From Africa to Australia: Expedition SO308 Investigates Trace Elements and Isotopes in the Indian Ocean

04 November 2024/Kiel/Durban. An international research expedition led by the GEOMAR Helmholtz Centre for Ocean Research Kiel set sail with the German research vessel SONNE into the largely unexplored Indian Ocean. This marine area is of great importance for the global climate and nutrient cycles in the ocean. During the nearly eight week voyage from Africa to Australia, the research team will investigate how trace elements and their isotopes — including essential micronutrients like iron, cobalt, and zinc — affect marine ecosystems and their capacity for CO₂ absorption.

The Indian Ocean is characterised by a large nutrient-poor gyre, considered an oligotrophic region or “ocean desert” with extremely low nutrient concentrations in surface waters. At the same time, large scale phytoplankton blooms regularly occur to the southwest of Madagascar, playing a vital role in ocean productivity and the carbon cycle. There is an insufficient supply of iron and nitrogen to the south Indian Ocean, which phytoplankton require for their growth, and therefore the biological sequestration of CO₂ from the atmosphere is insufficient. In the region between Mozambique and Australia, this expedition will for the first time conduct a detailed investigation into how these micro- and macronutrients enter the ocean, circulate, and become available to marine life.

“The results of this expedition will significantly expand our knowledge of the Indian Ocean and demonstrate how this marine region influences global cycles and the climate,” says Chief Scientist Dr Eric Achterberg, Professor of Chemical Oceanography at the GEOMAR Helmholtz Centre for Ocean Research Kiel.

Distribution, Origin, Transport, and Significance of Trace Elements

The expedition has four central research objectives. First, it will assess the distribution and chemical speciation of trace elements and isotopes. The team will collect samples from the water's surface down to depths of 5,000 metres to determine the exact concentrations and chemical binding forms of micronutrients like iron, manganese, and zinc, as well as isotopes such as neodymium, thorium and plutonium.

Another objective is to identify the sources of the trace elements: A variety of trace elements enter the ocean through atmospheric dust, continental inflows such as the Zambezi river, ocean sediments or hydrothermal sources from the oceanic crust. The scientists aim to investigate the contribution of these different sources to the total trace element inventory of the ocean.

A third focus of the expedition is on investigating the transport of these trace elements within the ocean. By using chemical tracers and oceanographic measurements, the team will analyse how circulation currents in the Indian Ocean transport nutrients over long distances until they eventually reach the surface water and nourish the marine food web.
Finally, the research team will examine the ecological and climatic significance of the trace elements. Since phytoplankton, the basis of the marine food web, relies on micronutrients and macronutrients, the expedition will explore how nutrient flows relate to plankton activity and what role the Indian Ocean plays as a carbon sink.

Route of the Research Voyage: From Durban to Fremantle

The route of the research team takes them initially from the Mozambique coast of the East African mainland to Madagascar, then on a straight east-west line from Madagascar to Australia. Nutrient sources and flows of trace elements and isotopes will be documented along both segments at about 50 stations. At a total of 15 so-called super stations, particularly elaborate sampling will take place. Here, in-situ pumps will be used to filter particles directly at various depths of up to 800 metres. In addition, shallow sediment cores will be sampled at every station.

“The Indian Ocean is a key system for global deep-water circulation and the supply of essential micronutrients to the surface layers of the ocean,” explains Professor Achterberg. “Nevertheless, the biogeochemistry of trace elements and chemical oceanography in this region are underrepresented. Our expedition will provide crucial data to better understand the chemical structure and dynamics of the Indian Ocean and to contextualise its contribution to the climate and the marine food web.”

The cruise is part of the internationally coordinated GEOTRACES programme. A total of 40 scientists are on board from 14 different nations. The research team includes scientist from GEOMAR, Constructor University, ZMT Bremen, HEREON, AWI, Woods Hole, ETH Zürich, IAEA Monaco, Zhejiang University, Stanford University, University of Chicago, University of Tasmania and Rostock University.

Expedition at a Glance:
Name: SONNE Expedition SO308
Chief Scientist: Prof. Dr. Eric Achterberg
Duration: 31.10.2024 - 22.12.2024
Start and End: Durban (South Africa) – Fremantle (Australia)
Area of Voyage: Indian Ocean

Background: Trace Elements and Isotopes
Trace elements and isotopes (TEIs) are substances that, although present at very low concentrations in seawater, play a crucial role in the ocean: they include important nutrients for microscopic organisms and influence biogeochemical processes. For example, phytoplankton require trace elements such as iron for growth, which in turn affects the entire food web and the ocean's ability to sequester carbon. In scientific research, isotopes of elements such as thorium or neodymium are also used as tracers to understand the movement of water masses and material cycles in the ocean.

Weitere Informationen:
http://www.geomar.de/n9661 Image material for Download
https://www.geotraces.org/ International GEOTRACES Programme
https://www.oceanblogs.org/ OCEANBLOGS
https://www.ldf.uni-hamburg.de/sonne/wochenberichte.html Expedition brochure and Wekly reports