Magnetic properties of surficial sediments in Lake Ogawara on the Pacific coast of northeastern Japan: spatial variability and correlation with brackish water stratification
© Hayashida et al. 2015
Received: 31 January 2015
Accepted: 14 October 2015
Published: 23 October 2015
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.
In this paper, we describe the magnetic properties of bottom sediments dredged from Lake Ogawara, which we measured by using mainly non-destructive methods. We present spatial variations of magnetic concentration parameters including the low-field magnetic susceptibility and anhysteretic remanent magnetization (ARM). Studies of the magnetic properties of lake sediments are widely used to reconstruct the past geomagnetic field and to investigate paleoenvironments (Thompson and Oldfield 1986; Evans and Heller 2003; Liu et al. 2012). For both of these purposes, it is important to understand the origin of magnetic minerals in sediments. The sources of magnetic minerals in coastal lakes can be complicated; for example, detrital input is controlled by hydrological processes in the watershed, while authigenic mineral formation is related to the aquatic productivity of lake water, and both of these may be affected by the influx of marine water and relative sea-level changes. In addition, aeolian dust may accumulate in sediments under a certain condition (Liu et al. 2015). Magnetic measurements of surficial deposits in lakes can therefore provide useful information on modern lake sediment formation processes and can assist researchers in interpreting magnetic proxies for paleoenvironments.
Although non-destructive measurements of magnetic concentration parameters provide a quick and efficient method for characterizing sediment samples, it is generally difficult to determine magnetic mineral species by using only such measurements. We therefore attempted to estimate the magnetic coercivity distribution and blocking temperatures of isothermal remanent magnetization (IRM) on selected samples. We found, however, that magnetic concentration parameters were reduced markedly during refrigerated storage of samples for 3 years after collection, particularly in samples from the deeper part of the lake. We suggest that metastable magnetic minerals decay after sample recovery.
We investigated the magnetic properties of bottom sediments with samples collected at Lake Ogawara in 2011 by Seto et al. (2012); these samples were collected with an Ekman–Birge bottom sampler at 112 sites at water depths of 0.75 to 24.7 m, mostly on a 1-km grid (Fig. 1). Details of water quality and sedimentological data will be reported elsewhere (Seto et al. in preparation). The bottom sediments were composed mainly of well-sorted fine to coarse sand at sites shallower than 6 m and of greenish black mud at deeper sites (Fig. 2a; Seto et al. 2012). We obtained samples for magnetic measurements by inserting plastic cubes (with one open face) with a 7-cm3 volume into the sediments dredged at 105 sites.
We measured a low-field magnetic susceptibility of the cubic specimens soon after sample recovery by using an MS3 susceptibility meter with a MS1B sensor (Bartington Instruments) operating at a frequency of 0.47 kHz. After correction for the diamagnetic susceptibility of the plastic sample holder and instrument calibration by use of paramagnetic standard samples, the volume and mass susceptibilities were calculated. We also measured a susceptibility at 4.7 kHz, but detected no significant frequency dependence. On pilot specimens from 40 sites, we carried out a stepwise acquisition of ARM up to 100 mT with a 0.1-mT biasing field and alternating field (AF) demagnetization of the ARM. For other sites, ARM was imparted to specimens at 100 mT AF. The ARM measurements were carried out in March 2012.
To investigate magnetic coercivity distributions, we also obtained isothermal remanence acquisition curves from eight selected samples in September 2012 and an additional five samples in April 2015. The IRM was progressively imparted up to 1.2 T by use of a pulse magnetizer (ASC, IM10-30). The IRM acquisition curves were analyzed by the method proposed by Kruiver et al. (2001) and Heslop et al. (2002). Measurements of ARM and IRM were made with a cryogenic magnetometer (Model 755R, 2G Enterprises), and the high-intensity IRM was measured with a spinner magnetometer (SMD-88, Natsuhara Giken).
In addition to the non-destructive measurements, in April 2015, we performed thermal demagnetization of a composite three-axis IRM (Lowrie 1990) with selected samples. Magnetizing fields of 1.2, 0.3, and 0.08 T were applied to the sample along the three perpendicular axes. Then, the sample was subjected to stepwise thermal demagnetization up to 710 °C. During this experiment, we found that the acquired magnetic intensity was lower than that measured in 2012, particularly in mud samples from the deeper sites, thus suggesting that there may have been a loss of magnetic minerals during refrigerated storage for about 3 years. We therefore re-measured low-field magnetic susceptibility and ARM for samples from all sites in April and May 2015.
Results and discussion
The spatial distributions of magnetic susceptibility and ARM intensity data obtained at the peak 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. The bottom sediments at these sites are dominated by medium sands (Fig. 2a), which suggests that magnetic minerals of detrital origin were transported into the lake both from the watershed to the southwest and by erosion of the sandbar on the Pacific coast near the mouth of the Takase-gawa River.
The shallow sites in the northeast part of the lake also showed the presence of a higher coercivity fraction, particularly in the Takase-gawa River and around its mouth (sites 1, 4, and 6 in Fig. 1; dotted lines in Fig. 3c). Other shallow-water sites close to the lakeshore were characterized by ARM with very low coercivity (Fig. 3d).
