Calcareous nannofossil evidence for Marine Isotope Stage 31 (1 Ma) in Core AND-1B, ANDRILL McMurdo Ice Shelf Project (Antarctica)

Global and Planetary Change 96–97 (2012) 75–86

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Global and Planetary Change j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g l o p l a c h a

Calcareous nannofossil evidence for Marine Isotope Stage 31 (1 Ma) in Core AND-1B, ANDRILL McMurdo Ice Shelf Project (Antarctica) Giuliana Villa a,⁎, Davide Persico a, Sherwood W. Wise b, Alessia Gadaleta a a b

Dipartimento di Scienze della Terra, Università di Parma, V. Usberti, 157A, 43100 Parma, Italy Department of Geology and the Antarctic Marine Geology Research Facility, Florida State University, FL 32306, USA

a r t i c l e

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Article history: Received 4 November 2008 Received in revised form 7 October 2009 Accepted 10 December 2009 Available online 13 January 2010 Keywords: Calcareous nannofossil Pleistocene Palaeoclimate MIS-31 ANDRILL Antarctica

a b s t r a c t ANDRILL Core AND-1B, recovered in the Western Ross Sea of Antarctica, has been examined in search of calcareous nannofossils. Exhaustive and detailed analyses of the interval from 86.61 to 98.99 mbsf revealed for the first time at an extreme southern high latitude (77.88° S) the presence of lower Pleistocene calcareous nannofossils, together with Tertiary and Upper Cretaceous reworked species. Other calcareous microfossils in the assemblage include, spicules of calciosponges and small foraminifers. The short normal magnetozone between 84.97 and 91.13 mbsf is correlated with the Jaramillo Subchron (C1r.1n) (Wilson et al., 2007). The presence of nannofossils in the biogenic interglacial sediments is consistent with an episode of warm surface waters and open-marine conditions during the Jaramillo subchron, at ~ 1 Ma, which corresponds with Marine Isotope Stage (MIS)-31 (Scherer et al., 2007; Naish et al., 2007). Climate proxies such as oxygen isotope stratigraphy and calcareous nannofossils at ODP Site 1165 (Pospichal, 2003; Villa et al., 2008) and the diatom assemblage in a shelly carbonate sequence at Cape Roberts 1 (Bohaty et al., 1998) also support a warming event during this time and suggest it extended around the Antarctic Continent. This in turn implies a total or partial collapse of the McMurdo Ice Shelf and a concurrent shift or temporary dissipation of the Polar Front (Antarctic Convergence) and Antarctic Divergence that currently serve as barriers to the influx of calcareous nannofossils to the margins of Antarctica. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Reconstruction of the evolution of the Antarctic Ice Sheet (AIS) is still a matter of debate. The AIS is composed of: (1) the East Antarctic Ice Sheet (EAIS), the major component that lies on continental crust and is thus considered relatively stable; and (2) the West Antarctic Ice Sheet (WAIS) that is grounded in part on oceanic crust below the sea level and is, therefore, more vulnerable to catastrophic collapse. The EAIS and WAIS developed at different times and by different mechanisms. EAIS glacial history began between the middle-late Eocene and early Oligocene (~ 36 Ma) (Birkenmajer, 1987, 1991; Prentice and Matthews, 1988; Barrett et al., 1991; Denton et al., 1991; Barrera and Huber, 1993). Its dimensions changed several times during the late Cenozoic, which influenced global climate and sea level. The various models of the nature and timing of EAIS development indicate its presence in the Oligocene. At this time, it advanced across the continental shelf at least along the eastern sector, but only evolved to its present size at ~15 Ma (Savin et al., 1975; Shipboard Scientific Party, 2000, ODP Leg 189).

⁎ Corresponding author. Tel.: + 39 0521905370. E-mail address: [email protected] (G. Villa). 0921-8181/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2009.12.003

