Mass accumulation rate of detrital materials in Lake Suigetsu as a potential proxy for heavy precipitation: a comparison of the observational precipitation and sedimentary record
© Suzuki et al. 2016
Received: 23 July 2015
Accepted: 18 January 2016
Published: 12 February 2016
In the densely populated region of East Asia, it is important to know the mechanism, scale, and frequency of heavy precipitation brought about during the monsoons and typhoons. However, observational data, which cover only several decades, are insufficient to examine the long-term trend of extreme precipitation and its background mechanism. In humid areas, the transport flux of a suspended detrital material through a river system is known to have an empirical power relationship with precipitation. Thus, the sedimentation flux of a fine detrital material could potentially be used as a proxy for reconstructing past heavy precipitation events. To test the idea that the sedimentation flux of detrital materials records past heavy precipitation events (e.g., typhoons), we focused on the detrital flux estimated from the annually laminated sediment of Lake Suigetsu, central Japan, which is capable of accurately correlating the age of detrital flux with the precipitation record. We first established a precise age model (error within ±1 year in average) beginning in 1920 A.D. on the basis of varve counting fine-tuned by correlation between event layers with historical floods. The flux of the detrital material (g/cm2/year) was estimated on the basis of Al2O3 content (wt%), dry bulk density (g/cm3), and sedimentation rate (cm/year) calculated from the age model. The detrital flux of background sedimentation showed a weak positive correlation with annual and monthly (June and September) precipitation excluding heavy precipitation that exceeded 100 mm/day. Furthermore, the thickness of instantaneous event layers, which corresponds to several maxima of detrital flux and is correlated with floods that occurred mainly during typhoons, showed a positive relationship with the total amount of precipitation that caused a flood event. This result suggests that the detrital flux maxima (deposition of event layers) record past extreme precipitation events that were likely associated with typhoons that hit the middle part of Honshu Island. Based on this result, the record of typhoon-caused flood events can go back to older period (e.g., last glacial period) on the basis of the occurrence, and thickness, or mass flux of event layers using long sediment cores from Lake Suigetsu.
The relationship represented by Eq. (1) is known as a rating curve [Arnell 1992; Kazama et al. 2005; Yang et al. 2007; Sadeghi et al. 2008; Kaji and Nihei 2014], where a and b are constants specific to an individual river basin. Since the water discharge of a river system is proportional to the precipitation in its river [Arnell 1992; Milliman and Syvitski 1992; Yang et al. 2007], we can estimate precipitation from the flux of the detrital material recorded in sediments. Because the river water discharge exponentially affects the detrital flux, this method should be especially sensitive to the heavy precipitation events. Therefore, this method is useful for sediments that have high-resolution and precise age-depth models and are found in regions that suffer from heavy precipitation events such as monsoons and typhoons. Thus, this method can be applied to lake sediments in Asia, which have high sedimentation rate and precise age models, and are subjected to heavy precipitation events.
Lake Suigetsu (35° 35′ N, 135° 53′ E) in central Japan is known to have annually laminated (varved) sediment from 70 to 11.6 ka, and also from ~350 years ago to the present [Fukusawa et al. 1994; Marshall et al. 2012; Nakagawa et al. 2012; Schlolaut et al. 2012]. Also, high-resolution 14C dating has been conducted during the past 52.8 kyrs (average resolution of ~100 years) [Staff et al. 2011; Bronk Ramsey et al. 2012], allowing the high-resolution estimation of detrital flux throughout the past 70 kyrs. Herein, we examined whether and how heavy precipitation events due to typhoons were recorded in the detrital materials of Lake Suigetsu sediments by comparing the flux of the detrital material reconstructed from the Lake Suigetsu sedimentary record and the observational precipitation record spanning the last 80 years.
