Changes in climate, vegetation, carbon storage, and stable carbon and nitrogen isotopes during the late-glacial and Holocene are investigated from two Kenai Peninsula, Alaska peatland sites. Pollen, spore, macrofossil, loss-on-ignition, and lithological analyses reveal significant disparities in vegetation patterns from other southern coastal Alaskan sites, particularly in the late-glacial and early Holocene, which suggest major atmospheric circulation changes. During the Younger Dryas chronozone on the Kenai Peninsula, Betula expanded, while graminoids contracted their range. This assemblage contrasts with other southern coastal Alaskan sites, which showed a reversion to herb tundra. An early Holocene fern peak on the Kenai Peninsula lasts from 11,500-8500 cal yr BP. Wet meadow vegetation imply a moist climate from 11,500-10,700 cal yr BP, which agrees with interpretations from southwestern Alaska, but differs from sites in southeastern Alaska, which record dry conditions. A drier climate after 10,700 cal yr BP suggests a more spatially uniform climate. We propose that this drier Kenai climate (10,700-8500 cal yr BP) signifies a weaker AL, but that high seasonality resulted in substantial snowfall, and enhanced glacial melt that sustained the fern proliferation, both on the lowlands and newly exposed land surface in the Kenai Mountains. Decreased insolation-induced seasonality results in climatic cooling after 8500 cal yr BP, with increased humidity from 8000-5000 cal yr BP. A drier interval punctuated with volcanic activity ensues 5000-3500 cal yr BP, followed by a cool moist climate, coincident with the Neoglaciation. Tsuga mertensiana expands after ∼1500 cal yr BP in response to the climate shift.
Two Kenai peatland sites, Horse Trail (HT) fen and Swanson Fen, analyzed for Holocene carbon storage, reveal temporal differences in carbon accumulation resulting from local hydrological disparities. The warm, moist early Holocene climate fostered carbon accumulation until 10,700 cal yr BP, when drier conditions limited carbon storage in Swanson Fen. HT fen appears to have had greater groundwater input, resulting in continuously high peat accumulation until 9000 cal yr BP. Decreased groundwater input at HT fen is thought to be related to lower recharge from glacial melt as Milankovitch-induced seasonality declined and climate cooling ensued. The reduced seasonality coincides with diminished carbon storage in both sites until the late Holocene, when cool, moist conditions enhanced Sphangum growth.
The stable carbon and nitrogen isotopic record from Swanson Fen reveals information about carbon and nitrogen source changes through time. Late-glacial depleted δ13C and δ15N values indicate that the use of respired carbon and combined soil nitrogen pools was high, suggesting that the peatland. A significant negative excursion in the carbon isotopic values at the Pleistocene-Holocene boundary may suggest increased methanogenesis resulting from thermokarst development and widespread peatland expansion. Nitrogen isotopic values near 0‰ suggest that the fen has been supported by atmospheric N-fixation for most of the Holocene.
|Advisor:||Peteet, Dorothy M.|
|School Location:||United States -- New York|
|Source:||DAI-B 70/02, Dissertation Abstracts International|
|Subjects:||Geology, Paleontology, Geochemistry|
|Keywords:||Alaska, Carbon, Climate, Holocene, Kenai Peninsula, Macrofossils, Peat, Pollen|
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