ORIGINAL RESEARCH published: XX 2020 doi: 10.3389/feart.2020.563453 1 58 2 59 3 60 4 61 5 62 6 63 7 64 8 Magnetostratigraphic Chronology of a 65 9 66 10 Cenozoic Sequence From DSDP Site 67 11 68 12 274, Ross Sea, Antarctica 69 13 70 14 Luigi Jovane 1*, Fabio Florindo 1,2,3,GaryWilson 4,5,Stephaniede Almeida Pecchiai Saldanha Leone 1, 71 15 Muhammad Bin Hassan 1,DanielRodelli 1 and Giuseppe Cortese 4 72 16 73 17 1Instituto Oceanográfico, Universidade de São Paulo, São Paulo, Brazil, 2Istituto Nazionale di Geofisica e Vulcanologia, Rome, 74 3 4 5 18 Italy, Institute for Climate Change Solutions, Pesaro e Urbino, Italy, GNS Science, Lower Hutt, New Zealand, Department of 75 Geology and Marine Science, University of Otago, Dunedin, New Zealand 19 76 20 77 21 New paleomagnetic results from the late Eocene-Middle Miocene samples from Deep Sea 78 22 Drilling Project Site 274, cored during Leg 28 on the continental rise off Victoria Land, Ross 79 23 Sea, provide a chronostratigraphic framework for an existing paleoclimate archive during a 80 24 81 25 key period of Antarctic climate and ice sheet evolution. Based on this new age model, the 82 26 cored late Eocene-Middle Miocene sequence covers an interval of almost 20 Myr (from 83 27 ∼35 to ∼15 Ma). Biostratigraphic constraints allow a number of possible correlations with 84 28 85 Edited by: the Geomagnetic Polarity Time Scale. Regardless of correlation, average interval sediment 29 86 Hagay Amit, accumulation rates above 260 mbsf are ∼6 cm/kyr with the record punctuated by a 30 87 Université de Nantes, France number of unconformities. Below 260 mbsf (across the Eocene/Oligocene boundary) 31 Reviewed by: 88 interval, sedimentation accumulation rates are closer to ∼1 cm/kyr. A major unconformity 32 Belén Oliva-Urcia, 89 33 Autonomous University of Madrid, identified at ∼180 mbsf represents at least 9 Myr accounting for the late Oligocene and 90 34 Spain Early Miocene and represent non-deposition and/or erosion due to intensification of 91 35 Chenglong Deng, 92 Antarctic Circumpolar Current activity. Significant fluctuations in grain size and 36 Chinese Academy of Sciences (CAS), 93 37 China magnetic properties observed above the unconformity at 180 mbsf, in the Early 94 38 *Correspondence: Miocene portion of this sedimentary record, reflect cyclical behavior in glacial advance 95 39 Luigi Jovane and retreat from the continent. Similar glacial cyclicity has already been identified in other 96 [email protected] 40 Miocene sequences recovered in drill cores from the Antarctic margin. 97 41 98 Specialty section: 42 Keywords: magnetostratigraphy, grain size, Ross Sea Antarctica, circumpolar current, Oligocene-Early Miocene 99 43 This article was submitted to 100 Geomagnetism and Paleomagnetism, 44 a section of the journal 101 45 Frontiers in Earth Science INTRODUCTION 102 46 103 Received: 18 May 2020 47 Accepted: 13 November 2020 Deep Sea Drilling Program (DSDP) Leg 28 (Figure 1) was designed to explore the long-term climatic, 104 48 Published: XX 2020 biostratigraphic and geological history of Antarctica and its environments (Hayes and Frakes, 1975). 105 49 Citation: Such geological records provide insights into modern and future climate sensitivity estimates, 106 50 Jovane L, Florindo F, Wilson G, de particularly for time periods characterized by the presence of continental ice sheets and a 107 51 Almeida Pecchiai Saldanha Leone S, paleogeography similar to modern (e.g., Markwick, 2007; Farnsworth et al., 2019). 108 52 Hassan MB, Rodelli D and Cortese G Earth’s climate underwent a stepwise shift from greenhouse to icehouse conditions during the 109 53 (2020) Magnetostratigraphic Cenozoic. Major ice sheets first appeared on Antarctica across the Eocene/Oligocene boundary co- 110 Chronology of a Cenozoic Sequence 54 incident with the earliest Oligocene Oi-1 oxygen isotope event (1.0‰ δ18O increase at ca. 33.55 Ma; 111 55 From DSDP Site 274, Ross 112 Sea, Antarctica. e.g., Miller et al., 1991; Zachos et al., 1996; Zachos et al., 2001; Miller, 2005a; Miller et al., 2005b; 56 Front. Earth Sci. 8:563453. Francis et al., 2009; Lurcock and Florindo, 2017, Westerhold et al., 2020). While long thought to be 113 57 doi: 10.3389/feart.2020.563453 associated with the early glaciation of Antarctica (Kennett, 1977), the Antarctic Circumpolar Current 114

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115 172 116 173 117 174 118 175 119 176 120 177 121 178 122 179 123 180 124 181 125 182 126 183 127 184 128 185 129 186 130 187 131 188 132 189 133 190 134 191 135 192 136 193 137 194 138 195 139 196 140 197 141 198 142 199 143 200 144 201 145 202 146 203 147 204 148 205 149 FIGURE 1 | Map of Ross Sea with schematic representation of present-day Antarctic Slope Current (dashed white line) and the continental shelf current flow 206 (orange lines) (Ainley and Jacobs, 1981; Whitworth et al., 1995; Orsi and Wiederwohl, 2009). Location of DSDP Site 274 is indicated by a red star. DSDP 270, AND-2A, 150 Eltanin 27–21 and CRP are also shown because they are reference cores in the area (respectively, Kulhanek et al., 2019, Jovane et al., 2019; Jovane et al., 2008; Roberts 207 151 et al., 2013). 208 152 209 153 210 did not appear until the Late Oligocene (Lyle, et al., 2007). Oligocene, however, the cooling during the Miocene was not 154 211 Throughout the Oligocene, Antarctic ice sheets experienced monotonic, but characterized by swings between cooler and 155 212 repeated cycles of advance and retreat, as indicated by large warmer climate (e.g., Miller et al., 1991; Zachos et al., 1997; 156 213 and rapid changes in global sea level (Kominz and Pekar, Zachos et al., 2001; Lear et al., 2004; Holbourn et al., 2005; 157 214 2001) and δ18O variations in deep-ocean sediment (Pälike Shevenell et al., 2008; Holbourn et al., 2013; Holbourn et al., 158 215 et al., 2006). Following the inception of the Antarctic 2014; Holbourn et al., 2015; Lear et al., 2015; Holbourn et al., 2018; 159 216 Circumpolar Current, the Oligocene terminated with another Liebrand et al., 2017; Westerhold et al., 2020). 160 217 transient global cooling event (Mi-1, at ca. 23 Ma), After the initial onset of and recovery from Mi-1, the Early 161 218 characterized by a ∼1‰ positive excursion in marine benthic Miocene experienced relatively warm intervals. The warming 162 219 foraminiferal δ18O records (e.g., Wilson et al., 2009; Beddow et al., cycles of the Early Miocene culminated in the Mid-Miocene 163 220 2016) and associated with a large-scale Antarctic ice sheet Climatic Optimum (MMCO) at around 17–14 Ma, 164 221 expansion at the Oligocene-Miocene transition. This event was immediately followed by the Middle Miocene Climate 165 222 dramatic in scale, with the East Antarctic Ice Sheet (EAIS) Transition (MMCT) at 14.2–13.8 Ma (Shevenell et al., 2004). 166 223 estimated to have grown from 50 to 125% of its present-day The transition, as recorded in deep-ocean δ18O variations, was a 167 224 size (e.g., Pekar et al., 2006). It was the first of a series of similar significant one: the MMCO was the warmest interval since the 168 225 “Mi-” oxygen isotope events that, while less extreme than Mi-1, Eocene, while the MMCT, and the subsequent cooling during the 169 226 were responsible for the progressive cooling and glaciation rest of the Miocene, produced temperatures colder than any 170 227 experienced by Antarctica during the Miocene. As in the previously observed in the Cenozoic. During this time, there 171 228