WAIS glacial history began during the early-middle Miocene (20– 15 Ma) as isolated ice caps separated by sea inlets that were gradually filled by glaciomarine sediments that allowed WAIS extension to develop through the thickening of ice shelves (Mercer, 1978). Two main hypotheses have emerged on the evolution and stability of the EIAS in pre-Quaternary time: (1) the ‘stabilist's’ view, that considers it to have been stable at least over the last 15 million years (Huybrechts, 1993; Sugden et al., 1993; Kennett and Hodell, 1995); and (2) the ‘dynamicist's’ view, which suggests that it reached stability at about 3 million years ago or less. The latter theory (Webb et al., 1984; Wilson et al., 1998; Harwood et al., 2000) asserts that the present ‘polar’ (as opposed to temperate) glacial conditions are relatively recent and that the AIS, and the WAIS in particular, have been highly dynamic, unstable and susceptible to nearly complete collapses during Plio-Pleistocene interglacial events (Mercer, 1978). In this view the EAIS also underwent major fluctuations up to Pliocene times implying considerable climate variations from the late Miocene through the Plio-Pleistocene (Escutia et al., 2003). This hypothesis is corroborated by the investigation of glacial–interglacial sequences in Plio-Pleistocene sediments from Prydz Bay (Anderson et al., 1991) and from the continental shelves of the Ross Sea and the Antarctic Peninsula (Alonso et al., 1992; Bart and Anderson, 1995; Scherer et al., 1998), and by recent studies that document the presence of calcareous nannofossil assemblages in sediment recovered

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from several locations around the Antarctic continent (Pospichal, 2003; Villa et al., 2003, 2005, 2008) that imply surface-water conditions different from the present. Coccolithophores are currently confined to latitudes lower than the Antarctic Divergence (AD) (63–65° S) (Findlay and Giraudeau, 2000; Gravalosa et al., 2008), south of which the present surface-water conditions are detrimental to the growth and preservation of these organisms. Accordingly, Quaternary sediments south of the AD have long been considered barren of calcareous nannofossils until recent new data from, piston cores (Villa et al., 2005), and ODP Leg 188 (Pospichal, 2003; Villa et al., 2008) instead revealed the presence of calcareous nannofossil assemblages at various sites located along the Antarctic continental margin. The presence of these microfossils in Quaternary sediments of periantarctic basins, their fluctuations and relationships with glacial–interglacial cycles are of great interest in attempts to reconstruct palaeoclimatic and palaeoceanographic conditions of the AIS of this time interval. During the austral summer of 2006, the ANDRILL Program recovered a 1284.87 m long succession of cyclic glaciomarine, terrigenous, volcanic and biogenic sediments from beneath the McMurdo Ice Shelf (MIS) (Fig. 1). The calcareous nannofossil assemblage provides useful information for characterizing the palaeoclimatic changes and behavior of the McMurdo Ice Shelf at about 1 Ma. 2. Oceanographic setting The Southern Ocean (SO) surrounds the entire Antarctic continent and includes the southern portions of the Atlantic, Pacific and Indian Oceans. It is characterized by a series of oceanic fronts and by an overall eastward flow defined as the Antarctic Circumpolar Current (ACC). Each front is identified by different specific temperature and salinity limits (except for local conditions [Holliday and Read, 1998]) that separate different water masses. The Subantarctic Front (SAF) (~45° S) and the Polar Front (PF) (~ 50° S) in particular are two

important biogeographical boundaries that delimit the Polar Frontal Zone (PFZ) (Whitworth and Nowlin, 1987; Pakhomov et al., 2000; Pollard et al., 2002). The Southern ACC Front (SACCF) south of the PF is the southern boundary of the Antarctic Zone (AZ) (Fig. 1). Gravalosa et al. (2008) document extant coccolithophore assemblages from the East Pacific sector of the SO (Bellinghausen and Amundsen Seas) and show clearly that they are more consistently present in this area than previously suggested, but are strongly associated with the frontal system. Coccolithophore abundance and diversity increase significantly in the proximity of the SAF and the PF where they are associated with a high nutrient supply to surface waters, but decrease within the PFZ and the AZ. South of the SACCF, however, coccolithophores are absent. 3. Materials and methods We examined the AND-1B drill core recovered beneath the McMurdo Ice Shelf northwest Ross Ice Shelf for calcareous nannofossils. We analyzed a total of 388 samples throughout the sequence, 100 of which contain calcareous nannofossils in intervals characterized by high diatom productivity (Scherer et al., 2007). A preliminary examination was performed on 278 smear slides taken at a low sampling resolution along the entire succession (9.45– 1231.70 mbsf). An exhaustive analysis was performed on an additional 110 samples taken every 8–10 cm from 86.61 to 98.99 mbsf and prepared using both standard smear-slide (Bown and Young, 1998) and settling (De Kaenel and Villa, 1996) techniques; the latter assures a uniform and homogeneous distribution of the calcareous nannofossils. Samples from 95.93 to 95.97 mbsf were taken every centimeter (Fig. 2). The sequence from 86.63 to 98.99 mbsf is defined by the transition from bioturbated volcanic sandstone and mudstone to glacial diamictite and volcanic sandstone (Krissek et al., 2007). In particular, the interval 86.63–92.50 mbsf is a highly fossiliferous clay and silty claystone-rich

