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The Taylor Dome Antarctic 18O Record and Globally Synchronous Changes in Climate
Quaternary Research 56, 289–298 (2001) doi:10.1006/qres.2001.2276, available online at http://www.idealibrary.com on
The Taylor Dome Antarctic 18 O Record and Globally Synchronous Changes in Climate1 Pieter M. Grootes,2 Eric J. Steig, Minze Stuiver, Edwin D. Waddington, and David L. Morse3 Department of Earth and Space Sciences and Quaternary Research Center, University of Washington, Seattle, Washington 98195 E-mail: [email protected]
and Marie-Jos´ee Nadeau Leibniz-Labor, Christian-Albrechts-Universit¨at, 24118 Kiel, Germany Received August 31, 1999
The 18 O/16 O profile of a 554-m long ice core through Taylor Dome, Antarctica, shows the climate variability of the last glacial– interglacial cycle in detail and extends at least another full cycle. Taylor Dome shares the main features of the Vostok record, including the early climatic optimum with later cool phase of the last interglacial period in Antarctica. Taylor Dome δ 18 O fluctuations are more abrupt and larger than those at Vostok and Byrd Station, although still less pronounced than those of the Greenland GISP2 and GRIP records. The influence of the Atlantic thermohaline circulation on regional ocean heat transport explains the partly “North Atlantic” character of this Antarctic record. Under full glacial climate (marine isotope stage 4, late stage 3, and stage 2), this marine influence diminished and Taylor Dome became more like Vostok. Varying degrees of marine influence produce climate heterogeneity within Antarctica, which may account for conflicting evidence regarding the relative phasing of Northern and Southern Hemisphere climate change. ° 2001 University of Washington. Key Words: Antarctica; Taylor Dome; ice core; oxygen isotope; climate change; thermohaline circulation. C
INTRODUCTION
Evidence of large and rapid glacial climate fluctuations in two ice cores (GISP2 and GRIP) from the summit area of the Greenland ice sheet (Johnsen et al., 1992; Dansgaard et al., 1993; Grootes et al., 1993) spurred a search for rapid fluctuations elsewhere to evaluate their extent and significance. Such fluc1
Oxygen isotope data with depth and time scale are available at the stable isotope laboratory home page at the University of Washington (depts. washington.edu/isolab) and the Leibniz-Labor, Christian-Albrecht University, Kiel (www.uni-kiel.de/leibniz). These and additional Taylor Dome data sets are also available from the World Data Center for Paleoclimatology at the National Geophysical Data Center (www.ngdc.noaa.gov/paleo). 2 Now at Leibniz-Labor, Christian-Albrechts-Universit¨ at Kiel, Germany. 3 Now at Institute for Geophysics, University of Texas, Austin, TX 78795.
tuations were soon observed in ocean sediment cores from the North Atlantic (Rasmussen et al., 1996, 1999; Bond et al., 1997; Sachs and Lehman, 1999) and North Pacific (Behl and Kennett, 1996). In contrast, long ice core records from Antarctica suggest a more gradual and moderate climate variability in the Southern Hemisphere (Bender et al., 1994). Results from an ice core from Taylor Dome, however, indicate a strong link between rapid climate fluctuations in the Northern and Southern Hemispheres. In January 1994, a 554-m ice core and 6 cm of basal debris were obtained from Taylor Dome, a small East Antarctic ice dome at the head of Taylor Valley, Southern Victoria Land (Fig. 1). Glaciological, geophysical, and meteorological field studies (Morse, 1997) were used to select the drill site at 1.5 km (3 ice depths) south of the flow divide over a bedrock plateau at 77◦ 470 4700 S, 158◦ 430 2600 E, ice surface elevation 2365 m. The basal temperature is −26◦ C (Clow and Waddington, 1996), and the ice is likely to have undergone simple flow (Morse, 1997), so that the geochemical paleoclimate records throughout the core are well preserved. A multidisciplinary group carried out an integrated core-sampling program (Grootes et al., 1994). Results have been reported for major ion concentrations, cosmogenic isotopes, stable isotopes, and trace gases (Steig et al., 2000 and references therein). The oxygen isotope record of the ice was measured at the Quaternary Isotope Laboratory of the University of Washington, using an automated Micromass 903 mass spectrometer with CO2 equilibration system. The results are expressed as δ 18 O, the relative deviation of the isotopic abundance ratio 18 O/16 O from that of standard mean ocean water (V-SMOW), given in per mil (‰) and have an overall precision of 0.14‰. In general, the more negative the δ 18 O values, the colder the climate (e.g., Jouzel et al., 1997). The oxygen isotope profile (Fig. 2) was measured twice, using independent sample sets. One, with increments of 1.0 m for 0–340 m and 0.5 m for 340–554 m depth, was taken
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FIG. 1. The location of Taylor Dome, and its two uppermost 25-m surface elevation contours with drill site (77◦ 470 4700 S, 158◦ 430 2600 E, elevation 2365 m) and geophysical survey line, are shown in relation to the McMurdo Dry Valleys, the Ross Sea, and the Ross Ice Shelf. The positions of the cores discussed are shown in the inset.
from the outside of the core. The other was obtained as a split from the melted 20-cm-increment chemistry samples at the University of New Hampshire, with gaps for a few sections of poor core quality. Agreement between the series over the full length of the core shows that the details of the oxygen isotope record are reproducible. We compare the Taylor Dome oxygen isotope record with the isotope records of Vostok and Byrd Station, Antarctica, and GISP2, Greenland. We use IS 1-23 to denote the interstadial Dansgaard/Oeschger periods (Dansgaard et al., 1993) and marine isotope stage (MIS) 1-9 for the marine paleoclimate record (Shackleton and Opdyke, 1973). The features of the Taylor Dome δ 18 O record suggest that climate changes were essentially synchronous in both hemispheres. Changes in ocean circulation heat transport may, however, have temporarily masked the global signal in certain regions of the ocean and in adjacent land areas, including eastern to central North America (e.g. Reasoner and Jodry, 2000), Greenland, Europe, and also some parts of the Southern Hemisphere.
TIME SCALE
Steig et al. (1998b) developed a time scale for the Taylor Dome core by correlating its gas record (CH4 and δ 18 O of O2 ) with the GISP2 ice core. After correction for offsets between ice age and gas age in both cores, the Greenland ages, based on counted annual layers back to 50,000 yr B.P. (before A.D. 1950; Meese et al., 1994), were transferred to the Taylor Dome ice core. Steig et al. (2000) provided a Taylor Dome time scale (termed st9810) that extends the gas-correlation results to ∼160,000 yr B.P. There are, however, too few dated gas control points to explore the timing of rapid isotopic shifts during the last ice age. Our new time scale follows st9810 back to 14,580 yr B.P. For deeper ice, we assume that major climate changes occurred essentially simultaneously at Taylor Dome and Vostok and can be correlated at the midpoints of major isotope changes. Therefore, the ice in each resulting depth-age segment comes only from either a warm period with presumably higher accumulation or a cold period with lower accumulation (Steig et al., 2000). Thus,
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of the isotope records decreases beyond 200,000 yr B.P., where the time interval spanned by individual samples in Taylor Dome is too large for faithful comparison. The new time scale is used to compare the Taylor Dome isotope record for the last glacial–interglacial cycle with records from Vostok (Petit et al., 1999) and Byrd Station (Beer et al., 1992), Antarctica, and from GISP2, Greenland (Grootes et al., 1993; Grootes and Stuiver, 1997), each on its own time scale (Fig. 4). Although dating precision and accuracy are still inadequate to prove or disprove synchrony of events of duration less than 1000 yr prior to 15,000 yr B.P., our new time scale addresses the concerns of Mulvaney et al. (2000) while clarifying the real and important differences between Taylor Dome and other Antarctic ice core records. THE ISOTOPE RECORD
Terminations II and I
FIG. 2. Oxygen isotope profiles of the 554-m-long ice core through Taylor Dome, Antarctica. Sample resolution for the coarse series is 1 m down to 314 m and 0.5 m below this depth; for the detailed series, it is 0.2 m over the full length. The transition into glacial ice is at about 370 m depth.
