Neogene tectonic and climatic evolution of the Western Ross Sea, Antarctica — Chronology of events from the AND-1B drill hole

Global and Planetary Change 96–97 (2012) 189–203

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Neogene tectonic and climatic evolution of the Western Ross Sea, Antarctica — Chronology of events from the AND-1B drill hole Gary S. Wilson a, b,⁎, Richard H. Levy c, Tim R. Naish d, c, Ross D. Powell e, Fabio Florindo f, Christian Ohneiser b, Leonardo Sagnotti f, Diane M. Winter g, Rosemary Cody d, Stuart Henrys c, Jake Ross h, Larry Krissek i, j, Frank Niessen k, Massimo Pompillio l, Reed Scherer e, Brent V. Alloway d, Peter J. Barrett d, Stefanie Brachfeld m, Greg Browne c, Lionel Carter d, Ellen Cowan n, James Crampton c, Robert M. DeConto o, Gavin Dunbar d, Nelia Dunbar ai, Robert Dunbar p, Hilmar von Eynatten q, Catalina Gebhardt k, Giovanna Giorgetti r, Ian Graham c, Mike Hannah d, Dhiresh Hansaraj d, David M. Harwood g, Linda Hinnov s, Richard D. Jarrard t, Leah Joseph u, Michelle Kominz v, Gerhard Kuhn k, Philip Kyle h, Andreas Läufer w, William C. McIntosh h, Robert McKay d, Paola Maffioli x, Diana Magens k, Christina Millan i, j, Donata Monien k, Roger Morin y, Timothy Paulsen z, Davide Persico aa, David Pollard ab, J. Ian Raine c, Christina Riesselman ac, Sonia Sandroni r, Doug Schmitt ad, Charlotte Sjunneskog ae, C. Percy Strong c, Franco Talarico r, Marco Taviani af, Giuliana Villa aa, Stefan Vogel e, Tom Wilch ag, Trevor Williams ah, Terry J. Wilson i, j, Sherwood Wise ae a

Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand Department of Geology, University of Otago, PO Box 56, Dunedin, New Zealand c GNS Science, PO Box 30‐368, Lower Hutt, New Zealand d Antarctic Research Centre, Victoria University of Wellington, PO Box 600, Wellington, New Zealand e Department of Geology & Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115, USA f Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, I-00143 Rome, Italy g ANDRILL Science Management Office, Department of Geosciences, University of Nebraska-Lincoln, Lincoln, NE 68588‐0340, USA h New Mexico Institute of Mining & Technology, Earth & Environmental Sciences, Socorro, NM 87801, USA i Byrd Polar Research Centre, The Ohio State University, Columbus, OH 43210, USA j Department of Geological Sciences, The Ohio State University, Columbus, OH 43210, USA k Alfred Wegener Institute, Department of Geosciences, Postfach 12 01 6, Am Alten Hafen 26, D-27515, Bremerhaven, Germany l Istituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola 32, I-56126 Pisa, Italy m Department of Earth and Environmental Studies, Montclair State University, Montclair, NJ 07043, USA n Department of Geology, Appalachian State University, Boone, NC 28608‐2067, USA o Department of Geosciences, University of Massachusetts, Amherst, MA 01003‐9297, USA p Department of Environmental Earth System Sciences, School of Earth Sciences, Stanford University, Stanford, CA 94305, USA q Department of Sedimentology and Environmental Geology, Geoscience Center Göttingen (GZG), Goldschmidtstrasse 3, Göttingen, Germany r Dipartimento di Scienze della Terra, Universita di Sienna, Via Laterina 8, I-53100, Sienna, Italy s Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA t Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, USA u Environmental Studies Program, Ursinus College, Collegeville, PA 19426, USA v Department of Geology, Western Michigan University, Kalamazoo, MI 49008, USA w Federal Institute for Geosciences & Natural Resources, BGR, Stilleweg 2, D-30655 Hannover, Germany x Università Milano-Bicocca, Dipartimento di Scienze Geologiche e Geotecnologie, Piazza della Scienza 4, I-20126 Milano, Italy y US Geological Survey, Mail Stop 403, Denver Federal Center, Denver, CO 80225, USA z Department of Geology, University of Wisconsin, Oshkosh, 800 WI 54901, USA aa Departimento di Scienze della Terra, Universita di Parma, Parco Aeres delle Scienze, 157 Parma, Italy ab Earth and Environmental Systems Institute, 2217 Earth-Engineering Science Bldg, University Park, PA 16802, USA ac Department of Geological and Environmental Sciences, School of Earth Sciences, Stanford University, Stanford, CA 94305, USA ad Department of Physics, Mailstop #615, University of Alberta, Edmonton, Alberta, Canada T6G 2G7 ae Antarctic Marine Geology Research Facility, Department of Geology, Florida State University, Tallahassee, FL 32306, USA af CNR, ISMAR — Bologna, Via Gobetti 101, I-40129 Bologna, Italy ag Albion College, Department of Geology, Albion, MI 49224, USA ah Columbia University, Lamont-Doherty Earth Observatory, Palisades, NY 10964, USA ai New Mexico Beurau of Geology and Mineral Resources, 801 Leroy Place, Socorro, NM 87801, USA b

⁎ Corresponding author at: Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand. 0921-8181/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2012.05.019

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a r t i c l e

G.S. Wilson et al. / Global and Planetary Change 96–97 (2012) 189–203

i n f o

Available online 7 June 2012 Keywords: Stratigraphic Drilling McMurdo Ice Shelf Chronostratigraphy Neogene Tectonics Ice Sheet history

a b s t r a c t Stratigraphic drilling from the McMurdo Ice Shelf in the 2006/2007 austral summer recovered a 1284.87 m sedimentary succession from beneath the sea floor. Key age data for the core include magnetic polarity stratigraphy for the entire succession, diatom biostratigraphy for the upper 600 m and 40Ar/39Ar ages for in-situ volcanic deposits as well as reworked volcanic clasts. A vertical seismic profile for the drill hole allows correlation between the drill hole and a regional seismic network and inference of age constraint by correlation with well‐dated regional volcanic events through direct recognition of interlayered volcanic deposits as well as by inference from flexural loading of pre‐existing strata. The combined age model implies relatively rapid (1 m/2–5 ky) accumulation of sediment punctuated by hiatuses, which account for approximately 50% of the record. Three of the longer hiatuses coincide with basin‐wide seismic reflectors and, along with two thick volcanic intervals, they subdivide the succession into seven chronostratigraphic intervals with characteristic facies: 1. The base of the cored succession (1275–1220 mbsf) comprises middle Miocene volcaniclastic sandstone dated at approx 13.5 Ma by several reworked volcanic clasts; 2. A late-Miocene sub-polar orbitally controlled glacial–interglacial succession (1220–760 mbsf) bounded by two unconformities correlated with basin‐wide reflectors associated with early development of the terror rift; 3. A late Miocene volcanigenic succession (760–596 mbsf) terminating with a ~ 1 my hiatus at 596.35 mbsf which spans the Miocene–Pliocene boundary and is not recognised in regional seismic data; 4. An early Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (590–440 mbsf), separated from; 5. A late Pliocene obliquity-controlled alternating diamictite and diatomite glacial–interglacial succession (440–150 mbsf) by a 750 ky unconformity interpreted to represent a major sequence boundary at other locations; 6. An early Pleistocene interbedded volcanic, diamictite and diatomite succession (150–80 mbsf), and; 7. A late Pleistocene glacigene succession (80–0 mbsf) comprising diamictite dominated sedimentary cycles deposited in a polar environment. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Interpretation of the late Neogene glacial and climatic history from terrestrial outcrops on the Antarctic continent is fraught with difficulty. Individual outcrops are temporally and spatially limited and usually preserve single events reflecting extreme warmth and glacial retreat or extreme cold and glacial advance (Denton et al., 1983; 1984; Webb et al., 1984; Webb and Harwood, 1991; Kennett and Hodell, 1993; Wilson, 1995; Marchant and Denton, 1996; Sugden and Denton, 2004; Lewis et al., 2010). This has led to conflicts in piecing together the Antarctica's Neogene history as broad interpretation of a single site seems to preclude alternative implications from other sites of similar age (Stroeven and Kleman, 1989; Barrett et al., 1992; Brook et al., 1995; Ivy-Ochs et al., 1995; Barrett et al., 1997; Wilson et al., 2002a). Thus, interpretations of Antarctic glacial history have relied on deep-sea benthic δ 18O records, which reflect both global deep-sea temperatures and ice volume variability. The δ 18O record implies a profound cooling in the middle Miocene (~14 Ma; Shackleton et al., 1993; Zachos et al., 2001), which has been correlated with Antarctic terrestrial evidence implying extinction of the biome at that time with persistence of polar conditions (Lewis et al., 2010). However, paired Mg/Ca ratios indicate a later cooling (~11 Ma; Billups and Schrag, 2002) with subsequent rewarming and the δ 18O record still shows obliquity-controlled variability throughout the Pliocene warm interval and prior to the onset of Northern Hemisphere glaciation (Lisiecki and Raymo, 2005), implying glacial/interglacial variability in the Antarctic ice sheets equivalent to 30% of the ice mass (Naish and Wilson, 2009). The AND-1B drill hole is the first in a series of holes to be drilled in McMurdo Sound under the ANDRILL programme (Wilson et al., this volume; Figs. 1 and 2), with the aim of recovering a record that spans the Neogene. The basins targeted for drilling by the programme have sufficient accommodation space that records are likely to have been preserved from erosion by grounded ice, even during peak glaciations of the LGM (Stuiver et al., 1981; Anderson et al., 2002; Denton and Hughes, 2002). Earlier drill holes at the western margin