Fig. 1. Location map of ANDRILL MIS drill site on the Ross Ice Shelf. Oceanographic fronts as reported by Barker and Thomas (2004): Southern Antarctic Circumpolar Current Front (SACCF); Polar Front (PF); Southern Antarctic Front (SAF). PFZ: Polar Front Zone, SAZ: Subantarctic Zone, AZ: Antarctic Zone. Dots represent ODP Sites mentioned in this work; stars represent previously studied piston cores that revealed the presence of nannofossils (Villa et al., 2005).

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Fig. 2. Distribution of calcareous nannofossil specimen counts from ANDRIL MIS Core AND-1B; litho-magnetostratigraphy on the left (Krissek et al., 2007; Wilson et al., 2007). For lithologic symbols see Fig. 3.

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diatomaceous ooze, including a short calcareous shelly debris interval (90.80–91.22 mbsf) with bryozoan, molluscan, foraminiferal fragments and nannofossils. These deposits are interpreted as a record of pelagic sedimentation in an open marine environment (Krissek et al., 2007); the common occurrence of open marine diatoms implies the same conclusion (Scherer et al., 2007). High-resolution calcareous nannofossil analyses were performed at 1250× magnification under crossed-polarized and phase-contrast light on a variable number of fields (1200–4000), equivalent to 37– 125 mm 2 on a slide.

4. Results Semi-quantitative analyses have revealed the presence of biogenic carbonate intervals that occur at different depths in the stratigraphic sequence (24–42 mbsf, 86–98 mbsf, 498–524 mbsf and 554– 556 mbsf), often in association with diatomites. The marked correspondence between diatom primary productivity and the presence of calcareous nannofossils reveals the utility of both microfossil groups for reconstructing surface-water temperature trends and sea-ice conditions in McMurdo Sound. Other calcareous microfossils include

Fig. 3. Correlation of Pleistocene magnetostratigraphy, nannofossil bioevents (Raffi et al., 2006), with litho-magnetostratigraphy (Krissek et al., 2007; Wilson et al., 2007), and biostratigraphic markers in ANDRILL Core AND-1B. Calibration of the magnetostratigraphy is according to Lourens et al. (2004).

