Samples from more than 3000 litres of sea water, 30 kilograms of frozen ice cores, frazil ice, snow and atmospheric dust. This is the precious cargo that a team of postgraduate students in Earth Sciences brought back after their recent winter cruise to Antartica on board the research vessel SA Agulhas II.
This was the second time that members of the environmental geochemistry research group, under the leadership of Prof Alakendra Roychoudhury and Dr Susanne Fietz, had the opportunity to collect samples during winter time. In the winter of 2017 they sailed along the so-called Indian sector (along 30°East), and this year they covered the Atlantic section (along the prime meridian, 0°East).
They hope to shed new light on what may happen to the chemistry of the ocean under warmer conditions in a future climate: “Marginal ice forms around the Antarctic each winter and thaws in spring and summer. This freezing-thawing process changes the distribution of micronutrients such as iron, zinc and other bioactive metals present in the ocean waters. We want to understand this change – how and where does it happen and where in the ice the micronutrients end up," Prof Roychoudhury explains.
Within the ice, as well as once released into the water, the available nutrients in ocean waters affect the growth and prevalence of different types of phytoplankton, as well as the types of phytoplankton. This has consequences for the global carbon budget, as well as the sustenance of krill and higher trophic levels in the region.
The team also collected dust samples over the open ocean to investigate the impact of dust as an external source of micronutrients. Dust deposition in the area is typically stronger in winter, but very little is known about how it affects the winter phytoplankton community.
To date, most sampling has been done during summer times.
Sampling in sub-zero temperatures
PhD-student Jean Loock says they sometimes faced temperatures of minus 20°C, and once the freezing cold wreaked havoc on the instruments' sensors: “The water inside the salinometer pumps would freeze in under a minute, just before we were planning to deploy it."
Because of the harsh weather conditions, a lot of time was spent in discussion with the chief scientist on board the ship, Prof Marcello Vichi from the University of Cape Town, Captain Bengu Knowlegde and ship-based marine forecasters from the South African Weather Service, to assess the viability of planned deployments, as well as trying to ensure that their arrival at designated locations coincided with the optimal weather windows.
“Sometimes this meant the difference between a five meter swell permitting stationary scientific operations, or a ten meter swell battering the ship and forcing us to postpone," Jeans says.
Deployments were especially risky in sea-ice: “Large ice floes continually bumped along the stationary ship as they moved about in the wind or currents. In one instance, the ice floes risk severing the winch cable attached to our sampling equipment. The captain, crew and our operations room personnel had to carefully position the ship and continually monitor the conditions to prevent this, sometimes deploying the powerful bow and stern thrusters of the ship to push away floating ice," he explains.
Back in the lab
Back in Stellenbosch, the precious samples are now safely stored in containers and a deep chest freezer, awaiting analysis by the team of five PhD and four MSc students, as well as three postdoctoral fellows.
Thousands of these sample bottles were carefully labeled ahead of the cruise. Dr Fietz explains: “We had to agree beforehand on abbreviations for each purpose. For example, TM stands for trace metals, and WC19 for Winter Cruise 2019, as we have similar bottles from previous years and seasons and they are not be confused! This is followed by GT, which stands for 'sample from GoFlo bottle', and then IE, which is the sampling station's name. Then follows our own designations. GFO1, for example, refers to the first triggered GoFlo sample, and diss-Al for dissolved aluminum, as this sample will be analysed by MSc student Tara de Jongh for dissolved aluminum."
So, in summary, a label reading “WC19-GTIE GFO1-diss-Al" refers to expedition, sampling device, coordinates, sample depth and purpose.
But while the samples are patiently waiting in storage, a few of the students have already left the continent again to attend and present their research at the Goldschmidt Conference in Spain, and next month Dr Fietz and PhD student Johan Viljoen are leaving for Spain to attend the International Geotraces Summer School, with Dr Fietz presenting a few lectures on the biogeochemical cycles of trace metals.
