Antarctic Science page 1 of 13 (2008) & Antarctic Science Ltd 2008 DOI: 10.1017/S0954102008000941

1 Review 56 2 57 3 Recent advances in understanding Antarctic climate evolution 58 4 MARTIN J. SIEGERT1*, PETER BARRETT2, ROBERT DECONTO3, ROBERT DUNBAR4,COLMO´ COFAIGH5, 59 5 SANDRA PASSCHIER6 and TIM NAISH2,7 60 6 61 1School of GeoSciences, University of Edinburgh, Grant Institute, Edinburgh EH9 3JW, UK 7 62 2Antarctic Research Centre, Victoria University of Wellington, PO Box 600, Wellington, New Zealand 8 3Department of Geosciences, 611 North Pleasant Street, 233 Morrill Science Center, University of Massachusetts, Amherst, 63 9 MA 01003-9297, USA 64 10 4Department of Geological and Environmental Sciences, 325 Braun Hall (bldg. 320), Stanford University, Stanford, CA 94305-2115, USA 65 5 11 Department of Geography, Durham University, Science Site, South Road, Durham DH1 3LE, UK 66 6 12 Department of Earth and Environmental Studies, Mallory Hall 252, Montclair State University, Montclair, NJ 07043, USA 67 7 13 Institute of Geological and Nuclear Sciences, PO Box 30368, Lower Hutt, New Zealand 68 *[email protected] 14 69 15 70 16 Abstract: Geological evidence shows that the ice sheet and climate in Antarctica has changed considerably 71 17 since the onset of glaciation around 34 million years ago. By analysing this evidence, important information 72 18 concerning processes responsible for ice sheet growth and decay can be determined, which is vital for 73 19 appreciating future changes in Antarctica. Geological records are diverse and their analyses require a variety 74 20 of techniques. They are, however, essential for the establishment of hypotheses regarding past Antarctic 75 21 changes. Numerical models of ice and climate are useful for testing such hypotheses, and in recent years 76 22 there have been several advances in our knowledge relating to ice sheet history gained from these tests. This 77 23 paper documents five case studies, employing a full range of techniques, to exemplify recent insights into 78 24 Antarctic climate evolution from modelling ice sheet inception in the earliest Oligocene to quantifying 79 25 Neogene ice sheet fluctuations and process-led investigations of recent (last glacial) changes. 80 26 81 27 Received 15 January 2007, accepted 29 August 2007 82 28 83 Key words: Cenozoic, environment, glacial history, ice sheet 29 84 30 85 31 86 Introduction 32 volume is a major driver of Earth’s climate and sea level, 87 33 Large ice sheets have existed on Antarctica since at least much effort has been expended in deriving numerical models 88 34 earliest Oligocene times (Wise et al. 1991, Barrett 1996), of its behaviour. Some of these models have been 89 35 approximately 34 million years ago. Since then it has successfully evaluated against modern conditions (Le Broqc 90 36 fluctuated considerably and has been one of the major 2007). Employing numerical models to evaluate past ice sheet 91 37 driving forces for changes in global sea level and climate. behaviour, using the record of changes in climate (inferred 92 38 The size and timing of these fluctuations has been the from ice cores, sedimentary facies, and seismic data), 93 39 subject of considerable debate. Knowing how large ice palaeoceanographic conditions (inferred from palaeoecology 94 40 masses and associated sea ice respond to external forcing is and climate proxies in ocean sediments) and palaeogeography 95 41 of vital importance, because ice volume variations change 1) (as recorded in landscape evolution), provides a powerful 96 42 global sea level on a scale of tens of metres or more, and 2) means by which quantitative process-led assessments of the 97 43 the capacity of ice sheets and sea ice as major heat sinks, cryosphere’s involvement in a variety of climate change 98 44 insulators and reflectors. It is thus important to assess the episodes can be established. Such assessment is critical to 99 45 stability of the cryosphere under a warming climate and predicting how the Antarctic Ice Sheet will respond to, and 100 46 higher atmospheric CO2 levels (IPCC 2001) when ice force, future environmental changes. 101 47 volumes may reduce, particularly as ice core records have Recognizing the importance of understanding past 102 48 yielded evidence of a strong correlation between CO2 in the changes in Antarctica to comprehending future changes, 103 49 atmosphere and palaeotemperatures (EPICA 2004). This the Scientific Committee on Antarctic Research has developed 104 50 concern is justified when CO2 levels are compared with a programme, entitled ACE (Antarctic Climate evolution), 105 51 temperature changes in the more distant past (Crowley & aimed at facilitating research in the broad area of Antarctic 106 52 Kim 1995, Pagani et al. 2005). For example, IPCC (2001) glacial and climate history. In this review we assess five 107 53 estimated global mean temperatures in 2300, from ‘best case’ areas of activity in which the ACE programme has been 108 54 projections of atmospheric CO2, that have not occurred on focused. These five examples are not meant to be an 109 55 Earth for over 50 Ma. Since variation in Antarctic ice exhaustive account of research undertaken on Antarctic 110

1 2 MARTIN J. SIEGERT et al.

