Julia S. Wellner

Progressive Cenozoic cooling and the demise of Antarctica’s last refugium

The Antarctic Peninsula is considered to be the last region of Antarctica to have been fully glaciated as a result of Cenozoic climatic cooling. As such, it was likely the last refugium for plants and animals that had inhabited the continent since it separated from the Gondwana supercontinent. Drill cores and seismic data acquired during two cruises (SHALDRIL I and II) in the northernmost Peninsula region yield a record that, when combined with existing data, indicates progressive cooling and associated changes in terrestrial vegetation over the course of the past 37 million years. Mountain glaciation began in the latest Eocene (approximately 37–34 Ma), contemporaneous with glaciation elsewhere on the continent and a reduction in atmospheric CO2 concentrations. This climate cooling was accompanied by a decrease in diversity of the angiosperm-dominated vegetation that inhabited the northern peninsula during the Eocene. A mosaic of southern beech and conifer-dominated woodlands and tundra continued to occupy the region during the Oligocene (approximately 34–23 Ma). By the middle Miocene (approximately 16–11.6 Ma), localized pockets of limited tundra still existed at least until 12.8 Ma. The transition from temperate, alpine glaciation to a dynamic, polythermal ice sheet took place during the middle Miocene. The northernmost Peninsula was overridden by an ice sheet in the early Pliocene (approximately 5.3–3.6 Ma). The long cooling history of the peninsula is consistent with the extended timescales of tectonic evolution of the Antarctic margin, involving the opening of ocean passageways and associated establishment of circumpolar circulation.

Retreat signature of a polar ice stream: sub-glacial geomorphic features and sediments from the Ross Sea, Antarctica

Abstract Three research expeditions to the Ross Sea, Antarctica resulted in collection of a dataset of more than 270 km of side-scan and chirp-sonar data, more than 330 km of swath bathymetry and 3.5 kHz data, and 24 cores within a glacially-carved trough. The former ice-stream flow path is divided into six zones, covering a distance of approximately 370 km, distinguished by unique stratigraphic signatures and geomorphic features. An erosional surface with thin, patchy lodgement till characterizes Zone 1. This region is interpreted as having experienced relatively high basal shear stress conditions. Zones 2, 3, and 4 are characterized by an erosional surface and thin, time-transgressive subglacial and grounding-line proximal deposits that include back-stepping moraines, flutes, transverse moraines, and corrugation moraines. These zones represent the transition between erosional and depositional regimes under the expanded LGM ice sheet; material eroded from the inner shelf was transported toward the outer shelf, possibly as a thin deforming till layer. The two outer zones are depositional and include maximum grounding-line (Zone 5) and pro-glacial deposits that were overridden subsequently by the ice sheet (Zone 6). Surface features include mega-scale glacial lineations, corrugation moraines, and iceberg furrows. Ice in these zones is interpreted as having experienced relatively lower basal shear stress, an extensional regime, and faster flow. This advance may have destabilized the ice sheet, initiating local draw-down and production of icebergs that furrowed the sea floor. Corrugation moraines are thought to represent annual retreat moraines, constraining the retreat rate of the ice sheet across the continental shelf to a consistent 40 to 100 m a−1.

Footprint of the Expanded West Antarctic Ice Sheet: Ice Stream History and Behavior

Evaluating the stability of the West Antarctic Ice Sheet and its regulating role in global climate and eustasy hinges on our ability to understand the interaction of ice streams and the bed on which they rest. Rapid streaming of ice is enabled by flow across a deformable till bed produced by the incorporation of basal meltwater into unconsolidated sedimentary material. These ice streams are shown to have flowed across extensive deformable till beds characterized by megascale glacial lineations composed of soft deformation till. The onset of rapid ice discharge occurs at the transition from crystalline bedrock to seaward-dipping sedimentary strata. In most locations, the deformed bed extends tens of kilometers to the outer continental shelf, which implies a thin ice sheet margin. Furthermore, most of the lateral boundaries of these ancestral ice streams were not constrained geologically, and there is evidence that these boundaries migrated a few tens of kilometers. The extent of the deformable till bed, the nature of the boundaries, and the location of grounding-zone wedges, which record grounding-line positions of individual ice streams, vary from trough to trough, implying unique ice advance and retreat histories. These are all critical parameters in glaciological models and, therefore, predictions of the West Antarctic Ice Sheet’s stability.

