open access

https://doi.org/10.14943/doctoral.k14196

https://hdl.handle.net/2115/96122

サンゴ骨格中の古環境指標及び成長パラメータに記録された生息環境の撹乱と石灰化応答

英語

2020-09-25

Hokkaido University

137p

Coral reefs are currently facing numerous disturbances that have multiple and mixed components of natural and anthropogenic factors at a local and global scale(i.e., climate change, temporary meteorological phenomena, human activity, and land-use change). Annual mean precipitation in the high latitudes and the equatorial Pacific are likely to increase, and extreme precipitation events will likely become more intense and more frequent. There is also an expected area with the potential occurrence of a large earthquake (Mw>8) and tsunami event.The intensity of human activities is also expected to increase. These factors would exacerbate the risk of reef disturbances. Despite this situation, there are many coastal areas in tropics and subtropics where periodic monitoring surveys and in situ environmental data are not available. While, a massive coral skeleton has the long-term archives of environmental change with a high temporal resolution, and they should be a useful tool for the quantitative evaluation for the impact of the reef disturbance and coral calcification response. It can be essential to the discussion of survival strategies for reef-building corals in which extreme rainfall occurrence, large earthquake-tsunami occurrence, and human activities will increase. Here we show the reef disturbance archives in coral sclerochronological proxies and the responses on skeletal calcification using the massive coral skeletons (e.g., Porites sp.). This study showed how natural (e.g., earthquake-related seafloor displacement, tsunami, heavy rainfall, river runoff, and mangrove systems) or anthropogenic (e.g., local industrial history and land use) factors lead to a reef disturbance, and how corals respond to these disturbances. The study sites and focuses were 1) Simeulue Island, Indonesia: Earthquake, seafloor displacement,and tsunami event, 2) Kikai Island, Japan: Tsunami event, 3) Amami-Oshima Island, Japan: Heavy rainfall, river flood event, and mangrove system, and 4) Amami-Oshima Island, Japan: Local industrial changes and land use, as summarized as below; 1) The remarkable signals of the 2004 and 2005 earthquake events (including coseismic seafloor displacement and tsunami) were observed on the surface of coral skeletons (the stress bands; the green band; incorporation of the small particles like clay materials), skeletal growth parameters (the significant decrease in the annual extension/calcification rate corresponding to the coseismic seafloor displacement; the significant mean difference between pre- and post-earthquake), and skeletal δ13C (step change corresponding to the coseismic seafloor displacement). The earthquake events led to the coral growth disturbance and the change in the coral light availability. Coral paleo-earthquake archive will provide us with local information on crustal deformation at the decimeter scale. 2) The remarkable signals of the 1911 tsunami event were observed on skeletal Ba/Ca (a high peak) and skeletal growth parameters (a significant decrease in the annual extension/calcification rate). This study showed the evidence fora new possibility of the skeletal Ba/Ca as a past-tsunami proxy. The tsunami-related sediment load in the seawater could temporarily cause low water visibility (i.e., high turbidity). Though the tsunami event occurs in the very short-term, the tsunami-related sediments will harm coral calcification, and then the annual skeletal calcification rate reflected the tsunami effect. 3) The remarkable signals of heavy rainfalls and river floods were observed on the skeletal Ba/Ca (a high peak in the isolated <2 years trend data) and temporary skeletal growth parameters(the stress bands; the increasing ΔSr/Ca). These results suggested the increase in the sediment load in the seawater due to river flood events. Especially when the extreme-heavy rainfall or devastating river flood occurred, skeletal Sr/Ca, skeletal Mg/Ca,and coral derived δ18Osw showed the temporal low-SST signal or low-SSS (due to the cold freshwater plume or the cold heavy precipitation). Moreover, the skeletal Ba/Ca record in this study was mainly controlled by the amount of the desorbed Ba2+(in the estuary) from the Ba-containing sediment transported by the rivers and mangrove forests. The skeletal Ba/Ca in the corals that inhabit near the mangrove area or estuarine reflects the mangrove system (affected by both tide level and precipitation) and SST variation on the seasonal scale. 4) The remarkable signals of local industrial history and land use were observed on the skeletal Ba/Ca (isolated >2 years trend data) and annual skeletal growth parameters. These proxies would reflect the local industry (e.g., traditional silk fabric)and land use for 46 years. The sensitivity (response) of the reef-building coral to sediment load would reflect the change in the rhythm of sediment outflow into the bay, corresponding to the local industrial history. These study cases suggested that the combined methodology, coral geochemical proxies with skeletal growth parameters, provides a quantitative scale of the impacts of natural and anthropogenic reef disturbance with a long time series. The skeletal geochemical proxies recorded natural/anthropogenic reef disturbance events, and coral calcification responded to these reef disturbances. The natural/anthropogenic disturbance event will be one of the controlling factors of coral calcification. Reef disturbance archives in the coral sclerochronological proxies will provide us the worthy key for understanding the skeletal calcification responses to the habitat environmental change and creating a sustainable healthy relationship in coral reefs and human society.

2020-09-25

甲第14196号

博士(理学)

(主査) 教授 永井 隆哉, 特任教授 竹下 徹, 講師 渡邊 剛, 助教 山崎 敦子（九州大学理学研究院）

理学院（自然史科学専攻）

博士論文

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