Oligocene–Miocene Antarctic continental weathering record and paleoclimatic implications, Cape Roberts drilling Project, Ross Sea, Antarctica

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Palaeogeography, Palaeoclimatology, Palaeoecology 260 (2008) 30 – 40 www.elsevier.com/locate/palaeo

Oligocene–Miocene Antarctic continental weathering record and paleoclimatic implications, Cape Roberts drilling Project, Ross Sea, Antarctica S. Passchier a,⁎, L.A. Krissek b a

Department of Earth and Environmental Studies, Montclair State University, 252 Mallory Hall, 1 Normal Ave, Montclair NJ 07043, USA b School of Earth Sciences, Ohio State University, 130 Orton Hall, 155 South Oval Mall, Columbus OH 43210, USA Received in revised form 16 March 2007; accepted 22 August 2007

Abstract The bulk chemistry of mudrocks in Ross Sea drillcores on the Antarctic inner shelf is re-evaluated to extract records of continental paleotemperatures and ice extent. The chemical index of alteration (CIA) of the sedimentary sequence derived from the composite of the Cape Roberts cores changes in accordance with pCO2 [Pagani, M., Zachos, J.C., Freeman, K.H., Tipple, B., Bohaty, S., 2005. Marked change in atmospheric carbon dioxide concentrations during the Paleogene, Science 309, 600–603]. The (K2O + Na2O)/ Al2O3 molar ratio of the mudrocks is used as a mean summer temperature proxy for continental Antarctica and it shows similarities with benthic Mg/Ca temperatures for the Southern Ocean [Billups, K., Schrag, D.P., 2002. Paleotemperatures and ice-volume of the past 27 myr revisited with paired Mg/Ca and stable isotope measurements on benthic foraminifera. Paleoceanography 17. doi 10.1029/2000PA000567]. The ratio is calibrated using intervals with significant pollen assemblages and the temperature proxy enables extrapolation of the paleoclimatic data over a larger portion of the drilled record. The CIA and (K + Na)/Al ratios support a persistent cooling and drying of climate through the Oligocene–Miocene transition. The results reinforce the strong coupling between East Antarctic continental temperatures and ice volume, with deep-sea temperatures and concentrations of atmospheric carbon over long timescales. © 2008 Elsevier B.V. All rights reserved. Keywords: Cenozoic; Antarctica; Continental; Chemical weathering; CIA

1. Introduction The purpose of this study is to provide an Antarctic terrestrial paleoclimatic record for the Oligocene and Early Miocene. Antarctica plays an important role in atmosphere–ocean interactions of the climate system ⁎ Corresponding author. Fax: +1 973 655 4072. E-mail address: [email protected] (S. Passchier). 0031-0182/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2007.08.012

due to its large surface area and the fact that it is situated in a near-polar position. From at least the Early Oligocene onward the continent periodically supported continental-scale ice sheets (Hambrey et al., 1991; Naish et al., 2001; Strand et al., 2003), but the magnitude and frequency of ice sheet expansions and reductions through the Late Cenozoic is poorly constrained by quantitative data from Antarctica. Modeling studies suggest that changes in pCO2 are a

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significant control on ice sheet inception (DeConto and Pollard, 2003). The East Antarctic continental weathering record is investigated here because it is strongly climate-controlled and because silicate weathering is a sink for CO2. Here previously published chemical records are analyzed to extract a quantitative paleoweathering signal and to identify major events of glacial expansion in the Oligocene and Miocene sequences drilled by the Cape Roberts drilling Project (CRP). CRP drilled three holes in the McMurdo Sound (Ross Sea) between 1997 and 1999 (Cape Roberts Science Team (CRST), 1998, 1999, 2000). The holes are situated on the margin of the Victoria Land basin, which forms part of the West Antarctic Rift system. Rifting and uplift of the nearby Transantarctic Mountain rift flank (Fig. 1) commenced ~ 55 Mya and glaciers have supplied sediments to the basin for at least the last 34 Ma. The sedimentary record in the Cape Roberts cores is strongly cyclic and represents expansions and reductions in the volume of the East Antarctic ice sheet (Powell et al., 1998, 2000, 2001; Fielding et al., 1998, 2000, 2001; Naish et al., 2001). The ice sheet, or an outlet glacier expanding through the Mackay sea valley from the Transantarctic Mountains, also grounded on the shelf several times (Passchier et al., 1998; Passchier, 2000;

