Marine Micronutrient Cycling between Sediments and Seawater: What controls the concentration and isotopic composition of micronutrient trace metals in

In this project you will use new and exciting experimental and analytical approaches to investigate how marine sediments help control the concentration and isotopic composition of micronutrient trace metals in seawater. This is important because trace metals are required by photosynthetic algae, and their abundance in seawater is intimately linked to the drawdown of atmospheric CO2 and the regulation of global climate.

Micronutrient trace metals are delivered to the oceans via a range of sources, including rivers, and end up being deposited into a number of sinks, including marine sediments. For many metals, marine sediments provide the most important sink, and sediment minerals provide the most important sedimentary hosts. In particular iron and manganese oxides are able to scavenge metals from the overlying water column and from sediment pore-waters and lock up metals over long timescales. As the sediments are buried and subducted, these metals eventually resurface on the continents and are once again weathered and transported back to the oceans by rivers. Over shorter timescales iron and manganese oxides can age and transform, and new research shows that during transformation these minerals can release some of their metal inventory. Moreover iron and manganese minerals can undergo partial dissolution as sediments become sub-oxic, which may also lead to a release of their metal load. In particular new research from our group shows that as the manganese oxide birnessite ages and transforms it can release up to half of its nickel content (Atkins et al., 2016). Nickel is an important micronutrient in seawater because it is required for photosynthesis and thus nickel concentrations and isotopic compositions in sediments and seawater might both influence and reflect modern and ancient primary productivity.

While it is clear for nickel that the aging and transformation of birnessite might have a drastic impact on nickel cycling, a number of important questions are still unclear:

Is nickel released from other sediment minerals as they age and transform?

Is nickel released from marine sediment minerals as they partially dissolve?

Are other micronutrient trace metals like cobalt, copper and zinc released from marine sediment minerals as they age and transform, and undergo partial dissolution?

How does the presence of sedimentary organic matter affect the uptake and release of micronutrient trace metals?

Does the uptake and release process impart an isotopic signal to micronutrient trace metals that we can use to track the global cycling of these important elements in the oceans?

In this project you will investigate the behaviour of nickel and other micronutrient trace metals during the aging, transformation and partial reductive dissolution of iron and manganese oxides, using a combined experimental and analytical approach, with the opportunity to use state-of-the-art nanoscale probes at Diamond Light Source synchrotron, and stable isotope facilities in the Isotope Geochemistry and Cosmochemistry Group at ETH Zurich.