111 166 112 167 113 168 114 169 115 170 116 171 117 172 118 173 119 174 120 175 121 176 122 177 123 178 124 179 125 180 126 181 127 182 128 183 129 184 130 185 131 186 132 187 133 188 134 189 135 Fig. 1. Ice volume (left) and corresponding ice sheet geometries (right) simulated by a coupled GCM ice sheet model in response to a slow 190 136 191 decline in atmospheric CO2 and idealized orbital cyclicity across the Eocene–Oligocene boundary. The sudden, two-step jump in ice volume 137 (left panel) corresponds to the Oi-1 event. The left panel shows simulated ice volume, extrapolated to an equivalent change in sea level and 192 138 the mean isotopic composition of the ocean (top). Arbitrary model years (left axis) and corresponding, prescribed atmospheric CO2 (right 193 139 axis) are also labelled. Atmospheric CO2 is shown as the multiplicative of pre-industrial (280 ppmv) levels. Ice sheet geometries (right 194 140 panels) show ice sheet thickness in metres. Black arrows correlate simulated ice volumes with the geometric evolution of the ice sheet 195 141 through the Oi-1 event (modified from DeConto & Pollard 2003b). 196 142 197 143 climate evolution, but they do provide a means of gauging (Tasmanian Passage) widened and deepened during the late 198 144 the variety of activities needed to gain a fuller appreciation Palaeogene and early Neogene (Lawver & Gahagan 1998), 199 145 of Antarctic glacial history. The review begins with a strengthening of the Antarctic Circumpolar Current and 200 146 discussion concerning the onset of glaciation in Antarctica Polar Frontal Zone were thought to have cooled the Southern 201 147 in the earliest Oligocene, with subsequent assessment of Ocean by limiting the advection of warm subtropical surface 202 148 ice sheet fluctuations during the Neogene. The review waters into high latitudes (Kennett 1977). A number of 203 149 continues with an account of ice sheet changes following ocean modelling studies have indeed shown that the opening 204 150 the last glacial maximum, and ends with an example for of both the Drake and Tasmanian gateways reduces poleward 205 151 how geological evidence can be used to characterise heat convergence in the Southern Ocean, cooling sea surface 206 152 modern ice sheet processes. temperatures by several degrees (Mikolajewicz et al. 1993, 207 153 Nong et al. 2000, Toggweiler & Bjornsson 2000). 208 154 Whereas the opening of the Tasmanian gateway broadly 209 Connection of CO2 and ice sheet inception at the 155 coincides with the earliest Oligocene glaciation event 210 Eocene–Oligocene boundary 156 (Oi-1) (Stickley et al. 2004), the tectonic history of the 211 157 Whereas the onset of major, continental-scale glaciation in Scotia Sea remains equivocal. Estimates for the opening of 212 158 the earliest Oligocene (Oi-1 event) has long been attributed Drake Passage range between 40 and 20 Ma (Barker & 213 159 to the opening of Southern Ocean gateways (Kennett & Burrell 1977, Livermore et al. 2004, Scher & Martin 214 160 Shackleton 1976, Kennett 1977, Exon et al. 2002), recent 2006), blurring the direct ‘cause and effect’ relationship 215 161 numerical modelling studies suggest declining atmospheric between the gateways and glaciation. Furthermore, recent 216 162 CO2 was the most important factor in Antarctic cooling atmosphere-ocean modelling (Huber et al. 2004) has shown 217 163 and glaciation. that the Tasmanian Gateway probably had a minimal effect on 218 164 As the passages between South America and the Antarctic oceanic heat convergence and sea surface temperatures around 219 165 Peninsula (Drake Passage), and Australia and East Antarctica the continent, because the warm East Australia Current does 220 ANTARCTIC CLIMATE EVOLUTION 3

221 not travel any further south if the gateway is open or closed. The geothermal heat flux (Pollard et al. 2005), Antarctic vegetation 276 222 gateway’s effect on East Antarctic climate and snowfall was also dynamics (Thorn & DeConto 2006), and Southern Ocean sea 277 223 showntobeminimal,pointingtosomeotherforcing(perhaps ice (DeConto et al. 2007) in the Eocene–Oligocene climatic 278 224 decreasing atmospheric CO2 concentrations) as the primary transition. 279 225 cause of Antarctic cooling and glaciation. The results of these studies can be summarized as follows. 280 226 One aspect of recent modelling has focused on the The timing of glaciation on East Antarctica was shown to be 281 227 development of coupled climate-ice sheet models capable sensitive to orbital forcing, mountain uplift, and continental 282 228 of running long (.106 yr), time-continuous simulations of vegetation, but only within a very narrow range of 283 229 specific climate events and transitions (DeConto & Pollard atmospheric CO2 concentrations around 2.8 times pre- 284 230 2003a). Simulations spanning the Eocene-Oligocene boundary industrial level - close to the model’s glaciation threshold. 