1. Accessing the Peatland Archive: Applying Peat Humification Analysis in Multi-Proxy Paleoenvironmental Research
Sullivan, Donald G., Department of Geography, University of Denver, Denver, CO 80208, Gilmore, Kevin P., Department of Geography, University of Denver, Denver, CO 80208,
Abstract
In the past few years multiple proxy approaches to reconstructing late Quaternary environmental changes have become relatively common. In this presentation we illustrate the integration of several techniques to recover paleoenvironmental information from peatlands in Colorado, including pollen analysis, bulk density and organic content determinations, and peat humification analysis. Our results to date indicate that the “peatland archive” (Charman 2002) preserves a potentially detailed record of environmental change at a decadal to subdecadal scale, although accurately interpreting the record presents several challenges. Among the questions we address in this research is whether warmer periods in the past are characterized by higher summer precipitation than cooler episodes. We attempt to answer this using a combination of data from pollen analysis, organic content determinations and peat humification analysis.
Since the 1970s, European researchers have used peat humification analysis to reconstruct late Quaternary climate changes (Aaby 1976), but the technique has not been widely used in North America. The relative rate of peat decomposition (humification) in wetlands is primarily dependent on the depth of the water table. During extended dry periods, a greater section of the peat will be above the water table (the acrotelm), and will decompose more rapidly than during moister periods. Peat humification uses simple techniques to determine the relative humification of peat samples through a core, providing a qualitative measure of peat decomposition, and indirectly, of water table fluctuation.
We present data from cores obtained from high elevation (>3100m asl) fens (minerotrophic peatlands) on Grand Mesa, in western Colorado, and from low elevation (<1800 m asl) fens located in the Chico Creek basin, in the Arkansas River drainage in eastern Colorado. The high elevation fens are larger than the lower elevation fens, and cores from these sites suggest that peat accumulated throughout the Holocene. At the lower elevation sites, the peat layers are often interbedded with mineral or lacustrine sediments. Peat began accumulating in the high elevation fens early in the Holocene, but has accumulated over only the last few thousand years in most of the eastern Colorado sites, although at least one site preserves a record for the last 8000 years. These eastern Colorado wetlands represent a previously unidentified resource for paleoenvironmental data from the High Plains.
Previous research on sediment cores from western Colorado suggests that organic content of lake sediments provides a relative paleotemperature proxy (organic content correlating positively with average temperature) (Doerner et al. 1998). Our results suggest that the percent organic content of the peat may also respond in phase with temperature fluctuations. Peat humification analysis and pollen data indicate that during relatively warmer periods (for example during the Medieval Warm Epoch and in the early Holocene), water tables in the fens was lowered, indicating effectively drier conditions. During cooler periods (the Little Ice Age), peat is less humified, indicating that conditions were wetter.
Above left: A “pocket fen” on Chico Creek. Center: Coring a “pocket fen”. Above right: Core from Church Camp Fen. Lower left: Preparing humification samples. Lower right: Filtering humification samples.
Grand Mesa
Chico Basin Ranch
Chico Creek 2 Site
Results
At the lower elevation sites, the peat layers are often interbedded with mineral or lacustrine sediments. Peat began accumulating in the high elevation fens early in the Holocene, but has accumulated over only the last few thousand years in most of the eastern Colorado sites, although at least one site preserves a record for the last 8000 years.
Previous research on sediment cores from western Colorado suggests that organic content of lake sediments provides a relative paleotemperature proxy (organic content correlating positively with average temperature) (Doerner et al. 1998). Our results suggest that the percent organic content of the peat may also respond in phase with temperature fluctuations.
The results of pollen analysis from Kannah Creek and Church Camp Fens on Grand Mesa indicate that during the Medieval Warm Period, when organic content is high, and transmissivity is low, suggesting warmer, drier conditions, spruce and sedge pollen percentages decrease, and there is an increase in desert-related taxa. In addition, AP:NAP decreases.
