Description: This work presents investigations of hydrothermally influenced sediments of the Bransfield Strait, an extensional marginal basin between the Antarctic Peninsula and the South Shetland lslands. Although the hydrothermal activity of Bransfield Strait has been documented since the mid-eighties, the first samples of warm, hydrothermally influenced sediments at Hook Ridge were obtained during the expeditions ANT-XV/2 with RN Polarstern in 1997/98 and NBP 99-04 with RN IB N. B.Palmer in 1999. The vent sites are characterized by white siliceous crusts on top of the sediment layer and by temperatures of up to 48.6°C. Pore waters are enriched in silica and sulfide and show low pH values. The very shallow depth of these vent sites (1050 m) and the influence of volcanic activity particularly control the concentration of pore fluid constituents. Chloride is depleted up to 20 % and the calculated hydrothermal endmember concentration is in the range of 1 to 84 mM. This fluid composition is attributed to phase separation, since other mechanisms for Cl depletion can clearly be ruled out. The depth of phase separation, inferred by using a relationship between Cl concentrations of the phase separated fluids and p,T conditions, is about 1450 m beneath the seafloor at Hook Ridge. Thus, phase separation takes place at subcritical conditions. Whereas the sediments on Hook Ridge are characterized by diffuse outflow of hydrothermal fluids, the hydrothermal impact on the sediments of the NE part of the adjacent King George Basin is shown in deeper sections of these cores (6 m). Depth profiles of elements that are not affected by diagenesis reveal diffusion controlled transport from a reaction layer below the sampling depth. The direct vicinity to the hydrothermally active volcanic edifice Hook Ridge suggests that this reaction layer is within the basal sediments that are heated by the hot crust underneath. This interpretation is confirmed by various geochemical characteristics of the pore fluids from which the temperature of the reaction zone is estimated to be ≤ 150°C. Analyses of boron and strontium isotopic compositions of the pore waters yield endmember signatures of 21 ‰ (δ 11 B) and 0.7028 (87Sr/86Sr) respectively. These findings indicate an intense interaction of the hydrothermal fluid with the sediments and the upper crust. Linear extrapolation of the chloride concentration versus magnesium yields an endmember concentration of 704 mM, which indicates the existence of a brine phase that corresponds to the vapor phase sampled at Hook Ridge. The investigations on the sediments and pore waters of Hook Ridge and King George Basin demonstrate that both locations are affected by the hydrothermal activity within the upper crust at the rift axis of Bransfield Strait. They are to be considered as two components of one hydrothermal system (see Figure 6-1 in the German summary for a schematic diagram). Since water column anomalies and a chimney-like fragment within the sediment provide promising evidence of active high temperature venting, manifestations of hot vents are likely to be discovered on upcoming cruises.