Hydrological, geographical, and climatological settings
A short gravity core named SG12-LM3 with a length of approximately 25 cm was recovered from the central part of Lake Suigetsu using a Limnos core sampler [Kansanen et al. 1991] on July 6, 2012. Because a Limnos sampler does not have any mechanical actuator, it can retrieve the sediment-water interface without any disturbance. To correlate and reveal the mechanism of the sedimentation of event layers, additional Limnos core sample was conducted at multiple sites on June 20 and 21, 2014. The sampling points are shown in Fig. 1c, and half-split scanned images are shown in Fig. 9. To determine the chemical composition of the suspended particle material from the Hasu River, 100 liters of river water was collected at the mouth of the Hasu River on October 1, 2012, immediately after the strong precipitation event that occurred during the 17th typhoon in 2012, which went through the central part of Honshu Island. The water was filtered using a Millipore filter (pore diameter = 0.45 μm), and the filtered suspended particle material of the Hasu River was retrieved from the filtrating paper using deionized water.
SG12-LM3 was sent to the University of Tokyo within 40 days after sampling and stored vertically in a refrigerator at 5 °C for half a year to let the top part of the sediments compact to avoid fluidization during core splitting. The core was then pushed into a tube of the same diameter that was assembled using two tubes cut in half. Next, two thin plastic boards attached together were inserted into the slit between the two half-cut tubes. Slab samples (20 × 5 × 0.7 cm) were taken from a half-split core and subjected to soft X-ray analysis using a SOFTEX CMB-2 in the filming condition of 40 kV/2 mA/5 min exposure time. The remaining part of the core was sliced into subsamples with thicknesses of 6 mm, corresponding to a resolution of 2–3 years. The sliced samples were weighed in wet condition (W w, g) and then freeze-dried for more than 30 h. The dried samples were weighed (W d) again. Grain density (GD, g/cm3) was measured for dried samples using a He pycnometer (AccuPyc 1330, Micrometrics Instrument Co.) at the Atmospheric and Ocean Research Institute at the University of Tokyo.
Measurement of major elements
From each dried sample (including suspended particle material sampled from the Hasu River), ~0.6 g was split and used to make a glass bead. Before making the glass bead, each sample was ignited, and the loss on ignition was calculated. X-ray fluorescence (XRF) analysis was conducted to determine the concentrations of 10 major elements (Al2O3, SiO2, TiO2, Fe2O3, MnO, P2O5, Na2O, CaO, MgO, and K2O) using an XRF spectrometer (PANalytical Axios) equipped with an Rh tube. The 1σ values of the measurements were ±0.046 % for SiO2, 0.0035 % for TiO2, 0.019 % for Al2O3, 0.0067 % for Fe2O3, 0.0018 % for MnO, 0.0058 % for MgO, 0.003 % for CaO, 0.0072 % for Na2O, 0.0057 % for K2O, and 0.0035 % for P2O5.
Measurements of radioactive nuclides
Using several sliced and dried samples, the radioactivities of 137Cs, 210Pb, and 214Pb were measured for 14 selected samples using an ORTEC GWL-120230-S HPGe (High-Purity Germanium) coaxial well photon detector system with an inner diameter of 17 mm and an active well depth of 40 mm at Hokkaido University. 137Cs is a fallout product of nuclear testing, which enables us to constrain the age of the 137Cs peak to the peak age of nuclear testing 1963 A.D. on the basis of the record of nuclear testing [Delaune et al. 1978].
Excess 210Pb, defined as the difference between the radioactivities of measured 210Pb and 214Pb, is a type of radioactive nuclide provided from the ground surface through the atmosphere that decays with a half-life time of 22.3 years. This nuclide can be used to estimate the mean mass accumulation rate (MAR, g/cm3/year) of the sediment assuming that sedimentation flux is constant. Here, 210Pb is assumed to come from the sediment itself and the ground surface. 214Pb is equal to the activity of sediment-originated 210Pb, assuming a constant mass accumulation rate (g/cm2/year) and the radiation equilibrium of the uranium series have been established [Kato et al. 2003]. One to two grams of a dried sample was weighed within an error of ±0.5 mg and placed into a plastic tube. The tube was then placed in the well-shaped Ge detector. The measurement time was 48 h. The gamma-ray spectra were obtained using a Seiko EG&G MCA7600 multichannel analyzer. The analytical errors (detection limits) of 137Cs, 210Pb, and 214Pb were 0.006 (0.018) Bq/g, 0.28 (0.83) Bq/g, and 0.010 (0.030) Bq/g, respectively.
