[MIS10-P34] Sedimentary disturbance as paleoenvironmental proxies recorded in Lake Suigetsu, central Japan
Keywords:Suigetsu, varve, bioturbation, image analysis
Lake Suigetsu (35°35'N, 135°53'E, 0 m a.s.l.), Fukui prefecture, central Japan, is one of the Mikata Five lakes, which measures 3 km east-west by 3 km north-south and the maximum water depth of ca. 34 m. Lake Suigetsu preserves annually laminated sediments over the last ca.70 kyr with a significant number of event layers. The visibility of the laminae is not homogeneous and varies depending on the age. Two long cores (SG93, SG06) have previously been recovered, and an exceptionally precise age model has been established through a combination of over 800 radiocarbon dates and high precision varve counting. In 2014, a new core (SG14) was recovered by the Fukui prefectural government from a point ca. 320 m to the east of the SG06 coring site. An age model of the SG14 core was established by visual correlation to the well-dated SG06 core using event layers.
Sedimentary disturbance intensity refers to the relative amount of disturbed sediments, indicating estimation of the initial condition is one of the key to the reconstruction. In this study, we reconstructed relative sedimentary disturbance from varved structures on the basis of image analysis using high-resolution sedimentary core images. Because the sediments oxidise rapidly after collection, we promptly cut the cores in half and took high-resolution photographs before the colours changed under controlled, uniform lighting conditions in a darkroom.
The disturbance of the lamination structures can be regarded as the dividing process of a line segment in a ‘broken-stick model’. When a line segment is divided at random points, the length of divided lines follows the exponential distribution with parameter lambda. Here, lambda value of exponential distribution is a reciprocal of average lamina length and can be regarded as an index of the sedimentary disturbance intensity. In this study, we measured natural lamina length of varved sediments from images and calculated lambda values as disturbance intensities.
Reconstructed lambda value fluctuations mirrors described facies change, resulting the index can be regarded as a better index for interpreting the facies. The value fluctuates with ten millennial-scale are synchronous with NGRIP δ18O fluctuation, especially 15 to 35 ka. The centennial to millennial fluctuations are sometimes are matched with that of NGRIP. These results indicate the lambda values probably reflect at least paleoclimate changes. As a result, reconstructed lambda values not only provide facies proxy but also are good indicator of the paleoenvironmental proxies.