our linearly interpolated depth–age relation should contain little error from changing accumulation rates associated with the correlated climate events. We tie the last glacial–interglacial δ 18 O cycle of Taylor Dome to the Vostok δD record on the time scale GT4 of Petit et al. (1999; Fig. 3a). The greatest difference with st9810 occurs between 25,000 and 14,580 yr B.P., where we correlate the Taylor Dome δ 18 O peak at 376.2 m, near 17,000 yr B.P. on the st9810 time scale, with the small δD peak at ∼ 24,000 yr B.P. at Vostok. Correlated calcium records provide a time scale for the δ 18 O rise that marks the end of the last glacial maximum (LGM). Following the argument of Mulvaney et al. (2000) that calcium (largely continental dust from long-range transport) “should show similar trends across the region,” we match the ∼ 50% dip in Taylor Dome calcium concentrations in the ice at 376.2 m (Mayewski et al., 1996; Steig et al., 2000) to its close counterpart at Dome C. We add three more tie points to the Dome C record that spread the 4.4-m section of very low accumulation rates between 24,000 and 14,580 yr B.P. at Taylor Dome to mimic the calcium pattern of Dome C (Fig. 3c). We tentatively extend the Vostok-correlation time scale beyond 300,000 yr B.P., MIS 9 (Fig. 3b), although the similarity
As shown in Figure 2, Termination II (the penultimate glacialto-interglacial transition) at Taylor Dome covers only 1 m of core, 23 m above bedrock. Because layer thinning from ice flow this close to bedrock is strong and because changes in accumulation rate and physical properties of the ice over the glacial– interglacial transition are uncertain, we cannot reliably estimate the time interval over which Termination II occurred. Termination I (the last glacial-to-interglacial transition) at Taylor Dome covers the depth interval 375–354 m. The initial δ 18 O increase looks particularly sharp, from a glacial −44.0‰ at 375 m depth to a Holocene −36.7‰ over just 4.4 m. This apparent sharpness may reflect extremely low accumulation rates during the LGM, when wind scouring may have been common (cf. Fisher et al., 1983). Indeed, the correlation of Taylor Dome with Dome C calcium implies that this δ 18 O increase is at first gradual and similar to that of other Antarctic cores. Near 16,000 yr B.P., however, even on our new time scale matched to Dome C Ca, the δ 18 O increase becomes steeper and may represent a real warming event (Fig. 3c). The first strong δ 18 O maximum (−36.7‰) is followed by 3‰ cooling (Figs. 2 and 3a). Seven methane tie points from 377.5 to 356.2 m provide clearly defined gas ages from 14,900 to 10,900 yr B.P. on the GISP2 layer-counted time scale (Steig et al., 1998b, 2000; Brook et al., 2000). These tie points show that the 3‰ cooling, although superficially similar to the Antarctic cold reversal (ACR) (Jouzel et al., 1995; Blunier et al., 1997, 1998), occurs later than the ACR at Byrd Station and, instead, is in phase with the Greenland cooling from the Allerød into the Younger Dryas (Steig et al., 1998b). This part of the time scale is not affected by the forced match between Dome C and Taylor Dome calcium records. Indeed, the appearance of this feature at Taylor Dome is corroborated by recent results from Dome C, which also show cooling synchronous with Greenland after ∼14,000 yr B.P. (Jouzel et al., in press). The steep increase in δ 18 O near 16,000 yr B.P., where Vostok δD and Dome C show only a small shoulder, may indicate a shift to more marine influences at Taylor Dome as discussed below.
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FIG. 3. Comparison of the Taylor Dome and Vostok isotope records on the Vostok GT4 time scale of Petit et al. (1999). (a) The last glacial–interglacial cycle. Arrows indicate where rapid changes of Taylor Dome δ 18 O (thin line) were correlated with Vostok and Dome C (Fig. 3c). (b) The full Taylor Dome δ 18 O record with a tentative Vostok correlation extending beyond 300 kyr B.P. (c) The Ca correlation of Taylor Dome (thin line) with Dome C (thick line) for the period 14,580 to 24,000 yr B.P.
Marine Isotope Stage 5e The last interglacial period (MIS 5e) is marked by heavy δ 18 O values between 526 and 531 m depth. The Taylor Dome δ 18 O record has an initial peak and a later shoulder that is similar to
the Vostok δD record (Petit et al., 1999; Fig. 3a). This similarity validates the fine structure of the isotope signal at Vostok. The peak-shoulder structure of an otherwise quiet MIS 5e, generally equated with the last interglacial period labeled “Eemian” in the Netherlands, thus appears real. The ∼ 2‰ (16‰) drop from
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peak to shoulder δ 18 O (δD) values probably corresponds to the intra-Eemian cooling event observed at Ocean Drilling Project (ODP) Site 658 off West Africa (Maslin et al., 1998) and the middle-Eemian cooling in the Norwegian Sea (Cortijo et al., 1994) and Europe (Litt et al., 1996; Cheddadi et al., 1998). This Eemian pattern is repeated in the Holocene: about 6000 yr B.P. δ 18 O at Taylor Dome fell rapidly by about 1.5‰ (Steig et al., 1998a; Steig, 1999). Aside from the peak-to-shoulder step, δ 18 O values for the last interglacial are fairly constant. The 0.2-m sample resolution at Taylor Dome, representing less than 1000 yr per sample in this interval, is sufficient to detect GRIP-like Eemian δ 18 O oscillations (Dansgaard et al., 1993), if they were in fact climate related. Their absence at Taylor Dome and Vostok adds Antarctic support to evidence from the Atlantic Ocean (e.g., McManus et al., 1994; Rasmussen et al., 1999) and the European continent (Litt et al., 1996; Cheddadi et al., 1998), for a stable Eemian climate. Marine Isotope Stages 5d to 4 The early-glacial period (MIS 5d to 4) of the Taylor Dome δ 18 O record validates the Vostok record of early-glacial Antarctic climate fluctuations by showing, prior to 80,000 yr B.P. (Fig. 4), oscillations similar in magnitude and fine structure to those at Vostok. In contrast, the oldest part of the Byrd Station record, before ∼ 85,000 yr B.P., shows no resemblance to Taylor Dome or Vostok, probably because of flow disturbance of this part of the Byrd core. The interval 80,000–60,000 yr B.P. (MIS 5a and 4) is similar in all three Antarctic isotope records and the GISP2 record. All show relatively long warm periods (IS 21, 20, 19 lasting 10400, 3000, and 1900 yr respectively on the GISP2 scale) broken by brief (900- and 1200-yr) and successively colder intervals. The similarities imply a global extent for early-glacial climate changes with a strong ∼ 20,000-yr precessional component (MIS 5c,a) and for the oscillating transition to MIS 4. Marine Isotope Stages 3 and 2 For the full-glacial period (MIS 3 and 2), 60,000 to 15,000 yr B.P., the Taylor Dome record shows that full-glacial climate variability in Antarctica was not limited to the major interstadials (equivalent with Greenland IS 17/16, 14, 12, and 8) but also included the minor interstadials documented in Greenland and North Atlantic records (Figs. 3 and 4). Taylor Dome δ 18 O fluctuations, like those of GISP2, are more abrupt and larger than those at Vostok and Byrd Station, even though the sampling resolution at Taylor Dome is lower. Smoothing of the δ 18 O signal during firnification at Vostok cannot explain this difference. The characteristic pattern of a major interstadial followed by three weaker, short ones found in the Greenland ice cores (IS 12-9 and IS 8-5; Johnsen et al., 1992; Dansgaard et al., 1993; Grootes et al., 1993; Stuiver and Grootes, 2000) and identified by Bond et al. (1993) in North Atlantic sediments, can also be seen at Taylor Dome, though for IS 12-9 the core is of poor quality. Minor interstadials are also reported by Bender et al. (1999)
FIG. 4. Comparison of ice core isotope records of climate shown on their respective time scales: Taylor Dome as discussed in text, Vostok GT4 of Petit et al. (1999), Byrd Station after Beer et al. (1992), and GISP2 after Meese et al. (1994) and Bender et al. (1994). Ice core isotope stages are labeled IS 1-21 after Dansgaard et al. (1993) and marine isotope stages MIS 1-6 after Shackleton and Opdyke (1973).
