Greenland palaeotemperatures derived from GRIP bore hole temperature and ice core isotope profiles

Oxygen isotope and palaeotemperature records from six Greenland ice‐core stations: Camp Century, Dye‐3, GRIP, GISP2, Renland and NorthGRIP

“…The absolute age of the Younger Dryas-Preboreal transition in the GRIP core (11 500 yr BP) was obtained by counting annual cycles in high-resolution Ca and ammonium ion records (Fuhrer et al, 1993) starting at the 8.2 ka cold event. Dating of the GRIP core further back in time was done by using two independent methods: (i) by a relatively simple iceflow model that uses two tie points to constrain two model parameters and most importantly a snow accumulation model based on the measured isotopic profile (Johnsen and Dansgaard, 1992;Johnsen et al, 1995) and (ii) by counting annual cycles in a continuous dust record back to 60 ka (Hammer et al, in press).…”

“…Calibration of decadal-to century-long temperature variations is possible only for the past several hundred years and is obtained from shallow core data (Johnsen, 1977;Cuffey et al, 1994). Deeper ice-core and borehole data provide information suitable only for calibrating millennial or longer term temperature variations (Dahl-Jensen and Johnsen, 1986;Cuffey et al, 1995;Johnsen et al, 1995;Cuffey and Clow, 1997). For the modelling of palaeotemperatures from the GRIP isotope profile we used the complete equation (2) and corrections were made for past ␦ 18 O variations of sea water.…”

“…For the GISP2 calibration (Cuffey et al, 1995), a linear dependency (␥ = 0) between T and ␦ 18 O was assumed, and no seawater correction was applied. Figure 4 shows the following important relationships between temperatures and ␦ 18 O ratios for Greenland precipitation: (i) the calculated cloud condensation temperature plotted against ␦ 18 O of the precipitate, based on a singlesource precipitation model for Greenland; (ii) the Greenland surface (spatial) temperature to ␦ 18 O relationship from equation (1) ; and the long-term borehole temperature-␦ 18 O trends (calibrations) obtained from (iii) the GISP2 (Cuffey et al, 1995) and (iv) the GRIP (Johnsen et al, 1995) deep boreholes. The offset between the GRIP and GISP2 calibration curves for the more negative ␦ 18 O values is due mainly to the omission of a seawater correction in the GISP2 calibration.…”

“…Stable isotope ratios in the water molecules of the ice ( 18 O/ 16 O or D/H-expressed herein as ␦ 18 O or ␦D, relative to that of SMOW, which is standard Mean Ocean Water), reflect the temperature of the cloud vapour at the time of snow formation, and hence, to a certain degree, the ambient air temperature (Dansgaard, 1964;Johnsen et al, 1995;Jouzel et al, 1997). Past anomalies of ambient surface temperatures are transmitted by thermal conduction into the deeper homogeneous ice.…”

“…Past anomalies of ambient surface temperatures are transmitted by thermal conduction into the deeper homogeneous ice. This means that borehole temperature profiles can be used either to measure directly the past temperature history of the ice (Dahl-Jensen and Johnsen, 1986;Dahl-Jensen, 1997), or to calibrate the measured borehole isotope variations by reference to modern relationships between isotope ratios and surface air temperatures (Cuffey et al, 1995;Johnsen et al, 1995).…”