High-resolution Holocene climate record from Maxwell Bay, South Shetland Islands, Antarctica

The highest resolution Holocene sediment core from the Antarctic Peninsula to date was collected during the first SHALDRIL cruise (NBP0502). Drilling yielded a 108.2-m-long core (87% recovery; site NBP0502–1B) from Maxwell Bay, South Shetland Islands. This high-resolution sediment record comes from a region that is currently experiencing dramatic climate change and associated glacial retreat. Such records can help to constrain the nature of past climate change and causal mechanisms, and to provide a context for evaluating current climate change and its impacts. The base of the drill site sampled till and/or proximal glacimarine sediments resting directly on bedrock. Glacimarine suspension deposits composed of dark greenish gray silty mud with variable diatom abundance and scattered very fine sand laminations make up the majority of the sedimentary section. Detailed sedimentological and geochemical analyses, including magnetic susceptibility, total organic carbon (TOC) content, carbon and nitrogen isotopic composition, pebble content, and biogenic silica content, allow subdivision of the glacimarine section into nine units, and seismic facies analyses resulted in the identification of six distinct seismic units. We used 29 radiocarbon ages to construct an age model and calculate sedimentation rates that vary by two orders of magnitude, from 0.7 mm/a to ?30 mm/a. Radiocarbon ages from glacimarine sediments just above the till date back to between 14.1 and 14.8 ka. Thus, ice was grounded in the fjord during the Last Glacial Maximum and eroded older sediments from the fjord. Following initial retreat of grounded ice from Maxwell Bay, the fjord was covered by a permanent floating ice canopy, probably an ice tongue. The highest sedimentation rate corresponds to an interval that contains abundant sand laminations and gravelly mud intervals and likely represents a melt-out phase or period of rapid glacial retreat from 10.1 ka to 8.2 ka. There is no evidence for an early Holocene climatic reversal, as recorded farther south at the Palmer Deep drill site. Minimum sea-ice cover and warm water conditions occurred between 8.2 and 5.9 ka. From 5.9 to 2.6 ka, there was a gradual cooling and more extensive sea-ice cover in the bay. After 2.6 ka, the climate varied slightly, causing only subtle variation in glacier grounding lines. There is no compelling evidence for a Little Ice Age readvance in Maxwell Bay. The current warming and associated glacial response in the northern Antarctic Peninsula appears to be unprecedented in its synchroneity and widespread impact.

Observed latitudinal variations in erosion as a function of glacier dynamics

Glacial erosion is fundamental to our understanding of the role of Cenozoic-era climate change in the development of topography worldwide, yet the factors that control the rate of erosion by ice remain poorly understood. In many tectonically active mountain ranges, glaciers have been inferred to be highly erosive, and conditions of glaciation are used to explain both the marked relief typical of alpine settings and the limit on mountain heights above the snowline, that is, the glacial buzzsaw. In other high-latitude regions, glacial erosion is presumed to be minimal, where a mantle of cold ice effectively protects landscapes from erosion. Glacial erosion rates are expected to increase with decreasing latitude, owing to the climatic control on basal temperature and the production of meltwater, which promotes glacial sliding, erosion and sediment transfer. This relationship between climate, glacier dynamics and erosion rate is the focus of recent numerical modelling, yet it is qualitative and lacks an empirical database. Here we present a comprehensive data set that permits explicit examination of the factors controlling glacier erosion across climatic regimes. We report contemporary ice fluxes, sliding speeds and erosion rates inferred from sediment yields from 15 outlet glaciers spanning 19 degrees of latitude from Patagonia to the Antarctic Peninsula. Although this broad region has a relatively uniform tectonic and geologic history, the thermal regimes of its glaciers range from temperate to polar. We find that basin-averaged erosion rates vary by three orders of magnitude over this latitudinal transect. Our findings imply that climate and the glacier thermal regime control erosion rates more than do extent of ice cover, ice flux or sliding speeds.