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Van der Meer and Hiemstra, 1998; Van der Meer, 2000). The sediment sources for the coarse-grained glacial facies were beyond and within the Transantarctic Mountains (Smellie, 1998, 2000, 2001; Talarico and Sandroni, 1998; Talarico et al., 2000; Sandroni and Talarico, 2001). The fine-grained facies represent deposition below wave base at higher relative sea level, with a low abundance of coarse fraction being delivered by ice rafting and eolian processes (DeSantis and Barrett, 1998; Barrett and Anderson, 2000; Barrett, 2001). Palynological data are sparse due to low concentrations of organic matter, as caused by the high sedimentation rates typical of the icehouse sequences around the Antarctic margin. The sparse occurrences that are present, however, indicate the presence of a partially open landscape dominated by small Nothofagus stands or sparse tundra vegetation (Raine, 1998; Askin and Raine, 2000; Raine and Askin, 2001; Prebble et al., 2006). Nesbitt and Young (1982) earlier demonstrated that climate has a strong influence on the bulk sediment composition of mudrocks. Using the ratios of major elements that are present in a wide variety of rock types, the degree of chemical weathering can be estimated and used as a climate proxy. Nesbitt and Young (1982) established the chemical index of alteration (CIA), which is defined as CIA = [Al 2 O 3 /(Al 2 O 3 + CaO ⁎ + Na 2 O + K 2 O)] × 100, where components are expressed as molar proportions, and CaO⁎ represents the amount of CaO fixed in silicate minerals. In environments that are dominated by physical weathering, CIAs are similar to those of fresh feldspars (33–50), whereas the average shale produced by accumulation of chemically weathered detritus has a CIA of 70– 75. In a study of 126 North American soils on a wide variety of parent materials, Sheldon et al. (2002) found that the CIAs of soil B horizons are statistically correlated to precipitation, whereas the (K2O + Na2O)/Al2O3 molar ratios (here abbreviated to (K + Na)/Al) correlate to temperature; these results are in accordance with theoretical experiments (White et al., 1999). These principles are applied here to interpret bulk detrital chemical records from CRP, by assuming that the fine-grained CRP sediments underwent erosion, transport, and deposition without significant additional weathering. 2. Materials and methods

Fig. 1. Location of the Cape Roberts Project drill holes (CRP).

Previously measured major element geochemical data for 185 samples from Krissek and Kyle (1998, 2000, 2001) are used to assemble a composite CIA record for the Cape Roberts cores. X-ray fluorescence measurements were carried out on bulk samples and on fine residues derived

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from sieving at 63 μm for micropaleontological analysis. Krissek and Kyle (1998) already demonstrated that the size separation resulting from sieving did not significantly affect the geochemical compositions measured. Here, the dataset is used to derive the chemical index of alteration and the (K + Na)/Al molar ratio after correcting the CaO content for the presence of CaCO3, and after removing a volcanic overprint present on the upper portion of the CRP2 and the entire CRP-1 core. These corrections are necessary so that the CIA can be used as a quantitative indicator of silicate weathering in the source area. Data from only the upper 340 m of the CRP-3 core were included, because authigenic smectite is present below this depth (Wise et al., 2001) and the associated diagenetic processes complicate the paleoclimatic interpretation of the bulk chemical data. The chronology of the cores is mainly based on biostratigraphy, Ar/Ar radiometric dating, Sr isotope dating and magnetostratigraphy (Florindo et al., 2005). Due to the presence of multiple hiatus, chronostratigraphic correlation is only possible with a resolution of 1–3 Ma, with b 1 Ma resolution only in restricted intervals. The magnetostratigraphic chronology (Florindo et al., 2005) shows that the CRP-1 and CRP-2 cores overlap by ca. 31 m. Although a gap of several tens of meters may separate the bottom of CRP-2 from the top of CRP-3, consistent sediment accumulation rates and lithofacies correlations are established by simply joining the top of CRP-3 with the bottom of CRP-2 (Florindo et al., 2005). The location of samples in the CRP-1, CRP-2 and CRP-3 cores is represented as cumulative meters drilled (cmd). 2.1. Carbonate correction Carbonate free CaO⁎ values are calculated by correcting total CaO values using the % carbonate measurements of Dietrich and Klosa (1998) and Dietrich et al. (2000, 2001). The carbonate percentages were derived from measuring the gas pressure through time in a reaction vessel, which is representative of the amount of CO2 set free during acid digestion. The nearest samples within each lithostratigraphic unit were used for the correction. No carbonate % was available within the lithostratigraphic unit of one bulk geochemical sample, so the average of the nearest overlying and underlying measurements was used. In the correction it is assumed that all carbonate is present as calcite. 2.2. Correction for a volcanic overprint In order to correct for a volcanic overprint on the bulk chemical record, Ti excess values were calculated for

