ALBERT

Description: George V Land (Antarctica) includes the boundary between Late Archean-Paleoproterozoic metamorphic terrains of the East Antarctic craton and the intrusive and metasedimentary rocks of the Early Paleozoic Ross-Delamerian Orogen. This therefore represents a key region for understanding the tectono-metamorphic evolution of the East Antarctic Craton and the Ross Orogen and for defining their structural relationship in East Antarctica, with potential implications for Gondwana reconstructions. In the East Antarctic Craton the outcrops closest to the Ross orogenic belt form the Mertz Shear Zone, a prominent ductile shear zone up to 5 km wide. Its deformation fabric includes a series of progressive, overprinting shear structures developed under different metamorphic conditions: from an early medium-P granulite-facies metamorphism, through amphibolite-facies to late greenschist-facies conditions. 40Ar-39Ar laserprobe data on biotite in mylonitic rocks from the Mertz Shear Zone indicate that the minimum age for ductile deformation under greenschist-facies conditions is 1502 ± 9 Ma and reveal no evidence of reactivation processes linked to the Ross Orogeny. 40Ar-39Ar laserprobe data on amphibole, although plagued by excess argon, suggest the presence of a ~1.7 Ga old phase of regional-scale retrogression under amphibolite-facies conditions. Results support the correlation between the East Antarctic Craton in the Mertz Glacier area and the Sleaford Complex of the Gawler Craton in southern Australia, and suggest that the Mertz Shear Zone may be considered a correlative of the Kalinjala Shear Zone. An erratic immature metasandstone collected east of Ninnis Glacier (~180 km east of the Mertz Glacier) and petrographically similar to metasedimentary rocks enclosed as xenoliths in Cambro-Ordovician granites cropping out along the western side of Ninnis Glacier, yielded detrital white-mica 40Ar-39Ar ages from ~530 to 640 Ma and a minimum age of 518 ± 5 Ma. This pattern compares remarkably well with those previously obtained for the Kanmantoo Group from the Adelaide Rift Complex of southern Australia, thereby suggesting that the segment of the Ross Orogen exposed east of the Mertz Glacier may represent a continuation of the eastern part of the Delamerian Orogen.

Description: The compositional record of the AND-2A drillcore is examined using petrological, sedimentological, volcanological and geochemical analysis of clasts, sediments and pore waters. Preliminary investigations of basement clasts (granitoids and metasediments) indicate both local and distal sources corresponding to variable ice-volume and ice-flow directions. Low abundance of sedimentary clasts (e.g., arkose, litharenite) suggests reduced contributions from sedimentary covers while intraclasts (e.g., diamictite, conglomerate) attest to intrabasinal reworking. Volcanic material includes pyroclasts (e.g., pumice, scoria), sediments and lava. Primary and reworked tephra layers occur within the Early Miocene interval (1093 to 640 metres below sea floor mbsf). The compositions of volcanic clasts reveal a diversity of alkaline types derived from the McMurdo Volcanic Group. Finer-grained sediments (e.g., sandstone, siltstone) show increases in biogenic silica and volcanic glass from 230 to 780 mbsf and higher proportions of terrigenous material c. 350 to 750 mbsf and below 970 mbsf. Basement clast assemblages suggest a dominant provenance from the Skelton Glacier - Darwin Glacier area and from the Ferrar Glacier - Koettlitz Glacier area. Provenance of sand grains is consistent with clast sources. Thirteen Geochemical Units are established based on compositional trends derived from continuous XRF scanning. High values of Fe and Ti indicate terrigenous and volcanic sources, whereas high Ca values signify either biogenic or diagenic sources. Highly alkaline and saline pore waters were produced by chemical exchange with glass at moderately elevated temperatures.

