Methodology:

Data was collected during the JR19002 Icebergs 3 cruise from Zodiac boats by hand. Surface seawater was collected into 500 mL bottles and then, on board under clean-room conditions, seawater was filtered over 0.4 um polycarbonate filters using a vacuum pump. Filters with sample were immediately frozen at -20 and transported back to the University of Southampton. Nine samples (3 from each glaciated bay) were studied using scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS) to determine the solid phase mineralogy and elemental composition. A small strip of sample filter was removed and transferred onto a conductive and adhesive carbon tape mounted on an aluminium SEM sample holder. Samples were coated with 20 nm of gold and placed on the stage of the microscope under low vacuum conditions. Chemical and morphological properties of the particles were determined by spot analysis and in some cases elemental mapping using an SEM (Carl Zeiss Leo 1450VP SEM) equipped with an Oxford Instruments X-Act 10 mm2 area SEM-Energy Dispersive Spectrometer. The SEM was set to a vacuum less than10 -4 mbar to avoid electrical charging, and analysis was performed at a working distance of 12 - 13 mm, with an accelerating voltage of 15 kV. Resolution was approximately 1 micrometer.

Data collection:

Carl Zeiss Leo 1450VP SEM

Oxford Instruments X-Act 10 mm2 area SEM-Energy Dispersive Spectrometer.

AZtec Energy

Data quality:

Quantification of element abundance was calibrated against the suite of SEM-EDS AZtec in-built standards, which necessitates all analyses being normalised to 100 percent, and elements of interest to have an atomic number more than 11, of which both conditions were met. The inbuilt AZtec EDS calibration setup requires routine calibration of the beam energy and the beam resolution, which was performed using a cobalt metal standard at 20 kV and a working distance of 15 mm.