CE01 - Milieux et biodiversité : Terre fluide et solide

Radiolarian silicon stable isotope Calibration: Development of a novel palaeo-proxy for marine silicon cycling – RadiCal

Radiolarian silicon stable isotope Calibration: Development of a novel palaeo-proxy for marine silicon cycling

Radiolarians are key players in the evolution of the Si cycle and have provided rich palaeontological records for a multitude of palaeoceanographic studies. These marine micro-organisms, which can produce intricate skeletons of silica, are very sensitive to changes in environmental conditions. The resistant silica-based skeletons of radiolarians are particularly useful in regions of importance for studying climate change where carbonate-based archives are poorly preserved (e.g. Southern Ocean).

The scientific objective of RadiCal is to develop the utility of radiolarians as a proxy for mid-depth marine DSi systematics for both modern and past environments.

The RadiCal project offers to develop radiolarians as a novel palaeo-proxy for marine silicon cycling by calibrating the silicon stable isotope composition (d30Si) of these organisms to their modern environment. This objective will be achieved by applying an innovative multidisciplinary approach evaluating the influence of the modern environment and taxonomy on the variability of radiolarian silicon isotope fractionation (1) in situ, (2) under laboratory-controlled conditions, and (3) from a core-top calibration study including a variety of different oceanographic basins within the Southern Ocean.

The RadiCal project combines laboratory-controlled experiments with in situ observations. Live radiolarian samples for RadiCal are collected from the Mediterranean Sea and Southern Ocean. Samples are collected by using plankton nets of various mesh sizes (64, 200, and 500 µm), Bottlenets, and Niskin bottles mounted on a rosette and attached to a CTD, which provide measurements of temperature, conductivity, salinity and light levels of the water column sampled. Biomass collected on filters are analysed post-campaign and post-culture for biogenic silica (BSiO2), particulate organic carbon and nitrogen (POC/PON) and chlorophyll (Chla) concentration. Nutrients (P, N, Si) are also analyzed on the filtered seawater. The DSi and BSiO2 samples collected are analysed for d30Si by MC-ICPMS using the PSO facilities. Core-top radiolarians were sampled from multiple cores from the Southern Ocean (Atlantic, Pacific, and Indian sector). For the experimental culture work (<1 week), BSiO2 production and Michaelis-Menten uptake kinetics are also being evaluated using the radioisotope 32Si in parallel to silicon stable isotope fractionation experiments.

To date, the main results of the RadiCal project highlight the importance of Rhizarians in the biogeochemical cycling of silicon in the marine environement.

A number of new collaborative projects, both nationally and internationally, have been developed in connection with the RadiCal project. These new collaborations highlight the scientific interest of studying the ecology, geochemistry and micropaleontology of siliceous radiolarians.

1. The silica cycle in the modern ocean (OSM San Diego)
2. Rhizarians and diatoms: their relative impact in the silicon cycle in the Southern Ocean (Ross Sea) (OSM Sand Diego)

During the geological history of the Earth, evolutionary competition for dissolved silicon in the ocean directly influenced changes in the global cycles of silicon, carbon, and other nutrients that regulate ocean productivity and ultimately the Earth’s climate. Radiolarians are key players of this evolution and, as such, have provided rich palaeontological records for a multitude of palaeoceanographic studies. These marine micro-organisms, which can produce intricate skeletons of silica, are very sensitive to changes in environmental conditions. The resistant silica-based skeletons of radiolarians are particularly useful in regions of importance for studying climate change where carbonate-based archives are poorly preserved (e.g. Southern Ocean) and offer a biogeochemical window into the mid-depth section (i.e. 30-500 m) of the marine environment. The mid-depth zone in the marine environment is currently inaccessible for Si cycle palaeo-reconstructions and its characterization is pivotal to the detection of glacial–interglacial changes in water column stratification, which plays a critical role during periods of abrupt climate change. Fortunately, radiolarians are common plankton found in this mid-depth zone. However, in contrast to other silicifying organisms found in the palaeo-record, such as deep-water sponges and surface dwelling diatoms (unicellular silicifying micro-algae), the use of radiolarian geochemistry to describe palaeoceanographic variation in dissolved Si (DSi) is very much in its infancy largely due to the challenges associated with their growth under laboratory-controlled conditions and limited knowledge on the factors that govern their contemporary biogeographic distribution.

The RadiCal project offers to develop Radiolarians as a novel palaeo-proxy for marine silicon cycling by Calibrating the silicon stable isotope composition (d30Si) of these organisms to their modern environment. This objective will be achieved by applying an innovative multidisciplinary approach evaluating the influence of the modern environment and taxonomy on the variability of radiolarian silicon isotope fractionation (1) in situ, (2) under laboratory-controlled conditions, and (3) from a core-top calibration study from a variety of different oceanographic basins within the Southern Ocean. RadiCal fosters the innovative and inter-disciplinary approach of the P.I., which aims to combine non-traditional stable isotope biogeochemistry with in situ observations, in vitro experimental culture experiments, and sediment core samples in order to answer questions regarding the role of marine silicifiers (e.g. radiolarians) on the global cycling of Si. The RadiCal project will also develop collaborations between French and foreign scientists, strengthen the link between internationally recognized laboratories, and aggregate a community of researchers with diverse expertise on silicifying organisms (e.g. radiolarians) while developing potential for further collaboration and development of larger scale projects (e.g. ERC). In addition, the project RadiCal will permit the training of a new generation of oceanographers using an innovative and multi-disciplinary approach.

Project coordination

Jill Sutton (LABORATOIRE DES SCIENCES DE L'ENVIRONNEMENT MARIN)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LEMAR LABORATOIRE DES SCIENCES DE L'ENVIRONNEMENT MARIN

Help of the ANR 234,468 euros
Beginning and duration of the scientific project: January 2019 - 36 Months

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