Browsing by Subject "Paleoredox"

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    Effects of American Colonial Settlement and Deforestation on Lacustrine Redox Conditions: Longterm Insights from Martin Lake, Indiana
    (2020-11) Henke, Alyssa Nicole; Gilhooly, William P., III; Bird, Broxton; Druschel, Gregory
    Colonial settlement of Indiana changed the environment in significant ways; the aim of this study is to quantify the impacts of settlement through the use of geochemical proxies including: % lithics; the carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S) isotope composition of organic matter; the elemental composition of carbon (TOC) and nitrogen (Ntot) in organic matter and their ratio (C/N); the δ34S of mineral sulfides (pyrite and acid volatile sulfides); and iron redox proxies. Lakes are a great recorder of aquatic-terrestrial linkages on both local and global scales. Martin lake’s watershed, in northeastern Indiana, was settled in 1840 by Euro-Americans, and since then clear shifts in lake chemistry are recorded in its sediments. A core spanning roughly the last 300 years taken from Martin Lake is the basis of this study. The impacts of settlement can be seen through the lenses of all the proxies that were used in this study. 1) Post-settlement deforestation increased erosion in Martin Lake’s watershed, increasing sedimentation rates and % lithics. 2) δ13C of organic matter reveals a pattern of deforestation and partial regrowth and agricultural use of land. 3) A pronounced increase in δ15N timed with the change in population at the time of settlement is consistent with the increased input of human or animal waste into Martin Lake. 4) TOC and C/N show an overall increase in the amount of organic matter within the lake caused by deforestation, and that the increased nutrient supply may have stimulated more in-lake productivity. 5) δ34S of mineral sulfides show that deforestation lead to an increase in the available sulfate pool of Martin Lake, which in combination with 6) an increase in FeHR created redox conditions in which pyrite formation was more favorable. These factors culminated in a transition in Martin Lake chemistry and redox cycling within the sediments.
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    A High-Resolution Study of Local Diagenetic Effects on the Geochemistry of the Late Ordovician Kope Formation
    (2022-09) Becerra, Evelyn S.; Gilhooly, William P., III; Licht, Kathy; Filippelli, Gabriel
    The Ordovician (485-444 Ma) was a highly dynamic period, characterized by significant evolutionary and climatic change. Paleozoic fauna which evolved during the Great Ordovician Biodiversification Event (GOBE) populated extensive epicontinental seaways. Major sea level fluctuations during The Hirnantian glaciation are believed to have led to a mass extinction event at the End Ordovician. However, a reassessment of Early Paleozoic fossil assemblages suggests the onset of extinctions began in the mid-Katian, ~3 million years before the Hirnantian. The Kope formation, within the North American succession of the Katian, was deposited during the peak biodiversification of the GOBE at the point which a biological crisis begins. The well-studied series of interbedded shale and fossiliferous limestone beds, deposited within a shallow epeiric sea, provide ideal sedimentological and paleontological context to interpret sediment geochemistry recorded at the onset of a global mass extinction. For a high-resolution section of the Kope, δ34Spyrite show an extraordinary range of variability, up to 64.5‰, with systematic oscillations throughout the core. The isotope signal represents a mix of pyrite formed at the time of deposition and during diagenesis. As sea levels fluctuated, the amount of sediment delivery influenced the connection of sediment porewaters to overlying seawater sulfate and the location of the sulfate reduction zone, which in turn, masked the primary signal. Reactive iron data suggest low oxygen concentrations in the water column, however fossil assemblages found throughout the Kope suggest otherwise. Changes in sedimentation can mask the water column signal, so these data also capture an aggregate signal. δ15Nbulk show an upsection decrease of 4.4‰, followed by a 3.4‰ increase. Though this excursion can be interpreted as a switch to increased denitrification in a low oxygen environment, the fossil record suggests the data capture localized diagenetic reactions that occur below an oxic water column. Perturbations in the ocean-climate system is often based on the interpretation of stable isotope excursions, and although excursions are diagnostic of changes to biogeochemical cycles, they may not fully account for diagenetic reactions that mask primary signals. The results from the Kope demonstrate strong localized, not global, controls on the sediment geochemistry.