Biological and climatic impacts of ocean trace element chemistry

09:05-09:30 Trace elements in seawater: advances from global measurement campaigns

Professor Robert Anderson, Lamont-Doherty Earth Observatory of Columbia University, USA

Abstract

Global campaigns began mapping the physical structure of ocean water masses nearly a century ago. Each major water mass can be linked to its region of formation through unique ranges of temperature and salinity. A half-century later the global distributions of chemical tracers of ocean circulation, measured together with the chemical byproducts of photosynthesis and respiration by the GEOSECS programme, illustrated the well-choreographed interplay between physics and biology in regulating the distributions of carbon and nutrients in the ocean.

A revolution in marine analytical chemistry, beginning in the 1970’s, provided the first glimpse of oceanographically consistent distributions of many trace elements and their isotopes. However, despite the heroic efforts of these chemists, a global picture of the distributions of trace elements remained out of reach for more than a quarter century.  Building on the GEOSECS legacy, the GEOTRACES programme was designed to remedy this situation by coordinating the efforts of marine geochemists worldwide to identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean. Fulfilling this mission will benefit other areas of ocean research, and the public at large, by defining the sources of trace nutrients that regulate ocean fertility and ecosystem health, the fate of many contaminants in the ocean, and the role of the ocean in past climate change.

This presentation will begin with selected examples of new insights gained from basin-wide distributions of trace elements and their isotopes. The second part will illustrate how the wealth of trace element and isotope data generated by GEOTRACES allows one to constrain the supply of trace elements using multiple independent techniques. This approach to the synthesis of GEOTRACES data will use aerosol (dust) deposition for illustration and segue into the next presentation.

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09:45-10:15 Atmospheric transport to the oceans of trace elements and micronutrients

Professor Tim Jickells, University of East Anglia, UK

Abstract

The atmosphere is now known to be an important route for the supply of various trace elements from the land to the ocean. The trace elements transported can include key micro and macro nutrients, as well as some pollutants and tracers. The transport of iron within crustal dust has particularly important biogeochemical impacts, but other trace elements may also play an important biogeochemical role. 

The solubility of trace elements in aerosols varies greatly. This partly reflects emission processes, with trace elements associated with high temperature (generally anthropogenic) emission processes tending to be more soluble than those associated with low temperature processes (such as dust transport from deserts). In addition solubility of some trace elements bound within aluminosilicate lattices appears to vary systematically across the globe, with highest solubilities in areas of lowest dust loadings, although the controls on these gradients are not well understood.  

In this presentation we will discuss the general patterns of atmospheric trace element transport, focussing particularly on the Atlantic Ocean, but also considering other ocean areas. We will also consider the patterns of trace element solubility in atmospheric aerosols and the possible solubility control mechanisms, deposition fluxes and the uncertainties associated with these, and finally touch on the biogeochemical impacts in so far as we know them now. 

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11:00-11:30 Solid river inputs and ocean margins as critical sources of elements to the oceans

Professor Catherine Jeandel, CNRS, Observatoire Midi-Pyrenees, France

Abstract

Land to ocean transfer of material largely controls the chemical composition of seawater and the global element cycles. Compilation of results on radiogenic isotopic tracers (Sr, Th, Nd) suggest that a significant fraction of the particulate material discharged by the rivers and deposited on the shelves and slopes, dissolves in seawater after its arrival in the oceans.  Moreover, data on non-traditional isotopes acquired in the framework of GEOTRACES (for example Ni, Mo or Zn) reveal that the oceanic budget of these elements is not balanced.  The mass of most elements arriving in the ocean via particulates exceeds that arriving via dissolved transport by at least a factor of 50. Consequently, we hypothesise that particulate material dissolution in the oceans may be the dominant mechanism contributing numerous elements to the ocean. We also suggest that estimates based on dissolved riverine transport alone may significantly underestimate the global element fluxes in the oceans. Discussing this issue is particularly interesting in the Atlantic Ocean, which is also subjected to the largest fresh water and dust inputs of the world (Amazon and Saharan respectively).

