ESTABLISHMENT OF A CHRONOSTRATIGRAPHIC MODEL FOR LATE PLEISTOCENE AND HOLOCENE LACUSTRINE SEDIMENTS OF CORE FL-00-9P FROM FLATHEAD LAKE, MONTANA

Late Pleistocene / Holocene lacustrine systems often rely heavily on radiometric carbon dating to establish the down core chronology. However, low carbon systems, such as oligotrophic lakes, have presented challenges to researchers seeking a high resolution chronostratigraphic framework to secure climatic interpretations. An alternative correlation dating method that has been gaining acceptance in lacustrine, marine, and terrestrial research is paleomagnetic secular variation (PSV). The two components of secular variation, inclination and declination, provide a time-series record of magnetic pole wandering that has been correlated globally.

In this study, we sought to establish the chronology for a paleoclimatic reconstruction derived from a low carbon piston core recovered from Flathead Lake, Montana. The core (FL-00-9P) contained no discrete carbon fragments and had limitations for use with bulk carbon dating due to an established terrestrial dead carbon signature creating an ~ 2,500 (cal. yr.) reservoir effect. The final chronology was a composite model created from multiple data sets, which was then fit to the data points using a polynomial curve, since sedimentation rates were not linear but varied over time. The chronostratigraphic model utilizes two well dated tephras, ²¹⁰Pb/¹³⁷Cs data, paleomagnetic secular variation records from two well-dated external localities, and secular variation correlations of discrete carbon dates from cores elsewhere in the lake. Utilizing the PSV data as our means of correlation, we were able to construct a chronostratigraphic model covering 14,000 cal. yr. BP from a core containing on average <1% organic carbon. We found that interbasinal and extrabasinal paleomagnetic records can be correlated with an uncertainty of ± 6.2% and ± 10.4%, respectively.