Biogeochemical impact of tropical instability waves in the equatorial Pacific

Tropical Instability Waves (TIW) have been suggested to fertilize the equatorial Pacific in iron leading to enhanced ecosystem activity. Using a coupled dynamical-biogeochemical model, we show that contrary to this suggestion, TIWs induce a decrease of iron concentration by 20% at the equator and by about 3% over the "TIW box" [90°W- 180, 5°N-5°S]. Chlorophyll decreases by 10% at the equator and 1% over the "TIW box". This leads to a decrease of new production up to 10% at the equator (4% over the "TIW box"). TIW-induced horizontal advection brings more iron-depleted water to the equator than it exports iron-rich equatorial water to the north. Additional iron decrease is caused by TIW-induced iron vertical diffusion. These two mechanisms are partly counter balanced by a decrease of iron biological uptake, driven by weaker phytoplankton concentration, and to a lesser extend by TIW- induced iron vertical advection

Contrasting responses of the ocean’s oxygen minimum zones to artificial re-oxygenation

Studies assessing potential measures to counteract the marine deoxygenation attributed to anthropogenic activities have been conducted in a few coastal environments and at regional scale, but not yet on a global scale. One way toward global scale artificial oxygenation would be to use oxygen produced as a by-product from hydrogen-production through electrolysis. The low-carbon footprint renewable production of hydrogen from offshore wind energy offers such a possibility. Here, we assessed the potential of this artificial oxygenation method on a global scale using a coupled physical-biogeochemical numerical model. The anthropogenic oxygen source scenario assumes worldwide adoption of hydrogen, considering demographic changes and the feasibility of offshore wind turbine deployment. Following this scenario, artificial oxygenation had a negligible effect on the overall oxygen inventory (an increase of 0.07%) but showed a reduction in the overall volume of Oxygen Minimum Zones (OMZs) between 1.1% and 2.4%. Despite the decrease in the mean OMZ volume globally, OMZs display distinct and contrasting regional patterns notably due to the oxygen impacts on the nitrogen cycle. Artificial oxygenation can inhibit denitrification resulting in a net gain of nitrate that promotes locally and remotely increased biological productivity and consequent respiration. Increased respiration could ultimately lead to an oxygen loss at and beyond injection sites as in the Tropical Pacific and Indian Ocean and particularly expand the Bay of Bengal OMZ. In contrast, the tropical OMZ shrinkage in the Atlantic Ocean is attributed to oxygen enrichment induced by advective transport into the OMZ, while the absence of denitrification in this area precludes any biochemical feedback effect on oxygen levels. These results suggest that the impacts of artificial oxygenation on oxygen concentrations and ecosystems are highly non-linear. It can produce unexpected regional responses that can occur beyond the injection sites which make them difficult to forecast.publishedVersio

Origin and mobility of Iron Age Gaulish groups in present-day France revealed through archaeogenomics

The Iron Age period occupies an important place in French history, as the Gauls are regularly presented as the direct ancestors of the extant French population. We documented here the genomic diversity of Iron Age communities originating from six French regions. The 49 acquired genomes permitted us to highlight an absence of discontinuity between Bronze Age and Iron Age groups in France, lending support to a cultural transition linked to progressive local economic changes rather than to a massive influx of allochthone groups. Genomic analyses revealed strong genetic homogeneity among the regional groups associated with distinct archaeological cultures. This genomic homogenisation appears to be linked to individuals’ mobility between regions as well as gene flow with neighbouring groups from England and Spain. Thus, the results globally support a common genomic legacy for the Iron Age population of modern-day France that could be linked to recurrent gene flow between culturally differentiated communities

Modélisation biogéochimique du pacifique équatorial

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Global oceanic simulations of trace elements

47th International Liege Colloquium on Marine Environmental Monitoring, Modelling and Prediction, 4-8 May 2015, Liège, Belgium.-- 1 pageThe biogeochemical fluxes and ecosystem functioning are tightly controlled bythe ocean dynamics and by a wide range of sources and sinks associated with biological, chemical and physical phenomena. Our knowledge of tracer distributions, which relies mostly on aliased observations in space and time, is flawed. It is mostly noticeable in models where parameterizations of subgrid processes, limited understanding of biogeochemistry, and imprecise atmospheric forcing hamper our capacity to realistically simulate the fluxes. One way to better constrain poorly known parameters in models is to use the integrated information contained in “proxy” tracers depending on only a few key processes. In this study, we will model two trace elements at a global scale and a spatial resolution of 1° with ocean dynamics model NEMO (Nucleus for a European Modeling of the Ocean) and compare them with data from the GEOTRACES international program in order to better understand the cycle of carbon and associated elements. As a first step, complementary numerical tracers and water mass ages have been computed to understand the circulation model behavior and to better describe the time scales associated with ventilation processes and continental shelves – open ocean exchanges : these simulations will be used in the analyze the results.Radium, a conservative tracer unaffected by chemistry, is emitted mainly by submarine groundwater discharge and sediments and removed only by radioactive decay. Because of its half-life of 5.75y, 228Ra is a good proxy of exchanges between the sediments and the open ocean. A consistent pattern will be searched for between model horizontal dynamics, radium data and boundary conditions. We will perform an inversion of the data into sediment fluxes by using the transport matrix of the model, and transposing it to reconstruct the transit time distribution and origin of water masses, as described in Khatiwala [1]. Thorium is produced by the decay of uranium, which is proportional to salinity, and removed by its own decay and by scavenging by particles, since thorium is much less soluble than uranium. With a half-life of 24.1h, on the same order of magnitude as the sinking time of large particles, 234Th is a proxy of biogenic particle dynamics and the carbon pump. A nonlinear inversion of the data will be performed to improve the biogeochemical parameterization of vertical particle dynamics. [1] Khatiwala, S. (2007). A computational framework for simulation of biogeo-chemical tracers in the ocean. Global Biogeochemical Cycles, 21, GB3001, doi:10.1029/2007GB002923Peer reviewe

Sedimentary particulate iron : the missing micronutrients ?

International audienc

An innovative approach of the surface layer sampling

International audienceRecent publications revealed that the mixed layer may present surface singularities for biogeochemical parameters. Those studies question the common view of a homogeneous mixed layer. However, the degree of ubiquity of these surface singularities and their horizontal structures remains largely unknown because of the lack of adequate instruments to sample the first centimeters of the ocean. Therefore, the development of a new type of instrument, an autonomous sailboat designed to be easily deployed and to sample quasi-simultaneously the top centimeters and the underlying water column has been started. This instrument, VAIMOS, will routinely measure continuously temperature and salinity as well as biogeochemical parameters: chlorophyll a (Chl-a), partial pressure of CO2, nutrients and dissolved oxygen. A first prototype has been built and seems to have successfully sampled a vertical gradient in Chl-a during its very first try at sea. These preliminary results sound promising and interests for VAIMOS are growing among the oceanographers community who foresees alternative use of those instruments

Spatial multi-modality as a way to improve both performance and interpretability of deep learning models to reconstruct phytoplankton time-series in the global ocean

International audienc