Carbon sink potential of eutrophic lake sediments: a reconstruction of the potntial in lakes Nakaumi and Shinji during the last about four hundred years

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Carbon sink potential of eutrophic lake sediments from lakes Nakaumi and Shinji was studied for recent and several hundred years environments using surface sediments and sediment cores.TOC (total organic carbon) contents of southern and central L. Nakaumi showed high sapropelic about 3.6~4.0% in the 1cm surface sediments. These values are constant from AD1997. TOC contents of all samples from L. Shinji showed also sapropelic and high about 3.5~4.3% except for eastern part. These TOC values in L. Shinji are not changed during the last 5 years. According to the result of C/N ratio (7-9) and py-GC-MS, origin of organic matter in both lakes is supposed to be predominant phyto-plankton. Sediment cores showed low TOC contents about 1% between AD1800 and 1900 in both L. Nakaumi and Shinji and TOC contents drastically increased from 1% to 4% after AD1900. Analysis of AD2003 data by our preliminary research indicates that TOC contents over 3.5% of surface sediments have a good positive correlat … More ion with H2S (ppm), showing an equation: H2S (ppm) = 13.9 * TOC (%) - 52.1 (TOC>3.5%). Atmospheric CO2 concentration on the northeastern Nakaumi was low about 360 ppm in average (April to May, 2013), which could depend on spring blooming of phyto-plankton. Impressive difference of the CO2 concentration between daytime and nighttime was up to 80 ppm. Average OCARs in the surface sediments of L. Nakaumi and Shinji are 7.0 mgC/cm2/y and 4.0 mgC/cm2/y，respectively based on the sedimentation rate reported by Kanai et al., (1998, 2002). As a result, L. Nakaumi and Shinji have a potential carbon sink of about 6,000 tonC/y and 3,200 tonC/y, respectively. Low OCARs of the cored sediments with low TOC about 1% in L. Nakaumi during 50-25 cm (1770-1890) and TOC about 0.7% in L. Shinji during 30-20cm (1830-1890), i.e. Little Ice Age, suggested 2,000 tonC/y and 1,300 tonC/y, respectively. These low values during cool climate are about one - third of the present potential. These brackish lakes are characterized as a quick response carbon sink for climate change. Less