Autocalibration based on dilution of a single concentrated standard is used for the determination of silicate in sea water by the modified molybdenum blue method.

The silicate (Si) molybdenum blue method was modified by combining oxalate and ascorbic acid into a single reagent and was used for determining Si in sea water samples. The first step of this automated assay protocol was designed to perform either a calibration by a single Si standard prepared in deionized (DI) water, or to dilute samples in the range of 0-160 µM Si to fit into 0-20 µM Si calibration range using a 20 cm flow cell. By designing the assay protocol to function in batch mode, the influence of salinity on calibration was eliminated, thus making the method suitable for analysis of samples collected in the open ocean, coastal areas, or rivers. Reproducibility and accuracy of this method were evaluated by analysis of certified sea water reference materials. Phosphate (P) does not interfere significantly if the Si:P ratio is 4:1 or larger. The limit of detection was 514 nM Si, r.s.d. 2.1 %, sampling frequency 40 s/h, reagent consumption 700 µL/sample, and using deionized water as the carrier solution.

How well can we quantify dust deposition to the ocean?

Deposition of continental mineral aerosols (dust) in the Eastern Tropical North Atlantic Ocean, between the coast of Africa and the Mid-Atlantic Ridge, was estimated using several strategies based on the measurement of aerosols, trace metals dissolved in seawater, particulate material filtered from the water column, particles collected by sediment traps and sediments. Most of the data used in this synthesis involve samples collected during US GEOTRACES expeditions in 2010 and 2011, although some results from the literature are also used. Dust deposition generated by a global model serves as a reference against which the results from each observational strategy are compared. Observation-based dust fluxes disagree with one another by as much as two orders of magnitude, although most of the methods produce results that are consistent with the reference model to within a factor of 5. The large range of estimates indicates that further work is needed to reduce uncertainties associated with each method before it can be applied routinely to map dust deposition to the ocean. Calculated dust deposition using observational strategies thought to have the smallest uncertainties is lower than the reference model by a factor of 2-5, suggesting that the model may overestimate dust deposition in our study area.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Hydrothermal impacts on trace element and isotope ocean biogeochemistry.

Hydrothermal activity occurs in all ocean basins, releasing high concentrations of key trace elements and isotopes (TEIs) into the oceans. Importantly, the calculated rate of entrainment of the entire ocean volume through turbulently mixing buoyant hydrothermal plumes is so vigorous as to be comparable to that of deep-ocean thermohaline circulation. Consequently, biogeochemical processes active within deep-ocean hydrothermal plumes have long been known to have the potential to impact global-scale biogeochemical cycles. More recently, new results from GEOTRACES have revealed that plumes rich in dissolved Fe, an important micronutrient that is limiting to productivity in some areas, are widespread above mid-ocean ridges and extend out into the deep-ocean interior. While Fe is only one element among the full suite of TEIs of interest to GEOTRACES, these preliminary results are important because they illustrate how inputs from seafloor venting might impact the global biogeochemical budgets of many other TEIs. To determine the global impact of seafloor venting, however, requires two key questions to be addressed: (i) What processes are active close to vent sites that regulate the initial high-temperature hydrothermal fluxes for the full suite of TEIs that are dispersed through non-buoyant hydrothermal plumes? (ii) How do those processes vary, globally, in response to changing geologic settings at the seafloor and/or the geochemistry of the overlying ocean water? In this paper, we review key findings from recent work in this realm, highlight a series of key hypotheses arising from that research and propose a series of new GEOTRACES modelling, section and process studies that could be implemented, nationally and internationally, to address these issues.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.