Roger Byrne

A 5000-year record of agriculture and tropical forest clearance in the Tuxtlas, Veracruz, Mexico

We present pollen and microscopic charcoal evidence from securely dated lake sediments, located in the Tuxtlas region of Veracruz, Mexico. Two periods of agricultural activity are recognized. The earliest phase reflects a low level of forest clearance and agriculture. The data provides a minimum date of 4830 cal. BP for the introduction of maize agriculture to the region. The second phase indicates intensive and prolonged human activity within the tropical forest. Commencing approximately 2600 years ago and ending a thousand years later, this episode reflects Pre-Classic to Middle Classic agricultural intensification. Coincident with this phase, we find evidence for drier climatic conditions. Subsequent to each episode of disturbance the forest regenerated within approximately 300 years.

Pollen evidence for historic sedimentation rates in California coastal marshes

The pollen of alien weeds and ornamentals is used to determine sedimentation rates in four California coastal salt marshes. The results indicate that during the present century sedimentation rates have been ca. 50 cm/100 years in southern California and 10 cm/100 years in central California. We interpret this regional contrast to be primarily the result of differences in land use history, although ‘natural’ environmental differences are apparently also involved. Radiocarbon dates indicate that pre-European sedimentation rates were ca. 10 cm/100 years in southern California and ca. 5 cm/100 years in central California.

Prehistoric agriculture and forest clearance in the Sierra de los Tuxtlas, Veracruz, Mexico

Abstract Analysis of the pollen and charcoal content of a 9.4 m sediment core from Lago Catemaco in the Sierra de los Tuxtlas of southern Veracruz, Mexico, has provided a record of prehistoric agriculture and forest clearance in the Olmec region. Problems with radiocarbon dating have precluded the establishment of a firm chronology, but the volcanic ash stratigraphy suggests that the core represents approximately the last three thousand years. Maize pollen was encountered in all but two of the samples analyzed, and is especially common in the middle section of the core. This section is also characterized by low arboreal pollen counts and high percentages of herbaceous taxa, particularly Ambrosia. We interpret this interval to represent a period of intensified agricultural activity during the Late Classic and Post‐classic. In the upper section of the core maize pollen counts are generally low, even in the near‐surface samples. The most striking feature of this section is a sharp increase in arboreal pollen...

Palynologic and geomorphic evidence for environmental change during the Pleistocene-Holocene transition at Point Reyes Peninsula, central coastal California

Abstract Three sites on the Point Reyes Peninsula, California, provide evidence of major environmental change during the Pleistocene-Holocene transition. A 12,300-yr-long pollen record from Coast Trail Pond reveals a change from closed canopy Pseudotsuga-Abies forest to coastal scrub and grassland about 10,000 yr B.P. The same change is also evident in pollen records from two sea cliff exposures that contain a series of buried soils interstratified with fluvial and debris-flow deposits. Eight radiocarbon dates from these exposures indicate that aggradation at both sites began ca. 12,000 yr B.P. and that the change from forest to scrub and grassland occurred between 10,300 and 9400 yr B.P. High percentages of alder pollen and bracken fern spores reflect repeated geomorphic disturbance at ca. 10,400 yr B.P. The occurrence of major aggradation accompanied by recurrent debris flows between ca. 12,000 and 10,000 yr B.P. suggests that this was a period of more frequent high-intensity storms.

Palaeolimnological evidence of late-Holocene settlement and abandonment in the Mirador Basin, Peten, Guatemala

Pollen, loss on ignition and magnetic susceptibility analyses provide a high-resolution palaeoenvironmental record from Lago Puerto Arturo, Peten, Guatemala. The presence of Zea pollen ~2650 BC provides a latest date for the arrival of maize agriculture to the region. The following 3600 years are marked by significant opening of the forest and episodic pulses of erosion. During the early Preclassic, around 1450 BC, all proxies indicate an abrupt increase in human activity, coincident with archaeological evidence of early settlement. Three discrete periods of decreased human activity are indicated by cessations of landscape disturbance. Such decreased human activity likely reflects periodic local population decline. These events coincide with times of cultural transition in the Maya lowlands and correspond to the terminal phases of the middle Preclassic, late Preclassic and late Classic periods. There is no evidence for human activity in the area following the late Classic abandonment.