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fragments of spicules of calciosponges and small foraminifers in variable concentrations. This report focuses on the results of the 110 samples from 86.61 to 98.99 mbsf (Fig. 3) that led to a significant find of Pleistocene calcareous nannofossils at such an extreme high southern latitude (77.88° S). This can be considered the first finding of coccoliths at latitudes higher than 75° S; in fact the other piston cores (Villa et al., 2005) or ODP sites (Wei and Wise, 1990, 1992; Pospichal, 2003; Villa et al., 2008) are located at lower latitudes, though south of the present day AD (Fig. 1). In particular, some of the species (small Gephyrocapsa spp., Pseudoemiliania lacunosa and Reticulofenestra asanoi) define a Pleistocene (Calabrian) assemblage. These are considered to be in situ and, therefore, reliable for biostratigraphic and palaeoecologic– palaeoceanographic reconstructions of the McMurdo Ice Shelf region. In addition, reworked Tertiary and Upper Cretaceous species in the assemblage may contribute to the reconstruction of sediment provenance. 4.1. Biostratigraphy Precise bioevents (such as first or last occurrences) cannot be distinguished because of the extreme scarcity of calcareous nannofossils, but some taxa nevertheless help define the biostratigraphy. The presence of P. lacunosa, whose extinction indicates the top of the biozone CN 14a (Okada and Bukry, 1980), suggests that the interval examined is older than 0.4 Ma. A more detailed age assignment is possible due to the presence of R. asanoi from 95.93 to 95.95 mbsf. Raffi et al. (2006) report its Lowest Consistent Occurrence (LCO) at 1.078 Ma in the Eastern Mediterranean ODP Site 967, and at 1.136 Ma at ODP Sites 925 and 926 (West Equatorial Atlantic), while they place its Highest Consistent Occurrence (HCO) at 0.901 Ma (East Mediterranean) and 0.905 Ma (ODP Sites 925, 926). Reale and Monechi (2005) relate its LCO at 1.17 Ma and its HCO at 0.87 Ma (ODP Hole 610A, Atlantic Ocean). Reticulofenestra asanoi represents here the most effective biostratigraphic marker as it greatly narrows the time span of the sequence in question (Fig. 3). The presence of very rare but only small Gephyrocapsa could be interpreted as one of the intervals in which medium to large Gephyrocapsa disappear temporarily, comparable to the Small Gephyrocapsa Zone of Gartner (1977). Among the species present R. asanoi allows us to confirm the assignment of the interval of normal polarity from 84.97 mbsf to 91.13 mbsf to the Jaramillo subchron (C1r.1n), which ranges from 0.988 to 1.072 Ma (Ogg and Smith, 2004). Such evidence supports a Pleistocene chronostratigraphic interpretation that places the sequence examined (86.61–98.99 mbsf) between the top of Jaramillo (0.988 Ma) (Lourens et al., 2004) and the LCO of R. asanoi (1.136 Ma) (Raffi et al., 2006) i.e. in the Calabrian Stages. The assemblage is also characterized by: Braarudosphaera bigelowii, Calcidiscus leptoporus, Coccolithus pelagicus, Dictyoccocites productus, Reticulofenestra haqii, Reticulofenestra minuta and Reticulofenestra minutula (Fig. 2) which are also compatible with this age assignment (Plates 1 and 2). 4.2. Reworked nannofossil taxa Though rare and discontinuous, the presence of reworked species from older sediments was detected at different depths in the Pleistocene interval. Reworked Cenozoic species consist of rare specimens of Discoaster sp. (86.61 mbsf), Chiasmolithus solitus (87.30 mbsf), Reticulofenestra wadeae (92.78 mbsf), cf. Geminilithella rotula (95.10 mbsf), Transversopontis sigmoidalis, and Reticulofenestra hampdenensis (95.70 mbsf). Cyclicargolithus floridanus, Dictyoccocites antarcticus, Reticulofenestra bisecta, Reticulofenestra daviesi, and Reticulofenestra dictyoda occur in more than one sample (Fig. 2). These species range in age from the Eocene to the early Miocene. This same age range was found in the fossiliferous content of the McMurdo erratics (Harwood

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and Levy, 2000), therefore the Cenozoic species could presumably have been supplied to the Pleistocene basin from erosion of strata beneath the Ross Ice Shelf. Reworked species from Cretaceous sediments were recognized as Stradneria crenulata (87.40 mbsf), Watznaueria barnesae (94.61 mbsf), and Reinhardtites levis (95.40 mbsf). Previous studies documented the presence of reworked Cretaceous nannofossils in the CIROS core (Monechi and Reale, 1997; Watkins, 2007) and at Prydz Bay (Pospichal, 2003), as well as reworked Cretaceous foraminifers and radiolarians in the Oligocene (Leckie and Webb, 1983) and Quaternary sediments of the Victoria Land Basin (Webb and Neall, 1972). 5. Discussion 5.1. Palaeoclimatic interpretation Calcareous nannofossils are good palaeoecologic indicators as their distribution is influenced by sea-water conditions in which they live. The presence of calcareous nannofossils in our samples suggests favorable conditions (temperature and light) in the surface waters for their productivity and preservation. As the lower temperature limit for thriving calcareous nannoplankton is about 2 °C (Burckle and Pokras, 1991; Findlay and Giraudeau, 2000; Villa et al., 2003), the presence of these organisms, though rare and discontinuous, in our biogenic interglacial sediments is consistent with a relatively intense warm episode in surface waters, characterized by summer temperatures higher than present in McMurdo Sound at ~1 Ma. In particular, C. leptoporus is said to tolerate temperature minima from 5.4 °C (Findlay and Giraudeau, 2000) to 11 °C (Samtleben et al., 1995). Recently, living specimens of C. leptoporus were retrieved as far south as the PF at a water temperature N 1.3 °C by Gravalosa et al. (2008), but not at their southernmost station south of the PF (65.50° S) where a monospecific assemblage of E. huxleyi var. kleijniae is present. South of the SACCF no coccolithophores were found, thus indicating that this front is the southern boundary for the presence and reproduction of coccolithophorids. Peaks of small Gephyrocapsa spp. during Pleistocene interglacial intervals have been documented at the Southeastern Atlantic ODP Site 1082 (Baumann and Freitag, 2004). Occurrences of C. pelagicus are described in the Nordic Seas by Samtleben et al. (1995), and Baumann et al. (2000) report a temperature tolerance of this species between 10 and 0 °C. Light is another vital factor to the development of coccolithophores since they are photosynthetic organisms. The calcareous nannofossils we observed, therefore, suggest open-marine conditions around the McMurdo Ice Shelf, with a considerable reduction in sea ice extent relative to the present day, as a consequence of the warm event that around 1 Ma should have affected the entire Ross Sea region. Our biostratigraphic and the magnetostratigraphic (Wilson et al., 2007) assignments identify the warm episode as the super-interglacial Marine Isotope Stage (MIS)-31 (Scherer et al., 2007), which represents the last significant global warm event, whose summer insolation values are, in both hemispheres, among the highest of the last 5 Ma. This unusual warm and prominent interglacial (1.085–1.055 Ma) was probably induced by a combination of high obliquity, eccentricity, and precession (Laskar et al., 2004; Scherer et al., 2008). It is marked by two Southern Hemisphere (SH) summer maxima, separated by a strong Northern Hemisphere (NH) summer peak at 1.07 Ma (DeConto et al., 2007), which coincides with the base of Jaramillo event (1.072 Ma). Of the two SH summer insolation maxima, the lower is stronger and occurs at 1.08 Ma (Laskar et al., 2004). The abundance pattern of the calcareous nannofossils through the study interval (Fig. 4) shows a continuous distribution in the lower portion of the succession (from 96.02 to 94.70 mbsf). C. leptoporus, which requires temperature minima higher than C. pelagicus, only