285 231 while accounting for decreasing CO2 concentrations and Once the glaciation threshold is approached, astronomical 286 232 orbital variability (DeConto & Pollard 2003b, Pollard & forcing can trigger sudden glaciation through non-linear 287 233 DeConto 2005), have led to the conclusions that 1) height/mass-balance and albedo feedbacks that result in 288 234 tectonically-forced changes in ocean circulation and heat the growth of a continental-scale ice sheet within 100 kyr 289 235 transport have only a small effect on temperature and glacial (Fig. 1). The timing of glaciation appears to be insensitive 290 236 mass balance in the Antarctic interior and (2) Southern Ocean to both expanding concentrations of seasonal sea ice and 291 237 gateways could only have triggered glaciation if the climate changes in geothermal heat flux under the continent. 292 238 system was already near a threshold. Considering the However, a doubling of the background geothermal heat 293 -2 239 sensitivity of polar climate to the range of CO2 concentrations flux (from 40 to 80 mW m ) does have a significant effect 294 240 likely to have existed over the Palaeogene–Neogene (Pagani on the area under the ice sheet at the pressure meltpoint 295 241 et al. 2005), CO2 probably played a fundamental role in (where liquid water is present), which may have had some 296 242 controlling Antarctica’s climatic and glacial sensitivity to a influence on the distribution and development of subglacial 297 243 wide range of forcings. This conclusion is supported by a lakes (Siegert & Dowdeswell 1996, Siegert et al. 2005). 298 244 number of numerical modelling studies exploring the role of Whereas these modelling studies have certainly improved 299 245 orbital variability (DeConto & Pollard 2003b), mountain uplift our understanding of the importance of atmospheric CO2 300 246 in the continental interior (DeConto & Pollard 2003a), concentrations relative to other Cenozoic forcing factors, 301 247 302 248 303 249 304 250 305 251 306 252 307 253 308 254 309 255 310 256 311 257 312 258 313 259 314 260 315 261 316 262 317 263 318 264 319 265 320 266 321 267 322 268 323 269 324 270 325 271 326 272 Fig. 2. Revised age model for Late Oligocene glacimarine cycles 9, 10 and 11 from CRP2/2A showing correlations to the high-resolution 327 273 ODP Site 1090 d18O record. 40Ar/39Ar ages on tephra allows cycles 11 and 10 to be correlated with individual Milankovitch-scale, 328 274 glacial-interglacial cycles within polarity chrons C7n2n and C7n1n, respectively. Cycle 9 is correlated with C6n3n. The Mi-1 glaciation and 329 275 the Oligocene-Miocene boundary correspond to the 3Ma-duration unconformity at the top of Cycle 9. 330 4 MARTIN J. SIEGERT et al.

331 several important model-data inconsistencies remain glacial evolution of the high southern latitudes will be the 386 332 unresolved. For example, long, time-continuous GCM-ice focus of future modelling and model-data comparisons. 387 333 sheet simulations of an increasing CO2 (warming) scenario, 388 334 show strong hysteresis once a continental ice sheet has 389 Orbital control on East Antarctic ice sheet dynamics 335 formed, in which the ice surface elevation reaches a 390 across the Oligocene–Miocene boundary 336 maximum where ice accumulation rates are low and 391 337 subsequently lowers under increasing surface accumulation, Orbital control of Northern Hemisphere ice sheet volume in 392 338 which leads to ice build-up (Pollard & DeConto 2005). In the Quaternary ice ages has been well established for a 393 339 these simulations, orbital forcing alone is not sufficient to quarter of a century (e.g. Mix & Ruddiman 1984, 394 340 produce the range of Palaeogene–Neogene ice sheet Shackleton et al. 1984, Ruddiman et al. 1989, Maslin et al. 395 341 variability (50–120% of modern Antarctic ice volumes) 1999). Now, through drilling off the Antarctic margin at 396 342 inferred from marine oxygen isotope records and sequence Cape Roberts it has been shown for the Antarctic Ice Sheet 397 343 stratigraphic reconstructions of eustasy (Zachos et al. 1992, for the period from 33 to 17 million years ago. Around 398 344 Pekar & DeConto 2006, Pekar et al. 2006). This points to 1500 m of strata in this age range were cored and 55 399 345 the importance of additional feedbacks (possibly related to sedimentary cycles identified, ranging from a few metres to 400 346 the marine carbon cycle, atmospheric CO2 or even non- over 60 m in thickness (Naish et al. 2001a). The cyclic 401 347 linear internal ice sheet processes) in controlling Cenozoic variation in lithology records both advance and retreat of 402 348 ice sheet variability. the ice margin and the fall and rise of sea level. Two latest 403 349 Several recent isotopic analyses of deep sea cores imply Oligocene cycles preserve volcanic ash layers whose ages 404 350 ice volumes during the peak Oligocene and Miocene link them with particular Milankovitch cycles around 24.0 405 351 glacial intervals that are too big to be accommodated by and 24.2 million years ago in the deep sea isotope record 406 352 East Antarctica alone (Coxall et al. 2005, Holbourn et al. (Zachos et al. 1997, Naish et al. 2001b) (see Fig. 2). While 407 353 2005, Lear et al. 2004). Furthermore new isotopic analyses calibration of the ice volume component of deep sea 408 354 of deep sea sediments of Eocene age are now being taken isotope