Jim Greenwood

Impact of eddies on surface chlorophyll in the South Indian Ocean

A unique feature of the subtropical South Indian Ocean is the existence of anticyclonic eddies that have higher chlorophyll concentrations than cyclonic eddies. Off Western Australia, this anomalous behavior is related to the seeding of anticyclonic eddies with shelf water enriched in phytoplankton biomass and nutrients. Further off-shore, two mechanisms have been suggested to explain the eddy/chlorophyll relationship: (i) eddies originating from the Australian coast maintain their chlorophyll anomaly while propagating westward; and (ii) eddy-induced Ekman upwelling (downwelling) enhances (dampens) nutrient supply in anticyclonic (cyclonic) eddies. Here we describe the relationship between eddies and surface chlorophyll within the South Indian Ocean, and discuss possible mechanisms to explain the anomalous behavior in light of new analyses performed using satellite chlorophyll data. We show that anticyclonic eddies exhibit higher surface chlorophyll concentration than cyclonic eddies across the entire South Indian Ocean basin (from 20 to 28°S), particularly in winter. Using Self Organizing Maps we analyze the chlorophyll patterns within anticyclonic eddies and cyclonic eddies and highlight their complexity. Our analysis suggests that multiple mechanisms may underlie the observed eddy/chlorophyll relationship. Based on Argo float data, we postulate the relationship may be partly related to seasonal adjustment of the mixed layer depth within eddies. Deeper mixing in anticyclonic eddies is expected to enhance nutrient supply to the mixed layer, while shallower mixing in cyclonic eddies is expected to reduce it. This could explain why the observed winter surface chlorophyll bloom is stronger in anticyclonic eddies than in cyclonic eddies.

Anticyclonic eddies are more productive than cyclonic eddies in subtropical gyres because of winter mixing

In subtropical ocean gyres, anticyclonic eddies increase surface nutrient injection and primary production during winter. Mesoscale eddies are ubiquitous features of ocean circulation that modulate the supply of nutrients to the upper sunlit ocean, influencing the rates of carbon fixation and export. The popular eddy-pumping paradigm implies that nutrient fluxes are enhanced in cyclonic eddies because of upwelling inside the eddy, leading to higher phytoplankton production. We show that this view does not hold for a substantial portion of eddies within oceanic subtropical gyres, the largest ecosystems in the ocean. Using space-based measurements and a global biogeochemical model, we demonstrate that during winter when subtropical eddies are most productive, there is increased chlorophyll in anticyclones compared with cyclones in all subtropical gyres (by 3.6 to 16.7% for the five basins). The model suggests that this is a consequence of the modulation of winter mixing by eddies. These results establish a new paradigm for anticyclonic eddies in subtropical gyres and could have important implications for the biological carbon pump and the global carbon cycle.

The kinetics of iodine disproportionation: a system of parallel second-order reactions sustained by a multi-species pre-equilibrium

Phosphate and borate buffers used commonly in the study of the disproportionation of iodine(0) into iodate (I(+5)) and iodide (I(−1)) catalyse the reaction. This offers significant advantages when the reaction is used to contain radio-iodine in a rupturing fission reactor, e.g., Three Mile Island. Here, such buffer catalysis has been investigated in solutions 0.20 mol l−1 in phosphate, borate, acetate or carbonate, with between 0–0.10 mol l−1 iodide. The reaction mechanism is modelled by spreadsheet, with parallel rate determining steps involving couplets such as HOI with I2OH−, all in pre-equilibrium with each other. At a given iodide concentration the kinetics are characterised by a peak in the graph of apparent rate constant versus pH. The peaks for hydroxide and each of the buffers except carbonate, coincided; carbonate displayed slightly anomalous behaviour at higher concentrations of iodide (0.10 mol l−1). The presence of a pKa in the pH domain occupied by the peak was not found to have any significant bearing on the result. The apparent rate constant for disproportionation increased linearly with phosphate concentration when the pH was that of the top of the peak. However, with the pH set on the shoulders of the peak catalysis increased initially, but then became saturated. It is not possible to ascribe, as in simpler systems, a single value to the catalytic activity of each buffer, which would apply across the entire reaction domain. In phosphate, a combination of parallel rate determining reactions involving I2OH−/IO−, I2OH−/HOI and HOI/IO− couplets was found to offer the best solution to the simultaneous fitting of the model to several sets of data. The reactivity of these couplets is investigated by means of molecular orbital theory.

Observational insights into chlorophyll distributions of subtropical South Indian Ocean eddies

The South Indian Ocean subtropical gyre has been described as a unique environment where anticyclonic ocean eddies highlight enhanced surface chlorophyll in winter. The processes responsible for this chlorophyll increase in anticyclones have remained elusive, primarily because previous studies investigating this unusual behavior were mostly based on satellite data, which only views the ocean surface. Here we present in situ data from an oceanographic voyage focusing on the mesoscale variability of biogeochemical variables across the subtropical gyre. During this voyage an autonomous biogeochemical profiling float transected an anticyclonic eddy, recording its physical and biological state over a period of 6 weeks. We show that several processes might be responsible for the eddy/chlorophyll relationship, including horizontal advection of productive waters and deeper convective mixing in anticyclonic eddies. While a deep chlorophyll maximum is present in the subtropical Indian Ocean outside anticyclonic eddies, mixing reaches deeper in anticyclonic eddy cores, resulting in increased surface chlorophyll due to the stirring of the deep chlorophyll maximum and possibly resulting in new production from nitrate injection below the deep chlorophyll maximum.

Latent disciplinal clashes concerning the batch dissolution of minerals, and their wider implications

Environmental context Mineral dissolution kinetics are important to understand natural processes including those increasingly used to store waste carbon dioxide and highly radio-active nuclides, and those involved in the amelioration of climate change and sea-level rise. We highlight a mistake made in the fundamental science that has retarded progress in the field for over 40 years. Its removal suggests improved ways to approach dissolution studies. Abstract Mineral dissolution kinetics are fundamental to biogeochemistry, and to the application of science to reduce the deleterious effects of humanity’s waste products, e.g. CO2 and radio-nuclides. However, a mistake made in the selection of the rate equation appropriate for use at the macro-scale of the aquatic environment has stymied growth in major aspects of the subject for some 40 years. This paper identifies the mistake, shows how it represents a latent disciplinal clash between two rate equations, and explores the misunderstandings that resulted from it. The paper also briefly explores other disciplinal clashes. Using the example of calcite dissolution, the paper also shows how the phenomenon of ‘non-ideal’ dissolution, which is prevalent in alumino-silicate mineral dissolution, as well as with calcite, has obscured the clash. The paper provides new information on plausible mechanisms, the absence of which has contributed to the problem. Finally, it argues that disciplinal clashes need to be minimised so that a rigorous description of dissolution at the large scale can be matched to findings at the atomic, or near-atomic, scale.