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occurs in this interval. This may have been induced by the stronger SH summer insolation maximum at 1.08 Ma (Fig. 5). During this insolation maximum, Flores and Sierro (2007) documented at ODP Site 1094 (53° S) an abrupt increase in calcareous nannofossil deposition, with high abundances of C. leptoporus, C. pelagicus, Helicosphaera carteri and Syracosphaera spp. Similar results were obtained by Maiorano et al. (2009) at ODP Site 1090. In the same time interval at ODP Site 1094, Scherer et al. (2008) provide evidence of a decrease of sea ice-related diatoms (indicative of sea ice reduction) that are also present within the shelly carbonate interval of CRP-1 (Bohaty et al., 1998). In AND-1B in the overlying interval from 94.70 to 88.70 mbsf, calcareous nannofossils decrease to lower abundances. Their presence still suggests, however, ice-free conditions and warm surface waters despite diminished SH insolation around the lower part of the Jaramillo subchron. The other increase in coccolith abundance in the upper portion of the succession from 88.70 to 87.40 mbsf, though discontinuous, may correspond with the younger and weaker SH insolation maximum of MIS-31, that occurred at about 1.06 Ma. In this interval C. leptoporus is absent, but C. pelagicus, which tolerates a lower temperature minima, is present. The extreme scarcity of calcareous nannofossils above this peak may suggest a return to colder surface-water conditions, unfavorable to the persistence of these organisms, and may represent the waning of the SH insolation period after 1.06 Ma. The same line of reasoning can be used to explain the scarcity of calcareous nannofossils below 96.02 mbsf, and from 25 to 42 mbsf. Modeling of the Antarctic margin by DeConto et al. (2007) suggested that sea-surface temperature during MIS-31 may have risen from 2° to 5 °C in conjunction with a 20-m eustatic-sea level rise (Raymo et al., 2006). It is therefore reasonable to hypothesize that MIS-31 circum-Antarctic warming may have led to a consistent reduction of Ross Ice Shelf at 1.07 Ma relative to its modern extent, and even to its collapse (Scherer et al., 2008). Comparing the relative duration and thickness of the Jaramillo Subchron (1.072–0.988 Ma, from 91.13 to 84.97 mbsf) with the duration of MIS-31 (1.085–1.055 Ma) and its thickness here inferred (96.74 to 86.61 mbsf), it becomes evident that the sedimentation rate is not constant, and that the portion between the bottom of Jaramillo subchron and the top of MIS-31 (1.072–1.055 Ma) should be thicker than the portion between the bottom of Jaramillo and the bottom of MIS-31 (1.072–1.085 Ma). However, by our interpretation the lower portion is much thicker. This discrepancy may be explained by invoking a very high sedimentation rate in the lower part, where two volcanic deposits (at 90.31–92.25 mbsf and 93.25–94.50 mbsf, respectively) have produced an interval of considerable thickness in a short amount of time. 5.2. Palaeoceanographic interpretation The unusual and striking presence of Pleistocene calcareous nannofossils at these extreme southern latitudes suggests a consid-

