Research paperAn automated method for varve interpolation and its application to the Late Glacial chronology from Lake Suigetsu, Japan
Highlights
► We establish a novel, automated method for varve interpolation. ► We describe Lake Suigetsu sediments (varve structure). ► We establish a varve chronology for Lake Suigetsu. ► We demonstrate the reliability of the interpolation by comparison with 14C data.
Introduction
Varved (annually laminated) sediments are palaeo-environmental archives that at the same time allow the construction of high precision age models, potentially down to a seasonal resolution (Brauer et al., 1999). However, a common problem is the occurrence of incompletely varved sections. Changes in the depositional environment may interrupt varve formation or result in partially indistinct records (Zolitschka et al., 2000), which therefore require interpolation. Commonly, interpolation is carried out manually, using sedimentation rate estimates from neighbouring, well varved sections. The main error source of this conventional interpolation approach is that sedimentation rates in compromised intervals (i.e. intervals with an incompletely developed varve record) and well varved intervals can be different. Also, the conventional interpolation cannot be applied to sediment profiles that do not show well varved intervals. The new approach presented here is based on an automated analysis of frequency distributions of seasonal layers from the compromised interval itself and therefore avoids this main problem associated with conventional varve interpolation. Moreover, since the interpolation method is computer based and automated it enables the reliable reproduction of a result, which is difficult to achieve when the interpolation is carried out manually. This novel approach is applied to the Lake Suigetsu sediment from the Last Glacial-Interglacial Transition (LGIT) (Table 1), which is an example of such an incompletely developed varve record.
The Suigetsu varves were first analysed by Kitagawa and van der Plicht, 1998a, Kitagawa and van der Plicht, 1998b, Kitagawa and van der Plicht, 2000, using a sediment core recovered in 1993 (SG93). They showed that, besides being annually laminated for much of its depth, the sediment also provides one of the most comprehensive atmospheric radiocarbon records, as it is rich in terrestrial leaf fossils. This makes it suitable for extending the atmospheric radiocarbon calibration model beyond the present IntCal tree-ring limit (12.55 ka cal BP (Reimer et al., 2009)) to >50 ka cal BP. However, the SG93 data significantly diverged from alternative, marine-based calibration datasets, due to gaps in the sediment profile and varve counting uncertainties (van der Plicht et al., 2004; Staff et al., 2010).
The Suigetsu Varves 2006 project aims to overcome the reported problems of the SG93 project. A new and continuous master profile was constructed (SG06), based on parallel cores from four bore holes, recovered in 2006 (Nakagawa et al., 2012). The varve interpolation program was devised to aid in the construction of an improved calendar age scale for the terrestrial SG06 radiocarbon calibration model. Therefore no information based on the 14C chronology can be used to complement the varve count as the varve chronology must be completely independent.
While this study focuses on the establishment of the new varve interpolation program and the results from microscopic varve counting, a second paper (Marshall et al., 2012) introduces an additional, novel and independent varve counting method utilising μXRF and X-radiography. The comparison of the results from the two methods, their individual strengths and weaknesses and the combination into the final Suigetsu varve chronology are given in Marshall et al. (2012). Hence, the LGIT varve chronology presented here, based on microscopic counting only, does not represent the final SG06 varve chronology.
Lake Suigetsu is situated in Fukui prefecture on the west coast of Honshu Island, central Japan. It is part of a tectonic lake system (Mikata Five Lakes) with the active Mikata fault running N–S less than 2 km to the east (Fig. 1). The lake is approximately 2 km in diameter and has a maximum water depth of 34 m (Nakagawa et al., 2005).
In AD 1664 a canal was built connecting Lake Suigetsu with Lake Kugushi (itself already connected to the sea), which resulted in the inflow of salt water into the previously fresh water lake and the subsequent formation of a chemocline between 3 and 8 m water depth, which now separates the lower salt water body and the upper fresh water layer (Masuzawa and Kitano, 1982; Kondo et al., 2009). Due to this artificial change in the hydrology, the majority of the Lake Suigetsu sediment formed under limnological conditions that are only partially comparable to those of the present.
The fresh water, that comprises the upper water body, flows into Lake Suigetsu from Lake Mikata through a shallow sill connecting the two lakes. Lake Mikata is fed by the Hasu river, which constitutes the only major fresh water source to the lake system. In this setting Lake Mikata acts as a natural filter for coarse detrital material from the Hasu river catchment. Therefore the sediment of Lake Suigetsu consists predominantly of autochthonous and authigenic material.
Section snippets
Varve description and counting
Sediment analysis and varve counting were carried out by thin section microscopy. For thin section preparation the LL-channel sediment sections (Nakagawa et al., 2012) were cut into 10 cm long segments and freeze dried. Afterwards the samples were impregnated with synthetic resin under vacuum. The blocks produced were glued to glass slides with the same resin and then ground and polished down to ≈20 μm (Brauer and Casanova, 2001). A petrographic microscope with magnification from 25× to 400×
Results
To anchor the floating SG06 varve chronology the SG06-1288 (U-Oki) tephra at 1288.0 cm composite depth (cd) (version 24 Aug 2009 (Nakagawa et al., 2012)) was used, which has a wiggle matched 14C age between 10.255 and 10.177 ka cal BP (at 95.4% probability range), with the median age being 10.217 ka cal BP (Staff et al., 2011).
Chronologically important micro-lithofacies boundaries occur at the onset of the LGIT interstadial (≈1729 cm cd) as well as the onset (≈1554 cm cd) and termination
Seasonal layer formation/preservation
The formation of mixed layers, which represent time windows of multiple years, is the main cause for the incomplete raw count. As already noted in the layer description, the mixed layer formation is possibly the result of years with a less pronounced or no stratification of the water body, and subsequently no seasonal lake overturn. This would still allow the production of the autochthonous and authigenic sediment components, but no mechanism for a separate deposition would be provided,
Conclusion
It has been shown that the new, automated varve interpolation method produces reliable results from varve records where on average as many as 50% of the varves are indistinguishable. Where manual interpolation approaches often suffer from subjectivity, the varve interpolation program (VIP) provides objectivity in the interpolation and error-estimation. Although the interpolation is automated, a detailed understanding of the data is necessary to evaluate the results and understand possible
Acknowledgements
We thank the German Research Foundation (DFG grants TA-540/3-1, BR 2208/7-1), the UK Natural Environment Research Council (NERC grants NE/D000289/1, NE/F003048/1, SM/1219.0407/001), the KAKENHI project of Japan (grant 211001002) and INTIMATE EU cost for funding. Furthermore, we thank two anonymous reviewers and S. Lauterbach for their constructive and helpful suggestions on the manuscript. We also thank H. Kitagawa and J. van der Plicht for inspiration for the project.
Editorial handling by: T.
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