Lithology and petrographical observation
Based on the macroscopic observation of a half-split core and the microscopic observation of smear slides, the Lake Suigetsu sediment is dominantly composed of diatom frustules, clay- to silt-sized detrital materials, organic materials, and black-colored minerals showing grain aggregation considered to be siderite or pyrite formed in the water column. Clear ~ mm-scale lamination composed of alternating black-gray (mainly diatom shells and organic material), brown (mainly diatom frustules and aggregated siderite), and gray (mainly detrital material) layers are observed throughout the core.
Next we show 4 steps showing procedure of constructing an age model. Varve counting was conducted based on soft X-ray images to construct an age model for the SG12-LM3 core. Each varve is defined as a set of a continuous lower light (high-density) layer and upper dark (low-density) layer observed in the soft X-ray image. The basal boundary of each varve was determined as the steepest point of the change in brightness change from the underlying dark (low-density) layer to the overlying light (high-density) layer; the former represents the organic amorphous material accumulated during summer, and the latter represents the siderite and/or clay layer accumulated during winter [Schlolaut et al. 2012]. Following the previous varve counting of Lake Suigetsu sediments (Schlolaut et al. 2012), when the varve does not have clear density contrast or is not horizontally continuous (hereafter called unclear varve), we consider that it formed at a rate of 1 layer per 0.5 ± 0.5 years (Additional file 1: Figure S1).
The laminae, which have sharp basal contact with the underlying layer and gradual contact with the overlying layer in the soft X-ray images and are characterized by higher density (light gray to white in the images) than adjacent laminae, were defined as event layers (Additional file 1: Figure S1). Event layers were excluded from the annual layer counting because they were considered to have accumulated in a short time on the basis of the bottom’s sharp contact. Because there was no clear lamination from depths of 17.3 to 19.1 cm, we considered this part as temporarily bioturbated and interpolated the sedimentation rate (cm/year) based on the average varve thickness in other parts (2.5 ± 1.2 mm/year).
Verification of varve count age using the 210 Pb and 137Cs results
As a second step, we used 210Pb and 137Cs to verify age model based on varve counting. Figure 2 D shows the depth profiles of excess 210Pb and 137Cs concentrations. The profile of 15 210Pb data against mass accumulation (g/cm2) was fit by a decaying curve, as shown in Fig. 2 F. The age model based on 210Pb (Fig. 2e) shows an age of 1925 A.D. (±3) at the bottom of the core. At the same point, varve counting suggested an age of 1930 A.D. (±10 varve count years). Therefore, the 210Pb-based age-depth model agrees with the varve count age model within the error.
The first appearance of 137Cs is known to correspond to the beginning of nuclear testing in 1954 A.D., and the peak 137Cs concentration corresponds to 1963 A.D., when nuclear testing was at its peak [Delaune et al. 1978]. In Fig. 2d, 137Cs appears (1954 A. D.) at a depth of 15.4–15.8 cm, while the depth of the varve count age of 1954 A.D. is at a depth of 15.2–20.4 cm. 137Cs has a clear peak (1963 A. D.) at a depth of 14.0–15.2 cm, while the depth of the varve count age of 1963 A.D. is 13.4–15.8 cm. These results show that the varve count age model is consistent with the 137Cs-based ages within the error (±10 years).