erable southward displacement or strong dissipation of the PF at the time of their deposition. The succession shows a short calcium carbonate-containing interval within a dominant biogenic siliceous lithology. Such a dramatic change in sedimentation is the consequence of extreme climate and surface water mass variations. The presence of biogenic carbonate intervals occur often in association with diatomites and not with diamictites. The distinct correspondence between diatom primary productivity and the presence of calcareous nannofossils suggests warm surface-water temperature and sea-ice free conditions in McMurdo Sound. The other intervals with biogenic carbonates (24– 42 mbsf, 86–98 mbsf, 498–524 mbsf and 554–556 mbsf) do not contain nannofossils, while in the discussed interval the presence of coccoliths indicate favorable conditions to their propagation, discussed in Villa et al. (2005). The coccolith assemblage, consisting mainly of C. leptoporus, C. pelagicus, and medium- and small-sized Noelaerhabdaceae (P. lacunosa, Gephyrocapsa and Reticulofenestra), is characteristic of the present-day Subantarctic Zone (SAZ; 35°–45° S). This necessarily implies a significant weakening of the PF during MIS-31 and the influx of calcareous nannoplankton (which is normally excluded from the margins of East Antarctica) into our study area. In addition, the present-day range of C. leptoporus is restricted north of the PF where it increases in number at both the PF and SAF, as documented by Gravalosa et al. (2008). Hence its presence in the AND-1B core further suggests that the PF moved considerably southward and/or weakened, at least during the time of the lower SH summer maximum (around 1.08 Ma). The southward displacement or partial or complete dissipation of the frontal system is probably linked to the increased sea-surface temperature, which in turn is the main factor that limits coccolithophore productivity (no coccolithophores currently occur below 1.3 °C). Hence, our results suggest that changes in the dynamics of the frontal system and the consequent nutrient availability affected the calcareous nannofossil abundance in the region. Maiorano et al. (2009) recognized, during MIS-31, at Site 1090, a southward migration of the Subtropical Front (STF).

5.3. Possible sources of reworked nannofossil taxa Reworked Cretaceous marine microfossils suggest the existence of a middle-late Cretaceous sea along the present Ross Sea continental margin. However, to our knowledge there has been no recovery of marine Upper Cretaceous materials from the TAM along the EAIS, but rather only from the Antarctic Peninsula and its nearby islands. Thus, we can hypothesize that there were Upper Cretaceous units capable of furnishing fine sediment to the margin during the early Pleistocene. Alternatively, due to their extreme small size, coccoliths eroded from distant Cretaceous formations may have been incorporated via aeolian transport into dust-rich glacial ice. The presence of reworked Cretaceous nannofossils in the apparently warmer intervals (at 94.6