the upper portion of CRP-2 and for CRP-1. Ti is a conservative element in earth surface processes, as is Al, and is unaffected by chemical weathering in most climatic settings (Young and Nesbitt, 1998). The Ti/Al ratio therefore is an excellent provenance indicator. TiO2 concentrations of N1.00 wt.% in many bulk samples in CRP-2 and CRP-1 indicate the addition of a basaltic volcanic component. A change in Ti/Al ratios with large fluctuations is observed above ca. 305 mbsf in CRP-2 (Krissek and Kyle, 2000), and coincides with the presence of sand-sized volcanic detritus of airborne and hydrothermal origin (Armienti et al., 2001; Smellie, 1998, 2000) and smectite with a hydrovolcanic source (Ehrmann et al., 2005). It is likely that the fine-grained volcanic material in the CRP cores was supplied by a basaltic volcanic source, situated to the south of the drillsite, and was carried to the basin by glacial transport and by currents without major chemical modification through subaerial weathering. As a result, this additional, unweathered, and rapidly cycled volcanic contribution should be removed so that it does not obscure the paleoclimatic record carried by the terrigenous sediments, which are appropriate for study using the CIA. By using the relatively constant Ti/Al ratios of the lower non-volcanic portion of the CRP-2 core as an indicator of the composition of the non-volcanic component, the Al contents of the volcanic-bearing samples can be used to calculate the excess wt. % TiO2 supplied by the volcanic component. Roser and Pyne (1989) report the average composition of 26 basanites from the McMurdo Volcanic Group; the ratios of TiO2 vs. CaO, Na2O and K2O in those basanites are used here to correct for the additions of CaO, Na2O, and K2O from the unaltered basaltic component. 2.3. Sample selection The fine residues (27 samples) from the upper 100 m of the CRP-1 core (Krissek and Kyle, 1998) were omitted due to contamination with a dispersant during the sieving process; this contamination produced high P2O5 contents. The dispersant was not used on any of the remaining fine residue samples. A further 20 samples were removed from the dataset due to problems in extracting a valid paleoclimatic signal, as described here: 1) two samples from an evolved tephra horizon (Lithostratigraphic Unit 7.2 in CRP-2) were removed, as well as a sample near a concentration of volcanic ash (278.8 mbsf in CRP-2). These three samples had anomalously high Na2O concentrations; 2) in 14 cases, the correction for carbonate % yielded negative CaO⁎ values in intervals with exceptionally high carbonate contents in the lower portion of CRP-2

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and in CRP-3 (Dietrich and Klosa, 1998; Dietrich et al., 2000, 2001); and 3) in the lower portion of CRP-2 and in CRP-3, three samples yielded high CaO values in the absence of high carbonate % in nearby samples. As a result, these high CaO values were judged to be unreliable. 3. Results The Oligocene–Miocene composite CIA and (K+ Na)/ Al records for the CRP cores are indicated in Fig. 2. The composite CIA record of the three cores shows a general upward decrease, but with a strong superimposed pattern of cyclic variations. The record can be subdivided into two main portions, separated by an unconformity at ~352 cmd (~307 mbsf in CRP-2). The lower portion shows highamplitude changes in CIAs, ranging between 42 and 67. Above ~352 cmd the curve flattens to values similar to those of Quaternary diamicts, with a minimum CIA of 38

