M. Thomassen

A new ice-core record from Lomonosovfonna, Svalbard: viewing the 1920-97 data in relation to present climate and environmental conditions

A new ice core record from Lomonosovfonna, Svalbard: viewing the data between 1920-1997 in relation to present climate and environmental conditions

A 1500 year record of accumulation at Amundsenisen western Dronning Maud Land, Antarctica, derived from electrical and radioactive measurements on a 120 m ice core

During the Nordic EPICA pre-site survey in Dronning Maud Land in 1997/1998 a 120 m long ice core was retrieved (76°00′S 08°03′W, 2400 m above sea level). The whole core has been measured using the electric conductivity measurement (ECM) and dielectric profiling (DEP) techniques, and the core chronology has been established by detecting major volcanic eruptions. In a nearby shallow core radioactive traces from nuclear tests conducted during the 1950s and 1960s have been identified. Altogether, 13 ECM and DEP peaks in the long core are identified as originating from specific volcanic eruptions. In addition two peaks of increased total β activity are identified in the short core. Accumulation is calculated as averages over the time periods between these dated events. Accumulation rate is 62 millimetres (w. eq./yr) for the last 181 years (1816 A.D. to present) and 61 mm w. eq./yr for the last 1457 years (540 A.D. to present). Our record shows an 8% decrease in accumulation between 1452 and 1641 A.D. (i.e. part of the Little Ice Age), compared to the long-term mean.

Reconstruction of the historical temperature trend from measurements in a medium-length borehole on the Lomonosovfonna plateau, Svalbard

Abstract A medium-length ice core was drilled at the ice divide on the Lomonosovfonna plateau (1230 m a.s.l.), Svalbard, in May 1997. As part of this project, temperature measurements were performed in the 120m deep borehole. At this site the ice thickness based on radar measurements is 126.5 m and the mean annual accumulation rate is 380 kg m–3. the measurements over the 15–120m depth interval show a nearly isothermal profile with a mean value of –2.8˚C and a standard deviation of 0.2˚C. the measurements reveal a temperature minimum at approximately 70m depth and a temperature gradient of 0.011 ±0.004˚Cm–1 near the bottom. the temperature minimum and relatively low temperature gradient cannot be explained in terms of a steady-state climate. Numerical calculations with a simple one-dimensional diffusion–advection model show that the temperature increased at a maximum rate of 0.02–0.025Ka–1 over the last 100 years, the total temperature increase amounting to 2.0–3.0K. Forcing the model with the observed record at Svalbard airport revealed that in the 19th century the surface temperature was at most 2.5 K lower, and that the instrumental observations started during a period with temperatures comparable to the end of the 19th century. the data are of particular interest for historical simulations since often no other temperature data are available in polar areas.

In situ produced 14C by cosmic ray muons in ablating Antarctic ice

Abstract Samples of a core (52 m) of ablating Antarctic ice were analysed for 14CO and 14CO2 by accelerator mass spectrometry. The data were compared with a 14C in situ production model that includes muon capture in addition to oxygen spallation by neutrons. The analysis reveals significant in situ 14C at depths below 10 m, which we attribute to 14C production by cosmic ray muons. The age of the ice was determined as 9.3±0.4 14C ka BP.

Very little in situ produced radiocarbon retained in accumulating Antarctic ice

Ice samples from Dronning Maud Land, Antarctica, were analyzed for 14 CO2 and 14 CO by accelerator mass spectrometry. Only a small amount (2%) of in situ produced radiocarbon was detected. The calibrated radiocarbon ages, corrected for in situ produced 14 C, are in fair agreement with age estimates obtained from stratigraphical methods added to a gas inclusion model. The ages of the entrapped air range from recent to ca. 1200 AD. ” 2000 Elsevier Science B.V. All rights reserved.

In-situ radiocarbon production by neutrons and muons in an antarctic blue ice field at Scharffenbergbotnen: a status report

In the radiocarbon accelerator mass spectrometry ( (super 14) C AMS) analysis of gases obtained in a dry extraction from a 52-m Antarctic ice core, we observed (super 14) CO (sub 2) and (super 14) CO concentrations decreasing with depth. The concentrations are explained in terms of in-situ production by neutrons and captured muons in ablating ice. The ratio of the (super 14) CO (sub 2) concentration to that of (super 14) CO has been found to be constant at 1.9+ or -0.3. The ablation rates obtained of 42+ or -18 cm.yr (super -1) and 40+ or -13 cm.yr (super -1) for the neutron and muon components, respectively, are about three times higher than observed from stake readings. The discrepancy may point to an incomplete extraction of the dry extraction method. Using the constant ratio in (super 14) CO (sub 2) and (super 14) CO concentrations we correct for the in-situ component in the trapped (super 14) CO (sub 2) and deduce an age of 10,300+ or -900 BP for the ice core.