Mercury enrichment in anthrosols and adjacent coastal sediments at a Classic Maya site, Marco Gonzalez, Belize

Abstract

Elevated concentrations of total mercury (THg) are found in the surface soils and flanking wetland sediments at the Classic Maya coastal site of Marco Gonzalez, Belize. Significant concentrations (up to 1.3 µg·g−1 dry mass) of THg occur in leaf litter‐rich soils, as well as in the artefact‐rich anthrosol spread over the vegetated mound site of structures and occupation debris. The abundance and spatial pattern of major and trace elements measured in the surface soils indicate both site‐scale controlling factors of topography, structures and vegetation on soil geochemistry as well as local highs in concentration compared with background, due to human activity. Geochemical stratigraphy of wetland sediment cores shows that a shift from carbonate‐reef sediments to mangrove peat in the 13th century AD was attended by an input of allogenic (mineral) elements, including mercury. A THg concentration peak (0.8 μg·g−1) in brackish pool sediment is 210Pb‐dated to 1960–1970 AD, but the incorporation of mercury in multiple cores adjacent to the site shows increasing mercury inputs to have occurred before, during Classic‐period Maya occupation and following the sites abandonment. Analysis of element values from site‐scale soil sampling, combined with results from off‐site cores, provides a numerical framework upon which outlier values of THg and other element spatial patterns can be assessed, especially the spatial co‐association of elements related to differences in soil–sediment matrices. Geochemical results from active soils developing from occupation deposits (anthrosols) and sediment cores open up questions concerning contemporary and past mercury accumulation at coastal Mayan sites, and the wider interaction of human and natural biogeochemical processes that occur in human‐modified soils and coastal wetland sediments.

An Archaeological Foundation to Soil Sustainability

“…Instead, the changes in fertility and the accumulation of deposits seem to reflect unintentional behaviour -the detritus of living, working and dying. Work carried out between 2013 and 2016 sponsored by the Leverhulme Trust (Graham et al 2017, Turner et al 2021 enabled us to provide evidence that the topsoil incorporated components from earlier occupations, particu- larly from the disintegration of pottery used to contain brine in salt processing and the fuel used to heat the brine and drive off water to produce salt cakes. Topsoil nutrient capacity was higher than expected in what otherwise would be a mangrove wetland.…”

Environmental legacy of pre-Columbian Maya mercury

“…The study mapped complex patterns of Hg enrichment across the site in surface soils and temporally via sediment cores. Turner et al (2021) found the greatest concentrations of total Hg were in organic matter-rich anthrosols (a peak value of 1.3 μg g −1 detected near the centre of the site, adjacent to Maya structures) and in a low organic anthrosol containing ceramic, charcoal and obsidian fragments (0.78 μg g −1 ). Significantly, a comparable peak concentration (0.8 μg g −1 ) corresponding with 20th century fossil-fuel combustion atmospheric indicators in a 210 Pb-dated core from a site-adjacent pool.…”

“…Mercury analysis of soils and sediments from Maya sites has been undertaken using a variety of analytical approaches meaning that direct comparisons between past datasets need to be undertaken with some caution (Turner et al, 2021). Most of the studies identified in our review detail the particle-size fraction of samples analysed for Hg content.…”

“…Employing a total sample decomposition approach (concentrated Aqua Regia leaching and measurement by CV-AFS), Macphail et al (2017) identified elevated total Hg (up to 0.5 μg g −1 ) in organic-rich surface soils from Maya structures (Op 13/2-3 (Str.19 and 8) and Structure 25) that related to underlying Hg-enriched anthropogenic deposits (fish bones). Turner et al (2021) conducted research across several geomorphic and archaeological contexts at Marco Gonzalez to test for the presence of mercury as a record of past Maya activity. The study mapped complex patterns of Hg enrichment across the site in surface soils and temporally via sediment cores.…”

“…Each of the ten Maya settlements in this review have one or more contexts with Hg concentrations above "background" levels. Another means of assessing the magnitude of ancient Maya Hg enrichment is to compare the Classic Period Maya Hg levels with modern guidelines for safe exposure to total Hg in soils and sediments, as Lentz et al (2020) at Tikal andTurner et al (2021) at Marco Gonzalez have recently. If we use the sediment TET value of 1 μg g −1 , then six of the ten ancient Maya sites in this review have at least one location with Hg values that exceed the TET.…”