High-precision analysis on annual variations of heavy metals, lead isotopes and rare earth elements in mangrove tree rings by inductively coupled plasma mass spectrometry

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Abstract

Annual variations from 1982 to 1999 of a wide range of trace elements and reconnaissance Pb isotopes (207Pb/206Pb and 208Pb/206Pb) were analyzed by solution ICP-MS on digested ash from mangrove Rhizophora apiculata, obtained from Leizhou Peninsula, along northern coast of South China Sea. The concentrations of the majority of elements show a weak declining trend with growth from 1982 to 1999, punctuated by several high concentration spikes. The declining trends are positively correlated with ring width and negatively correlated with inferred water-use efficiency, suggesting a physiological control over metal-uptake in this species. The episodic metal concentration-peaks cannot be interpreted with lateral movement or growth activities and appear to be related to environmental pollution events. Pb isotope ratios for most samples plot along the ‘Chinese Pb line’ and clearly document the importance of gasoline Pb as a source of contaminant. Shale-normalised REE + Y patterns are relatively flat and consistent across the growth period, with all patterns showing a positive Ce anomaly and elevated Y/Ho ratio. The positive Ce anomaly is observed regardless of the choice of normaliser, in contrast to previously reported REE patterns for terrestrial and marine plants. This pilot study of trace element, REE + Y and Pb isotope distribution in mangrove tree rings indicates the potential use of mangroves as monitors of historical environmental change.

Introduction

Dendrochronological trace element studies have been reported for many decades with varied outcomes, some positive [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] and some less encouraging [11], [12], [13], [14], [15], [16], with success or otherwise related to species, analytical methodology and elemental uptake mechanisms [13], [15]. For example, studies of conifer trees more often report positive results than those using hardwood trees [13]. Nabais et al. [17] presented a detailed review of over 200 publications in this field and discussed the potential of using annual trace element concentrations in tree rings as pollution monitors.

Dendrochronological studies continue to be important, because when successful, they provide complementary data to other natural recorders such as corals from tropical oceans [18] and ice cores from high latitudes [19]. Furthermore, tree rings have significant spatial and/or temporal advantages over sediment cores due to their wide distribution, continuous growth (sometimes on the order of 1000 years [20], [21]) and in many species, a clear annual boundary (light-colored early wood or spring wood and darker-colored late wood or summer and autumn wood) providing an accurate chronology.

Mangroves are environmentally significant, living in the coastal and estuarine zone and providing a buffer against the ocean, fish breeding sites and an efficient sink for terrigenous sediments and anthropogenic pollutants [22]. Yu et al. [23] first suggested that Rhizophora apiculata from Leizhou Peninsula (∼20°10–30′N, 110°E), on the northern coast of South China Sea has clear seasonal rings and that its alpha-cellulose δ13C can provide a record of annual sea level change. Seasonal growth rings were also identified by Verheyden et al. [24] in mangrove Rhizophora mucronata from the Kenyan coast (Gazi Bay: 39°30′E, 4°25′S; and Mida Creek: 39°59′E, 3°21′S). If mangrove tree rings also preserve trace element concentrations, they will have significance for monitoring natural and anthropogenic induced environmental change. Towards this aim, we report annual trace element, reconnaissance lead (Pb) isotope and REE + Y variations in mangrove tree rings and explore their environmental significance.

Section snippets

Location, sampling and experiment

The Leizhou Peninsula (see Fig. 1 of [25] experiences the SE Asian tropical monsoon climate regime. From June to August, the region is affected by the southwest summer monsoon and tropical cyclones, which bring warm and wet tropical air masses to the region, resulting in increased precipitation. During the winter season, from October to March, the region is affected by the northeast winter monsoon, which brings cold and dry continental air to the peninsula, leading to reduced precipitation.

Results

The average absolute concentration (Table 1, relative to wood) order for the measured trace elements is: Sr (1921 ± 705 ppb) > Ag (231 ± 113 ppb) > Cr (26 ± 88 ppb; 1.5 ± 1.4 ppb after the exclusion of 1986 and 1997 high values) > Zn (14.7 ± 4.9 ppb) > Cu (10.9 ± 2.9 ppb) > Ba (9.3 ± 4.0 ppb) > Li (5.2 ± 1.4 ppb) > Ni (2.7 ± 3.8 ppb; 1.9 ± 2.2 ppb after the exclusion of 1986 and 1997 highs) >Co (1.9 ± 0.6 ppb) > Ti (1.1 ± 0.5 ppb) > Mo (0.8 ± 2.0 ppb; 0.27 ± 0.36 ppb after the exclusion of 1986 and 1997 highs) > Pb (0.51 ± 0.40 ppb) > Sn (0.45 ± 0.23 ppb) > V (0.17 ±

Trace elements

Fig. 1 outlines the fluctuations in the concentrations of the 21 trace elements and total REEs over the mangrove life span, i.e. from 1982 to 1999. They exhibit the following features:

Conclusions

We performed a detailed pilot study on annual variations of 20 trace elements (Li, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Sr, Mo, Ag, Cd, Sn, Sb, Ba, Pb, Tl, Th and U), fourteen rare earth elements and yttrium and lead isotope ratios (207Pb/206Pb and 208Pb/206Pb) from mangrove tree rings (from 1982 to 1999), for the purpose of exploring the potential of using R. apiculata for environmental monitoring and conclude that:

  • (1)

    The annual concentrations of most elements show declining trends with time,

Acknowledgements

This work was funded by the National Natural Science Foundation of China (Nos. 40231009 and 40572102), the Chinese Academy of Sciences innovation project (No. KZCX2-YW-318), an ARC international linkage fellowship (LX0559831) and an ARC discovery grant (DP0773081). We thank the anonymous reviewer for very constructive comments and suggestions for the improvement of this manuscript.

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