Dartmouth Institute for Health Policy and Clinical Practice
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Water Column Controls on Magnetic Mineral Formation and Preservation
Get PDFIron-bearing minerals in the sedimentary record are frequently used to reconstruct past depositional environments. This is accomplished through geochemical, magnetic, and mineralogical analyses placed in geological context. These interpretations, however, depend on our understanding of how iron minerals form in nature. To better understand the formation of magnetic iron minerals in redox-stratified depositional settings, the study of modern analogs is crucial to unraveling environmental context no longer available from geological deposits. In this study, I utilized four anoxic and sulfidic stratified lakes within a 100-mile radius to examine the effects of small variations in lake properties on magnetic mineral formation and preservation. I combined geochemical, magnetic, and microscopic analyses to characterize the water columns, sediments, and mixing statuses of these sites. Two of these lakes were previously known to be meromictic and the other two I identified as potentially meromictic. Inter-site comparison suggests that pH of the overlying water column, trophic status, detrital inputs, and redox oscillations each influence magnetic mineral formation and preservation. Magnetic properties of particulates filtered from the water column and lake-bottom sediments along with trace element analyses, gyroremanent magnetization (GRM) acquisition, and transmission electron microscopy (TEM) support the identification of greigite in three of the lakes (Barbadoes, Black, and Trout Ponds). These results provide insight into the factors controlling magnetic mineral formation and preservation in freshwater, low-iron lakes, and may have implications for interpreting the sediment records of ancient lakes or oceans
