ESTABLISHING A GRAINSIZE SAMPLE PROCESSING METHOD: ISOLATING CLASTIC GRAINS AND REMOVING EVAPORITE MINERALS TO ENABLE PALEO-ENVIRONMENTAL INTERPRETATIONS

Grainsize analysis can be used to make paleo-environmental inferences as particle size distributions are dependent on the type of depositional environment and transport process. Specifically, grainsize analysis of the clastic sediment fraction from ephemeral lake cores can aid paleo-climate interpretations of hydrological balances because the geologic processes that govern lake sediment deposition during lake expansion (deep water environments) and contractions (shallow to dry lake conditions) filter different grain sizes. However, one complication that arises when working with sediment from evaporate basins is the high concentration of evaporite deposits that can be left behind as water level drops. These evaporite minerals (e.g. halite, gypsum, anhydrite, etc.) are of little value for grainsize analysis because they reflect crystallization history and may not reflect depositional energy or paleo-transport conditions. Moreover, evaporite minerals form in situ and encapsulate the clastic material of interest—which indicate transport to and within the lake, as well as depositional environment. Thus, evaporite minerals should be removed to isolate the clastic materials. To do this, we built upon standardized grainsize pretreatment methods which remove organics with hydrogen peroxide, biogenic silica with sodium bicarbonate, and by adding some additional steps (acidic baths and DI rinses) to eliminate the evaporites minerals while retaining the clastic sediments. We verified our new methodology using a series of experiments and with SEM & EDs, XRD, and grainsize analysis using a coulter LS-13 230. Sediment samples come from core samples collected at Searles Lake, CA—a location where we plan to develop a record of lake expansion and contraction in the future. It appears that our method not only eradicates the evaporites, but also preserves the grainsizes of the clastic materials.