T. Thorsteinsson

Bio: T. Thorsteinsson is an academic researcher from Alfred Wegener Institute for Polar and Marine Research. The author has contributed to research in topics: Ice core & Ice stream. The author has an hindex of 6, co-authored 8 publications receiving 2554 citations.

High-resolution record of Northern Hemisphere climate extending into the last interglacial period.

Abstract: High-resolution record of Northern Hemisphere climate extending into the last interglacial period

Modelling fabric development along the GRIP ice core, central Greenland

Abstract: The preferred c-axes orientation (fabric) observed in cold polar ice is induced by intracrystalline slip only when the grain-boundary migration rate is low enough (i.e. corresponding to grain growth or rotation recrystallization regimes). Fabrics reflect the entire thermomechanical history of the ice and strongly influence its mechanical behaviour. Large viscoplastic anisotropy is always associated with pronounced fabrics. We use a viscoplastic self-consistent (VPSC) polycrystal deformation model to calculate fabric development. In this model, stress- and strain-rate fields are not uniform within the polycrystal and both equilibrium and compatibility conditions are fulfilled. We compare fabrics measured on thin sections along the GRIP ice core (central Greenland) with those calculated down to a depth of 2800 m. Behaviours predicted by uniform stress and uniform strain bounds are presented for comparison. Predictions of the VPSC model are in close agreement with measurements within the upper 650 m, which corresponds to the entire grain-growth zone. Deeper clown, the simulated fabric strength appears to be too high. A simple calculation shows that this discrepancy may be fully attributed to the effects of rotation recrystallization.

Flow properties of the ice from the Greenland Ice Core Project ice core: The reason for folds?

Abstract: Long-term deformation tests on ice from the Greenland Ice Core Project (GRIP) deep ice core show that ice from the different climate zones in the ice core has flow properties correlated with the concentrations of impurities in the sample. The deformation tests are performed by uniaxial unconfined compression at -16 degrees C with an octahedral compression stress of 3 bars. The ice samples are compressed for 1/2 to 3 years until the tertiary strain rate is reached. It is believed that by the end all downhole flow conditions are forgotten and that the ice sample has settled in a state determined by the applied stress and temperature conditions. All samples are tested under the same stress and temperature conditions so the resulting deformation rates and final ice crystal size and fabrics can only differ due to varying impurity concentrations. The results show that ice from cold climatic periods with high concentrations of impurities deforms more slowly than ice from warm climatic periods in compression. When tertiary creep is reached, the crystal size is smaller in the cold ice than in the warm. The ice from warmer climatic periods with lower concentrations of impurities deforms at a factor of 2-3 times more rapidly in compression. The tertiary steady state crystal size is increased by 50% and the ice crystals have oriented more favorably for the applied compression in the warm ice, which is believed to be the reason why the strain rates are greater here than in the cold ice. In the bottom 200 m of the GRIP ice core, zones are observed with folds on the scale of 1-8 cm. An investigation of the ice layers in and around the folds shows that the layers are composed of ice from different climatic zones. The folding is believed to result from the different flow and rheological properties of the layers involved in the folding structures.

Properties of ice crystals in NorthGRIP late- to middle-Holocene

Abstract: Detailed measurements of crystal outlines and fabrics have been per- formed on 35000 crystals in fifteen 10 620cm 2 vertical thin sections from the North Greenland Icecore Project (NorthGRIP) ice core, evenly distributed in the depth interval 115^880m.The crystals exhibit important changes over this period. As the ice gets older the meancrystalareaincreasestowardsaconstantvalue,the shape of thecrystalsbecomes increasingly irregular, and the area distribution of crystals develops from a single log- normal distribution into a bimodal lognormal distribution. The c-axis fabric of the ice shows a smooth development of an increasingly stronger vertical fabric with depth, and the formationofaweak vertical girdle. Already inthe younger samplesthefabric israther strongly orientedtowardsvertical.The fabric andthe area of individual crystals are found not to correlate. A simple model, which takes into account the vertical strain of the ice, is applied in an attemptto determine the crystal growth rate at NorthGRIP.