Description: The site for CRP-3, 12 km east of Cape Roberts (77.006°S; 103.719°E)was selecte to overlap the lower Oligocene strata cored in nearby CRP-2/2A, and to sample the oldest strata in the Victoria Land Basin (VLB) for Paleogene climatic and tectonic history. As it transpired there was underlap of the order of 10s of metres. CRP-3 was cored from 3 to 939 mbsf (metres below the sea floor), with a core recovery of 97%. Coring took place from October 9 to November 19, 1999, on 2.0 to 2.2 m of sea ice and through 295 in of water. The Cenozoic strata cored were mostly g1acially influenced marine sediments of early Oligocene age, though they may be earliest Eocene near the base, where at 823 mbsf Devonian Beacon sandstone was encountered. Following CRP-1 and CRP-2/2A, CRP-3 completes the coring of 1500 m of strata on the western margin of the VLB. Core fractures and other physical properties, such as sonic velocity, density and magnetic susceptibility, were measured throughout the core. Down-hole logs for these and other properties were taken from 20 down to 900-919 mbsf. Also, vertical seismic profile data were gathered from shots offset both along strike and up dip from the hole. Sonic velocities in CRP-3 are close to 2.0 km/s in the upper 80 m, but become significantly faster below 95 mbsf, averaging 3.2+0.6 km/s to the bottom of the hole. An exception to this is an interval of dolerite conglomerate from 790 to c. 820 mbsf with a velocity of c. 4.5 km/s. Dip of the strata also increases down-hole from 10° in the upper 100 m to around 22° at the bottom. Over 3000 fractures were logged through the hole, and borehole televiewer imagery was obtained for most of the hole for orienting core and future stress field analysis. Two high-angle crush zones, interpreted as faults, were encountered at c. 260 and c. 540 mbsf, but no stratigraphic displacement could be recognised. A third fault zone is inferred from a low angle shear zone in the upper part of a coarse dolerite conglomerate from 790 to 805 mbsf. Temperature gradient was found to be 28.5°.km-1. Basement strata cored from 823 mbsf to the bottom of the hole are largely light-reddish brown medium-grained sandstone (quartz-cemented quartzarenite) with abundant well-defined parallel lamination. These features are comparable with the middle Devonian part of the Beacon Supergroup, possibly the Arena Sandstone. This interval also includes a body of intrusive rock from 901 to 920 mbsf. It has brecciated contacts and is highly altered but some tholeiitic affinity can be recognised in the trace element chemistry. Its age is unknown. Post-Beacon sedimentation began on deeply eroded quartzarenite with the deposition of a thin sandstone breccia and conglomerate, probably as terrestrial talus, followed by dolerite conglomerate and minor sandstone of probable fluvial origin to 790 mbsf. Sedimentation continued in a marine setting, initially sandstone and conglomerate, but above c. 330 mbsf the strata include mudstone and diamictite also. The older sandstone and conglomerate beds are seen as the products of rapid episodic sedimentation. They are interpreted by some as the product of glaciofluvial discharge into shallow coastal waters, and others as a result of sediment gravity flows, perhaps glacially sourced, into deeper water. The core above c. 330 mbsf has facies that allow the recognition of cyclic sequences similar to those in CRP-2A. Fourteen unconformity-bounded sequences have been recognised from 330 mbsf to the sea floor, and are interpreted in terms of glacial advance and retreat, and sea level fall and rise. Detailed lithological descriptions on a scale of 1 :20 are presented for the full length of the core, along with core box images, as a 300 page supplement to this issue. The strata cored by CRP-3 are for the most part poorly fossiliferous, perhaps as a consequence of high sedimentation rates. Nevertheless the upper 200 m includes several siliceous microfossil- and calcareous nannoplankton-bearing intervals. Siliceous microfossils, including diatoms, ebrideans, chrysophycean cysts and silicoflagellates are abundant and well-preserved in the upper 67 m - below this level samples are barren or poorly preserved, but contain residual floras that indicate assemblages were once rich. No siliceous microfossils were found below 193 mbsf. Calcareous nannofossil have a similar distribution but are generally well preserved. Foraminifera, marine and terrestrial palynomorphs, and marine macrofossils were found consistentlsy down to c. 330 mbsf and sporadically to 525 mbsf. The taxa suggest marine deposition in water depth of c. 50 to 120 m. Below 525 mbsf no microfossils were found, apart from mudstone with similar marine and terrestrial palynomorphs at 781 mbsf, and rare miospores in the conglomerate below 790 mbsf. The terrestrial miospore record, which include several species of Nothofagus and podocarpaceous conifers, suggest low diversity woody vegetation, implying a cold temperate to periglacial climate for the hinterland throughout the period recorded by CRP-3. Important components of the warmer Eocene flora, known from erratics in southern McMurdo Sound, are missing, through the dominance of smectite in clay from strata below 650 mbsf suggests that the landscape prior to the timne of deposition had experienced a more temperate weathering regime. Biostratigraphy for ihe upper part of CRP-3 is provided by diatoms and calcareous nannofossils. The first appearance of Cavitatus jouseanus at 48 mbsf suggests an age of arround 31 Ma for this horizon. The last appearance of Transverspontis pulcheroides at 114 mbsf in an interval of relatively high abundance indicates a reasonably sound age for this horizon at 32.5 ± 0.5 Ma. The absence of particular resistant diatoms that are older than 33 Ma supports an age that is younger than this for the upper 200 m of CRP-3. Marine palynomorphs, which occur sporadically down to 525 mbsf and in a single occurrence at 781 inbsf, have biostratigraphical potential once the many new species in this and other CRP cores are described, and F0 and LO datums established. The mudstone at 781 mbsf has a new clinocyst species, rare Lejeunecysta cysts and a variety of acritarchs and prasinophytes, a varied marine assemblage that is quite different from and presumably younger than the well known Transantarctic Flora of mid to late Eocene age. On this basis and for the moment we conclude that the oldest strata in CRP-3 are earliest Oligocene (or possibly latest Eocene) in age - c. 34 Ma. Over 1l00 samples were taken for magnetic studies. Four magnetozones were recognisd on the basis of NRM intensity and magnetic susceptibility, reflecting the change in sediment composition between quartz sand-dominated and dolerite-dominated. For this report there was time only to produce a magnetostratigraphy for the upper 350 m. This interval is largely of reversed polarity (5 normal intervals total 50 of the 350 m), in contrast to the dominantly normal polarities of CRP-2/2A, and is inferred to be Chron C12R. This extends from 30.9 to 33 Ma. consistent with the biostratigraphic datums from the upper part of CRP-3. The lower limit of reversed polarity has yet to be established. The short period normal events are of interest as they may represent cryptochrons or even polarity changes not recognised in the Geomagnetic Polarity Time Scale. Erosion of the adjacent Transantarctic Mountains through the Kirkpatrick Basalt (Jurassic tholeiitic flows) and dolerite-intruded Beacon Supergroup (Devonian-Triassic sandstone) into granitic basement beneath is recorded by petrographical studies of clast and sand grain assemblages from CRP-3. The clasts in the lower 30 m of the Cenozoic section are almost entirely dolerite apart from a few blocks from the Beacon Supergroup beneath. Above this, however, both dolerite and granitoids are ubiquitous, the latter indicating that erosion had reached down to granitic basement even as the first sediment was accumulating in the VLB. No clasts or sand grains of the McMurdo Volcanic Group were found, but rare silt-size brown volcanic glass occurs in smear slides through most of CRP-3, and is interpreted as distal air fall from alkaline volcanism in northern Victoria Land. Jurassic basalt occurs as clasts sporadically throughout the sequence: in the sand fraction they decline upwards in abundance. The influence of the Devonian Beacon Supergroup is most striking for the interval from 600 to 200 mbsf, where quartz grains, from 10 to 50% of them rounded, dominate the sand fraction. Laminae of coal granules from the overlying Permian coal measures in all but the upper 150 in of the CRP-3 sequence show that these also were being eroded actively at this time. CRP-3 core completed the stratigraphical sampling of the western margin of the VLB by not only coring the oldest strata (Seismic Unit V5) but also the basin floor beneath. This has several important tectonic implications: - most of the Kirkpatrick Basalt and the Beacon Supergroup with the sills of Ferrar Dolerite have been eroded by the time down-faulting displaced the Beacon to form the basin floor. - matching the Beacon strata at the bottom of CRP-3 with the equivalent strata in the adjacent mountains suggests c. 3000 m of down-to-the-east displacement across the Transantarctic Mountain Front as a consequence of rifting and subsequent tectonic activity. - the age of the oldest Cenozoic strata in CRP-3 (c. 34 Ma), which are also the oldest strata in this section of the VLB, most likely represents the initiation of the rift subsidence of this part of the West Antarctic Rift System. This age for the oldest VLB fill is much younger than previously supposed by several tens of millions of years, but is consistent with newly documented sea floor spreading data immediately north of the northern Victoria Land continental margin. These new data sets will drive a re-evaluation of the relationship between initiation of uplift of the Transantarctic Mountains (currently c.55 Ma) and VLB subsidence.