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229 was a sizable EAIS, as inferred from δ18O, sea level curves and de Astronomia, Geofísica e Cienciasˆ Atmosféricas da Universidade 286 230 direct sedimentological evidence from the Antarctic continent de São Paulo (IAG USP) in a magnetically shielded room using a 2 G 287 231 itself (e.g., Lewis et al., 2007). Enterprises magnetometer (Model 755–4K). Grain size, magnetic 288 232 In this paper, we present the first magnetostratigraphic susceptibility and rock magnetic analyses were conducted at the 289 233 chronology of a late Eocene-Early Miocene record from a Centro Oceanográfico de Registros Estratigráficos (CORE) at the 290 234 sediment core (DSDP Leg 28, Site 274) obtained from offshore Instituto Oceanográfico da Universidade de São Paulo (IO USP) 291 235 Cape Adare in the Ross Sea Sector of the Antarctic continent in using a Microtrac Bluewave-SIA, an AGICO MFK1-FA 292 236 1973. The new age model will allow further studies of this unique Kappabridge, and a Vibrating Sample Magnetometer (VSM) 293 237 core to produce future paleoenvironmental reconstructions MicroMag 3,900 Princeton-Lake Shore Cryotronics, respectively. 294 238 focusing on the discharge of sediments from the catchment 295 239 areas in this region of the Ross Sea (e.g., Sagnotti, et al., 1998; 296 240 Grain Size 297 Jovane et al., 2004; Jovane et al., 2019; Roberts et al., 2013). Samples were prepared by crushing ∼0.2 g of bulk sediment in an 241 298 agate mortar. The crushed samples were further disaggregated 242 299 using 10% hydrogen peroxide to remove organic matter. After 243 300 SITE LOCATION AND LITHOLOGY 48 h of reaction, the samples were passed through 2 mm sieves 244 301 and dried in an oven. After drying, sodium pyrophosphate was 245 302 DSDP Site 274 was drilled during Expedition Leg 28 in February added to prevent the particles from flocculating. The Microtrac 246 303 1973, on the continental rise off Victoria Land, Ross Sea, about Bluewave equipment was used to measure the particles by laser 247 ° 304 250 km north-northeast of Cape Adare (Figure 1) (68 59.81S, diffraction in water, with their size being calculated according to 248 ° ′ 305 173 25.64 E; water depth of 3,326 m). A 421-m thick, largely the modified Mie theory for non-spherical particles (Chýlek et al., 249 306 terrigenous, sedimentary section was drilled with an average core 1976). To establish the basic statistical parameters of particle size, 250 307 recovery of 66%. as well as particle size distribution, the GRADISTAT v. 4.0 251 308 A graben-like structure, with relief of several hundred meters program, developed by Blott and Pye (2001), was used to 252 309 (Houtz and Meijer, 1970), lies to the southwest of the site and establish the basic statistical parameters of particle size, as well 253 310 probably served as an effective barrier to the downslope transport as particle size distribution. 254 of terrigenous material. Seafloor magnetic lineations in the area 311 255 and detailed biostratigraphic studies suggested a late Eocene/ 312 256 Early Oligocene age for the base of the sedimentary cover (Hayes Mineral Magnetic Properties 313 257 et al., 1975; Hayes and Frakes, 1975). However, paleomagnetic Rock magnetic analyses were conducted on 21 selected samples 314 258 analysis has not been carried out on those cores until this study. distributed along the studied interval to estimate downcore 315 259 For this study, half-core samples were obtained from the variations in the composition, concentration, and grain size of 316 260 International Ocean Discovery Program (IODP) Gulf Coast magnetic minerals. These analyses included volume normalized 317 fi κ 261 Repository from 320 m below sea floor (mbsf) (close to the low- eld magnetic susceptibility ( ), hysteresis loops, with 318 fi 262 contact with basement) to 123 mbsf. This interval comprises maximum elds of 0.5 T, isothermal remanent magnetization 319 263 (IRM) imparted in progressively (100 acquisition steps) to 320 silty claystone, diatom-detrital silty clay ooze and diatom-rich fi 264 silty clay (Hayes et al., 1975; Hayes and Frakes, 1975). An saturation (SIRM), or to the maximum attainable direct eld 321 265 unconformity was identified at 180.5 mbsf by Frakes (1975) of 1.8 T. IRM curves were subdivided into discrete coercivity 322 266 and interpreted to represent a time interval between late components, as in Robertson and France (1994), with cumulative 323 267 Oligocene (Core 20) and Early-Middle Miocene (Core 19). log-Gaussian (CLG) functions, using the spreadsheet developed 324 268 by Kruiver et al. (2001) and Heslop et al. (2002). 325 269 In addition, we investigated the temperature dependence of 326 270 MATERIAL AND METHODS magnetic susceptibility using a MFK1-FA equipped with CS4 327 271 furnace (AGICO) Kappabridge, to discriminate ferromagnetic 328 272 For the present study, DSDP Site 274 working half core samples mineralogy. Samples were measured up to a maximum 329 ° 273 were provided by the IODP Gulf Coast Repository from which we temperature of 700 C in an argon atmosphere, in order to 330 274 collected 8 cm3 cubic samples, using a non-magnetic double blade minimize thermochemical alteration. 331 275 saw. The core was dry but preservation was excellent. 332 276 Two hundred and sixty samples were taken from the center of the Demagnetization 333 277 working halves between 316 mbsf (Core 34) and 123 mbsf (Core 14), In order to remove secondary NRM components, an Alternating 334 278 with a sample collected at least every meter (depending on the core Field (AF) demagnetization technique was employed, with 335 279 recovery, drilling-induced disturbanceandwheresufficient material stepwise demagnetization at peak AF fields of 2.5, 5, 10, 15, 336 280 was available). Sediment samples from the interval between 320 and 20, 30, 40, 50, 60, 70, 80, 90 and 100 mT. Vector component 337 281 180 mbsf are stiffer and more consolidated, while those from the diagrams (Zijderveld, 1967) were used to display stepwise 338 282 interval between 180 and 123 mbsf are softer and less consolidated. demagnetization data. Characteristic remanent magnetization 339 283 Samples were oriented only with respect to vertical direction, (ChRM) directions were determined using principal 340 284 therefore absolute paleomagnetic declinations could not be component analysis (PCA) with linear best fits calculated from 341 285 determined. Paleomagnetic analyses were performed at Instituto three or more demagnetization steps using the PuffinPlot 342

