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A HIGH RESOLUTION CLIMATE RECORD FROM THE NORTHGRIP DEEP ICE CORE

SCHMIDT, KAREN G  University of Copenhagen.
Dahl-Jensen, Dorthe  University of Copenhagen.
Johnsen, Sigfus J  University of Copenhagen.
NorthGRIP members,  .

The North Greenland Icecore Project (NorthGRIP) deep drilling operation reached bedrock at 3085 m in July 2003, making the NorthGRIP core the longest ice core ever drilled in Greenland. The drill site is located at 75.10ºN, 42.32ºW, which is 324 km NNW of the summit of Greenland where the Greenland Icecore Project (GRIP) and Greenland Ice Sheet Project 2 (GISP2) deep ice cores were drilled. NorthGRIP was initiated in 1995 as a joint international programme involving Denmark, Germany, Japan, Belgium, Sweden, Iceland, the U.S.A., France and Switzerland. The main goal was to obtain undisturbed high-resolution information about the Eemian climate period (115-130 kyr BP). The records from GRIP and GISP2 are different and disturbed in the ice covering this period. Internal radio-echo sounding layers showed that NorthGRIP is located on a gently sloping ice ridge with very flat bedrock and has a lower annual accumulation than central Greenland, resulting in a higher age of the basal ice. In the Eemian the climate was several degrees warmer than present conditions, making it an important period to study for evaluation of possible future climate scenarios with an anthropogenic global warming.

The NorthGRIP camp was established in 1996, where shallow drilling, casing and construction of the deep drill setup was accomplished. In 1997, the drill got stuck at 1371 m depth and could not be recovered. This core is referred to as the NGRIP1 ice core. In 1998 a new bore hole was prepared and casing was made. During the 1999 and 2000 field seasons, a depth of 2930 m was reached, before the drill got stuck. This time, though, the drill was successfully recovered. In the 2001 season the drill was repeatedly stuck and recovered 4 times due to increased bore hole temperature near the base of the ice sheet. In 2003 bedrock was reached at a depth of 3085 m.

The NorthGRIP core contains the first continuous record of the end of the Eemian and the start of the last glacial period recorded in a deep Greenland ice core. The bottom of NorthGRIP is essentially undisturbed and annual layers are relatively thick, a situation caused by basal melting which results from a unexpectedly high geothermal heat flow along the NorthGRIP flow line. The ice from the NorthGRIP ice core contains about 1 cm thick annual layers in the period of the transition from the Eemian to the last ice age, making the NorthGRIP records the best resolved climate series from this period ever recorded in Greenland. The NorthGRIP core will make it possible to investigate the duration of the onset of the ice age and how the atmospheric circulation changed from year to year and thereby get valuable insight of the dynamics of the climate system at the onset of an ice age. The precise dating will also provide the opportunity for using it as an important link between the northern and the southern hemisphere. The detailed measurements have been initiated and we hope to have significant results within a year.

The NorthGRIP isotopic record covers the Holocene, the entire last glacial period, and part of the last interglacial, the Eemian. The NorthGRIP core can be cross dated with the GRIP core down to 105 kyr BP using the isotope curves together with the methane concentrations and the oxygen isotopic composition of air in the air bubbles. Older ice can be dated using these gas concentrations and comparing them to the corresponding Vostok profile. Below Dansgaard-Oeschger event 24 the NorthGRIP record is compared to the planktonic oxygen isotope record from marine core MD95-2045 drilled at the Iberian coast. Based on similarities between the two records and 1D ice modelling, the basal part of the NorthGRIP record is dated to 123 kyr BP. The lower 60 m of ice core contains ice from 115 kyr BP to 123 kyr BP, corresponding to the end of the Eemian. The results from the isotope measurements imply significantly warmer (approx. 5ºC) climate in the Eemian compared to the present and that the temperatures slowly decreased during a 5000 yr period before suddenly dropping to ice age conditions.

Using the continuous flow analysis (CFA) it is possible to measure the concentration of trace materials in ice cores at a very high depth resolution. During the field season of 2000 the ice core from the depth interval 1400-2930 m was analysed for its contents of Ca2+, Na+, NH4+, NO3-, SO42-, H2O2, HCHO, and insoluble microparticles (mainly mineral dust). Also, the electrical conductivity of the melted ice sample was measured contiuously. In addition, the visual stratigraphy of the ice was recorded continuously with sub-millimeter resolution using a dedicated line scanner. The depth resolution of the CFA equipment is approx. 1–2 cm, making it possible to detect changes in concentration and timing of the trace substances on a sub-annual scale in the last part of the glacial period and during the Bølling-Allerød – Younger Dryas – Preboreal oscillation. This part of the ice core can thus be dated by identification and counting individual annual layers in the ice core based on a multi-parameter approach. It is believed that the timescale produced for this part of the core will have an uncertainty of a few percent or less. For the deeper parts of the core, annual layers will be detectable from the CFA data in the relatively warm interstadials due to the increased accumulation, and with improved CFA measuring techniques it should be possible to detect annual layers during the stadials as well.

In the field many additional measurements were made. The electrical properties of the ice was recorded with Dielectric Profiling (DEP) and Electrical Conductivity Measurements (ECM), and the borehole was logged using several independent methods. In the laboratory the physical properties of the ice are studied and deformation experiments performed.

When bedrock was reached at NorthGRIP, basal water flooded the lower 45 m of the hole. From the measured temperature profiles it was known that the base of the ice sheet was at or very near the pressure melting point, but the sudden and rapid flooding was unexpected. A distinct layer of liquid water was not observed in the radar profiles taken during site selection, so the water is apparently contained in a sediment layer under the ice. The drill came up covered with refrozen basal melt water and a 2 kg reddish colored piece of ice was attached to the end of the drill head. The recovered basal ice had a high contents of gasses. The hydrological system under the ice cap may have been isolated from the surface for 2 million years. At this time, we know little about the basal water ice at NorthGRIP, including why it is pink, what the gas is, and if it contains signs of bacterial life. Samples are examined at the moment.

Acknowledgements: The NorthGRIP project is directed and organized by the Department of Geophysics, University of Copenhagen, Denmark. It is being supported by funding agencies in Denmark (SNF), Belgium (FNRS-CFB), France (IFRTP and INSU/CNRS), Germany (AWI), Iceland (RannIs), Japan (MEXT), Sweden (SPRS), Switzerland (SNF) and the United States of America (NSF).



Figure 1. Picture from the NorthGRIP camp just after bedrock was reached. Basal water had entered the bore hole and the drill was brought to the surface. Under the drill a piece of reddish refrozen basal water can be seen.


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