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and short-term increases to CIA values as high as 57. CIA values are similar in the lower portions of the overlap between CRP-1 and CRP-2, but an offset appears near the top of the overlapping sections. The composite (K + Na)/Al record shows a slight decrease in values at ~ 840 cmd (~ 174 mbsf in CRP-3) and an increase at ~ 352 cmd with values staying N 0.55 above 315 cmd. Large values in the (K + Na)/Al record occur at ~ 1004 cmd (338 mbsf) and ~ 884 cmd (217 mbsf) in CRP-3 and at 646–626 cmd (602– 580 mbsf), ~ 561–549 cmd (~ 516–503 mbsf) and ~ 480–472 cmd (435–426 mbsf) in CRP-2, as well as 0–100 cmd, in the CRP-1 core (CRST, 1999, 2000). 4. Discussion In soils, the molar ratios of Al, K, Na, and Ca are affected by precipitation and temperature. In North

Fig. 2. Composite chemical records in the CRP cores plotted relative to cumulative core depth (After Barrett, 2008; chronology after Florindo et al., 2005).

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American soils developed on a wide variety of parent materials, the (K + Na)/Al molar ratios of the soil B horizons are statistically correlated to temperature (Sheldon et al., 2002). Experimental studies of silicate weathering in granitoid rocks, as well as relationships between cation discharge and annual mean temperature in natural granitoid watersheds, also show a strong positive relationship between the release of K and Na and temperature (White et al., 1999). In cold environments, biotite weathering is the principal cause of K-loss from granitoid weathering profiles, whereas Na is released from plagioclase. In contrast, Ca does not show a positive correlation with temperature in the experimental studies (White et al., 1999). Ca (and Mg) concentrations are probably affected by ion exchange during the initial stages of biotite weathering, and by the effects of other processes, such as acidification. In cold climatic conditions, such as in the foreland of a glacier, chemical weathering is dominated by loss of Ca (Anderson et al., 2000), and glacial tills deposited upon glacial advance after deglaciation can be strongly depleted in CaO⁎ (Roy et al., 2004). Rates of chemical weathering in the foreland of a glacier can be significant due to the relatively large reactive surface area of the finely comminuted glacial materials (Sharp et al., 1995; Burkins et al., 1999). Sheldon et al. (2002) found that CIAs of soil B horizons in American soils correlate with precipitation, suggesting that the availability of moisture is another important limiting factor in the leaching of Ca. In the CRP record, most of the samples with CIAs N60 have CaO⁎ contents b 1, which is strongly depleted relative to average crustal abundances (Wedepohl, 1995). The CRP samples show less depletion of Na2O and negligible depletion in K2O. These results suggest that chemical weathering is mainly affecting calcic plagioclase, which is consistent with weathering in the foreland of a glacier or ice sheet (Anderson et al., 2000). The loss of Ca, as indicated by the CIA, requires exposure of surfaces to chemical weathering, and possibly to moisture from snow melt; as a result, the loss of Ca is controlled by ice extent. Nesbitt and Young (1982) calibrated the CIA using chemical analyses from sedimentary rocks in a variety of climatic settings. Pleistocene tills generally produce CIA values of 40–55, whereas proglacial mudstones and varved sediments typically have values of 55–65. The relatively low CIAs in the CRP cores, with a maximum of 67 in only one sample, are interpreted to indicate that glacial conditions prevailed throughout deposition of the entire Oligocene and lower Miocene CRP sequence. CIA values up to 67 are still considerably lower than

