|Narrative summary of dataset||Here we provide data from the Ross Ice Shelf ocean cavity. Location - The HWD2 Camp was established in October of 2017 at 80o 39.497’S, 174o 27.678’E where the ice is moving seaward at around ~600 m a-1 and is sourced from the Transantarctic Mountains. Profiling Instruments - Profiling was primarily conducted with an RBR Concerto CTD (conductivity-temperature depth) profiling instrument, and this was cross-calibrated against irregular profiles with an RBR Duet (pressure and temperature only), a SBE37 MicroCat CTD as well as moored SBE37 MicroCat CTDs. The RBR unit is small and has suitable sensor capability (temperature and conductivity accuracies of ±0.002°C and ±0.003 mS cm-1). Its conductivity cell design is not prone to fouling by ice crystals, making it ideal for work in the sometimes crystal-laden borehole conditions. We were inconsistent in how we mounted the CTD on its protective frame and this appeared to make small difference in the conductivity signal (resulting in an ~0.03 psu variation). This was post-corrected based on the essentially invariant mooring data from the lower water column as well as SBE37 cross-calibration profile data. Because of the potential for sediment contamination of the sensors, the profiles were mostly conservative in their proximity to the sea floor. On several occasions, profiles were conducted all the way to the sea floor. The temperature and salinity are presented in EOS-80 in order to compare with available data. Eighty three profiles are provided here (ctd_HWD2_*.dat). In addition, limited microstructure profiling was conducted to provide insight into some of the mixing details. The profiles were conducted by lowering the instrument to the ice base then commencing a sequence of three up-down “yo-yos” before returning to the surface and downloading. A data segment is included here (VMP_HWD2.dat). There were some challenges registering the vertical coordinate for the profiles. The melting of the borehole generates a trapped pool of relatively fresh water. The interface between this and the ocean should be near the base of the hole or a little higher – with seawater intrusion. However, there were some instances where the interface was at a higher pressure (i.e. apparently in the open water column). The best explanation for this is that the water in the borehole is not at static equilibrium for some period after initial melting. We use 34.3 psu as a cut-off, in addition to a pressure criterion to identify the top of the useful oceanic profile. It is also not inconceivable that water was being ejected from the hole, but it is unlikely that this would have impacted in the consistent observed pattern. Instrumented Mooring - The mooring instruments at HWD2-A comprised 5 Nortek Aquadopp single point current meters in titanium housings reporting to the surface (30-minute interval, Table SI-Three) via an inductive modem to a Sound-9 data logger and Iridium transmitter. The current meter measurements were corrected to account for the 138o magnetic declination offset (i.e. the south magnetic pole is to the north-west of the field site). Five files are provided here (HWD2_Init_rcm*.dat4). Details in: Stevens C, Hulbe C, Brewer M, Stewart C, Robinson N, Ohneiser C and Jendersie J, 2020. Ocean mixing and heat transport processes observed under the Ross Ice Shelf controls its basal melting, accepted PNAS, May 2020.
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|Summary of processing methodology||The HWD2 Camp was established in October 2017 following from a site surveyin 2015. On 5 December 2017 the camp location was 80°39.497'S, 174°27.678'E where the ice is moving seaward at around~600 m a-1(49) and is sourcedfrom the Transantarctic Mountains (50). Ground-based seismic surveying hasrevealed the large-scale subshelf bathymetry (51), altimetry provides esti-mates of ice shelf draft (52), and cavity volume estimated from numericalrepresentations suggest a volume of around 130×103km3(4). However,significant uncertainty remains in the cavity geometry (53). This has impli-cations for topographically guided ocean circulation, at both the seafloorand basal boundary.The HWD2 site is in a modest local minimum in terms of ice thickness. Two 25-cm initial diameter boreholes were melted usinga hot water drill (based on a design described in ref. 54). The first borehole(HWD2-A) was used for initial camera inspection and a conductivity–temperature–depth (CTD) profile after which an instrumented mooring wasdeployed. The mooring included Nortek current meters and Seabird SBE 37CTD instruments. A second borehole (HWD2-B), 500 m to the south along anice flow trajectory, was kept open for 12 d (9 to 20 December 2017) and usedfor all subsequent CTD profiles as well as bottle-sampling for oxygen isotopeanalysis. The majority of the profiling was with an RBR Concerto CTD andwas sufficiently well resolved to allow estimation of overturn scales. SomeStevens et al.PNAS|July 21, 2020|vol. 117|no. 29|16803EARTH, ATMOSPHERIC,AND PLANETARY SCIENCESDownloaded by guest on November 26, 2020
additional profiles were recorded with a Rockland microstructure profiler.|