A MINERAL MAGNETIC AND SCALED-CHRYSOPHYTE PALEOLIMNOLOGICAL STUDY OF 2 NORTHEASTERN PENNSYLVANIA LAKES - RECORDS OF FLY-ASH DEPOSITION, LAND-USE CHANGE, AND PALEORAINFALL VARIATION

A combined mineral magnetic and scaled chrysophyte study of lake sedim ents from Lake Lacawac and Lake Giles in northeastern Pennsylvania was conducted to determine the effects of land-use and sediment source ch anges on the variation of pH, conductivity, and alkalinity inferred fr om biotic changes. Ten 30-40 cm long gravity cores were collected from Lake Lacawac and three from Lake Giles. Isothermal remanent magnetiza tions (IRMs) were given to the lake sediments in a 1.3 T magnetic fiel d to measure magnetic mineral concentration variations. IRM acquisitio n experiments were conducted to identify magnetic mineralogy. The bedr ock, soils and a peat bog on the shores of Lake Lacawac were also samp led for magnetic analysis to determine possible lake sediment sources. The top 10 cm of sediment collected from Lakes Lacawac and Giles was two to four times more magnetic than deeper sediment. Pb-210 dating su ggests that this intensity increase commenced circa 1900. SEM images o f magnetic extracts from the highly magnetic sediments indicates the p resence of magnetic fly ash microspheres from fossil fuel burning elec tric power generation plants. The similarity in magnetic coercivity in the top 8 cm lake sediments and in the peat bog supports an atmospher ic source for some of the magnetic minerals in the youngest lake sedim ents. The highly magnetic sediments also contain an antiferromagnetic mineral in two cores closest to Lake Lacawac's southeastern shore. Thi s magnetic mineral is only present deep in the soil profile and would suggest erosion and significant land-use changes in the Lacawac waters hed as another cause for the high magnetic intensities (concentrations ) in the top 10 cm of the lake sediments. The most significant changes in the scaled chrysophyte flora occurred immediately above the 10 cm level and were used to infer a doubling of the specific conductivity b etween circa 1910 and 1929. These variations also support land-use cha nges in the Lacawac catchment at this time. A similar shift in the sca led chrysophte flora was not observed in the top of Lake Giles, howeve r, distinct changes were found in the deeper sections of the core coup led with a smaller peak in magnetic concentration. Fourier analysis of the Pb-210-dated lake sediment magnetics indicates the presence of a 50 year period, low amplitude variation in the Lake Lacawac, Lake Gile s, and Lake Waynewood (Lott et al., 1994) magnetic concentration recor ds. After removal of the land-use/fly ash magnetic concentration peak by Gaussian filtering, the 50 year variation correlates strongly from lake to lake even though the lakes are in different watersheds separat ed by up to 30 km. When this magnetic variation is compared with Gauss ian-filtered rainfall variations observed in New York City and Philade lphia over the past 120-250 years there is a strong correlation sugges ting that magnetic concentration variations can record regional rainfa ll variations with an approximately 50 year period. This result indica tes that magnetics could be used to document regional variations in cl imatic change.