Results Continued
Pollen samples taken from sediments deposited during the Little Ice Age, when organic content is low, and transmissivity is high, indicating cooler, wetter periods, spruce and sedge pollen percentages increase, and Sarcobatus and Cheno-Am pollen decrease. There is also an increase in AP:NAP, suggesting an expansion of forest taxa at the expense of desert shrubs.
Data from cores from both Grand Mesa and Chico Basin Ranch show evidence of a cool, wet interval preceding the Medieval Warm Period. We tentatively correlate this with the middle Audubon neoglacial advance (Benedict 1985). A previously unidentified period of low transmissivity and high organic content dated 100 BC-AD 400 at Chico Basin Ranch and 350 BC-AD 250 on Grand Mesa is designated here the “Terminal Archaic Drought.” This event correlates with a hypothesized increase in prehistoric population that occurred at the transition between the Archaic and Formative periods both east and west of the Continental Divide. Drought, in addition to increasing population, may have provided a forcing mechanism that resulted in the cultural innovations that define the beginning of the Formative Period. Additional AMS dates are anticipated that will clarify the tentative chronologies presented here.
AD 1560
Introduction
In this presentation we illustrate the integration of several techniques to recover paleoenvironmental information from peatlands in Colorado, including pollen analysis, bulk density and organic content
determinations, and peat humification analysis.
The relative rate of peat decomposition (humification) in wetlands is primarily dependent on the depth of the water table. During extended dry periods, a greater section of the peat will be above the water table (the acrotelm), and will decompose more rapidly than during moister periods. Peat humification analysis uses simple techniques to determine the relative humification of peat samples through a core, providing a qualitative measure of peat decomposition, and indirectly, of water table fluctuation.
Research Questions
In this research we assess the sensitivity and reproducibility of the paleoenvironmental record preserved in peat deposits in Colorado. Among the questions we address in this research is whether warmer periods in the past are characterized by higher summer precipitation than cooler episodes.
Conclusions
Our results to date indicate that the “peatland archive” (Charman 2002) preserves a potentially detailed record of environmental change at a decadal to subdecadal scale, although accurately interpreting the record presents several challenges.
Peat humification analysis and pollen data indicate that the Medieval Warm Epoch was relatively warm, and the water tables in the fens were lower, indicating effectively drier conditions. During cooler periods (the Little Ice Age), peat is less humified, indicating that conditions were wetter than during the Medieval Warm Epoch.
We tentatively correlate a prolonged cool wet period and a prolonged warm, dry period with the previously identified middle Audubon neoglacial advance and a previously unrecognized “Terminal Archaic Drought”, respectively. Evidence of these episodes is present in both the high and low elevation records. The eastern Colorado wetlands represent a previously unidentified resource for paleoenvironmental data from the High Plains.
Methods
The cores used in this project were obtained from high elevation (>3100m asl) fens (minerotrophic peatlands) on Grand Mesa, in western Colorado, and from low elevation (<1800 m asl) fens located in the Chico Creek basin, in the Arkansas River drainage in eastern Colorado.
Cores were obtained using a square-rod piston corer. In the laboratory, subsamples taken at 1 cm intervals were analyzed for bulk density, organic content, using loss on ignition, and peat humification, using NaOH digestion and a spectrophotometer. Pollen analysis was completed on selected samples to compare with results of the humification and organic content determinations.
REFERENCES
Aaby, B Cyclic climatic variations over the past 5500 years reflected in raised bogs. Nature 263:
Benedict, J.J Arapaho Pass: glacial geology and archaeology at the crest of the Colorado Front Range. Ward, CO: Center for Mountain Archeology.
Charman, D Peatlands and Environmental Change. West Sussex, England: John Wiley and Sons.
Doerner, J., D.G. Sullivan, and C. Briles Late Quaternary eolian deposition in Colorado subalpine lakes. In Dust Aerosols, Loess Soils, & Global Change (Alan Busacca, ed), Washington State University Publication No. MISC0190, pp