those of average shales, whose CIAs are 70–75 (Nesbitt and Young, 1982); even in the mudstones, these lower CIAs indicate the presence of a small proportion of physically weathered debris. These interpretations agree with Ehrmann et al. (2005), who document evidence of physical weathering in clay mineral compositions for the upper Oligocene–lower Miocene interval above ~ 330 mbsf in CRP-3 (~ 998 cmd). 4.1. Lower Oligocene sequence (33–29 Ma) The long-term trend of the (K + Na)/Al ratios in the Lower Oligocene sequence is characterized as a relatively flat curve with superimposed short-term fluctuations. The lowest (K + Na)/Al ratios, indicating the highest temperatures, occur between 700 and 800 cmd at the top of CRP-3. CIAs are low in the center of this interval indicating reduced chemical weathering, ice expansion, and dryer conditions. No evidence was found of anomalous grain-size or provenance in these strata to explain the apparent discrepancy between (K + Na)/Al ratios and CIAs. The interval between 700 and 800 cmd is pervasively brecciated, but it is unclear how the associated postdepositional processes (e.g., Passchier et al., 1998) may have altered the chemical composition of the sediment. High peaks in the (K + Na)/Al ratios, indicating low temperatures, associated with relatively high (N 60) but declining CIAs, indicating increasingly more reduced chemical weathering are found in CRP-3 and the lower portion of CRP-2. The samples were taken from beds that occur adjacent to sandstones sourced from coarsegrained plutonic rocks, most likely the Granite Harbour Intrusives on the coast of Victoria Land (Polozek, 2000; Smellie, 2000). The apparent discrepancy between high (K + Na)/Al ratios and relatively high CIAs of mudstones in this interval is caused by the combination of glacial rock flour from a granitic source and a contribution of chemically weathered material. The combined supply of an unaltered granitic component and a weathered fine fraction can be explained by glaciation, with outlet glaciers eroding basement in the nearby Transantarctic Mountains but sufficient exposure of ice-free surfaces to allow chemical weathering; the ice-free surfaces may have been located at lower elevations along the Ross Sea coast. This proposal is supported by the clay mineralogy of these intervals, characterized by high proportions of smectite (Ehrmann et al., 2005), and by the palynology of the strata. The lowermost strata in CRP-2/2A and the upper portion of CRP-3 contain a pollen assemblage indicating woody vegetation of Nothofagus trees and shrubs, with a

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possible modern analogue in the Magellanic region of southernmost South America. In the Magellanic region today, mean summer temperature range is 10–12 °C (Raine and Askin, 2001; Prebble et al., 2006). The cyclic variations in the CIAs coincides with the cycles identified in sequence stratigraphic analyses with maxima up to 67 occurring within the “highstand” mudstones and a sharp decline in CIA in diamict units

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near each sequence boundary. The cyclical variations in the CIAs therefore probably represent the same orbital scale glacial–interglacial cyclicity (Naish et al., 2001). The amplitude of the CIA cycles further quantifies the magnitude of the glacial–interglacial variability, which is large in the Lower Oligocene sequence. The maximum CIA ratio of 67 occurs at 386 cmd, and values slowly decrease upsection to the unconformity at

Fig. 3. Correlation of Cape Roberts chemical records to pCO2 (Pagani et al., 2005), and Mg/Ca temperature of Billups and Schrag (2002). All records in timescale of Berggren et al. (1995).

