Ground ice hydrochemical composition of permafrost core ICDP5011-3

The continuous sediment record from Lake El'gygytgyn in the northeastern Eurasian Arctic spans the last 3.6 Ma and for much of this time permafrost dynamics and lake level changes have likely played a crucial role for sediment delivery to the lake. Changes in the ground-ice hydrochemical composition (d18O, dD, pH, electrical conductivity, Na+, Mg2+, Ca2+, K+, HCO3-, Cl-, SO4-) of a 141 m long permafrost record from the western crater plain are examined to reconstruct repeated periods of freeze and thaw at the lake edge. Stable water isotope and major ion records of ground ice in the permafrost reflect both a synsedimentary palaeo-precipitation signal preserved in the near-surface permafrost (0.0-9.1 m core depth) and a post-depositional record of thawing and refreezing in deeper layers of the core (9.1-141.0 m core depth). These lake marginal permafrost dynamics were controlled by lake level changes that episodically flooded the surfaces and induced thaw in the underlying frozen ground. During times of lake level fall these layers froze over again. At least three cycles of freeze and thaw are identified and the hydrochemical data point to a vertical and horizontal talik refreezing through time. Past permafrost thaw and freeze may have destabilised the basin slopes of Lake El'gygytgyn and this has probably promoted the release of mass movements from the lake edge to the deeper basin as known from frequently occurring turbidite layers in the lake sediment column.

Specific conductivity and hydrogen ions measured on two ice core (NEEM and NGRIP)

A stratigraphy-based chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core has been derived by transferring the annual layer counted Greenland Ice Core Chronology 2005 (GICC05) and its model extension (GICC05modelext) from the NGRIP core to the NEEM core using 787 match points of mainly volcanic origin identified in the electrical conductivity measurement (ECM) and dielectrical profiling (DEP) records. Tephra horizons found in both the NEEM and NGRIP ice cores are used to test the matching based on ECM and DEP and provide five additional horizons used for the timescale transfer. A thinning function reflecting the accumulated strain along the core has been determined using a Dansgaard-Johnsen flow model and an isotope-dependent accumulation rate parameterization. Flow parameters are determined from Monte Carlo analysis constrained by the observed depth-age horizons. In order to construct a chronology for the gas phase, the ice age-gas age difference (Delta age) has been reconstructed using a coupled firn densification-heat diffusion model. Temperature and accumulation inputs to the Delta age model, initially derived from the water isotope proxies, have been adjusted to optimize the fit to timing constraints from d15N of nitrogen and high-resolution methane data during the abrupt onset of Greenland interstadials. The ice and gas chronologies and the corresponding thinning function represent the first chronology for the NEEM core, named GICC05modelext-NEEM-1. Based on both the flow and firn modelling results, the accumulation history for the NEEM site has been reconstructed. Together, the timescale and accumulation reconstruction provide the necessary basis for further analysis of the records from NEEM.