GoKawiilIceberg imaging and sampling
During expedition PS126 of RV Polarstern, the opportunity arose to visit a large iceberg in the vicinity of HAUSGARTEN carrying numerous dark stones. The iceberg was accessed by helicopter from the ship on 14 June 2021 at the coordinates 78° 35.66′ N, 3° 32.92′ W, allowing us to collect samples of the transported stone material.
A series of overlapping, downward-facing images of the iceberg and its stone load were recorded from the helicopter at an altitude of about 100 m using a Canon EOS digital camera with a lens operated at about 50 mm. The images were stitched together using Agisoft Photoscan Pro. Stone piles placed approximately 5 m apart by field personnel served as a size reference for both individual images and the final mosaic (Extended Data Fig. 2a). Additional close-range images of stone clusters were acquired on the iceberg using a Nikon D850 with a 50 mm lens, with a ruler included in each frame for scale. The planar area of 20 randomly selected stones per image (n = 500 in total) was measured using ImageJ. Furthermore, stones were sampled to determine their mineralogical composition. Representative specimens spanning the observed morphological range were collected for classification (Extended Data Fig. 2c).
An ice core (9 cm in diameter) was collected from the upper 2 m of the iceberg (after snow removal) using a hand auger with an electric drill (Kovacs Enterprise). The coring location was about 50 m away from the nearest stone pile to avoid stone interference during drilling. Three replicate measurements of electrical conductivity were made from sub-cores 27–57 cm long (n = 6) to determine the salinity of the ice. Identical methods were used to assess the salinity of two sea ice floes nearby for reference (78° 54.11′ N, 3° 9.39′ W and 79° 01.44′ N, 5° 42.49′ W).
Seafloor observations
Images were recorded from the seafloor at HAUSGARTEN station EG-IV (2,500 m depth) using either the OFOS or the Ocean Floor Observation and Bathymetry System between 2015 and 2021 (ref. 49). The exact transect location varied by year due to ice conditions (Extended Data Fig. 1b). In 2015 and 2017, the same transect was surveyed, allowing for a comparison between years in an area of seafloor about 5,400 m2. OFOS included a downward-facing camera (Canon EOS), strobes that flashed to illuminate each image and downward-facing laser points that served as a size scale for the image and stone plan area. Telemetry data showed distance to the seafloor, and all images were recorded at a target altitude of 1.5 m. OFOS images were recorded automatically every 30 s while the ship transited at 0.5 knots. Images that were too bright, too dark or at an anomalous altitude (that is, outside the range of 1.3– 1.6 m) were considered ineligible for analysis.
We observed glacial dropstones on the seafloor in OFOS images, ranging from 0.6 cm to 73 cm. The vast majority of stones on the deep seafloor north of 45° N are glacial in origin50. Near-bottom currents in the bathyal Fram Strait are <10 cm s−1, not fast enough to uncover buried stones or bedrock14,21.
An exception occurs at a steep reef in the eastern Fram Strait, where stronger near-bottom currents expose bedrock and mobilize locally derived stones51. However, this feature lies several hundred kilometres from station EG-IV, where our imagery was collected. The stones observed on both the seafloor and the visited iceberg consist of shale and quartz (Extended Data Fig. 2). Taken together, the geological, bathymetric and oceanographic context indicates that stones observed on the seafloor at station EG-IV represent glacial dropstones.
The densities of stones in 30 randomly selected images from the beginning, middle and end of the transect were calculated each year (total of 90 images per year). Dropstones and dropstone-associated fauna were annotated in each image using the online tool BIIGLE (Bio-Image Indexing and Graphical Labelling Environment; biigle.de; ref. 52). OFOS images recorded at 3 m altitude reliably show seafloor features and fauna >1 cm across49. Images in this study were recorded at about 1.5 m altitude, allowing for visualization of fauna and features to 0.6 cm across. Fauna visible in our OFOS images included sponges, bryozoans, tunicates, anemones, soft corals, serpulid polychaetes, barnacles, sea stars and crinoids. Some taxa could be identified based on taxonomic voucher samples collected in previous studies throughout Fram Strait53. Taxa for which no voucher had been identified were given morphotype descriptors. The densities of stones and of each dropstone-associated species in 2015 and 2017 were calculated by dividing the number in each image by the planar area of that image. Plan areas of 250 stones in randomly selected 2015 and 2017 images (n = 250 per year) were measured using the rectangle select tool in BIIGLE. Plan areas of 500 stones from randomly selected 2021 images were measured using the straight line tool for measurement of length and width in ImageJ.
Dropstones are typically deposited on the seafloor in clusters, and slight variations in the swing of the OFOS camera could cause these clusters to appear in one year but not another. To avoid biasing the results by a small number of images containing dense stone clusters, we removed outlier images with anomalously high stone densities (>5 stones per m2) before analysis (n = 1 image, 2015; n = 3 images, 2017). Differences in univariate metrics (that is, stone density, fauna density) were evaluated using parametric analysis of variance (ANOVA) tests or non-parametric Kruskal–Wallis tests, when the assumptions of ANOVA were violated. Chi-square tests were used to test for differences in the size distributions of stones on the iceberg and on the seafloor and between years (2015 and 2017). Statistical analyses were conducted in the R environment using the packages vegan, pairwiseAdonis and ggplot2.
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