The labile fraction of suspended matter in the Loire River (France): multi-element chemistry and isotopic (Rb–Sr and C–O) systematics

Abstract

Suspended particulate matter transported by the Loire River (France) comprises mainly quartz and K-feldspar during periods of high river flow, with an increase in calcite contents during periods of low flow. We studied the labile fraction (leached by HCl 0.2 N) associated with the suspended matter transported by the upper Loire River. The acid extractable matter (AEM) concentrations in the Loire exhibit a wide range from 8% during high-flow conditions in the river to 47% during the low-flow conditions. The dispersion of AEM data differentiates two fields; one related to a domain where CaCO 3 precipitation is possible, and the other related to a domain where erosion is dominant. The concentrations of trace elements and REE fluctuate widely and generally decrease with increasing AEM concentrations. A middle REE (MREE) enrichment over the LREE and HREE is observed which can be related to Fe–Mn oxide coatings developed on the clastic particles. The 87Sr/ 86Sr ratio varies from 0.710582 to 0.711472 and displays two different trends when plotted against AEM abundance, again reflecting varying contributions from two main end-members. The hydrous Fe–Mn oxides have the lowest 87Sr/ 86Sr ratio (≈0.7105) and the carbonates have the highest 87Sr/ 86Sr ratio (≈0.7115). The highest 87Sr/ 86Sr ratios of AEM, obtained during the period of authigenic calcite precipitation, agree with values measured for the dissolved Sr over the same period. During the period where erosion dominates, the 87Sr/ 86Sr ratio of the hydrous Fe–Mn oxide end-member diverges from the mean value of the dissolved load of the Loire River. This implies that the oxides were formed in water with a lower 87Sr/ 86Sr ratio, suggesting that (i) the location of oxide precipitation is upland, and (ii) the oxides conserve their Sr isotope signature during transport by the river and do not re-equilibrate with local waters during transport. Stable carbon and oxygen isotope compositions of the carbonate fraction of the AEM during low-flow conditions are consistent with authigenic calcite formation in isotopic equilibrium with Loire River water. At high-flow conditions we observe an isotopic enrichment that may reflect a higher proportion of atmospheric CO 2 within the river waters at this time, or a contribution of detrital carbonate material from marine sediments exposed in the watershed, or secondary calcite derived from eroded soils.