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343 paleomagnetic analysis application (Lurcock and Wilson, 2012; grain size is greater and ranges between 10 and 12 phi 400 344 Lurcock and Florindo, 2019). (0.98–0.24 μm). Between 180 and 160 mbsf (Cores 19–18, 401 345 respectively) the average grain size is finer between 11 and 402 346 12 phi (0.48–0.24 μm) and coarsens between 160 and 130 mbsf 403 347 RESULTS (Cores 17–14) to values ranging between 9.7 and 11.7 phi 404 348 (1.20–0.30 μm). The average grain size is finer in the lower 405 349 Grain Size interval of the core (between ∼300 and ∼180 mbsf; Cores 406 350 The lithology of the studied interval is predominantly clay. The 34–20) with values ranging between 11 and 11.5 phi 407 351 average grain size is 11.16 phi (0.435 μm), while the mode is (0.48–0.34 μm), with a slight coarsening in average grain size 408 352 11.17 phi (0.432 μm), with a standard deviation of 0.33. The in the interval ∼320–∼304 mbsf (Cores 34–33). Abrupt changes 409 353 coarsest average grain size (9.87 phi; 1.065 μm) is found at in grain size at ∼262, ∼240 and ∼224 mbsf suggest the possibility 410 354 158.09 mbsf. The finest average grain size (12.41 phi; 0.181 μm) of minor unconformities in the sedimentary succession. 411 355 was observed at 162.11 mbsf. Several samples have multiple peaks 412 356 in the grain size distribution (Figure 2). Magnetic Properties 413 − 357 A drop-stone, 2 cm in diameter, was found at 319.74 mbsf, and Magnetic susceptibility (κ) values range widely between 1.8 × 10 6 414 − − 358 additional ice rafted debris (IRD) particles greater than 5 mm in and 92.9 × 10 6 SI with a mean value of 10.9 × 10 6 SI (Figure 2). 415 359 diameter were identified at 258.5 and 247.5 mbsf. Frakes (1975) Below the unconformity at ∼180 mbsf, the highest κ values occur 416 360 also reported the presence of a coarse fraction in Cores 19 and 18 in samples between 280 and 210 mbsf and are associated with 417 361 (180.5 and 161.5 mbsf, respectively) interpreted to reflect ice IRD intervals with coarser sediments and larger clasts. Above 418 362 rafting events. 180 mbsf, κ gradually increases reaching peak values at 140 mbsf. 419 363 We observed a lower proportion of clay and a higher IRM acquisition curves obtained for the samples were 420 364 proportion of sand in the interval between 180 and 130 mbsf separated into four coercivity components (Figure 3). Both 421 365 (Cores 19–14, respectively), where the variability of the average samples (at 218.92 and 285.41 mbsf) show a component with 422 366 423 367 424 368 425 369 426 370 427 371 428 372 429 373 430 374 431 375 432 376 433 377 434 378 435 379 436 380 437 381 438 382 439 383 440 384 441 385 442 386 443 387 444 388 445 389 446 390 447 391 448 392 449 393 450 394 451 395 452 396 453 397 454 398 FIGURE 2 | Downcore variations of (A) NRM intensity, (B) low-field magnetic susceptibility (κ), (C) average grain size and (D) sediment grain size distribution in a 455 contour. Hiatuses and IRD horizons are highlighted as pink and light blue, respectively. 399 456

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457 514 458 515 459 516 460 517 461 518 462 519 463 520 464 521 465 522 466 523 467 524 468 525 469 526 470 527 471 528 472 529 473 530 474 531 475 532 476 533 477 534 FIGURE 3 | Rock magnetic properties of two selected samples at. (A), (E) hysteresis loops before (red) and after (green) subtracting the paramagnetic contribution, 478 by applying an automatic linear slope correction. (B), (F) backfield and IRM saturaton acquisitions. (C), (G) IRM acquisition curves gradient with the separated 535 479 components. Black component: low coercivity detrital magnetite, blue and green components: SD magnetite; orange components: hematite. (C), (F) thermomagnetic 536 480 curves, the red curve represents the heating portion and the blue line the cooling portion of the cycle. 537 481 538 482 539 483 540 very low coercivity (mean 8 mT) and dispersion parameter of peak at about 520°C and a major inflection at 579°C, calculated 484 541 0.30, which corresponds to coarse detrital magnetite with with the two-tangent method of Grommé et al. (1969), and 485 542 different coercivity or activation of superparamagnetic (SP) consistent with chemically pure magnetite (Thompson and 486 543 particles dispersed in the material (Heslop et al., 2004; Oldfield, 1986). A κ-T curve for sample at 285.41 mbsf 487 544 Abrajevitch and Kodama, 2011; Savian et al., 2014; Savian (Figure 3F) has a sharp drop at 565°C consistent with 488 545 et al., 2016). The two middle-coercivity components show magnetite probably mixed with titano-magnetite (Ozima and 489 546 values consistent with single domain (SD) magnetite (mean Larson, 1970). The cooling curve for sample at 285.41 mbsf has 490 547 coercivity of 23 mT, dispersion parameter 0.23 in samples higher overall κ than the heating curve, which indicates 491 548 218.92 mbsf and 45 mT, dispersion parameter 0.19 in sample production of new magnetic phases during heating while 492 549 285.41 mbsf, respectively). Those components are assumed to sample at 218.92 mbsf maintain similar values. 