for Vostok and by Blunier and Brook (2001) for Byrd, but their expression in these cores is less clear than at Taylor Dome. Shading in Figure 5 highlights the δ 18 O pattern of IS 8 to 5 at Taylor Dome and GISP2 on their respective time scales. A relative shift of 3200 ± 100 yr—within the uncertainty of the time scales at this point in the records—brings for IS 8 to 6 a close match between the δ 18 O maxima of TD and the sudden increases at GISP2, similar to the CH4 -based match between Byrd and Greenland of Blunier and Brook (2001). HEMISPHERIC AND INTERHEMISPHERIC CLIMATE COMPARISON
Under present interglacial conditions, Taylor Dome receives much of its precipitation from cyclonic systems from the nearby Ross Sea (Morse, 1997; Morse et al., 1998) and is thus sensitive to local oceanic conditions near Antarctica. In contrast, the high East Antarctic plateau (Fig. 1) favors outflowing katabatic winds and a strong zonal circulation that together impede poleward lower tropospheric transport. These conditions effectively
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NADW formation during stadials and vigorous NADW flow and heat transport during interstadials (Rasmussen et al., 1996, 1999) led to large and rapid changes in isotope values in Greenland. The effects of switching NADW formation on and off will be felt almost instantaneously along the full thermohaline circulation. Because NADW is warmer than Circumpolar deep water (CPDW), with which it mixes in the South, regional warming and sea ice melt may occur in those coastal areas where CPDW upwelling is significant when NADW formation is switched on (Steig et al., 1998b). Greenland is thus linked to Antarctica, but more so to near-coastal Taylor Dome than to continental Vostok. Regional oceanic influence may also account for differences among the isotope minima and maxima in Figure 4. The minima of GISP2, Vostok, and Taylor Dome show a fairly uniform decrease over MIS 5b-4 and MIS 3-2. However, interstadial– maximum δ 18 O values at GISP2 stay close to −37‰ for all but IS 2, while Vostok maxima gradually decline. At Taylor Dome, the early interstadials IS 17, 16, 14, and 12 during MIS 3 stay fairly warm, as in GISP2, but the maxima of IS 20, 19, and 11-2 decrease and join the trend of the continental Vostok record. Evidently, during stadials, when thermohaline heat transport was greatly reduced, both hemispheres experienced deFIG. 5. Taylor Dome and GISP2 20-cm δ 18 O values for IS 8–5 on their clining minima, modulated by longer term Milankovich-driven respective time scales. Shading highlights idealized isotope profile for these cooling. Vigorous NADW formation and heat transport during Dansgaard/Oeschger cycles. Dashed lines indicate correlation between δ 18 O the warmer parts of the glacial, however, kept both GISP2 inmaxima at Taylor Dome and jumps at GISP2 as suggested by Byrd Station and terstadials and Taylor Dome interstadials relatively warm. Thus Greenland methane data (Blunier and Brook, 2001). the “North Atlantic” character of the Taylor Dome core for the Younger Dryas (Steig et al., 1998b) also appears to have preisolate Vostok from nearby oceanic influences (Koster et al., vailed for much of the last glacial period. The muted character 1992). At present, Taylor Dome is outside the central East Antarc- of the climate variability in most Antarctic ice cores, espetic zone of impeded tropospheric transport (Raper et al., 1984; cially Vostok, in comparison with Greenland ice cores thereMosley-Thompson et al., 1990). However, expansion of circum- fore reflects Greenland’s sensitivity to changes in North Atlantic polar circulation (Mayewski et al., 1996) and advance of the oceanic heat transport in contrast with the isolation of central Ross Sea ice sheet after MIS 5a and during MIS 3 (Morse et al., Antarctic sites from the influences of upwelling CPDW. The 1998) may have reduced Ross Sea cyclonic activity and marine climate records from some coastal Antarctic sites, such as Law influence at Taylor Dome. This led to a large decrease in ac- Dome, may be similarly muted in character if they are not locumulation at Taylor Dome (Steig et al., 2000), coupled with cated close to areas of deep water upwelling. Thus, hemispheric a change in its spatial distribution (Morse et al., 1998), which differences in the amplitude of climate variability are not diagis in addition to the general, Antarctic-wide change related to nostic of the hemispheric origin of climate change. temperature. The differences between interstadial and stadial accumulation at Taylor Dome are thus amplified by the changINTERHEMISPHERIC CLIMATE SYNCHRONY ing importance of marine influence at this site. At Vostok, in Various recent publications have emphasized the differences contrast, regional marine influence is always so subdued that between Northern and Southern climate records to argue for smaller interstadials rarely stand out from climatic noise. The correspondence between the GISP2 and the Taylor Dome either a simple antiphase (“seesaw”) relationship between the δ 18 O record, which is better than between GISP2 and any other hemispheres, or a Southern Hemisphere lead for stadial–interstaAntarctic isotope record to date (Figs. 4 and 5), probably reflects dial climate changes (Crowley, 1992; Stocker, 1998; 1999; a variable degree of influence at Taylor Dome of heat transport Broecker, 1997). The most prominent support for this are the driven by North Atlantic deep water (NADW) formation. The results of Blunier et al. (1997, 1998) showing that the AntarcGreenland Summit records are closely linked to conditions in the tic cold reversal (ACR) at Byrd Station preceded the Younger North Atlantic, especially northward surface heat transport as- Dryas by about 1000 years. Similarly, warming for the intersociated with NADW formation (Rasmussen et al., 1996, 1999; stadials IS 8 and IS 12 was gradual in Antarctica and started Bond et al., 1997; Sachs and Lehman, 1999; van Kreveld et al., more than 1000 yr before the abrupt warming observed in the 2000 and references therein). The alternations between weak GRIP record (Blunier et al., 1998; Blunier and Brook, 2001). In
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FIG. 6. Surface currents of the global thermohaline circulation causing regional warming (solid) or cooling (dashed) under present-day conditions.
