| Sediment cores from Lake Tokun were used to reconstruct paleoenvironmental and paleoclimate changes in the Copper River delta area, Alaska. A 4.2-m-long sediment sequence was analyzed for organic matter, chlorophyll-a, magnetic susceptibility, and bulk density. The chronology was based on radiocarbon ages and short-lived isotopes. Distinct changes in the sedimentary succession record major paleoenvironmental changes during the Holocene. Prior to 8.8 ka, Lake Tokun received meltwater from the Martin River Glacier at a time when sea level had transgressed at least 6 km inland of the present shoreline likely due to isostatic depression upon deglaciation. From 8.8 to 0.7 ka, Lake Tokun was a shallow lake receiving sporadic pulses of rock flour when Martin River overtopped its channels. At ∼735-1215 AD, rock flour deposited in Lake Tokun represents an aggrading outwash plain as the Martin River Glacier expanded and then deposited its terminal moraine. The proximal end of the associated outwash plain dammed Lake Tokun, deepening the lake and enhancing sediment deposition. The lake attained its present depth about 1215 AD and rock flour has not reentered the lake since then. The uppermost sedimentary unit (150 cm thick) was analyzed at 0.5 cm resolution (average of 3 year per sample) using VNIR reflectance spectroscopy to infer the concentration of chlorophyll-a (chl-a). Instrumental weather data from Cordova (1917 to 2009) show a strong inverse correlation (r2 = 0.49; p = 0.03) between August precipitation and sedimentary chl-a content in Lake Tokun. Storms beginning in the late summer and persisting through the fall are important in controlling peak discharges and the subsequent transport of allochthonous material into Lake Tokun. Winter (DJF) temperature is also inversely correlated with chl-a content (r2 = 0.33; p = 0.04), indicating that warm winters, which tend to be wet, also lead to enhanced runoff, which carries mineral matter that dilutes the chl-a content of the lake sediment. Decreased chl-a values suggest increased runoff during the early to middle Little Ice Age (LIA; 1215-1650 AD). Increased chl-a values during the late LIA (1650-1850 AD) are suggestive of decreased runoff. This study demonstrates that multi-proxy analysis of lake sediments is an effective method for inferring past paleoenvironmental and paleoclimate change. |
