Formation, Transformation and Trace-element scavenging of Minerals in Deep-Sea Ferromanganese crusts: Implications for marine biogeochemical cycles and paleoceanography

Amy Atkins

Supervisors: Dr Caroline Peacock & Dr Sam Shaw

Funding

• Part NERC/ Part SOEE

Academic background Background

• 2006-2009: B.Sc. Chemistry -University of Huddersfield

• 2009-2010: M.Sc. Geochemistry - University of Leeds

• M.S.c. Project : Mineralogical and physiochemical controls on Ni uptake by manganese minerals

• Deep-sea hydrothermal vent fluids provide a major source of dissolved Fe and Mn to the deep oceans

• Dissolved species react with oxidised seawater to precipitate abundant Fe/Mn (hydr)oxide minerals that form ferromanganese crusts at the sediment-water interface

• Precipitates are rich in manganese mineral phases such as, birnessite and todorokite

• These mineral phases have high sorptive capacities and strongly scavenge trace elements from seawater

Background

Images courtesy of nasa.gov

• The formation of these minerals and their scavenging of trace-elements is a key control on the concentration of trace-elements in seawater and ocean sediments

• Minerals play a key role in the biogeochemical cycling of trace-elements, including micro-nutrients, at both the regional and global scales.

• As the minerals sorb trace-elements from seawater over time, they provide a ready depository for trace-metal chemical information over the entire history of their formation

• These chemical signatures may be useful in terms of reconstructing aspects of seawater composition over the earths history.

Why do we care ?

Project Aims & Objectives

• Investigate birnessite and todorokite formation at the molecular level

• Examine questions surrounding the transformation of the minerals on the ocean floor

• Determine how these minerals sorb key trace-elements from seawater on a molecular scale.

• Provide a fundamental new understanding of manganese mineralogy

• Provide valuable insight into biogeochemical trace-element cycles in the ocean.

• Design a series of lab experiments to examine the formation of birnessite and todorokite in the deep ocean.

• Employ X-ray spectroscopy and diffraction techniques to investigate the transformation of these mineral phases over time and with temperature and pH.

• Perform batch sorption experiments on synthetic mineral phases

• Characterise the samples at the molecular level using synchrotron radiation.

• Utilise computational modelling programs to aid interpretation of laboratory results.

Methodology

Any Questions ?

(A) Electrostatic outer-sphere sorption in the Hx-birnessite interlayers and todorokite tunnels. (B) Bidentate inner-sphere complexation to the MnOH sites on Hx-birnessite and todorokite. (C) Tridentate inner-sphere complexation above the Mn2O vacancy sites on Hx-binessite. (D) Structural incorporation.