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CLEAR LAKE SEDIMENTS: ANTHROPOGENIC CHANGES IN PHYSICAL SEDIMENTOLOGY AND MAGNETIC RESPONSE
Author(s) -
Osleger David A.,
Zierenberg Robert A.,
Suchanek Thomas H.,
Stoner Joseph S.,
Morgan Sally,
Adam David P.
Publication year - 2008
Publication title -
ecological applications
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.864
H-Index - 213
eISSN - 1939-5582
pISSN - 1051-0761
DOI - 10.1890/06-1469.1
Subject(s) - geology , diagenesis , sediment , sedimentary depositional environment , paleolimnology , debris , grain size , sedimentology , mercury (programming language) , geochemistry , paleontology , oceanography , geomorphology , structural basin , computer science , programming language
We analyzed the sedimentological characteristics and magnetic properties of cores from the three basins of Clear Lake, California, USA, to assess the depositional response to a series of land use changes that occurred in the watershed over the 20th century. Results indicate that distinct and abrupt shifts in particle size, magnetic concentration/mineralogy, and redox conditions occur concurrently with a variety of ecological and chemical changes in lake bed sediments. This coincidence of events occurred around 1927, a datum determined by an abrupt increase in total mercury (Hg) in Clear Lake cores and the known initiation of open‐pit Hg mining at the Sulphur Bank Mercury Mine, confirmed by 210 Pb dating. Ages below the 1927 horizon were determined by accelerator mass spectrometry on 14 C of coarse organic debris. Calculated sedimentation rates below the 1927 datum are ∼1 mm/yr, whereas rates from 1927 to 2000 are up to an order of magnitude higher, with averages of ∼3.5–19 mm/yr. In both the Oaks and Upper Arms, the post‐1927 co‐occurrence of abrupt shifts in magnetic signatures with color differences indicative of changing redox conditions is interpreted to reflect a more oxygenated diagenetic regime and rapid burial of sediment below the depth of sulfate diffusion. Post‐1927 in the Oaks Arm, grain size exhibits a gradual coarsening‐upward pattern that we attribute to the input of mechanically deposited waste rock related to open‐pit mining activities at the mine. In contrast, grain size in the Upper Arm exhibits a gradational fining‐upward after 1927 that we interpret as human‐induced erosion of fine‐grained soils and chemically weathered rocks of the Franciscan Assemblage by heavy earthmoving equipment associated with a road‐ and home‐building boom, exacerbated by stream channel mining and wetlands destruction. The flux of fine‐grained sediment into the Upper Arm increased the nutrient load to the lake, and that in turn catalyzed profuse cyanobacterial blooms through the 20th century. The resulting organic biomass, in combination with the increased inorganic sediment supply, contributed to the abrupt increase in sedimentation rate after 1927.