Showing papers by "Sigfus J Johnsen published in 2001"

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

Abstract: Oxygen isotope variations spanning the last glacial cycle and the Holocene derived from ice-core records for six sites in Greenland (Camp Century, Dye-3, GRIP, GISP2, Renland and NorthGRIP) show strong similarities. This suggests that the dominant influence on oxygen isotope variations reflected in the ice-sheet records was regional climatic change. Differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios. Palaeotemperature estimates have been obtained from the records using three approaches: (i) inferences based on the measured relationship between mean annual δ18O of snow and of mean annual surface temperature over Greenland; (ii) modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile, or (iii) by using Monte Carlo simulation techniques. The third of these approaches was adopted to reconstruct Holocene temperature variations for the Dye 3 and GRIP temperature profiles, which yields remarkably compatible results. A new record of Holocene isotope variations obtained from the NorthGRIP ice-core matches the GRIP short-term isotope record, and also shows similar long-term trends to the Dye-3 and GRIP inverted temperature data. The NorthGRIP isotope record reflects: (i) a generally stronger isotopic signal than is found in the GRIP record; (ii) several short-lived temperature fluctuations during the first 1500 yr of the Holocene; (iii) a marked cold event at ca. 8.2 ka (the ‘8.2 ka event’); (iv) optimum temperatures for the Holocene between ca. 8.6 and 4.3 ka, a signal that is 0.6‰ stronger than for the GRIP profile; (v) a clear signal for the Little Ice Age; and (vi) a clear signal of climate warming during the last century. These data suggest that the NorthGRIP stable isotope record responded in a sensitive manner to temperature fluctuations during the Holocene. Copyright © 2001 John Wiley & Sons, Ltd.

An oceanic cold reversal during the last deglaciation.

Abstract: A detailed deuterium excess profile measured along the Dome C EPICA (European Project for Ice Coring in Antarctica) core reveals the timing and strength of the sea surface temperature changes at the source regions for Dome C precipitation. We infer that an Oceanic Cold Reversal took place in the southern Indian Ocean, 800 years after the Antarctic Cold Reversal. The temperature gradient between the oceanic moisture source and Antarctica is similar to the Dome C sodium profile during the deglaciation, illustrating the strong link between this gradient and the strength of the atmospheric circulation.

High-resolution analyses of an early Holocene climate event may imply decreased solar forcing as an important climate trigger

Abstract: Early Holocene lacustrine, tree-ring, ice-core, and marine records reveal that the Northern Hemisphere underwent a short cooling event at 10 300 calendar yr B.P. (9100 14 C yr B.P.). The records were compared on a common high-resolution time scale and show that the event lasted less than 200 yr, with a cooling peak of 50 yr, and the event coincides with a distinct Holocene thermohaline disturbance recognized in the North Atlantic Ocean. In spite of wellknown freshwater forcings at the time of the event, the negligible difference between the modeled D 14 C record, based on the GISP2 (Greenland Ice Sheet Project 2) 10 Be data, and the measured values, does not allow for detectable D 14 C changes related to global ocean ventilation. We can, however, show that the onset of the cooling coincides with the onset of one of the largest Holocene 10 Be flux peaks. This finding may imply that the climate system is more

Deuterium excess record from central Greenland over the last millennium: Hints of a North Atlantic signal during the Little Ice Age

Abstract: A stacked water isotope record for Summit, central Greenland, has been established covering approximately the last 900 years. Measurements of δ18O and δD in the GRIP deep ice core and the 230-m core, S93, allow the reconstruction of a millennial record of the deuterium excess in a near-annual resolution. A short period of particularly high values of the Deuterium Excess at the beginning of the fourteenth century may be associated with the medieval warm period (MWP). The Little Ice Age (LIA) might be represented by a 100-year period of very low excess values in the sixteenth and seventeenth centuries. Using a simple isotope model, δ18O and deuterium excess are interpreted in terms of surface temperature variations over central Greenland and over the subtropical North Atlantic, Greenland's principal vapor source region. An estimated cooling of −0.7°C of subtropical sea surface temperatures (SSTs) during the Little Ice Age and a warming of 0.6°C during the short warm period in the medieval is in agreement with previous studies. Over periods of about 100 years, an antiphasing between gradually decreasing δ18O and increasing deuterium excess is observed. Interannual-to-decadal-scale variability associated to the North Atlantic Oscillation may be responsible for this anti-phase relationship. An alternative explication is a North-South oscillation in North Atlantic sea surface temperatures associated with short-term changes in the thermohaline circulation.