PS124 - Weekly Report No. 7 | 15 - 21 March 2021

The biogeochemistry of the water column

[22. March 2021] 

There are several reasons why the Southern Ocean and the Weddell Sea are extraordinary. Not only are its ice formation and unique fauna awe inspiring, but it also plays a crucial role in regulating our planet’s climate.

This week, the report focuses on the interaction between micronutrient supply, biological production and climate. The oceans absorb massive amounts of the greenhouse gas carbon dioxide (CO2), which is generated, amongst other things, from anthropogenic activities, such as the burning of fossil fuels.

Responsible for this absorption process are phytoplankton. These tiny, mostly single celled, plants, turn CO2 into biomass, through photosynthesis. When they die or are eaten, this organic carbon is either recycled in the water column or transported down to the bottom of the ocean where some of it can be buried (Fig. 1). This transport of carbon is termed the biological pump and moves ~10,000,000,000 t of carbon from the surface into the deep ocean every year where it can remain for a long time. Thus the ocean acts as a giant buffer and regulator for the planet’s climate.

In order to grow, phytoplankton require not only light and CO2 but also nutrients. While there are plenty of the major nutrients like nitrogen, phosphorus, and silica present in the Southern Ocean, micronutrients, such as trace metals and vitamins, are scarce (Fig. 2). Especially iron has been described as the primary growth limiting micronutrient in the Southern Ocean. This is because even though iron is the fourth most abundant element in the earth’s crust, in an oxic ocean it exists in a form not readily accessible to phytoplankton. Thus, phytoplankton blooms are typically confined to areas close to the Antarctic Peninsula, downstream of islands or governed by episodic trace metal deliveries such as melting icebergs or atmospheric dust depositions. The concentrations we are trying to measure are equivalent to measuring a sugar cube dissolved in Lake Constance, which explains our ever present fear of contamination. We are, after all, on a ship consisting of 11,000,000 kg of steel and every speck of dust could inflate our iron measurements or contaminate our experiments.

 

Since the efficiency of the biological pump in the Southern Ocean, with consequences for global climate, is governed to a large degree by the availability by iron, iron sources need to be better understood. This is especially important in order to fine tune climate models. Also, the degree to which iron and other micronutrients alter plankton community composition in combination with changing light regimes are important to investigate. In order to address these questions, the AWI invested in infrastructure and expertise, allowing for trace metal clean sampling, experiments and sample processing on board of Polarstern. While previous cruises successfully sampled and conducted experiments from near surface waters using trace metal clean techniques, on PS124 we were able to, for the first time, use the new trace metal clean CTD rosette (Fig, 3). This CTD is equipped with 24 Teflon-coated GoFlo bottles, allowing us to profile the entire water column. Our team consists of biogeochemists specialized in pelagic production and benthic processes, and we work closely with other groups which investigate sea ice, the physical characteristics of water masses as well as benthic flux processes.

 

As the onboard meteorologist predicts increasing winds at the German Antarctic Station Neumayer III, we stop major parts of our scientific program in good time. The additional freight from Neumayer III, requires our containers to be packed, closed, and stowed away, and the labs to be cleaned.  After breaking through a 1-km wide compact belt of sea ice, Polarstern reaches the station’s northern ‘pier’ in the afternoon of 17 March (Fig. 4). Since loading is planned for early next morning, sea ice physics and physical oceanography take the opportunity for work on the fast ice of Atka Bay while the rest takes a walk on the icy side. Due to CORONA regulations, this time we are not allowed to visit Neumayer III.

 

At late afternoon on 19 March all freight and winter overs of last season as well as the scientists and technicians, who arrived with PS123, are on board (Fig. 5). The days gained by the early departure are used to recover a nearby mooring despite increasing winds and higher wave crests and to approach a new position for the delivery of a biogeochemistry mooring in the central Weddell Sea.

 

PS124 sends regards from the ice free but shaky north eastern Weddell Sea. Its more than 5000-m deep waters still keep some secrets even after decades of research.

 

Hartmut H. Hellmer (Chief Scientist)

Contact

Science

Hanna Sophie Knahl
hanna.knahl@awi.de

Scientific Coordination

Ingo Schewe
+49(471)4831-1709
Ingo Schewe

Assistant

Sanne Bochert
+49(471)4831-1859
Sanne Bochert