contrast, the results from Taylor Dome, obtained with the same techniques (Steig et al., 1998b) imply that stadial–interstadial climate changes extend synchronously, or nearly so, to both poles, as do climate changes on Milankovich time scales. These seemingly contradictory results indicate that there are differing regional expressions of changes in ocean heat transport. Strengthening of oceanic heat transport by the thermohaline circulation during rapid warming events recorded at GISP2 leads to regional warming in areas affected by currents such as the Brazil, Agulhas, and North Atlantic currents (Fig. 6), which transport heat away from the equator. Areas affected by surface currents, such as the Peru and Benguela currents, which draw cool water away from the poles, experience simultaneous cooling. Results from ice cores therefore support neither a simple in phase nor an antiphase relationship between the Northern and Southern Hemispheres. Our most complete picture of the regional heterogeneity that occurs during a major climate change is provided by data from the last glacial–interglacial transition. In Greenland ice cores, a large δ 18 O increase at 14,700 yr B.P., marking the end of cold, glacial isotope values, is followed by a return to glacial values during the Younger Dryas (Fig. 4). Terrestrial climate records from Western Europe show a similar strong warming (Mangerud et al., 1974). The 14,700-yr-B.P. warming comes 3000 to 5000 yr after clear signs of a gradual warming in the Antarctic Byrd Station and Vostok cores (Fig. 4) (Sowers and Bender, 1995) and the start of global sea-level rise (Fairbanks, 1989; Hanebuth et al., 2000). This time difference, at first glance, implies a cli-
matic lead of the Southern over the Northern Hemisphere during the last deglaciation. Warming in Greenland ice core records and other North Atlantic records began much earlier, however, around 21,000 yr B.P. when summer insolation at 60◦ N began to increase (Bard et al., 1997). This warming was simultaneous with warming in Antarctica (Grootes and Stuiver, 1997; McCabe and Clark, 1998; Stocker, 1999) dated at 21,000 yr B.P. by Sowers and Bender (1995). Various other records show that deglaciation in the Northern Hemisphere began well before the Bølling warming at 14,700 yr B.P. The glacial–geologic record of Andøya, an island off northern Norway, shows that glacial retreat began ∼19,000 yr B.P. and continued with small interruptions until the Holocene (Vorren et al., 1988), similar to temperature in Antarctica (Fig. 4). The same holds for the northern and eastern margins of the Fennoscandian ice sheet in Russia (Faustova, 1984). Atlantic sediment cores (Sarnthein et al., 1995) indicate a strong meltwater influence originating off northern Norway ∼18,000 yr B.P. This meltwater suppressed NADW formation and associated northward heat transport in the North Atlantic, which led to regional cooling that delayed the deglaciation in Greenland (Grootes and Stuiver, 1997; Alley and Clark, 1999 and references therein; Stocker, 1999) until 14,700 yr B.P. On our new time scale, Taylor Dome did not experience a major jump from glacial to interglacial conditions synchronously with Greenland, as suggested by Steig et al. (1998b) but warmed gradually like Byrd and Vostok. Yet, Taylor Dome δ 18 O does show rapid increases after 16,000 and 14,500 yr B.P., the latter roughly
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synchronous with the Bølling warming at the end of Heinrich event 1 (H1). Similar rapid increases also appear in the new West Antarctic Siple Dome δD record (J. W. C. White and E. J. Steig, unpublished data). Oceanographic conditions during H5 and H4 were similar to those during H1, with a greatly reduced or halted NADW formation (Rasmussen et al., 1996; Cortijo et al., 1997; Sachs and Lehman, 1999; van Kreveld et al., 2000 and references therein). The regional expressions of climate change were also similar. The transitions from H5, H4, and H1 to the major interstadials IS 12, IS 8, and IS 1/Bølling-Allerød, respectively, all show sharp increases in δ 18 O in the Greenland Summit area (Johnsen et al., 1992, Grootes and Stuiver, 1997). Importantly, the reported leads of Byrd Station, and of sediment cores RC11-83 (Charles et al., 1996) and GeoB 1711 (Vidal et al., 1999) at IS 12 and IS 8 are all based on comparisons with the North Atlantic in a cold “Heinrich” mode where northward heat transport is suppressed by a greatly reduced NADW formation. The duration of H5 and H4 (1070 yr and 1700 yr respectively; Meese et al., 1994) agrees with the observed ∼1500-yr leads. These three transitions were all followed, however, by gradual, but apparently synchronous, cooling for both Greenland and Byrd Station and a gradually decreasing intensity of the NADW IcelandScotland overflow (Rasmussen et al., 1996). Importantly, Byrd not only leads GISP2 in warming during H5, H4, and H1, but the same relationship—delayed rapid warming at GISP2 followed by cooling that is synchronous at both GISP2 and Byrd—also holds for IS 21, 20, 19, 17, and 14 and for the minor interstadials IS 11, 10, 7, 6, 5, although for these the delays are shorter (Blunier and Brook, 2001), making this a general feature. Figure 5 shows that the same is true for the comparison of Taylor Dome with GISP2. Minor ice rafted detritus (IRD) events preceding the minor interstadials (Bond and Lotti, 1995) indicate that the observations for H5, H4, and H1, regarding the interactions of ice, ocean circulation, and climate may have general validity for D/O-type climate changes (van Kreveld et al., 2000). Ocean–atmosphere general circulation models support a regional differentiation of global climate change as suggested by our simplified oceanic heat transport in Figure 6. In addition to the Atlantic north–south temperature seesaw, they also show antiphasing of temperature changes around Antarctica, such that parts of the Indian Ocean and the Pacific Ocean, especially the Ross Sea sector, are in phase with the North Atlantic (Schiller et al., 1997; Manabe and Stouffer, 2000). These model results agree with the spatial distribution of records having a “Northern” or a “Southern” climate response during the last deglaciation (Alley and Clark, 1999). The importance of the global thermohaline circulation (THC) heat transport for regional climate is also evident at the North Pacific ODP Site 883, where weakening of the THC leads to sea-surface temperature increases in antiphase with their North Atlantic counterparts (Kiefer et al., 2001). Latitudinal heat transport, mainly through THC, thus produces globally synchronous climate changes not only on the orbital time scales of the Milankovitch cycles, but also on the
millennial and, possibly, centennial time scales of stadials and interstadials. The regional character of oceanic heat transport, however, creates regional differences in climate response, including apparent antiphase behavior and climate leads and lags, the character of which is not global but depends on which specific records are being compared. CONCLUSIONS
The Taylor Dome ice core yields a detailed record of climate fluctuations, possibly reaching back as far as MIS 9, and links the Greenland and Antarctic records of past global climate. The direct marine influence at Taylor Dome during interglacial and warmer interstadial phases provides a record of the strength of the thermohaline circulation and thereby a correlation with Greenland. During MIS 5a-4 and MIS 3-2, a stronger circumpolar atmospheric circulation and expansion of the Ross Sea ice sheet separated Taylor Dome from marine influences and gave the record a more continental character, making the smaller interstadials difficult to distinguish. The differences between the Taylor Dome and the Vostok and Byrd Station records indicate regional Antarctic climate differences and suggest the need to reevaluate the significance of evidence for leads or lags of interhemispheric climate changes. The asynchrony of climate warming that is sometimes apparent between Byrd Station and GRIP and comparable differences between the timing of climate change in South and North Atlantic sediment cores are real, but they are related to the interruption of northward Atlantic heat transport during Heinrich and other meltwater events. The climatic expression of those interruptions is regional and should not be interpreted as a phase difference between the entire Southern and Northern Hemispheres. ACKNOWLEDGMENTS We thank the U.S. National Science Foundation, Office of Polar Programs, for financial support of the Taylor Dome ice core project (Grant nos. 89-15924, 93-16162, 94-21644), the PICO drillers (Polar Ice Coring Office, then University of Alaska, Fairbanks) for their dedication to core quality and reaching bedrock, the ASA (Antarctic Support Associates) personnel for camp support and logistics, Squadron VXE6 and the New York Air National Guard (TAG 109) for air transport, the many field party members for their able assistance, and T. Saling for the isotope measurements.
