Description: Oxygen and hydrogen isotopic ratio samples were collected from sea ice and water samples. Coring and sampling were completed at six locations on July 25 for measurement of salinity and stable oxygen isotopic composition to better understand the meltwater sources for under-ice meltwater and false bottoms. Cores were collected using a corer (9 cm inner diameter; Mark II coring system, Kovacs Enterprises, US), and ice samples were collected from the bottom 10 centimeters of the ice core in 5 cm increments (0-5 and 5-10 cm above the bottom) and from the false bottom ice. Under-ice meltwater layer samples and water from directly below the false bottom were collected using a peristaltic pump. In order to minimize contamination from coring activities, under-ice meltwater samples from the meltwater layer in the void space between ice and false bottom were collected prior to coring through the false bottom. Surface melt pond water samples were collected by dipping plastic sample cups (rinsed with milliQ water) in the melt ponds where cores were collected, prior to ice coring. In total, ice samples were collected from six cores –– five of which included false bottoms –– and water samples were collected from four surface melt ponds, three under-ice melt water layers, and seawater beneath false bottoms at four locations. Locations are shown on map (Figure 2) as pink dashes. Ice samples were melted onboard the R/V Polarstern, and salinities of all water and ice samples were measured onboard using a calibrated YSI model 30 probe (salinity is given in PSS-78 scale, unitless). Vials for oxygen isotope composition were shipped to the AlfredWegener Institute (AWI) ISOLAB Facility in Potsdam, Germany, where they were analyzed for stable water isotopes with Finnigan MAT Delta-S mass spectrometers using equilibration techniques. The oxygen isotope composition is given as per mil difference relative to VSMOW (‰, Vienna Standard Mean Ocean Water), with an internal 1 sigma error better than 0.1‰ for delta 18O (Meyer et al., 2000). See attached diagram for a visualization of where the samples were collected.