Holocene evolution of the Tabasco delta – Mexico: impact of climate, volcanism and humans

Abstract

This research revealed the impact of climate, volcanism and humans on the late Holocene evolution of a tropical delta in southern Mexico. Palynological, tephrochronological, limnological, geomorphological and sedimentological techniques have been applied to reconstruct the evolution of the Usumacinta-Grijalva delta coast over the past 6000 years. Observed changes are correlated with climate, volcanism, and land use changes, partly reconstructed from a multiproxy study of sediments from lake Tuspan. The geomorphological and sedimentological study of the extensive beach-ridge plain was supported by LiDAR data, 14C and OSL dating, GPR measurements, grain-size analyses, and chemical fingerprinting of volcanic glass and pumice fragments. The new detailed chronology of the beach-ridge plain established in this study shows that the exceptionally long sequence of around 500 subsequent beach ridges was formed due to ample long-term riverine sediment supply, partly related to a high availability of easily weatherable Los Chocoyos ignimbrites in the catchment of the Usumacinta River. The elevation of the beach ridges appears not to be determined by sea level fluctuations or changes in storm activity as previously accepted, but relates to the variability in fluvial sediment supply, reflecting decadal-scale precipitation changes within the river catchment. A centennial-scale variability in precipitation during the Pre-Classic Period was found that significantly correlates with the North Atlantic δ14C atmospheric record, with a comparable periodicity of ~500 years, indicating an important role of North Atlantic atmospheric-oceanic forcing on precipitation in the Maya Lowlands. A dramatic change from dry to wet conditions occurred during the Middle Pre-Classic period, around the well-known 850 BCE (2.8 ka BP) event. This wet period was likely unfavourable for agricultural intensification in the Central Maya Lowlands, and may have delayed the development of Maya civilization in this area until the start of the Late Pre-Classic period around 400 BCE. Multiple cores from the back-barrier plain were described and analysed on a range of proxies, including pollen, diatoms, macroremains, charred plant fragments, organic matter content, and chemical elements. Age control was based on a total of 74 AMS 14C dated samples and two distinct tephra layers. Combining different dating techniques enabled the dating of one of these tephralayers to 546 ± 10 CE. Chemical fingerprinting of volcanic glass isolated from this layer indicate El Chichón as the source volcano. The eruption is a likely candidate for the 540 CE sulfur spike in the bipolar ice-core data, and may had a severe environmental impact on Maya societies. The pollen records indicate that a freshwater marsh developed in the back-barrier flood basin since the closure of the barrier system around 4200 and 1800 BCE in the western and eastern part of the study area. The average rate of sediment accumulation (~0.5 mm yr-1) probably exceeded the rate of sea-level rise, resulting in an overall decrease in water depth and hydroperiod, promoting the natural succession of wetland plant communities from open water to marsh and tree communities. Periods of increased salinity (most noticeable from the expansion of mangrove vegetation and an increase in diatom species that prefer oligosaline conditions) are related to river avulsions and salt water intrusion through deteriorating river channels.