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The Taylor Dome Antarctic 18 O Record and Globally Synchronous Changes in Climate1 Pieter M. Grootes,2 Eric J. Steig, Minze Stuiver, Edwin D. Waddington, and David L. Morse3 Department of Earth and Space Sciences and Quaternary Research Center, University of Washington, Seattle, Washington 98195 E-mail: [email protected]
and Marie-Jos´ee Nadeau Leibniz-Labor, Christian-Albrechts-Universit¨at, 24118 Kiel, Germany Received August 31, 1999
The 18 O/16 O profile of a 554-m long ice core through Taylor Dome, Antarctica, shows the climate variability of the last glacial– interglacial cycle in detail and extends at least another full cycle. Taylor Dome shares the main features of the Vostok record, including the early climatic optimum with later cool phase of the last interglacial period in Antarctica. Taylor Dome δ 18 O fluctuations are more abrupt and larger than those at Vostok and Byrd Station, although still less pronounced than those of the Greenland GISP2 and GRIP records. The influence of the Atlantic thermohaline circulation on regional ocean heat transport explains the partly “North Atlantic” character of this Antarctic record. Under full glacial climate (marine isotope stage 4, late stage 3, and stage 2), this marine influence diminished and Taylor Dome became more like Vostok. Varying degrees of marine influence produce climate heterogeneity within Antarctica, which may account for conflicting evidence regarding the relative phasing of Northern and Southern Hemisphere climate change. ° 2001 University of Washington. Key Words: Antarctica; Taylor Dome; ice core; oxygen isotope; climate change; thermohaline circulation. C
INTRODUCTION
Evidence of large and rapid glacial climate fluctuations in two ice cores (GISP2 and GRIP) from the summit area of the Greenland ice sheet (Johnsen et al., 1992; Dansgaard et al., 1993; Grootes et al., 1993) spurred a search for rapid fluctuations elsewhere to evaluate their extent and significance. Such fluc1
Oxygen isotope data with depth and time scale are available at the stable isotope laboratory home page at the University of Washington (depts. washington.edu/isolab) and the Leibniz-Labor, Christian-Albrecht University, Kiel (www.uni-kiel.de/leibniz). These and additional Taylor Dome data sets are also available from the World Data Center for Paleoclimatology at the National Geophysical Data Center (www.ngdc.noaa.gov/paleo). 2 Now at Leibniz-Labor, Christian-Albrechts-Universit¨ at Kiel, Germany. 3 Now at Institute for Geophysics, University of Texas, Austin, TX 78795.
tuations were soon observed in ocean sediment cores from the North Atlantic (Rasmussen et al., 1996, 1999; Bond et al., 1997; Sachs and Lehman, 1999) and North Pacific (Behl and Kennett, 1996). In contrast, long ice core records from Antarctica suggest a more gradual and moderate climate variability in the Southern Hemisphere (Bender et al., 1994). Results from an ice core from Taylor Dome, however, indicate a strong link between rapid climate fluctuations in the Northern and Southern Hemispheres. In January 1994, a 554-m ice core and 6 cm of basal debris were obtained from Taylor Dome, a small East Antarctic ice dome at the head of Taylor Valley, Southern Victoria Land (Fig. 1). Glaciological, geophysical, and meteorological field studies (Morse, 1997) were used to select the drill site at 1.5 km (3 ice depths) south of the flow divide over a bedrock plateau at 77◦ 470 4700 S, 158◦ 430 2600 E, ice surface elevation 2365 m. The basal temperature is −26◦ C (Clow and Waddington, 1996), and the ice is likely to have undergone simple flow (Morse, 1997), so that the geochemical paleoclimate records throughout the core are well preserved. A multidisciplinary group carried out an integrated core-sampling program (Grootes et al., 1994). Results have been reported for major ion concentrations, cosmogenic isotopes, stable isotopes, and trace gases (Steig et al., 2000 and references therein). The oxygen isotope record of the ice was measured at the Quaternary Isotope Laboratory of the University of Washington, using an automated Micromass 903 mass spectrometer with CO2 equilibration system. The results are expressed as δ 18 O, the relative deviation of the isotopic abundance ratio 18 O/16 O from that of standard mean ocean water (V-SMOW), given in per mil (‰) and have an overall precision of 0.14‰. In general, the more negative the δ 18 O values, the colder the climate (e.g., Jouzel et al., 1997). The oxygen isotope profile (Fig. 2) was measured twice, using independent sample sets. One, with increments of 1.0 m for 0–340 m and 0.5 m for 340–554 m depth, was taken
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FIG. 1. The location of Taylor Dome, and its two uppermost 25-m surface elevation contours with drill site (77◦ 470 4700 S, 158◦ 430 2600 E, elevation 2365 m) and geophysical survey line, are shown in relation to the McMurdo Dry Valleys, the Ross Sea, and the Ross Ice Shelf. The positions of the cores discussed are shown in the inset.
from the outside of the core. The other was obtained as a split from the melted 20-cm-increment chemistry samples at the University of New Hampshire, with gaps for a few sections of poor core quality. Agreement between the series over the full length of the core shows that the details of the oxygen isotope record are reproducible. We compare the Taylor Dome oxygen isotope record with the isotope records of Vostok and Byrd Station, Antarctica, and GISP2, Greenland. We use IS 1-23 to denote the interstadial Dansgaard/Oeschger periods (Dansgaard et al., 1993) and marine isotope stage (MIS) 1-9 for the marine paleoclimate record (Shackleton and Opdyke, 1973). The features of the Taylor Dome δ 18 O record suggest that climate changes were essentially synchronous in both hemispheres. Changes in ocean circulation heat transport may, however, have temporarily masked the global signal in certain regions of the ocean and in adjacent land areas, including eastern to central North America (e.g. Reasoner and Jodry, 2000), Greenland, Europe, and also some parts of the Southern Hemisphere.
TIME SCALE
Steig et al. (1998b) developed a time scale for the Taylor Dome core by correlating its gas record (CH4 and δ 18 O of O2 ) with the GISP2 ice core. After correction for offsets between ice age and gas age in both cores, the Greenland ages, based on counted annual layers back to 50,000 yr B.P. (before A.D. 1950; Meese et al., 1994), were transferred to the Taylor Dome ice core. Steig et al. (2000) provided a Taylor Dome time scale (termed st9810) that extends the gas-correlation results to ∼160,000 yr B.P. There are, however, too few dated gas control points to explore the timing of rapid isotopic shifts during the last ice age. Our new time scale follows st9810 back to 14,580 yr B.P. For deeper ice, we assume that major climate changes occurred essentially simultaneously at Taylor Dome and Vostok and can be correlated at the midpoints of major isotope changes. Therefore, the ice in each resulting depth-age segment comes only from either a warm period with presumably higher accumulation or a cold period with lower accumulation (Steig et al., 2000). Thus,
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of the isotope records decreases beyond 200,000 yr B.P., where the time interval spanned by individual samples in Taylor Dome is too large for faithful comparison. The new time scale is used to compare the Taylor Dome isotope record for the last glacial–interglacial cycle with records from Vostok (Petit et al., 1999) and Byrd Station (Beer et al., 1992), Antarctica, and from GISP2, Greenland (Grootes et al., 1993; Grootes and Stuiver, 1997), each on its own time scale (Fig. 4). Although dating precision and accuracy are still inadequate to prove or disprove synchrony of events of duration less than 1000 yr prior to 15,000 yr B.P., our new time scale addresses the concerns of Mulvaney et al. (2000) while clarifying the real and important differences between Taylor Dome and other Antarctic ice core records. THE ISOTOPE RECORD
Terminations II and I
FIG. 2. Oxygen isotope profiles of the 554-m-long ice core through Taylor Dome, Antarctica. Sample resolution for the coarse series is 1 m down to 314 m and 0.5 m below this depth; for the detailed series, it is 0.2 m over the full length. The transition into glacial ice is at about 370 m depth.