of the Ross Sea targeted the pre-Neogene succession (Figs. 1 and 2; Barrett et al., 1987; Wilson et al., 1998; Florindo et al., 2005) and drill holes from New Harbour had recovered Pliocene to recent valley sediments (Fig. 1; Barrett and Hambrey, 1992; Ishman and Rieck, 2002; Levy et al., 2012; Ohneiser and Wilson, this volume; Winter et al., this volume). The AND-1B drill hole project was strategically placed to recover a history of the McMurdo/Ross Ice Shelf and the West Antarctic Ice Sheet, which is the more vulnerable component of the system (Naish et al., 2007a; Rignot et al., 2008). 2. Setting The AND-1B drill core was recovered from beneath the McMurdo Ice Shelf, 9 km from New Zealand's Scott Base (Fig. 1). The McMurdo Ice Shelf is an extension of the Ross Ice Shelf into southern McMurdo Sound between White and Black islands to the south and Ross Island to the north. At the drill site, the ice shelf is ~ 82 m thick and moving westwards at 30 cm/day (Falconer et al., 2007). McMurdo Sound is, on average, around 600 m deep, but in the vicinity of Ross Island, including where the AND-1B site (917 m water depth) is located, it plunges to between 900 m and 1000 m in depth in a moat surrounding the island (Fig. 1). Geologically, McMurdo Sound lies at the southern end of the Victoria Land Basin, an eastward dipping and thickening half-graben, which accommodates several kilometres of strata. The Victoria Land Basin lies within the West Antarctic Rift and much of the strata in the basin relate to the main phase of rifting in the Late Eocene–Oligocene (Fielding, 2008). The upper kilometre of the strata is Neogene in age and is related to renewed rifting associated with the Terror Rift (Cooper et al., 1987; Fielding, 2008). Neogene volcanism in the Erebus Volcanic Province is also linked to the Terror Rift, with quasi-periodic eruptive phases from Mount Morning (~19 Ma; Armstrong, 1978) to Mount Erebus (1.3 Ma to present; Esser et al., 2004; Fig. 1). The heat associated with the volcanism as well as the rift itself resulted in weakened crust (elastic thickness, Te = 2 to 5 km; Aitken et al., 2012), which is modelled to have resulted in ~ 500 m of accommodation space associated with Ross

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191

Fig. 1. Location of the key features discussed in the text. AND-1B = the ANDRILL McMurdo Ice Shelf (SMS) Project drill hole, AND-2A = the ANDRILL Southern McMurdo Sound Project drill hole (Harwood and Florindo, 2009), DVDP-10 and ‐11 = Dry Valley Drilling Project drill holes 10 and 11, CIROS-1 and ‐2 = Cenozoic Investigations in the Ross Sea drill holes 1 and 2, MSSTS-1 = McMurdo Sound Sediment and Tectonic Studies drill hole 1, CRP-1, -2 and ‐3 = Cape Roberts Project drill holes 1, 2 and 3, B = Mount Bird, BI = Black Island, D = Mount Discovery, E = Mount Erebus, M = Mount Morning, MB = Minna Bluff, T = Mount Terror, W = White Island, blue line = coastline.

3. AND-1B lithostratigraphy

Island loading, predominantly mounts Terror (>1.75 Ma) and Erebus (b1.3 Ma; Aitken et al., 2012). The prognosis for drilling was that the upper part of the succession was likely to be deposited in relatively deepwater in the Ross Island flexural moat and that the lower part of the succession would relate to more modest depths and rates of accommodation space development associated with Terror Rift subsidence (Horgan et al., 2005; Naish et al., 2007a).

The 1285 m AND-1B drill core can be subdivided according to rock type and arrangement of rock types in interbedded successions (Fig. 3). Diamictite is the most common rock type in the core. It is variably clast-rich to clast-poor and stratified to massive with sandy or muddy matrix. Where massive and overlying a zone of deformed

Victoria Land Basin AND-2A

Transantarctic Mountains Pre

West

Neo

gen

Terror Rift

e Ri

Hut Point Peninsula

AND-1B

ft St

rata

East

Fig. 2. Schematic structural–stratigraphic cross section across the Victoria Land Basin showing the stratigraphic extent of the AND-1B drill hole (after Naish et al., 2007a). The crosssection is defined from interpretations of seismic reflection data integrated with previous drill hole data (Cooper et al., 1987; Fielding et al., 2008). Coloured surfaces are regionally mapped disconformity surfaces in the Neogene Victoria Land Basin; green = the mid-Miocene Rh reflector of Fielding et al. (2008). AND-2A — the ANDRILL Southern McMurdo Sound (SMS) Project drill hole (Harwood and Florindo, 2009).

Depth (mbsf)

LSU

G.S. Wilson et al. / Global and Planetary Change 96–97 (2012) 189–203

Age

192

Pleistocene

0

1

100

3 300

400

Pliocene

Diamictite cycles with thin mudstone interbeds, interpreted to represent a subglacial environment with little or no subglacial melting

2

200

Upper Succession

Mud Sand Gravel

Diamictite / Diatomite cycles, interpreted to represent varying subglacial to open marine environments (G/I cycles) with limited subglacial melting

Diatomite with some IRD, interpreted to represent open marine environment

4 500

600

5

Volcanics 700

900

Miocene

Lower Succession

800

6

cycles, interpreted to represent varying subglacial to open marine environments (G/I cycles) with subglacial melting

1000

1100

1200

7

Diamictite cycles with thin mudstone interbeds, interpreted to represent a subglacial environment with little or no subglacial melting

