The undisclosed paleoclimatic record of northern mesoamerica: A pedostratigraphic approach for environmental reconstruction

Highlights

• Northern Mesoamerican paleosols provide useful indicators for paleoenvironmental reconstructions.

• Main pedofeatures are illuviation (humid conditions) and erosional (landscape instability).

• Multivariate ordination strengthens observed pedofeatures relationships.

• Paleoclimate change played several roles in driving pedogenesis in Quaternary.

• Humid and arid climates cycles have prevailed in the Aguascalientes Valley.

Abstract

This article presents the results of a research on paleosols formation conditions in the Aguascalientes Valley, northern Mesoamerica, where paleoenvironmental reconstructions have been scarce. Records of environmental and climatic changes, as well as the alteration of the landscape are expressed in the memory of the soil. Soil micromorphology is the most efficient method to identify these changes. In paleosols, micromorphological features reflecting a certain soil forming process can be related to environments in which those processes took place. Paleoenvironmental reconstruction of the region consisted in identifying pedofeatures and other present soil attributes, grouping features into pedogenic facies under constant environmental conditions, examining the groundmass in order to detect features caused by erosion or sedimentation, and finally establishing a hierarchy between the different pedogenic facies, including erosional and sedimentary features if necessary. Specific statistical data analysis disclosed stratigraphical relationships. The results we obtained, such as illuvial clay coatings, redoximorphic nodules, and hypocoatings, revealed the most representative pedofeatures that developed under humid climate. Under a semiarid climate, representative pedofeatures that were revealed included micritic coatings and hypocoatings. A paleosol with ferruginous nodules and hypocoatings in groundmass, and at least one phase of dusty and high-birefringence yellow clay illuviation is proposed as stratigraphic marker for this area of the Aguascalientes Valley. Evidence indicates the sequence of events in the area was: (i) a large period of humid climate; (ii) a short period of climate change, from humid to semiarid, followed by subhumid; and (iii) a large period of semiarid climate, including the current one. The paleosols of the Aguascalientes Valley to a large extent preserve major trends in history and provide further evidence to define the 11 phases of stability (pedogenesis) in 8 cycles and to detect the interlayer phases of instability (sediment transport/deposition).

Introduction

So far, studies that focus on the western and northern regions of Mesoamerica are scarce; most geoarchaeological research focused on the southern and central regions of the area. The Aguascalientes Valley, in northern Mesoamerica, is of particular interest for several fields of study. The presence of ancient settlers between 650 and 900 A.D. has been proven through archaeomagnetic dating (Cejudo Ruiz et al., 2019). Archaeological studies have also identified pre-Hispanic sanctuaries in small, isolated hills (Macías Quintero and Villagrana Prieto, 2015). Surveys of fossil identification reveal Pleistocene equids (Barrón-Ortiz et al., 2008) and turtles (Mooser, 1980) in the local fauna of El Cedazo during the Pleistocene. Absence of a detailed paleoenvironmental reconstruction is considered a weakness, which makes the subject of this study highly significant in Mesoamerican history, given that it also provides an interesting case study of landscape change in the region.

Paleoenvironmental analysis allows to trace the history of a particular site and identify environmental changes resulting from natural or anthropogenic events. The analysis of long chronological periods permits us to identify the dynamics of ecosystems at different times, even if some of the past climatic circumstances are similar to current ones (Borejsza and Frederick, 2009; Lowe and Walker, 2014).

Research related to paleoenvironmental reconstructions draws from multiple records, including lacustrine sediments, pollen, isotopic records, and rock magnetism, which have been extensively used in paleoecological studies (Berres, 2000; Lozano-García and Vazquez-Selem, 2005; Trombold and Israde-Alcantara, 2005).

Nevertheless, in areas where (undisturbed) lake sediments are unavailable, paleoecological and paleoenvironmental studies have focused on the analysis of paleosols (González-Arqueros et al., 2013; McClung de Tapia et al., 2003; Tabor and Myers, 2015). Soils formed in past landscapes and environments are a valuable tool for paleoenvironmental reconstructions, and yield proxy indicators to define climatic characteristics (Retallack, 2005), as well as chronological and environmental features.

Records of environmental and climatic changes and landscape alterations are reflected in the soil memory and can be recognized through the study of soil micromorphology. Observations of paleosol micromorphology facilitate to identify analogous modern soil types and reveal processes responsible for soil pedogenesis on paleolandscapes (Tabor and Myers, 2015). Kühn et al. (2018) highlighted it is the most efficient method to detect persistent pedogenic properties, which can subsequently be grouped into different pedogenic facies (Fedoroff et al., 1990). Therefore, the recognition of pedofeatures in the Aguascalientes Valley paleosols will provide information on the environmental dynamics; and will enable to look for signs of humid or arid climate records, as well as evidence of erosion, transport, and deposition of materials in thin sections of soil. Moreover, this evidence will indicate whether soils were formed in situ (landscape stability) or are transported soil materials (pedosediments - landscape instability).

The goal of this study is to reconstruct the subsequent phases of environmental change in the Aguascalientes Valley through an analysis of geopedological proxies. More particularly, the intention is to identify and expose a sequence of landscape stability and instability, and to assess the climate records imprinted in the soil memory by micromorphological features. Ultimately, the research therefore seeks to reveal the most representative pedofeatures developed under humid and semiarid climate.