The coral δ15N record of terrestrial nitrate loading varies with river catchment land use
We analysed the nitrogen isotopes in two coral cores (δ15Ncoral) from the mouth of the Todoroki River, Ishigaki Island, Japan, to examine whether the δ15Ncoral reflects the run-off of nitrate related to the land use in the river catchment. The two coral cores were used to examine the seasonal variation in δ15Ncoral for 14 years (CORE1; 1993–2007) and the annual variation of δ15Ncoral for 52 years (CORE2; 1958–2010). In CORE1, the 5-month running mean of δ15Ncoral was positively correlated with that of monthly precipitation, excluding all strong precipitation events (>150 mm d−1). In CORE2, the δ15Ncoral mean in the earlier period (1958–1980) was 1.0 ‰ greater than that in the later period (1981–2010). The annual averages of δ15Ncoral are positively correlated with the total precipitation in the rainy season (May–June) for both time periods. The difference in the δ15Ncoral between the earlier and later periods is probably caused by the land use changed from paddy fields with 15N-rich manure to sugar cane fields in the early 1980s. Although some uncertainties still remain regarding the precision of δ15N coral proxy records, this study emphasises the clear potential for their use in reconstructing terrestrial nitrate discharge records from corals.
KeywordsCoral skeletons Nitrogen isotopes Nitrate Land use River discharge Coral reefs
Coral and water sampling for this study were supported by CREES members of Hokkaido University, Toshiaki Tanaka, Teppei Sagawa, Satoshi Maekawa, and the WWF Coral Reef Conservation and Research Centre. Coral core cutting was supported by Masataka Ikeda. The research was funded by a Grant-in-Aid for Scientific Research on Innovative Areas ‘Coral reef science for symbiosis and coexistence of human and ecosystem under combined stresses’ (#20121004) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. A. Yamazaki was supported by Grants-in-Aid for JSPS research fellows.
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