glacimarine facies, the diamict is interpreted to represent subglacial deposition and grounding of the ice shelf on the sea floor (Krissek et al., 2007; McKay et al., 2012). Where stratified, the diamict may reflect a range of environments from proximal to the grounding line to distal settings where clasts originate from iceberg rain out. Diatomite is common in the upper 600 m of the drill core. It is generally thinly laminated to bioturbated with occasional dispersed clasts and intraclasts and interpreted to represent pelagic sedimentation. Upper sections are often sheared by emplacement of the overlying diamictite. Mudstone is common in the lower 500 m of the drill core and is generally fine-grained variably massive to stratified, and occasionally interstratified with sandstone. The mudstone is interpreted, generally, to reflect more distal glaciomarine environments, although deposits can be found in proximal settings where sediment gravity flows or settling is involved (Krissek et al., 2007). More than 50 disconformities interpreted to represent glacial surfaces of erosion were observed in the AND-1B drill core. They are recognised by the juxtaposition of facies types at distinctive surfaces, sometimes with rip-up clasts in overlying units and/or angular truncation and deformation of underlying units (Krissek et al., 2007). The upper 85 m of the drill core is predominantly diamict and glacial surfaces of erosion are generally recognised by deformation and truncation of underlying units (McKay et al., 2012). Likewise, the interval between 1063 mbsf and 1220 mbsf is also predominantly diamict. Both of these intervals are interpreted to represent deposition in polar conditions beneath a grounded marine ice sheet in the Ross Embayment during glacials with conditions cold enough to allow for an expansive ice shelf during interglacials (McKay et al., 2009; 2012). The AND-1B succession is divided into upper and lower successions by an expanded 169 m thick volcanic interval between 590 mbsf and 759 mbsf (Fig. 3). DiRoberto et al. (2010) subdivided the interval into (1) a lower sequence attributed to epiclastic gravity flow turbidite processes consistent with abundant active volcanism that occurred at a site distal with respect to the AND-1B drill site, and (2) an upper subsequence comprising mainly interbedded tuff, lapilli tuff, volcanic diamictite and lava flow, attributed to recurring phases of submarine to emergent volcanic activity that occurred proximal to the AND-1B drill site. Above the volcanic succession, the AND-1B record is represented by sedimentary cycles of variable thickness that comprise a basal massive diamict, which gives way to a grounding line retreat succession of variably stratified sands and muds with dispersed clasts overlain by an open-water diatomite. Cycles vary between 10 and 25 m in thickness and are separated by glacial surfaces of erosion (Krissek et al., 2007; McKay et al., 2012). They are interpreted to reflect glacial/interglacial advance and retreat in a subpolar to polar environment with sediment starved conditions during interglacials (McKay et al., 2009; 2012). An 83-m thick diatomite unit reflects an expanded period of open ocean deposition between 376 mbsf and 459 mbsf. A thin interval of volcaniclastic sediment between 438 and 440 mbsf may subdivide the thick diatomite into two units. Below the 590–759 mbsf volcanic succession, the record is represented by unconformity bounded sedimentary cycles of variable thickness that comprise diamictite and mudstone facies, which differ from the cycles above the 590–759 mbsf volcanic succession in the absence of diatomite. These cycles are interpreted to represent a subpolar environment with significant melt water and terrigenous sediment delivery during interglacial conditions, similar to facies seen today in Greenland (McKay et al., 2009; 2012). 4. Chronostratigraphic data

Volcanics

4.1. Palaeomagnetism

8

Fig. 3. Lithostratigraphy of the AND-1B drill core. Lithostratigraphic units (LSU) after Krissek et al. (2007). Orange = volcanic sandstone, mudstone; green = diamictite; grey = terrigenous siltstone and sandstone; yellow = diatomite. Timescale based on the GPTS of Ogg and Smith (2004), adjusted to Gibbard et al. (2010).

Discrete palaeomagnetic samples were collected on average at 1-m spacing from the 1285 m length of the AND-1B drill core (Wilson et al., 2007a). Where possible, samples were taken from fine-grained lithologies, but where samples were taken from diamictite or other coarse

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units care was taken to sample the matrix and avoid clasts. Samples were subject to stepwise thermal demagnetisation at 50 °C increments to a peak temperature of 650 °C following a pilot study undertaken on paired samples taken every ~10 m down-core (Wilson et al., 2007a; Fig. 4A–E). Most samples exhibited a drilling induced overprint, either as a strong vertical (Fig. 4A and B) or radial low unblocking temperature component (Wilson et al., 2007a). The characteristic remanent magnetism (ChRM) was usually identified between 200 °C and 600 °C on vector component diagrams (Wilson et al., 2007a; Fig. 4).

A magnetic polarity stratigraphy was constructed for the AND-1B drill core from all stepwise-demagnetised samples where a characteristic remanence could be isolated (990 out of 1309; 76% of samples; Fig. 5). Characteristic remanences could not be isolated in the remainder of the samples either because they contained clasts or were too coarse grained to yield a ChRM (Fig. 4E and F; Wilson et al., 2007a). Samples between 676 mbsf and 683 mbsf were not included in the analysis as they were taken from a mudstone breccia. Magnetozones were assigned where contiguous samples had similar inclinations in line with the expected Geocentric Axial Dipole (GAD) field, taking into

A) AND-1B, 690 metres, silty claystone N,U 4

B) AND-1B, 767 metres, clayey siltstone N,U 1

N,U Thermal demagnetisation 5 NRM: 4.07 Am x10 -2 PCA Dec: 221.57° PCA Inc: -73.74° MAD: 5.44°

AF demagnetisation NRM: 3.88 Am x10-2 PCA Dec: 176.64° PCA Inc: -82.91° MAD: 2.73°

193

N,U 6

AF NRM: 9.25 Am x10

Thermal

-2

NRM: 6.06 Am x10-2 PCA Dec: 146.79° PCA Inc: 67.98° MAD: 3.86°

2 vertical horizontal

4

5

Am x10 -2

Am x10 -2

vertical horizontal

E,N 0.2 1

7

700 Temperature (C°)

100 AF strength (mT)

100 AF strength (mT)

N,U 20

AF NRM: 1.43 Am x10-1 PCA Dec: 204.12° PCA Inc: -74.99° MAD: 1.82°

10 5 W,S

Thermal

N,U 10

NRM: 1.68 Am x10 -1 PCA Dec: 161.72° PCA Inc: -70.18° MAD: 5.79°

E,N 5

vertical horizontal

700 Temperature (C°)

D) AND-1B, 999 metres, silty claystone

C) AND-1B, 867 metres, silty claystone N,U 15

vertical horizontal

Am x10 -2

E,N 1

Am x10 -2

vertical horizontal

E,N 0.5

E,N

AF NRM: 8.98 Am x10-2 PCA Dec: 313.76° PCA Inc: -73.62° MAD: 5.14°

vertical horizontal

N,U 8

vertical horizontal

E,N 2

Thermal NRM: 6.55 Am x10-2 PCA Dec: 308.72° PCA Inc: -82.63° MAD: 4.95°

vertical horizontal

E,N 2

700 Temperature (C°)

E) AND-1B, 1192 metres, silty claystone AF NRM: 6.91 Am x10-2 PCA Dec: 119.59° PCA Inc: -84.72° MAD: 3.71°

vertical horizontal

E,N 2

N,U 8

Thermal

vertical horizontal

E,N 2

W,S

N,U

Thermal NRM: 2.45 Am x10-4

5

20

700 Temperature (C°)

700 Temperature (C°)

G) AND-1B, 720 metres, siltstone N,U 20

Thermal NRM: 1.98 Am x10-2

vertical horizontal

E,N 5 2

2.5

Am x10 -2

Am x10-2

100 AF strength (mT)

20

NRM: 6.40 Am x10-2 PCA Dec: 171.76° PCA Inc: -85.73° MAD: 4.95°

8

8

F) AND-1B, 1091 metres, diamict

Am x10-4

N,U 8

100 AF strength (mT)

Am x10-2

100 AF strength (mT)

8 Am x10-2

1 Am x10-1

Am x10-1

2 Am x10-1

10 1.5

700 Temperature (C°)

700 Temperature (C°)

Fig. 4. Orthogonal vector component diagrams of demagnetisation of representative samples from the 690–1285 m interval of the AND-1B drillcore: alternating and thermally demagnetised samples from 690 mbsf (A); 767 mbsf (B); 867 mbsf (C); 999 mbsf (D); 1192 mbsf (E); and thermally demagnetised samples from 1091 mbsf (F) and 720 mbsf (G).

194

G.S. Wilson et al. / Global and Planetary Change 96–97 (2012) 189–203

Mag Sus (SI)

NRM Int (A/m) 0 0.05 0.1 0.15 0.2 0.25 0.3

NRM Inc (°) -90 -60 -30

0

ChRM Inc (°)

30 60 90

0

N1

51.69 73.09 91.42

55.51 84.97

R1 191.75 200

N2 251.67

R2 346.13

N3

400

R3

439.96 452.88 459.19 517.34

N4 596.35 600

Depth (mbsf)

R4

630.12 637.52 654.33

N5 R5 N6

R6

800

N7

706.70 725.35 733.48

749.47

815.60 827.83 853.09 867.37 878.12 895.72 903.60 945.37 948.69 979.53

1000

R7

1036.38 1049.66 1061.05 1073.76 1112.36

N8 1200

R8

1400

0

0.005 0.01 0.015 0.02

-90 -60 -30

0

1270.20

30 60 90

Fig. 5. Downcore variations in magnetic properties and log of magnetic polarity zonation for the AND-1B drill core. NRM Int = intensity of natural remanent magnetisation; mag sus = low-frequency magnetic susceptibility; NRM Inc = inclination of natural remanent magnetisation; ChRM Inc = inclination of characteristic remanent magnetisation after sample demagnetisation. For polarity log: black = normal polarity, white = reversed polarity. Magnetozones are defined by several contiguous samples of the same polarity and discrete samples of differing inclination are not included in the analysis (see text for discussion). Refer to figure 3 for the lithology key and description.