records (Pekar et al. 2006, Pekar & DeConto 2006) 409 355 to imply periods of significant ice cover in both Polar indicates orbital-duration eustatic sea level variations of 410 356 Regions (Tripati et al. 2005). These observations suggest 10–40 m at this time, it had hitherto not been possible to 411 357 that either our interpretations of the proxy data are faulty, evaluate these inferred changes from direct evidence for 412 358 or episodic, bipolar glaciation occurred much earlier than coeval oscillations of sea level and changes in the volume 413 359 currently accepted (Eldrett et al. 2007). These, among of the Antarctic ice sheets. Naish et al. (in press) used 414 360 other unresolved controversies related to the climatic and a grain size-derived palaeobathymetry curve (Dunbar et al. 415 361 416 362 417 363 418 364 419 365 420 366 421 367 422 368 423 369 424 370 425 371 426 372 427 373 428 374 Fig. 3. Trends in climate proxies from the 429 375 Cape Roberts section for the period 430 376 from 34–17 Ma, compared with the 431 377 composite deep-sea oxygen isotope 432 378 curve of Zachos et al. (2001). The 433 temperature estimates on the right are 379 434 for interglacial periods and for mean 380 435 summer monthly (December, January, 381 February) temperature. For the last two 436 382 million years it is based on the 437 383 temperature records from Scott Base, 438 384 Ross Island, since 1957 (-5ºC). See 439 385 Barrett (in press) for further explanation. 440 ANTARCTIC CLIMATE EVOLUTION 5

441 496 442 497 443 498 444 499 445 500 446 501 447 502 448 503 449 504 450 505 451 506 452 507 453 508 454 509 455 510 456 511 457 512 458 513 459 514 460 515 461 516 462 517 463 518 464 Fig. 4. Geological cross-section of McMurdo Sound from seismic stratigraphy and drill hole data (Naish et al., 2006). 519 465 520 466 521 467 in press), to determine the amplitude of eustatic sea level McMurdo Ice Shelf (MIS, forms the north-west corner of 522 468 fluctuations, represented by glacimarine cycles in the Cape the Ross Ice Shelf). The MIS drillcore represents the 523 469 Roberts core. Their approach estimated the eustatic sea level longest and most complete (98% recovery) geological 524 470 contribution to the palaeobathymetry curve by using a record from the Antarctic continental margin to date, and 525 471 simple back-stripping approach to constrain total subsidence, will provide a key reference record of climate and ice sheet 526 472 decompacted sediment accumulation and glacio-isostasy. variability through the Late Neogene (Naish et al. 2007). 527 473 The resulting eustatic estimates were consistent with the Late Drilling in Southern McMurdo Sound in late 2007 aims to 528 474 Oligocene d18O to sea level calibrations of Pekar et al. extend the record back to 20 million years (Harwood et al. 529 475 (2006), and show that eustatic sea level fluctuated between 2006). Together, the Cape Roberts and ANDRILL cores 530 476 around 10 and 40 m, and represented ice volume fluctuations will provide an unprecedented palaeoenvironmental record 531 477 involving 15% to 60% of the present day Antarctic Ice for this part of the Antarctic margin for the last 34 million 532 478 Sheet. This work also implies a d18O calibration for the years through 3500 m of strata (Fig. 4). 533 479 Mi-1 glacial excursion that supports a significant expansion 534 480 of ice on Antarctica, perhaps equivalent to 120% of the 535 Neogene major advance and retreat episodes of the 481 present day East Antarctic Ice Sheet, with an attendant fall 536 East Antarctic Ice Sheet 482 in global sea level of ~50 m. 537 483 A further conclusion from the Cape Roberts record is that In the 1980s, studies of the Sirius Group and 538 484 coastal temperature declined progressively through Oligocene geomorphological investigations in the Transantarctic 539 485 and early Miocene time (Barrett 2006, Fig. 3). This is at Mountains led to the development of ‘dynamic’ versus 540 486 odds with the initial interpretation of the Zachos et al. (2001) ‘stable’ ice sheet hypotheses, representing widely 541 487 synthesis of the Cenozoic oxygen isotopic record, where a contrasting views of Neogene Antarctic climate and glacial 542 488 major shift of the d18Ovaluesat25 Ma was interpreted as dynamics (Webb et al. 1984, Denton et al. 1984). While 543 489 a warming of the oceans. This has been found to result from comprehensive studies of landscape evolution in the 544 490 splicing records from high and low latitudes (and cold to McMurdo Sound region support a persistent ice sheet in 545 491 warm water masses, Pekar et al. 2006). A revised global central East Antarctica through Neogene time (Sugden & 546 492 Cenozoic proxy temperature record is now overdue. Denton 2004), preserved fragments of a pre-mid Miocene 547 493 The ANDRILL Program has successfully recovered a landscape carved by temperate ice have been recognized 548 494 1285 m long succession of cyclic glacimarine sediment (Hicock et al. 2003) and the extreme relief of the 549 495 with inter-bedded volcanic deposits from beneath the Transantarctic Mountains has been linked to continued 550 6 MARTIN J. SIEGERT et al.

