Matthieu Roy-Barman

Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

Abstract. Dissolved manganese (Mn) is a biologically essential element. Moreover, its oxidised form is involved in removing itself and several other trace elements from ocean waters. Here we report the longest thus far (17u202f500u202fkm length) full-depth ocean section of dissolved Mn in the west Atlantic Ocean, comprising 1320 data values of high accuracy. This is the GA02 transect that is part of the GEOTRACES programme, which aims to understand trace element distributions. The goal of this study is to combine these new observations with new, state-of-the-art, modelling to give a first assessment of the main sources and redistribution of Mn throughout the ocean. To this end, we simulate the distribution of dissolved Mn using a global-scale circulation model. This first model includes simple parameterisations to account for the sources, processes and sinks of Mn in the ocean. Oxidation and (photo)reduction, aggregation and settling, as well as biological uptake and remineralisation by plankton are included in the model. Our model provides, together with the observations, the following insights: – The high surface concentrations of manganese are caused by the combination of photoreduction and sources contributing to the upper ocean. The most important sources are sediments, dust, and, more locally, rivers. – Observations and model simulations suggest that surface Mn in the Atlantic Ocean moves downwards into the southward-flowing North Atlantic Deep Water (NADW), but because of strong removal rates there is no elevated concentration of Mn visible any more in the NADW south of 40°u202fN. – The model predicts lower dissolved Mn in surface waters of the Pacific Ocean than the observed concentrations. The intense oxygen minimum zone (OMZ) in subsurface waters is deemed to be a major source of dissolved Mn also mixing upwards into surface waters, but the OMZ is not well represented by the model. Improved high-resolution simulation of the OMZ may solve this problem. – There is a mainly homogeneous background concentration of dissolved Mn of about 0.10–0.15u202fnM throughout most of the deep ocean. The model reproduces this by means of a threshold on particulate manganese oxides of 25u202fpM, suggesting that a minimal concentration of particulate Mn is needed before aggregation and removal become efficient. – The observed distinct hydrothermal signals are produced by assuming both a strong source and a strong removal of Mn near hydrothermal vents.

Urbanization impact on sulfur content of groundwater revealed by the study of urban speleothem-like deposits: Case study in Paris, France

Speleothem-like deposits that develop underground in urban areas are an archive of the environmental impact of anthropic activities that has been little studied so far. In this paper, the sulfate content in shallow groundwater from northern Paris (France) is compared with the sulfur content in two 300-year-old urban carbonate deposits that grew in a historical underground aqueduct. The present-day waters of the aqueduct have very high sulfur and calcium contents, suggesting pollution from gypsum dissolution. However, geological gypsum levels are located below the water table. Sulfur content was measured by micro-X-ray fluorescence in these very S-rich carbonate deposits (0.5 to 1% of S). A twofold S increase during the second half of the 1800s was found in both samples. These dates correspond to two major periods of urbanization above the site. We discus three possible S sources: anthropic sources (industries, fertilizers…), volcanic eruptions and input within the water through gypsum brought for urbanization above the studied site (backfill with quarry waste) since the middle of the 19th century. For the younger second half of the studied section, S input from gypsum brought during urbanization was confirmed by the study of isotopic sulfur composition (δ34S=+15.2‰ at the top). For the oldest part, several sulfur peaks could be related to early industrial activity in Paris, that caused high local air pollution, as reported in historical archives but also to historical gypsum extraction. This study provides information on the origin and timing of the very high SO42- levels measured nowadays within the shallow groundwater, thus demonstrating the interest in using carbonate deposits in urban areas as a proxy for the history of urbanization or human activities and their impact on water bodies.

A global scavenging and circulation ocean model of thorium-230 and protactinium-231 with improved particle dynamics (NEMO–ProThorP 0.1)

Abstract. In this paper we set forth a 3-Dnocean model of the radioactive trace isotopes 230Th nandxa0 231Pa . The interest arises from the fact that these isotopesnare extensively used for investigating particle transport in the ocean andnreconstructing past ocean circulation. The tracers are reversibly scavengednby biogenic and lithogenic particles. Our simulations of 230Th and 231Pa are based on thenNEMO–PISCES ocean biogeochemistry general circulation model, which includesnbiogenic particles, namely small and big particulate organic carbon, calciumncarbonate and biogenic silica. Small and big lithogenic particles from dustndeposition are included in our model as well. Their distributions generallyncompare well with the small and big lithogenic particle concentrations fromnrecent observations from the GEOTRACES programme, except for boundarynnepheloid layers for which, as of today, there are no non-trivialnprognostic models available on a global scale. Our simulations reproducen 230Th and 231Pa dissolved concentrations: they comparenwell with recent GEOTRACES observations in many parts of the ocean.nParticulate 230Th and 231Pa concentrations arensignificantly improved compared to previous studies, but they are still toonlow because of missing particles from nepheloid layers. Our simulationnreproduces the main characteristics of the 231Pa∕230Th rationobserved in the sediments and supports a moderate affinity ofn 231Pa to biogenic silica as suggested by recent observationsnrelative to 230Th . Future model development may further improve understanding, especially whennthis will include a more complete representation of all particles, includingndifferent size classes, manganese hydroxides and nepheloid layers. This cannbe done based on our model as itsnsource code is readily available.