Biological and climatic impacts of ocean trace element chemistry

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.

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.

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.

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.

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. 

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.

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.

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.