Plate 1. Fig. 1 Small Gephyrocapsa sp. LM, X nicols. AND-1B, 95.10 mbsf. Figs. 2 and 3 Reticulofenestra minutula. (2) LM, X nicols. AND-1B, 95.95 mbsf; (3) LM, X nicols. AND-1B, 95.95 mbsf; Fig. 4 Reticulofenestra asanoi. LM, X nicols. AND-1B, 95.95 mbsf. Figs. 5–8 Pseudoemiliania lacunosa. (5) LM, X nicols. AND-1B, 95.93 mbsf; (6) LM, X nicols. AND-1B, 95.96 mbsf; (7) LM, X nicols. AND-1B, 95.97 mbsf; (8) LM, X nicols. AND-1B, 95.94 mbsf. Figs. 9 and 20 Dictyoccocites antarcticus. (9) LM, X nicols. AND-1B, 91.66 mbsf; (20) LM, X nicols. AND-1B, 92.19 mbsf. Figs. 10, 13, 16, 17 and 18 Dictyoccocites productus. (10) LM, X nicols. AND-1B, 92.19 mbsf; (13) LM, X nicols. AND-1B, 96.02 mbsf; (16) LM, X nicols. AND-1B, 96.02 mbsf; (17) LM, X nicols. AND-1B, 86.61 mbsf; (18) LM, X nicols. AND-1B, 86.61 mbsf. Figs. 11 and 12 Reticulofenestra sp. LM, X nicols. AND-1B, 95.00 mbsf.; (12) LM, X nicols. AND-1B, 96.02 mbsf; Figs. 14,15 and 19 Reticulofenestra minutula (14) LM, X nicols. AND-1B, 96.02 mbsf; (15) LM, X nicols. AND-1B, 96.02 mbsf.; (19) Reticulofenestra minutula LM, X nicols. AND-1B, 92.19 mbsf. Plate 2. (see on page 10) Figs. 1–6 Coccolithus pelagicus. (1,2) LM, X nicols-CF. AND-1B, 95.50 mbsf; (3,4) LM, X nicols-CF. AND-1B, 91.22 mbsf; (5,6) LM, X nicols-CF. AND-1B, 90.50 mbsf. Fig. 7 Coccolithus pelagicus with bar LM, X nicols. AND-1B, 91.50 mbsf. Fig. 8 Braarudosphaera bigelowii LM, X nicols. AND-1B, 88.38 mbsf. Figs. 9–10 Reticulofenestra minutula. (9) LM, X nicols. AND-1B, 96.02 mbsf; (10) LM, X nicols. AND-1B, 96.02 mbsf. Figs. 11 and 12 Dictyoccocites antarcticus. (11) LM, X nicols. AND-1B, 93.03 mbsf; (12) LM, X nicols. AND-1B, 93.03 mbsf. Fig. 13 Reticulofenestra asanoi. LM, X nicols. AND-1B, 95.93 mbsf. Fig. 14 Calcispongiae spicule LM, X nicols. AND-1B, 95.95 mbsf. Fig. 15 Transversopontis sigmoidalis LM, X nicols. AND-1B, 95.70 mbsf. Fig. 16 Dictyoccocites antarcticus LM, X nicols. AND-1B, 91.22 mbsf. Fig. 17 Discoaster sp. LM, CF. AND-1B, 86.61 mbsf. Fig. 18 Reticulofenestra hampdenensis LM, X nicols. AND-1B, 95.70 mbsf. Fig. 19 Stradneria crenulata LM, X nicols. AND-1B, 87.40 mbsf. Fig. 20 Reinhardtites levis LM, X nicols. AND-1B, 95.40 mbsf.

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Plate 1.

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Plate 2 (caption on page 80).

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a

b

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c

Fig. 4. Calcareous nannofossil abundance curves plotted against litho-magnetostratigraphy in Core AND-1B (Krissek et al., 2007; Wilson et al., 2007, left column): a: In situ-nannofossil total abundance; b: reworked Cenozoic specimens; c: reworked Cretaceous specimens. For lithologic symbols see Fig. 3.

and 87.4 mbsf) (Fig. 4), could reflect a major melting of such dust-rich glacial ice and its consequent supply to the basin. 6. Conclusions The extensive analyses outlined in this paper demonstrates the potential for acquiring an unprecedented record of Pleistocene calcareous nannofossils at high southern latitudes. A diatomaceous unit recovered at AND-1B was assigned to the Pleistocene (Calabrian) by calcareous nannofossil biostratigraphy and magnetostratigraphy. The precise age assignment, the presence of calcareous microfossils and open-sea diatoms, allow correlation of this interval with the super-interglacial stage MIS-31 (Naish et al., 2007; Scherer et al., 2007). MIS-31 was characterized by a strong precessional signal (Scherer et al., 2008) prior to the eccentricity-controlled cycles (from 41 to 100 ka climate cycle dominance) that followed the mid-Pleistocene climate transition (Flores and Sierro, 2007). MIS-31 is composed of two SH insolation maxima that we correlate with two intervals of higher abundances of calcareous nannofossils. This warm episode led to an extension of nannoplankton ranges towards the extreme southern latitudes where they cannot survive today. Deep water formation in the North Atlantic during that time may have decreased significantly as a consequence of fresh water derived from the partial melting of the NH ice cap. Global bottom water

circulation, therefore, could have decelerated during MIS-31, causing in the SO a southward displacement of the PF and the SAF, and possibly their partial or complete dissipation. The result of the weakening of the bottom water circulation would be a southward extension of warmer water masses, allowing nannoplankton to extend beyond their present day operating barriers. Integration of our data with recent results obtained by the same approach at different sites along the Antarctic margin allows us to trace a more complete picture of Pleistocene climate history around Antarctica. Coring offshore Cape Roberts in the Ross Sea (CRP-1) recovered a 2-m-thick (33.82–31.89 mbsf) lithostratigraphic unit composed of unconsolidated biogenic carbonate, that represents the deposition of sediments during a single interglacial. Paleontological evidence related this unit to warmer than present surface-water temperatures and reduced sea-ice extent (Bohaty et al., 1998). The magnetostratigraphy indicates that the base of the Jaramillo lies within the unit that was then correlated with MIS-31 of the deep-sea oxygen isotope record (Scherer et al., 2008; Naish et al., 2009). At Prydz Bay ODP Site 1165, the presence of Pleistocene calcareous nannofossils, through multidisciplinary inferences, also delineates a warming event, interpreted as MIS-31 (Villa et al., 2008). All of this evidence supports the idea that the effect of the warming event recognizable as MIS-31 was not constricted to certain individual basins, but probably extended around the Antarctic continent. This in

