Gary S. Wilson

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).

Magnetobiostratigraphic chronology of the Eocene—Oligocene transition in the CIROS-1 core, Victoria Land margin, Antarctica: Implications for Antarctic glacial history

In 1986, cores were obtained to a depth of 702 m (with 98% recovery) from the CIROS-1 drill hole beneath the Ross Sea on the Victoria Land margin. Glaciogene sediments identified near the base of the hole mark the earliest known record of Antarctic glaciation. Initial biostratigraphic analysis indicated that the lower 336 m of the core is early Oligocene in age, and that the upper 366 m is of late Oligocene‐early Miocene age. Recently, the chronology of the CIROS-1 core has been questioned. We developed a magnetostratigraphy for the lower 400 m of the CIROS-1 core to clarify the chronology. Our magnetobiostratigraphic results indicate that the base of the CIROS-1 core is early-late Eocene in age (corresponding to Chron C16r; ca. 36.5 Ma). We identify the Eocene-Oligocene boundary at about 410‐420 m, within a 20-m-thick, poorly stratified, bioturbated sandy mudstone. This makes the CIROS-1 core the highest latitude site (77.1°S) from which this datum event has been recognized. At 366 m, a 4 m.y. hiatus, which lies immediately beneath fluvial sediments, accounts for most of Chrons C11 and C12. We recognize three major climatic episodes in the CIROS-1 core: (1) the late Eocene (34.5‐36.5 Ma, 430‐702 m), when relatively warm conditions dominated and there were high sedimentation rates and some glacial activity; (2) the late Eocene‐early Oligocene boundary interval (28.5‐34.5 Ma, 340‐430 m), which was a transition from relatively warm to cooler conditions that coincided with glacial intensification, sea-level fall, and subaerial erosion of the shelf; and; (3) the late Oligocene‐ early Miocene (22‐28.5 Ma, 50‐340 m), when large-scale glaciation dominated the region and glaciers grounded across the continental shelf. From correlation with global oxygen isotope and sea-level records, we infer that the Antarctic climate and surrounding oceans cooled after separation of Australia and Antarctica and development of deep-water circulation between them. This marked the onset of the Eocene‐Oligocene transition at ca. 34.5 Ma. A major East Antarctic ice sheet did not develop until the early-late Oligocene boundary, toward the end of the Eocene‐Oligocene transition (ca. 28.5 Ma). Outlet glaciers did not breach the Transantarctic Mountains and ground across the Ross Sea Shelf until 0.5 m.y. later (ca. 28 Ma).

Magnetostratigraphic, lithostratigraphic and tephrostratigraphic constraints on Lower and Middle Pleistocene sea-level changes, Wanganui Basin, New Zealand

Wanganui Basin, North Island, New Zealand, contains a complex sedimentary record of Lower and Middle Pleistocene sea-level changes. Palaeomagnetic results allow identification of the Matuyama/Brunhes transition, the Jaramillo Subchron and the Cobb Mountain Subchron. Correlations of rhyolitic tuff horizons across the basin are consistent with magnetostratigraphic correlations between sections. Isothermal plateau fission-track (ITPFT) ages of 1.05 ± 0.05 Ma and 1.63 ± 0.15 Ma on two tuffs (Potaka Pumice and Pakihikura Pumice, respectively) are consistent with the interpreted magnetostratigraphy and also with the astronomically tuned timescale of ODP Site 677. Magnetostratigraphy and ITPFT ages allow correlation of sedimentary cycles at Wanganui with odd-numbered oxygen isotope stages 17–31 in deep-sea cores. The stratotype section for the New Zealand Castlecliffian Stage is shown to be incomplete relative to other studied sections in the basin. Below stage 31, the character of the cyclothems changes at Wanganui, from marine dominated, to a greater representation of non-marine and estuarine strata. This change may be partly related to a change in amplitude and frequency of climatic cycles identified in oxygen isotope records, but the primary cause is likely to be increased basinal uplift.

The neogene east antarctic ice sheet: A dynamic or stable feature?

Abstract This paper is a review of the history and behaviour of the East Antarctic ice sheet during the late Neogene and is a response to the publication of Sugden et al. (1993a), ‘The Case for a Stable East Antarctic Ice Sheet’, in Geografiska Annaler , 75A (4). The Antarctic ice sheet is a critical factor in the global climate and oceanographic system. Knowledge of its historical development is critical to understanding global palaeoceanographic and palaeoclimatic systems and their development. The ‘Webb-Harwood hypothesis’ proposes that the ice sheet. while being present in Antarctica for more than 40 Ma, was, until recently (ca. 2.5 Ma), a dynamic feature, waxing and waning under a more temperate climatic and glaciologic regime. The ‘stabilistic hypothesis’ proposes that the current ice sheet has existed in its present cold and polar form for at least the last 14 Ma. ‘The Case for a Stable East Antarctic Ice Sheet’ is the clearest statement of the stabilistic argument to date. The authors find support for their viewpoint by presenting evidence, from several geologic data bases, that does not support their own null ‘meltdown hypothesisr, which they deem to be equivalent to the ‘Webb-Harwood hypothesis’, thereby they conclude supporting the ‘stabilistic hypothesis’. However, their conclusions are weakened by: failure to reject their null hypothesis, conflict between the ‘meltdown’ and ‘Webb-Harwood’ hypotheses, infraction of geological laws, and poor application of stratigraphic principles. Several issues are highlighted here as important for direction of future research and discussion concerning Late Cenozoic Antarctic glacio-climatic history.