551 erosion in the valley floors while the summit tops remained glacial source occurred periodically as short-term events 606 552 frozen (Stern et al. 2005). Based on compositional studies until the middle Pleistocene (Joseph et al. 2002). 607 553 of the glacigenic sedimentary rocks themselves, Passchier The combined studies of ODP cores and field studies of 608 554 (2001, 2004) concluded that the Sirius Group could have the Pagodroma Group provide evidence of major shifts in 609 555 been deposited concurrently with stepwise glacial the position of the grounding-line of the Lambert Glacier 610 556 denudation of the Transantarctic Mountains. through the Late Neogene. Numerical modelling studies of 611 557 Nevertheless, recent drilling by the Ocean Drilling erosion and sediment supply suggest that, besides climate, 612 558 Program in Prydz Bay, field studies of the Pagodroma continued excavation and overdeepening of the glacial 613 559 Group exposed on land, and numerical modelling studies, trough during ice advance phases is an important factor 614 560 provide evidence for a more complex behaviour of the controlling the dynamics of the Lambert Glacier in the late 615 561 Neogene East Antarctic Ice Sheet. The importance of large Pleistocene (Taylor et al. 2004, O’Brien et al. 2007). 616 562 outlet glaciers as major drainage pathways of the East Complete deglaciation as proposed in the dynamic ice 617 563 Antarctic Ice Sheet is apparent in RADARSAT data (Jezek sheet hypothesis has not been demonstrated in any of the 618 564 2003) and from the presence of Neogene trough mouth datasets, and recent PRISM ice sheet reconstructions show 619 565 fans, represented by sediment wedges on the continental the East Antarctic interior remaining ice covered (Hill et al. 620 566 slopes seaward of large glacial troughs (O’Brien & Harris 2007). However the modelling does show significant ice 621 567 1996, Bart et al. 1999, 2000). loss at the margins, consistent with the retreat of fast 622 568 The Lambert Glacier is the largest fast flowing outlet flowing outlet glaciers associated with recognizable 623 569 glacier in the world and drains c. 12% of the East Antarctic changes in sea level at continental margins elsewhere. 624 570 Ice Sheet into Prydz Bay. During advances of the Lambert Recent studies with coupled ocean-atmosphere general 625 571 Glacier to the shelf break, glacigenic debris flows built up circulation models also re-emphasize the role of Antarctic 626 572 a trough mouth fan on the continental slope. ODP Leg 188 terrestrial ice cover and sea ice extent during periods of 627 573 drilled through the upper portion of the Prydz Channel warming and the need to improve our knowledge about 628 574 trough mouth fan at Site 1167, revealing evidence of Neogene ice configurations (Haywood & Valdes 2004). 629 575 repeated advances of the Lambert Glacier across the Prydz 630 576 Bay shelf until the middle Pleistocene (Passchier et al. 631 Synchroneity of late deglacial ice retreat from widely 577 2003, O’Brien et al. 2004). The Pagodroma Group occurs 632 separated areas of Antarctica’s continental margin 578 200–500 km landward of Prydz Bay and consists of 633 579 massive diamicts and boulder gravels deposited in an ice The nature and timing of the last large-scale, rapid warming 634 580 proximal environment near a grounding-line, and stratified event in Antarctica is an especially interesting target for 635 581 facies representing more distal iceberg deposition. The scientific research given projections of significant warming 636 582 depositional environments are considered to be analogous in the centuries ahead (IPCC 2007). The retreat of 637 583 to the modern fjords of East Greenland with fast flowing Antarctica’s ice sheet following the last glacial maximum 638 584 polythermal tidewater glaciers (Hambrey & McKelvey (LGM) has been studied for more than 30 years via marine 639 585 2000a). The formations have ages ranging from early geology and continental glacial geomorphology (Anderson 640 586 Miocene (or possibly Oligocene) to Pliocene–Pleistocene 1999, Anderson et al. 2002, Domack et al. 2006, and 641 587 (Hambrey & McKelvey 2000a, Whitehead et al. 2003) and many others), and yet many questions remain regarding the 642 588 indicate periods of significant glacial retreat (Hambrey & timing, speed, and style of ice retreat. These questions are 643 589 McKelvey 2000b, Passchier & Whitehead 2006, Whitehead directly applicable to projections of climate and ice sheet 644 590 et al. 2006). behaviour into the future. How fast can Antarctica’s ice 645 591 Studies combining results from ODP Site 1165 off Prydz sheet retreat during periods of warming (particularly as 646 592 Bay and seismic data show that changes in margin future warming may be greater than at any period since the 647 593 architecture at c. 3 Ma are related to changes in glacial Pliocene)? Is the style of past retreat suggestive of constant 648 594 thermal regime (Rebesco et al. 2006, Passchier 2007). and steady sea level rise or do we see evidence of abrupt 649 595 Indeed, stable isotope studies (Hodell & Venz 1992), and short-lived yet rapid intervals of ice retreat? How much did 650 596 interpretations of siliceous microfossils (Bohaty & Antarctic glacial ice melting contribute to the global ocean 651 597 Harwood 1998, Whitehead et al. 2005) indicate low sea ice meltwater pulses of the last deglaciation? 652 598 concentrations and relatively high sea surface temperatures Antarctic marine geologists have recently collected 653 599 in the early Pliocene with a cooling trend occurring from expanded Holocene sedimentary sections from continental 654 600 the middle Pliocene onward. Previously, based on results shelf basins by drilling and ultra-long piston coring (Crosta 655 601 from ODP Site 745 in the East Kerguelen sediment drift, et al. 2005, Leventer et al. 2006, Anderson et al. 2006). 656 602 Joseph et al. (2002) had argued that a stable East Antarctic By dating the biogenic sediments (indicative of marine 657 603 Ice Sheet had established itself during the middle Pliocene. productivity) or other open marine sediments immediately 658 604 However, they also found that enhanced sediment overlying the LGM diamict, it is possible to estimate the 659 605 accumulation from a less stable, wet-based, East Antarctic timing of ice retreat from outer and mid-shelf regions 660 ANTARCTIC CLIMATE EVOLUTION 7

