Ross D. Powell

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

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, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. 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. 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 and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model 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 under conditions of elevated CO2.

Glacimarine processes and inductive lithofacies modelling of ice shelf and tidewater glacier sediments based on Quaternary examples

Abstract Three variables control the type of glacial debris source and where and when debris is introduced to an ocean: (1) type of glacial ice source (ice sheet or valley glacier); (2) condition of a grounded glacier (melting/freezing or frozen base and cold or warm internal ice); and (3) type of glacier front (ice shelf or tidewater). Using these variables in different combinations eight glacimarine regimes are postulated to exist. By integrating information from Holocene sediment and Pleistocene glacimarine sequences with the eight inductive regimes, lithofacies relationships are predicted and modelled. The models show a complexity of glacimarine lithofacies. The importance of recognizing this complexity lies in realistic interpretations of ancient sequences. Ice shelf sediment may be limited in the geologic record because of tectonic and climatic constraints required for ice shelf formation. Alternatively, in some tidewater glacier regimes larger volumes of glacimarine sediment are produced than in ice shelf regimes. Ice-proximal sediment is distinctive, especially near subglacial meltwater stream effluxes. Ice shelves and tidewater glaciers may produce similar ice-proximal lithofacies (e.g. morainal bank associations and laminites) in certain regimes. However, details of lithofacies may aid in distinguishing ice shelf from tidewater regimes. Ice shelf and tidewater iceberg zone sediment can be distinguished from each other if basal and high-level englacial debris can be distinguished. Ice shelf zone sediment out from the grounding line has a basal debris source. High-level englacial debris is deposited in more distal ice shelf zones resulting in sediment by-pass. In corresponding positions of a tidewater glacier regime, iceberg rafted debris comprises mixed basal and high-level englacial debris. Deeper water zones receive large volumes of sediment when a melting/freezing-base grounding line is at the edge of a continental shelf. When the grounding line retreats sediment supply may be drastically reduced because of a back-sloping continental shelf and trapping by morainal banks. Cool water bioclastic carbonates may accumulate once a grounding line has retreated. Although ice shelf iceberg zones are more extensive than those in tidewater regimes, ice shelf icebergs have low debris content and resulting ice-rafted debris may be scarce. The smallest volume of sediment is produced by a frozen-base ice sheet ending as a tidewater front, and the largest volume by a melting/freezing-base valley, tidewater glacier. Glacimarine sediment has a relatively high preservation potential near continental interiors or margins. However, at continental margins mass flow (olistostrome) and tectonic (melange) deposits may be preserved with glacimarine diamictites. These deposits may be difficult to distinguish from each other, especially if metamorphosed such as at a convergent margin. The models presented do not fully represent the glacimarine environment, because many other variables are omitted. However, the principal concepts of generating glacimarine lithofacies associations are considered. Continued research will verify if each regime exists and will provide data on those regimes that exist today to compare and modify these models.

Glacimarine sedimentary processes, facies and morphology of the south-southeast Alaska shelf and fjords

Abstract High precipitation from Gulf of Alaska air masses can locally reach up to 800 cm a−1. This precipitation on tectonically active mountains creates cool-temperate glaciation with extremely active erosion and continuously renewed resources. High basal debris loads up to 1.5 m thick of pure debris and rapid glacial flow, which can be more than 3000 m a−1, combine to produce large volumes of siliciclastic glacimarine sediment at some of the highest sediment accumulation rates on record. At tidewater fronts of valley glaciers, sediment accumulation rates can be over 13 m a−1 and deltas commonly grow at about 106 m3 a−1. Major processes influencing glacimarine sedimentation are glacial transport and glacier-contact deposition, meltwater (subaerial and submarine) and runoff transport and deposition, iceberg rafting and gouging, sea-ice transport, wave action and storm reworking, tidal transport and deposition, alongshelf transport, sliding and slumping and gravity flows, eolian transport, and biogenic production and reworking. Processes are similar in both shelf and fjord settings; however, different intensities of some processes create different facies associations and geometries. The tectonoclimatic regime also controls morphology because bedrock structure is modified by glacial action. Major glacimarine depositional systems are all siliciclastic. They are subglacial, marginal-morainal bank and submarine outwash, and proglacial/paraglacial-fluvial/deltaic, beach, tidal flat/estuary, glacial fjord, marine outwash fjord and continental shelf. Future research should include study of long cores with extensive dating and more seismic surveys to evaluate areal and temporal extent of glacial facies and glaciation; time-series oceanographic data, sidescan sonar surveys and submersible dives to evaluate modern processes; biogenic diversity and production to evaluate paleoecological, paleobiogeographic and biofacies analysis; and detailed comparisons of exposed older rock of the Yakataga Formation to evaluate how glacial style has evolved over 6.3 Ma.

