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Evidence for the Thermal Bleaching of Porites Corals From 4.0 ka B.P. in the Northern South China Sea
Author(s) -
Xu Shendong,
Yu Kefu,
Tao Shichen,
Wu ChungChe,
Wang Yinghui,
Jiang Wei,
Wang Shaopeng,
Shen ChuanChou
Publication year - 2018
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1002/2017jg004091
Subject(s) - porites , coral , reef , oceanography , zooxanthellae , coral bleaching , geology , sea surface temperature , paleontology , symbiosis , bacteria
Coral bleaching is becoming a serious issue for coral reefs under the stress of global warming. However, whether it has occurred in the past in times of thermal stress remains unclear. Moreover, an understanding of historic coral bleaching events would greatly improve our insight into the adaptive capabilities of corals under such stresses. It is known that Porites corals, a massive coral, have relatively high levels of symbiotic zooxanthellae and a strong thermal tolerance when compared with most other corals (and particularly branched corals). Thus, growth hiatuses and/or mortality surfaces of fossil Porite s may be used to indicate past ecological or environmental stress events, such as severe bleaching. In this study, monthly geochemical and isotopic environmental proxies of four fossil Porites corals with well‐preserved growth hiatuses and mortality surfaces (aged 3,800–4,200 years before 2013 A.D.), collected from Wenchang fringing reef, Hainan Island, Northern South China Sea were analyzed. Specifically, the Sr/Ca, δ 18 O, and δ 13 C were measured with a monthly resolution for each sample. These environmental proxies were used to reconstruct the sea surface temperature (SST), sea surface salinity (SSS), and physiological activity of the coral at the time when the growth hiatuses and/or mortality surfaces occurred. These data were subsequently used to determine the causes of coral mortality and growth discontinuity in each case. The results show that growth hiatuses and mortalities mainly occurred in summer, with high SST (31–34°C) and SSS (32.8–38.4). In addition, abrupt negative shifts of 2–3‰ in δ 13 C were observed in almost all of the surfaces of growth hiatus and mortality, indicating a dramatically reduced level of photosynthetic activity in symbiotic zooxanthellae. Because of the above reasons, we conclude that the frequently observed mortality and growth discontinuity of Porites corals from the mid‐Holocene is evidence for thermal bleaching events in the past. That is, coral bleaching has occurred 3,800–4,200 years ago and is not a new phenomenon.