PALEOLIMNOLOGIC VARIABILITY OF MULTIPLE LAKE BASINS: WALDEN POND, MA

This study assesses potential biases in paleolimnologic sedimentary archives associated with basin-specific variability within a given lake. We use Walden Pond, MA for our study since it is a relatively small lake with three basins of varying depth and internal catchment size (area of lake bottom contributing to sediment deposition). Geophysical data reveal that Holocene lacustrine sediment thicknesses are > 10 m in the western basin (149,079 m², z_max= 30.1 m), > 7 m in the central basin (55,720 m², z_max = 20 m), and > 5.5 m in the eastern basin (39,537 m², z_max = 16 m). The western and central basins are composed of clayey silts while silty sands occur within the eastern basin. Magnetic susceptibility (MS) rises at 20 cm down core in the western basin and at 12 cm down core in the central and eastern basins. However, the MS of the western and central basins declines in the upper 8 cm, while the eastern basin MS remains elevated until 2 cm. Bulk isotope analysis of δ³⁴S in the top 28 cm are significantly more depleted in the deep western basin as compared to the shallow eastern basin (1.07‰ vs. 2.97‰; t-test: p = 0.004). δ¹⁵N increases in the upper 17 cm in the western basin and in the upper 8 cm in the eastern basin. Mean total % Carbon in the top 28 cm is significantly higher in the western basin as compared to the eastern basin (17.4% vs. 15.1%; t-test: p = 0.016). Using the base of the Ambrosia- supported anthropogenic MS peak as a chronostratigraphic horizon and the offset increase of δ¹⁵N between basins, we interpret sedimentation rates to be higher in the western basin, which is consistent with geophysical results. The coeval declines in MS in the western and central basins as well as δ³⁴S depletion in the western basin may be explained by reduction diagenesis associated with water column stratification and oxygen depletion in the hypolimnion of these deeper basins. Increased % C in the western basin suggests higher preservation of organic matter under dysoxic conditions, which is supported by excellent preservation of dinoflagellate cysts and other algal palynomorphs in the deep basin core. We conclude that a deeper basin (> 20 m) results in higher sediment accumulation rates, greater likelihood of lakebed anoxia, and higher organic matter preservation.