Phytoliths, stable carbon isotopes and micromorphology of a buried alluvial soil in Southern Mexico: A polychronous record of environmental change during Middle Holocene

Abstract

The reconstruction of the Maya Lowlands vegetational history and its paleoecological implications during the Middle-Late Holocene was carried out using phytolith analysis and organic matter stable isotopic ratios. Three sections located in a terrace of the Usumacinta River were studied: Tierra Blanca I, Tierra Blanca II, and El Pochote. They consisted of a series of alluvial sediments alternating with paleosols that contained artifacts belonging to the Formative, Classic, and Postclassic periods according to the Mesoamerican chronology. Our results showed that the Middle Holocene paleosols had well developed vertic features with −16‰ δ¹³C values, which evidence seasonal-semiarid conditions. However, phytolith content recorded a predominance of C3 plants, and paleosol micromorphology showed a set of features related to an alternation of humid and drier conditions. Therefore, stable isotope composition reflects the integral signal of the last period of soil formation. Phytoliths and soil micromorphology correspond to a more extensive record that represents the successive stages of pedogenetic and environmental changes.

Introduction

The Holocene is considered a period of high climatic variability. Mayewski et al. (2004) have recorded several events of climatic changes throughout the Holocene (9000–8000, 6000–5000, 4200–3800, 3500–2500, 1200–1000 and 600–150 cal. BP), which were characterized by polar cooling, tropical aridity and changes in atmospheric circulation. The evidence of these climatic changes was determined using different proxies such as ice cores, marine sediments, and the study of the extent and retreat of glaciers. Some of these time-intervals coincide with significant disruptions of cultural events, for example the collapse of Maya civilization (AD 800–1000).

The studies of paleoenvironmental changes in the Maya Lowlands are mostly based on lacustrine sequences (Rosenmeier et al., 2002, Hodell et al., 2005, Lozano-García et al., 2007), and to a lesser proportion on the paleopedological records of Belize, Guatemala (Beach et al., 2008, Beach et al., 2009, Beach et al., 2011), and the Peninsula of Yucatán in Mexico (Sedov et al., 2007, Fedick et al., 2008, Cabadas et al., 2010). However, recent investigations have linked human impact to the changes in the environmental conditions of that region, including the effects of farming in soil degradation, erosion, and modifications in the sedimentation rate. The majority of these studies have been done in Belize, Guatemala and Honduras (Beach et al., 2006, Beach et al., 2009, Webster et al., 2007, Velez et al., 2011). Natural or human induced changes inside an ecosystem can produce a great impact in the vegetation.

Phytoliths and carbon stable isotopes are considered important proxies for paleovegetation records that can be found in organic soil horizons. They are commonly used as an indirect method to register changes in environmental conditions and human activity. Stable isotope carbon ratios are indicators of the different types of vegetation that can be present in an area, because they are related to the metabolic pathways of plants (C3/C4/CAM; Cerling and Quade, 1993). On the other hand, phytolith morphology is a parameter for the classification of C3/C4 vegetation types. These structures correspond to silica cells that form when the plant is alive and can remain in the soil for long periods of time due to their resistance to weathering.

The analyses of stable isotope carbon ratios in paleosols have been revealed to be an important tool as an indicator of paleovegetation. In Teotihuacán, Central Mexico, Lounejeva et al. (2006) studied the changes in vegetation for the last 20,000 years. In the Maya Lowlands, Johnson et al. (2007a) used this method to register agricultural activity, finding high values of δ¹³C (−16.6‰) in paleosols with moderate drainage. In archeological sites, such as Piedras Negras and Yaxchilán, Guatemala, similar δ¹³C values were obtained and interpreted as evidence of human activity (Johnson, 2004, Johnson et al., 2007b). In contrast, the use of phytoliths in Holocene paleoenvironmental studies for the Maya area has been more limited, although in other regions of Mesoamerica, an important paleovegetation phytolith record was registered (McClung de Tapia et al., 2008). In addition, this method allowed the recognition of the earliest maize cultivation in Mesoamerica (Pohl et al., 2007).

In this paper, we use the phytolith morphology and the stable carbon isotope composition to reconstruct the changes in the paleovegetation for the Middle to Late Holocene paleosols of the Maya Lowland. These proxies, paired with the paleosol micromorphological features, give new insights to determine the influence of climate vs. human induced modifications.