5:15 PM - 6:45 PM
[AHW22-P20] Origin and budget of black carbon in sediments of the Akkeshi Bay, the eastern Hokkaido, Japan
Keywords:black carbon, marine sediments, Akkeshi Bay, eastern Hokkaido
Silicate weathering on land is an acid-base reaction between rocks and precipitation in which atmospheric carbon dioxide is dissolved, reducing atmospheric carbon dioxide over long time scales, while oxidation of organic carbon in sedimentary rocks is thought to be a carbon release source on a comparable scale. Although marine sediments are the ultimate sinks in the Earth’s carbon cycle, fine-grained sedimentary rocks such as black shale exposed on land could be a source of carbon. Such sedimentary rocks contain persistent organic matter, which is expected to be redeposited intact in marine sediments.
Although the organic carbon in marine sediments is thought to be a mixture of marine biogenic and terrestrial plant sources, some of the organic carbon may also be derived from terrestrial sedimentary rocks, which can remain sequestered from the atmosphere for long periods of time, and a quantitative understanding of the carbon cycle in the Earth's surface layer is needed to understand the carbon cycle. Such organic carbon is often referred to as black carbon (BC), which can also be of combustion origin, and is estimated to account for 5-30% of the organic carbon in marine sediments. Therefore, the mineral composition, total organic carbon (TOC), and stable carbon isotope ratios were determined for marine sediments, shales in the hinterland, and fluvial sediments in Akkeshi Bay and surrounding area in the eastern Hokkaido, Japan. In addition, we measured BC which could be originated from biomass burning and shales using chemo-thermal oxidation, and tried to differentiate them using stable carbon isotope ratios to infer their origins. In addition, the origin of organic carbon (OC = TOC - BC) was also inferred.
The mineral composition of the sediments in the Akkeshi Bay is explained by the mixing of sediments outside the bay, outcrops in the vicinity, and beach sand. On the other hand, the TOC in the sediments of the Akkeshi Bay is more abundant than that in the outcrops and beach sands, suggesting that marine biogenic and fluvial sources are the main sources of TOC. The stable carbon isotope ratio of TOC in marine sediments is -22.9--21.0‰, while the stable carbon isotope of OC, excluding the BC contribution, is -25.0 --19‰. The BC accounts for 13-60% of the sediment TOC. Stable carbon isotope of BC ranged from -27.8--13.6‰ in marine sediments, suggesting a mixture of BCs from shale with high value and terrestrial biomass burning with low value. However, comparison with mineral compositions also suggests the presence of other sources.
Although the organic carbon in marine sediments is thought to be a mixture of marine biogenic and terrestrial plant sources, some of the organic carbon may also be derived from terrestrial sedimentary rocks, which can remain sequestered from the atmosphere for long periods of time, and a quantitative understanding of the carbon cycle in the Earth's surface layer is needed to understand the carbon cycle. Such organic carbon is often referred to as black carbon (BC), which can also be of combustion origin, and is estimated to account for 5-30% of the organic carbon in marine sediments. Therefore, the mineral composition, total organic carbon (TOC), and stable carbon isotope ratios were determined for marine sediments, shales in the hinterland, and fluvial sediments in Akkeshi Bay and surrounding area in the eastern Hokkaido, Japan. In addition, we measured BC which could be originated from biomass burning and shales using chemo-thermal oxidation, and tried to differentiate them using stable carbon isotope ratios to infer their origins. In addition, the origin of organic carbon (OC = TOC - BC) was also inferred.
The mineral composition of the sediments in the Akkeshi Bay is explained by the mixing of sediments outside the bay, outcrops in the vicinity, and beach sand. On the other hand, the TOC in the sediments of the Akkeshi Bay is more abundant than that in the outcrops and beach sands, suggesting that marine biogenic and fluvial sources are the main sources of TOC. The stable carbon isotope ratio of TOC in marine sediments is -22.9--21.0‰, while the stable carbon isotope of OC, excluding the BC contribution, is -25.0 --19‰. The BC accounts for 13-60% of the sediment TOC. Stable carbon isotope of BC ranged from -27.8--13.6‰ in marine sediments, suggesting a mixture of BCs from shale with high value and terrestrial biomass burning with low value. However, comparison with mineral compositions also suggests the presence of other sources.