493 550 represent low-coercivity SD magnetite and high-coercivity SD 494 551 magnetite, probably of biogenic origin (Egli, 2004; Jovane et al., 495 Demagnetization Behavior 552 2012; Yamazaki , 2012; Heslop et al., 2004; Rodelli et al., 2019). NRM intensities (Figure 2A) for the DSDP 274 succession 496 − − 553 The component characterized by mean coercivities of 100 and range widely between 9.36 × 10 2 and 6.58 × 10 5 A/m about a 497 − 554 280 mT (dispersion parameter 0.15 and 20, respectively), are mean of 4.72 × 10 3 A/m. At the base of the sequence 498 555 interpreted as hematite (Kruiver et al., 2001; Abrajevitch et al., (320–310 mbsf), values are higher than for the rest of the 499 556 2009; Rodelli et al., 2019). The overall contribution to the IRM in sequence below the unconformity, then increase again 500 557 sample 218.92 mbsf from the SD magnetite is 72, and 9% is linked above 180 mbsf. Glaciomarine sediments at the base are 501 558 instead to detrital magnetite. Hematite only represents 19% of the coarser and contain larger clasts that can increase 502 559 total contribution. IRM acquisition curves for a sample at intensities and lead to non-reliable paleomagnetic behavior 503 560 285.41 mbsf, typical of samples with lower κ, was fitted with (e.g., unstable remanence, low inclination or multi-component 504 561 only four components (Figure 3E): the low coercivity SD magnetizations). For these reasons, samples at the base were 505 562 magnetite component is missing, and the hard SD magnetite excluded from magnetostratigraphic interpretations. 506 563 is less abundant, corresponding to only 64% of the magnetic Some samples exhibit a low coercivity horizontal component 507 564 signal, and the detrital magnetite is more abundant (18%). of remanence (Figure 4) interpreted to be a drilling induced 508 565 Hematite is slightly less abundant (18%). radial overprint related to strong magnetic fields in the cutting- 509 566 A κ-T curve for sample at 218.92 mbsf (Figure 3C) shows shoe and/or core barrel (e.g., Acton et al., 2002; Florindo et al., 510 567 increasing values up to about 330°C, which can be interpreted as 2003). In most cases, this is removed by AF demagnetization in a 511 568 maghemite (Rodelli et al., 2018; Rodelli et al., 2019), possibly peak field of 20 mT, but in some cases the horizontal overprint is 512 569 related to partial oxidation of magnetite. This is followed by a harder and persists to higher coercivities (>80 mT; Figure 4D) 513 570

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571 628 572 629 573 630 574 631 575 632 576 633 577 634 578 635 579 636 580 637 581 638 582 639 583 640 584 641 585 642 586 643 587 644 588 645 589 646 590 647 591 648 592 649 593 650 594 651 595 652 596 653 597 654 598 655 599 656 600 657 601 658 602 659 603 660 604 661 605 662 606 663 607 664 608 665 609 666 610 667 611 668 612 669 613 670 614 671 615 672 616 673 617 674 618 675 619 676 620 677 621 678 622 679

623 FIGURE 4 | AF (alternating field) demagnetization behavior for eight representative samples. For the vector component diagrams, open (closed) symbols represent 680 624 projections onto the vertical (horizontal) plane. The red dashed lines represent PCA fits that indicate the ChRM direction for each sample. A), F), B) examples of reverse 681 625 polarity. C), H), E), G) examples of normal polarity; Samples A), E) G) are noisy but it is possible to identify a ChRM. Sample D) has a radial overprint that resists AF 682 626 demagnetization. The cores were not azimuthally oriented and declinations are, therefore, arbitrary. See text for discussion. 683 627 684

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685 and prevents the isolation of a ChRM. In a few samples, following conditions: 1) samples with a radial overprint 742 686 characterized by the lowest NRM intensities, this overprint (Figure 4D); 2) single samples showing a ChRM with an 743 687 could not be removed by AF demagnetization. We cannot rule inclination of opposite polarity to that of the previous and 744 688 out that drying may have further hardened the radial overprint. following samples; 3) samples with anomalous low inclination 745 689 The ChRM is identified in most samples, regardless of polarity, (shallower than ±25°) were also not considered (Figure 4G); 4) 746 690 between AFs of 20 and 70 mT (Figure 4). Approximately half of samples containing IRD which do not demagnetize due to the 747 691 the ChRMs are directed to the origin of vector component plots presence of high-coercivity minerals and 5) samples with 748 692 (Figures 4A,C) and half follow a trajectory toward a high overprints (drilling overprint (Fuller et al., 1998) or core- 749 693 coercivity component not removed by peak AFs of 100 mT splitting overprint (Wilson et al., 2000; Florindo et al., 2001) 750 694 (Figures 4F,G). The high coercivity component of samples that could not be removed by low field AF demagnetization. 751 695 containing a small amount of hematite is assumed to be a Mean ChRM inclinations were computed separately for 752 696 secondary chemical remanent magnetism and unanchored normal and reverse polarity populations using a maximum 753 697 ChRM components are reported. likelihood method (Arason and Levi, 2010) as implemented in 754 698 the PuffinPlot application (Lurcock and Florindo, 2019). The 755 699 Magnetozones normal polarity intervals have a mean inclination of −55.01° (N 756 700 ° ° 757 Of the total of 287 samples, 65.51% (188) were used to determine 56, α95 6.67 , θ63 27.03 ), the reverse polarity intervals have a 701 ° ° ° 758 the magnetic polarity zonation, with discrete magnetozones mean of 68.3 (N 132, α95 4.45 , θ63 27.8 )(Figure 5). 702 defined as an interval where at least two consecutive samples Some samples have lower inclination than the expected 759 703 show polarities different from adjacent intervals. The other 99 Geocentric Axial Dipole (GAD) inclination (±64.5° at 20 Ma 760 704 samples were discarded according to the following criteria. and ±63.9° at 40 Ma; Torsvik et al., 2012) for this period and for 761 705 Samples were discarded from the final analysis if they met the this site latitude. A post depositional tilt of ∼5° the strata is 762 706 763 707 764 708 765 709 766 710 767 711 768 712 769 713 770 714 771 715 772 716 773 717 774 718 775 719 776 720 777 721 778 722 779 723 780 724 781 725 782 726 783 727 784 728 785 729 786 730 787 731 788 732 789 733 790 734 791 735 792 736 793 737 794 738 795 739 FIGURE 5 | Histograms of paleomagnetic inclinations from discrete samples for DSDP Site 274 after AF demagnetization. Mean ChRM inclinations (shown in blue), 796 α 740 and associated 95, were computed separately for normal and reverse polarity populations using a maximum likelihood method (Arason and Levi, 2010). The expected 797 741 inclinations for the GAD are shown in red for this period and for the site latitude (Torsvik et al., 2012). 798

