Abstract
To examine limnological conditions in Lake Ogawara on the Pacific coast of northwestern Japan, we investigated the magnetic properties of dredged bottom sediment originally collected from the lake in the summer of 2011. We used non-destructive methods to measure the low-field magnetic susceptibility shortly after sampling, and anhysteretic remanent magnetization (ARM) was assessed in 2012 and 2015. The ARM acquisition and demagnetization curves from littoral sites showed several patterns that reflect the provenance of the sediments. At water depths below 10 m, the magnetic susceptibility and ARM of greenish black mud with high organic content decreased considerably with the increase in water depth, but ARM increased slightly at water depths greater than 16 m. We also found that the magnetic concentrations of mud samples were reduced markedly during a period of storage for about 3 years. We attributed these reductions to diagenetic loss of magnetic minerals, which had been enhanced at deeper sites. It is possible that the ARM carriers in deeper areas were derived from authigenic formation of iron sulfide or from deposition of suspended matter in the hypolimnion water. We propose that the magnetic properties of surficial sediments are controlled by limnological stratification of the brackish lake water, thus possibly providing an analog for down-core variations of magnetic parameters associated with the modification of magnetic minerals during reductive diagenesis.
Highlights
Lake Ogawara is a brackish coastal lagoon on the Pacific coast of Aomori Prefecture, northwestern Japan (Fig. 1)
The spatial distributions of magnetic susceptibility and anhysteretic remanent magnetization (ARM) intensity data obtained at the peak alternating field (AF) of 100 mT (Fig. 2b, c) showed higher values along the lakeshore at water depths less than 5 m, and this area included the sites around the mouth of the Shichinohe-gawa and Sadoro-gawa rivers in the southwest part of the lake and the sites near the Takase-gawa River in the northeast part
For the sites at water depths greater than 10 m, where lake floor sediments consist of greenish black mud with very high total organic carbon (TOC) contents, the magnetic susceptibility and ARM decreased with increasing water depth, the initial measurements in 2012 showed a slight increase in the ARM for water depths greater than 16 m
Summary
Lake Ogawara is a brackish coastal lagoon on the Pacific coast of Aomori Prefecture, northwestern Japan (Fig. 1). The lake is connected to the Pacific by the Takase-gawa River and an artificial sluice (the Takase-gawa sluice). The average water depth of the lake is about 11 m, and in the central part of the lake, the water depth ranges from 15 to 25 m. There is a year-round halocline at 10–15 m depth, and the bottom to middle waters are anoxic (Fujiwara et al 2003a). The lake basin is filled with about 20 m of clayey to sandy sediments that were deposited over the last 10,000 years (Inagaki et al 2012).
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