Weathering processes, catchment geology and river management impacts on radiogenic (87Sr/86Sr) and stable (δ88/86Sr) strontium isotope compositions of Canadian boreal rivers

Radiogenic (Sr-87/Sr-86) and stable (delta Sr-88/86) strontium isotope compositions spanning a calendar year are reported for rivers from across subarctic Canada that drain contrasting lithologies ranging from Precambrian bedrock (Koksoak, Great Whale and La Grande rivers of Northern Quebec) to carbonate and clastic Phanerozoic sedimentary rocks of the Western Interior Platform (Nelson River, of central and western Canada). The Sr-87/Sr-86 isotopic compositions of the river waters reflect the underlying geology, with rivers draining the Precambrian Shield having higher Sr-87/Sr-86 ratios (0.727-0.734) than the Phanerozoic dominated Nelson River (0.713). The stable strontium isotope values (delta Sr-88/86) range from 0.26 to 0.39 parts per thousand, with the values for the Nelson River overlapping those of the other three. Rivers that have not been developed for hydroelectric power show a seasonal variation in the Sr-87/Sr-86 ratios, whereas those that have been diverted or dammed show little or no seasonal variation due to increased residence time of their water in hydroelectric reservoirs. The three rivers from Northern Quebec show discrete ranges in their Sr-87/Sr-86 and delta Sr-88/86 isotope compositions that correlate with the Ca/Sr and Na/Sr ratios of the river water. These correlations are interpreted to reflect differential chemical weathering of felsic versus mafic source rocks and/or of surficial sediment vs bedrock sources.