L. Monin

Particulate barium tracing of significant mesopelagic carbon remineralisation in the North Atlantic

21 The remineralisation of sinking particles by prokaryotic heterotrophic activities is important for controlling oceanic 22 carbon sequestration. Here, we report mesopelagic particulate organic carbon (POC) remineralisation fluxes in the 23 North Atlantic along the GEOTRACES-GA01 section (GEOVIDE cruise; May-June 2014) using the particulate 24 biogenic barium (excess barium; Baxs) proxy. Important mesopelagic (100–1000 m) Baxs differences were observed 25 along the transect depending on the intensity of past blooms, the phytoplankton community structure and the physical 26 forcing, including downwelling. The subpolar province was characterized by the highest mesopelagic Baxs content (up 27 to 727 pmol L), which was attributed to an intense bloom averaging 6 mg Chl-a m between January and June 2014 28 and by an intense 1500 m-deep convection in the central Labrador Sea during the winter preceding the sampling. This 29 downwelling could have promoted a deepening of the prokaryotic heterotrophic activity, increasing the Baxs content. 30 In comparison, the temperate province, characterized by the lowest Baxs content (391 pmol L), was sampled during 31 Biogeosciences Discuss., https://doi.org/10.5194/bg-2017-400 Manuscript under review for journal Biogeosciences Discussion started: 4 October 2017 c

Systematic REE Partitioning in Cubo-Dodecahedral Fluorite from Belgium Revealed by Cathodoluminescence Spectral Imaging and Laser Ablation-ICP-MS

Geochemistry of the rare earth elements (REE) in fluorite is widely used in ore geology. However, bulk analysis may be biased by compositional heterogeneity in crystals. Here we report a systematic REE partitioning in cubo-dodecahedral fluorite crystals from Belgium using cathodoluminescence (CL) spectral imaging and laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) analysis. The light REE (LREE) are markedly enriched, and heavy REE (HREE) depleted in the |110| sector relative to the |100| sector. The partition coefficient K |110|/|100| is >10 for LREE and <1 for HREE. Very contrasting REE patterns and ratios are thus recorded from the same crystal. CL can be used for imaging compositional heterogeneities since REE are known CL activators in fluorite. However, CL imaging is found of limited application when the REE lines are masked by the strong Eu2+ emission. Spectral imaging of other REE lines solved this problem by producing sharp and contrasted compositional images.