from 2.5 C isotherm as defined in Figure 3. Solid and open arrows mark assumed cold and warm surface water currents. Large numbe rs label isotherms C). ( Figure 5. Maximum extent of sea ice and SIMMAX SST in the North Atlantic during LGM winter. Sea ice cover per analogy to (up to 0.4 C isotherm) is hatched in blue; additional sea ice per analogy to the Little Ice Age (up to 0.75 C isotherm) is hatched in gray, as defined in Figure 3. Large numbers label isotherms ( C) and 25-6