our linearly interpolated depth–age relation should contain little error from changing accumulation rates associated with the correlated climate events. We tie the last glacial–interglacial δ 18 O cycle of Taylor Dome to the Vostok δD record on the time scale GT4 of Petit et al. (1999; Fig. 3a). The greatest difference with st9810 occurs between 25,000 and 14,580 yr B.P., where we correlate the Taylor Dome δ 18 O peak at 376.2 m, near 17,000 yr B.P. on the st9810 time scale, with the small δD peak at ∼ 24,000 yr B.P. at Vostok. Correlated calcium records provide a time scale for the δ 18 O rise that marks the end of the last glacial maximum (LGM). Following the argument of Mulvaney et al. (2000) that calcium (largely continental dust from long-range transport) “should show similar trends across the region,” we match the ∼ 50% dip in Taylor Dome calcium concentrations in the ice at 376.2 m (Mayewski et al., 1996; Steig et al., 2000) to its close counterpart at Dome C. We add three more tie points to the Dome C record that spread the 4.4-m section of very low accumulation rates between 24,000 and 14,580 yr B.P. at Taylor Dome to mimic the calcium pattern of Dome C (Fig. 3c). We tentatively extend the Vostok-correlation time scale beyond 300,000 yr B.P., MIS 9 (Fig. 3b), although the similarity
As shown in Figure 2, Termination II (the penultimate glacialto-interglacial transition) at Taylor Dome covers only 1 m of core, 23 m above bedrock. Because layer thinning from ice flow this close to bedrock is strong and because changes in accumulation rate and physical properties of the ice over the glacial– interglacial transition are uncertain, we cannot reliably estimate the time interval over which Termination II occurred. Termination I (the last glacial-to-interglacial transition) at Taylor Dome covers the depth interval 375–354 m. The initial δ 18 O increase looks particularly sharp, from a glacial −44.0‰ at 375 m depth to a Holocene −36.7‰ over just 4.4 m. This apparent sharpness may reflect extremely low accumulation rates during the LGM, when wind scouring may have been common (cf. Fisher et al., 1983). Indeed, the correlation of Taylor Dome with Dome C calcium implies that this δ 18 O increase is at first gradual and similar to that of other Antarctic cores. Near 16,000 yr B.P., however, even on our new time scale matched to Dome C Ca, the δ 18 O increase becomes steeper and may represent a real warming event (Fig. 3c). The first strong δ 18 O maximum (−36.7‰) is followed by 3‰ cooling (Figs. 2 and 3a). Seven methane tie points from 377.5 to 356.2 m provide clearly defined gas ages from 14,900 to 10,900 yr B.P. on the GISP2 layer-counted time scale (Steig et al., 1998b, 2000; Brook et al., 2000). These tie points show that the 3‰ cooling, although superficially similar to the Antarctic cold reversal (ACR) (Jouzel et al., 1995; Blunier et al., 1997, 1998), occurs later than the ACR at Byrd Station and, instead, is in phase with the Greenland cooling from the Allerød into the Younger Dryas (Steig et al., 1998b). This part of the time scale is not affected by the forced match between Dome C and Taylor Dome calcium records. Indeed, the appearance of this feature at Taylor Dome is corroborated by recent results from Dome C, which also show cooling synchronous with Greenland after ∼14,000 yr B.P. (Jouzel et al., in press). The steep increase in δ 18 O near 16,000 yr B.P., where Vostok δD and Dome C show only a small shoulder, may indicate a shift to more marine influences at Taylor Dome as discussed below.
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FIG. 3. Comparison of the Taylor Dome and Vostok isotope records on the Vostok GT4 time scale of Petit et al. (1999). (a) The last glacial–interglacial cycle. Arrows indicate where rapid changes of Taylor Dome δ 18 O (thin line) were correlated with Vostok and Dome C (Fig. 3c). (b) The full Taylor Dome δ 18 O record with a tentative Vostok correlation extending beyond 300 kyr B.P. (c) The Ca correlation of Taylor Dome (thin line) with Dome C (thick line) for the period 14,580 to 24,000 yr B.P.
Marine Isotope Stage 5e The last interglacial period (MIS 5e) is marked by heavy δ 18 O values between 526 and 531 m depth. The Taylor Dome δ 18 O record has an initial peak and a later shoulder that is similar to
the Vostok δD record (Petit et al., 1999; Fig. 3a). This similarity validates the fine structure of the isotope signal at Vostok. The peak-shoulder structure of an otherwise quiet MIS 5e, generally equated with the last interglacial period labeled “Eemian” in the Netherlands, thus appears real. The ∼ 2‰ (16‰) drop from
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peak to shoulder δ 18 O (δD) values probably corresponds to the intra-Eemian cooling event observed at Ocean Drilling Project (ODP) Site 658 off West Africa (Maslin et al., 1998) and the middle-Eemian cooling in the Norwegian Sea (Cortijo et al., 1994) and Europe (Litt et al., 1996; Cheddadi et al., 1998). This Eemian pattern is repeated in the Holocene: about 6000 yr B.P. δ 18 O at Taylor Dome fell rapidly by about 1.5‰ (Steig et al., 1998a; Steig, 1999). Aside from the peak-to-shoulder step, δ 18 O values for the last interglacial are fairly constant. The 0.2-m sample resolution at Taylor Dome, representing less than 1000 yr per sample in this interval, is sufficient to detect GRIP-like Eemian δ 18 O oscillations (Dansgaard et al., 1993), if they were in fact climate related. Their absence at Taylor Dome and Vostok adds Antarctic support to evidence from the Atlantic Ocean (e.g., McManus et al., 1994; Rasmussen et al., 1999) and the European continent (Litt et al., 1996; Cheddadi et al., 1998), for a stable Eemian climate. Marine Isotope Stages 5d to 4 The early-glacial period (MIS 5d to 4) of the Taylor Dome δ 18 O record validates the Vostok record of early-glacial Antarctic climate fluctuations by showing, prior to 80,000 yr B.P. (Fig. 4), oscillations similar in magnitude and fine structure to those at Vostok. In contrast, the oldest part of the Byrd Station record, before ∼ 85,000 yr B.P., shows no resemblance to Taylor Dome or Vostok, probably because of flow disturbance of this part of the Byrd core. The interval 80,000–60,000 yr B.P. (MIS 5a and 4) is similar in all three Antarctic isotope records and the GISP2 record. All show relatively long warm periods (IS 21, 20, 19 lasting 10400, 3000, and 1900 yr respectively on the GISP2 scale) broken by brief (900- and 1200-yr) and successively colder intervals. The similarities imply a global extent for early-glacial climate changes with a strong ∼ 20,000-yr precessional component (MIS 5c,a) and for the oscillating transition to MIS 4. Marine Isotope Stages 3 and 2 For the full-glacial period (MIS 3 and 2), 60,000 to 15,000 yr B.P., the Taylor Dome record shows that full-glacial climate variability in Antarctica was not limited to the major interstadials (equivalent with Greenland IS 17/16, 14, 12, and 8) but also included the minor interstadials documented in Greenland and North Atlantic records (Figs. 3 and 4). Taylor Dome δ 18 O fluctuations, like those of GISP2, are more abrupt and larger than those at Vostok and Byrd Station, even though the sampling resolution at Taylor Dome is lower. Smoothing of the δ 18 O signal during firnification at Vostok cannot explain this difference. The characteristic pattern of a major interstadial followed by three weaker, short ones found in the Greenland ice cores (IS 12-9 and IS 8-5; Johnsen et al., 1992; Dansgaard et al., 1993; Grootes et al., 1993; Stuiver and Grootes, 2000) and identified by Bond et al. (1993) in North Atlantic sediments, can also be seen at Taylor Dome, though for IS 12-9 the core is of poor quality. Minor interstadials are also reported by Bender et al. (1999)
FIG. 4. Comparison of ice core isotope records of climate shown on their respective time scales: Taylor Dome as discussed in text, Vostok GT4 of Petit et al. (1999), Byrd Station after Beer et al. (1992), and GISP2 after Meese et al. (1994) and Bender et al. (1994). Ice core isotope stages are labeled IS 1-21 after Dansgaard et al. (1993) and marine isotope stages MIS 1-6 after Shackleton and Opdyke (1973).
