Tim R. Naish

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.

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).

Sequence stratigraphy of sixth-order (41 k.y.) Pliocene–Pleistocene cyclothems, Wanganui basin, New Zealand: A case for the regressive systems tract

This study is based on a late Pliocene and early Pleistocene (approximately 2.6‐1.7 Ma) succession about 1 km thick of 20 sixth-order (41 k.y. duration) cyclothems of shelf origin exposed in the Rangitikei River valley in the eastern part of Wanganui basin. The cyclothems correlate with δ 18 O isotope stages 100‐58, and each 41 k.y. glacialinterglacial stage couplet is represented by an individual depositional sequence comprising transgressive, highstand, and regressive systems tracts. Unlike most examples inferred from the stratigraphic record, these systems tracts were deposited during phases of known sea-level cycles indicated by the contemporary oxygen isotope ice-volume curve. Because of the high rate of subsidence of Wanganui basin, glacioeustatic sea-level falls during most cycles were not of sufficient magnitude to expose the outer shelf. Thus, the Rangitikei section provides an exceptional example of regressive strata deposited landward of the contemporary shelf break. Simple one-dimensional modeling shows that moderate to high rates of basin subsidence (1‐2 mm/yr) and low rates of sedimentation (<0.2 mm/yr) during transgressions combined to produce an accommodation surplus at the relative highstand. This surplus accommodation was infilled during the late highstand and ensuing fall partly by aggradational, highstand systems tract shelf siltstone, and chiefly by strongly progradational shoreface sediments of the regressive systems tract. Rangitikei regressive systems tracts are distinguished from forced regressive systems tracts (sensu Hunt and Tucker, 1992) by their different stratal geometry. By definition, forced regressive systems tracts display an erosional contact with the underlying highstand systems tracts and typically occur as a series of downstepped disjunct shoreline wedges stranded on the shelf and/or slope. In contrast, regressive systems tracts exhibit a gradational lower contact, above which parasequences are stacked in a strongly progradational pattern terminated by the superjacent sequence boundary. Cyclothems display two types of motif termed Rangitikei dt (depositional transgression), and Rangitikei nt (nondepositional transgression), which include the following architectural elements in ascending stratigraphic order: (1) a basal sequence boundary that is coincident with either the transgressive surface of erosion, which displays small-scale (up to 50 cm) erosional relief and may be penetrated by the ichnofossil Ophiomorpha, or its deeper water correlative conformity; (2) either a thick (5‐30 m) transgressive systems tract comprising a deepening upward nearshore to inner shelf, mixed carbonate-siliciclastic lithofacies succession (depositional transgression), or a thin (<2 m) transgressive systems tract comprising condensed fossiliferous facies deposited on the sediment-starved offshore shelf (nondepositional transgression); (3) a sharp downlap surface separating condensed fossiliferous facies of the transgressive systems tract from terrigenous siltstone of the superjacent highstand systems tract; (4) a highstand systems tract comprising a 10‐20-m-thick interval of aggradational, shelf siltstone; and (5) a thick (up to 45 m) progradational inner shelf to shoreface lithofacies assemblage ascribed to the regressive systems tract. Condensed shell beds are associated with intrasequence and sequence-bounding discontinuities, and, together with the sedimentological and stratal characteristics of the sequences, are important indicators of stratigraphic architecture. Four types of shell bed are associated with surfaces formed by four different types of stratal termination; onlap, backlap, downlap, and flooding surface shell beds (cf. Kidwell, 1991) are associated, respectively, with the transgressive surface of erosion, “apparent truncation” at the top of the transgressive systems tract, the downlap surface, and local marine flooding surfaces. A fifth shell-bed type, termed a compound shell bed, forms in offshore environments where the downlap surface converges with the sequence boundary, and elements of both the downlap and the backlap shell beds become mixed or superposed. The shell beds mark zones of stratal attenuation and can be used as surrogates for seismic discontinuities when applying sequence stratigraphic concepts at outcrop scale.

A review of the Milankovitch climatic beat: template for Plio–Pleistocene sea-level changes and sequence stratigraphy

The Milankovitch theory of climate change predicts that global ice volume, and hence sea-level changes, were controlled by long-term quasi-periodic variations in the earth’s orbital parameters (obliquity, precession and eccentricity). d 18 O records from deep-sea cores are a proxy for sea-level changes and have an orbitally tuned chronology covering the last 5 Ma. The sea-level signal in d 18 O data from east equatorial Pacific core V19-30 is well calibrated with sea-level data from coral terraces on Huon Peninsula, New Guinea, over the last 140 ka, and with less certainty back to 340 ka. Over the last 140 ka, the sea-level contribution to benthic glacial‐interglacial isotopic variation is about 1.2‐1.3‰ or 0.011‰ m 1 , and for core V19-30 the glacial-age temperature contribution from deep ocean cooling of 1.7oC is 0.4‰. Independent constraints on Late Pliocene sea-level changes interpreted from shallow marine continental margin records indicate that the sea-level d 18 O calibration may not have been the same over the last 2.6 Ma, and the temperature correction is unlikely to have been the same in all glacial periods and all ocean settings. Nevertheless, the astronomically tuned isotopic records from deep-sea cores provide an accurate chronology and approximate the magnitudes of sea-level changes over the last 2.6 Ma, against which the facies architecture of stratigraphic sequences can be analysed and the concepts of sequence stratigraphy properly evaluated.