Fine-tuning based on the correlation between event layers and flood events
Estimation of age uncertainty using a Bayesian approach
To verify the reliability of the fine-tuned age model, we conducted Bayesian analysis on the basis of study by Bronk Ramsey (2009) using OxCal software (http://c14.arch.ox.ac.uk). We used sets of varve counts and their errors sandwiched by two neighboring control points (a set is called “si = sample interval”, Additional file 1: Figure S2A, Additional file 2: Table S2) to estimate the age difference between centers of neighboring “si” using Bayesian analysis. In the analysis, varve counts and its errors are used as prior probability, and age of control points (Table 1) are used for constraints. The results (Additional file 1: Figure S2B, Additional file 2: Table S3) indicate that our estimations of the errors in ages for the fine-tuned age model are in agreement with the posterior probability estimates of the Bayesian analysis. Therefore, we verified the fine-tuned age model. We did not use a Bayesian age model because it does not provide annual resolution. For the error estimation, we adopted a probability estimation based on Bayesian analysis because it narrows the age estimation error (Additional file 2: Table S1).
Estimation of detrital flux
XRF data of the SG12LM3 samples and suspended material from the Hasu River
Distance from the core top (cm)
XRF data (wt%)
F det varied widely from 10 to ~100 mg/cm2/year (Fig. 5 D); in samples including event layers, F det shows maxima ranged from 10 to 100 mg/cm2/year, whereas it ranged from 10 ~ 35 mg/cm2/year in samples not including event layers. F det reached the maximum at 1965 A.D., when this region received over 400 mm of continuous precipitation within 1 week; this was the most significant precipitation event in the past 100 years and was caused by two typhoons that directly hit the area. The variations in F det were closely associated with SR, suggesting that SR (nearly equal to the varve thickness (+ event layers)) had a stronger effect on F det than C det or DBD. The samples with an extremely short interval (less than 1 year) were excluded in the following discussion because of the large estimation error of the F det due to the large uncertainty of the SR (SG12LM3-22, depth = 12.55 to 13.1 cm, red shaded in Fig. 5). The flux of total detrital material calculated here was 5- to 10-times larger than the eolian dust fluxes in SW Honshu and the south-central part of the Sea of Japan, which were estimated as less than 2 mg/cm2/year by observation [Osada et al. 2014] and analysis of sediment core [Irino and Tada 2000; Nagashima et al. 2007], respectively. Therefore, herein, we did not separate the contribution of eolian dust.
Results and discussion
Relationship between detrital flux and precipitation in Lake Suigetsu
The F det values of samples without event layers showed a weak positive correlation (R = 0.43) with annual precipitation. However, except for one sample characterized by high precipitation and high F det values, the correlation is not so clear.
Relationship of event layers and flood events
A highly precise age model of the near-surface sediments of Lake Suigetsu was established on the basis of varve counting combined with a verification of the age estimation using excess 210Pb and 137Cs dating. The model was fine-tuned by correlating event layers and observed flood events. Using sedimentation rates calculated from the developed age model, XRF data, and dry bulk density data, the flux of detrital material was reconstructed with a resolution of 2–3 years. The estimated detrital flux of background sedimentation showed a weak positive correlation with annual and monthly (June and September) precipitation excluding heavy precipitation that exceeded 100 mm/day. Furthermore, event layer thickness corresponded to the maxima of detrital flux was positively related to the total amount of precipitation during a flood event [(thickness of event layer: mm) = 0.34 + 0.0023 × (precipitation: mm); R = 0.74]. These results indicate that the detrital flux maxima (deposition of event layers) record the occurrence and magnitude of flood events caused by typhoons. The typhoons that trigger event layer deposition tend to pass through the northwest part of Honshu Island, where they receive an abundant vapor supply from the Sea of Japan.
We would like to express our thanks to SG12 project members, the drilling team of Seibushisui Co. Ltd., and staffs at Jomon Museum in Wakasa town for sampling and safekeeping during the collection of SG12LM cores. Mr. Kojima in Jomon Museum gave us compiled data of historical floods in Wakasa town. We also thank Dr. Ashi and Dr. Omura of AORI, University of Tokyo, for allowing us to use the He pycnometer. Dr. Yoshida and Mr. Kobayashi of the University of Tokyo helped us operate the XRF spectrometer. The authors would like to thank Enago (www.enago.jp) for the English language review. This research is supported by the JSPS KAKENHI Grant KIBAN-S Number 23221002.
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