661 Table I. Radiocarbon-based estimates of the date of onset of the most recent rapid deglacial ice retreat from shelf basins in East and West Antarctica. 716 14 14 § 662 Location Core/sample Water depth C age (yrs BP) C calendar age (yrs BP) 717 663 718 East Antarctic Margin-Prydz (68846’S, 76841’E) ODP site 740 807 m 10 700a 10 800 Æ 200 664 East Antarctic Margin (68845.1’S, 76842.1’E) JPC-25 848 m 10 625 Æ 35b 10 548 Æ 273 719 665 East Antarctic Margin (66855.9’S, 63807.3’E) JPC-43B 465 m 11 770 Æ 45c 11 450 Æ 300 720 d 666 East Antarctic Margin (67830’S, 658E) multiple cores 400–500 m 11 000 11 000 Æ 200 721 West Antarctic Penin. (64851.7 S, 64812.5 W) ODP site 1098 1011 m 11 700 Æ 75e 11 510 Æ 300 667 ’ ’ 722 Western Ross Sea–Coulman area (748S, 1728E) Multiple cores 400–900 m 11 000f ~11 000 Æ 400 668 723 a 669 Domack et al. (1991), corrected for reservoir age but not calibrated. 724 bLeventer et al. (2006), scaphapod carbonate, uncorrected 14C age. 670 725 cStickley et al. (2005), acidified organic matter, uncorrected 14C age. 671 dHarris & O’Brien (1998), Sedwick et al. (1998, 2001), corrected for reservoir age but not calibrated. 726 672 eDomack et al. (2001), Dunbar et al. (2002), interpolated age of base of laminated biogenic unit. 727 f 673 Domack et al. (1989), based on many cores, corrected for surface ages. 728 g 674 Calibrated calendar year ages are derived through calibration using CALIB 4.2 and 5.0 (Stuiver et al. 2005). 729 §Reservoir ages for the ODP 1098 were accomplished using the variable reservoir ages of van Beek et al. (2002). 675 730 676 731 677 (Licht et al. 1996, Leventer et al. 2006). At present there are crossed, such as sea level or temperature rise, which forced 732 678 still relatively few dates from these kinds of deposits and in a continent-wide response. The significance of this rapid 733 679 fact there has been much focus in the literature on retreat event is threefold. 734 680 establishing the timing and extent of maximum ice advance 1) It suggests that a global or hemispheric forcing agent 735 681 during the LGM and the general character of deglaciation was responsible for rapid loss of ice rather than 736 682 (e.g. Anderson et al. 2002) rather than the nature of regionally variable fluctuations in local energy 737 683 discrete periods of rapid ice retreat. balance or the dynamics of ice and ice streams 738 684 Radiocarbon dating of Antarctic marine sediments is interacting with bedrock and the ocean. 739 685 plagued by a variety of difficulties, including variable 740 686 water column carbon reservoir ages and sedimentary 2) The timing coincides with estimates for the initiation of 741 687 reworking that results in the mixing of older and younger global meltwater pulse 1B (taken as 11 500 to 11 000 742 688 materials (Domack et al. 1989, Berkman & Forman 1996, calendar years BP, after Fairbanks 1989), allowing for 743 689 van Beek et al. 2002). However, independent estimates of the possibility that loss of Antarctic ice contributed 744 690 reservoir age corrections are becoming available (Van Beek significantly to this event rather than accepting that 745 691 et al. 2002) as are palaeomagnetic intensity age virtually all meltwater came from the Northern 746 692 determinations for the deglacial interval (Brachfeld et al. Hemisphere ice sheets. 747 693 2003) that serve to improve our ability to estimate the 3) Synchroneity implies the sudden release of large 748 694 timing of key events on the Antarctic shelf during volumes of freshwater into the Southern Ocean, 749 695 deglaciation. For example, Table I shows age estimates for raising the possibility of significant oceanic 750 696 the most recent onset of rapid deglacial ice retreat from stratification and concomitant changes in productivity, 751 697 Prydz Bay, the Ross Sea, Mac.Robertson Land, and the nutrient fluxes, and the control of atmospheric CO2 752 698 western shelf of the Antarctic Peninsula. To this list we add levels by the Southern Ocean. 753 699 a palaeomagnetic age estimate of 10 700 Æ 500 yrs BP for 754 700 the conclusion of the transition from grounded ice to a 755 Subglacial processes and flow dynamics of former 701 floating Larsen Ice Shelf (64847.1’S, 60821.5’W, 901 m 756 Antarctic ice streams from marine geology and 702 water depth; Brachfeld et al. 2003). 757 geophysics 703 The developing view is one of the onset of rapid and 758 704 synchronous retreat of ice from widely separated regions of Recent marine geophysical and geological research from the 759 705 Antarctica’s continental shelf beginning at 11 500 Antarctic continental shelf has resulted in significant 760 706 calendar years BP and lasting for up to 1000 years. This advances to our understanding of the extent, timing and 761 707 apparent synchroneity is unexpected, given previous dynamic behaviour of the West Antarctic Ice Sheet and the 762 708 inferences of large geographic asynchroneity in the timing Antarctic Peninsula Ice Sheet during the last glacial 763 709 of maximum glacial advance and subsequent early maximum, as well as the processes and conditions at the 764 710 deglacial history along Antarctica’s margin (Anderson former ice sheet bed. This research indicates extensive ice 765 711 et al. 2002). However, a rapid retreat of ice from sheets in West Antarctica and the Antarctic Peninsula at the 766 712 widespread regions of Antarctica’s continental shelf last glacial maximum. The ice sheet was positioned at, or 767 713 beginning about 11 500 years ago is not necessarily close to, the shelf edge around the Peninsula, and in the 768 714 inconsistent with observations of asynchroneity earlier Bellingshausen Sea and Pine Island Bay (Anderson et al. 769 715 during the last deglaciation. A threshold may have been 2002, O´ Cofaigh et al. 2002, 2005a, Lowe & Anderson 770 8 MARTIN J. SIEGERT et al.

