Strontium isotope variation and carbonate versus silicate weathering in rivers from across Alaska: Implications for provenance studies

The characterization of strontium (Sr) isotopic (87Sr/86Sr) variation in surface waters of Alaska (AK) has significant implications for provenance studies aiming to track movement patterns of animals. This study presents 87Sr/86Sr ratios, concentrations of Sr, Ca, Mg, Na and K, and selected molar ratios (Sr/Na, Ca/Na, Mg/Na and Ca/Sr) from 61 rivers from across AK to characterize regional patterns in i) 87Sr/86Sr variation and ii) carbonate versus silicate weathering influencing 87Sr/86Sr ratios. 87Sr/86Sr ratios range from 0.70422–0.74041 ± 0.00009. Rivers north of the Denali Fault exhibit more radiogenic and variable 87Sr/86Sr ratios (0.70763 to 0.74041) than rivers to the south (0.70422 to 0.70895), reflecting the tectonic growth of southern AK via accretion of relatively young mafic-rich exotic island arcs along North Americaʹs continental margin during the Mesozoic. Within interior AK an east–west gradient exists. Radiogenic 87Sr/86Sr ratios (> 0.725) in east-central AK and relatively low ratios (< 0.709) in western AK reflect AKʹs progressive western growth along the ancestral North American miogeocline from the Precambrian to Mesozoic. High relief rivers draining north of the Brooks Range are more radiogenic (> 0.711) than lowland rivers (< 0.709). Elemental ratios indicate carbonate weathering has the strongest influence on 87Sr/86Sr ratios in high relief watersheds north of the Denali Fault and in watersheds of south-central AK. Elemental and 87Sr/86Sr ratios indicate silicate weathering is important across AK, but is most influential in lowland basins north of the Denali Fault and in southwestern AK. This study illustrates the large Sr isotopic heterogeneity across AK on multiple spatial scales and provides necessary baseline information for provenance studies.