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799 856 800 857 801 858 802 859 803 860 804 861 805 862 806 863 807 864 808 865 809 866 810 867 811 868 812 869 813 870 814 871 815 872 816 873 817 874 818 875 819 876 820 877 821 878 822 879 823 880 824 881 825 882 826 883 827 884 828 885 829 886 830 887 831 888 832 889 833 890 834 891 835 892 836 893 837 894 838 895 839 896 840 897 841 898 842 899 843 900 844 901 845 902 846 903 847 904 848 905 849 906 850 907 851 908 852 909 853 910 854 FIGURE 6 | Downcore variations of NRM intensity, ChRM inclination, and magnetic polarity zonation (black denotes normal polarity and white denotes reversed 911 polarity). 855 912

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913 implied by the difference in normal and reversed polarity AGE MODEL 970 914 inclinations. 971 915 We recognize eight normal and eight reversed magnetozones Biostratigraphic Data 972 916 (Figure 6). The base of magnetozone N1 is not identified due to No further biostratigraphic information has been reported for the 973 917 an interval of undefined polarity at the base of the core with most 316–123 mbsf interval (Cores 34–14) since the original 974 918 samples characterized by radial overprints. The upper part of biostratigraphic first and last occurrence information provided 975 919 magnetozone N1 is well defined by four normal polarity by the Shipboard Report (Table 1; Hayes et al., 1975). Here, we 976 920 samples. Samples 313.21 (normal polarity) and 311.4 (reversed update those occurrences accounting for more recent age and 977 921 polarity) mbsf define the boundary of magnetozone N1 with R1. occurrence data from other Southern Ocean drill holes in order to 978 922 Between 312.3 and 302.38 mbsf magnetozone R1 is clearly correlate our magnetic polarity stratigraphy with the 979 923 identified, although it includes several samples with noisy data Geomagnetic Polarity Time Scale (GPTS) (Figure 7). 980 924 and others that do not demagnetize. Magnetozone N2 is well Planktonic foraminifera occur exclusively within Core 21 981 925 defined by three normal polarity samples between 302.82 (top of (190–199.5 mbsf) Introduction (92–88 cm) and Material and 982 926 magnetozone R1) and 297.72 mbsf, which is the base of Methods (73–69 cm), including Globigerina (Subbotina) 983 927 magnetozone R2. Between 298.4 and 280.09 mbsf, magnetozone angiporoides (LO 29.18–30.28 Ma, FO 40.40–41.89 Ma; Olsson 984 928 R2 includes a long series of samples with reversed polarity with et al., 2006; Wade et al., 2018) and Globoturborotalita 985 929 only occasional samples that do not demagnetize. The boundary labiacrassata Jenkins (LO 21.12–22.44 Ma, FO 30.28–32.10 Ma; 986 930 between magnetozones N3 and R3 lies within an 8-m-interval Spezzaferri et al., 2018). There is some indication that the 987 931 where polarity could not be determined due to samples in which assemblage may be reworked, but a penecontemporaneous 988 932 the radial overprints do not demagnetize. The interval between ca. redeposition was suggested by Kaneps (1975, Table 1). 989 933 270 and 241.08 mbsf includes two intervals of well-defined Calcareous nannofossils are present only in isolated horizons 990 934 reversed polarity: magnetozones R3 and R4, with a long interval and burrows in the interval covered by Cores 28–21 (266–190 991 935 magnetozone N4 where polarity is unclear with several samples mbsf), but only in Core 21 (199.5–190 mbsf), Introduction,93cm 992 936 not demagnetizing and others with persistent overprints. and Material and Methods, 70 cm, was the assemblage sufficient 993 937 Magnetozone N5 is defined at its base between 241.08 and for an age determination (Chiasmolithus altus assemblage) 994 938 240.49 mbsf and at its top between samples 236.37 and (Burns, 1975; Bukry, 1975, Table 1). Fioroni et al. (2012),ina 995 939 235 mbsf. Magnetozones R5, N5 and N4 contain several Southern Ocean calibration, attributed the Chiasmolithus altus 996 940 samples that do not fully demagnetize. The boundary biozone to the time interval from chrons C12r to C8n, which was 997 941 between magnetozones R5 and N6 is not well defined due to later confirmed by Kulhanek et al. (2019). Dictyococcites scrippsae 998 942 an interval including some samples that do not demagnetize and (middle Eocene to Upper Oligocene; Bukry and Percival, 1971), 999 943 are noisy and have radial overprints. Magnetozone N6 is Coccolithus pelagicus (Danian to present; Wallich, 1877) and 1000 944 represented by four samples but also contains some samples Reticulofenestra species (Ypresian to present, Fioroni et al., 1001 945 that do not demagnetize. The boundary between magnetozones 2015) were also found in the same samples. 1002 946 N6 and R6 is at 219.83 mbsf; at the midpoint between samples of Diatoms are generally abundant with preservation ranging 1003 947 opposite polarity at 220.74 and 218.92 mbsf. Magnetozone N7 from fair/poor (Cores 19–1) to excellent (Cores 34–20). Down 1004 948 includes only two samples and is separated from magnetozone through Core 19 (180.5–171 mbsf), diatoms are of Miocene age 1005 949 R6 by an interval of questionable polarity conditions containing (Denticula antarctica Zone) and below this, through Core 34 1006 950 several noisy samples that do not demagnetize and have radial (323–313.5 mbsf), of Oligocene age (Pyxilla prolongata; See 1007 951 overprints. The interval of the core above 210 mbsf includes a Figures 8A,B of McCollum, 1975, Table 1). 1008 952 well-defined N-R-N-R pattern of polarity, with the boundary Ciesielski (1975) identified silicoflagellates in the interval 1009 953 between magnetozones N7 and R7 at 206.70 mbsf between between Cores 19–13, from 179 to 114 mbsf, with this interval 1010 954 samples 206.94 and 206.47 mbsf. Magnetozone R7 includes containing an assemblage of low diversity and abundance, poor 1011 955 several noisy samples that do not demagnetize and have preservation, and broken specimens. Distephanus speculum and 1012 956 radial overprints. The boundary between magnetozones R7 D. boliviensis are identified throughout this interval providing 1013 957 and N8 is at 175.87 mbsf midway between a reversed sample poor age-diagnostic forms (Guiry and Guiry, 2011). However, the 1014 958 at 177.53 mbsf and a normal sample at 174.22 mbsf. The interval comprising Cores 34 to 20 (321.5–181 mbsf) displays a 1015 959 boundary between magnetozone N8 and R8 is identified at uniform assemblage of silicoflagellates with excellent preservation 1016 960 160.65 mbsf between samples at 162.11 and 159.20 mbsf. and high abundance. Mesocena apiculata, Dictyocha deflandrei 1017 961 Although most of the magnetozone boundaries were s.l, Distephanus cf. D. boliviensis, Distephanus crux, Corbisema 1018 962 identified, some of the boundaries, particularly in the lower apiculata, and Naviculopsis trispinosa (See Tables 6C, D of 1019 963 interval of the core, could not be located precisely because of Ciesielski, 1975)define this interval as belonging to the 1020 964 intervals with uncertain polarity due to strong radial overprints, Dictyocha deflandrei Zone, which is subdivided into two 1021 965 samples that do not demagnetize and others which were subzones: 1) Dictyocha deflandrei Subzone (Cores 30–20) 1022 966 truncated by sedimentary unconformities identified by abrupt characterized by the presence of Dictyocha frenguellii and 2) 1023 967 grain size changes. The boundaries between magnetozones N6 Mesocena apiculata subzone (Cores 34–30) (Guiry and Guiry, 1024 968 and R6, R4 and N5, and within N4 occur within sedimentary 2011, Table 1). 1025 969 unconformities. 1026