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11:45-12:15 Modelling the transport of trace elements: the mantle He example

Professor Reiner Schlitzer, Alfred Wegener Institute, Germany

Abstract

One of the most exciting discoveries of the GEOTRACES observational programme are the pronounced deep-ocean trace element plumes originating from hydrothermal vents along geologically active ocean ridges found in all ocean basins. The shape and extent of the plumes, as well as the actual concentrations found in the plumes are the combined result of biogeochemical and physical processes, with water-mass circulation and mixing playing a crucial role. Inferences about source strengths or reaction rates in the plumes therefore depend on knowledge of the physical transports and often require the use of models. In this talk I concentrate on the ‘classic’ example of mantle He that is released together with the trace elements along deep ocean ridges, but, as a noble gas, is not affected by complicating chemical reactions. A global ocean model with realistic chlorofluorocarbon and radiocarbon distributions is used to simulate distributions of 3He and 4He. In agreement with observations this model produces separate westward spreading He plumes south and north of the Equator in the deep eastern Pacific. Results for adapted He source strengths and pathways of the mantle He to the ocean surface are presented. These results help the interpretation of the more complicated biogeochemically active trace elements, such as Fe, Zn, Mn and Al.

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13:30-14:00 Local and global-scale interactions between micro-nutrient cycles and phytoplankton

Dr Mick Follows, Massachusetts Institute of Technology, USA

Abstract

At the local scale, Resource Ratio Theory suggests that the relative supply rates of key resources control community composition and nutrient concentration. Interpreting global-scale data sets with this framework we find clear relationships between the relative rates of delivery of iron, nitrogen and phosphorus to the surface ocean and the biogeography of nitrogen fixation. Considering the global-scale, we hypothesise a feedback that maintains marine trace metal abundances to be co-limiting with macro-nutrients over long times-scales, suggesting that there is ‘just enough’ iron in the ocean.

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14:15-14:45 Ecosystem responses to dust and ash input to the surface ocean

Dr Mark Moore, University of Southampton, UK

Abstract

Atmospheric transport and deposition of land derived particulate material represents a key connection pathway between terrestrial and oceanic systems. In particular, atmospheric deposition of both desert dust and volcanic ash can influence oceanic systems through providing upper ocean ecosystems with a range of nutrients, including biologically essential trace metals such as iron. Emphasising the multi-nutrient characteristics of particle associated inputs, alongside microbial nutritional requirements, this talk will briefly review evidence for significant impacts of dust and ash on marine ecosystems using a combination of results from small scale experiments, observations of discrete events and analysis of apparent relationships between biogeochemical tracers in the context of simple theory and numerical models. A distinction is drawn between the short term regional impacts which may result from discrete episodic event scale processes and apparent large scale biogeochemical partitioning of the upper ocean which is argued to partially reflect geographical variations in major aerosol sources. However, we argue that the consequences of predominantly event scale ash deposition and more chronic dust borne nutrient inputs both need to be considered in the context of large scale spatial variability in oceanic nutrient deficiency, as can be illustrated through analysis of the new data becoming available through the GEOTRACES programme.

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15:30-16:00 Hydrothermal inputs of micronutrients as a control on ocean productivity

Dr Alessandro Tagliabue, University of Liverpool, UK

Abstract

Hydrothermal vents were only discovered in the late 1970s thanks to advances in undersea technology. Around the same time, new sampling techniques demonstrated that the levels of the micronutrient iron in the ocean were much lower than previously thought. From this the now well established role of iron in shaping the distribution and magnitude of ocean productivity emerged. After their discovery, hydrothermal systems were quickly acknowledged as having extremely high iron concentrations but, equally, their influence on the oceanic iron inventory was quickly dismissed as negligible. In recent years a combination of observational and modelling studies have led to this paradigm being reversed and hydrothermal activity is being recognised as an important component of the ocean iron cycle. As iron regulates biological activity over large parts of the ocean, hydrothermal inputs may therefore play an important role in shaping the distributions and magnitude of ocean productivity in regions such as the Southern Ocean. Recent results highlight the relative importance of different ocean ridge systems in sustaining rates of ocean productivity that can direct future fieldwork. In this talk I will review recent developments in this rapidly expanding field and highlight the key emerging questions.