G.S. Wilson et al. / Global and Planetary Change 96–97 (2012) 189–203

account secular variation for the AND-1B site (Fig. 5). Magnetozone boundaries are placed at the midpoint between samples of opposite polarity unless an obvious stratigraphic discontinuity or Glacial Surface of Erosion (GSE) separates magnetozones, in which case the boundary was placed at the disconformity. Normal and reversed magnetozones are generally equally distributed in the AND-1B record (Fig. 5) with no correlation between polarity and lithology type. While it is not possible to conduct a true reversals test, normal and reversed characteristic inclinations (uncorrected for stratal tilt) are equally distributed throughout the core (Fig. 6). Wilson et al. (2007a, 2007b) presented a preliminary magnetostratigraphy for the upper 700 m of the AND-1B core. Here we revise that magneto stratigraphy and extend it to the base of the core (1284.87 mbsf). In the upper 700 m, additional samples have allowed us to infill intervals where sampling gaps prevented the assignment of polarity in the preliminary age model and to refine the depth of reversal boundaries. For ease of description, magnetozones are numbered down core according to the predominant polarity.

A) NRM inclination

Number of samples

GAD

300

GAD

350

250

200

150

100

-80

-60

-40

-20

0

20

40

60

80

40

60

80

Inclination (°)

B) ChRM Inclination

Number of samples

GAD

GAD

150

100

50

0

Poor core recovery and poor core condition in the upper 32 m of the core prevented the collection of additional samples. However, from 40 Ar/39Ar results (Ross et al., this volume), we assume that the upper 73.09 m of the core (Magnetozone N1) represents the Brunhes Normal Chron. A thin reversed polarity interval represented by only two samples between 51.69 and 55.51 mbsf is interpreted to reflect a short polarity excursion. Reversed Magnetozone R1 (73.09–191.75 mbsf) includes a thin Normal Polarity interval tightly constrained by multiple samples. Normal Magnetozone N2 is the same as Magnetozone N3 in Wilson et al. (2007a) with additional samples allowing better definition of the base of the magnetozone at 251.67 mbsf. Additional samples have infilled sampling gaps in the interval between 350 and 450 mbsf demonstrating that normal Magnetozone N3 (N4 in Wilson et al., 2007a) is a single magnetozone and refining the base of the magnetozone slightly to 439.96 mbsf. The upper boundary of normal Magnetozone N4 (517.34) is also slightly refined from Wilson et al. (2007a; Magnetozone N6). The lowest magnetozone (N8) identified in Wilson et al. (2007a) labelled Magnetozone N5 here is now extended to 706.70 mbsf. Magnetozones R5 (706.70–725.35 mbsf) and N6 (725.35– 749.47 mbsf) lie within the lower part of the 590–759 mbsf volcanic interval with some variability in inclination directions from the distributed coarse volcaniclastic sediments. Between 749.47 and 853.09 mbsf polarity is predominantly reversed (Mag-netozone R6) with a thin interval of normal polarity between 815.60 and 827.83 mbsf. Similarly, normal polarity Magnetozone N7 (853.09– 979.53 mbsf) contains two thin reversed polarity intervals (867.37–878.12 mbsf and 945.37–948.69 mbsf, respectively) and reversed polarity Magnetozone R7 contains two thin normal polarity intervals (1036.38–1049.66 mbsf and 1061.05–1073.76 mbsf, respectively). Towards the base of the core, a distinctive thick normal polarity magnetozone (N8) extends between 1112.36 and 1270.20 mbsf. The basal 14.67 m of the AND-1B core (1270.20– 1284.87 mbsf) is reversed polarity Magnetozone R8. 4.2. Diatom biostratigraphy

50

0

195

-80

-60

-40

-20

0

20

Inclination (°) Fig. 6. Histograms of NRM and ChRM palaeomagnetic inclinations without correction for stratal tilt for the AND‐1B drillcore. Median inclinations are lower than expected at the latitude of the AND-1B drill site (78°S; ±83° inclination). The slight shallowing may be due to the borehole not being vertical or alternatively to tilted of the strata. The lower-angle ChRM inclinations generally relate to samples from transitional polarity as well as discrete samples with opposing polarity from the magnetozone they lie within.

The upper 586 m of the AND-1B drill core contains multiple diatomaceous units that in turn contain a diverse and abundant diatom flora (Scherer et al., 2007; Sjunneskog and Winter, this volume; Winter et al., this volume). Interstratified diamictite units were mostly barren but occasionally yielded reworked forms. Thus many highest and lowest occurrence diatom datums in the AND-1B drill core are local occurrences, may be facies controlled, and are not true (regional) first and last appearances. In a few cases, individual diatomaceous units comprise assemblages of taxa that range through the entire unit. These diatom assemblage units offer biostratigraphic age control (Winter et al., this volume) but provide poorer constraints than horizons that contain lowest and highest occurrences. Productive intervals all occur above 586 mbsf (Fig. 7), with units of significant thickness and generally greater number of lowest and highest occurrences occurring between 150 mbsf and 586 mbsf. Only one thin interval (58.15–58.90 mbsf) was productive in the upper ~ 85 m of the drill core and interpreted to be more distal from the ice margin (McKay et al., 2012). Ages of regional first and last appearances of individual taxa (Table 1) were derived from an updated comprehensive integrated database of Southern Ocean occurrences via constrained optimisation (e.g. Sadler, 2004, 2007; Cody et al., 2008). Cody et al. (this volume) conducted a CONOP-based analysis to include chronostratigraphic data from AND-1B. Three new southern ocean age models were produced for diatom events and include updated average and total range age models and a new hybrid range model (Cody et al., this volume). These new age models were applied to the AND-1B drill core to constrain a correlation envelope within which the final line of correlation (this paper) plots. Each new age model was established through four iterative CONOP runs, during which all results converged toward the hybrid model. Therefore,

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Pleistocene

51.69 73.09 91.42

191.75

251.67

55.51 84.97

A1 A A2 D1 P1 D3 A3 D4 U1 D2 P2 2 P3 A4 U2 D5 A5 D6 D10 D7 D8 D9 D11 P4 D12-15 P5 D17

U3 U4 U5 U6

D16 D18

346.13

P6

439.96 452.88 459.19

D19 7 P7 U7 P8 D20 P9 D21 D22 A6

517.34

D23 D25 596.35 630.12 637.52 654.33

P10 D24 D26 U8

P11 A7

706.70 725.35 749.47

815.60 827.83

U9 A8

853.09 867.37 878.12

945.37 948.69 979.53

1036.38 1049.66 1061.05 1073.76 1112.36

1270.20

A9

U10

Fig. 7. Age model for the AND-1B drill core based on diatom lowest and highest occurrences (D), 40Ar/39Ar ages on in-situ and reworked volcanic material (A) and magnetic polarity reversal stratigraphy (P or +). Individual datums are outlined in Table 1. Solid grey line represents unique correlation. Dashed grey line represents mean sediment accumulation rate based on polarity reversal stratigraphy only. Timescale based on the GPTS of Ogg and Smith (2004), adjusted to Gibbard et al. (2010). Refer to figure 3 for the lithology key and description.

diatom event ages derived from the hybrid model are utilised for the final AND-1B age model (Table 1). The CONOP analysis also identified a hiatus of ~800 ky at ~440 mbsf not recognised in the preliminary age model (Wilson et al., 2007a), co-incident with a thin volcaniclastic horizon (Fig. 7). 4.3.

