Lionel Carter

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.

Response of surface water masses and circulation to Late Quaternary climate change east of New Zealand

A series of cores from east of New Zealand have been examined to determine the paleoceanographic history of the late Quaternary in the SW Pacific using planktonic foraminiferal data. Distinct shifts of species can be seen between glacial and interglacial times especially south of Chatham Rise east of South Island. Foraminiferal fragmentation ratios and benthic/planktonic foraminiferal ratios both show increased dissolution during glacials, especially isotope stage 2 to the south of Chatham Rise. The present-day Subtropical Convergence appears to be tied to the Chatham Rise at 44°S, but during glacial times this rise separated cold water to the south from much warmer water to the north, with an associated strong thermal gradient across the rise. We estimate that this gradient could have presented as much as an 8°C temperature change across 4° of latitude during the maximum of the last ice age. There is only weak evidence of the Younger Dryas cool event, but there is a clear climatic optimum between 8 and 6.4 ka with temperatures 1°–2°C higher than the present day. The marine changes compare well with vegetational changes on both South and North Island.

Recent sedimentation beneath the Deep Western Boundary Current off northern New Zealand

The triangular shaped region defined by Chatham Rise in the south, the Louisville Seamount Chain in the east and the Pacific/Australian plate boundary in the west is one of the most active sedimentary regions of the world ocean. The SW Pacific Deep Western Boundary Current (DWBC), with a transport of 20 Sv, flows through the area producing regions of scour, nepheloid layers and typical depositional bedforms. Sediment is supplied to the region by turbidity currents directly via Hikurangi Channel and by the DWBC, which removes material from Bounty Fan south of Chatham Rise, as well as by fallout of volcanic ash and pelagic biogenic material. The CaCO3 content of the sediments is very much controlled by terrigenous dilution, with Chatham Rise having ∼ 60%, but Hikurangi Plateau (near the Channel), sited well above the 4750 m-deep CCD, having only ∼ 20%. Although there is much evidence of scour around pinnacles and in scoured, possibly furrowed, mud deposits seen on 3.5 kHz profiles, dilute nepheloid layers, slow geostrophic and measured velocities, photographic and sediment grain-size evidence do not indicate fast flows. Mudwaves are of the irregular, vertically migrating type (rather than progressive antidunes), suggesting flows < ∼ 0.10 m s−1. In common with other Southern Ocean areas, flows may have been strongest during glacial times. Hikurangi Channel delivers its turbidity currents into the path of the DWBC, which sweeps them along to create a deep-sea fan with the characteristics of a contourite drift, here termed a fan-drift. This is mantled with migrating, climbing mudwaves, which are ascribed to deposition from episodic turbidity currents travelling at more than 0.1 m s−1. The fan-drift forms on the right-hand side of a boundary channel along the foot of the 1000 m-high Rapuhia Scarp. It is interpreted as a large right-bank levee formed by turbidity currents overspilling the Channel, which also owes its origin to scour by the DWBC. The turbidite deposits are dominantly mud and the evidence of scour/deposition patterns around a swath-mapped seamount indicate the depositing flows to be consistent with the DWBC flow direction, demonstrating entrainment of the turbidity currents by the deep geostrophic flow. Patterns of sedimentation around the Louisville Seamount Chain indicate action of a strong filament of the DWBC on the east side of the seamounts at least as far north as 37°S, and a relative intensification of the current on the west side of the seamounts north of 38°S.

The opening of Cook Strait: Interglacial tidal scour and aligning basins at a subduction to transform plate edge

Abstract Cook Strait, the central seaway through the axial ranges of New Zealand, has many inferred origins. A compilation of marine geological and geophysical datasets suggests that Cook Strait developed when five sedimentary basins at a rapidly changing, obliquely convergent, plate boundary were moved into line and were linked by strong tidal scour in middle Pleistocene times. The basins relate partly to oblique subduction north of Cook Strait and partly to intercontinental transform to the south. Prior to opening, muddy, subduction pull-down and foreland basins in the northwest were separated from equally quiet water, rotating, forearc to transform basins in the southeast. The land barrier between them narrowed and was finally breached as subduction-related basins migrated southwards and transform-related basins extended northwards in response to rotation and divergent branching of a major transcurrent fault. Following breaching, a history of alternating scour and quiet water deposition is recorded by a series of deep, irregular unconformities in muddy basin fill. Scour is correlated with interglacial periods of high sea level when land barriers were submerged and strong tides, caused by a 140° phase difference at either end of the strait, eroded muddy sediments deposited in glacial periods when an emergent landbridge limited tidal exchange.