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1027 TABLE 1 | Biostratigraphic data from previous studies in the DSDP Initial Reports -Hayes et al. (1975), Radiolaria (Chen, 1975), Diatoms (McCollum, 1975), Silicoflagellates 1084 1028 (Ciesielski, 1975), Planktonic Foraminifera (Kaneps, 1975), Calcareous nannoplankton (Bukry, 1975; Burns, 1975) - used to correlate of the DSDP Site 274 magnetic 1085 1029 polarity zonation with the Geomagnetic Polarity Time Scale. Ages for radiolarian datum levels were assigned, where available, by the composite ordering of events and hybrid 1086 age model output from constrained optimization (CONOP) analyses of Southern Hemisphere data sets and Ocean Drilling Program (ODP) Site 744 (Florindo et al., 2013). 1030 Ages for all other events are as reported in the biozonation by Takemura and Ling (1997), or from other reports, such as Weaver (1983). The age calibrations of the events 1087 1031 reported in Funakawa and Nishi (2005) and Takemura and Ling (1997) were converted to the Gradstein et al. (2012) timescale from the original Berggren et al. (1995) age 1088 1032 calibration, using the converter at: http://www.odsn.de/cgi-bin/conv_ts.pl; (A) 27.1–27.5, (B) 28.2–28.3, (C) ?28.28–29.40, (D) 28.9–29.1, (E) 30.48-?33.06, (F) 36. 1089 1033 2–36.4. 1090 1034 Event and source References Base Depth Top Depth Datum (Ma) or References of Datum 1091 1035 (mbsf) (mbsf) period of occurrence or occurrence 1092 1036 (Myr) 1093 1037 1094 FORAMINIFERA ——— — 1038 (Kaneps, 1975) ——— — 1095 1039 Globigerina (Subbotina) angiporoides (F1) 190.90 193.71 LO 29.18–30.28 Olsson et al., 2006; Wade et al., 2018 1096 1040 Globoturborotalita labiacrassata Jenkins (F2) 190.90 193.71 LO 21.12–22.44 Spezzaferri et al., 2018 1097 1041 —————1098 Calcareous NANNOFOSSILS ——— — 1042 (Bukry, 1975; Burns, 1975) ——— — 1099 1043 Chiasmolitus altus (N1) 190.93 197.00 HO 25 ma (C12r to C8n) Fioroni et al., 2012; Kulhanek et al., 2019 1100 1044 Dictyococcites scrippsae 190.93 197.00 Mid Eocene/Late Oligocene Bukry and Percival, 1971 1101 1045 Coccolithus pelagicus 190.93 197.00 Danian/present Wallich, 1877 1102 1046 Reticulofenestra sp. 190.93 197.00 Ypesian/present Fioroni et al., 2015 1103 RADIOLARIA ——— — 1047 (Chen, 1975) ——— — 1104 1048 Antarctissa deflandrei ( A. conradae) 128.62 — FO 13.25 Florindo et al., 2013 1105 1049 Cyrtocapsella isopera (R1) 128.62 — LO Early/Middle Miocene Weaver, 1983 1106 1050 Eucyrtidium cienkowskii 128.62 — LO 8.40 Florindo et al., 2013 1107 — 1051 Eucyrtidium cienkowskii (R2) 161.50 FO 19.47 Florindo et al., 2013 1108 Cycladophora bicornis (Theocalyptra bicornis 165.10 — FO 12.19 Florindo et al., 2013 1052 Cyrtocapsella isopera 177.95 — FO Early/Middle Miocene Weaver, 1983 1109 1053 Larcopyle frakesi (Prunopyle frakesi) (R3) 183.95 — LO 22.08 Florindo et al., 2013 1110 1054 Lithomelissa robusta (R4) 183.95 — LO 26.499–26.93 Funakawa and Nishi, 2005 (A) 1111 1055 Lithomelissa sphaerocephalis (R5) 183.95 — LO 27.747–27.876 Funakawa and Nishi, 2005 (B) 1112 — – 1056 Lithomelissa challengerae (R6) 183.95 LO ?27.855 29.182 Takemura and Ling, 1997 (C) 1113 Cyclampterium milowi ( C. longiventer) (R7) 183.95 — LO 28.492–28.768 Funakawa and Nishi, 2005 (D) 1057 Calocyclas semipolita (?) (R8) 183.95 — LO 30.592-?33.159 Takemura and Ling, 1997 (E) 1114 1058 Dendrospyris stabilis (R9) 333.63 — FO 36.561–36.757 Funakawa and Nishi, 2005 (F) 1115 1059 POLLEN ——— — 1116 1060 (Ciesielski, 1975) ——— — 1117 Leiofusa 133.00 171.00 Devonian Couper, 1960 1061 Classopollis torosus 133.00 171.00 Jurassic-Early Cretaceous Couper, 1960 1118 1062 Areosphaeridium diktyoplokus 199.50 218.50 Late Eocene Wilson et al., 1998 1119 1063 Spinidinium aperturum 199.50 218.50 Oligocene-Paleocene Fensome et al., 1993 1120 1064 Deflandrea asymmetrica 199.50 218.50 Oligocene-Paleocene Levy and Harwood, 2000 1121 1065 Thalassiphora cf. pelagica 199.50 218.50 Oligocene-Late Cretaceous Hall, 1977 1122 Aiora fenestrata 199.50 218.50 Early Oligocene-Late Eocene Hall, 1977 1066 Nothofagus 199.50 218.50 Oligocene-Eocene Couper, 1960 1123 1067 DIATOMS ——— — 1124 1068 (McCollum, 1975) ——— — 1125 1069 Hemialus sp. 1 180.80 313.80 LO 22–30 Ma Olney et al., 2007 1126 — 1070 Pseudopyxilla americana 180.80 313.80 LO 13.8 Ma 1127 Pyxilla sp. 180.80 313.80 LO 24.22 Scherer et al., 2000 1071 Rhizosolenia barboi 146.80 313.80 LO 20 - FO 22 Ma Olney et al., 2007 1128 1072 Rhizosolenia hebetata f. hiemalus 190.30 294.80 HO 21 Ma Scherer et al., 2000 1129 1073 Stephanopyxis (sp.) 153.40 — LO 22 Ma Olney et al., 2007 1130 1074 Trinacria excavata 123.90 313.80 FO 24.1 Ma Olney et al., 2007 1131 ——— — 1075 SILICOFLAGELLATES 1132 (Bukry, 1975; Ciesielski, 1975) ——— — 1076 Dictyocha sp. 180.50 322.00 Eocene Mandra and Mandra, 1970 1133 1077 Mesocena apiculata 180.50 322.00 Late Oligocene-Miocene Bukry, 1974 1134 1078 Corbisema apiculata 180.50 322.00 Maastrichtian Cornell, 1974 1135 1079 Corbisema hastata 180.50 322.00 Cretaceous/Paleocene Bukry, 1974 1136 Corbisema triacantha 180.50 322.00 Eocene-Miocene Bukry, 1974 1080 Mesocena oamaruensis 180.50 322.00 Late Eocene Mandra and Mandra, 1971 1137 1081 Distephanus sp. 180.50 322.00 Eocene Mandra and Mandra, 1970 1138 1082 1139 1083 1140

