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Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2 These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

Quaternary processes and environmental changes are often difficult to assess in remote subantarctic islands due to high surface erosion rates and overprinting of sedimentary products in locations that can be a challenge to access. We present a set of high-resolution, multichannel seismic lines and complementary multibeam bathymetry collected off the eastern (leeward) side of the subantarctic Auckland Islands, about 465 km south of New Zealand's South Island. These data constrain the erosive and depositional history of the island group, and they reveal an extensive system of sediment-filled valleys that extend offshore to depths that exceed glacial low-stand sea level. Although shallow, marine, U-shaped valleys and moraines are imaged, the rugged offshore geomorphology of the paleovalley floors and the stratigraphy of infill sediments suggests that the valley floors were shaped by submarine fluvial erosion, and subsequently filled by lacustrine, fjord, and fluvial sedimentary processes. [ABSTRACT FROM AUTHOR]

Palaeomagnetic and rock magnetic measurements on sediment cores from Integrated Ocean Drilling Program (IODP) Sites U1331 and U1332 in the equatorial Pacific are used to examine changes in the relative palaeointensity (RPI) of the geomagnetic field during the Oligocene and Eocene. The main magnetic carrier of these sediments is interpreted to be partially oxidized magnetite, which varies in concentration by less than about a factor of six along the stratigraphic section. In some cases, these variations occur as abrupt changes or fluctuations. The largest change in lithology and rock magnetic properties occurs across the Eocene/Oligocene (E/O) boundary, with the radiolarian-rich late Eocene sediments having higher magnetic concentration than the nannofossil-rich early Oligocene sediments. RPI estimates computed from ratios of the natural remanent magnetization to anhysteretic remanent magnetization and isothermal remanent magnetization appear to be insignificantly influenced by the lithologic differences within either the Oligocene or Eocene but are significantly influenced by the differences between the Eocene and Oligocene sediments. We thus estimate the RPI separately for the Oligocene and Eocene intervals. For both sites, the RPI records always have minima at chron boundaries. The records also show large fluctuations between highs and lows even during stable polarity periods. These features are similar to what has been observed for RPI records spanning the past 3 Myr, suggesting persistent geomagnetic field behaviour since the Eocene. The two new records show consistent variation with the RPI reported from Site U1333, which is in the vicinity of Sites U1331 and U1332. The RPI variations during the Oligocene at Site U1332 are also generally consistent with those reported from the South Atlantic. One interesting feature is that the RPI is consistently high throughout most of Chron C12r (31.021–33.232 Ma). In addition, Cryptochron C18n.1n-1, which is characterized by common marked intensity lows in the RPI records, is observed at the age of 39.094–39.114 Ma for Site U1331 and 38.958–38.983 Ma for Site U1332. [ABSTRACT FROM PUBLISHER]

We present new environmental magnetic data from the Southwest Pacific spanning the middle Miocene climatic transition (circa 16.2–13.5 Ma), which indicate a dynamic South Pacific Oceanic system with two periods of increasing and decreasing current strength in response to orbitally paced ice volume variations. A magneto-biostratigraphic age model provides age control for magnetic susceptibility and Anisotropy of magnetic susceptibility data from a mudstone succession in Western Southland, New Zealand. Magnetic susceptibility in this succession reflects changes in carbonate content, which we suggest is proxy for ocean productivity or sea level variations and therefore a proxy for ice volume changes. Timeseries analyses of the magnetic susceptibility record reveals eccentricity and obliquity paced ocean productivity cycles. Anisotropy of magnetic susceptibility data carry a record of deep current strength in response to ice volume changes. Our data show that periods of increased ice volume resulted in the northward migration of erosive currents in this sector of the Pacific and that large, dynamic ice sheets may have persisted in Antarctic during the middle Miocene climatic optimum. [ABSTRACT FROM AUTHOR]

This study describes 16 well-dated, terrestrial glacial sedimentary cycles deposited during astronomically paced climate cycles from the termination of the Miocene Climatic Optimum (MCO) through the middle Miocene Climate Transition (MMCT) (15.1-13.8 Ma) in the Friis Hills, Transantarctic Mountains, Antarctica. Three locations were continuously cored (79% recovery) to a maximum depth of 50.48 m through a succession of interbedded till sheets and fossil-bearing, fluvio-lacustrine sediments. A composite chronostratigraphic framework is presented for the cores based on the previous mapping, a seismic refraction survey that defines basin geometry, and a new, integrated age model based on paleomagnetic stratigraphy that is constrained by radioisotopic 40Ar/39Ar numeric ages on two newly identified silicic tephra. The paleoecologic and sedimentologic characteristics of organic-rich lithologies are relatively consistent up-section, which implies that successively younger interglacial deposits during the MMCT represented broadly similar environmental and climatic conditions. During these interglacials, the Friis Hills hinterland was likely ice-free. Major disconformities in the section suggest a transition to colder climates, and after ca. 14.6 Ma, thicker, more extensive and erosive ice cover occurred across the Friis Hills during glacial episodes. Diamictites in the upper three cycles suggest that climate cooled and became drier after ca. 14.2 Ma. However, cyclical retreat of the ice and a return to warm climate conditions during interglacials continued through ca. 13.9 Ma. These direct records reflect a highly variable East Antarctic Ice Sheet margin but show that the ice margin became progressively more extensive during successive glacial intervals, which is consistent with a cooling trend toward more glacial values in the far-field benthic foraminifera δ18O proxy ice volume and temperature record. Age constraints show that glacial-interglacial variability at the terrestrial margin of the East Antarctic Ice Sheet was primarily paced by astronomical precession (~23 k.y.) through the onset of the MMCT (15-14.7 Ma). Precession-driven cycles are modulated by short-period (~100 k.y.) eccentricity cycles. Intervals of maximum eccentricity (high seasonality) coincide with sedimentary cycles comprising thin diamictites and relatively thick interglacial sandstone and mudstone units. Intervals of minimum eccentricity (low seasonality) coincide with sedimentary cycles comprising thick diamictites and relatively thin interglacial sedimentary deposits. Major disconformities in the Friis Hills succession that span more than ~100 k.y. reflect episodes of expansion of erosive ice across, and well beyond, the Transantarctic Mountains and coincide with nodes in eccentricity (~400 k.y.). These relationships suggest that during relatively warm intervals in the middle Miocene, the East Antarctic Ice Sheet expanded and contracted over 100 k.y. cycles, while its margins continued to fluctuate at higher (~23 k.y.) frequency. After 14.5 Ma, obliquity is the dominant frequency in δ18O records, marking a period during which large regions of the Antarctic Ice Sheet grounded in marine environments. [ABSTRACT FROM AUTHOR]