Cultural implications of late Holocene climate change in the Cuenca Oriental, Mexico

Significance Researchers have long invoked drought to explain the demise of many pre-Columbian Mesoamerican sites. However, the climatic history of many regions of Mesoamerica remains poorly understood. This includes the region around Cantona, a large fortified city in highland Mexico that was abandoned between 900 CE and 1050 CE. We used stable isotopes and elemental concentrations from lake sediments to reconstruct past climate, and found evidence of regional aridity between 500 CE and 1150 CE. In the initial phase of drought, Cantona’s population grew, possibly as a result of regional political instability. However, by 1050 CE, long-term environmental stress likely contributed to the city’s abandonment. Our work highlights the interplay of environmental and political factors in past human responses to climate change. There is currently no consensus on the importance of climate change in Mesoamerican prehistory. Some invoke drought as a causal factor in major cultural transitions, including the abandonment of many sites at 900 CE, while others conclude that cultural factors were more important. This lack of agreement reflects the fact that the history of climate change in many regions of Mesoamerica is poorly understood. We present paleolimnological evidence suggesting that climate change was important in the abandonment of Cantona between 900 CE and 1050 CE. At its peak, Cantona was one of the largest cities in pre-Columbian Mesoamerica, with a population of 90,000 inhabitants. The site is located in the Cuenca Oriental, a semiarid basin east of Mexico City. We developed a subcentennial reconstruction of regional climate from a nearby maar lake, Aljojuca. The modern climatology of the region suggests that sediments record changes in summer monsoonal precipitation. Elemental geochemistry (X-ray fluorescence) and δ18O from authigenic calcite indicate a centennial-scale arid interval between 500 CE and 1150 CE, overlaid on a long-term drying trend. Comparison of this record to Cantona’s chronology suggests that both the city’s peak population and its abandonment occurred during this arid period. The human response to climate change most likely resulted from the interplay of environmental and political factors. During earlier periods of Cantona’s history, increasing aridity and political unrest may have actually increased the city’s importance. However, by 1050 CE, this extended arid period, possibly combined with regional political change, contributed to the city’s abandonment.

A paleoecological record from a late classic Maya reservoir in the north Petén

Aguada Zacatal is a reservoir located within a bajo 4 km west ofNakbe, Peten, Guatemala. It is 100 m in diameter and the surrounding berm is approximately 1 m tall. The small Classic period site of Zacatal is adjacent to the aguada. The reser voirs artificial lining prevents dry season desiccation and enhances microfossil preservation. In 1998 a 335 cm sediment core was taken from the center of the reservoir and analyzed for pollen, microscopic charcoal, and total organic matter. Core chronology, based on twoAMS radiocarbon determinations, shows the record covers the period from A.D. 695 to pre sent. Only the upper 113 cm contained well-preserved microfossils. The pollen record clearly documents a period of agri cultural activity followed by abandonment and forest succession. In the agricultural period (A.D. 695-840), corn pollen is found in conjunction with disturbance indicators. After the abandonment at approximately A.D. 840, the record is domi nated by aquatic pollen types and corn pollen is absent. This shift in pollen spectra represents the end of the Classic period.

An automated charcoal scanner for paleoecological studies

Abstract An automated imaging system is described that determines fragment numbers, areas, and size classes of microscopic charcoal in pollen preparations. Charcoal counts made with the scanner are similar to counts made using standard visual methods of charcoal estimation.

Anthropogenic Burning on the Central California Coast in Late Holocene and Early Historical Times: Findings, Implications, and Future Directions

Abstract In this final paper, we summarize the results of the eco-archaeological project, address five research questions concerning anthropogenic burning on the central California coast in Late Holocene and early historical times, and outline plans for future research.

A Paleolimnological Record of Late Holocene Vegetation Change from the Central California Coast

Abstract In this paper, we present results of the analysis of the pollen and microscopic charcoal content of a sediment core from Skylark Pond near Point Año Nuevo in Santa Cruz County, California. The core covers approximately the last 3,000 years and is of interest because Skylark Pond is located only 1.8 km from Quiroste Valley State Cultural Preserve, an important ceremonial and habitation location for the historic Quiroste tribe containing numerous late Holocene archaeological sites. The results show an increase in fire activity from the fifteenth century to the present. Peaks in charcoal at ca. AD 1425 along with subsequent high charcoal abundance indicate either small, frequent fires ignited by humans or large natural conflagrations. Significant changes after European colonization include increases in grass, oak, and bracken fern, all of which indicate the opening of the redwood forest by logging and ranching. Historic period charcoal peaks can be attributed to the burning of logging slash.