I am a geochemist measuring the natural variability micronutrient trace metals (notably Iron, Fe) that cycle between oceans and sediments. In particluar, I seek to identify and quantify the mechanisms and rates that exchange trace metals between rocks, minerals and the ocean. Many of these processes occur during early diagenesis, while other moderating effects may occur in the near-sediment ‘benthic’ or ‘bottom’ boundary layer (BBL). These combined processes ultimately control the nutrition available for marine eco-systems, and can thereby moderate the global cycling and storage of the greenhouse gas CO2. My interests encompass the entire ocean floor, from shallow coasts to deep basins, volcanic and hydrothermal realms. I have other collaborative interests that overlap here too. Briefly, for example, the preservation of chemcial signals from the ocean (e.g. Cd and Ba isotopes or lipid biomarkers) that may be used as proxies in sediments to reconstruct past conditions, will also depend on early diagenetic and BBL processes.
Seafloor recycling of micronutrients
Diagenetic reactions occur accross the entire ocean floor; where fragments of the continents and underwater volcanic terrains settlle to the seabed with a rain of biological debris, and undergo all sorts of mineral altering reactions long-before they are ultimately preserved as sedimentary rocks. Iron (Fe) undergoes vigorous transformations on the seafloor; Individual Fe atoms might be recycled many hundreds of times between solid minerals and dissolved species, and a small, vital yet but poorly quantified fraction of dissolved Fe is known to leak back in to the oceans, before its ultimate burial.
Identifying the mechanisms
“Dissolved” Fe is not all alike in the ocean, so identifying it’s true chemical form can provide essential clues about its origin, and behaviour in the oceans. I’ve previously foucsed on the early diagenetic transformations of Fe and Mn, and discovered irregular occurences of these metals as colloids (or nanoparticles) in sediment pore waters, with implications for the patchy formation and release of these “dissolved” micro-nutrient metals to the oceans from oxidative weathering.
Dissolved iron isotopes carry signatures that are symptomatic of their dissolution and precipitation histories in ocean sediments. So by measuring Fe isotopes in sediment porewaters we can also learn how iron might has been dissolved – with or without a chemical reduction mecahnism. Source signatures like this also help to refine isotope mass-balance quantifications of iron in the oceans.
With collegues and a doctoral student at University of Southampton, we exlporing similar refinments to hydrothermal plume Fe isotope systematics, added to which we’re employing the use of a synchrotron based scanning X-ray microscopy at Diamond Light Source, to study Fe colloids formed in the Southern Ocean.
Measuring the rates
The quantification of benthic micronutrient fluxes is needed to understand the biological functioning of the ocean. Despite a good knowledge of the chemical requirements for life, and a growing picture of these chemcial distributions in the ocean thanks to the GEOTRACES Programme, our assessment of the origins and rates of micronurient inputs is remarkably poor, meaning we still cannot accurately assess the ocean’s biological sensitivty to changing micronutrient regimes.
I use pore water concentration profiles with ancillary data (e.g porosity, oxygen, salinity, temperautre, advection…) to quantitaivlely esimate the flux of dissolved Fe and Mn released during early diagensis. Ex situ and in situ sediment incubations evaluate these approaches and can simulate the affects of environmental variables (e.g. oxygen, pH or seawater tubidity). Pore water depth profiles provide a first-order quantification of flux where suitable gradients can be measured, however this is not readily achieved in the ocean (if at all) for many micronutrient metals (e.g. Cd, Zn, Co, Ni and Cu). Added to this, a number of spatio-temporal and bio-physcial dynamics limit the suitability of porewater fluxes to basin-wide extrapolations. Altogether, this has motivates exploration in ways to quanfiy miconutrient fluxes.
Using nuclear detection to quantify micronutrient flux
Even for Fe, which has recieved much attention in recent years, we have no simple approach to measure the result of multiple processes which serve to enhance (e.g. seasonal carbon supply, bio-irrigation, diapyncal mixing) and supress (e.g. oxidation, particulate scavenging, stratification) the magnitude of a benthic Fe flux to the overlying water-column.
Through a NERC-funded Independent Research Fellowship, I am attempting to utilise the distribition of radioactivity from naturally occuring Radium isotopes in combination with dissolved and particlate samples to derive net fluxes across the ocean’s bottom bounary layer for a variety of micronutrient trace metals. The result is an ongoing development of “MAP”s (Minature Autonomous Pumps), designed for trace-metal clean operation in partcle-rich environments up to 5km depth in the ocean, with engineering support from the National Oceanography Centre in Southampton.
View Selected Publications
Aquilina, A., Homoky, W.B., Hawkes, J.A., Lyons, T.W. and Mills, R.A., (2014) Hydrothermal sediments are a source of water column Fe and Mn in the Bransfield Strait, Antarctica
Homoky, W.B,, John S.G., Conway, T., and Mills, R.A. (2013). Distinct iron isotope signatures and supply from marine sediment dissolution. Nature Communications 4:2143
Homoky, W.B., Severmann, S., McManus, J., Berelson, W. M., Riedel, T., Statham, P. J., Mills, R. A. (2012) Dissolved oxygen and suspended sediments regulate the benthic flux of iron from continental margins. Marine Chemistry 134-135: 59-70.
Homoky, W.B., Hembury, D.J., Hepburn, L.E., Mills, R.A., Statham, P.J., Fones, G.R. and Palmer, M.R. (2011) Iron and manganese diagenesis in deep sea volcanogenic sediments and the origins of pore water colloids. Geochimica et Cosmochimica Acta, 75(17): 5032-5048.
Homoky, W.B., Severmann, S., Mills, R.A., Statham, P.J. and Fones, G.R. (2009). Pore-fluid Fe isotopes reflect the extent of benthic Fe redox recycling: evidence from continental shelf and deep-sea sediments. Geology, 37(8): 751-754.