Insights into glacial processes from micromorphology of silt-sized sediment

Abstract

Silt-rich meltwater plume deposits (MPDs) analyzed from marine sediment cores have elucidated relationships that are clearly connected, yet difficult to constrain, between subglacial hydrology, ice-marginal landforms, and grounding-zone retreat patterns for several glacial catchments. Few attempts have been made to infer details of subglacial hydrology, such as flow regime, geometry of drainage pathways, and mode(s) of sediment transport through time, from grain-scale characteristics of MPDs. Using sediment samples from MPD, till, and grounding-zone proximal diamicton collected offshore of six modern and relict glacial catchments in both hemispheres, we examine grain shape distributions and microtextures (collectively, grain micromorphology) of the silt fraction to explore whether grains are measurably altered from their subglacial sources via meltwater action. We find that 75 % of all imaged grains (n = 9400) can be described by 25 % of the full range of measured shape morphometrics, indicating grain shape homogenization through widespread and efficient abrasive processes in subglacial environments. Although silt grains from MPDs exhibit edge rounding more often than silt grains from tills, grain surface textures indicative of fluvial transport (e.g., v-shaped percussions) occur in only a modest number of grains. Furthermore, MPD grain surfaces retain several textures consistent with transport beneath glacial ice (e.g., straight or arcuate steps, (sub)linear fractures) in comparable abundances to till grains. Significant grain shape alteration in MPDs compared to their till sources is observed in sediments from glacial regions where (1) high-magnitude, potentially catastrophic meltwater drainage events are inferred from marine sediment records and (2) submarine landforms suggest supraglacial melt contributed to the subglacial hydrological budget. This implies that quantifiable grain shape alteration in MPDs could reflect a combination of high-energy flow of subglacial meltwater, persistent sediment entrainment, and/or long sediment transport distances through subglacial drainage pathways. Integrating grain micromorphology into analysis of MPDs in site-specific studies could therefore aid in distinguishing periods of persistent, well-connected subglacial discharge from periods of sluggish or disorganized drainage. In the wider context of deglacial marine sedimentary and bathymetric records, a grain micromorphological approach may bolster our ability to characterize ice response to subglacial meltwater transmission through time. This work additionally demonstrates that glacial and fluvial surface textures are retained on silt-sized quartz grains in adequate amounts for microtexture analysis, which has heretofore been conducted exclusively on the sand fraction. Therefore, grain microtextures can be examined on silt-rich glaciogenic deposits that contain little to no sand as a means to evaluate sediment transport processes.