Seasonal variation of mercury concentration of ancient olive 1 groves of Lebanon. 2

. This study aimed to investigate the seasonality of the mercury (Hg) concentration of olive trees foliage, an 21 iconic tree of the Mediterranean basin. Hg concentrations of foliage, stems, soil surface, and litter were analyzed on 22 monthly basis in ancient olive trees growing in two groves in Lebanon, Bchaaleh and Kawkaba (1300 and 672 m.a.s.l 23 respectively). A significantly lower concentration was registered in stems (~7-9 ng/g) with respect to foliage (~35-48 24 ng/g) in both sites with the highest foliage Hg concentration in late winter-early spring and the lowest in summer. It 25 is noteworthy that olive fruits also have low Hg concentration (~7-11 ng/g). The soil has the highest Hg content (~62-26 129 ng/g) likely inherited through the cumulated litter biomass (~ 63-76 ng/g). A good covariation observed between 27 our foliage Hg time-series analysis and those of pCO 2 and atmospheric Hg concentrations of the atmospheri Northern


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Mercury (Hg) is among the most widely distributed potentially toxic metals polluting the Earth (Briffa et al. 2020). It 37 is found as all heavy metals naturally on the Earth's crust reservoir and in the atmosphere through the natural long-38 term Hg biogeochemical cycle (i.e., volcanic activities, geological weathering). This metal is easily modified into 39 several oxidation states and it can also be spread through many ecosystems (Boening 2000). The natural Hg cycle has 40 been modified due to anthropogenic activities (i.e., mining, smelting, soil erosion due to deforestation, gold extraction,

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Two monumental olive groves were chosen in the context of their historical and agricultural importance, 137 since these two sites are considered to contain olive trees more than 1400 years old and are still productive. percentages are 52.8 %, 38.7 % and 10.7 % respectively. Soil pH is 7.07 ± 0.26 with organic matter and calcium 145 carbonate contents are 1.7 % and 38.3 % respectively (Yazbeck et al. 2018). In this study, soil profiles of carbon and 146 nitrogen contents were analyzed. Organic carbon contents decreased with soil depth from about 4 % at 0-1 cm (Soil 147 surface) to 2.7 % at 30-60 cm. The total nitrogen is about 0.3 % at 1cm depth and 0.2 % at 30-60 cm depth. The olive 148 trees are located on two terraces. The first terrace is at 1.5 meter above the road level while the second is at the road 149 level. They are maintained by the municipality for the last four decades as an endowment property. Precipitation 150 average ranges between 229 and 392 mm in winter and between zero and less than 2 mm/season in summer, while 151 average temperature is between 4 and 8 °C in winter and between 20 and 23 °C in summer and average relative 152 humidity of 63% (data extracted from LARI climatic data) (Table S1, Figure S1).

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The village is at about 36 km from Chekka town located at a lower altitude (0-200 m.a.s.l.) nearby the sea (Figure 1 and nitrogen at the 0-1 cm and at 0-30 cm decrease from about 9.0 % to 2.2 % and from 0.9 % to 0.3 % for the carbon 174 and nitrogen respectively. Average precipitation ranges between 215 and 374 mm in winter and drop to almost zero 175 mm in summer, while average temperature is between 7 and 11 °C in winter and between 21 and 27 °C in summer 176 and relative humidity of 61% (data extracted from LARI climatic data) (Table S1, Figure S1).

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The village has to its east the Hasbani river, originated from the north-western slopes of Mount Hermon in Hasbaya

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In Bchaaleh grove, foliage registered its highest Hg concentration during winter and spring with 61.8 ± 7.6 ng/g and 282 55.1 ± 12.5 ng/g respectively, and its lowest Hg amount during summer and autumn with 41.5±12.7 ng/g and 44.4 ± 283 6.2 ng/g, respectively. A seasonal effect on foliage and stems was registered (p-value<2.2*10 -16 ; Figure 2a

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In Kawkaba, the highest Hg concentrations for foliage and stems were registered in spring with 51.8 ± 4.5 ng/g, 11.7 291 ± 6.7 ng/g respectively (Table 2, Figure 2b

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In the upper terrace of Bchaaleh grove, the foliage average Hg concentration of BCO4 and BCO1 varied between 42.4 315 ± 11.5 ng/g and 44.6 ± 13.3 ng/g respectively showing no significant difference (p-value = 0.8225). In the lower 316 terrace of the same site, foliage average Hg concentrations of trees BCO12 and BCO9 were found to vary from 45.6 317 ± 12.7 ng/g to 60.7 ± 12.7 ng/g respectively ( Figure 2a)

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The litter showed higher Hg concentration than that in foliage in both Bchaaleh (62.9 ± 17.8 ng/g) and Kawkaba (75.7 388 ± 20.3 ng/g) (Table 1). This has been also described by Rea et al., (1996)  For each site, Hg contents in stems exhibit a narrow range between the different trees except tree BCO9, which had 451 the highest stem values. We speculate that this higher Hg content is the adjunction of chemical products as fertilizer 452 on the plot 549 belonging to a different owner ( Figure S2) likely between fall and winter. It was observed by (Zhao &   The authors declare that they have no known competing financial interests or personal relationships that could have 516 appeared to influence the work reported in this paper. 517 518 519