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

Glacimarine processes at grounding-line fans and their growth to ice-contact deltas

Abstract Grounding-line fans originate from subglacial and basal stream tunnels at grounding lines of glaciers terminating in a marine environment. During melt seasons discharge forms a turbulent jet beyond the efflux. Over the initial zone of flow establishment in the jet, it may remain in contact with the sea floor for up to 13.4D (D-tunnel diameter) along a runout distance, if velocity is constant. Plug flow in this zone deposits marine outwash sediment commonly having chaotic texture and graded and welded contacts. Texture may vary rapidly because of discharge pulses, and the jet may move laterally to produce sheet or cut-and-fill geometries. Imbricate gravel is deposited near the efflux, then farther out are sheet or weakly channelized finer-grained gravels and sands. In some instances a migrating barchanoid bar forms at the detachment zone and forms large-scale trough cross-beds. Beyond the detachment zone the turbulent jet becomes vertical and sediment cascades from it in a ‘veil’. This sediment is added to by flows from continuous failures of bed load sediment in the detachment zone. Resulting deposits are interstratified sediment gravity flows and thick, coarse end-members of cyclopsams. The jet changes to a plume when inertial forces become less than buoyancy forces. Turbulent vorticies within the jet can re-entrain particles into a buoyant plume. The plume remains vertical, although continuously spreading, until reaching neutral buoyancy. Fan depocentres have been observed to accumulate at over 106 m3 a−1 in temperate glacial areas. Eventually, at quasi-stable grounding lines, fans may aggrade to sea level to form ice-contact deltas. At incipient deltas when the delta plain is intertidal, coarse sediment is redistributed to the prodelta with each tidal cycle. Intense prodelta rhythmites mark the transition of a fan into a delta.

Interlaminated ice-proximal glacimarine sediments in Muir Inlet, Alaska

Muir Inlet in Glacier Bay, Alaska, is a glacial fjord receiving a tremendous volume of sediment annually. The rate of sediment accumulation is greatest proximal to Muir Glacier (about 9 m yr−1) and decreases away from the glacier. The primary sediment sources are meltwater streams discharging at subglacial and ice-marginal positions to form overflows, interflows, and underflows (continuous turbidity currents). Overflows and interflows interact with diurnal tidal currents and their volume and sediment concentration varies diurnally and annually with meltwater discharge. These effects produce cyclic deposits of a thin fine-grained sand or silt lamina that grades normally to a thicker poorly to very poorly sorted mud lamina. This lamina couplet is termed a cyclopel. Underflows are suggested to occur in this glacimarine environment because of conditions unique to subglacial fluvial systems. Underflow deposits occur only in proximal positions (<0.5 km from glacier face), and are coarse-grained, reverse to normal graded, and exhibit an increase in sorting and sand content up-layer. Ice-rafted debris (identified as particles >177 μm) is ubiquitous, though low (<5% by weight), and occurs as isolated particles, frozen pellets, or as lenses that in cores may have a lamina appearance. Proximally, ice-rafted debris is difficult to identify because proximal sediment is often as coarse-grained. Deposited sediment may be reworked by tidal currents, and sediment gravity flows. Depositional processes operating in Muir Inlet produce interlaminated sand/silt/clay that characterizes sediment proximal to a glacier and fines seaward to mud. Sediment is classified into one of three sediment types: 1. (1) Type I sediment is very fine grained (mean 8.65–7.17 o), low in sand (0.1–11.2%), and very poorly to poorly sorted. It is the dominant sediment type in Muir Inlet, and is transported by plumes and deposited by suspension settling. 2. (2) Type II sediment is fine- to coarse-grained (mean 6.70–3.12 o), low to high in sand (5.1–86.6%), and very poorly to moderately sorted. It represents reworked sediment, proximal plume deposits, or coarse-grained laminae of cyclopels. 3. (3) Type III sediment is coarse-grained (mean 3.89–2.38 o), high in sand (58.0–100.0%), and poorly to well sorted. It is deposited by sediment gravity flows or underflows.

In situ observations of floc settling velocities in Glacier Bay, Alaska

Abstract In situ floc settling velocities and diameters of particles ranging in size from 0.63 to 5.05 mm equivalent circular diameter were measured under a buoyant discharge plume by deploying a bottom-tripod-mounted Floc Camera Assembly (FCA) in Tarr Inlet, Glacier Bay, Alaska. These observations were used to estimate floc effective densities. Three results emerge from this work. First, fits of settling velocity and effective density to diameter are consistent with expressions published for other environments, suggesting that common controls on floc size and settling velocity operate across diverse marine environments. Second, the raw data show considerable scatter, with upper and lower 95% prediction intervals on settling velocity and excess density differing by about a factor of 7. Analysis of sources of error suggests that the variability is caused by differences in component-grain composition among flocs and turbulent stirring within the stilling box. Third, bin-averaged effective densities and settling velocities are highly correlated with diameter. Thus, while it is not possible, based on diameter, to predict accurately the settling velocity of a single floc, it is possible to estimate the mean settling velocity of a population of like-sized flocs.

