First-Cycle Grain Weathering Processes: Compositions and Textures of Sea Glass from Port Allen, Kauai, Hawaii

Abstract

Abstract Chemical and mechanical weathering textures on siliciclastic grains have provided important information concerning depositional processes and environments, yet understanding is limited by the multicycle origin of most sedimentary deposits. Sea glass grains sampled from a beach in Port Allen, Kauai, Hawaii, were analyzed for mechanical and chemical weathering features using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, and X-ray photoelectron spectroscopy (XPS). As a result of a proximal dumping-ground source, the sea glass offers a unique opportunity to study first-cycle grain weathering processes in a moderate- to high-energy shoreline environment. Grain-size analysis indicates that the largest grains are located behind natural barriers, such as outcrops and boulders. The best-sorted samples are from the lower to middle foreshore environment, whereas moderately sorted deposits are located closer to the backshore where storm waves predominate. Although there is no preferred distribution of different-colored sea glass grains on the beach, nonfrosted grains predominate over frosted varieties, suggesting constant input of fresh material. Grain surface textures can be divided into mechanically dominated and chemically dominated weathering groups. Evidence of mechanical weathering is provided by conchoidal fractures, crescentic gouges, and straight grooves, whereas chemical weathering is indicated by c-shaped cracks, halite (salt) and silica precipitation, and solution pits. Combinations of these features demonstrate that both weathering processes work together to degrade the grains. XPS depth profiling of individual grains indicates that chemical weathering occurs at different grain depths, with leaching of sodium from the glass surfaces. This study provides a rare opportunity to relate grain compositions and textures with depositional and weathering processes, due to the first-cycle origin of the sea glass. The results show that using XPS and SEM techniques is a broadly applicable approach toward unraveling chemical and mechanical breakdown of sediment in beach environments.