Source-to-sink particle size and shape analyis of fine-grained fluvial and eolian sediment along the Matanuska River, Alaska: evaluating the role of glaciofluvial systems in the formation of loess deposits

ABSTRACT Despite the importance of loess in Earth’s climate system, aspects of loess formation between glacial silt formation and terminal eolian deflation, i.e., the transport, sorting, and distribution of glacially generated sediment by glaciofluvial systems, remain poorly understood. To address this, we present the first high-resolution dataset of source-to-sink particle size and shape to constrain the physical characteristics of fluvially transported fines in a modern proximal glaciofluvial depositional system (south-central Alaska). From the Matanuska Glacier terminus to Cook Inlet estuary (∼ 85 km downstream), fine-grained sediment was sampled from bar-top fines of the Matanuska River, as well as from tributaries that drain diverse (glacial and nonglacial) terrain along the transect. To capture the particle size and shape characteristics of eolian sediment in the system, six Holocene loess deposits and four stationary dust traps were sampled from strategic locations along the Matanuska River. Dual particle size and shape analysis was performed using the SYNC (by Microtrac), and geospatial analysis in ArcGIS Pro 3.3 was performed to classify Matanuska River tributaries as either glacial or nonglacial (defined by a 3% cutoff of glacial coverage in their respective watersheds). We underscore the critical utility of multidimensional statistical analysis in working with high-dimensional granulometry (particle size and shape) datasets due to its unique ability to recognize variance and separation between defined classes. Our results show that fine-grained sediment in the main Matanuska River is silt-dominant, clusters closely with dust-trap, loess, and glacial-tributary sediment classes, and lacks progressive longitudinal (along-transect) morphological variability. We interpret this uniformity to reflect that suspended-load silt (generated by glacial erosion) overwhelms the system, that this sediment is not influenced substantially by downstream sorting, fining, or tributary mixing, and that this sediment can be deflated (as observed during field sampling) at any distance along the transect to source continental loess in the Matanuska Valley. These findings have implications for understanding analogous processes in ancient glacial systems, including the role of fluvial systems in the formation of loess deposits. Our data build on the arguments of Smalley et al. (2009) and others that the distribution of silt by large rivers is a critical step in forming major loess deposits, with the observation that such glaciofluvial systems are inundated by suspended load sediment with the same size and shape characteristics as loess, at least in the proximal system. This suggests that the particles that form loess deposits do not necessarily require (and may diagnostically not experience) substantial sorting by rivers. Furthermore, this has implications for fluvial channel bar analogs in ancient, proximal proglacial fluvial systems, which we would expect to include a higher proportion of grain shapes characteristic of loess, including angular to subangular, silt-size particles as opposed to rounded sand grains more typical of non-glaciogenic or eolian (dune)-influenced fluvial systems. Our results generally support the usefulness of particle shape data in detecting variations in sediment-transport history, although distinguishing glacial from nonglacial depositional paleoenvironments from these data is more of a challenge. Enabled by the advent of high-resolution and rapid dynamic image analysis (DIA) technology, this study is one of few in the initial steps of developing a robust database of particle size and shape from a broad climatic and geomorphic variety of modern fluvial analogs that is required to reconstruct deep time paleoenvironmental conditions. It is also the first source-to-sink study to focus on particle shape (in the fine-grained, < 50 µm, fraction), which is imperative for distinguishing between glacial outwash and nonglacial fluvial–deltaic sedimentation in historical systems, given that fines produced by glacial grinding are a hallmark of glacial systems.