Collapse of a giant iceberg in a dynamic Southern Ocean marine ecosystem: in situ observations of A-68A at South Georgia

• Published in: Progress in Oceanography
• Main Authors: Tarling, Geraint A., Thorpe, Sally E., Henley, Sian F., Burson, Amanda, Liszka, Cecilia M., Manno, Clara, Lucas, Natasha S., Ward, Freyja, Hendry, Katharine R., Woodward, E. Malcolm S., Wootton, Marianne, Abrahamsen, E. Povl
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2024
• Subjects: Southern Ocean
• Online Access: http://nora.nerc.ac.uk/id/eprint/536938/; https://nora.nerc.ac.uk/id/eprint/536938/1/1-s2.0-S0079661124001034-main.pdf; https://www.sciencedirect.com/science/article/pii/S0079661124001034

Large icebergs (>20 km long) are responsible for most of the freshwater discharged into the Southern Ocean. We report on in situ and satellite observations made during the break-up phase around South Georgia of the giant tabular iceberg A-68A. The in situ measurements were obtained during a 4-day visit by a research vessel in February 2021, where physical, chemical and biological measurements were made at a range of distances away from the main and subsidiary icebergs. These results were compared to a far-field station 133 km away. Upstream of the iceberg field, water column structure was similar to ambient water although there was evidence of iceberg-associated phytoplankton as a likely remnant of the passage of the icebergs. Nevertheless, enhancement of primary productivity along the path of the icebergs was not resolved in either in situ or monthly mean satellite observations. There was a considerable brash-ice field moving ahead of the icebergs which limited the number of downstream sampling stations. One downstream station within 2 km of iceberg A-68P showed several ice-melt influenced features that distinguished it from most other stations. Firstly, there was a strong stratified meltwater influenced layer that reached to around 120 m. This had the effect of deepening underlying water masses, with the core of the temperature minimum layer around 50 m deeper than elsewhere. Secondly, there was evidence of rapid downward displacement of both particulate material and certain phytoplankton taxa that may be a further result of this water mass deepening. Thirdly, macronutrient profiles were altered, with concentrations of nitrate, silicic acid and phosphate characteristic of deeper layers being found closer to the surface and a dilution of the ambient nutrient pool just above the iceberg draft that we ascribe to meltwater released from basal melting. Meanwhile, nutrient recycling processes associated with organic matter remineralisation were also modified by the physical restructuring of the water column and ...