Abstract
Peat cores are valuable archives of past environmental change because they accumulate plant organic matter over millennia. While studies have primarily focused on physical, ecological, and some biogeochemical proxies, cores from peatlands have increasingly been used to interpret hydroclimatic change using stable isotope analyses of cellulose preserved in plant remains. Previous studies indicate that the stable oxygen isotope compositions (δ18O) preserved in alpha cellulose extracted from specific plant macrofossils reflect the δ18O values of past peatland water and thereby provide information on long-term changes in hydrology in response to climate. Oxygen isotope analyses of peat cellulose (δ18Ocellulose) have been successfully developed from peat cores that accumulate the same species for millennia. However, to fully exploit the potential of this proxy in species-diverse fens, studies are needed that account for the isotopic variations caused by changes in dominant species composition. This study assesses variation in δ18O values among peatland plant species and how they relate to environmental waters in two fens informally named Horse Trail and Goldfin, located on the leeward (dry) and windward (wet) side, respectively, of the climatic gradient across the Kenai Peninsula, Alaska. Environmental water δ18O values at both fens reflect unmodified δ18O values of mean annual precipitation, although at Goldfin standing pools were slightly influenced by evaporation. Modern plant (mosses and Carex (sedges)) δ18Ocellulose values indicate that all Carex are higher (~2.5 ‰) than those of mosses, likely driven by their vascular structure and ecophysiological difference from non-vascular mosses. Moss δ18Ocellulose values within each peatland are similar among the species, and differences appear related to evaporation effects on environmental waters within hummocks and hollows. The plant taxa-environmental water δ18O differences are applied to the previously determined Horse Trail Fen untreated bulk δ18O record. Results include significant changes to inferred millennial-to-centennial scale hydroclimatic trends where dominant taxa shift from moss to Carex, indicating that modern calibration datasets are necessary for interpreting stable isotopes from fens, containing a mix of vascular and nonvascular plants. Accounting for isotopic offsets through macrofossil analysis and modern plant-water isotope measurements opens new opportunities for hydroclimatic reconstructions from fen peatlands.
Highlights
Peat core records have long served as geologic archives of paleoenvironmental change, using a range of biological, physical, and biogeochemical proxies
To further examine oxygen isotope variations of plant cellulose within Alaskan fens, the goals of this study are to (a) determine the δ18O values of modern peatland environmental waters and evaluate their relationship to precipitation and groundwater by comparison with local surface water δ18O values, including Kenai Peninsula lakes, rivers and streams, and the global meteoric water line (GMWL) (Rozanski et al, 1993; Anderson et al, 2016); (b) compare δ18Ocellulose values of plant species with the δ18O values of their environmental waters to determine whether species δ18O values significantly differ; and (c) evaluate the range of δ18Ocellulose values exhibited by different peatland-plant species spatially
Horse Trail fen (HTF) values from all sampling locations were nearly identical and plot near the intersection between the Local Evaporation Line (LEL), defined by the Kenai lakes measured for this study (Figure 1, Table 1), which are mostly located in the western lowlands (Figure 1), and the GMWL (Tables 1–3; Figure 2)
Summary
Peat core records have long served as geologic archives of paleoenvironmental change, using a range of biological, physical, and biogeochemical proxies. Many Alaska peatlands are primarily groundwater-sourced fens with high species diversity through space and time. The utility of this proxy was tested in one Alaska fen core by analyzing δ18O changes relative to the plant macrofossils comprising the peat (Jones et al, 2014), and while isotopic shifts occurred that were consistent with other regional records (Fisher et al, 2004; Anderson et al, 2005), questions remain about how closely tied the shifts were to hydroclimate changes vs other factors, and most plant species shifts in response to changes in peatland hydrology
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