TREMOR INSU EC2CO

Rationale

New Caledonia's coastal soils are characterised by the fact that they are essentially made up of laterites, which have been mined for decades for their content of Trace Metal Elements (TMEs), such as nickel, chromium, cobalt and manganese. Because of their high organic matter productivity, mangroves are a major reservoir of Dissolved Organic Matter (DOM) that can be transported to the ocean over long distances.

The observations carried out lead us to question the sink/source role of these ecosystems in relation to the dynamics of DOM at the land-sea interface, downstream of the New Caledonian mining massifs. The aim of the TREMOR project (2018-2020) is to provide answers based on the fluorescence properties of a fraction of the MOD, called Fluorescent Dissolved Organic Matter (FDOM).

TREMOR aims more particularly to better understand the origin, sources and sinks of the MODF identified in the rivers and lagoon of New Caledonia and to assess the influence of this MODF on the dissemination and toxicity of TMEs within the coastline downstream of mining sites. Comparisons with mangroves in other regions (Brazil, Fiji...) in relation to their geodynamics are envisaged due to the involvement of some team members in other regions of the world. TREMOR is also contributing to the development of a standardized method (spectrofluorometry) that can be used to quickly and cheaply identify certain traces of anthropization (sewage, fires...) in fresh and marine waters.

Approach, method and skills mobilized

This project mobilizes various skills in biogeochemistry, biology, geochemistry, chemistry and optics. Water samples are taken from the river and the lagoon, from the sediments (pore water), filtered online (micropore 0.2 μm) and stored until their analysis by spectrofluorometry. The 3D fluorescence emission/excitation spectra are analysed using a software developed (PARAFAC) to digitally extract the contributions of different fluorophores (natural, anthropic...) and to compare them with those found in other environments or periods.

Quenching laboratory experiments (fluorescence attenuation by complexation effect by successive addition in logarithmically increasing concentration) with some ETMs (Cu and Ni) allow the calculation of fluorophore complexation constants and finally give an indication of the capacity of the fluorescent CDMO to form complexes and trap ETMs.

Field of study

The study site is made up of two rivers: the Temala River, which drains volcano-sedimentary terrain, and the Coco River, which drains the ultrabasic lateritic massif of Koniambo). For each river, the samples correspond to a continuum river (stations T1 and C1) - mangrove (stations T2/T3 and C2/C3) - mouth (stations T4 and C4) - lagoon (stations T5 and C5) in the bays of Chasseloup and Vavouto, respectively.

First results

The first available results show that the Coco and Temala rivers have different geochemical signatures related to the geological and pedological nature of the drained land. For both sites, the concentrations of ETM increase strongly between the river and the mangrove, and are very low in the lagoon. Among the identified fluorophores, three are common to the two rivers studied (humic type in the rivers and mangroves, fulvic type at the mouths and lagoon). The complexing properties of fluorophores with respect to Ni and Cu were confirmed. All these results suggest that MODF certainly plays a role in the dispersion (river dilution effect), or non-dispersion (creation of autochthonous fluorophores in the mangrove), of the ETMs at the river-mangrove-lagoon interface downstream of the ultrabasic massifs. However, a particular phenomenon of aggregation/flocculation of MODF seems to occur during the mixing of fresh and salt water at the Coco site, which remains to be studied.

Outlook for TREMOR

The mechanisms of aggregation/flocculation of MODF will be more particularly studied in the laboratory at the end of 2019 (thanks to the support of a 3.5-month fixed-term contract). A campaign in March 2020 (thanks to the support of a lecturer from the University of Toulon, MIO, by CRESICA) will enable further study of the particular behaviour of MODF at the Coco site, by sampling the salinity gradient and focusing more specifically on the phase-particulate dissolved phase relationship. A final mission in October 2020 will be used to characterize the specific fluorophore of the mangrove, and valorize all the results.

Map of stations sampled in the Temala and Coco rivers flowing into the Koné lagoon in the Northern Province of New Caledonia
Copyright : Farid Juillot