Borehole temperatures reveal details of 20th century warming at Bruce Plateau, Antarctic Peninsula

Abstract

Two ice core boreholes of 143.74 m and 447.65 m (bedrock) were drilled during the 2009–2010 austral summer on the Bruce Plateau at a location named LARISSA Site Beta (66°02' S, 64°04' W, 1975.5 m a.s.l.). Both boreholes were logged with thermistors shortly after drilling. The shallow borehole was instrumented for 4 months with a series of resistance thermometers with satellite uplink. Surface temperature proxy data derived from an inversion of the borehole temperature profiles are compared to available multi-decadal records from weather stations and ice cores located along a latitudinal transect of the Antarctic Peninsula to West Antarctica. The LARISSA Site Beta profiles show temperatures decreasing from the surface downward through the upper third of the ice, and warming thereafter to the bed. The average temperature for the most recent year is −14.78 °C (measured at 15 m depth, abbreviated <i>T</i><sub>15</sub>. A minimum temperature of −15.8 °C is measured at 173 m depth and basal temperature is estimated to be −10.2 °C. Current mean annual temperature and the gradient in the lower part of the measured temperature profile have a best fit with an accumulation rate of 1.9 × 10<sup>3</sup> kg m<sup>−2</sup> a<sup>−1</sup> and basal heat flux (<i>q</i>) of 88 mW m<sup>−2</sup>, if steady-state conditions are assumed. However, the mid-level temperature variations show that recent temperature has varied significantly. Reconstructed surface temperatures (<i>T</i><sub>s</sub>=<i>T</i><sub>15</sub> over the last 200 yr are derived by an inversion technique. From this, we find that cold temperatures (minimum <i>T</i><sub>s</sub>=−16.2 °C) prevailed from ~1920 to ~1940, followed by a gradual rise of temperature to −14.2 °C around 1995, then cooling over the following decade and warming in the last few years. The coldest period was preceded by a relatively warm 19th century at <i>T</i><sub>15</sub> ≥ −15 °C. To facilitate regional comparisons of the surface temperature history, we use our <i>T</i><sub>15</sub> data and nearby weather station records to refine estimates of lapse rates (altitudinal, adjusted for latitude: Γ<sub>a(l))</sub>. Good temporal and spatial consistency of Γ<sub>a(l))</sub> over the last 35 yr are observed, implying that the climate trends observed here are regional and consistent over a broad altitude range

Geothermal heat flow in Antarctica: current and future directions

“…Thirdly, if the borehole doesn't reach bedrock, and similarly to the previous method, a thermal model is required to constrain GHF (e.g. Zagorodnov et al, 2012). In the methods where modelling is required, the variables are modified within constraints determined for the location until the modelled temperature profile best fits the measurements (Fig.…”

“…the ice sheet needs to be stationary and frozen to the bed). This limits the available GHF estimates to Vostok (Salamatin et al, 1998), Dome Fuji (Hondoh et al, 2002), Law Dome (Dahl-Jensen et al, 1999), South Pole (Price et al, 2002), Marie Byrd Land (Clow et al, 2012;Engelhardt, 2004;Gow et al, 1968), and the Antarctic Peninsula (Mulvaney et al, 2012;Nicholls and Paren, 1993;Zagorodnov et al, 2012) (Fig. 15).…”

Bellingshausen Sea ice extent recorded in an Antarctic Peninsula ice core

“…The site experiences virtually no melting, and only three very thin (<3 mm) melt layers were observed in the entire core. Borehole temperatures reveal that the average temperature for the most recent year is −14.78°C (measured at 15 m), the minimum temperature of −15.8°C occurs at 173 m, and the ice is frozen to the bed with a basal temperature estimated to be −10.2°C [ Zagorodnov et al, ].…”

Centuries of intense surface melt on Larsen C Ice Shelf

“…Deuterium content analysis of a 1000-year ice core from James Ross Island (JRI), 350 km to the north-east of Cabinet Inlet, shows mean annual temperatures to have been rising steadily for the past 600 years (Mulvaney et al, 2012;Abram et al, 2013) but that within this period AD 1777 marked the end of one of two significant warming intervals. This warming interval repeatedly produced temperatures equivalent to those seen in the latter half of the 20th century and may have conditioned the ice in Cabinet Inlet to a density at which surface ponding could occur, in a manner similar to that which has probably been occurring since the recent AP warming which began in the mid-1950s (Zagorodnov et al, 2012;Turner et al, , 2016. In other words, we find that the formation of a deep U3 section at CI-22 coincided with an anomalously warm period during the first half of the 18th century.…”

“…Isotopic analyses of ice cores from Ferrigno on the coast of West Antarctica (Thomas et al, 2013) and Gomez on the southwestern AP (Thomas et al, 2009) as well as of borehole temperatures on the Bruce Plateau (Zagorodnov et al, 2012) also indicate a warming trend since the 1950s. The Southern Hemisphere annular mode (SAM) or the Antarctic oscillation index describes the difference in zonal-mean geopotential heights between mid-and high latitudes which drives the strength and latitude of the subpolar westerly winds.…”