| The Foram Index(FI)is an indicator that uses the content of benthic foraminifera functional groups(algae symbiotic species,opportunistic species,non-autotrophic species)in the sediments of coral reef areas to evaluate the ecological environment health status of coral reef areas.It has been widely used in the ecological environment assessment of global coral reef areas.So far,the application of this index in China is still very few,and it has not been applied to the study of coral reef ecological status in geological history.In this paper,the modern surface sediments of coral reef areas such as Weizhou Island,Sanya Luhuitou Island,Xisha Islands(Qilianyu Island,Dongdao Island,Shanhu Island,Jinyin Island,Huaguang Reef,Panshi Island,Zhongjian Island,Yinyu Island,Quanfu Island,Yagong Island and Shiyu Island),Zhongsha Islands Huangyan Island and Nansha Islands(Nanxun Reef and Dongmen Reef)in the northern South China Sea were used as materials.Through the analysis of the characteristics of benthic foraminiferal types,the FI of different coral reef areas was calculated,and its spatial variation and ecological significance were evaluated with ecological data.The changes of FI in the past 2600 years were analyzed and its ecological significance was evaluated by using the columnar sediments drilled from Lingyang Reef in Xisha Islands.In addition,this paper also tested foraminifera shell diameter and other parameters that can generally reflect the temperature trend to explore the causes of past ecological environment changes.The FI of 20 modern sediment samples and 607 columnar sediment samples were calculated.The foraminifera shell diameter and element content of 190 columnar sediments in 284 columnar sediment samples were tested.The paper draws the following main conclusions:(1)The modern FI in the coral reef area of the South China Sea is significantly different on the spatial scale,and increases with the decrease of latitude.The high FI corresponds to the ecological environment characteristics such as low nutrition,high coral coverage and high coral recruitment.The FI of different reef areas in the South China Sea fluctuated between 2.9 and 5.1,with an average of 4.2.The foraminifera index of Weizhou Island,Dongsha Islands,Sanya Luhuitou,Xisha Islands,Huangyan Island and Nansha Islands were 2.9,2.9,4.4,4.5,3.5 and 5,respectively,showing an increasing trend with decreasing latitude.The FI of different islands and reefs in Xisha Islands fluctuated between 2.6 and 5.2,with an average of 4.0.Comparing the FI with the ecological data of each coral reef area,it was found that the FI was significantly negatively correlated with Chl-a and POC,and significantly positively correlated with living coral coverage,SST and coral recruitment.The coral reef area with high FI corresponds to relatively healthy ecological environment characteristics such as low nutrition,relatively high living coral coverage and relatively high coral replenishment.In general,the FI of the coral reef area in the South China Sea has a good indication of the ecological environment of the reef area.(2)The core sediments of Antelope Reef in Xisha Islands showed that the FI fluctuated in the past 2600 years,with a range of 4.1-7.9,with an average of 5.9,indicating that the coral reef of Antelope Reef was generally healthy in the past 2600 years.The foraminifera index of Lingyang Reef in the past 2600 years showed a fluctuating downward trend and then a fluctuating upward trend,including three obvious rising periods:2380-1628 a BP,1212-572 a BP,252-92 a BP;three obvious rapid decline periods:1628-1212 a BP,572-252 a BP,1858 AD-present.Based on the indication relationship of FI to the health status of coral reefs,this paper believes that the coral reefs of antelope reefs in Xisha Islands are generally healthy in the past 2600 years.(3)The three rapid decline periods(1628-1212 a BP,572-252 a BP,1858 AD-present)of the antelope reef foraminifera index in the past 2600 years indicate the cyclical deterioration of the ecological environment in the historical period.The first two decline periods(1628-1212 a BP and 572-252 a BP)correspond to the cold climate stages of the historical period,namely the Dark Ages Cold Period(DACP)and the Little Ice Age(LIA),respectively.The literature records show that there was more precipitation and stronger winter monsoon in the Xisha area during this period.This paper speculates that the rapid decline of foraminifera index in these two periods is related to the climatic characteristics of the same period,that is,strong winter monsoon leads to the increase of atmospheric dust and heavy rainfall leads to the increase of atmospheric wet deposition,which further leads to the increase of nutrients and the rapid growth of macroalgae(coral growth is inhibited)in the antelope reef area;The rapid decline of FI since 1858 AD corresponds to the rapid degradation of coral reef ecosystem in Xisha Islands in recent decades.The possible reasons include the increase of nutrient concentration(caused by the increase of macroalgae)in antelope reef area caused by the intensification of modern human activities and the increase of atmospheric nitrogen deposition flux.(4)Based on the reconstruction of benthic foraminiferal shell diameter,the sea surface temperature of Xisha Islands in the past 2600 years showed fluctuation and alternation of cold and warm,which had a good correspondence with the change of FI.According to the relationship between the diameter of benthic foraminifera shell and sea temperature,it is concluded that the sea temperature of the antelope reef in the Xisha Islands in the past 2600 years fluctuates between 26.4℃ and 28.1℃,with an average of 27.4℃.The change trend of temperature is generally consistent with the known 2.3 ka cold event,Rome warm period,dark age cold period,medieval warm period,small ice period and modern warm period.On the centennial scale,the foraminifera index is consistent with the reconstructed sea surface temperature changes.The high and low values of the foraminifera index are roughly corresponding to the warm and cool periods,respectively.The positive correlation between the Ca content and the foraminifera index(consistent with the sea temperature change)also indicates that the foraminifera index is higher during the warm period.(5)In the past 2600 years,the FI and the sea temperature reconstructed by benthic foraminiferal shell diameter showed significant periodic changes,which were consistent with ENSO and other periods,indicating that ENSO activities had a certain impact on the ecological environment of coral reefs in Xisha Islands.The sea surface temperature reconstructed by the diameter of benthic foraminifera shell has 30.3a,33.7a,42.0a,and 47.9a cycles,and the foraminifera index has 45.3a and 33.0a cycles,which are consistent with the 35-45 year low-frequency variation cycle of ENSO.It is speculated that during the El Ni o period,the Xisha sea area was not conducive to the growth of macroalgae in the coral reef area(relatively conducive to the growth of corals)due to the weak winter monsoon,low primary productivity of surface seawater and low nutritional status of water bodies,so the foraminifera index was higher;during the La Nina period,the Xisha Islands had more precipitation,stronger winter monsoon and higher water nutritional status,which was conducive to the growth of macroalgae(hindering the growth of corals),so the FI in the coral reef area decreased.The contents of Fe and Ba,which indicate the nutritional characteristics of seawater,are negatively correlated with the foraminifera index.It also shows that the foraminifera index of Xisha Islands is related to seawater nutrients,which indirectly proves the impact of ENSO activities on the coral reef ecological environment of Xisha Islands. |
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