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350 cmd. Within this interval an intraformational breccia is found (CRST, 1999), with a possible origin by meltwater hydrofracturing in a temperate proglacial setting (Passchier, 2000). Temperate, wet-based continental glacial conditions interrupted by interglacials with reduced ice sheet coverage are therefore proposed for the lower Oligocene sequence. 4.2. Upper Oligocene–Lower Miocene sequence (25–17 Ma) Low CIA values at ~ 350 cmd (~ 305 mbsf) and ~ 187 cmd (~ 142 mbsf) in the CRP-2/2A cores coincide with unconformity surfaces, ice-proximal sedimentary facies, and interpreted ice grounding events near the drill site identified visually during initial core characterization (Powell et al., 2000; Passchier, 2000). The chemical ratios at these levels indicate a supply of fresh, unaltered detritus to the basin in a climatic setting that was dominated by physical weathering. These glacial strata are interbedded with mudstones with CIAs b 57, indicating slightly reduced glacial conditions with limited chemical weathering in the foreland of the ice sheet. The palynology of the strata provides evidence of cooling in the Late Oligocene–Early Miocene, with a mean January temperature at sea level of 2–7 °C for vegetation mainly consisting of flowering plants and bryophytes, similar to a present-day herb tundra of the present Antarctic Peninsula (Raine, 1998). Evidence of partial cover of low-diversity woody vegetation, found in certain intervals of both the CRP-1 and CRP-2/2A cores, is consistent with a mean January temperature of 7–10 °C (Raine, 1998; Askin and Raine, 2000). Viewing the (K + Na)/Al ratios as a temperature proxy, the stepwise increase at ~ 352–315 cmd and the continuously low values above 315 cmd (~ 25 Ma) support the transition to cooling conditions identified in other datasets. The fact that CIAs drop from a range of 42–67 below this level to a range of 38–57 above this boundary can also be regarded as indicating a transition to dryer climates. Dryer conditions are also documented by the marked decrease in smectite above ~ 270 mbsf in CRP-2 (~ 315 cmd; Ehrmann et al., 2005), since the formation of smectite requires humid climatic conditions. 4.3. Chemical index of alteration and ice extent The CIA record of the Cape Roberts cores shows a correspondence to the Oligocene–Miocene CO2 record of Pagani et al. (2005) in both general pattern and in details (Fig. 3). Overall, CIAs are generally high in the

earliest Oligocene, as are CO2 concentrations. In detail, maxima in the CIA at 20–20.5 Ma, 24.5–25 Ma, and 27.5–28 Ma correspond to maxima in the CO2 concentrations, whereas the CIA minima at ~ 25 Ma and ~ 29.5 Ma correspond to lows in the CO 2 concentrations. This correlation may be explained by the fact that levels of atmospheric CO2 influenced weathering rates on the Antarctic coast; these influences may have included direct effects on the acidity of the precipitation and on the temperature and precipitation patterns that influenced soil formation. Indirectly, pCO2 may also have influenced silicate weathering by affecting plant growth and the distribution of organic matter in soils. On the other hand, the correlation between the CIAs and CO2 may have been driven by changes in weathering rates on the Antarctic continent, where periodic glacial ice coverage and interglacial exposure affected the CO2 content of the atmosphere. The CIA record also shows similarities to the δ18Osw curve derived from studies of deep-sea cores (Lear et al., 2000, 2004; Billups and Schrag, 2003), and an apparent sea level curve constructed from New Jersey margin stratigraphy (Kominz and Pekar, 2001) which confirms that the three proxies are mainly controlled by the extent of ice cover in Antarctica. The Late Oligocene deglaciation at ~ 26 Ma identified by Billups and Schrag (2003), which is also visible in the apparent sea level curve of Kominz and Pekar (2001) may be partly represented by a hiatus in the Cape Roberts cores (Fig. 3); however, evidence of partial deglaciation is shown in the CIA record at ~ 27.5 Ma by a CIA value of 67. In view of the limited resolution of the chronology for 25–31 Ma in the CRP core (Florindo et al., 2005), it is possible that the level presently dated as ~ 27.5 Ma actually correlates to the ~ 26 Ma event (Fig. 4). If that revised correlation is correct, then the hiatus at ~ 306 mbsf in CRP2/2A is not as large as the estimated maximum of 2 Ma that was used to construct Fig. 3. Unfortunately, the existing chronology of the CRP cores is not sufficiently robust to resolve lead and lag relationships; improving this chronology should be a focus of future work. Minima in the CIAs (b55) indicate glacial events of continental proportions at ~ 33 and 29.5 Ma, and possibly at 31 Ma as discussed above (Fig. 3). The Oi-1 event of Miller et al. (1991) probably occurs beyond the limit of the record. The glacial event with grounded ice near the drill site recorded at 490 cmd in the CRP-2 cores occurs at ~ 29.5 Ma and coincides with a dramatic decrease in pCO2 (Pagani et al., 2005) and a lowstand of considerable magnitude in the Phanerozoic sea level curve constructed by Miller et al. (2005). Other

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Table 1 Sample depths (mbsf) for calibration of pollen data and bulk geochemistry

Fig. 4. Correlation of CIA (upper solid line) with sea level proxies from Lear et al. (2004) for the Oligocene. The gray graph is an apparent sea level curve based on New Jersey margin stratigraphy by Kominz and Pekar (2001). The black solid and dashed lines are corrected and uncorrected δ18O seawater curves from benthic foraminifera at ODP Site 1218. Note that the chronology of the CIA records is poorly constrained for the upper Oligocene (Florindo et al., 2005) and that the hiatus may be smaller than the 2 Ma indicated.

