Age Models Of Seabird Remains From The Qilian Yu In The Xisha Archipelago And Its Ecological Implications

Reef islands of the Xisha Archipelago,South China Sea,were formed during the mid-late Holocene.Due to lack of precise dating methods,we know little about the ecological evolution of the Xisha islands.In this study,we analyzed ornithogenic sediments of islands in the Qilian Yu of the Xisha Archipelago,and collected four sediment profiles from these islands?each labled as ZS2,BD1,ZD1and ND1,as well as 94cm,137cm,103cm,121cm in length,separately?in AD2015.Based on radiocarbon dated seabird subfossils retrieved from ornithogenic sediments in these profiles,such as seabird bones,fish bones,egg shells and fish scales etc.,we have successfully constructed age-depth models of these profiles.Meanwhile,we have also reconstructed the eco-environmental revolution processes of these four reef islands in the past nearly 1000 years ago,combined with reflectance spectrum and geochemical analysisOn the one hand,we have compared traditional mathematic fittings with Byesian age-depth models,such as Bpeat,OxCal,Bacon models etc.Here we chose Clam age-depth model to discuss sedimentation rates,as well as seabird population changes of these four islands in the Qilian Yu.It turns out that the sedimentation rates tend to be faster at the bottom and slower down in the upper surface,and the mean accumulation rate is 1.15±0.47 mm?a^-1,which is apparently lower than current sea level rise in the Xisha area.This implies that global change could possibly pose a threat to the safety of islands in the Xisha Arichipelago.On the other hand,we have conducted geochemical analysis on ornithogenic sediments in ZS2 profile,and performed primary component analysis?PCA?and R-mode cluster analysis.The result indicates that the combination of Ag,As,Cd,Co,Cu,P,Se,Zn elements could be defined as seabird bio-elements of Qilian Yu in the Xisha islands.By comparing the PC1 scores with the guano ratio obtained from reflectance spectrum of ZS2 profile,we have found that they matched very well,implying feasibility of using reflectance spectrum to reconstruct seabird population changes.In the end,the seabird population on the Qilian Yu could generally experienced three periods:slowly growing,stably thriving and rapidly falling.In addition,we regard the timing of seabird bones remaining at the bottom of each profile as the initial time of seabirds landing,thus the earliest occupation of the Zhaoshu Island,Beidao Island,Zhongdao Island,and Nandao Island by seabirds bagan no later than AD 1172-1290,AD 984-1140,AD 413-684,AD 550-664,seperately.Besides,the ornithogenic layer in the middle of each profile can be seen as the booming period of seabirds on the islands,so the seabird population reaching its peak on the Zhaoshu Island is between AD 1336 and AD 1811;the thriving period of the seabirds on the Beidao Island is from AD 1459 to AD 1888;the population peak of the seabirds on the Zhongdao Island is between AD 777 to AD 1560;the corresponding peak period of seabird population on the Nandao Island is between AD 1503 and AD 1804.Finally,it is found that a relatively cool climate is more favorable for seabirds in the Qilian Yu.