Subsea permafrost in Arctic shelf seas is largely understudied.In order to characterize its properties andto understand its dynamics during the Quaternarypast, a joint Russian-German sea-ice-based drillingcampaign (COAST I) was undertaken in springApril 2005 in the western Laptev Sea, Siberia. Permafrostoutcrops had already been studied in summer2003 and represent the upper terrestrial counterpartof the submarine sequences. Here, we presentdetailed hydrochemistry and stable water isotopedata of ground ice and pore water samples from boththe subsea and the terrestrial part as tracers for permafrostdegradation under subsea conditions.The study site was located on the coast of CapeMamontov Klyk (73.61°N, 117.18°E), where a 12km long coring transect of five cores was drilled includingfour offshore cores at different distancesfrom the coast, and one terrestrial core.Figure 1: Position of the COAST I drilling transectThe sampled coastal cliff of about 25 m heightabove the sea level (m a.s.l.) is an additional datasource for the sequences of the upper part of coreC1. The coring reached a maximal depth below sealevel (b.s.l.) of 77 m in the core C2 located furthestfrom the coast (~12km from the coast).Borehole temperature logging showed increasingground temperatures seawards from about -12 °C inC1 on land up to -1 °C in the marine cores, pointingto thermal degradation of relict terrestrial permafrostunder marine influence.Hydrochemical and stable water isotope data fromground ice in ice-bonded deposits or from pore waterin cryotic deposits reveal clear evidence forchemical degradation of relict terrestrial permafrostunder subsea conditions. The marine influence actsdownwards via conductive heat transfer, via pressure-gradient and via concentration-gradient diffusionof warm, saline and isotopically heavy sea waterinto the ground. Accordingly, relict terrestrialpermafrost shows low ionic contents (measured aselectrical conductivity, EC) that increase under marineinfluence to values much more than 10 mS cm-1.The respective stable water isotopes (δ18O, δD) ofrelict terrestrial permafrost are generally lighter than-15 for δ18O and -150 for δD. In all four marinecores, the infiltration of marine waters into underlyingformer terrestrial permafrost following the Holocenesea transgression is mirrored by abrupt shifts inhydrochemical and stable isotope parameters in transitionzones between degrading and non-degradedpermafrost. However, the cryolithologically observedborders between cryotic, but not ice-bondedversus ice-bonded sediments do not correspond tothe shifts evident in hydrochemical and stable waterisotope data.Based on current coastal erosion rates of 4.5 to 5myear-1 at Cape Mamontov Klyk, the furthest offshorelocated coring site (C2) was likely flooded at about2500 years ago showing an increase in boreholetemperature of more than 10°C during that time.Taking into account the reconstructed timing of inundationon the Western Laptev Shelf and the depthof non-degraded terrestrial subsea permafrost, amaximum infiltration (or permafrost degradation)rate of 0.7 to 1.3 cm year-1 seems to be reliable.