A 700-year record of mercury in avian eggshells of Guangjin Island, South China Sea
Research highlights
► Eggshell Hg is a marker for past mercury deposition in marine environment. ► This is a Hg record from ancient sequential eggshell samples. ► The 700-year record of eggshell Hg is closely related to human activities. ► Eggshell Hg suggests the increase of Hg production in Asia over the past decades.
Introduction
Bioaccumulation, particularly of lipid-soluble contaminants, is quite common in aquatic ecosystem and thus a minor change of a pollutant in low trophic organism can be amplified by several orders of magnitude. As a heavy metal, mercury is a pervasive pollutant that accumulates in organisms throughout the food chain and is highly toxic (Morel et al., 1998). Mercury levels in fish-eating birds are remarkably higher than those of fish due to trophic transfer in aquatic ecosystems and this may threaten the survival and multiplication of seabirds (Boening, 2000, Pietrelli and Biondi, 2009).
Monteiro and Furness (1995) proposed that seabirds were ideal monitors of mercury in the marine environment. Furness and Camphuysen (1997) further suggested that seabirds acted as monitors for marine environment. Although mercury levels in different avian tissues are rather different (Dietz et al., 2009), a number of studies have demonstrated that avian tissues, such as feather, muscle, egg content, etc. could be ideal biomonitors for heavy metal (Hg, Pb, Cu, As, Se etc.) pollution (Morera et al., 1997, Burger, 1994, Santos et al., 2006). The eggs could also be used as a biomonitor for persistent organic matter, such as PCB (Pereira et al., 2009). The use of eggshell as a biomonitor has been emphasized by scientists (Ayas, 2007, Mora, 2003, Dauwe et al., 1999). Mercury is deposited into various tissues according to principles of solubility and chemical binding opportunities. High Hg levels in avian tissues lead to increased Hg in eggshell, and Hg at high concentrations can be embryotoxic to chicks and may cause cracked eggs due to reduced eggshell strength or eggshell thinning (Lundholm, 1995). This increases incubation failure and ultimately impacts bird population. For example, Wiemeyer et al. (1984) found that high Hg in the diet resulted in few eggs and higher embryo mortality. However, the majority of prior studies have focused on fresh eggshells or samples from recent decades (Nygard, 1998, Pereira et al., 2009). Few researchers have analyzed ancient eggshell samples and most of the ancient avian hard tissues used in prior studies were collected from museums and they were discrete samples (Dietz et al., 2009), because these materials are rarely available and difficult to preserve in nature. Recently, Yin et al. (2008) discussed the distribution of several heavy metals (Hg, Cu, Pb, Zn) in ancient avian tissues, including eggshells, bones and feathers. Sun et al. (2006) revealed a relationship between human civilization and seal hair Hg in a sequence, but none of previous studies have tested sequential eggshell samples. One of the major advantages of our study is that the eggshell samples we used are from a genuine environmental archive.
Several studies have revealed that the anthropogenic contribution to Hg levels significantly increased in recent times. For instance, Dietz et al. (2009) reviewed numerous studies regarding Hg pollution in both fresh and ancient avian hard tissues in the Arctic, and found that there had been an increase of human-induced Hg in marine animals over the past several hundred years. It is well known that Asia has become the largest contributor of anthropogenic atmospheric Hg in recent decades (Li et al., 2009, Yang et al., 2010), but such record in avian hard tissues in Asia remains insufficient.
The Xisha archipelago is located in central South China Sea. The environmental changes of the Xisha Archipelago have drawn increasing attention in recent years (Liu et al., 2005, Liu et al., 2006, Liu et al., 2008a, Sun et al., 2005, Sun et al., 2007). There were once numerous birds on the Xisha islands (Exploration Group of Xisha Islands of Institute of Soil Science of CAS, 1977), although the seabirds on most of those islets are now almost extinct. We collected a sediment core from Guangjin Island during a field investigation in 2008 and a few well preserved eggshell fragments were found throughout the profile. These eggshells from a sequence provide information about how mercury deposition has changed with time. In the present study, we attempt to reconstruct mercury deposition over the past few hundred years from these eggshells and identify the driving forces which have impacted on eggshell Hg levels.
Section snippets
Study area
The Xisha Archipelago (15°47′–17°08′N, 110°10′–112°55′E) consists of more than 40 islets, sandbanks and reefs (Fig. 1, Hainan Ocean Administration, 1999). Based on the observed data from the Yongxing weather station, annual mean air temperature and annual rainfall of the Xisha Islands are 26–27 °C and 1500 mm respectively. From June to November, the Xisha Islands are subject to the effect of the southwest monsoons, tropical cyclones of high frequency from intense convergent convection, and
Chronology for the ornithogenic sediment profile GJ3
210Pb, 226Ra and 137Cs activity concentrations versus depth are plotted in Fig. 2 (A, B). The 210Pb/226Ra equilibrium was reached at 9 cm of the profile and the excess 210Pb activities did not decline exponentially with depth (Fig. 2C). Since a non-exponential pattern is acceptable for the Constant Rate of Supply (CRS) model (Appleby, 2001), we tried this dating model for the age calculation here. As shown by the record of 137Cs in the profile GJ3 (Fig. 2B), there is an evident peak at 6 cm
Conclusions
A 700-year record of Hg in well preserved eggshells has been obtained from ornithogenic sediments on Guangjin Island, South China Sea. The Hg concentration in ancient eggshells was at least one order of magnitude larger than that of seawater, testifying to the biomagnification of Hg in marine ecosystems. There was little evidence for the suggestion that the fluctuation of Hg concentration in ancient eggshells could be due to post-depositional diagenetic changes or trophic level transfer of
Acknowledgements
This work was funded by National Natural Science Foundation of China (Nos. 40730107 and 40876096), the National Basic Research Program of China (973 Program, No 2010CB428902), the Ministry of Education of China’s Ph.D. Programs Foundation for the new teachers, and the Knowledge Innovation Program of CAS (KZCX2-EW-QN50). We would like to thank Patterson W.P. and Bedient K. for assistance with the isotope analysis, and Ms Geraldine Carton for language help. All members of field study, including
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