164-7 Late Holocene Anthropogenic Influences on Volcanic Soil Chemistry in the Southern Tuxtlas, Mexico

Session: Advances and Applications of Geochemistry in Archaeology

Presenting Author:

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Abstract:

Soil formation at archaeological sites in tropical volcanic regions is complex, and disentangling natural from human-influenced weathering remains a challenge. We present isotopic, micro-geochemical, and micromorphological evidence from a soil transect that spans Mazapa, a Late Postclassic archaeological site in southern Veracruz, Mexico with evidence of modern grazing, to trace chemical changes in soil development over the past ~3,000 years. Radiocarbon ages and δ¹³C of soil organic matter were used to build a two-end member model of varying C4 and C3 biomass contributions during the Late Holocene. The isotope data show a decline in C4 vegetation from 66 % 2,800 years ago to 43% 500 years ago. This trend tracks with archaeological evidence, and we interpret this shift to document terminal Post Classic site abandonment and forest regeneration, resulting in an increasing C3 contribution to the soil organic matter. From 500 to as early as 45 years ago, the C4 biomass increases up to 78 % and reflects the Colonial to modern deforestation and grazing practices. The dominance of recent C4 biomass documented in our study is consistent with recent work showing that these modern Southern Gulf Lowland landscapes have ~75 % C4. We compared this trend with soil geochemistry using an Atlas X micro-XRF to investigate deconvoluted weathering patterns in the soil matrix, pore fillings, and lapilli with depth. Elemental mapping and calculation of a Mg-alteration index (Al/(Al+Mg)*100) shows that the lapilli alteration mirrors δ¹³C depth trends. Maximum potential Mg-alteration of lapilli coincides with the interval of lowest C4 contribution (inferred forest regrowth). This increase in weathering also coincides with a transition from slight (6.2-6.4 soil pH) to moderate (5.8-5.9 soil pH) soil acidity. Maximum alteration in pore fillings also occurs in this C3 interval. Forest regeneration and subsequent deforestation and grazing during the Colonial to modern land use may have accelerated lapilli weathering and the development of more mature pore fillings. Our results indicate that the volcanic soils of southern Veracruz, Mexico have a legacy of Late Holocene anthropogenic land use where more recent impacts may have accelerated weathering.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6264

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