771 826 772 827 773 828 774 829 775 830 776 831 777 832 778 833 779 834 780 835 781 836 782 837 783 838 784 839 785 840 786 841 787 842 788 843 789 844 790 845 791 Fig. 5. Representative geophysical and 846 792 geological records of palaeo-ice stream 847 793 flow and deposition from bathymetric 848 troughs on the Antarctic continental 794 849 shelf. a. Swath bathymetry shaded relief 795 850 image of mega-scale glacial lineations 796 formed in sediment at the mouth of the 851 797 Ronne Entrance, Bellingshausen Sea. 852 798 b. Swath bathymetry showing sea-floor 853 799 morphology as a shaded relief image in 854 800 middle-outer Marguerite Trough, 855 801 Antarctic Peninsula. Note subglacial 856 802 meltwater channels and bedrock 857 803 drumlins. The drumlins become highly 858 804 attenuated downflow and evolve into 859 sedimentary lineations. c. Core log and 805 860 shear strength plot of sub-ice stream 806 861 sediments, Marguerite Trough. Note the 807 low shear strength massive till underlain 862 808 by high shear strength (. 98 kPa) (stiff) 863 809 till. d. TOPAS sub-bottom profiler 864 810 record from the Ronne Entrance 865 811 showing acoustically transparent 866 812 sediment unit (soft till) sitting above a 867 813 prominent basal reflector (arrowed). 868 814 This profile is located perpendicular to 869 815 the former direction of ice flow. 870 Modified from O´ Cofaigh et al. (2005a, 816 871 2005b). 817 872 818 873 819 2002, Heroy & Anderson 2005, Evans et al. 2006). In these Anderson 2002, Gilbert et al. 2003). This evidence 874 820 areas large glacial troughs extend across the continental shelf, includes elongate subglacial bedforms such as drumlins 875 821 and sedimentary and geomorphic evidence from these and mega-scale glacial lineations orientated along trough 876 822 troughs indicates that they were occupied by grounded long axes (Fig. 5a & b). 877 823 palaeo-ice streams during, or immediately following, the Mega-scale glacial lineations can attain lengths of greater 878 824 last glacial maximum (e.g. Canals et al. 2000, 2002, than 20 km within the troughs and are characteristically 879 825 Wellner et al. 2001, Camerlenghi et al. 2001, Lowe & formed in a weak (0–20 kPa) porous and deformable till 880 ANTARCTIC CLIMATE EVOLUTION 9

881 layer (Fig. 5c & d) (Wellner et al. 2001, Shipp et al. 2002, Antarctic ice streams are noticeably different (Davis et al. 936 882 O´ Cofaigh et al. 2002, 2005b, 2007, Dowdeswell et al. 2005). Such variability may be due to differences in ocean- 937 883 2004, Evans et al. 2005, Hillenbrand et al. 2005). Such temperature at the ice-water interface across the ice sheet 938 884 weak tills have been identified and mapped in all the margin (e.g. Payne et al. 2004). 939 885 palaeo-ice stream troughs investigated to date. They tend to 940 886 be confined to the troughs and are not widely observed in 941 Summary and future activities 887 the inter-trough areas. The association of this weak porous 942 888 till layer with highly elongate subglacial bedforms implies Analysis of geological evidence, often in conjunction with 943 889 that the rapid motion of these ice streams was facilitated, at numerical modelling studies, has over the past few years 944 890 least in part, by subglacial deformation of the soft bed. generated substantial insights into the Cenozoic history of 945 891 Geophysical data also indicate significant transport of the Antarctic ice sheet and climate. 946 892 subglacial till towards former ice stream termini The traditional explanation for the genesis of ice in 947 893 (O´ Cofaigh et al. 2007). Antarctica related the tectonic opening of the Drake 948 894 Subglacial geology exerted a major control on ice stream Passage with the development of the Antarctic Circumpolar 949 895 development. A transition from crystalline bedrock to a Current and the isolation of Antarctic climate. Numerical 950 896 sedimentary substrate within these troughs characteristically modelling suggests, however, that while the timing of this 951 897 marks the onset of streaming flow. However, in Marguerite first ice sheet, at around 34 million years, is likely to have 952 898 Trough (the cross-shelf bathymetric trough emanating from been connected with a combination of the onset of the 953 899 Marguerite Bay on the west side of the Antarctic circumpolar current and orbital forcing, ice sheet formation 954 900 Peninsula), streaming flow appears to have commenced was an inevitable consequence of declining atmospheric 955 901 over the crystalline bedrock by enhanced basal sliding, CO2 concentrations (and associated global cooling) that 956 902 with the highest flow velocities occurring over the occurred throughout most of Cenozoic time. 957 903 sedimentary substrate further downflow by, at least in part, A significant achievement, from drilling at the Antarctic 958 904 subglacial deformation. This indicates spatial variation in margin off Cape Roberts, has been to show the cyclic 959 905 the mechanism of rapid flow beneath individual ice expansion and contraction of the Antarctic ice sheet from 960 906 streams. Subglacial meltwater channels eroded into its inception at 34 Ma almost to the middle Miocene 961 907 crystalline bedrock in Pine Island Bay and Marguerite Bay transition, with sea level varying on a scale of tens