The multi-millennial Antarctic commitment to future sea-level rise

Atmospheric warming is projected to increase global mean surface temperatures by 0.3 to 4.8 degrees Celsius above pre-industrial values by the end of this century. If anthropogenic emissions continue unchecked, the warming increase may reach 8–10 degrees Celsius by 2300 (ref. 2). The contribution that large ice sheets will make to sea-level rise under such warming scenarios is difficult to quantify because the equilibrium-response timescale of ice sheets is longer than those of the atmosphere or ocean. Here we use a coupled ice-sheet/ice-shelf model to show that if atmospheric warming exceeds 1.5 to 2 degrees Celsius above present, collapse of the major Antarctic ice shelves triggers a centennial- to millennial-scale response of the Antarctic ice sheet in which enhanced viscous flow produces a long-term commitment (an unstoppable contribution) to sea-level rise. Our simulations represent the response of the present-day Antarctic ice-sheet system to the oceanic and climatic changes of four representative concentration pathways (RCPs) from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. We find that substantial Antarctic ice loss can be prevented only by limiting greenhouse gas emissions to RCP 2.6 levels. Higher-emissions scenarios lead to ice loss from Antarctic that will raise sea level by 0.6–3 metres by the year 2300. Our results imply that greenhouse gas emissions in the next few decades will strongly influence the long-term contribution of the Antarctic ice sheet to global sea level.

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.

Facies development and sequence architecture of a late Quaternary fluvial-marine transition, Canterbury Plains and shelf, New Zealand: implications for forced regressive deposits

Abstract The Canterbury Plains, South Island, New Zealand, comprise a c. 7500 km2 coarse-grained, braidplain that accumulated during Quaternary glacio-eustatic, sea-level fluctuations. The adjacent Canterbury Bight shelf covering c.13,000 km2, comprises coeval shelf–slope deposits, that are punctuated by advances of the braidplain onto the shelf during periods of sea-level fall. This study examines the sedimentological and stratal characteristics of outcropping last glacial braidplain deposits, and then traces oscillations in the position of the fluvial-marine transition over several late Quaternary sea-level cycles using high-resolution seismic reflection profiles of the Canterbury shelf and slope. Outcropping last glacial Burnham Formation sediments display numerous, aggradationally stacked massive and cross-stratified gravel deposits with minor intercalated sand and mud. The gravels accumulated as longitudinal bars and channel fills within an extensive braidplain succession, with some evidence of frozen ground conditions during deposition based on sedimentological features. High-frequency (3.5 kHz) seismic reflection data of the subsurface Canterbury shelf identify up to seven unconformity-bound, Milankovitch-duration depositional sequences. These sequences are inferred to correlate with successive 100-ka, sea-level cycles spanning Oxygen Isotope Stages 16 to 1 (last c. 700 ka). Each sequence displays a distinctive stratigraphic motif comprising four recurring seismic units: 1. Basinward of the glacial maximum shoreline, wedge-shaped units displaying steeply dipping clinoforms that onlap the continental slope are interpreted as “perched lowstand deltas” belonging to the lowstand prograding wedge systems tract (LST). 2. Irregular hummocky units up to 10 m thick, containing high-amplitude discontinuous reflectors, are interpreted as representing stranded coastal deposits of the transgressive systems tract (TST). 3. Low-amplitude seismic units which offlap and downlap onto the TST, infilling local paleotopography, and interpreted as comprising fine-grained marine sediments of the highstand systems tract (HST). 4. Basinward thickening units (up to 40 m thick), containing a strongly progradational series of offlapping, inclined (0.5–1.0°), high-amplitude reflectors, that downstep towards the basin are interpreted as coarse-grained, fluvio-deltaic sediments, similar to the last glacial Burnham Formation, deposited during glacio-eustatic sea-level fall, or forced regression. We assign this unit to the regressive systems tract (RST), which displays a gradational lower boundary overlain by a sharp planar regionally extensive sequence boundary or ravinement surface. 2-D forward stratigraphic modelling, constrained by outcrop and seismic data, indicates that rivers of the Canterbury region did not incise during eustatic sea-level fall. This may be the case elsewhere, too, where a coastal plain is flanked by a lower gradient shelf. On the Canterbury shelf, fluvial incision did not occur during Quaternary forced regressions, but instead, subaerial accommodation was created and filled in by thick, fluvio-deltaic deposits, as contemporary rivers graded to the glacial maximum shoreline. Incision was restricted to three zones: (1) The lowstand shelf break, where canyons of limited extent formed by nickpoint retreat; (2) the transgressive coastline, where rivers incised due to coastal erosion; and (3) the inner braidplain adjacent to the Southern Alps, where degradation was caused by tectonic uplift.