40

Ar/ 39Ar ages

The AND-1B drill core contains primary volcanic deposits (lava, lapilli tuff and tuff) at five separate horizons (Table 1). The volcanic interval that separates the core into upper and lower successions (unit 5; 583–759 mbsf) contains a 2.81 m thick subaqueous lava

flow at 646.49–649.30 mbsf (Pompilio et al., 2007), which yielded a 6.48 ± 0.13 Ma age (Ross et al., this volume). In the upper succession, a lapilli tuff at 85.27–85.87 mbsf yielded an inverse isochron age of 1.1014 ± 0.008 Ma and basaltic tephra at 112.02– 112.97 mbsf and 136.12–137.22 mbsf yielded isochron ages of 1.633 ± 0.057 Ma and 1.683 ± 0.055 Ma, respectively (Ross et al., this volume). Volcanic clasts, while reworked into the succession, afford maximum age constraint for the units in which they occur (Table 1; Fig. 7). In the upper succession, volcanic clasts at ~17 mbsf and ~53 mbsf in subunit 1.1 yielded ages of 0.310±0.039 Ma and 0.726± 0.052 Ma, respectively, and a pumiceous clast ~481 mbsf yielded an age of 4.800 ±0.076 Ma for

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197

Table 1 Chronostratigraphic data for the AND-1B drill core; A = 40Ar/39Ar ages; D = diatom datums; LO = lowest occurrence, HO = highest occurrence; P = palaeomagnetic datum; GSE = glacial surface of erosion (as identified by Krissek et al., 2007 and McKay et al., 2012); U = non-genetic angular unconformity. Depths for 40Ar/39Ar ages are sample depths. Depths for glacial surfaces of erosion and other unconformities are observed depths. Depth for diatom datums is the sample that the species either first appeared in (LO) or last appeared in (HO). Depth of palaeomagnetic datums is the midpoint between samples of opposing polarity, unless co-incident with an unconformity or GSE, bracketed ages indicate correlation to GPTS by polarity matching only. Diatom errors reported are from Cody et al. (2008) and magnetic datum errors are from Ogg and Smith (2004). Event

Datum

Depth (mbsf)

Age (Ma)

Error (+/− Ma)

Source

A1 A2 D1 P1 P2 A3 D2 D3 D4 P3 U1 U2 A4 A5 D5 D6 U3 D7 D8 U4 D9 D10 D11 P4 D12 D13 D14 D15 U5 U6 P5 D16 D17 D18 P6 D19 P7 U7 D20 D21 D22 P8 P9 A6 P10 D23 D24 D25 D26 P11 U8 (P12) (P13) A7 (P14) (P15) (P16) (P17) U9 (P18) A8 (P19) (P20) (P21) (P22) (P23) (P24) U10 A9

40

17.17 52.8 56.67 73.09 84.97 85.53 86.9 86.9 86.9 91.13 99.58 109.42 112.51 136.21 149.6 150.7 150.73 151.3 158.9 163.65 164.1 169.8 189 191.75 193.7 194.96 200.7 201.4 211.40 233.48 251.67 251.76 290.74 290.74 346.13 437.59 439.96 440 440.12 440.45 456 452.86 459.19 481.8 517.34 570.13 574.04 583.64 583.64 596.35 596.35 630.12 637.52 648.37 654.33 706.7 725.35 749.47 763.12 815.6 822.78 827.83 853.09 979.53 1036.38 1112.36 1270.2 1274.00 1279.02

≤ 0.31 ≤ 0.726 ≥ 0.21 0.781 0.988 1.014 ≥ 0.5 ≥ 1.54 ≥ 1.6 1.072 > 1.08 b 1.63 1.63 1.68 ≥ 1.83 ≥ 1.86 1.69–2.48 ≥ 1.89 ≥ 1.96 1.945–2.20 2.23 1.98 ≥ 2.73 2.581 ≤ 2.66 ≤ 2.77 ≤ 2.81 ≤ 2.85 > 2.64 b 2.93 3.032 ≥ 2.97 ≤3 ≤ 3.5 3.33 ≤ 3.86 3.596 3.60–4.34 ≤ 3.57 ≥ 3.55 ≤ 4.7 4.493 4.631 4.8 4.799 ≤ 4.77 4.98 4.96 5.5 ≤ 4.896 4.896–5.78 6.033 6.252 6.48 6.436 6.733 7.14 7.212 7.26–7.81 8.254 8.53 8.3 8.769 9.098 9.312 9.987 11.04 10.82–b 13.57 ≤ 13.57

0.039 0.052 0.14 0.001 0.001 0.008

Ross et al. (this volume) Ross et al. (this volume) Cody et al. (this volume) This paper This paper Ross et al. (this volume) Cody et al. (this volume) Cody et al. (this volume) Cody et al. (this volume) This paper Krissek et al. (2007), this paper Krissek et al. (2007), McKay et al. (2009), this paper Ross et al. (this volume) Ross et al. (this volume) Cody et al. (this volume) Cody et al. (this volume) Krissek et al. (2007), McKay et al. (2009), this paper Cody et al. (this volume) Cody et al. (this volume) McKay et al. (2009), this paper Cody et al. (this volume) Cody et al. (this volume) Cody et al. (this volume) This paper Cody et al. (this volume) Cody et al. (this volume) Cody et al. (this volume) Cody et al. (this volume) Krissek et al. (2007), this paper Krissek et al. (2007), this paper This paper Cody et al. (this volume) Cody et al. (this volume) Cody et al. (this volume) This paper Cody et al. (this volume) This paper Krissek et al. (2007), this paper Cody et al. (this volume) Cody et al. (this volume) Cody et al. (this volume) This paper This paper Ross et al. (this volume) This paper Cody et al. (this volume) Cody et al. (this volume) Cody et al. (this volume) Cody et al. (this volume) This paper Krissek et al. (2007), this paper This paper This paper Ross et al. (this volume) This paper This paper This paper This paper Krissek et al. (2007), this volume This paper Ross et al. (this volume) This paper This paper This paper This paper This paper This paper Krissek et al. (2007), this volume Ross et al. (this volume)

Ar/39Ar, max age from clast Ar/39Ar, max age from clast HO Rouxia leventerae Brunhes-Matuyama transition Top Jaramillo Subchron 40 Ar/39Ar, K-feldspar HO Actinocyclus ingens HO Thalassiosira torokina HO Actinocyclus karstenii Base Jaramillo Subchron Glacial surface of erosion Glacial surface of erosion 40 Ar/39Ar, basaltic glass 40 Ar/39Ar, basaltic glass HO Thalassiosira inura HO Thalassiosira kolbei Glacial surface of erosion HO Thalassiosira webbi HO Actinocyclus fasciculatus Glacial surface of erosion HO Actinocyclus maccollumii HO Thalassiosira vulnifica HO Synedropsis creanii Gauss–Matuyama transition LO Actinocyclus fasciculatus LO Rouxia leventerae LO Shionodiscus tetraoestrupii var. reimeri LO Actinocyclus maccollumii Glacial surface of erosion Glacial surface of erosion Top Kaena Subchron HO Fragilariopsis praeinterfrigidaria LO Thalassiosira vulnifica LO Thalassiosira elliptipora Base of Mammoth Subchron HO Fragilariopsis interfrigidaria Gilbert–Gauss transition Unconformity LO Rhizosolenia harwoodii HO Fragilariopsis fossilis LO Thalassiosira striata Top Nunivak Subchron Base Nunivak Subchron 40 Ar/39Ar, K-feldspar Top Sidufjall Subchron LO Fragilariopsis curta LO Fragilariopsis praeinterfrigidaria LO Thalassiosira complicata LO Shinodiscus tetraoestrupii Base Sidufjall Subchron Unconformity Top C3An.1n Base C3An.1n 40 Ar/39Ar age from groundmass concentrate Top C3An.2n Base C3An.2n Top C3Bn Base C3Bn Glacial surface of erosion Top C4r.1n 40 Ar/39Ar, K-feldspar Base C4r.1n Top C4An Base C4An Top C4Ar.1n Top C5n.2n Base C5n.2n Unconformity 40 Ar/39Ar, max age from clast 40

0.02 0.03 0.001

0.06 0.06 0.15 0.14 0.13 0.09 0.03 0.12 0.02 0.001 0.02 0.01 0.02

0.001 0.03 0.01 0.01 0.001 0.01 0.001 0.01 0.01 0.03 0.001 0.001 0.076 0.001 0.03

0.001

0.13

0.53

0.13

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subunit 4.2. In the lower succession, a volcanic clast at ~823 mbsf provides a maximum age of 8.53±0.51 Ma for subunit 6.1, and three volcanic clasts yield a maximum depositional age of 13.57 ±0.13 for the base of the AND-1B drill core (Ross et al., this volume). 5. Integrated chronostratigraphy of the AND-1B drill core

to the GPTS. This interval includes the Brunhes normal polarity chron and the Jaramillo normal polarity subchron implying lower sedimentation rates (~ 0.1 m/ky) and a hiatus of ~500 ky distributed between GSE 9 (99.58 mbsf) and GSE 11 (109.42 mbsf). The Jaramillo occurs between 84.97 mbsf and 91.13 mbsf and encompasses a thin diatomite interval (87–90 mbsf), which represents peak warmth in MIS stage 31 (Villa et al., this volume).

