Composition and mineralogy of suspended sediment in the fluvio‐estuarine zone of the Loire River, France

Abstract

The fluvio‐tidal transition of suspended sediment in terms of mineralogy and composition in the Loire River drainage basin, the largest French river basin, was investigated in the fluvial zone at Montjean and in the tidal zone at Mauves‐Thouaré, for a complete seasonal cycle. At Montjean, where the river experiences unidirectional flow, the composition and mineralogy (especially clays and clay minerals) of river suspended material (RSM) are governed by the river discharge, upstream contributions, climatic conditions and microbiological activities. However, due to reversing tidal and river currents at Thouaré, in the zone of tidal dynamics, these relations are changed. In the downstream direction sand and clay content in the RSM decrease while the silt content increases. Among clay minerals, between these two observation stations, montmorillonite remains constant, kaolinite diminishes, and the other minerals increase downstream. Combustible material (organic) and nitrate (NO3) contents in the RSM increase whereas the phosphate (PO43‐) and CaCO3 contents decline considerably during transport. At the head of the tidal zone, where the river flow encounters the tidal influence, there is a relatively stationary water mass (tidal slack) where sands, clays, phosphate, carbonate and silica are deposited by physical and chemical processes. Physical sedimentation takes place by simple gravitational deposition (sands), and by sorting and complicated differential settling (clays); chemical sedimentation takes place by precipitation (calcite‐CaCO3; apatite‐Ca5 (PO4)3 (OH,F,Cl); coagulation of dissolved silica‐SiO2) in connection with seasonal algal bloom and eutrophic events.

Carbonate dissolution in the turbid and eutrophic Loire estuary

“…This makes the estuarine MTZ fluid-mud system an efficient biogeochemical reactor for carbonate dissolution. Using a mixing model based on rare earth-element analysis in Loire estuarine suspensions, Négrel (1997) showed that the proportion of fluviatile suspensions in the estuary represented 80 to 90% all along the first Values from Billen et al (1986), Meybeck et al (1988) and this study consistent at ±10% c Values from Billen et al (1986), Meybeck et al (1988) d Values from Manickam et al (1985), Meybeck et al (1988) and Grosbois et al (2001) consistent at ±10% e Average value observed in the estuary, this study Table 1. Typical particulate organic and inorganic carbon concentrations (POC and PIC, respectively, expressed as % of suspended particulate matter [SPM]) in the Loire riverine and estuarine suspensions.…”

“…Indeed, the total PIC we measured during our cruise in river suspensions (2.1%) exactly matches values reported previously during the same late summer period, among which ca. 1.7% is authigenic PIC and 0.4% is detrital PIC (Manickam et al 1985, Grosbois et al 2001. Thus, the residual value we measured in the MTZ (0.4%) corresponds to the detrital PIC in river suspensions, which strongly suggests that (similarly to algal-POC) only authigenic PIC was lost at the river-estuary transition, detrital PIC (similarly to detrital POC) having a conservative behaviour.…”

“…It is probably one of the most eutrophic large rivers in the world, with chl a concentrations exceeding 100 µg l -1 , and algal POC representing 60% of the POC in summer . In addition to a high load of algal POC, a second effect of eutrophication in the Loire River is the chemical precipitation of authigenic carbonates in the river waters, when primary production raises the pH above 8.5 (Manickam et al 1985). As a consequence, authigenic carbonates represent up to 37% of the river suspensions in summer , Grosbois et al 2001.…”

C, N, P and S in Rivers: From Sources to Global Inputs

“…There is also a resulting precipitation of calcite when pH exceeds 8.4 (Manickam et al, 1985). Autochthonous calcite may represent up to 30 % of the summer inorganic TSS.…”

“…Average values of these two seasons for both carbon contents and rate of exports are presented on Figures I and 8. Meybeck et al (1988) and Manickam et al (1985). The limit CaC0 3 content of 4 % in TSS is interpreted to represent only detrital calcite.…”