The stratigraphic signature of the late Cenozoic Antarctic Ice Sheets in the Ross Embayment

A 1284.87-m-long sediment core (AND-1B) from beneath the McMurdo sector of the Ross Ice Shelf provides the most complete single section record to date of fluctuations of the Antarctic Ice Sheets over the last 13 Ma. The core contains a succession of subglacial, glacimarine, and marine sediments that comprise ∼58 depositional sequences of possible orbital-scale duration. These cycles are constrained by a chronology based on biostratigraphic, magnetostratigraphic, and 40 Ar/ 39 Ar isotopic ages. Each sequence represents a record of a grounded ice-sheet advance and retreat cycle over the AND-1B drill site, and all sediments represent subglacial or marine deposystems with no subaerial exposure surfaces or terrestrial deposits. On the basis of characteristic facies within these sequences, and through comparison with sedimentation in modern glacial environments from various climatic and glacial settings, we identify three facies associations or sequence “motifs” that are linked to major changes in ice-sheet volume, glacial thermal regime, and climate. Sequence motif 1 is documented in the late Pleistocene and in the early Late Miocene intervals of AND-1B, and it is dominated by diamictite of subglacial origin overlain by thin mudstones interpreted as ice-shelf deposits. Motif 1 sequences lack evidence of subglacial meltwater and represent glaciation under cold, “polar”-type conditions. Motif 2 sequences were deposited during the Pliocene and early Pleistocene section of AND-1B and are characterized by subglacial diamictite overlain by a relatively thin proglacial-marine succession of mudstone-rich facies deposited during glacial retreat. Glacial minima are represented by diatom-bearing mudstone, and diatomite. Motif 2 represents glacial retreat and advance under a “subpolar” to “polar” style of glaciation that was warmer than present, but that had limited amounts of subglacial meltwater. Sequence motif 3 consists of subglacial diamictite that grades upward into a 5- to 10-m-thick proglacial retreat succession of stratified diamictite, graded conglomerate and sandstone, graded sandstone, and/or rhythmically stratified mudstone. Thick mudstone intervals, rather than diatomite-dominated deposition during glacial minima, suggest increased input of meltwater from nearby terrestrial sources during glacial minima. Motif 3 represents Late Miocene “subpolar”-style glaciation with significant volumes of glacially derived meltwater.

Antarctic and Southern Ocean influences on Late Pliocene global cooling.

The influence of Antarctica and the Southern Ocean on Late Pliocene global climate reconstructions has remained ambiguous due to a lack of well-dated Antarctic-proximal, paleoenvironmental records. Here we present ice sheet, sea-surface temperature, and sea ice reconstructions from the ANDRILL AND-1B sediment core recovered from beneath the Ross Ice Shelf. We provide evidence for a major expansion of an ice sheet in the Ross Sea that began at ∼3.3 Ma, followed by a coastal sea surface temperature cooling of ∼2.5 °C, a stepwise expansion of sea ice, and polynya-style deep mixing in the Ross Sea between 3.3 and 2.5 Ma. The intensification of Antarctic cooling resulted in strengthened westerly winds and invigorated ocean circulation. The associated northward migration of Southern Ocean fronts has been linked with reduced Atlantic Meridional Overturning Circulation by restricting surface water connectivity between the ocean basins, with implications for heat transport to the high latitudes of the North Atlantic. While our results do not exclude low-latitude mechanisms as drivers for Pliocene cooling, they indicate an additional role played by southern high-latitude cooling during development of the bipolar world.

Suspended sediment transport and deposition of cyclically interlaminated sediment in a temperate glacial fjord, Alaska, U.S.A.

Abstract Cyclically interlaminated sediment is a distinctive lithofacies within the sediment package of temperate glacial fjords. The tidewater terminus of McBride Glacier is at the head of a small fjord that receives abundant suspended sediment and is a site of rapid accumulation of cyclically interlaminated sediment. Buoyant sediment-laden meltwater rises from a subglacial stream at the base of the glacier and mixes with fjord water to produce a thick, brackish overflow. Peak suspended sediment concentrations occur beneath the surface of the overflow at 3 to 10 m depth. Particle release from the overflow is controlled by semi-diurnal tidal fluctuations. The major vertical flux of suspended sediment is initiated at low tide because of low horizontal current velocities and reduced vertical eddy velocities. Sorting occurs as the particles settle; sand and coarse silt settle as single grains and finer flocculated particles settle as turbid layers at a rate between 2.5 m/h and 10.9 m/h. Each low water produces a couplet of a coarser grained lamina that is sorted as coarser single grains settle, and a finer grained lamina of flocculated particles in turbid layers. Individual couplet thicknesses and particle size decrease with distance from the discharge source. Semi-diurnal tides produce two couplets each day. Interstratified with these tidal rhythmites are coarser and poorly sorted laminae deposited by sediment gravity flows and coarse laminae contributed by peak daily discharge.