Late Holocene glacial advance and ice shelf growth in Barilari Bay, Graham Land, west Antarctic Peninsula

Three marine sediment cores were collected along the length of the fjord axis of Barilari Bay, Graham Land, west Antarctic Peninsula (65°55′S, 64°43′W). Multi-proxy analytical results constrained by high-resolution geochronological methods ( 210 Pb, radiocarbon, 137 Cs) in concert with historical observations capture a record of Holocene paleoenvironmental variability. Our results suggest early and middle Holocene (>7022–2815 cal. [calibrated] yr B.P.) retreated glacial positions and seasonally open marine conditions with increased primary productivity. Climatic cooling increased sea ice coverage and decreased primary productivity during the Neoglacial (2815 to cal. 730 cal. yr B.P.). This climatic cooling culminated with glacial advance to maximum Holocene positions and expansion of a fjord-wide ice shelf during the Little Ice Age (LIA) (ca. 730–82 cal. yr B.P.). Seasonally open marine conditions were achieved and remnant ice shelves decayed within the context of recent rapid regional warming (82 cal. yr B.P. to present). Our findings agree with previously observed late Holocene cooling and glacial advance across the Antarctic Peninsula, suggesting that the LIA was a regionally significant event with few disparities in timing and magnitude. Comparison of the LIA Antarctic Peninsula record to the rest of the Southern Hemisphere demonstrates close synchronicity in the southeast Pacific and southern most Atlantic region but less coherence for the southwest Pacific and Indian Oceans. Comparisons with the Northern Hemisphere demonstrate that the LIA Antarctic Peninsula record was contemporaneous with pre-LIA cooling and sea ice expansion in the North Atlantic–Arctic, suggesting a global reach for these events.

Geomorphology and age of the Oxygen isotope stage 2 (last lowstand) sequence boundary on the northwestern Gulf of Mexico continental shelf

Abstract The sequence boundary associated with the last glacial-eustatic lowstand was mapped across the northwestern Gulf of Mexico continental shelf. The geomorphology of incised fluvial valleys varies widely across the shelf. These differences are due to differences in shelf physiography and the interval of the eustatic cycle the valleys were occupied. Incision begins during the falling limb of sea level and results in terraced valleys. Rivers that abandoned their valleys during the fall in sea level to cut new valleys during the lowstand generally have u-shaped profiles. Incised valleys connected to turbidite systems only occurred in two valleys (the Colorado and Rio Grande), but this may be because sea level did not fall below the shelf break during the last eustatic cycle. Some valleys deepen in an offshore direction, others become shallower. The timing of fluvial incision was constrained using radiocarbon dates so that incision can be tied directly to the sea-level curve for the last glacial-eustatic cycle. The results show that the fluvial incision occurred throughout the falling limb of sea level and lowstand; however, maximum incision occurred during the lowest position of sea level. The resulting surface has significant relief, extends across the shelf, and has time significance. The associated conformable surface, on the other hand, is much harder to recognize and occurs at different stratigraphic levels relative to different shelf-margin deltas.

Antarctic Shallow Drilling Project provides key core samples

The understanding of Antarcticas climate, cryosphere, and biosphere evolution is limited, which is due in part to the paucity of outcrops and cores that record changes during the Tertiary. This problem has been partially rectified using conventional drill ships, such as the JOIDES Resolution, but access to key areas of the continental margin has been restricted because of the inability of these ships to operate in ice-covered waters. While researchers are restricted in their ability to acquire long cores in ice-prone areas, nature has provided an alternative method. During past glacial maxima, ice sheets advanced onto the continental shelf, eroding deeply into the stratigraphic section. Several areas of the shelf are characterized by seaward dipping strata a few meters beneath the seafloor. However, these older strata are overlaid by glacial sediments that have proven to be impenetrable by standard piston coring. The Shallow Drilling Project (SHALDRIL) began with the idea of using an icebreaking vessel as the drill platform so that drilling could be conducted in areas that are inaccessible by standard drill ships.Though, even icebreaking vessels with a drill stem hanging beneath them are vulnerable to ever-present changes in sea state, winds, and ice conditions. Thus, the approach, by necessity, needs to be one of ‘drill and run.’ Two cruises, in the austral falls of 2005 and 2006, have shown the efficacy of this approach.

