C. Atkins

Bio: C. Atkins is an academic researcher from Victoria University of Wellington. The author has contributed to research in topics: Glacier & Ice sheet. The author has an hindex of 15, co-authored 27 publications receiving 926 citations.

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

Abstract: 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 today1, the Antarctic ice sheets may have been unstable2, 3, 4, 5, 6, 7. 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 cycles8, 9, 10. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets11. 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 event5).

Cold glaciers erode and deposit: Evidence from Allan Hills, Antarctica

Abstract: Here we report previously undescribed features of erosion and deposition by a cold (polar) glacier. A recent study challenged the assumption that cold glaciers neither slide nor abrade their beds, but no geological evidence was offered. The features we describe include abrasion marks, subglacial deposits, glaciotectonically deformed substrate, isolated blocks, ice-cored debris mounds, and boulder trains, all products of a recent cold ice advance and retreat. Mapping these features elsewhere in Antarctica will document recent shifts in the East Antarctic Ice Sheet margin, providing new insight on regional mass-balance changes.

Aeolian sediment flux from sea ice into Southern McMurdo Sound, Antarctica

Abstract: Aeolian sediment accumulates on the surface of annual sea ice in Southern McMurdo Sound (SMS). It is sourced from ice-free areas around the sound and is released from the sea ice during annual melting and sinks to the sea floor. However, the volume, composition and contribution to overall sedimentation in the region have not previously been quantified. Here, we present the results of a systematic, quantitative field study of aeolian sediment on the surface of sea ice in SMS carried out during the drilling of the ANDRILL 2-2A drillhole during the austral summer of 2007. Our results reveal a surprisingly large volume of aeolian sediment (between 7.8 and 24.0 tonnes of aeolian sediment per km 2 ) forming an elongate lobe extending north from the McMurdo Ice Shelf onto the sea ice surface. Detailed particle size analysis reveals distinctive fine sand and silt modes that progressively fine down wind, suggesting that the sediment is transported under two distinct wind transport processes. XRF grain composition analysis indicates that the material is predominantly volcanic and is sourced primarily from a large area of loose debris on the surface of the McMurdo Ice shelf. Comparison with modern sediment recovered from the sea floor shows a remarkable similarity in grain size distributions and suggests that the sea floor is dominated by aeolian sediment that has settled through the water column. Our calculations indicate that this process produces a sea floor sedimentation rate of up to 1.53 cm/ky in the local area. This new dataset quantifies the aeolian sediment flux in SMS and highlights the contribution of aeolian deposition to the modern sedimentary record in SMS. Furthermore, it draws attention to the recently demonstrated link between iron-bearing aeolian sediment release from sea ice and the triggering of phytoplankton blooms each spring.

Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric CO2.

Abstract: The sudden, widespread glaciation of Antarctica and the associated shift towards colder temperatures at the Eocene/Oligocene boundary (approximately 34 million years ago) (refs 1-4) is one of the most fundamental reorganizations of global climate known in the geologic record. The glaciation of Antarctica has hitherto been thought to result from the tectonic opening of Southern Ocean gateways, which enabled the formation of the Antarctic Circumpolar Current and the subsequent thermal isolation of the Antarctic continent. Here we simulate the glacial inception and early growth of the East Antarctic Ice Sheet using a general circulation model with coupled components for atmosphere, ocean, ice sheet and sediment, and which incorporates palaeogeography, greenhouse gas, changing orbital parameters, and varying ocean heat transport. In our model, declining Cenozoic CO2 first leads to the formation of small, highly dynamic ice caps on high Antarctic plateaux. At a later time, a CO2 threshold is crossed, initiating ice-sheet height/mass-balance feedbacks that cause the ice caps to expand rapidly with large orbital variations, eventually coalescing into a continental-scale East Antarctic Ice Sheet. According to our simulation the opening of Southern Ocean gateways plays a secondary role in this transition, relative to CO2 concentration.