The ARM acquisition curves of clayey and silty sediments from sites at water depths greater than 10 m were mostly saturated above 60 mT (Fig. 3e, f), although two sites near the Takase-gawa River mouth (sites 19 and 29 in Fig. 1; dotted lines in Fig. 3e) showed hard coercivities similar to those of samples from the northeast of the lake (Fig. 3c). Note that the MDFs of samples from 10 to 17 m water depth were less than 24 mT, but those from 17 to 24 m water depth were 25–28 mT.
Water quality observations in the summer of 2011 (Fig. 4; Seto et al. 2012) clearly showed a thermocline and halocline from 8 to 18 m water depth. Thus, Seto et al. (2012) identified an epilimnion (0–8 m), metalimnion (8–18 m), and hypolimnion (deeper than 18 m) in Lake Ogawara. Waters of the metalimnion and hypolimnion were anoxic to euxinic, and there was a downward decrease of susceptibility and ARM in the metalimnion, where dissolved oxygen was depleted (Fig. 4). The total organic carbon (TOC) contents of bottom sediments at water depths greater than 5 m were generally very high (4 to 9 %). We propose, therefore, that the decrease of magnetic concentration parameters with increasing water depth represents the loss of magnetic minerals through diagenetic dissolution under reductive conditions.
Depletion of magnetic oxides and the formation of authigenic iron sulfide associated with reductive diagenesis have been widely observed in association with down-core variations of the magnetic properties of hemipelagic and coastal marine sediments (e.g., Karlin 1990; Bloemendal et al. 1993; Liu et al. 2004; Rowan et al. 2009). Similar modifications of magnetic properties have been reported in studies of core samples from lake sediments in various regions (e.g., Anderson and Rippey 1988; Snowball 1993; Nowaczyk et al. 2001). The variations of magnetic properties of the bottom sediments with water depth in Lake Ogawara (Fig. 4) represent a profile analogous to such down-core variations. We suggest that diagenetic loss of magnetic minerals has occurred in the surficial sediments on the lake floor.
Rapid loss of magnetization during storage has been reported for organic-rich lake sediment cores (Oldfield et al. 1992) and for anoxic marine sediments (Richter et al. 1999; Roberts et al. 1999; Yamazaki et al. 2000). Such modifications of magnetic properties are known as “storage diagenesis” (Oldfield et al. 1992). As proposed by the studies cited above, the loss of magnetic material in our samples might reflect dissolution of fine-grained magnetite or decay of a metastable iron-sulfide such as greigite.
There are two possible explanations for the origin of the magnetic minerals that were ARM carriers at the deeper sites and lost by storage diagenesis. The water quality data (Fig. 4) revealed that hypolimnion water deeper than 18 m was characterized by high turbidity and high chlorophyll a concentrations (Fujiwara et al. 2003b; Seto et al. in preparation), which implies that fine-grained magnetic minerals might have been supplied in suspension in the hypolimnion water delivered from the Pacific Ocean. However, ARM acquisition and demagnetization curves for sediments from the deep sites (Fig. 3f) were different from those observed at the shallow sites around the Takase-gawa River (Fig. 3c). Thus, it is more plausible that authigenic formation of iron sulfide occurred in the deeper lake floor sediments, which are overlain by anoxic water with high salinity and high organic matter content. This assumption is consistent with the occurrence of greigite in Holocene lake sediments such as those in Loch Lomond (Snowball and Thompson 1990). It is difficult, however, to clarify the magnetic mineralogy of the ARM carriers from the available data because it has been affected by storage diagenesis, which suggests that freezing of samples immediately after recovery is crucial for future environmental magnetic studies of organic-rich sediments.
Our measurements of the magnetic susceptibility and ARM of dredged sediment samples from Lake Ogawara showed that medium sand samples at shallow littoral sites are characterized by high magnetic concentrations. The results of stepwise acquisition and demagnetization of ARM showed several patterns that possibly represent differences in the provenance of the sediments and different water depths. For the sites at water depths greater than 10 m, where lake floor sediments consist of greenish black mud with very high TOC contents, the magnetic susceptibility and ARM decreased with increasing water depth, although the initial measurements in 2012 showed a slight increase in the ARM for water depths greater than 16 m. The ARM carriers were characterized by a narrow coercivity range around 40 mT, but this component was considerably reduced after sample storage for about 3 years. Although it is not clear whether the ARM carriers were the result of authigenic formation of iron sulfide or the deposition of material suspended in the hypolimnion water, we suggest that the magnetic properties of the surficial sediments on the lake floor are controlled by both their location in the basin and the limnological stratification of the brackish lake water. However, we conclude that freezing of samples immediately after recovery to avoid the effects of storage diagenesis is crucial for future environmental magnetic studies of organic-rich sediments. The results of our study may provide an analog for down-core variations of magnetic parameters associated with modification of magnetic minerals during reductive diagenesis.
We thank Yuko Okazaki, Hiromi Nakashima, Mutsumi Akimitsu, Katsuya Gotanda, Megumi Saito, Takeshi Haraguchi, Yoshitsugu Shinozuka, and Junko Kitagawa for their enthusiastic support during the fieldwork in Lake Ogawara. We also thank two anonymous reviewers and the editor, Yuhji Yamamoto, for valuable comments to improve the manuscript. This work was supported in part by grants from the Japanese Ministry of Education, Culture, Sports, Science and Technology (KAKENHI Grant Nos. 15K05321, 21101002, and 26101002).
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