Sediment chronology in Antarctic deglacial sediments: Reconciling organic carbon 14C ages to carbonate 14C ages using Ramped PyrOx

We present the first study which directly compares carbonate radiocarbon (14C) dates with the Ramped PyrOx (RP) radiocarbon dating technique within a single sediment core, and we confirm the utility for the latter constructing chronologies of high latitude, Holocene marine margin sediment successions. Historically, the heavily detrital nature of Antarctic margin sedimentary organic material and lack of carbonate preservation have made these sediments difficult to date accurately. Here, we use archived cores with existing foraminiferal ages to compare with RP dates at equal or similar depth intervals. The lowest temperature RP splits were integrated over narrower intervals than in previous studies to reduce the amount of mixing with older, more thermochemically stable end-members during pyrolytic decomposition. Ages of the low-temperature RP splits coincide with their corresponding carbonate counterparts, suggesting that the RP 14C dating method is a reliable alternative to carbonate dates in sediments where carbonates are absent or not sufficiently preserved for 14C dating. The rarity of calcareous material in most Antarctic sediments often obligates the use of the bulk acid insoluble organic (AIO) fraction of the sediment, which can be problematic because of contamination by older carbon. The bulk AIO 14C ages, which are calculated using the weighted arithmetic mean of all RP splits of individual samples, show that age reversals and biases can occur using bulk AIO dates for age models because of variable proportions of pre-aged organic matter down-core. The application of the RP dating method can reduce these effects to produce a more reliable chronology that is statistically identical to the foraminiferal dating chronology.

Latitudinal variation in glacial erosion rates from Patagonia and the Antarctic Peninsula (46°S−65°S)

We use extensive sedimentary and marine geophysical data to derive sediment volume−based millennial time-scale glacial erosion rates ( Ē ) from glacially influenced fjords and bays across a broad latitudinal transect, from central Patagonia (46°S) to the Antarctic Peninsula (65°S), and to determine how glacial erosion rates change with increasing latitude and decreasing atmospheric temperatures. We also calculate million-year time-scale erosion rates for the western Antarctic Peninsula cordillera and inner continental shelf from seismic stratigraphic analysis of the continental margin. These results are complemented by erosion rates derived from existing thermochronology data sets (apatite fission-track and apatite [U-Th]/He) for both Patagonia and the Antarctic Peninsula regions. Despite considerable regional variability, our results show a clear trend of decreasing Ē with increasing latitude. Millennial Ē values span two orders of magnitude, from 0.02 mm/yr for Illiad glacier on Anvers Island, Antarctica (∼64.5°S), to 0.83 mm/yr for San Rafael glacier in northern Patagonia (∼46.5°S). Regional averages are three times higher for the Patagonian areas than the Antarctic Peninsula areas. This trend is interpreted to result from a general decrease in temperature and water availability at the ice-bedrock interface. For the Antarctic Peninsula study sites, erosion rates are highly clustered around 0.1 mm/yr, with the exception of Maxwell Bay, for which the Ē value is 0.36 mm/yr. In Patagonia, erosion rates are more variable than in the Antarctic Peninsula, with Ē ranging between 0.14 mm/yr (Europa glacier area) and 0.83 mm/yr (San Rafael glacier area). This regional variability in Ē is interpreted as due to differences in hypsometry and bedrock resistance to erosion. Million-year time-scale Ē values derived from thermochronology ages also decrease with latitude, with maximum values decreasing from ∼0.9−1.1 mm/yr north of 46°S to ∼0.1−0.2 mm/yr south of 48°S in Patagonia, and reaching ∼0.2−0.3 mm/yr in the Antarctic Peninsula. The sediment-based million-year time-scale Ē estimates for the western Antarctic Peninsula cordillera indicate that glacial erosion rates increased by 25%−30% after 5.3 Ma, from ∼0.09 mm/yr (5.3−9.5 Ma) to ∼0.11−0.12 mm/yr (<5.3 Ma). For Patagonia, the decrease in long-term erosion rates south of ∼46°S is interpreted to result from relatively long periods of slow glacial erosion associated with the ice masses having been colder (subpolar) on the southern Patagonian cordillera, and having eroded at rates comparable to those we obtained for the Antarctic Peninsula. These long-term erosion rates are 1−2 orders of magnitude lower than estimates based on recent sediment yields, highlighting the transient nature of high-sediment-flux events. However, our sediment volume−derived millennial time-scale Ē closely approximates the maximum values of tectonic time-scale Ē values derived from thermochronology ages. Our combined millennial and million-year time-scale glacial erosion data quantify the significant decrease in rates of glacially driven denudation at geological (tectonic) and millennial time scales with increasing latitude from Patagonia to the Antarctic Peninsula, highlighting the influence of climate on mountain denudation.