Estimates of lake core ages in Northeast U.S. to qualify diatom reference samples
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The Northeast Lakes Sediment Diatom Collaborative (NLSDC) is committed to improving the management of northeastern U.S. lakes by assessing their current and past ecological integrity using sediment diatoms. Researchers from U.S. EPA and state agencies compiled a dataset with sediment cores from 601 Northeast lakes. All diatom samples were collected from the top and bottom of the sediment cores. The diatom assemblage information inferred from the top of the core represents present-day lake conditions. Historical lake conditions, representing reference-quality conditions, are indicated by the diatom assemblage at the bottom of the core. The age of the bottom sample allows the reference sample to be characterized as indigenous era, early agricultural era, or industrial era. The depth of the core can be associated with the age of the bottom sample based on the observed or derived sediment accumulation rate (SAR) that is specific to each lake. The purpose of this project was to estimate the SAR of lakes and back-calculate the core depth necessary to reach indigenous or early agricultural times. An existing global model of SAR based on historical land use and climate data (Baud et al. 2021) was compared to observed core chronology data (from 210Pb, 137Cs, or 14C isotope analysis, pollen analysis, or sediment varve analysis) in a subset of 106 Northeast lakes. We found that the Baud equation overpredicted SARs for most lakes (RMSE = 2.36 mm/yr) but the overprediction was not consistent enough to apply a constant correction factor. Therefore, we did not recommend using the equation to estimate SARs in Northeast lakes. However, we were able to contextualize the recorded bottom core depths. Distributional data from the 601 Northeast lakes and the 106 lakes with core chronology data suggest that sediment core depths ≥ 45cm would often represent sediments deposited earlier than AD 1800, which would be pre-industrial and before intensive agricultural in many places in the Northeast U.S. In addition, we used lake characteristics and U.S. EPA LakeCat data to explain variability in observed SARs. Present-day land use and human impacts in lake watersheds had positive correlations with observed SARs in Northeast lakes.