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16:15-16:45 Ecosystem controls on the oceanic cycling of trace metals

Dr Maite Maldonado, University of British Columbia, Canada

Abstract

Marine phytoplankton (microscopic algae) annually convert 45 gigatons of CO₂ to organic carbon via photosynthesis in surface waters. This primary production sustains the majority of life in the oceans, and plays a critical role in the global carbon cycle. Over the past two decades, there has been significant research interest in trace metals (e.g. Fe, Cu and Co) as important controls on marine productivity, and a growing awareness that plankton can greatly influence oceanic trace metal cycling.

In this talk, I will illustrate how the marine biogeochemical cycling of trace metals is closely linked to ecosystem functioning. I will first review key mechanisms by which plankton affect the concentration, distribution and speciation of trace elements in the sea. For example, phytoplankton can concentrate bioactive metals intracellularly by more than a million fold relative to dissolved concentrations in seawater, and can influence the redox speciation of metals through cell-bound oxidases and reductases. I will also present recent findings describing how organisms in upper trophic levels (e.g. krill, whales and seabirds) concentrate, retain and excrete trace elements, thus impacting the cycling and residence time of metals in surface waters. Emerging analytical techniques are providing information on the differential trace metal stoichiometry of organisms across entire food webs, while traditional methods have provided valuable data on metal assimilation efficiencies in many organisms. By incorporating these data in mass balance ecosystem models, we are now well poised to significantly advance our understanding of ecosystem-level controls on trace metal cycling in the ocean.

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09:00-09:30 Radiogenic isotope tracers of present and past ocean circulation

Dr Tina van der Flierdt, Imperial College London, UK

Abstract

The deep ocean contains ~60 times more carbon than the atmosphere and a large part of the carbon exchange takes place in areas of deep water formation and upwelling. Therefore small changes in the rate of deep water formation are likely to have a large impact on the atmospheric carbon budget. Hence it is – and has been – of great interest to decipher the relationship between climate change and ocean circulation. While we have ample ways to look at ocean circulation in the modern ocean, we struggle to reconstruct physical oceanography in the past. Neodymium (Nd) isotopes have been considered a promising ‘quasi-conservative’ water mass tracer for many years, and have been used extensively to reconstruct ocean circulation, on million year, glacial-interglacial, and even more rapid (sub)millennial timescales. 

While published Nd isotope records show intriguing co-variations with other climate indicators, our understanding of the modern cycle of Nd in the ocean has been rather poor. A good understanding of modern processes is, however, critical before a palaeo tracer can be applied with confidence to study the past. In this talk I will assess where we stand on using Nd isotopes as a palaeo-circulation tracer by scrutinising the wealth of new data collected on modern seawater in the context of the international GEOTRACES programme, which more than doubled the observational database over the past five years. Evaluating this data set reveals critical and novel information to be considered for palaeo interpretations, with implications on the ocean’s role in climate change.

09:45-10:15 Transition metal isotopes as tracers of oceanic metal budgets and cycling

Professor Derek Vance, ETH Zürich, Switzerland

Abstract

The GEOTRACES programme has come at a propitious time, when we are newly able to study trace metal isotope variations in seawater. This contribution will review the progress of this new endeavour, with an emphasis on two distinct but related applications: 

(1) One of the most dramatic features of ocean chemistry is the extreme transfer of metals from the surface to the deep associated with nutrient-like behaviour. For some metals this internal cycling is associated with isotope fractionation. But for others even near complete uptake in the photic zone does not come with a significant isotope effect. An understanding of this dichotomy can only emerge from a more active pursuit of process studies that focus on phytoplankton uptake mechanisms and their controls, including the interplay between different trace metals. We suggest that some oceanic hubs, in particular the high-latitude oceans, are proving to be just as key to trace metals as they are for major nutrient distributions.