Integrated tephrochronology and magnetostratigraphy for cyclothemic marine strata, Wanganui Basin: Implications for the Pliocene-Pleistocene boundary in New Zealand

Abstract The middle Pliocene to earliest Pleistocene part of the Rangitikei River section in Wanganui Basin contains at least six distal tephras—five are rhyolitic (Kowhai, Eagle Hill, Ohingaiti, Waipura and Vinegar Hill tephras) and one is andesitic (Mangamako Tephra). All tephras occur as macroscopic layers and are enclosed within cyclothemic marine sediments that accumulated in shoreface to outer shelf water depths under conditions of oscillating sea level driven by glacio-eustacy. The glass and ferromagnesian compositions of the silicic tephras imply derivation from calc-alkaline rhyolitic sources, either or both of Taupo Volcanic Zone and southern Coromandel Volcanic Zone. The Waipuru and Vinegar Hill tephras have isothermal plateau fission-track (ITPFT) ages of 1.87 ± 0.15 Ma and 1.75 ± 0.13 Ma, respectively. Paleomagnetic results allow identification of (i) the Gauss/Matuyama boundary located ca. 70 m below the top of the Mangaweka Mudstone; and (ii) the top of the Olduvai Subchron, placed at the base of the Waipuru Shellbed. The ITPFT ages of the tephras are consistent with the interpreted magnetostratigraphy and also with astronomically tuned timescales. Correlation of the Rangitikei sequences to the ODP site 846 δ18O record indicates that each sequence corresponds to a glacial/interglacial isotope stage couplet, and allows ages to be interpolated for tephras not dated directly. Integration of the new ages for the Waipuru and Vinegar Hill tephras, the magnetostratigraphy, and the sequence correlations to the astronomically tuned oxygen isotope timescale, lead us to place the Pliocene-Pleistocene boundary, as defined at the Vrica stratotype, at the base of the highstand systems tract siltstone of Sequence 17 in the Rangitikei section, 60 m below the Vinegar Hill Tephra.

An integrated sequence stratigraphic, palaeoenvironmental, and chronostratigraphic analysis of the Tangahoe Formation, southern Taranaki coast, with implications for mid-Pliocene (c. 3.4-3.0 Ma) glacio-eustatic sea-level changes

Abstract Sediments of the mid‐Pliocene (c. 3.4–3.0 Ma) Tangahoe Formation exposed in cliffs along the South Taranaki coastline of New Zealand comprise a 270 m thick, cyclothemic shallow‐marine succession that has been gently warped into a north to south trending, low angle anticline. This study examines the sedimentologic, faunal, and petrographic characteristics of 10 Milankovitch‐scale (6th order), shallow‐marine depositional sequences exposed on the western limb of the anticline. The sequences are recognised on the basis of the cyclic vertical stacking of their constituent lithofacies, which are bound by sharp wave cut surfaces produced during transgressive shoreface erosion. Each sequence comprises three parts: (1) a 0.2–2 m thick, deepening upwards, basal suite of reworked bioclastic lag deposits (onlap shellbed) and/or an overlying matrix supported, molluscan shellbed of offshore shelf affinity (backlap shellbed); (2) a 5–20 m thick, gradually shoaling, aggradational siltstone succession; and (3) a 5–10 m thick, strongly progradational, well sorted “forced regressive” shoreline sandstone. The three‐fold subdivision corresponds to transgressive, highstand, and regressive systems tracts (TSTs, HSTs, and RSTs) respectively, and represents deposition during a glacio‐eustatic sea‐level cycle. Lowstand systems tract sediments are not recorded because the outcrop is situated c. 100 km east of the contemporary shelf edge and was subaerially exposed at that time. Well developed, sharp‐ and gradational‐based forced regressive sandstones contain a variety of storm‐emplaced sedimentary structures, and represent the rapid and abrupt basinward translation of the shoreline on to a storm dominated, shallow shelf during eustatic sea‐level fall. Increased supply of sediment from north‐west South Island during “forced regression” is indicated from petrographic analyses of the heavy mineralogy of the sandstones. A chronology based on biostratigraphy and the correlation of a new magnetostratigraphy to the magnetic polarity timescale allows: (1) identification of the Mammoth (C2An.2r) and Kaena (C2An. 1r) subchrons; (2) correlation of the coastal section to the Waipipian Stage; and (3) estimation of the age of the coastal section as 3.36–3.06 Ma. Qualitative assessment of foraminiferal census data and molluscan palaeoecology reveals cyclic changes in water depth from shelf to shoreline environments during the deposition of each sequence. Seven major cycles in water depth of between 20 and 50 m have been correlated to individual 40 ka glacio‐eustatic sea‐level cycles on the marine oxygen isotope timescale. The coastal Tangahoe Formation provides a shallow‐marine record of global glacio‐eustasy prior to the development of significant ice sheets on Northern Hemisphere continents, and supports evidence from marine δ18O archives that changes in Antarctic ice volume were occurring during the Pliocene.

