| Sediment cores from Upper and Lower Whitshed Lakes located near Cordova, Alaska, were used to create a tephrochronology and to interpret environmental and climatic changes during the past 15,000 years. Age models were developed for both lakes using radiocarbon dates, and profiles of short-lived isotopes near the surface of the cores. The major-oxide glass geochemistry of seven unique tephras was analyzed and used along with magnetic susceptibility (MS) stratigraphy to correlate four tephras between the two lakes. The correlated tephras were used to synchronize the age models, which reduced the uncertainty in both the age models and the age estimates of the tephras.;Several proxies for productivity were analyzed in the cores: organic matter (OM), biogenic silica (BSi), and spectrally inferred chlorophyll-a. Deglaciation occurred by 14.6 ka, and the early part of the record is characterized by low OM and moderately high BSi values. All proxies of productivity are low during the Younger Dryas (12.5-11.6 ka), indicating a return to colder unproductive conditions. From 10.9-2.7 ka, BSi in both lakes is generally high and stable. From 2.7-1.7 ka, Upper Whitshed Lake BSi declined from 30% to 10% and the relative concentration of chlorophyll-a decreased markedly, likely due to a reduction in nitrogen input as alder (Alnus) was replaced by conifers. In Lower Whitshed Lake, a much smaller shift occurs at the same time (from 30% to 22%). Upper Whitshed Lake BSi is negatively correlated with summer precipitable water as represented by the local grid point in a climate reanalysis product, suggesting that the reduction in BSi after 2.7 ka could also be driven by increased summer cloudiness (reduced photosynthesis) or increased summer precipitation (increased sedimentation, diluting BSi).;The paired-lake comparison allows for an investigation of the replicability and uncertainty of proxies analyzed in both lakes. Millennial-scale patterns within BSi and OM are similar between the two lakes, and these proxies correlate significantly between the two lakes (R2 = 0.18, p = 0.020, and R2 = 0.44, p = 0.001, respectively). However, the BSi records contain substantially more variability between the records compared to the signal that is common to the records, and the signal-to-noise ratio is less than 1. This suggests that, for lakes in these settings, a large portion of BSi variability is not climatically driven. |
