The large-scale oceanic distribution of trace elements: disentangling preformed contributions, regenerative processes, subsurface sources and sinks

Elements present in seawater in quantities so small that they do not affect salinity are called "trace elements". In spite of their low abundances, trace metals can play disproportionally large roles in controlling the dynamics of marine ecosystems. This is because some are necessary for the proper functioning of important enzymes and proteins and must thus be supplied in sufficient quantities to maintain phytoplankton populations. There are regions in the ocean where key metals, particularly iron (Fe), are lacking in proportion to the other nutrients, what limits biological productivity. Why some micronutrients are lacking in some region and not others is not fully understood; the processes that govern the natural cycles of these metals are not well known. This gap in understanding is partly due to the difficulty of measuring trace metals in the ocean: trace metals are present in very small quantities and water samples are taken from metallic research ships, making measurements prone to contamination. Reliable techniques to routinely measure trace metals in seawater have only been recently developed. Thanks to them, the accuracy of the data and data coverage over the oceans have improved dramatically over the last few years. With this project, I aim to improve the general understanding of the cycling of trace metals, particularly the micronutrients, by analyzing the newest and most complete trace metal databases available.

The difficulty when trying to interpret measurements of dissolved trace metal concentrations, or other nutrients, in the deep sea is that one cannot easily distinguish between the amount that is present because it has been transported to the point of sampling from somewhere else and the amount that has been added or removed due to local, internal processes. Yet, one must be able to isolate that later component to quantify and interpret the influence of subsurface biogeochemical processes on trace metal cycles. Without this ability, one's interpretation of the measured concentration field could be wrong, mistaking transport phenomena for internal cycling mechanisms. This work will directly address this issue by applying statistical deconvolution techniques to explicitly quantify the amount that is transported. By taking the difference between the measured concentrations and the calculated transported component, it is possible to quantify the fraction that is due to biogeochemical processes and map these residual quantities.

One of the most important processes influencing the distribution of trace metals in the sea is "scavenging"; that is the propensity for dissolved metals to stick to particles and sink along with them. Scavenging affects metals more than other nutrients. It is an important process because it is omnipresent (particles are everywhere) and can redistribute the metals within the ocean interior. It is hypothesized that if scavenging is strong, or operating for a long time, the scavenging process can fractionate metals relative to the other nutrients. When layers that are affected strongly by this process are transported back to the surface, they will bring with them waters that are depleted in the metal relative to the other nutrients. If the metal abundance is too low, this will limit surface productivity. Preliminary modeling experiments support the view that scavenging exerts a first order control on the distribution of some metals, such as thorium, beryllium, the rare earth elements and aluminium. It is, however, not clear how much micronutrient metals scavenge and if this effect is able to explain the distribution and characteristics of micronutrient-limited regions. This project will test this hypothesis. First, the statistical deconvolution results from the data will inform on the degree of fractionation imposed by scavenging on each metal. Secondly, models will be used to simulate scavenging and the fractionation process and quantify the influence on surface ecosystems.

The benefits of this work will extend beyond advances in academic research. The following areas will be particularly concerned.

i. Environment and climate change: problems and remediation strategies
a. Iron fertilization: Iron addition was proposed as a geo-engineering solution to pump carbon out of the atmosphere into the ocean. It is not clear how to quantify the benefits of such schemes and how to weight the negative aspects, such as the possible development of anoxic conditions and their influence on sea life. This project will provide a better understanding of iron remineralization on large scales, on interior iron transport and on the relationships between iron and other metals.

b. Acidification: Changing seawater pH affects metal speciation and bioavailability. pH and redox conditions also control sedimentary metal cycling and the dissolution of carbonates. Acidification changes the lysocline depth and can influence sediment-water column metal exchange. This project will provide a baseline assessment of current relationships between pH-gradients in the sea and ocean interior metal cycling.

c. Ecosystems: Micronutrients such as Fe, Zn, Cd, Mn and Ni are important to maintain phytoplankton growth and thus to support the marine ecosystem. By analysing the large-scale subsurface signature of these metals, the project will provide a new understanding of the supply of micronutrients to the surface ocean. A better understanding of ecosystems will help better manage fisheries and international marine resources.

ii. Deep-sea resource exploitation and management
a. Deep-sea mining: It has been known for decades that marine deposits are rich in valuable metals. About 30 years ago, deep-sea mining was deemed not profitable; it was cheaper to exploit land-based resources. Today, our economies rely on access to critical metals. With prices going up and the risk that some might use metal supply as geopolitical leverage in international negotiations, countries and corporations are looking again in the potential of deep-sea mining. The results from this project will provide a large-scale picture of the processes controlling the deep-sea distribution of metals, information that will help identify, evaluate and, in time, mitigate the influence of deep-sea mining on ecosystems and on the metal budget of the ocean.

iii. Earth system modeling and forecasting
a. Carbon export: The flux of particulate carbon is a critical quantity to assess the strength of the biological pump and its role in the global carbon cycle. It is, however, a difficult quantity to observe and model. No model currently simulates the dynamics of marine particles with any confidence. Since some metals scavenge on particles, tracing the interior distribution of metals helps constrain particles dynamics, the particle flux and its spatial pattern, provided the role of transport on concentrations can be quantified. This work will explicitly quantify the role of advection. By focusing on modeling the distribution of metals better, one will achieve a better understanding and representation of carbon export and ultimately of the fate of atmospheric CO2 and of the functioning of ecosystems. This will be pursued through Oxford's relationship with the Met-Office.

b. Ocean interactions with its boundaries: Our understanding of the interactions between submerged landmasses (continental shelves, mid-ocean ridges, seamounts, bottom boundary layer) and the ocean is generally poor. Our ability to model these interactions is commensurate with our understanding of these processes. This work will estimate of the fraction of trace elements that cannot be explained by advection. It will then be possible to interpret this "residual" fraction using a combination of trace metals and ancillary isotopic data. By investigating the spatial gradient of these residuals, the project will contribute a much greater understanding of these land-ocean interactions.