Mud sedimentation on the continental shelf at an accretionary margin—Poverty Bay, New Zealand

Abstract Sediments on the continental shelf, atop the accretionary prism of the eastern North Island, are dominated by mud. This situation reflects a highly erodible provenance of soft Tertiary sediments, active tectonism, meteorological extremes, and, in historical times, changing land use. Off Poverty Bay, mud is supplied by the Waipaoa River, New Zealands fourth largest river in terms of sediment supply. Under normal conditions, suspended sediment is dispersed as surface or hypopycnal plumes that have a net northeastward or southward dispersal along the shelf, mainly in response to the prevailing wind‐driven circulation. During extreme floods with return periods of 10 years or more, fluvial suspended sediment concentrations are probably high enough to form subsurface or hyperpycnal plumes that move and disperse under gravity and shelf currents. After the 100 year Cyclone Bola event of 1988, reef communities of the inner shelf were temporarily inundated by a fluid mud layer. Surficial sediments and 3.5...

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.

Source, sea level and circulation effects on the sediment flux to the deep ocean over the past 15 ka off eastern New Zealand

The last post-glacial transgression and present highstand of sea level were accompanied by a reduction in the terrigenous flux to the deep ocean bordering the active convergent margin off the eastern North Island of New Zealand. Although in accord with long-established models of highstand shelf deposition, new data from giant piston core MD97 2121 (2314 m depth) reveal that the flux also varied with terrigenous supply and palaeocirculation. Between 15 and 9.5 ka, the flux reduced from 33 to 20 g/cm2/ka as supply declined with an expanding vegetation cover, and mud depocentres became established on the continental shelf. An increase from 20 to 27 g/cm2/ka during 9.5–3.5 ka coincided with a strengthened East Cape Current which probably introduced sediment from fluvial and shelf sources in the north. The flux profile shows no immediate response to the establishment of modern sea level ∼7 ka. However, accumulation decreased from 3.5 to 1 ka as more sediments were retained on the shelf, possibly under wind-strengthened, along-shelf currents. Over the last 1 ka, the flux decline halted under increased terrigenous supply during anthropogenic development of the land. Despite the proximity of the North Islands Central Volcanic Region, major eruptions caused only brief increases (centuries duration) in the terrigenous flux through direct deposition of airfall and possibly fluvial redistribution of onshore volcanic deposits. Frequent earthquakes also had little short-term effect on accumulation although such events, along with volcanism, probably contribute to the long-term high flux of the region. The other measured flux component, biogenic carbonate, reached maxima of 6 g/cm2/ka between 11 and 8.5 ka when nutrient-bearing waters of the East Cape Current dominated the palaeoceanography. After these peaks, carbonate accumulation declined gradually to modern levels of ∼3 g/cm2/ka.

Correlation, dispersal, and preservation of the Kawakawa Tephra and other late Quaternary tephra layers in the Southwest Pacific Ocean

Abstract Voluminous rhyolitic eruptions and prevailing westerly winds have dispersed late Quaternary ash from the Taupo Volcanic Zone (TVZ) of the North Island, New Zealand, across the Southwest Pacific Ocean. We identify the Taupo (1850 14C years), Waimihia (3280 yr), Rerewhakaaitu (14 700 yr), and Kawakawa (22 590 yr) Tephra layers in deep ocean cores, mainly on the basis of their stratigraphic position, radiometric age, and glass shard chemistry. Approximately 25 km3 of Taupo Tephra were dispersed ENfE at least 650 km from the TVZ whereas c. 22 km3 of Waimihia Tephra and c. 14 km3 of Rerewhakaaitu Tephra travelled over 500 km to the east. In contrast, at least 400 km3 of Kawakawa Tephra occur out to 1400 km southeast of the TVZ. Such widespread dispersal is not only a function of the size of the Kawakawa eruption, but is also influenced by the strong wind regime during the last glaciation as manifest by high aeolian quartz contents of sediments encasing the tephra. More ash appears to have deposited of...

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.

Antarctic contribution to meltwater pulse 1A from reduced Southern Ocean overturning

During the last glacial termination, the upwelling strength of the southern polar limb of the Atlantic Meridional Overturning Circulation varied, changing the ventilation and stratification of the high-latitude Southern Ocean. During the same period, at least two phases of abrupt global sea-level rise--meltwater pulses--took place. Although the timing and magnitude of these events have become better constrained, a causal link between ocean stratification, the meltwater pulses and accelerated ice loss from Antarctica has not been proven. Here we simulate Antarctic ice sheet evolution over the last 25 kyr using a data-constrained ice-sheet model forced by changes in Southern Ocean temperature from an Earth system model. Results reveal several episodes of accelerated ice-sheet recession, the largest being coincident with meltwater pulse 1A. This resulted from reduced Southern Ocean overturning following Heinrich Event 1, when warmer subsurface water thermally eroded grounded marine-based ice and instigated a positive feedback that further accelerated ice-sheet retreat.