CIA minima that are supported by unconformity surfaces and evidence of grounded ice near the drill site (Passchier, 2000; Van der Meer, 2000) occur at 350 cmd (~ 305 mbsf) and ~ 187 cmd (~ 142 mbsf) in the CRP-2/2A cores and are dated at ~ 25 and 23 Ma. Further CIA minima are present at ~ 24 and 17.5 Ma. The ~ 24, ~ 23, and ~ 17.5 Ma events likely correspond to the Mi-1 events identified in oxygen isotope curves (Miller et al., 1991). Two intervals at approximately 20.5 and 18.4 Ma indicate more advanced conditions of chemical weathering. The 20.5 Ma event is accompanied by slightly higher CO2 concentrations in the atmosphere (Pagani et al., 2005). The 18.4 Ma maximum does not correspond to an increase in CO2, but occurs a few meters above an ashfall in CRP-1 (~ 117 mbsf). The maximum weathering event at 18.4 Ma may represent a local phenomenon, since deposition of ash on soil profiles enhances chemical weathering by its acidifying effect (Burkins et al., 1999). However, it should be noted that this interval also yields slightly higher abundances of Nothofagus flemingi pollen, possibly indicating improved conditions for plant growth or reworking of Eocene/Oligocene pollen assemblages (Raine, 1998).

Core

Pollen (mbsf)

Chemistry (mbsf)

(K + Na)/Al

Mean Jan. T (°C)

CRP-1 CRP-1 CRP-1 CRP2/2A CRP2/2A CRP2/2A CRP2/2A CRP2/2A CRP3 CRP3

58.43 99.02 116.45 255.51 263.39 296.09 299.32 575.36–575.37 106.22–106.24 190.77–190.79

56.83 99.52 116.43 255.39 263.32 296.32 299.49 575.45 106.07 190.77

0.68027 0.69918 0.513288 0.584718 0.566306 0.557206 0.533075 0.623086 0.556078 0.51025

4.5 4.5 11 11 11 8.5 8.5 8.5 11 11

paleotemperatures estimated from the vegetation types reconstructed from palynological data. Intervals of core were selected where samples for geochemistry and palynology are sufficiently close to make a direct comparison between the records, and where pollen and spores are sufficiently abundant to allow vegetation reconstructions with some confidence (Table 1). Three vegetation types were reconstructed from pollen and spore assemblages in the CRP cores (Raine, 1998; Askin and Raine, 2000; Raine and Askin, 2001; Prebble et al., 2006): 1) a woody vegetation of Nothofagus trees and shrubs with a mean summer temperature range of 10– 12 °C; 2) partial cover of low-diversity woody vegetation with a mean summer temperature of 7–10 °C; 3) a vegetation type mainly consisting of flowering plants and bryophytes with a mean summer temperature of 2–7 °C. A negative correlation between (K + Na)/Al ratios and vegetation type was found (Fig. 5). The equation for the regression line then was used to calculate a mean summer

4.4. Average summer paleotemperatures Mean summer temperatures for the CRP sequence were derived by calibrating the (K + Na)/Al ratios with

Fig. 5. Correlation between (K + Na)/Al ratio and mean summer temperature estimated from palynological data.