for Vostok and by Blunier and Brook (2001) for Byrd, but their expression in these cores is less clear than at Taylor Dome. Shading in Figure 5 highlights the δ 18 O pattern of IS 8 to 5 at Taylor Dome and GISP2 on their respective time scales. A relative shift of 3200 ± 100 yr—within the uncertainty of the time scales at this point in the records—brings for IS 8 to 6 a close match between the δ 18 O maxima of TD and the sudden increases at GISP2, similar to the CH4 -based match between Byrd and Greenland of Blunier and Brook (2001). HEMISPHERIC AND INTERHEMISPHERIC CLIMATE COMPARISON
Under present interglacial conditions, Taylor Dome receives much of its precipitation from cyclonic systems from the nearby Ross Sea (Morse, 1997; Morse et al., 1998) and is thus sensitive to local oceanic conditions near Antarctica. In contrast, the high East Antarctic plateau (Fig. 1) favors outflowing katabatic winds and a strong zonal circulation that together impede poleward lower tropospheric transport. These conditions effectively
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NADW formation during stadials and vigorous NADW flow and heat transport during interstadials (Rasmussen et al., 1996, 1999) led to large and rapid changes in isotope values in Greenland. The effects of switching NADW formation on and off will be felt almost instantaneously along the full thermohaline circulation. Because NADW is warmer than Circumpolar deep water (CPDW), with which it mixes in the South, regional warming and sea ice melt may occur in those coastal areas where CPDW upwelling is significant when NADW formation is switched on (Steig et al., 1998b). Greenland is thus linked to Antarctica, but more so to near-coastal Taylor Dome than to continental Vostok. Regional oceanic influence may also account for differences among the isotope minima and maxima in Figure 4. The minima of GISP2, Vostok, and Taylor Dome show a fairly uniform decrease over MIS 5b-4 and MIS 3-2. However, interstadial– maximum δ 18 O values at GISP2 stay close to −37‰ for all but IS 2, while Vostok maxima gradually decline. At Taylor Dome, the early interstadials IS 17, 16, 14, and 12 during MIS 3 stay fairly warm, as in GISP2, but the maxima of IS 20, 19, and 11-2 decrease and join the trend of the continental Vostok record. Evidently, during stadials, when thermohaline heat transport was greatly reduced, both hemispheres experienced deFIG. 5. Taylor Dome and GISP2 20-cm δ 18 O values for IS 8–5 on their clining minima, modulated by longer term Milankovich-driven respective time scales. Shading highlights idealized isotope profile for these cooling. Vigorous NADW formation and heat transport during Dansgaard/Oeschger cycles. Dashed lines indicate correlation between δ 18 O the warmer parts of the glacial, however, kept both GISP2 inmaxima at Taylor Dome and jumps at GISP2 as suggested by Byrd Station and terstadials and Taylor Dome interstadials relatively warm. Thus Greenland methane data (Blunier and Brook, 2001). the “North Atlantic” character of the Taylor Dome core for the Younger Dryas (Steig et al., 1998b) also appears to have preisolate Vostok from nearby oceanic influences (Koster et al., vailed for much of the last glacial period. The muted character 1992). At present, Taylor Dome is outside the central East Antarc- of the climate variability in most Antarctic ice cores, espetic zone of impeded tropospheric transport (Raper et al., 1984; cially Vostok, in comparison with Greenland ice cores thereMosley-Thompson et al., 1990). However, expansion of circum- fore reflects Greenland’s sensitivity to changes in North Atlantic polar circulation (Mayewski et al., 1996) and advance of the oceanic heat transport in contrast with the isolation of central Ross Sea ice sheet after MIS 5a and during MIS 3 (Morse et al., Antarctic sites from the influences of upwelling CPDW. The 1998) may have reduced Ross Sea cyclonic activity and marine climate records from some coastal Antarctic sites, such as Law influence at Taylor Dome. This led to a large decrease in ac- Dome, may be similarly muted in character if they are not locumulation at Taylor Dome (Steig et al., 2000), coupled with cated close to areas of deep water upwelling. Thus, hemispheric a change in its spatial distribution (Morse et al., 1998), which differences in the amplitude of climate variability are not diagis in addition to the general, Antarctic-wide change related to nostic of the hemispheric origin of climate change. temperature. The differences between interstadial and stadial accumulation at Taylor Dome are thus amplified by the changINTERHEMISPHERIC CLIMATE SYNCHRONY ing importance of marine influence at this site. At Vostok, in Various recent publications have emphasized the differences contrast, regional marine influence is always so subdued that between Northern and Southern climate records to argue for smaller interstadials rarely stand out from climatic noise. The correspondence between the GISP2 and the Taylor Dome either a simple antiphase (“seesaw”) relationship between the δ 18 O record, which is better than between GISP2 and any other hemispheres, or a Southern Hemisphere lead for stadial–interstaAntarctic isotope record to date (Figs. 4 and 5), probably reflects dial climate changes (Crowley, 1992; Stocker, 1998; 1999; a variable degree of influence at Taylor Dome of heat transport Broecker, 1997). The most prominent support for this are the driven by North Atlantic deep water (NADW) formation. The results of Blunier et al. (1997, 1998) showing that the AntarcGreenland Summit records are closely linked to conditions in the tic cold reversal (ACR) at Byrd Station preceded the Younger North Atlantic, especially northward surface heat transport as- Dryas by about 1000 years. Similarly, warming for the intersociated with NADW formation (Rasmussen et al., 1996, 1999; stadials IS 8 and IS 12 was gradual in Antarctica and started Bond et al., 1997; Sachs and Lehman, 1999; van Kreveld et al., more than 1000 yr before the abrupt warming observed in the 2000 and references therein). The alternations between weak GRIP record (Blunier et al., 1998; Blunier and Brook, 2001). In
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FIG. 6. Surface currents of the global thermohaline circulation causing regional warming (solid) or cooling (dashed) under present-day conditions.
