Implications for post-comminution processes in subglacial suspended sediment using coupled radiogenic strontium and neodymium isotopes

Abstract

Enhanced physical weathering rates in subglacial systems promote high levels of comminution, transport, and deposition of fine-grained sediment within the subglacial drainage network. The impact of shifts in sediment loads due to variations in meltwater flux, and their effects on downstream ecosystems, remains poorly quantified and places a fundamental importance on our ability to characterize subglacial depositional environments. Here, for the first time, we assess the seasonal evolution of the subglacial suspended sediment using coupled radiogenic strontium (87Sr/868 Sr) and neodymium (143Nd/1449 Nd) isotopic ratios with elemental ratios, and in-situ measurements. Weathering rates in fluvial and riverine systems have been traditionally assessed using radiogenic isotopic tracers: 143Nd/144Nd ratios relate to the crustal age whereas 87Sr/8611 Sr ratios relates to both age and preferential mineral dissolution. Relative shifts in these ratios allow us to characterize distinct sediment transport networks. We apply this technique to the Lemon Creek Glacier (LCG), Alaska, USA and the Athabasca Glacier (AG), Alberta, CA. At the LCG, the 143Nd/144Nd values range from εNd of -4.6 (0.9) to -8.7 (0.2), which suggests a poorly mixed sediment flux. However, the greatest period of variability may correlate with the drainage of a supraglacial lake and suggests caution should be exerted in timescale 143Nd/14418 Nd provenance studies that may be affected by climatic disturbances. In contrast, limited variation is observed within the AG 143Nd/14419 Nd seasonal record. A consistent, direct relation between the Rb/Sr elemental ratio and the 87Sr/8620 Sr ratio enables us to unravel incongruent weathering trends in the radiogenic Sr record. Correlation between the 87Sr/8622 Sr and total discharge suggests the process is partially controlled by mantling of the bedrock, which can be detected using post-comminution ages. While the subglacial structure may be enabled by the subglacial till beneath the AG, our study supports the use of Sr-Nd as a new proxy in the subglacial environment.