Radiogenic and Stable Strontium Isotope Variation in Rivers Draining the Canadian Precambrian Shield

English Abstract:

We present new radiogenic (87Sr/86Sr) and stable (δ88Sr/86Sr) strontium isotope data from four rivers of the Canadian Precambrian Shield. Radiogenic Sr isotope ratios reflect the contributions of different crustal sources, while the stable strontium isotope ratios can be used to calculate weathering rates. All four rivers flow into Hudson Bay or Hudson Strait. Three rivers drain the Precambrian Shield of Northern Quebec (The Koksoak, Great Whale and La Grande Rivers). The fourth one, the Nelson River, drains the Precambrian Shield of Northern Manitoba and Phanerozoic sedimentary deposits, through supplies from Lake Winnipeg and the North and South Saskatchewan Rivers. The Nelson, La Grande and Koksoak have one or more hydroelectric dams, while the Great Whale River remains free of hydroelectric development. Radiogenic strontium isotopes provide constraints on the geology of the terrain underlying the drainage basin whereas stable strontium isotopes fractionate in response to precipitation and dissolution reactions. Combining these radiogenic and stable isotope systems leads to a better understanding of the weathering of the Canadian Shield. Preliminary data reveals that the difference in the geology of the drainage basin of the Nelson River is evident from lower 87Sr/86Sr ratios (0.713) compared to the other three rivers (0.728-0.735). Seasonal variations in the 87Sr/86Sr ratios are evident in some of the rivers and absent in others. Preliminary stable strontium isotope data (δ88Sr/86Sr) exhibits variations between 0.25 and 0.35 per mil, values that are consistent with those measured in rivers from elsewhere in the world.