The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0-55 cal kBP)

Abstract: Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.

High-resolution record of Northern Hemisphere climate extending into the last interglacial period.

Abstract: High-resolution record of Northern Hemisphere climate extending into the last interglacial period

The holocene Asian monsoon : links to solar changes and North Atlantic climate

Abstract: A 5-year-resolution absolute-dated oxygen isotope record from Dongge Cave, southern China, provides a continuous history of the Asian monsoon over the past 9000 years. Although the record broadly follows summer insolation, it is punctuated by eight weak monsoon events lasting approximately 1 to 5 centuries. One correlates with the "8200-year" event, another with the collapse of the Chinese Neolithic culture, and most with North Atlantic ice-rafting events. Cross-correlation of the decadal- to centennial-scale monsoon record with the atmospheric carbon-14 record shows that some, but not all, of the monsoon variability at these frequencies results from changes in solar output.

High-resolution carbon dioxide concentration record 650,000–800,000 years before present

Abstract: Changes in past atmospheric carbon dioxide concentrations can be determined by measuring the composition of air trapped in ice cores from Antarctica. So far, the Antarctic Vostok and EPICA Dome C ice cores have provided a composite record of atmospheric carbon dioxide levels over the past 650,000 years. Here we present results of the lowest 200 m of the Dome C ice core, extending the record of atmospheric carbon dioxide concentration by two complete glacial cycles to 800,000 yr before present. From previously published data and the present work, we find that atmospheric carbon dioxide is strongly correlated with Antarctic temperature throughout eight glacial cycles but with significantly lower concentrations between 650,000 and 750,000 yr before present. Carbon dioxide levels are below 180 parts per million by volume (p.p.m.v.) for a period of 3,000 yr during Marine Isotope Stage 16, possibly reflecting more pronounced oceanic carbon storage. We report the lowest carbon dioxide concentration measured in an ice core, which extends the pre-industrial range of carbon dioxide concentrations during the late Quaternary by about 10 p.p.m.v. to 172-300 p.p.m.v.

A new Greenland ice core chronology for the last glacial termination

Abstract: [1] We present a new common stratigraphic timescale for the North Greenland Ice Core Project (NGRIP) and GRIP ice cores. The timescale covers the period 7.9–14.8 kyr before present and includes the Bolling, Allerod, Younger Dryas, and early Holocene periods. We use a combination of new and previously published data, the most prominent being new high-resolution Continuous Flow Analysis (CFA) impurity records from the NGRIP ice core. Several investigators have identified and counted annual layers using a multiparameter approach, and the maximum counting error is estimated to be up to 2% in the Holocene part and about 3% for the older parts. These counting error estimates reflect the number of annual layers that were hard to interpret, but not a possible bias in the set of rules used for annual layer identification. As the GRIP and NGRIP ice cores are not optimal for annual layer counting in the middle and late Holocene, the timescale is tied to a prominent volcanic event inside the 8.2 kyr cold event, recently dated in the DYE-3 ice core to 8236 years before A. D. 2000 (b2k) with a maximum counting error of 47 years. The new timescale dates the Younger Dryas-Preboreal transition to 11,703 b2k, which is 100–150 years older than according to the present GRIP and NGRIP timescales. The age of the transition matches the GISP2 timescale within a few years, but viewed over the entire 7.9–14.8 kyr section, there are significant differences between the new timescale and the GISP2 timescale. The transition from the glacial into the Bolling interstadial is dated to 14,692 b2k. The presented timescale is a part of a new Greenland ice core chronology common to the DYE-3, GRIP, and NGRIP ice cores, named the Greenland Ice Core Chronology 2005 (GICC05). The annual layer thicknesses are observed to be log-normally distributed with good approximation, and compared to the early Holocene, the mean accumulation rates in the Younger Dryas and Bolling periods are found to be 47 ± 2% and 88 ± 2%, respectively.