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temperature from the (K + Na)/Al ratios of the other samples, including samples from intervals where pollen and spores were absent or sparse due to high sedimentation rates or low productivity. Although the uncertainty of the temperature estimates from the palynological calibration is quite large, the relationship between temperature and (K + Na)/Al ratios is similar to the one derived statistically for North American soils based on measured mean annual temperatures (Sheldon et al., 2002). Applying the linear equation of Sheldon et al. (2002) to the CRP record yields temperature estimates ca. 3 degrees lower at higher temperatures, although the two sets of estimates converge at temperatures b 5 °C. This difference between the two sets of paleotemperature estimates can be explained by the fact that the palynological calibration yields mean summer temperatures, whereas the equation of Sheldon et al. (2002) is based on measurements of mean annual temperature with a bias towards more temperate climatic conditions. The temperature record derived from the (K + Na)/Al ratios of the Cape Roberts cores indicates relatively cool mean summer temperatures (~ 10 °C) in the earliest Oligocene, but otherwise relatively constant temperatures of ~ 12 °C, followed by a cooling trend starting at ~ 24 Ma and progressing into the Early Miocene (Fig. 3). This long-term temperature trend is similar to the composite Mg/Ca temperature trend for the deep-ocean of Lear et al. (2000), which shows stable temperatures through the Oligocene and very slow cooling across the Oligocene–Miocene transition. The Early Miocene portion of the Mg/Ca temperature record at Southern Ocean Site 747 (Billups and Schrag, 2002) indicates a slow warming, opposite to the cooling trend observed in CRP (Fig. 3), but short-term cooling events at ~ 21, ~ 19.5 and ~ 18.5 Ma at Site 747 correspond to low mean summer temperatures in East Antarctica. The discrepancies between the Mg/Ca and CRP temperature records can be attributed to differences in the resolution and/or sensitivity of the two records, the limited accuracy of the CRP chronology, changes in the Mg and Ca concentrations of seawater through time and variability in Mg/Ca temperature records between ocean basins (Billups and Schrag, 2003). Moreover, continental temperatures in Antarctica could be affected by significant snow accumulation before ice reaches sea level; this change may have had less impact on deep-sea temperatures. 5. Conclusions Glacial conditions in East Antarctica intensified from the Oligocene to the Miocene. This is apparent in both

the CIA record and the (K + Na)/Al record of the Cape Roberts cores: 1) Large quantities of physically weathered glacial debris were periodically supplied to the Victoria Land basin through the Transantarctic Mountains in the Early Oligocene, indicating major expansions of continental-scale ice sheets. 2) Cyclical variations of the CIA record document episodes of glacial expansion alternating with periods of moderate chemical weathering on the Antarctic coast, with ice still present further inland. 3) The CIA and (K + Na)/Al ratios suggest persistent cooling and drying through the Oligocene–Miocene transition, in agreement with clay mineralogical and palynological datasets derived from the same cores (Ehrmann et al., 2005; Raine, 1998; Askin and Raine, 2000). 4) Similarities between the CIA, ice volume proxies (Kominz and Pekar, 2001; Lear et al., 2000, 2004) and pCO2 (Pagani et al., 2005) records, and between the Antarctic (K + Na)/Al record and the composite Mg/Ca temperature record for the deep-sea (Lear et al., 2000; Billups and Schrag, 2002), highlight linkages and possible feedbacks between the atmospheric, oceanic, and lithospheric components of the Oligocene/Miocene climate system. Acknowledgements This paper is a contribution to the SCAR Scientific Research Program Antarctic Climate Evolution (ACE). The Global Education Center and the College of Science and Mathematics at Montclair State University provided a travel grant to present this study at the 2006 SCAR conference “Antarctica in the Earth System” in Hobart, Tasmania. Feedback given by several members of the Cape Roberts and ANDRILL Science Teams is highly appreciated. Peter Barrett kindly provided the lithological column for Fig. 3. Two anonymous reviewers are thanked for helpful comments that improved the paper. References Anderson, S.P., Drever, J.I., Frost, C.D., Holden, P., 2000. Chemical weathering in the foreland of a retreating glacier. Geochim. Cosmochim. Acta 64, 1173–1189. Armienti, P., Tamponi, M., Pompilio, M., 2001. Sand provenance from major and trace element analyses of bulk rock and sand grains from CRP-2/2A, Victoria Land Basin, Antarctica. Terra Antart 8, 569–582. Askin, R.A., Raine, J.I., 2000. Oligocene and early Miocene terrestrial palynology of the Cape Roberts drillhole CRP-2/2A, Victoria Land Basin, Antarctica. Terra Antart 7, 493–501.

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