Constraints on the amplitude of Mid-Pliocene (3.6–2.4 Ma) eustatic sea-level fluctuations from the New Zealand shallow-marine sediment record

Ice-volume calibrations of the deep-ocean foraminiferal δ18O record imply orbitally influenced sea-level fluctuations of up to 30 m amplitude during the Mid-Pliocene, and up to 30 per cent loss of the present-day mass of the East Antarctic Ice Sheet (EAIS) assuming complete deglaciation of the West Antarctic Ice Sheet (WAIS) and Greenland. These sea-level oscillations have driven recurrent transgressions and regressions across the worlds continental shelves. Wanganui Basin, New Zealand, contains the most complete shallow-marine Late Neogene stratigraphic record in the form of a continuous cyclostratigraphy representing every 41 and 100 ka sea-level cycle since ca 3.6 Ma. This paper presents a synthesis of faunally derived palaeobathymetric data for shallow-marine sedimentary cycles corresponding to marine isotope stages M2–100 (ca 3.4–2.4 Ma). Our approach estimates the eustatic sea-level contribution to the palaeobathymetry curve by placing constraints on total subsidence and decompacted sediment accumulation. The sea-level estimates are consistent with those from δ18O curves and numerical ice sheet models, and imply a significant sensitivity of the WAIS and the coastal margins of the EAIS to orbital oscillations in insolation during the Mid-Pliocene period of relative global warmth. Sea-level oscillations of 10–30 m were paced by obliquity.

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.

THE RELATIONSHIP BETWEEN SHELLBED TYPE AND SEQUENCE ARCHITECTURE : EXAMPLES FROM JAPAN AND NEW ZEALAND

Abstract Examples of lithology, fossil content and taphonomic features of shellbeds and intervening less fossiliferous intervals are presented from four Plio–Pleistocene successions (Shimosa Group, Boso Peninsula, Omma Formation, Hokuriku area, Japan, and Okehu, Kai-iwi, and Shakespeare groups in Wanganui, and the Rangitikei Group along the Rangitikei River in New Zealand). As for pre-Pliocene 3rd- and 4th-order depositional sequences, Plio–Pleistocene 5th- to 7th-order depositional sequences contain a variety of shellbeds which are often associated with surfaces or intervals that are characterized by sedimentary condensation, omission or erosion (e.g. sequence boundaries, ravinement surfaces, downlap surfaces and condensed sections). Stratigraphic patterns of shellbed type tend to be similar and repetitive within a basin and a locality. This demonstrates that a specific palaeogeography played an important role in determining the nature of shellbeds. For example, shellbeds formed in the context of toplap are common only in the Shimosa Group, which was deposited in a moderately sheltered sea, the palaeo-Tokyo Bay. Toplap shellbeds are rare in other sequences formed in more open conditions. Despite the variability resulting from such basin characteristics, common styles of shellbeds can be recognized that formed under conditions of marine onlap, backlap, downlap and toplap. Each type of shellbed has a characteristic fossil composition and taphonomy. Onlap and toplap shellbeds contain low-diversity macrobenthic associations including Glycymeris, Mercenaria, Paphies or other bivalves having robust shells, which are often abraded or fragmented. Backlap shellbeds, which are equivalent to the condensed section formed at the maximum transgression, are characterized by dominance of epifaunal macrobenthos such as bryozoa, brachiopoda, pectinid and ostreid bivalves, preserved in a slightly cemented, glauconitic muddy matrix. In contrast to fossils in such condensed sections, the shell density and species diversity of downlap shellbed associations are rather low, and in a few examples the macrobenthic association was buried rapidly in a lower unit of the highstand systems tract (HST) stratigraphically located above the condensed sections. Variations in the stratigraphic distribution of shellbed types are reflected in symmetrical and asymmetrical sequence architectures. Symmetrical sequences have roughly the same thickness of transgressive systems tracts (TST) and highstand systems tracts (HST), and have well segregated shellbeds that were formed during marine onlap and backlap. Asymmetrical cycles have very thin TSTs and much thicker HSTs, and are characterized by the amalgamation of condensed onlap and backlap shellbeds. Such contrasting cycle architectures are interpreted to reflect inner (symmetrical) and outer (asymmetrical) shelf palaeodepositional settings. The amalgamated onlap/backlap shellbeds appear to be common in Plio–Pleistocene sequences. Owing to the short duration of glacio-eustatic sea-level changes with dominant frequencies of 20,000, 40,000 or 100,000 years, shellbeds in the Plio–Pleistocene are relatively simple and thin compared to those formed in ordinary third-order depositional sequences. Infauna-dominated benthic associations are generally more common than in third-order cycles, and epifaunal associations facilitated by taphonomic feedback on sediment-starved shell-gravel substrates occur only in the condensed section corresponding to maximum transgression in most Plio–Pleistocene sequences.