Effects of American Colonial Settlement and Deforestation on Lacustrine Redox Conditions: Longterm Insights from Martin Lake, Indiana

Abstract

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.