(2) Isotope systems can help us to quantify and understand whole ocean transition metal budgets, including inputs and outputs, and the processes that control them. In a larger-scale perspective an important emerging theme concerns the contrasting isotopic impact of open ocean, oxic, versus marginal, anoxic/sulphidic, sinks from the dissolved pool to sediment. The contrast has implications for understanding ocean chemistry in deep time, and may turn out to be one of the more important applications of newly-developing transition metal isotope systems in earth science.

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11:00-11:30 U-series rate-meters for ocean processes

Professor Gideon Henderson FRS, University of Oxford, UK

Abstract

The distribution of trace-elements in the ocean is controlled by their input, removal and internal biogeochemical cycling. Assessing the rates at which these processes move trace metals is fundamental to understanding the modern distribution, the supply of trace metals to photic surface waters, and the response of ocean biogeochemistry to change. The differential solubility of the isotopes in the U and Th decay chains, coupled to their wide range of half-lives, provide many tools with potential to quantify the rates of processes involved in trace-metal cycling. 

Insoluble Th has recently been shown to be a tracer for dust input, combining long-lived ²³²Th with shorter-lived ²³⁰Th to quantify the rates of modern dust addition averaged over a few years.  The ²³⁰Th removed from seawater accumulates in underlying sediment where it provides a proxy for the rate of sedimentation and of sediment dissolution, and is used to normalise sedimentary ²³¹Pa concentration to provide a controversial proxy for the rates of past ocean circulation. The shorter lived ²³⁴Th can be used to assess the downward flux of trace elements due to biological producitivity. 

Soluble isotopes of Ra also provide powerful rate information, particularly related to the rates of mixing and advection in the modern ocean. This talk will overview recent and new data illustrating the power of U-series isotopes to quantify the operation of ocean trace-element cycles.

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11:45-12:15 Nitrogen isotope variations as tracers of marine nitrogen biogeochemistry

Dr Karen Casciotti, Stanford University, USA

Abstract

Nitrogen (N) isotopes are a powerful tool for tracking N cycling in past and present marine environments. Many of the biogeochemical processes that comprise the marine N cycle lead to isotopic fractionation between pools of N, and the intensities of these activities are recorded in the isotope ratios of the substrates and products of the reactions. Denitrification, the microbial reduction of nitrate to nitrite and gaseous products, occurs in subsurface oxygen deficit zones (ODZs) and is a dominant process in the marine N budget. Its activity leads to removal of fixed (bioavailable) N, potentially lowering marine productivity on a variety of space and time scales. Denitrification also leads to enrichment of ¹⁵N in nitrate, and the effects of this process can be observed long distances from ODZs. These signals can be mixed and advected out of the ODZ, or may be upwelled to the surface and incorporated into particulate organic matter. This transfer of ODZ signals into phytoplankton biomass and preservation in sediments leads to historical records of the intensity of water column denitrification where N uptake in surface waters is complete. N isotope data in nitrate and nitrite from a series of cruises to the Eastern Tropical South Pacific (ETSP), including GEOTRACES GP16, are used to more closely investigate the pathways of N transformation in the ETSP, as well as the transmission and alteration of N isotope signals beyond the oxygen deficient waters.

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13:30-14:00 Trace metal biogeochemistry and bioavailability in an acidifying ocean

Professor François Morel, University of Princeton, USA

Abstract

Because it directly affects chemical speciation, the acidification of the ocean influences the bioavailability of trace elements and their biogeochemical cycling. Here we focus on two biologically essential trace metals, iron (Fe) and zinc (Zn). The limited laboratory and field data available indicate that the availability to phytoplankton of Fe decreases with decreasing pH while that of Zn sometimes decreases and sometimes increases.  

The decrease in Fe bioavailability at low pH is often fully explained by the change in Fe speciation, namely the decrease in the concentration of the Fe(OH)n(3-n)+ species.  But when all the Fe is bound to strong chelating agents (and remains so as the pH decreases), it is presumably the response of the Fe uptake system of phytoplankton to acidification that is responsible for the change in bioavailability.   

