Responses of Arctic permafrost peatlands to climate changes over the past millennia

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Title: Responses of Arctic permafrost peatlands to climate changes over the past millennia
Author: Zhang, Hui
Thesis level: Doctoral Dissertation
ISBN: 978-951-51-4457-7
Abstract: Northern circumpolar permafrost peatlands store ~300 Pg of organic carbon (C), and play a critical role in regulating global biogeochemical cycles. Amplified warming in high-latitude regions is threatening this large C stock, because permafrost thawing will expose previously frozen C to decomposition. Permafrost thaw-induced hydrological changes together with warming will influence plant photosynthesis and decomposition processes and thus C accumulation patterns. However, it is still unresolved how C accumulation, warming and associated hydrological changes are interlinked. In this study, I used 14 peat records from four sites in northeast European Russia and Finnish Lapland to reconstruct permafrost peatland vegetation, hydrology and C dynamics. The studied records were dated by radiocarbon (14C) and lead (210Pb) methods. In order to reconstruct hydrology, i.e. water-table depth (WTD), I first developed a new modern testate amoeba-WTD training set. The training set represented different habitats and comprised 145 surface peat samples collected from the four study sites. The training set data were then applied to six peat records to reconstruct last millennium hydrological changes. Plant macrofossil analysis was also conducted for these six cores to study changes in the vegetation, habitat conditions and permafrost dynamics, and to evaluate testate amoeba-based WTD reconstructions. In order to calculate and model C accumulation patterns, bulk density and C content (occasionally loss on ignition) analyses were carried out for 14 cores. C modelling enabled the C capacity analyses of peat that was accumulated during recent centuries vs. older peat layers. Several environmental variables, such as nitrogen (N) content, C/N ratio, WTD, plant functional types and summer temperature were used to evaluate allogenic C accumulation forcing in the past. The data show that testate amoebae are powerful indicators of hydrological conditions in permafrost peatlands. Testate amoeba and plant macrofossil reconstructions suggest that permafrost peatlands are sensitive to climate changes. Warm climate phases (Medieval Climate Anomaly: MCA, and warming since 1850 AD) caused permafrost thawing and temporarily wet conditions, but peat surfaces later desiccated due to accelerated evapotranspiration. In turn, during the cool climate phase of the Little Ice Age (LIA) peat surfaces mainly remained dry due to peat surface uplift causing desiccation of the peat surface. However, the proxies indicate occasional wetter than MCA habitat conditions possibly due to decreased evapotranspiration. This again highlights the importance of evapotranspiration in regulating eco-hydrological feedback mechanisms in permafrost peatlands. The studied peatlands have been persistent C sinks during mid- to late-Holocene with an average accumulation rate of 10.80 - 32.40 g C m-2 yr-1 during this period. Apparent C accumulation rate (ACAR) analyses suggest inconsistent response to warming. The data from Russia indicate that the Holocene Thermal Maximum stimulated faster ACARs, while the MCA did not. The likely reason is that permafrost aggradated only during the late Holocene. Moreover, sometimes relatively high ACARs were dated to the LIA, suggesting that ACARs were controlled more by other factors than by cold climate per se. In conclusion, the current data suggest there is no single environmental factor that alone drives C accumulation. This is manifested by the pattern where in some of the studied permafrost peatlands, warming since 1850 AD has increased C accumulation rates while elsewhere there is a slight decrease in C accumulation over the same period. These divergent trends suggest that there are alternative response directions to warming in the future and that also an overall decrease in the C sequestration ability may occur for permafrost peatlands.
Date: 2018-09-21
Rights: This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.

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