5.1. Constraints (Table 1) Correlation of the magnetozones identified in the AND-1B core with the Geomagnetic Polarity Time Scale (GPTS; Ogg and Smith, 2004) is constrained as follows: 40Ar/ 39Ar ages on in-situ volcanic deposits provide key pinning points for the age model; 40Ar/ 39Ar ages on volcanic clasts provide a maximum age for the horizon at which they occur in the drill core. Lowest and highest diatom occurrence events provide “younger than” or “older than” constraints, respectively. In this case, ages are derived from the Southern Ocean diatom event database — hybrid age model for the relevant species (Cody et al., this volume; Levy et al., 2012). Likely hiatuses in the AND-1B succession are also derived from the diatom data set where several lowest and highest occurrence events representing a range of ages occur at, or proximal to, the same stratigraphic horizon. Finally, the magnetic polarity stratigraphy is correlated to the GPTS within the 40Ar/ 39 Ar ages and diatom-based biostratigraphic constraints outlined above (Fig. 7). Where a one-to-one match is possible, ages are assigned to discrete reversal boundaries. Physical evidence of unconformities in the core, either from glacial surfaces of erosion (Krissek et al., 2007) or from disconformities in volcanic successions (Pompilio et al., 2007) is also taken into account where hiatuses are implied from the diatom biostratigraphy or magnetostratigraphy in concert with physical evidence of erosion. Assignment of depths for datums varies slightly by datum type. 40 Ar/ 39Ar ages are reported to the depth that the dated sample was retrieved. Diatom datums are reported to the depth of the next sample above or below that, which did not or did contain the species, respectively. Magnetic polarity reversals are reported to the midpoint between samples of opposing polarity, unless an obvious candidate, such as a GSE lies between samples of opposing polarity. 5.2. Construction of the age model (Fig. 7) 5.2.1. Pleistocene (0–191.75 mbsf) The definition of Pleistocene applied in this paper adheres to the new 2.58 Ma age assigned to the base of the Pleistocene (Gibbard et al., 2010). Locating the Plio–Pleistocene boundary in the AND-1B record is relatively straightforward as the 2.58 Ma age is coincident with the top of the Gauss (C2An.1n) magnetochron. Actinocyclus fasciculatus and A. maccollumii both have short ranges that span the Gauss/Matuyama boundary (Cody et al., 2008, this volume; Levy et al., 2012; Table 1) and hence constrain the correlation of the normal to reverse polarity boundary at 191.75 mbsf to the C2An.1n/C2r.2r reversal (2.58 Ma), which in turn marks the Pliocene/Pleistocene boundary in the AND-1B drill core. The stratigraphic interval from the Pliocene/Pleistocene boundary to U4 at 163.65 mbsf contains A. maccollumii and must correlate with Chron C2r.2r. The highest occurrences of both A. maccollumii and Thalassiosira vulnifica are recorded within the stratigraphic interval immediately beneath U4 and imply a hiatus of ~250 ky across the unconformity. The highest occurrence datums of A. fasciculatus, T. webbii, T. kolbei and T. inura are recorded below and above U3 (150.73 mbsf). These data and the absence of a normal magnetozone imply a hiatus of at least 170 ky at that level. Between 150 and 100 mbsf, two 40Ar/ 39Ar ages constrain the correlation to the lower part of C1r.3r and imply a relatively high sediment accumulation rate (0.5 m/ky). Above 100 mbsf, several 40Ar/ 39 Ar ages constrain correlation of the magnetic polarity stratigraphy

5.2.2. Pliocene (191.75–583.64 mbsf) Diatom biostratigraphy between 583.64 mbsf and 191.75 mbsf provides tight constraints on the age model for the Pliocene interval. Fifteen diatom events, with well-constrained Southern Ocean ages are recognised in this interval. At 583.64 mbsf, the first appearances of T. complicata and Fragilariopsis praeinterfrigidaria constrain the base of the Pliocene interval of the core to C3n.3n (Sidufjall) and accordingly the normal magnetozone between 517.34 mbsf and 596.35 mbsf is correlated with C3n.3n. The base of the magnetozone is coincident with an unconformity recognised in the volcanic succession at 596.35 mbsf. The lowest occurrence datum of T. striata at 456 mbsf constrains correlation of the normal magnetozone between 459.19 mbsf and 452.86 mbsf to the Nunivak subchron. The top of the Nunivak normal interval is followed closely by the lowest occurrence of F. interfrigidaria and Rhizosolenia harwoodii at 440.12 mbsf which implies a hiatus of ~800 ky between them (Cody et al., this volume; Winter et al., this volume; Levy et al., 2012). The hiatus accounts for C3n.1n and is coincident with a thin volcaniclastic interval and an angular unconformity at ~440 mbsf. Above the unconformity, the normal polarity magnetozone between 346.13 mbsf and 439.96 mbsf is correlated with Chron C2An.3n (lower Gauss) and the overlying reversed polarity magnetozone (251.67–346.13 mbsf) must represent an amalgamation of the Kaena (C2An.1r) and Mammoth (C2An.2r) subchrons. An average sedimentation rate of 0.17 m/ky is implied for the Pliocene interval of the AND-1B core, but with a significant percent of the record represented by unconformities, interval sedimentation rates are likely to be higher. 5.2.3. Miocene (583.64–1284.87 mbsf) The Miocene interval of the AND-1B core lacks diatom event datums and thus is only constrained by 40Ar/ 39Ar ages and magnetic polarity stratigraphy. The 6.48 ± 0.13 Ma 40Ar/ 39Ar age on a lava flow at 648.37 mbsf and the 40Ar/ 39Ar age of 8.53 ± 0.51 Ma on a volcanic clast at ~ 823 mbsf constrain that interval of the core to chrons C3A– C4 of the GPTS. The number of magnetozones in this interval implies an average sedimentation rate of 0.15 m/ky but an exact match to the GPTS is not possible, although a hiatus is implied at GSE 36 (763.12 mbsf) with amalgamation of two reversed polarity chrons most likely accounting for much of Chron C4n. The thick normal polarity magnetozone between 1112.36 mbsf and 1270.20 mbsf most likely correlates with Chron C5n.2n implying a similar average sedimentation rate (0.15 m/ky) to that level. The 40Ar/ 39Ar ages on three volcanic clasts from 1279.02 mbsf yield a maximum depositional age of 13.57 ± 0.13 Ma for the base of the AND-1B drill core (Ross et al., this volume) and imply a significant hiatus (~2.75 my) in an observed angular unconformity close to the base of the volcanic interval at 1274 mbsf. 6. Discussion 6.1. Tectonic evolution of the Victoria Land Basin Regional seismic surveys (Cooper et al., 1987; Brancolini et al., 1995; Bartek et al., 1996; Wilson et al., 2004; Horgan et al., 2005; Betterly et al., 2007; Johnston et al., 2008) provide insight into the development of the Victoria Land Basin (VLB) through the identification of contiguous stratal packages and the disconformable relationships

