14:39 〜 14:57
[BPT01-05] Sedimentary geochemical record of human-induced environmental changes in the estuarine lake Nakaumi, southwest Japan
キーワード：海跡湖、湖底堆積物、人為汚染負荷、CNS、重金属類、中海
We investigated the distribution of organic- and abiotic- geochemical records in sediment from brackish lake Nakaumi to put the consequences of natural and artificial environmental factors affecting the lake environment into a larger context.
The eastern part of the Shimane Prefecture is a region of intense human activities since ancient times, and these activities are well documented in historical records. Lake Nakaumi is a brackish lagoon, which is located at the estuary of the Hii River system in the lower reaches of Lake Shinji-ko and is isolated from the Japan Sea by the Yumigahama peninsula. So, this maritime lake with geography enclosed setting is recording the effects of human activities in the lake sediments with a high temporal resolution.
We established the stratigraphy of bottom sediments at site Nk-3C in Lake Nakaumi based on three sediment cores using lithofacies, soft X-ray photography and CNS elemental analysis. Using the age model inferred from 137Cs, 210Pb and 14C dating, the sediment is estimated to be deposited for the past 600 years, and the sedimentation rates ranges between 0.24 and 0.38 cm/yr.
The C/N, which follows a generally increasing trend in the 18th century, indicating a relative increase of terrestrial organic materials. We suggest that a displacement in river course of the Hii and/or Iinashi rivers in the middle 17th century and resulting progradation of their deltas enhanced the dilution effect on C and N due to an increase in sedimentation rate at the core site. The sharp rise of C, N, and S after late 19th century reflects the eutrophication of Lake Nakaumi, caused by an array of anthropogenic impacts, such as the rapid increase of nutrient inputs due to industrialization and population growth, and the semi-enclosure of the lake due to land reclamation in the Sakai Strait, the outlet of Lake Nakaumi.
We also investigated the vertical distribution of abiotic elements mainly heavy metals by using XRF analyzer and ICP-MS. There, we found a significant trend in Cu and Mo, which can be the distinct time indicators for the bottom sediment in Lake Nakaumi. Cu sharply increased from the 19th century, which is earlier than initiation of heavy metal increase in common sediments from Japanese urban bays (e.g. Yasuhara et al., 2007) due to industrial development at the beginning of 20th century. It reflects the Cu loads to the lake through the River Iu from Homan-zan copper mine which started in middle 19th century and reached their peak in early 20th century. Mo displayed three peaks in the early 1900s, 1940-50 and 1960-70, which indicated remarkable correspondence with the changes in domestic demand for molybdenum. Molybdenum, which was used as an additive element in steel, was mined in the Daito mine from 1912 to 1985 on the upper reaches of the Hii River. This mine accounted for most of the Mo supply in Japan at that period. Accordingly, these trends in Mo is considered to be reflected by the load from this mine.
The pollution in Lake Nakaumi can be linked to copper mining activity in the catchment area from the 18th century. Interestingly, the clear trend in heavy metal pollution (Cu, Pb, Zn and As) predates eutrophication and also nationwide trends of heavy metals. Most of the metals peaked around 1970 CE and decreased in general due to the decline of industrial activities after the rapid economic growth and emission regulations, but Pb, Zn, As and In displays recent increasing trends. In our presentation we will also discuss the trends of other heavy metals, including the possibility of long-distance transport from Asian continents.
Reference:
Yasuhara, M. et al. (2007) Limnology and Oceanography, 52: 1633–1644.
The eastern part of the Shimane Prefecture is a region of intense human activities since ancient times, and these activities are well documented in historical records. Lake Nakaumi is a brackish lagoon, which is located at the estuary of the Hii River system in the lower reaches of Lake Shinji-ko and is isolated from the Japan Sea by the Yumigahama peninsula. So, this maritime lake with geography enclosed setting is recording the effects of human activities in the lake sediments with a high temporal resolution.
We established the stratigraphy of bottom sediments at site Nk-3C in Lake Nakaumi based on three sediment cores using lithofacies, soft X-ray photography and CNS elemental analysis. Using the age model inferred from 137Cs, 210Pb and 14C dating, the sediment is estimated to be deposited for the past 600 years, and the sedimentation rates ranges between 0.24 and 0.38 cm/yr.
The C/N, which follows a generally increasing trend in the 18th century, indicating a relative increase of terrestrial organic materials. We suggest that a displacement in river course of the Hii and/or Iinashi rivers in the middle 17th century and resulting progradation of their deltas enhanced the dilution effect on C and N due to an increase in sedimentation rate at the core site. The sharp rise of C, N, and S after late 19th century reflects the eutrophication of Lake Nakaumi, caused by an array of anthropogenic impacts, such as the rapid increase of nutrient inputs due to industrialization and population growth, and the semi-enclosure of the lake due to land reclamation in the Sakai Strait, the outlet of Lake Nakaumi.
We also investigated the vertical distribution of abiotic elements mainly heavy metals by using XRF analyzer and ICP-MS. There, we found a significant trend in Cu and Mo, which can be the distinct time indicators for the bottom sediment in Lake Nakaumi. Cu sharply increased from the 19th century, which is earlier than initiation of heavy metal increase in common sediments from Japanese urban bays (e.g. Yasuhara et al., 2007) due to industrial development at the beginning of 20th century. It reflects the Cu loads to the lake through the River Iu from Homan-zan copper mine which started in middle 19th century and reached their peak in early 20th century. Mo displayed three peaks in the early 1900s, 1940-50 and 1960-70, which indicated remarkable correspondence with the changes in domestic demand for molybdenum. Molybdenum, which was used as an additive element in steel, was mined in the Daito mine from 1912 to 1985 on the upper reaches of the Hii River. This mine accounted for most of the Mo supply in Japan at that period. Accordingly, these trends in Mo is considered to be reflected by the load from this mine.
The pollution in Lake Nakaumi can be linked to copper mining activity in the catchment area from the 18th century. Interestingly, the clear trend in heavy metal pollution (Cu, Pb, Zn and As) predates eutrophication and also nationwide trends of heavy metals. Most of the metals peaked around 1970 CE and decreased in general due to the decline of industrial activities after the rapid economic growth and emission regulations, but Pb, Zn, As and In displays recent increasing trends. In our presentation we will also discuss the trends of other heavy metals, including the possibility of long-distance transport from Asian continents.
Reference:
Yasuhara, M. et al. (2007) Limnology and Oceanography, 52: 1633–1644.