Description: Petrologic descriptions will follow a division related to both grain size and lithology like that adopted in Cape Roberts investigation (Cape Roberts Science Team 2000, hdl:10013/epic.28287.d001, and references therein). First, we describe the petrology of the sand fraction of the core. Second, we report the distribution of clasts with diameter larger than 2 millimeter (mm) and the petrology of basement clasts. Third, we focus on the occurrence and petrology of McMurdo Volcanic Group sediments, which represent the largest component in the core. The final section continues with the compositional characterization of bulk sediments by continuous X-ray fluorescence measurement (XRF scan) integrated by carbonate geochemistry and concludes with the porewater geochemistry. All of these data will contribute to infer palaeoclimatic information and to understand provenance history of detritus, changes in depositional environment, and spatial and temporal evolution of magmatism in McMurdo Ice Shelf area.

Description: Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth's orbital geometry control the ice ages (Hays et al., 1976, doi:10.1126/science.194.4270.1121), fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles (Raymo and Huybers, 2008, doi:10.1038/nature06589). Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the 'warmer-than-present' early-Pliocene epoch (~5-3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming (Solomon et al., 2007). Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ~40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth's axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ~3° C warmer than today ( Kim and Crowley, 2000, doi:10.1029/1999PA000459) and atmospheric CO2 concentration was as high as ~400 p.p.m.v. (van der Burgh et al., 1993, doi:10.1126/science.260.5115.1788, Raymo et al., 1996, doi:10.1016/0377-8398(95)00048-8). The evidence is consistent with a new ice-sheet/ice-shelf model (Pollard and DeConto, 2009, doi:10.1038/nature07809) that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt (Huybers, 2006, doi:10.1126/science.1125249) under conditions of elevated CO2.