EXPLORING VIVIANITE AS AS PROXY FOR PALEOENVIRONMENTAL CONDITIONS IN A MICHIGAN KETTLE LAKE

Flat Iron Lake (43°07'26.3"N 85°23'06.0"W) is situated in the glaciated rolling hills physiographic region of western Michigan. Flat Iron Lake is geomorphically characteristic of a kettle lake, suggesting that the sedimentary archive contains a post-glacial paleoenvironmental record. This research aims to evaluate the viability of the FIL sediments as an archive of environmental change - insight into the various components of the sediment will further elucidate how this lake has functioned historically. Here we focus on the mineral record, specifically. We retrieved sediment cores from 3 nearshore locations and one deep location with a Livingston coring device. Individual drives were split and described using smear slides and visual observations. The sediment from the deep part of the lake is dominated by sub-millimeter laminated silty sapropel. Alternating laminations of light colored calcite and dark colored organic laminations, and layers of charcoal were observed. The final drive (refusal) collected unstratified silty clay diamict, which we interpret to represent the original lake basin. Upon splitting the cores, vivianite [Fe2+3(PO4)2•8(H2O)] appeared as it oxidized from translucent to white, then light blue, and eventually turned opaque and black. Vivianite occurred as ~1-3 mm sized nodules throughout the laminated sapropel. In smear slides, vivianite manifests as either globs of clay-sized crystals, or euhedral single or grouped and bladed crystals. Vivianite was rare in cores taken from the shallow regions of Flat Iron Lake but occurred continuously in the deep sediments. We assess the possible range of conditions for vivianite formation in Flat Iron Lake sediments, such as geochemical and trophic conditions, which leads us to interpret groundwater as a strong influence throughout the lake’s history. However, diagenesis is a potential complicating factor, and given that vivianite is indicative of complicated redox cycles in the lake bottom, we conclude that vivianite’s occurrence must be considered along with other minerals (e.g. calcite) as proxies in the sedimentary record in Flat Iron Lake.