Evidence of recent sea-level rise and the formation of a classic Maya canal system inferred from Boca Paila cave sediments, Sian Ka'an biosphere, Mexico

Abstract

Cave sediments along the eastern coast of the Yucatán Peninsula contain important records of paleoenvironmental change that have not been fully explored. Reconstructing environmental changes in Boca Paila lagoon reveals details about sea level, flooding of the Sian Ka'an Biosphere, and the timeline of occupation at Muyil, an important Classic Maya maritime trading site. Three sediment cores (BP1, BP2, and BP3) were collected from a cave system beneath Boca Paila lagoon in the Sian Ka'an Biosphere. Radiocarbon dating, geochemical (X-Ray Fluorescence Core Scanning, δ13C, C/N), and microfossil (foraminifera, diatoms, pollen) analyses were performed. The combined results show three distinct phases of coastal evolution. Phase 1 (1157 BCE or earlier), an upland area with mangrove associate Conocarpus erectus, grasses, and ferns, is characterized by: organic-rich detrital peat; a relative absence of foraminifera and diatoms; organic geochemistry results within terrestrial ranges (δ13C values of −28‰ to −26‰); and low Sr/Ca, Si/Ti, and Ti/K ratios. These indicate dry conditions at the karst surface. Phase 2 (1157 BCE – 312 CE), a shallow wetland, is represented by: an increase in weathering products (Ti/K) and diatom productivity (Si/Ti); more positive δ13C values (−27‰ to −22‰) and decreasing C/N ratios; and increased marine foraminifera (e.g., Ammodiscus sp.). These indicate more open water conditions. Phase 3 (>312 CE), a wetland/lagoon environment, is characterized by: carbonate-rich marl; a greater diversity of foraminifera (Ammonia spp., Elphidium spp., Rosalina spp., and Bolivina spp.), diatoms (Cyclotella meneghiniana, Craticula spp., Amphora spp., Hyalosynedra laevigata, and Grammatophora spp.) and pollen (from mangroves, ferns, grasses, palms, and pine); increased Si/Ti and Sr/Ca values; and mixed marine and terrestrial organic geochemistry values (δ13C values of −22‰ to −20‰). These indicate increased input of marine organic sediment during sea-level rise. Sea-level and climate records support the interpretation of a dry upper karst environment prior to ∼1157 BCE, with sea-level rise forming shallow (<50 cm) wetlands by ∼312 CE. Previous archaeological analysis estimates that the first settlers arrived at Muyil ∼350 BCE, but that population expansion and construction of most structures occurred during the Postclassic (925–1550 CE). Sea-level rise would have been an important factor in the expansion of coastal settlements and trade routes; continued sea-level rise after ∼312 CE allowed for the formation of deeper lagoons and channels connecting the coast to Muyil and other inland sites which would likely not have been navigable prior to the Early Classic period (250–600 CE) as they would be too shallow. This study highlights the impacts of environment on society, as well as the importance of karst cave systems for obtaining paleoenvironmental records.