Sequence stratigraphy: methodology and nomenclature

Abstract: The recurrence of the same types of sequence stratigraphic surface through geologic time defines cycles of change in accommodation or sediment supply, which correspond to sequences in the rock record. These cycles may be symmetrical or asymmetrical, and may or may not include all types of systems tracts that may be expected within a fully developed sequence. Depending on the scale of observation, sequences and their bounding surfaces may be ascribed to different hierarchical orders. Stratal stacking patterns combine to define trends in geometric character that include upstepping, forestepping, backstepping and downstepping, expressing three types of shoreline shift: forced regression (forestepping and downstepping at the shoreline), normal regression (forestepping and upstepping at the shoreline) and transgression (backstepping at the shoreline). Stacking patterns that are independent of shoreline trajectories may also be defined on the basis of changes in depositional style that can be correlated regionally. All stratal stacking patterns reflect the interplay of the same two fundamental variables, namely accommodation (the space available for potential sediment accumulation) and sediment supply. Deposits defined by specific stratal stacking patterns form the basic constituents of any sequence stratigraphic unit, from sequence to systems tract and parasequence. Changes in stratal stacking patterns define the position and timing of key sequence stratigraphic surfaces. Precisely which surfaces are selected as sequence boundaries varies as a function of which surfaces are best expressed within the context of the depositional setting and the preservation of facies relationships and stratal stacking patterns in that succession. The high degree of variability in the expression of sequence stratigraphic units and bounding surfaces in the rock record means ideally that the methodology used to analyze their depositional setting should be flexible from one sequence stratigraphic approach to another. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture. The purpose of this paper is to emphasize a standard but flexible methodology that remains objective.

Antarctic climate change and the environment

Abstract: The Antarctic climate system varies on timescales from orbital, through millennial to sub-annual, and is closely coupled to other parts of the global climate system. We review these variations from the perspective of the geological and glaciological records and the recent historical period from which we have instrumental data (the last 50 years). We consider their consequences for the biosphere, and show how the latest numerical models project changes into the future, taking into account human actions in the form of the release of greenhouse gases and chlorofluorocarbons into the atmosphere. In doing so, we provide an essential Southern Hemisphere companion to the Arctic Climate Impact Assessment.

A revised tropical to subtropical Paleogene planktonic foraminiferal zonation

Abstract: New biostratigraphic investigations on deep sea cores and outcrop sections have revealed several shortcomings in currently used tropical to subtropical Eocene plank­ tonic foraminiferal zonal schemes in the form of: 1) mod­ ified taxonomic concepts, 2) modifiel:l/different ranges of taxa, and 3) improved calibrations with magnetostratig­ raphy. This new information provides us with an op­ portunity to make some necessary improvements to ex­ isting Eocene biostratigraphic schemes. At the same time, we provide an alphanumeric notation for Paleo­ gene zones using the prefix 'P' (for Paleocene), 'E' (for Eocene) and '0' (for Oligocene) to achieve consistency with recent short-hand notation for other Cenozoic zones (Miocene ['M'], Pliocene [PL] and Pleistocene [PTD. Sixteen Eocene (E) zones are introduced (or nomen­ claturally emended) to replace the 13 zones and subzones of Berggren and others (1995). This new zonation serves as a template for the taxonomic and phylogenetic studies in the forthcoming Atlas of Eocene Planktonic Forami­ nifera (Pearson and others, in press). The 10 zones and subzones of the Paleocene (Berggren and others, 1995) are retained and renamed and/or emended to reflect im­ proved taxonomy and an updated chronologic calibra­ tion to the Global Polarity Time Scale (GPTS) (Berggren and others, 2000).' The PaleocenelEocene boundary is correlated with the lowest occurrence (LO) of Acarinina sibaiyaensis (base of Zone El), at the top of the trun­ cated and redefined (former) Zone P5. The five-fold zonation of the Oligocene (Berggren and others, 1995) is modified to a six-fold zonation with the elevation of (former) Subzones P21a and P21b to zonal status. The Oligocene (0) zomil' components are re­ named and/or nomenclaturally emended.