The recent climatic change of subarctic zone recorded in core sediments of Lake Abashiri in the east part of Hokkaido, Japan
Abstract
In the coastal area of the Sea of Okhotsk in the east part of Hokkaido located to for subarctic zone, many brackish-water lakes are distributed. Especially, the Okhotsk brackish-water lake group around Abashiri City is constituted by major lake in Japan such as Lake Abashiri, Lake Mokoto, Lake Tofutsu, and Lake Notoro. The each lake shows a different present environment and history. Therefore, the change that is common in those lakes seems to be the change concerning the climate. In this study, recent environment change in Abashiri region (after the Little Ice Age) is discussed by sedimentologic and geochemical high-resolution analysis of the sediment cores collected from the Lake Abashiri. The water column of Lake Abashiri has a distinct halocline around 5m depths, and is divided into oligohaline surface waters and polyhaline bottom water by its halocline. The bottom water in Lake Abashiri shows the euxinic conditions throughout the annual. Therefore, surface sediment of below water depth 5m shows the black organic mud with the lamination. The 10AB-5C core collected from Lake Abashiri shows the length of 332cm. This core is composed of muddy sediment with a distinct lamination through all horizons. The Ta-a tephra (AD 1739) and Ko-c2 tephra (AD 1694) are found at the horizon of 250 cm, and 291 cm, respectively. Sedimentation rate based on these ages was 0.92cm/yr between Ko-c2 tephra and Ta-a tephra, and was 0.91cm/yr between surface and Ta-a tephra. Lamina set of 44 was recognized between Ko-c2 tephra and Ta-a tephra. This is suggested that this set is annual lamina. In 10AB-5C core, total organic carbon (TOC) contents, total sulfur (TS) contents, and C / N ratios were revealed by CNS elemental analysis. And the content of major elements were revealed by XRF elemental analysis. The change of iron (Fe) content synchronized with that of TS content. However, in the peak of TS contents around the horizon of 60cm, the Fe content does not change enough. It is considered that Fe is preserved the state of pyrite in this horizon. Phosphorus (P) contents show relative high values in top 20cm. The case of euxinic environment, phosphorus is not preserved in sediments by elution. This case is considered that phosphorus is preserved in the sediment due to the supply of excess phosphorus. This is evidence of eutrophication in Lake Abashiri. The horizons of 113-130cm and 60-70cm showing the relatively high lightness show a high P content and a low TS contents. This is considered that bottom environment of Lake Abashiri became to oxidative condition by desalination during these horizon. This change may be caused by slight regression with global cooling.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMPP41A1978S
- Keywords:
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- 1051 GEOCHEMISTRY / Sedimentary geochemistry;
- 1650 GLOBAL CHANGE / Solar variability;
- 4235 OCEANOGRAPHY: GENERAL / Estuarine processes;
- 4942 PALEOCEANOGRAPHY / Limnology