G.S. Wilson et al. / Global and Planetary Change 96–97 (2012) 189–203

of those packages. Neogene basin-wide unconformities have been mapped into the southern McMurdo Sound area through the seismic network. The early Neogene succession is represented by a sheet-like stratal package blanketing and thickening slightly into the VLB, but with no abrupt lateral changes in thickness (Fielding et al., 2008). The lower part of the succession was sampled by the Cape Roberts Project CRP-1 and CRP-2/2A drill holes (Cape Roberts Science Team, 1998; 1999), which dates the lower boundary (reflector Re in Fielding et al., 2008) at the Oligocene–Miocene boundary (Wilson et al., 2002b). The upper boundary (reflector Rg in Fielding et al., 2008) was not sampled by CRP-1, although it afforded some age constraint in that it must be younger than the youngest Neogene strata sampled by CRP-1 (~17 Ma; Roberts et al., 1998). The early Neogene sedimentary cycles sampled by CRP drilling are truncated (Fielding et al., 1998) and are interpreted to reflect more passive subsidence than in the underlying Oligocene rift succession (Fielding et al., 2008). Reflector Rg (Fielding et al., 2008) reflects a basin-wide unconformity with an angular discordance increasing westward, which Fielding et al. (2008) interpreted to reflect renewed rifting (Terror Rift) with accommodation space outpacing sedimentation. In a vertical seismic profiling (VSP) experiment at the AND-1B site, Naish et al. (2007b) mapped reflector Rg into the AND-1B drill hole at ~ 1220 mbsf. We now contend that reflector Rg coincides with the 2.75 my unconformity at 1274 mbsf (Fig. 8) and that the volcanic clasts in the base of the AND-1B drill core date the top of the passive subsidence to 13.57 ± 0.13 Ma. The Terror Rift succession, itself, comprises a number of stratal packages and dissecting unconformities. Reflector Rh is lapped onto by volcanic massifs of the Erebus volcanic province including White Island, which implies an age of 7.6 Ma for the unconformity (Cooper et al., 2007; Fielding et al., 2008). Strata between Rg and Rh represent an eastward thickening succession in the central part of the VLB (Fielding et al., 2008). Naish et al. (2007b) mapped reflector Rh into the AND-1B succession at the base of the volcanic sequence that subdivides the AND-1B into an upper and lower succession. Our age model indicates a short duration unconformity coincident with a GSE at 763.12 mbsf overlain by the volcanic sequence dated at 7.26– 7.81 my, which is consistent with the age of the early White Island volcanics (7.6 Ma; Cooper et al., 2007). Strata between Rh and Ri reflect an unconformity bounded set (Fielding et al., 2008). Reflector Ri was mapped into the AND-1B succession within the lower part of diatomite unit 4.1, coincident with a ~ 800 ky (4.4–3.6 my) unconformity beneath a thin volcaniclastic interval at ~440 mbsf. Clinoform strata above reflector Ri thicken to the east and west and are linked to large-scale regression across the shelf (Fielding et al., 2008) and increased sediment flux to the VLB due to more dynamic EAIS outlet glaciers under warmer-than-present Pliocene climate (Golledge and Levy, 2011; Levy et al., 2012). Reflector Rk mapped into AND-1B at 150 mbsf (Naish et al., 2007b) represents an ~ 500 ky hiatus distributed across two disconformities and may be related to flexural loading from Ross Island (mainly mounts Terror and Erebus). The depth and proximity of the moat alone (500+ m) imply weakened lithospheric crust associated with Erebus Province volcanism. This has been confirmed by Aitken et al. (2012) who have calculated an elastic thickness of between 2 and 5 km from gravity observations on the McMurdo Ice Shelf. In turn, this implies additional flexural accommodation from earlier Ross Archipelago volcanoes that may have influenced the nature of sediment accumulation recovered by the upper AND-1B succession. 6.2. Climatic evolution of the AND-1B succession The age model identifies several unconformities and defines sediment accumulation rates that imply climatic control of accumulation in the AND-1B succession. In the lower AND-1B succession (Late Miocene), average sedimentation rates imply orbital control on

199

depositional cyclicity. The interval between 1063 mbsf and 1220 mbsf is predominantly diamictite interpreted to reflect cold polar conditions (McKay et al., 2009) with cycle thickness implying orbital control on cyclicity. Above 1063 mbsf, this gives way to unconformity-bounded diamictite–mudstone cycles interpreted to reflect a subpolar environment with significant meltwater (McKay et al., 2009). Thinning of the cycles with the same average sedimentation rate implies higher frequency cyclicity. The unconformity at the base of the AND-1B late Miocene succession is likely tectonically controlled and related to the onset of renewed Terror rifting and accommodation space likely increased at the AND-1B site. The late Miocene succession may well reflect a more proximal setting due to the early rift phase, but this explains neither the increase in frequency, nor the change in style of sedimentation. The benthic δ 18O record does not offer much insight either, as the Late Miocene benthic signal (Zachos et al., 2001) is fairly low in amplitude and is superimposed on a long term cooling. However, Billups and Schrag (2002), using Mg/Ca palaeothermometry, note a cooling at ~ 11 Ma followed by a warming after ~10 Ma and Miller et al. (1996) identified large transient glaciations (Mi4 and Mi5) reflecting a possible orbital control on glaciation (Fig. 9). The Pliocene AND-1B record includes well developed sub-polar to polar diamictite–diatomite sedimentary cycles separated by glacial surfaces of erosion. The age model predicts that these cycles are 40 ky in duration indicating obliquity-controlled glacial interglacial advance and retreat of the West Antarctic/Ross Ice Sheet (Naish et al., 2009). An extended deglacial phase is indicated by the late Pliocene 83-m thick (376–459 mbsf) diatomite (unit 4.1; Krissek et al., 2007), punctuated by a cooling at 3.6–3.3 Ma represented by a short hiatus in the AND-1B core correlated with reflector Ri, which Fielding et al. (2008) interpreted to represent a basin-wide unconformity driven by sea level fall. Above reflector Rk, the Pleistocene record is predominantly diamictite (Krissek et al., 2007) and following a thin diatomaceous interval indicating open water conditions over the site at the MIS-31 interglacial (Villa et al., this volume), polar conditions persisted with ice grounding at the AND-1B site. 7. Conclusions The AND-1B drill core record recovered from beneath the McMurdo/Ross Ice Shelf indicates large transient polar glaciation in the middle Miocene followed by a general cooling trend in ice sheet thermal regime and ice sheet/shelf extent throughout the late Neogene. Integrated magnetostratigraphy, biostratigraphy and 40Ar/ 39 Ar ages provide a chronology for the evolution of the succession, which through a VSP experiment (Naish et al., 2007b) allows correlation with basin-wide seismic reflectors (Fielding et al., 2008). A thick volcanic succession between 587 and 759 mbsf separates the 1284.87 m drill core record into an upper and lower succession. The base of the volcanic interval also coincides with a mid-late Miocene (~7.6 Ma) basin-wide seismic reflector, which separates the terror rift succession into upper and lower parts, with the upper part extensively intruded by volcanic activity (Fielding et al., 2008; DiRoberto et al., 2010). The base of the AND-1B drill hole is dated at 13.57 ± 0.13 Ma from three volcanic clasts at 1279.02 mbsf. However, the magnetostratigraphic age model indicates a 2.75 my unconformity at 1274.00 mbsf and that the lower succession is restricted to the late Miocene. Depositional facies and sedimentary cyclicity through the lower succession are interpreted to represent a warming from polar subglacial conditions to subpolar conditions accompanied by abundant meltwater at the AND-1B drill site (McKay et al., 2009). The age model implies orbitally controlled glacial–interglacial cyclicity throughout. Above the volcanics, a ~ 445 m-thick Pliocene succession indicates periods of extended open water in the Ross Sea, with obliquity controlled glacial–interglacial cyclicities from grounded ice over

200

Pleistocene

Overdeepened Shelf records large-scale Late Pleistocene glaciations 51.69 73.09 91.42

251.67

84.97

A1 A A2 D1 P1 D3 A3 D4 D2 P2 2 P3 A4 D5 A5 D6 D10 D7 D8 D9 D11 P4 D12-15 P5 D17

Rk

Passive subsidence / uniform sedimentation

D16 D18

Rj?

