Utilizing calcite‐bound phosphate to reconstruct historical phosphorus loading to marl lake ecosystems

Reconstructing historical phosphorus (P) concentrations in lakes has been a long‐standing problem for paleolimnology. The most commonly applied method is the use of diatom‐inferred total P models, but there are well‐documented limitations to these approaches. Authigenic calcite co‐precipitated with P is a promising geochemical indicator of ambient P conditions in the water column through time. We tested two approaches for isolating the P content associated with authigenic calcite. The first capitalizes on a well‐known sequential extraction method but incorporates a physical separation to increase the specificity of the extraction. The second uses scanning electron microscopy and energy‐dispersive x‐ray spectroscopy (SEM‐EDS) to target P concentrations of authigenic calcite. These methods were evaluated using calcite‐rich sediments from Utah Lake in north‐central Utah, USA. Utah Lake is a eutrophic lake with a well‐documented increase in P pollution due to rapid population growth and wastewater inputs. Both methods showed increased up‐core concentrations of calcite‐P that match historical P loading records; however, a relatively high P detection limit reduced the effectiveness of the semi‐quantitative SEM‐EDS method. Overall, we demonstrate that the methods outlined here are feasible for reconstructing historical P concentrations in lakes with sufficient sedimentary authigenic calcite and appropriate sedimentary environments.