Welcome to GEOTRACES
GEOTRACES is an international programme which aims to improve the understanding of biogeochemical cycles and large-scale distribution of trace elements and their isotopes in the marine environment. Scientists from approximately 35 nations have been involved in the programme, which is designed to study all major ocean basins over the next decade.
GEOTRACES Sections. For more information please click here. In red: Planned Sections. In yellow: Completed Sections. In black: Sections completed as GEOTRACES contribution to the IPY. Download the map.
Radium quartet reveals no less than four main processes along the GEOTRACES North Atlantic Ocean section (30°N)
- Published on Friday, 23 January 2015 14:48
The four radium (Ra) isotopes (224Ra, 223Ra, 228Ra, 226Ra, "radium quartet") are produced in situ via decay of their insoluble thorium isotope parents in sediments from the continental margins and deep-sea and then released to the ocean. In the ocean, their distributions are controlled by particle removal (226Ra) and radioactive decay with four different half-lives. These properties make the "quartet" an invaluable tracer of coast-to-ocean processes and a water mass spreading chronometer.
Thanks to a dense and beautiful data set documenting the radium quartet along the 30°N GEOTRACES US section (GA03), Charette and co-authors (2015, see reference below) were able to identify:
- a Mediterranean outflow spreading rate of 0.52-0.60 cm/s derived from 228Ra,
- evidence of substantial sediment/water interaction in the benthic boundary layer along the oxygen minimum zones
- decoupling between 223Ra and the other Ra isotope sources over the mid-Atlantic Ridge, and
- significant continental inputs (e.g. submarine groundwater discharge) in the western Atlantic.
Last but not least, they conclude that the 228Ra inventories in the upper water column have remained constant over the past 40 years, which suggests that submarine groundwater discharge (the primary 228Ra source) is steady-state for the North Atlantic on decadal time scales.
Figure. Box average 0-1000 m inventories (15° x 15°) of 228Ra (x 1010 atoms m-2) for samples collected on the GEOTRACES Atlantic section (GT) (2010-2011) and the Transient Tracers in the Ocean cruises (TTO) (1981-1986). Each solid black dot is a TTO station, each red x is a GT station. Modified from Charette et al. (in press) Click here to view the figure larger.
When direct mapping of diatoms reveals unexpected fate of trace metals in the twilight zone
- Published on Tuesday, 20 January 2015 21:51
Twining and co-authors (2014, see reference below) used synchrotron x-ray fluorescence mapping to measure macronutrients such phosphorus (P), sulphur (S), and silicon (Si), and also trace metals like iron (Fe), nickel (Ni) and zinc (Zn), in individual cells of a diatom specie during a spring bloom off New Zealand. They clearly show that P, S, Zn and Ni are released faster than Fe and Si from sinking cells in the upper 200 m. Although the metals are co-located with P and S at the surface, the scheme changes deeper. The relationships with P and S become weak while an association of Fe with Si appears, suggesting re-adsorption when particles are settling. Exciting results revealing that ratios of dissolved Fe to macronutrients in the water column likely underestimate stoichiometries in sinking cells.
Figure. Element maps (P, S, Fe ,and Zn) and associated scatterplots of Fe and S concentrations in each pixel of the scans for two diatom cells collected from 30m or 200m following a spring bloom off New Zealand. The scatterplots show that Fe and S are spatially decoupled from each other when the diatom cells degrade as they sink through the upper water column. S is lost more readily from the cells, while Fe appears to be retained or is re-scavenged. Scale bar indicates 10um for each cell. Adapted from Twining et al. (2014). Click here to view the figure larger.
Iron isotopes in the Equatorial Pacific Ocean: when the dissolved phases are heavier than the particulate ones
- Published on Friday, 16 January 2015 14:09
A detailed study of the dissolved and particulate iron (Fe) concentrations and isotopes off the Papua New Guinea coast is proposed. Regarding the sources, iron isotopic composition (δ56Fe) values reveal that the aerosols are heavier than the average crustal value while the sediments and river/volcano waters display δ56Fe similar to crustal values.
Surprisingly, all along the vertical profiles (down to 1000m) the dissolved phase is almost systematically heavier than the particulate one (see figure below). This likely reflects equilibrium exchanges between the dissolved and particulate iron. These interactions seem to result in a net non reductive release of dissolved iron. The intensity of this release suggests this process could play an important role on the global scale.
Figure. Dissolved (triangles) and particulate (circles) Fe isotopic composition profiles in ‰ in three different stations in the Western Equatorial Pacific Ocean: (a) in Vitiaz Strait, (b) close to the Papua New Guinea coast downstream the Sepik river, and (c) close to New Ireland coast. The difference between dissolved and particulate iron isotopic composition (Δ56FeDFe - PFe) is represented by the colored area. It shows that the dissolved phase is almost systematically heavier (more positive values) than the particulate one with Δ56FeDFe - PFe = + 0.27 ± 0.25‰ (2SD, n = 11). Click here to view the figure larger.
Unprecedented set of dissolved manganese data in the North Atlantic Ocean (US GEOTRACES cruise)
- Published on Thursday, 08 January 2015 16:17
Manganese (Mn) is an essential nutrient for biological growth. In the ocean, manganese distribution is sensitive to several processes: redox conditions, photochemistry, biological activity, abiotic scavenging, and also eolian, hydrothermal and sedimentary sources. All of them are conditioning the concentration of dissolved Mn in the ocean vertical profiles as shown in the east-west ocean section proposed by Wu and co-workers across the Subtropical North Atlantic Ocean (see figure below).
Their simple model calculation suggests that the main actors determining the distribution are:
- In the surface waters (0-40m): eolian Mn(II) deposition and in-situ photochemical reduction of dioxide of manganese (MnO2).
- Below the mixed layer (40-200m): the intensity of sunlight available for in-situ MnO2 photochemical reduction.
- Between 200 and 700 m: regeneration preformed Mn in the source water and lateral inputs from hydrothermal and sedimentary sources.
- Below 700 m: lateral inputs from hydrothermal and sedimentary sources become predominant.
Figure. Vertical distribution of manganese (Mn) along a section across Subtropical North Atlantic. Warm colours (red, orange, etc.) indicate high concentrations. Click here to view the figure larger.
- UPDATED - Sampling and Sample-handling Protocols for GEOTRACES Cruises (Cookbook)
- What is controlling the copper isotopic composition in oceanic waters?
- GEOTRACES Japanese Cruise in the South Pacific Ocean
- Seasonal iron supply in the Southern Ocean is dominated by winter mixing
- Why is the deep ocean zinc isotopic signature so heavy?