84 G. Villa et al. / Global and Planetary Change 96–97 (2012) 75–86 Fig. 5. Litho-magnetostratigraphy (Krissek et al., 2007; Wilson et al., 2007) and the nannofossil abundance pattern in Core AND-1B with their tentative correlation with the Southern Hemisphere summer insolation maxima during MIS-31 (Laskar et al., 2004) (horizontal bands); MIS-31, as recognized by the calcareous nannofossil distribution, is indicated by gray vertical bar. Correlation with MIS-31 as recognized by oxygen isotopes and calcareous nannofossils at Site 1090 (Maiorano et al., 2009) and Site 1165 (Villa et al., 2008). For lithologic symbols see Fig. 3.

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turn could imply that both the EAIS and WAIS have not experienced stable conditions since the late Neogene (Kennett and Barker, 1990; Barrett, 2003). Instead the McMurdo Ice Shelf underwent a partial or total collapse slightly before 1 Ma as it was susceptible to insolation-driven warming, here recognized through the calcareous nannofossil signal that in turn is an expression of the two insolation maxima at about 1.08 and 1.06 Ma, respectively, and preceding a high-latitude cooling of East Antarctica at about 1.0 Ma (Raymo et al., 2006). Acknowledgements This study was supported by the Italian Programma Nazionale di Ricerche in Antartide (PNRA) and an ANDRILL subcontract from the University of Nebraska—Lincoln to SWW. The ANDRILL-MIS Science Team and the Chief Scientists Tim Naish and Ross Powell are thanked for encouragement and assistance. We are grateful to Jim Pospichal for his useful and pleasant revision, and to an anonymous reviewer for his helpful comments. The ANDRILL project is a multinational collaboration between the Antarctic programs 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; Raytheon Polar Services Corporation supported the science team at McMurdo Station and the Crary Science and Engineering Laboratory. 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, NZ Foundation for Research, Science and Technology and the Royal Society of NZ Marsden Fund, the Italian Antarctic Research Programme, the German Research Foundation (DFG) and the Alfred Wegener Institute Polar and Marine Research. Appendix 1 Calcareous nannofossils cited in this report: Braarudosphaera bigelowii (Gran and Braarud, 1935) Deflandre, 1947a Calcidiscus leptoporus (Murray and Blackman, 1898) Loeblich and Tappan, 1978 Chiasmolithus solitus (Bramlette and Sullivan, 1961) Locker, 1968 Coccolithus pelagicus (Wallich, 1877) Schiller, 1930 Cyclicargolithus floridanus (Roth and Hay in Hay et al., 1967) Bukry, 1971 Dictyoccocites antarcticus (Haq, 1976) Backman, 1980 Dictyoccocites productus (Kamptner, 1963) Backman, 1980 Discoaster sp. Tan Sin Hok, 1927 Geminilithella rotula (Kamptner, 1956) Backman, 1980 Gephyrocapsa small (b3–5 μm) Kamptner 1943 Pseudoemiliania lacunosa (Kamptner, 1963) Gartner, 1969 Reinhardtites levis Prins and Sissingh in Sissingh, 1977 Reticulofenestra asanoi Sato and Takayama, 1992 Reticulofenestra bisecta (Hay et al., 1966) Roth, 1970 Reticulofenestra daviesii (Haq, 1968) Haq, 1971 Reticulofenestra dictyoda (Deflandre in Deflandre and Fert, 1954) Stradner in Stradner and Edwards, 1968 Reticulofenestra hampdenensis Edwards, 1973 Reticulofenestra haqii Backman, 1978 Reticulofenestra minuta Roth, 1970 Reticulofenestra minutula (Gartner, 1967) Haq and Berggren, 1978 Reticulofenestra spp. Hay, Mohler and Wade, 1966 Reticulofenestra wadeae Bown, 2005 Stradneria crenulata (Bramlette and Martini, 1964) Noël, 1970 Transversopontis sigmoidalis Locker, 1967 Watznaueria barnesae (Black, 1959) Perch-Nielsen, 1968

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