Quantifying fluxes and processes in trace-metal cycling at ocean boundaries

Fluxes of heat and water at mid-ocean ridges are partitioned between focussed and diffuse flow along mid-ocean ridge axes and through the flanks of mid-ocean ridges.  While the axial heat and volume fluxes are a small proportion of the whole, geophysically, they are of particular interest to marine geochemistry for two reasons.  First, it is only at high temperature vents that a number of TEIs are mobilised into solution and, hence, can be released into the ocean interior.  Second, the turbulent mixing processes active in deep sea hydrothermal plumes are so extensive, it has been estimated that the residence time for the entire deep ocean volume with respect to entrainment into hydrothermal plumes may be directly comparable to the timescales for thermohaline circulation. Thus, the possibility arises that processes associated with deep sea hydrothermal venting might impact upon global scale biogeochemical budgets. My talk will discuss these possibilities, with particular reference to Fe, an essential micronutrient, which is approximately 1 million fold enriched in hydrothermal fluids compared to deep ocean seawater. The presentation will draw upon the latest finding from two directly relevant programmes – SCOR Working Group 135 and the US GEOTRACES East Pacific Zonal Transect.

All seafloor sediments undergo diagenetic reactions with the potential to mobilise constituent metals and promote the release of biologically essential trace metal micronutrients to the ocean. Quantifying this input and understanding how it changes are vital goals for simulating past and future ocean conditions. In this talk, I will explain the need to discern the mechanisms responsible for the release of trace metals to the ocean, in addition to measuring the rates of these processes, to effectively parameterise models of ocean primary production and climate. I will present the enduring evidence and our empirical basis for incorporating sedimentary iron (Fe) fluxes into ocean biogeochemical models, followed by recent evidence for additional processes that govern the release of Fe from sediments – processes which are presently ill quantified and unaccounted for. The challenges outlined for benthic Fe flux quantification concern other bio-essential trace metals too, such as manganese (Mn), nickel (Ni), copper (Cu) and zinc (Zn). Pore water profiles provide a first-order perspective to evaluate flux, while more intensive micronutrient and chemical tracer observations through the ocean’s bottom boundary layer might yield a better measure of the net flux to link with ocean transect data. In any case, without mechanistic knowledge of trace metal mobilisation and transfer to the ocean, a more accurate representation and projection of these inputs in ocean biogeochemical models is a difficult task. To this end, GEOTRACES-linked process studies are now emerging, and promise to bring exciting new knowledge and opportunities in these areas.

Continental shelves are a highly productive part of the oceans, playing a key role for both marine life and human activities. We know that they can easily be affected by a changing climate. They are also a particularly challenging system in marine trace element chemistry. The shelves are very strong sources for trace elements due to river runoff, dry deposition, submarine groundwater discharge (SGD), pore water fluxes, redox and salinity gradients, bioirrigation, and many other processes. These source terms, some of them extremely variable in space and time, are encountered by nearly equally strong and variable sink terms, like bioproductivity, precipitation reactions or fishing, just to name a few. The net flux is therefore a difference of two very large and variable terms, resulting in large uncertainties. In addition, discussing and quantifying individual parts of this budget is often problematic due to slightly different uses of terminology: is advective pore water flux the same as SGD? What are point sources, what are diffuse sources (and where does SGD stand here)? Where do you draw a boundary between suspended particle transport and sediment transport? This talk will aim to highlight a few of the main sources and sinks, uncertainties in their determination and challenges, as well as identifying areas where a closer look at definitions might help to improve our understanding of fluxes across the continental shelf.

There is huge scope in using stable and radiogenic isotope systems to trace and quantify elemental fluxes to and from the ocean. The use of Nd isotopes as a sedimentary source tracer is well known, while new precise measurements of Pb isotopes in seawater reveal the changing contributions of Pb from natural and anthropogenic dust sources (Bridgestock et al., 2015). Increasingly, the spotlight has shifted towards the information recorded by stable metal isotope systems, like Fe and Zn. The dissolved isotopic composition of Fe in the Atlantic has been used to differentiate four iron sources to this basin, with the aerosol dust signature of Sub-Saharan Africa calculated to be of particular significance (Conway & John, 2012). Quantifying oceanic Fe sources is driven by its importance as a (co-)limiting micronutrient. Zinc may be of similar importance to Fe in controlling the efficiency of global carbon cycling, but behaves rather differently in the oceanic realm, and less is known about its oceanic cycle. New data will be presented, highlighting the importance of an output of light Zn isotopes to organic-rich continental margin sediments to the global mass balance of Zn.

Observations of the distributions of trace elements and isotopes (TEIs) in the ocean have been used to infer boundary fluxes and in situ reaction rates. Because the distributions of reactive (non-conservative) properties depend on both physical transport as well as in situ biogeochemical processes, diagnosing the former from the distributions of conservative TEIs (whether they be transient or radioactive) can lead to estimates of the unknown boundary fluxes and in situ reaction rates. Such an approach has been used in the past to obtain relatively robust first order estimates, but some care should be exercised in recognising its limitations. I present a simple example of this.

The challenges in this approach may be divided into two categories

(1) Sampling adequacy: is the spatial-temporal coverage internally sufficient to resolve the relevant scales and the end-member concentrations? Have the correct properties been measured and to adequate accuracy?

(2) Model competence: does the model (whether diagnostic or prognostic) incorporate the appropriate physical mechanics? Have the biogeochemical processes been correctly parameterised?

The first challenge is a function of experiment design, which is in turn governed by cost and logistics. In reality, for GEOTRACES the die is cast so it becomes a question of tailoring objectives to the observations at hand. The second challenge raises an important synthesis strategy for GEOTRACES: making sure that geochemists and physicists work together to make the most of this exciting dataset.