Tracking seasonal subglacial drainage evolution of alpine glaciers using radiogenic Nd and Sr isotope systematics: Lemon Creek Glacier, Alaska

Abstract:

The transport pathways of water beneath a glacier are subject to change as melt seasons progress due to variability in the balance between basal water pressure and water flux. Subglacial hydrology has been well studied, but the understanding of spatial distribution is less well constrained. Whereas radiogenic isotopic tracers have been traditionally used as proxies to track spatial variability and weathering rates in fluvial and riverine systems, these techniques have yet to be applied extensively to the subglacial environment and may help resolve ambiguity in subglacial hydrology. Research has shown the ¹⁴³Nd/¹⁴⁴Nd values can reflect variation in source provenance processes due to variations in the age of the continental crust. Correlating the ¹⁴³Nd/¹⁴⁴Nd with other radiogenic isotope systematics such as strontium (⁸⁷Sr/⁸⁶Sr) provides important constraints on the role of congruent and incongruent weathering processes. Our study presents the application of Nd and Sr systematics using isotopic ratios to the suspended load of subglacial meltwater collected over a single melt season at Lemon Creek Glacier, USA (LCG). The time-series data show an average εNd ~ -6.83, indicating a young bedrock (~60 MYA). Isotopic variation helps track the seasonal expansion of the subglacial meltwater channels and subsequent return to early season conditions due to the parabolic trend towards less radiogenic Nd in June and towards more radiogenic Nd beginning in mid-August. However, the high variability in July and early August may reflect a mixture of source as the channels diverge and derive sediment from differently aged lithologies. We find a poor correlation between ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr (R²= 0.38) along with a slight trend towards more radiogenic ⁸⁷Sr/⁸⁶Sr values with time ((R²= 0.49). This may indicate that, even as the residence time decreases over the melt season, the LCG subglacial system is relatively stable and that the bedrock is congruently weathered. Our study suggests that the ¹⁴³Nd/¹⁴⁴Nd is a useful tool for tracking sediment source and hydrological dynamics in the subglacial environment.