346.13

P6

439.96 452.88 459.19

D19 P7 7 P8 D20 P9 D21 D22 A6

517.34

D23 D25 596.35 630.12 637.52 654.33

Onset of RossIsland volcanism/ loading (~2 Ma)

- reduced clastic input / increased biogenic input - Pliocene warm interval - glacial retreat from McMurdo Sound - superimposed orbital glacioeustatic cyclity

Ri >0.25 m.y. basin-wide unconformity - progressively truncates older succession towards western margin of VLB / TAM front

P10 D24 D26 P11 A7

Onset of White Island volcanism (~8 Ma)

706.70 725.35 749.47

815.60 827.83

A8

Rh >0.4 m.y. basin-wide unconformity - increase in angular discordance towards western margin of VLB / TAM front

853.09 867.37 878.12

945.37 948.69 979.53

Eastward thickening sedimentary succession with east dipping clinoform geometries

1036.38 1049.66 1061.05 1073.76

- superimposed orbital glacioeustatic cyclity

1112.36

1270.20

A9

Inception of Terror Rift (~13 Ma)

Rg ~2.75 m.y. basin-wide unconformity

Fig. 8. Correlation of the AND-1B record basin-wide seismic reflectors (Rg, Rh, Ri, Rj and Rk from Fielding et al., 2008) and interpretation of tectonic and volcanic events in basin history. Timescale based on the GPTS of Ogg and Smith (2004), adjusted to Gibbard et al. (2010)Refer to figure 3 for the lithology key and description.

G.S. Wilson et al. / Global and Planetary Change 96–97 (2012) 189–203

191.75

55.51

Pleistocene

Benthic δ18 O (Zachos et al., 2001) δ18 O (o/oo)

P5 D17

346.13

439.96 452.88 459.19

D16

Late Pliocene

D18 P6

D19 P7 7 P8 D20 P9 D21 D22 A6

517.34

D23 D25 596.35 630.12 637.52 654.33

Early Pliocene

P10 D24 D26 P11

A7

2

G.S. Wilson et al. / Global and Planetary Change 96–97 (2012) 189–203

251.67

Rk

g

191.75

84.97

A1 A A2 D1 P1 D3 A3 D4 D2 P2 2 P3 A4 D5 A5 D6 D10 D7 D8 D9 D11 P4 D12-15

5

Rj?

Coo lin

73.09 91.42

55.51

Orbital (Obliquity) control on glacial advance/retreat Distal WAIS / RIS grounding and retreat

51.69

4

3

0

Orbital (Eccentrocity) control on glacial advance/retreat Proximal WAIS grounding

4

Ri

Mid Pliocene Cooling event 6

Volcanics

706.70 725.35

A8

853.09 867.37 878.12

945.37 948.69 979.53

1036.38 1049.66 1061.05 1073.76 1112.36

Late Miocene

1270.20

A9

8

Rh

Subsidence

815.60 827.83

Orbital (Eccentricity/obliquity) control on glacial advance/retreat Proximal EAIS tidewater glaciers Too warm/turbid for diatom accumulation

749.47

10

12

Rg 14 2

2.5

3

3.5

4

4.5

5

0/ 00

201

Fig. 9. Correlation of the AND-1B record basin-wide seismic reflectors (Rg, Rh, Ri, Ri and Rk from Fielding et al., 2008) and interpretation of climatic events in basin history. Benthic δ18O curve is from Zachos et al. (2001). Timescale based on the GPTS of Ogg and Smith (2004), adjusted to Gibbard et al. (2010). Refer to figure 3 for the lithology key and description.

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the AND-1B drill site to modelled collapse of the West Antarctic ice sheet (Pollard and DeConto, 2009). A mid-Pliocene unconformity at 3.60–4.34 my is correlated with a regional seismic reflector interpreted to represent a drawdown of sea level (Fielding et al., 2008) and hence a potential cooling. The Pleistocene record (0–150.73 mbsf) includes extremes of climate, from peak warmth and marine ice sheet collapse associated with MIS-31 (Villa et al., this volume; DeConto et al., this volume) to extreme polar conditions in the Late Pleistocene with ice grounding in 900 m of water at the AND-1B site and out to the Antarctic shelf edge (Anderson et al., 2002) in glacials and extensive ice shelves in interterglacials. Acknowledgements The ANDRILL project is a multinational collaboration between the Antarctic programmes of Germany, Italy, New Zealand and the United States. Antarctica New Zealand is the project operator and developed the drilling system in collaboration with Alex Pyne at Victoria University of Wellington and Webster Drilling and Enterprises Ltd. Antarctica New Zealand supported the drilling team at Scott Base, and Raytheon Polar Services Corporation supported the science team at McMurdo Station and the Crary Science and Engineering Center. The ANDRILL Science Management Office at the University of Nebraska-Lincoln provided science planning and operational support. Scientific studies are jointly supported by the US National Science Foundation (NSF), NZ Ministry of Science and Innovation (MSI) and the Marsden Fund, the Italian Antarctic Research Programme (PNRA), the German Research Foundation (DFG) and the Alfred Wegener Institute for Polar and Marine Research (AWI). References Aitken, A.R.A., Wilson, G.S., Jordan, T., Tinto, K., Blakemore, H., 2012. Flexural controls on late Neogene basin evolution in southern McMurdo Sound, Antarctica. Global and Planetary Change 80–81, 99–112. Anderson, J.B., Shipp, S.S., Lowe, A.L., Wellner, J.S., Mosola, A.B., 2002. The Antarctic Ice Sheet during the Last Glacial Maximum and its subsequent retreat history: a review. Quaternary Science Reviews 21, 49–70. Armstrong, R.L., 1978. K–Ar dating: Late Cenozoic McMurdo Volcanic Group and Dry Valley glacial history, Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics 21, 685–698. Barrett, P.J., Hambrey, M.J., 1992. Plio–Pleistocene sedimentation in Ferrar Fiord, Antarctica. Sedimentology 39, 109–123. Barrett, P.J., Elston, D.P., Harwood, D.M., McKelvey, B.C., Webb, P.-N., 1987. MidCenozoic record of glaciation and sea-level change on the margin of the Victoria Land Basin, Antarctica. Geology 15, 634–637. Barrett, P.J., Adams, C.J., McIntosh, W.C., Swisher III, C.C., Wilson, G.S., 1992. Geochronological evidence supporting Antarctic deglaciation three million years ago. Nature 359, 816–818. Barrett, P.J., Bleakley, N.L., Dickinson, W.W., Hannah, M.J., Harper, M.A., 1997. Distribution of siliceous microfossils on Mount Feather, Antarctica, and the age of the Sirius Group. In: Ricci, C.A. (Ed.), The Antarctic Region, Geological Evolution and Processes. Terra Antartica Publication, Siena, pp. 763–770. Bartek, L.R., Henrys, S.A., Anderson, J.B., Barrett, P.J., 1996. Seismic stratigraphy of McMurdo Sound, Antarctica: implications for glacially influenced early Cenozoic eustatic change? Marine Geology 130, 79–98. Betterly, S.J., Speece, M.A., Levy, R.H., Harwood, D.M., Henrys, S.A., 2007. A novel over-sea-ice seismic reflection survey in McMurdo Sound. Terra Antartica 14, 97–106. Billups, K., Schrag, D.P., 2002. Paleotemperatures and ice volume of the past 27 Myr revisited with paired Mg/Ca and 18O/16O measurements on benthic foraminifera. Paleoceanography 17, 1003, http://dx.doi.org/10.1029/2000PA000567. Brancolini, G., Busetti, M., Marchetti, A., DeSantis, L., Zanolla, C., Cooper, A.K., Caochrane, G.R., Zayatz, I., Belyaev, V., Knyazev, M., Vinnikovskaya, O., Davey, F.J., Hinz, K., 1995. Descriptive text for the seismic stratigraphic atlas of the Ross Sea, Antarctica. In: Cooper, A.K., Barker, P.F., Brancolini, G. (Eds.), Geology and Seismic Stratigraphy of the Antarctic Margin. : Antarctic Research Series, Vol. 68. American Geophysical Union, Washington, pp. A271–A286. Brook, E.J., Brown, E.T., Kurz, M.D., Ackert Jr., R.P., Raisbeck, G.M., Yiou, F., 1995. Constraints on age, erosion, and uplift of Neogene glacial deposits in the Transantarctic Mountains determined from in situ cosmogenic 10Be and 26Al. Geology 23, 1063–1066. Cape Roberts Science Team, 1998. Initial report on CRP-1, Cape Roberts Project, Antarctica. Terra Antartica 5, 1–187.

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