DIATOM INFERRED ENVIRONMENTAL RECONSTRUCTION OF TWO VOLCANIC CRATER LAKES WITHIN NEWBERRY VOLCANO, OREGON

Newberry Crater, Oregon, is a low shield volcano that includes two meromictic volcanic lake systems within close proximity to each other. Paulina Lake and East Lake show significant chemical profile differences. Both lakes have subaqueous inputs of volcanic fluids, but fluids that enter East Lake are primarily hydrogen sulfide (H₂S) and carbon dioxide (CO₂) with minor amounts of mercury, while volcanic fluids that enter Paulina Lake are carbonate-rich and have abundant iron (Fe), arsenic (As), silicon (Si) and phosphorus (P). From each lake, multiple sediment cores were collected to establish a diatom assemblage record. Changes in the diatom record from lake sediments indicate long-term fluctuations in lake level, productivity, and other conditions. Planktic diatom communities are commonly found in open water away from shore. Benthic diatom communities thrive in shallow water where light can penetrate to the sediment-water interface. Diatom records were created from a high or low resolution counting scheme. In Paulina Lake, Stephanodiscus excentricus, a planktonic species, was the most abundant diatom. S. excentricus is often found in waters that exhibit pH values around 7 and is an indicator of eutrophication and phosphorus. In East Lake, a variety of benthic species were most abundant in the sediments. The most abundant species include Cocconeis placentula, Rhoicosphenia abbreviata, and Pseudostaurosira brevistriata. C. placentula is common and found in circumneutral to alkaline waters. R. abbreviata and P. brevistriata are found in eutrophic systems with alkaline pH. A planktic-benthic ratio was created for each sediment core to gather evidence for fluctuations in lake level. To further track fluctuations in lake level and diatom habitat areas, we created 3-D models of each lake and the surrounding volcanic crater. Diatom habitat area models can be used to predict relative changes in the available area for benthic and planktic diatom communities, and then can be compared against changes in planktic-benthic ratios in fossil assemblages from sediment cores to improve interpretations of past lake-level fluctuations. Research in these lakes is significant because it offers a unique opportunity to investigate two volcanic lakes that are close geographically but are chemically distant.