Marine Shelf Ice from the Filchner-Ronne Ice Shelf: A sleeping iron giant?

• Applicant: Walter Geibert, Ph.D., since 3/2023
• Subject Area: Oceanography ; Mineralogy, Petrology and Geochemistry
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 442696514

Biological productivity in the Polar Southern Ocean (PSO) - like in all other oceans - depends on the availability of nutrients. While the main nutrients, such as nitrate, phosphorus or silicate, are highly abundant in the PSO, important micronutrients needed for the use of the main nutrients are lacking in large areas. Therefore, there are regions in PSO with varying levels of productivity. The regions with relatively high productivity appear to be linked to the availability of micronutrients such as iron and manganese. One example is the west side of the Antarctic Peninsula, where sediments can release these nutrients close to the sea surface. On the eastern side of the peninsula, in the Weddell Sea, enormous algal blooms occur in periodically ice-free areas, which indicate high productivity and characterise the local ecosystem. This productivity must be linked to a micronutrient source that has not yet been fully determined. I have preliminary self-assessed data suggesting that so-called marine ice, as known from "green icebergs" and found underneath the Filchner-Ronne Ice Sheet, contains up to a thousand times more iron and manganese than dissolved in the surface water of the Weddell Sea, and could therefore be considered as a source of deficient nutrients. Marine ice is formed by a freezing process below the ice shelf edge, which is pushed from land to sea by glacier movement. The freezing process initially results from thawing of the ice shelf at the transition from the continent to the sea. As a result, the surrounding seawater becomes less saline and small ice flakes form at temperatures close to the freezing point. These platelets rise because of their lower density than the surrounding water, allowing particles to adhere to them, and then freeze as bubble-free marine ice below the ice shelf. This process takes place over hundreds of years, forming a marine ice layer up to 200 m thick. Through the Alfred Wegener Institute in Bremerhaven, I have access to 2 ice cores from the Filchner-Ronne ice shelf in the southern Weddell Sea, the lower parts of which consist of an approximately 60 or 165 m long sequence of marine ice. The wide geographical extent of the marine ice below the ice shelf is documented by seismic measurements. I propose a geochemical investigation of these ice cores to find out if marine ice represents a previously neglected nutrient source for productivity in the Weddell Sea and also in the whole PSO. For this purpose, I will analyse dissolved trace metals, especially iron and manganese, but also many others, as well as the particulate composition in the ice. I will use different analytical methods such as mass spectrometry, emission spectrometry and electron microscopy. With the help of these methods, I will be able to show the previously unknown geochemical composition of marine ice, determine the origin of trace metal enrichment and evaluate its potential as an important nutrient source.

• DFG Programme: Infrastructure Priority Programmes
• Subproject of: SPP 1158: Infrastructure area - Antarctic Research with Comparative Investigations in Arctic Sea Ice Areas
• Ehemaliger Antragsteller: Torben Stichel, Ph.D., until 3/2023