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1141 1198 1142 1199 1143 1200 1144 1201 1145 1202 1146 1203 1147 1204 1148 1205 1149 1206 1150 1207 1151 1208 1152 1209 1153 1210 1154 1211 1155 1212 1156 1213 1157 1214 1158 1215 1159 1216 1160 1217 1161 1218 1162 1219 1163 1220 1164 1221 1165 1222 1166 1223 1167 1224 1168 1225 1169 1226 1170 1227 1171 1228 1172 1229 1173 1230 1174 1231 1175 1232 1176 1233 1177 1234 FIGURE 7 | Age vs. depth plot with correlation of DSDP Site 274 polarity zonation to the GPTS of Gradstein et al. (2012). Hiatuses defined from the 1178 sedimentological data and biostratigraphical datum are represented following the legend. Magnetostratigraphic model A is in dark red; models B and E are in red; models 1235 1179 C and D are in orange. 1236 1180 1237 1181 1238 1182 Chen (1975) originally reported radiolarian occurrences at Site sp, Eucyrtidium sp. This interval is assigned an age of Chattian 1239 1183 274. Many of the species he found or described are however or older, based on Lithomelissa challengerae, Lithomelissa 1240 1184 junior synonyms (see Table 1 for their correct names, along robusta, Lithomelissa sphaerocephalis, Lithomelissa tricornis, 1241 1185 with a reinterpretation of his reported occurrences of Prunopyle frakesi and Prunopyle hayesi (see Table 9B, Chen, 1242 1186 bioevents at Site 274). A very typical modern to Pliocene 1975; Table 1). 1243 1187 sequence of radiolarian events is observed in cores 11–1, On the basis of radiolarian assemblages and their comparison 1244 1188 followed by several late to middle Miocene LADs occurring with foraminiferal and calcareous nannofossil assemblages, Chen 1245 1189 in Core 12 (Antarctissa deflandrei,5.87Ma,Cycladophora (1975) hypothesized a hiatus between cores 20 1246 1190 spongothorax,8.62Ma,Actinomma golownini,10.87Ma, (190.0–180.5 mbsf) and 19 (180.5–171.0 mbsf). This 1247 1191 Cycladophora golli, 14.92 Ma). In the interval between Cores interpretation seems to be supported by his data, with the 1248 1192 19–15 (180.5–133 mbsf), radiolarian associations are Miocene and younger portion of the sequence being well 1249 1193 dominated by Cyrtocapsella isopera indicating a middle to constrained based on radiolarian marker species which are 1250 1194 early Miocene age (see Table 9A, Chen 1975; Weaver, well calibrated, and an older section starting with Core 20 1251 1195 1983). Associations are completely different in the interval (sample 20R-3, 40–50 cm) which is substantially different, 1252 1196 between cores 34 and 20, where abundant radiolarians are older and much more uniform in its radiolarian assemblage 1253 1197 present, such as Amphisphaera sp,Calocyclassp,Cenosphaera (Table 1). The LADs recorded in Core 20 include Larcopyle 1254

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1255 frakesi (22.08 Ma), Lithomelissa sphaerocephalis C5Cn and C5Cr (B) chrons C5Cr, C5Dn and C5Dr. and (C) 1312 1256 (23.35–24.73 Ma), Lithomelissa challengerae (?28.28–29.40 Ma), chrons C5Dr., C5En and C5Er, respectively (Figure 7). However, 1313 1257 and Calocyclas (?) semipolita (30.48-?33.06 Ma) which, while sediment accumulation rates are quite high for all reconstructions 1314 1258 clearly identifying this core as substantially older than Core (between 2 and 5 cm/kyr), similar to the sediment accumulation 1315 1259 19, make it difficult to assign it a well-constrained age. rates observed beneath the unconformity. 1316 1260 Our new age model implies that the unconformity at 180 mbsf 1317 1261 accounts for between 9 Myr at its shortest and 11 Myr at its 1318 1262 Correlation to the Geomagnetic Polarity longest, which is significantly shorter than the >15 Myr identified 1319 1263 Time Scale from biostratigraphy only in the original age depth model of 1320 1264 Correlation of the paleomagnetic polarity zonation with the Hayes et al. (1975). 1321 1265 GPTS (Gradstein et al., 2012) provides an age interpretation According to our correlation and calculation of the 1322 1266 between the upper Eocene and the Middle Miocene (from Chron sedimentary accumulation rate, the studied section 1323 1267 C15n to C5Br) (Figure 7). encompasses ∼20 Myr. For the interval 180–120 mbsf (cores 1324 1268 Biostratigraphic data identify a major unconformity at 19–14), we produced the following estimates of mean 1325 1269 ∼180 mbsf (Hayes et al., 1975) separating Early/Late Oligocene sedimentation rates for the two most likely age 1326 1270 strata below from Early/Middle Miocene strata above. More reconstructions: (model A) 2.0 cm/kyr (C5B-C5C) (model B) 1327 1271 recent constraints on age data (Table 1) restrict the interval of 5.1 cm/kyr (C5C-C5D) and (model C) 3.3 cm/kyr (C5D-C5E). 1328 1272 core beneath the unconformity to the late Eocene–Early The interval between 320 and 180 mbsf (cores 34–20) has a 1329 1273 Oligocene and above the unconformity to the late mean sediment accumulation rate of 1.5 cm/kyr (model D) and 1330 1274 Early–Middle Miocene (Figure 7). 1.4 cm/kyr (model E) with a mean of 0.8 cm/kyr between chrons 1331 1275 Beneath the unconformity, magnetozones N1 to R7 are C12n and C15n. The highest sedimentation rate is observed 1332 1276 correlated to chrons C15–C9 as follows (Figure 7): Between during C5Cn (following model B) with 5.1 cm/kyr and lowest 1333 1277 315 and 281 mbsf the normal magnetozones N1 and N2 are sediment accumulation rate is observed during Chron C12n 1334 1278 correlated with chrons C15n and C13n, respectively, and reversed 0.7 cm/kyr. 1335 1279 magnetozones R2 and R2 with chrons C13r and C12r, 1336 1280 respectively. This correlation implies a sedimentation rate of 1337 1281 ∼1 cm/kyr for the late Eocene/Early Oligocene interval of the DISCUSSION 1338 1282 core with the Eocene/Oligocene boundary at ∼304 mbsf between 1339 1283 Core 33 and Core 32. Two correlations are considered between We present a new magnetostratigraphic chronology for an upper 1340 1284 280 and 180 mbsf (magnetozones N3–R7; Figure 7). The younger Eocene to Middle Miocene sedimentary section cored during 1341 1285 correlates magnetozones N3. N5, R5, N6, R6, N7 and R7 with DSDP Leg 28 (Site 274) on the continental rise off Victoria Land, 1342 1286 chrons C12n, C11n.2n, C11n.1r, C10n.2n, C10n.1r, C10n.1n and Ross Sea. We confirm a significant hiatus between chrons C9r and 1343 1287 C9r (model D), respectively and implies that the last appearances C5Cr/C5Dr/C5Er, i.e., from the earliest Chattian to the mid 1344 1288 of radiolarians Cyclampterium milowi and Calocyclas semipolita Burdigalian (Figure 7), and its duration is significantly shorter 1345 1289 are almost one million year younger than their currently than previous estimates. 