McPherson, Rebecca; Kanzow, Torsten; Hoppmann, Mario; Allerholt, Jacob; Reifenberg, Simon F (2024): Raw and converted under-ice CTD data obtained during RV POLARSTERN expedition PS131 ("ATWAICE") to the marginal ice zone north of Svalbard in July/August 2022 [dataset]. PANGAEA, https://doi.org/10.1594/PANGAEA.971721

During RV Polarstern expedition PS131 to the marginal ice zone north of Svalbard in July – August 2022, we used two RBRconcerto3 handheld CTD probes (serial numbers 210909 and 060676) for recording pressure, seawater temperature, and conductivity/salinity directly under drifting sea ice floes. The instruments were lowered on a rope through drilled holes in the ice or in leads, i. e. open water next to the respective ice floe. Our scientific interest was to characterize the shallow meltwater stratification in a melting sea ice regime. The measurements were taken during repeated visits of three ice floes during their drift across Yermak Plateau, and during one additional ice station at the western slope of the plateau. A detailed characterization of the ice floes can be found in the cruise report. We sampled multiple sites on each ice floe. After drilling a hole through the ice, ice thickness and draft were measured, and two or three shallow (~10m) water column profiles were collected in quick succession. These profiles are defined as one set for a given site. Both probes were configured to sample at 16 Hz frequency. With an ascending and lowering speed of approximately 20 cm/s, this yields a vertical resolution of 1 to 2 cm. In several instances, we visited a site at a later time during the same floe visit, and sampled through the same hole again. Therefore, a single site for a given floe visit can have two sets of profiles. This is documented well in the metadata. In addition to the classical CTD downcast mode, where the instrument was simply lowered through a hole, we also tried for the first time to use the instrument in an ascending mode in order to increase the data quality close to the ice-ocean interface. We modified one of the probes by attaching a collar with hollow plastic cylinders, which made the CTD positively buoyant in water. We then lowered the instrument with an attached weight through a hole or in a lead, released the weight (by pulling on the line), and let the instrument ascend freely. Due to the ice drift, the instrument typically reached the ice bottom or lead surface a few meters away from the drilled hole. This strategy enabled us to minimize the influence of the hole on the stratification in the shallowest centimeters below the ice. A more detailed description is provided in the cruise report (Kanzow et al 2023). The respective mode is indicated in the provided iceCTD-metadata.xlsx file, together with other relevant metadata. In addition to the under-ice data from the floes, we provide data from a calibration cast, for which both instruments were attached to the CTD rosette from the ship. The corresponding CTD cast, PS131_035_06, is also published on PANGAEA (https://doi.pangaea.de/10.1594/PANGAEA.966267). These data can be used for setting offsets for salinity and temperature. The data submitted here are not corrected or otherwise processed, except for the conversion to .txt files. A processed version of the data set will be linked to this submission in due time.

We are grateful to the captain, crew, and scientific staff of Polarstern expedition PS131 for their great field support.