For Zn, which does not form stable hydroxide complexes at circumneutral pH, the decrease in binding to organic chelators at low pH should increase Zn bioavailability as is seen in some field and many laboratory experiments. However, the formation of bioavailable weak Zn complexes can reverse this trend. Both the increase and decrease in Zn bioavailability at decreasing pH reflect a change in chemical lability that can be quantified by electrochemistry. 

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14:10-14:40 The ocean mercury cycle and its anthropogenic perturbation

Dr Carl Lamborg, University of California, Santa Cruz, USA

Abstract

The toxic metal mercury is present only at trace levels in the ocean, but it accumulates in fish at concentrations high enough to pose a threat to human and environmental health. Human activity has dramatically altered the global mercury cycle, resulting in loadings to the ocean that have increased by at least a factor of three from pre-anthropogenic levels, and these loadings may continue to increase as a result of higher atmospheric emissions and other factors related to global environmental change. The impact that these new loadings, as well as previously released ‘legacy’ mercury, will have on the production of methylated mercury (the form that accumulates in fish) is unclear. The GEOTRACES program and other work currently underway, is shedding important additional light on the amount and distribution of pollution mercury in the ocean, as well as the factors affecting methylation of mercury. This information, when combined with modelling and anthropogenic emissions estimates will refine our predictions for mercury conditions in the ocean and suggests the best ways to avoid a future of inedible fish.

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15:20-15:50 Anthropogenic Pb isotopes as pollutants and tracers of ocean processes

Professor Ed Boyle, Massachusetts Institute of Technology, USA

Abstract

Anthropogenic emissions of Pb into the atmosphere (dominantly from leaded gasoline consumption, but also and increasingly due to coal combustion, smelting and high temperature industrial processes) have increased the flux of lead into the ocean by about an order of magnitude. Hence most of the Pb in the ocean is anthropogenic. Pb enters the ocean at the surface and then is dispersed by ocean currents and ‘scavenging’ onto (and perhaps release from) sinking particles within the water column. GEOTRACES now has Pb data for at least five sections with at least one from each ocean basin. I will use data on dissolved and particulate Pb concentrations, Pb isotope ratios, and Pb-210 to show how this data – and earlier data showing the temporal evolution of Pb in the ocean – illuminate transport of Pb from different sources through the ocean by currents and sinking particles. In particular, I will demonstrate that most of the variability of particulate Pb in the mid-latitude Atlantic Ocean can be modelled by two-phase partition coefficient reversible exchange, and estimate the extent to which this process alters the distribution of lead from that imposed by the physical circulation of the ocean.

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16:00-16:30 The GEOTRACES contribution to the ocean iron fertilisation geoengineering debate

Professor Philip Boyd, Institute for Marine and Antarctic Studies, Australia

Abstract

There is increasing focus, by national academies and the IPCC, on evaluating whether geoengineering (also termed climate intervention) can mitigate warming and/or rising atmospheric CO₂ concentrations. Of the wide range of Solar Radiation Management and Carbon Dioxide Removal approaches discussed so far, ocean iron fertilisation has been the most thoroughly appraised. This assessment was made possible by the availability of datasets from 12 mesoscale (~1000 km²) ocean iron-enrichment studies, funded to investigate the role of changes in iron supply in altering Earths’ climate in the geological past. The unprecedented scale of these manipulations and their multidisciplinary scope offer penetrating insights into the challenges, benefits, costs and side-effects of geoengineering.

Since 2009 GEOTRACES has provided an in-depth study of ocean iron biogeochemistry that has shed light on several unknowns – highlighted in Boyd et al. (2007) Science – that have hindered the debate into iron fertilisation as a geoengineering approach. The GEOTRACES global survey lines have resulted in an improved understanding of iron biogeochemistry in the modern ocean. For example, the discovery of new sources of iron, how they are dispersed, and hence their geographical sphere of influence. GEOTRACES process studies have provided complementary datasets, such as the joint measurement of key properties in the iron and carbon cycles including Fe:C ratios that are valuable to modellers exploring ocean iron fertilisation across a range of scales. In this presentation I will use the findings from the first phase of GEOTRACES to reframe some of the questions central to the ocean geoengineering debate.

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