contrast, the results from Taylor Dome, obtained with the same techniques (Steig et al., 1998b) imply that stadial–interstadial climate changes extend synchronously, or nearly so, to both poles, as do climate changes on Milankovich time scales. These seemingly contradictory results indicate that there are differing regional expressions of changes in ocean heat transport. Strengthening of oceanic heat transport by the thermohaline circulation during rapid warming events recorded at GISP2 leads to regional warming in areas affected by currents such as the Brazil, Agulhas, and North Atlantic currents (Fig. 6), which transport heat away from the equator. Areas affected by surface currents, such as the Peru and Benguela currents, which draw cool water away from the poles, experience simultaneous cooling. Results from ice cores therefore support neither a simple in phase nor an antiphase relationship between the Northern and Southern Hemispheres. Our most complete picture of the regional heterogeneity that occurs during a major climate change is provided by data from the last glacial–interglacial transition. In Greenland ice cores, a large δ 18 O increase at 14,700 yr B.P., marking the end of cold, glacial isotope values, is followed by a return to glacial values during the Younger Dryas (Fig. 4). Terrestrial climate records from Western Europe show a similar strong warming (Mangerud et al., 1974). The 14,700-yr-B.P. warming comes 3000 to 5000 yr after clear signs of a gradual warming in the Antarctic Byrd Station and Vostok cores (Fig. 4) (Sowers and Bender, 1995) and the start of global sea-level rise (Fairbanks, 1989; Hanebuth et al., 2000). This time difference, at first glance, implies a cli-
matic lead of the Southern over the Northern Hemisphere during the last deglaciation. Warming in Greenland ice core records and other North Atlantic records began much earlier, however, around 21,000 yr B.P. when summer insolation at 60◦ N began to increase (Bard et al., 1997). This warming was simultaneous with warming in Antarctica (Grootes and Stuiver, 1997; McCabe and Clark, 1998; Stocker, 1999) dated at 21,000 yr B.P. by Sowers and Bender (1995). Various other records show that deglaciation in the Northern Hemisphere began well before the Bølling warming at 14,700 yr B.P. The glacial–geologic record of Andøya, an island off northern Norway, shows that glacial retreat began ∼19,000 yr B.P. and continued with small interruptions until the Holocene (Vorren et al., 1988), similar to temperature in Antarctica (Fig. 4). The same holds for the northern and eastern margins of the Fennoscandian ice sheet in Russia (Faustova, 1984). Atlantic sediment cores (Sarnthein et al., 1995) indicate a strong meltwater influence originating off northern Norway ∼18,000 yr B.P. This meltwater suppressed NADW formation and associated northward heat transport in the North Atlantic, which led to regional cooling that delayed the deglaciation in Greenland (Grootes and Stuiver, 1997; Alley and Clark, 1999 and references therein; Stocker, 1999) until 14,700 yr B.P. On our new time scale, Taylor Dome did not experience a major jump from glacial to interglacial conditions synchronously with Greenland, as suggested by Steig et al. (1998b) but warmed gradually like Byrd and Vostok. Yet, Taylor Dome δ 18 O does show rapid increases after 16,000 and 14,500 yr B.P., the latter roughly
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synchronous with the Bølling warming at the end of Heinrich event 1 (H1). Similar rapid increases also appear in the new West Antarctic Siple Dome δD record (J. W. C. White and E. J. Steig, unpublished data). Oceanographic conditions during H5 and H4 were similar to those during H1, with a greatly reduced or halted NADW formation (Rasmussen et al., 1996; Cortijo et al., 1997; Sachs and Lehman, 1999; van Kreveld et al., 2000 and references therein). The regional expressions of climate change were also similar. The transitions from H5, H4, and H1 to the major interstadials IS 12, IS 8, and IS 1/Bølling-Allerød, respectively, all show sharp increases in δ 18 O in the Greenland Summit area (Johnsen et al., 1992, Grootes and Stuiver, 1997). Importantly, the reported leads of Byrd Station, and of sediment cores RC11-83 (Charles et al., 1996) and GeoB 1711 (Vidal et al., 1999) at IS 12 and IS 8 are all based on comparisons with the North Atlantic in a cold “Heinrich” mode where northward heat transport is suppressed by a greatly reduced NADW formation. The duration of H5 and H4 (1070 yr and 1700 yr respectively; Meese et al., 1994) agrees with the observed ∼1500-yr leads. These three transitions were all followed, however, by gradual, but apparently synchronous, cooling for both Greenland and Byrd Station and a gradually decreasing intensity of the NADW IcelandScotland overflow (Rasmussen et al., 1996). Importantly, Byrd not only leads GISP2 in warming during H5, H4, and H1, but the same relationship—delayed rapid warming at GISP2 followed by cooling that is synchronous at both GISP2 and Byrd—also holds for IS 21, 20, 19, 17, and 14 and for the minor interstadials IS 11, 10, 7, 6, 5, although for these the delays are shorter (Blunier and Brook, 2001), making this a general feature. Figure 5 shows that the same is true for the comparison of Taylor Dome with GISP2. Minor ice rafted detritus (IRD) events preceding the minor interstadials (Bond and Lotti, 1995) indicate that the observations for H5, H4, and H1, regarding the interactions of ice, ocean circulation, and climate may have general validity for D/O-type climate changes (van Kreveld et al., 2000). Ocean–atmosphere general circulation models support a regional differentiation of global climate change as suggested by our simplified oceanic heat transport in Figure 6. In addition to the Atlantic north–south temperature seesaw, they also show antiphasing of temperature changes around Antarctica, such that parts of the Indian Ocean and the Pacific Ocean, especially the Ross Sea sector, are in phase with the North Atlantic (Schiller et al., 1997; Manabe and Stouffer, 2000). These model results agree with the spatial distribution of records having a “Northern” or a “Southern” climate response during the last deglaciation (Alley and Clark, 1999). The importance of the global thermohaline circulation (THC) heat transport for regional climate is also evident at the North Pacific ODP Site 883, where weakening of the THC leads to sea-surface temperature increases in antiphase with their North Atlantic counterparts (Kiefer et al., 2001). Latitudinal heat transport, mainly through THC, thus produces globally synchronous climate changes not only on the orbital time scales of the Milankovitch cycles, but also on the
millennial and, possibly, centennial time scales of stadials and interstadials. The regional character of oceanic heat transport, however, creates regional differences in climate response, including apparent antiphase behavior and climate leads and lags, the character of which is not global but depends on which specific records are being compared. CONCLUSIONS
The Taylor Dome ice core yields a detailed record of climate fluctuations, possibly reaching back as far as MIS 9, and links the Greenland and Antarctic records of past global climate. The direct marine influence at Taylor Dome during interglacial and warmer interstadial phases provides a record of the strength of the thermohaline circulation and thereby a correlation with Greenland. During MIS 5a-4 and MIS 3-2, a stronger circumpolar atmospheric circulation and expansion of the Ross Sea ice sheet separated Taylor Dome from marine influences and gave the record a more continental character, making the smaller interstadials difficult to distinguish. The differences between the Taylor Dome and the Vostok and Byrd Station records indicate regional Antarctic climate differences and suggest the need to reevaluate the significance of evidence for leads or lags of interhemispheric climate changes. The asynchrony of climate warming that is sometimes apparent between Byrd Station and GRIP and comparable differences between the timing of climate change in South and North Atlantic sediment cores are real, but they are related to the interruption of northward Atlantic heat transport during Heinrich and other meltwater events. The climatic expression of those interruptions is regional and should not be interpreted as a phase difference between the entire Southern and Northern Hemispheres. ACKNOWLEDGMENTS We thank the U.S. National Science Foundation, Office of Polar Programs, for financial support of the Taylor Dome ice core project (Grant nos. 89-15924, 93-16162, 94-21644), the PICO drillers (Polar Ice Coring Office, then University of Alaska, Fairbanks) for their dedication to core quality and reaching bedrock, the ASA (Antarctic Support Associates) personnel for camp support and logistics, Squadron VXE6 and the New York Air National Guard (TAG 109) for air transport, the many field party members for their able assistance, and T. Saling for the isotope measurements.
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