of 962 908 (O´ Cofaigh et al. 2002, 2005b, Lowe & Anderson 2003) metres. This might have been suspected from Milankovitch 963 909 demonstrate the development of organised drainage frequency patterns in the d18O deep sea isotope record, but 964 910 systems and the evacuation of meltwater beneath these ice it could not be demonstrated until drill sites were so sited 965 911 streams. In the case of Mertz Trough in East Antarctica, as to record not only the changes in sea level but also the 966 912 McMullen et al. (2006) show that meltwater evacuation advance and retreat of the ice edge. 967 913 occurred during deglaciation. The behaviour of the ice sheet since the middle Miocene is 968 914 A variety of glacial geomorphic features imaged on less clear, though fragmentary evidence from ancient 969 915 geophysical records and supplemented by investigations of landscapes and selective erosion by outlet glaciers, support 970 916 core sedimentology indicate that the rate of ice stream the view of an East Antarctic ice sheet from around 14 Ma 971 917 retreat varied between different bathymetric troughs. For with a persistent cold core and considerable fluctuations at 972 918 example, in Marguerite Trough subglacial till is overlain by the margin. Crucial new data for improving the chronology 973 919 a thin unit of (de)glacial sediment, and pristine mega-scale of climatic events, and providing a better record of their 974 920 glacial lineations recording former streaming flow along the character will be collected from the Antarctic margin in the 975 921 trough are not overprinted by moraines (Fig. 5). This McMurdo region during the current phase of ANDRILL 976 922 suggests that during deglaciation, the Marguerite Trough (Naish et al. 2006, Harwood et al. 2006). 977 923 ice stream underwent rapid floatation and collapse across During the Late Pliocene and throughout the Pleistocene, 978 924 much of its bed (O´ Cofaigh et al. 2005b), and it contrasts the Antarctic ice sheet may have been subject to less intense 979 925 with slower ice stream recession in the Larsen-A region changes. Nonetheless, geological records still reflect 980 926 (Evans et al. 2005), the Bellingshausen (O´ Cofaigh et al. modifications in ice volume, for example, since the LGM. 981 927 2005a) and Ross seas (Shipp et al. 2002, Mosola & Sedimentary cores across the Antarctic continental shelf 982 928 Anderson 2006), and in Mertz Trough on the Wilkes Land show an unusual synchronous retreat at around 11 500 983 929 continental margin (McMullen et al. 2006). This implies calendar years BP, which could be associated with a global 984 930 marked variations in the response of Antarctic palaeo-ice meltwater pulse at this time. Thus, the decay of ice from an 985 931 streams to climate warming during regional deglaciation LGM condition to its present-day form may have had 986 932 and demonstrates that retreat of marine-based ice sheets is worldwide rapid sea level and ocean chemistry consequences. 987 933 not necessarily uniformly rapid even in areas of reverse Whereas geological records inform us about the size and 988 934 bed slope. This appears sensible given satellite shape of the past ice sheets in Antarctica, they can also 989 935 observations of ice surface changes at the margins of instruct us about the dynamic processes controlling ice 990 10 MARTIN J. SIEGERT et al.

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Earth and Planetary Science Letters, 1003 the ice cover and its connection with ice sheet 1058 224, 33–44. 1004 development, on past and present large-scale ice sheet BOHATY, S.M. & HARWOOD, D.M. 1998. Southern Ocean paleotemperature 1059 1005 dynamics and stability, on the role of sub-glacial water in variation from high-resolution silicoflagellate biostratigraphy. Marine 1060 1006 the ice sheet system and on the influence of ice sheet Micropaleontology, 33, 241–272. 1061 1007 evolution on Antarctic biology. BRACHFELD, S., DOMACK, E., KISSEL, C., LAJ, C., LEVENTER, A., ISHMAN, S., 1062 GILBERT, R., CAMERLENGHI,A.&EGLINGTON, L.B. 2003. Holocene 1008 1063 In 2004 the Scientific Committee on Antarctic Research history of the Larsen-A Ice Shelf constrained by geomagnetic 1009 (SCAR) recognized the importance of understanding past paleointensity dating. Geology, 31, 749–752. 1064 1010 changes in Antarctica with the establishment of its CAMERLENGHI, A., DOMACK, E., REBESCO, M., GILBERT, R., ISHMAN, S., 1065 1011 Antarctic Climate Evolution (ACE) scientific research LEVENTER, A., BRACHFIELD,S.&DRAKE, A. 2001. Glacial morphology 1066 and post-glacial contourites in northern Prince Gustav Channel (NW 1012 programme. This programme, in conjunction with the other 1067 Weddell Sea, Antarctica). Marine Geophysical Research, 22, 417–443. 1013 SCAR research programmes (SALE - Subglacial Antarctic 1068 CANALS, M., URGELES,R.&CALAFAT, A.M. 2000. Deep sea-floor evidence of 1014 Lake Environments, AGCS - Antarctica in the Global past ice streams off the Antarctic Peninsula. Geology, 28, 31–34. 1069 1015 Climate System, and EBA - Evolution and Biodiversity in CANALS, M., CASAMOR, J.L., URGELES, R., CALAFAT, A.M., DOMACK, E.W., 1070 1016 Antarctica), aims to further integrate numerical models BARAZA, J., FARRAN,M.&DE BATIST, M. 2002. 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