Abstract
We have developed a new approach to quantitatively reconstruct past changes in evaporation based on compound-specific hydrogen isotope ratios of vascular plant and Sphagnum biomarkers in ombrotrophic peatland sediments. We show that the contrast in H isotopic ratios of water available to living Sphagnum (top 20 cm) and in the rooting zone of peatland vascular plants can be used to estimate “ƒ”—the fraction of water remaining after evaporation. Vascular plant leaf waxes record H isotopic ratios of acrotelm water, which carries the D/H ratio signature of precipitation and is little affected by evaporation, whereas the Sphagnum biomarker, C 23 n-alkane, records H isotopic ratios of the water inside its cells and between its leaves, which is strongly affected by evaporation at the bog surface. Evaporation changes can then be deduced by comparing H isotopic ratios of the two types of biomarkers. We calibrated D/H ratios of C 23 n-alkane to source water with lab-grown Sphagnum. We also tested our isotopic model using modern surface samples from 18 ombrotrophic peatlands in the Midwestern United States. Finally, we generated a 3000-year downcore reconstruction from Minden Bog, Michigan, USA. Our new record is consistent with records of other parameters from the same peatland derived from different proxies and allows us to differentiate precipitation supply and evaporative loss.
Highlights
The balance of precipitation and evaporation is a critical climate parameter that has enormous impact on natural vegetation and agriculture, as well as drinking water and sanitation infrastructure (IPCC WG II, 2007)
The slope of the lines fit to the sphagnum water sample data is much lower (4.7), indicating that sphagnum water has been subjected to substantial evaporative enrichment
We propose a new method to quantitatively reconstruct the fraction of surface water remaining after evaporation, f, from the surface of an ombrotrophic peat bog. This value is calculated using a Rayleigh model of hydrogen isotope enrichment between water used by the living Sphagnum in the peatland and water in the acrotelm used by vascular plants of the peatland
Summary
The balance of precipitation and evaporation is a critical climate parameter that has enormous impact on natural vegetation and agriculture, as well as drinking water and sanitation infrastructure (IPCC WG II, 2007). The drought-sensitivity, site density, and sampling depth of the tree ring record vary in time and space, and are temporally shallow in the humid regions of eastern North America Because they are isolated from the local groundwater, all moisture input to ombrotrophic peatlands comes from precipitation. Sediments from Sphagnum-dominated ombrotrophic peatlands have been widely used as archives of hydrologic balance because of the controlling role of peatland hydrology over the vegetation assemblage (Barber et al, 2003), microbial communities (Booth, 2007), and degree of decomposition of organic material at the surface (Caseldine et al, 2000) Paleoenvironmental indicators such as testate amoebae, plant macrofossils, measurements of humification, and plant biomarker distributions have been used to extract this hydrologic information from the sediment record (Barber et al, 2003; Booth and Jackson, 2003; Booth et al, 2006; Nichols et al, 2006). Our results indicate that the hydrogen isotopic difference between the biomarkers of vascular plants and Sphagnum can be used as a proxy for past changes in evaporation
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