Identification of a Waipawa Formation equivalent in the upper Te Uri Member of the Whangai Formation - implications for depositional history and age

Abstract Stable isotopes and biomarkers have identified a unit with similar organic geochemistry to the Waipawa Formation, in the upper Te Uri Member of the Whangai Formation, exposed in the Akitio River, at Tawanui, southern Hawkes Bay, New Zealand. At Tawanui, the uppermost greensand of the Te Uri Member contains a large positive 813C isotopic excursion from ‐27.0%o to ‐20%e and an increase in total organic carbon from 0.1% to 1.0%. Biomarker analyses demonstrate a similar C30 sterane fingerprint to other deposits of the Waipawa Formation. We propose that the uppermost greensand of the Te Uri Member at Tawanui is a condensed stratigraphic equivalent of the Waipawa Formation at nearby Angora Stream and other East Coast Basin localities. This correlation demonstrates that Waipawa Formation is middle Teurian (middle Paleocene) and precedes the late Paleocene thermal maximum event by c. 5 m.y. The likely upwelling event that resulted in deposition of the Waipawa Formation was geographically widespread but probably restricted to the outer shelf/upper slope. In places, biogenic activity prevented the preservation of organic carbon in equivalent condensed stratigraphic intervals. Localised restriction of upwelling and black shale deposition may be demonstrated by the occurrence of a thick black shale at Angora Stream only c. 10 km from the coeval greensands at Tawanui. Alternatively, Oligocene‐Miocene east‐west shortening and structural reorganisation in the East Coast Basin may have juxtaposed facies that were originally many tens of kilometres apart. Our correlation also implies that the Te Uri Member is diachronous. It may have been on the outermost shelf to upper slope during lowstand conditions, where it is oldest, to higher on the shelf during transgression and highstand conditions.

Stratigraphy of the Awatere Group, Marlborough, New Zealand

The Awatere Group comprises siliciclastic marine and freshwater sediments of Late Miocene and Pliocene age that unconformably overlie basement rocks in the lower Awatere Valley, Marlborough, New Zealand. The Awatere Group has marked stratigraphic and lateral lithologic variations which make it difficult to adequately subdivide. This paper presents results of a detailed stratigraphic study of the Awatere Group, with an emphasis on the Upton Formation. The nomenclature associated with, and the definitions of, the Medway, Upton and Starborough Formations that comprise the Awatere Group are revised and members are defined within each formation. Detailed descriptions of units are presented to facilitate field recognition and to provide a framework for lithological correlation within the Awatere Group. This work thus comprises a new working definition of the stratigraphy of the Awatere Group.

Paleomagnetic lab established in Antarctica

The worlds southernmost paleomagnetic laboratory was established at McMurdo Station, Antarctica, in early October as part of the Cape Roberts Project [Cape Roberts Project International Steering Committee, 1994]. The laboratorys location at McMurdo Station (166°40′10″E, 77°50′18″S) is close to Hut Point, where British explorer Robert Falcon Scott established a base in the early 1900s. The facility is housed in two rooms at the Albert P. Crary Science and Engineering Center. From the laboratory, there is a superb view of southern McMurdo Sound with Black Island, Minna Bluff, Mt. Discovery to the south (where Scotts expedition for the South Pole first traversed), and the Royal Society Range of the Transantarctic Mountains. The lab is equipped with two spinner magnetometers, a thermal demagnetizer, an alternating field demagnetizer, a susceptibility meter, and an impulse magnetizer.

Warm Climates in Earth History

Global warming and global environmental change are two key, inter-related topics that receive near-constant attention in the international press. Why? Because the political agencies that direct national and international economies are reluctant to admit that we may be conducting our own global scale experiment in atmospheric pollution. Perhaps they are right to do so. However, the arguments can only be tested properly by carefully documenting the natural climate system and by comparing recent and predicted future regional and global climate change with high-resolution geologic records of past changes and reorganization in response to climatic forcing. Geologic records on their own, though, are limited in their regional and global application and can only be properly applied to understanding the global climate system by integration with computer models and simulations.