1346 1290 identified last occurrences. The older correlation links Our new age model also identifies a four-fold increase in 1347 1291 magnetozones N5, R5, N6, R6, N7 and R7 with chrons C12n, sedimentation rate on the Antarctic margin after the Eocene/ 1348 1292 C11r, C11n.2n, C11n.1r, C11n.1n and C10r (model E) and has a Oligocene boundary (Figure 7). Previous drilling in McMurdo 1349 1293 smaller lag and consequently more consistent with the radiolarian Sound (southern Victoria Land) also identified a significant early 1350 1294 late occurrence data but leaves magnetozone N3 with no Oligocene unconformity (Wilson et al., 1998) and a significant 1351 1295 corresponding chron in the GPTS. While we expect to see increase in sediment accumulation rates across the Eocene/ 1352 1296 magnetozones truncated and potentially missing from the Oligocene boundary (Wilson et al., 1998; Wilson et al., 2000; 1353 1297 DSDP Site 274 record, magnetozone N3 is represented by Hannah et al., 2001). The increase in sedimentation rate has been 1354 1298 several samples and multiple normal polarity chrons, and we linked to the onset of Antarctic ice sheet growth across the 1355 1299 would not expect to see an additional magnetic polarity interval Eocene/Oligocene boundary (Salamy and Zachos, 1999; 1356 1300 not identified in the GPTS. Regardless, we observe that the Galeotti et al., 2016). We confirm those findings across the 1357 1301 sediment accumulation rate in the mid Oligocene interval of wider Antarctic shelf and slope. The unconformity at 1358 1302 the core (280–180 mbsf) is more than four times that of the lower 180 mbsf, is coincident with other significant Late Oligocene 1359 1303 Eocene-Oligocene boundary interval, with 25% of the time unconformities reported from the Ross Embayment (Florindo 1360 1304 interval accounted for by distributed unconformities as et al., 2005) but extends into the mid Miocene. We relate the long 1361 1305 identified by marked grain size changes (Figures 2, 7). unconformity to be linked to the early evolution of the Antarctic 1362 1306 Above the unconformity at 180 mbsf, the interval between 180 Circumpolar Current which was established in the Late Oligocene 1363 1307 and 120 mbsf (cores 19–14), has few biostratigraphical and intensified into the Miocene after the growth of Antarctic ice 1364 1308 constraints (Hayes et al., 1975, Table 1), and we identify three sheets at the Eocene-Oligocene Boundary (Barker et al., 2007; 1365 1309 possible magnetostratigraphic age model reconstructions: where Lyle et al., 2007; Jovane and Verosub, 2011). Sedimentation did 1366 1310 R8 N8 and upper part of R7 correspond to (A) chrons C5Br, not begin again at DSDP Site 274 until the Antarctic Polar Front 1367 1311 1368

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1369 migrated northward coincident with Mid Miocene Glacial This new age model will allow further studies of this unique core 1426 1370 intensification (Nelson and Cook, 2001). to reconstruct paleoenvironmental studies focusing on the discharge 1427 1371 The unconformity at ca. 180 mbsf could also be partially of sediments from the catchment areas in this region of the Ross Sea. 1428 1372 related to the formation of the Adare Basin, which 1429 1373 accommodates the sediments from the West Antarctica Rift 1430 1374 System and forms a barrier for the arrival of sediments to the DATA AVAILABILITY STATEMENT 1431 1375 site (Granot et al., 2010, 2013). 1432 1376 Regardless of the exact explanation of the unconformity, these The raw data supporting the conclusions of this article will be 1433 1377 insights are invaluable because they demonstrate that the Early made available by the authors, without undue reservation. 1434 1378 Miocene is characterized by long-term fluctuations of the 1435 1379 Antarctic Ice Sheets (Levy et al., 2016; Levy et al., 2019; 1436 1380 Jovane et al., 2019; Evangelinos et al., 2020) and that, prior to AUTHOR CONTRIBUTIONS 1437 1381 the Middle Miocene Climatic Optimum (∼15 Ma), there was a 1438 1382 period of climatic instability modulated by long-term variations LJ: paleomagnetic measurements, data analysis, discussion and 1439 1383 with phases of bottom current intensification which either eroded writing; FF: discussion and writing; DR: paleomagnetic 1440 1384 or did not allow the deposition of sediments in the deep parts of measurements, data analysis, and writing; SL: grain size 1441 1385 the basin. measurements, data analysis, and writing; MH: 1442 1386 paleomagnetic measurements, data analysis, and writing; 1443 1387 GC: radiolaria and writing; GW: data analysis, discussion 1444 1388 CONCLUSION and writing. 1445 1389 1446 1390 We provided new paleomagnetic results from the late Eocene- 1447 1391 Middle Miocene samples from DSDP Site 274, cored during Leg ACKNOWLEDGMENTS 1448 1392 28 on the continental rise off Victoria Land, Ross Sea. 1449 1393 We thank the editor Hagay Amit and the reviewers for useful 1450 1394 - Based on our new age model, the cored late Eocene-Middle comments and corrections which greatly improved the 1451 1395 Miocene sequence covers an interval of almost 20 Myr, from manuscript. The Deep Sea Drilling Project samples and 1452 1396 ∼35 to ∼15 Ma. data were provided by the International Ocean Discovery 1453 1397 - We confirm and better define a major unconformity Program (sample requests: 043328-IODP and 069232- 1454 1398 identified at ∼180 mbsf which represents at least 9 Myr IODP). The samples were collected at the UC Davis 1455 1399 from the earliest Chattian to the mid Burdigalian. It may Paleomagnetism Laboratory as part of NSF grant OPP 06- 1456 1400 represent non-deposition and/or erosion due to 17194 to K. L. Verosub. LJ, MBH and DR acknowledge 1457 1401 intensification of Antarctic Circumpolar Current activity. funding from Fundação de Amparo á Pesquisa do Estado 1458 1402 - Significant fluctuations in grain size and magnetic properties de São Paulo (FAPESP) grant 2016/24946-9 and 2018/17061- 1459 1403 are observed above the unconformity (above 180 mbsf) 6. LJ is also financed by the Coordenação de Aperfeiçoamento 1460 1404 interpreted as cyclical behavior in glacial advance and de Pessoal de Nível Superior - Brasil (CAPES)—Finance Code 1461 1405 retreat from the continent. 001. The data are available from the authors upon request. 1462 1406 1463 1407 1464 1408 REFERENCES Beddow, H. M., Liebrand, D., Sluijs, A., Wade, B. S., and Lourens, L. J. (2016). 1465 1409 Global change across the oligocene-miocene transition: high-resolution stable 1466 isotope records from IODP site U1334 (equatorial pacific ocean): the OMT at 1410 Abrajevitch, A., and Kodama, K. (2011). Diagenetic sensitivity of paleoenvironmental site U1334. Paleoceanography 31, 81–97. doi:10.1002/2015PA002820 1467 1411 proxies: a rock magnetic study of Australian continental margin sediments. Berggren,W.A.,Kent,D.V.,Swisher,C.C.,III.,andAubry,M.-P.(1995).“A 1468 1412 Geochem. Geophys. 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Frontiers in Earth Science | www.frontiersin.org 16 November 2020 | Volume 8 | Article 563453