Abstract In January 2009, on its way from Cape Town to South America (at around 49.5 ∘ S and 25 ∘ W), the German research vessel Polarstern entered a region with a dense cover of icebergs and broken-off chunks of ice up to a few meters in size, largely hidden in a thick fog, a result of the microclimate created by the large agglomeration of icebergs and broken-off glacial ice. Melting of glacial ice led to a cooling (by up to 8 ∘ C) and freshening (by up to 2.5 psu) of the surface ocean with perturbations reaching down to about 75 m depth. The observed surface ocean equilibrium fugacity of CO 2 , fCO 2 , dropped to values that were over 100  µ atm lower than atmospheric fCO 2 values. Given the similar concentrations in chlorophyll a and the macronutrients nitrate and phosphate inside and outside the perturbed areas, influences of potential iron input from disintegrating icebergs on phytoplankton productivity and fCO 2 can be ruled out. The low fCO 2 values, compared to adjacent regions, can be attributed to thermodynamic effects, i.e. mostly increase of CO 2 solubility with decreasing temperature with a smaller contribution from the dilution due to freshwater inputs. Based on these observations, we consider the potential impact on atmospheric fCO 2 by the release of an armada of icebergs during Heinrich events.