Peat‐based climate reconstructions run into murky waters?
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Peatlands are globally important ecosystems that serve as archives of past environmental change. Peatlands form over thousands of years from the accumulation of decaying plants and hold water, or in some cases, purely rainwater. Therefore, external processes, such as climate, as well as internal processes, such as the rates of peat growth and decay, control the water table in peatlands. However, throughout the previous century and particularly over the past decade, paleoclimatologists have increasingly relied on reconstructions of the water table in rain‐fed peatlands to infer changes in rainfall through the Holocene period (the past ∼12,000 years), ignoring the potentially important role of internal processes.Keywords:
Table (database)
Rainwater Harvesting
Peatlands provide a widespread terrestrial archive of Holocene environmental change. The taphon omy of peat is relatively simple, the range of evidence and proxies is wide, and dating methods have become more accurate and precise, such that the potential temporal resolution of records is high. Although long estab lished, the use of peatlands as archives of Holocene change has undergone phases of decline and resurgence. Here, the variable exploitation of the peat archive is explored, and recent developments in peatland science as applied to Holocene records are reviewed with reference to the collection of papers in this Special Issue of The Holocene, which are arranged in four key themes: (1) records of Holocene climatic change; (2) peatland dynamics; (3) carbon accumulation; and (4) implications for conservation and management. The changing acceptance of peatlands as archives of Holocene climatic change is attributed to developments in understanding of the peatland system and geographical differences in the history of Holocene research. Recent developments in biological and geochemical proxies combined with improvements in chronological techniques have resulted in renewed interest in peatland palaeoclimate records. Peatlands are an important global carbon pool and it is clear that climate has influenced the efficiency of long-term carbon sequestration by these systems. Climate has also had an impact on the biodiversity and condition of peatlands, which creates problems in discerning cause and effect in sites affected by human activities, and in targeting remedial management. It is concluded that particular strengths of the archive are the current diversity of peat-based palaeoenvironmental research and the potential for multiproxy analyses to be applied to a range of research issues. Mire-based investigations can complement research in other realms, and are deserving of greater attention from researchers of other archives.
Environmental change
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Abstract A high‐resolution study of bulk properties in a peat sequence from the Xinjiang Altai Mountains of northwestern China has allowed reconstruction of local variations in peat properties and peat C and N accumulation rates (CAR and NAR) during the Holocene. Analyses of peat bulk density, loss on ignition, and concentrations of total organic carbon (TOC) and total nitrogen (TN) and their elemental ratios and stable isotopic values suggest that changes in peat‐forming vegetation types during different parts of this epoch are the major factors responsible for the variations of peat properties in this sequence. The long‐term peat CAR has been 25.4 ± 7.7 (SD) g C/m 2 /yr, with lower values during the early Holocene and higher accumulations during the late Holocene, which is opposite to the Holocene variations in CAR in other northern peatlands. In contrast, the long‐term peat NAR is 1.5 ± 0.5 (SD) g N/m 2 /yr and is higher during the early and middle Holocene and lower during the late Holocene as in other northern peatlands. However, unlike other northern peatlands, long‐term peat NAR does not vary with the CAR, which is influenced by the peat density and accumulation rate. Variations in long‐term peat C and N accumulations in the Altai Mountains can be attributed to changes in primary productivity, in the dominant plant types and in peat decomposition caused by changes in both regional Holocene climate and local conditions.
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Throughout northeast China, the widely distributed peatlands have formed a large carbon (C) pool. However, the relationship between peatland initiation and climate controls is still poorly documented and understood. Understanding the responses of these C‐rich ecosystems to past climate change will provide useful insights into projecting the fate of peatland C in the future. In this study, we present a detailed historical reconstruction of peatland development in northeast China based on 312 basal peat dates, and examine the relationship between Holocene peatland dynamics and climate sensitivity. Our results indicate that peatland initiation started in the early Holocene, and that the majority of peatlands were initiated by and developed during the late Holocene. After the most intensive initiation period of 4.2–0.8 ka, the rate of peatland development slowed, which was concomitant with decreasing insolation and monsoon intensity. The widespread peatland initiation in the late Holocene might have been caused by the cool and moist climate patterns. The optimum timing of the peatland development was not uniform across northeast China, and these spatio‐temporal differences indicate the influences of regional climate and terrain on peatland initiation. Peat‐core data show variations in the long‐term apparent rate of C accumulation ( LORCA ) during the Holocene, with an average rate of 37.2 g C m −2 a −1 . The peak LORCA occurred during 10.5–9.0 ka, probably in response to higher temperatures and stronger East Asia summer monsoon intensities. Both temperature and humidity are important factors influencing the peatland initiation and C dynamics in this region.
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Carbon fibers
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Northern peatlands represent one of the largest biospheric carbon (C) reservoirs; however, the role of peatlands in the global carbon cycle remains intensely debated, owing in part to the paucity of detailed regional datasets and the complexity of the role of climate, ecosystem processes, and environmental factors in controlling peatland C dynamics. Here we used detailed C accumulation data from four peatlands and a compilation of peatland initiation ages across Alaska to examine Holocene peatland dynamics and climate sensitivity. We find that 75% of dated peatlands in Alaska initiated before 8,600 years ago and that early Holocene C accumulation rates were four times higher than the rest of the Holocene. Similar rapid peatland expansion occurred in West Siberia during the Holocene thermal maximum (HTM). Our results suggest that high summer temperature and strong seasonality during the HTM in Alaska might have played a major role in causing the highest rates of C accumulation and peatland expansion. The rapid peatland expansion and C accumulation in these vast regions contributed significantly to the peak of atmospheric methane concentrations in the early Holocene. Furthermore, we find that Alaskan peatlands began expanding much earlier than peatlands in other regions, indicating an important contribution of these peatlands to the pre-Holocene increase in atmospheric methane concentrations.
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This paper proposes a novel approach using basal peat ages and carbon (C) accumulation profiles from the world’s major peatland regions to decompose C flux terms from time-dependent C pool data observed from peat cores. Our peat-data syntheses show that the total peat C pools are 547 GtC, 50 GtC, and 15 GtC for northern, tropical and southern peatlands, respectively. The modeled net C balance (NCB) has a mean value of 41.8 TgC/yr for northern peatlands during the Holocene, ranging from 83.1 TgC/yr in the early Holocene around 9 ka (1 ka = 1000 cal. yr BP) to 21.5 TgC/yr around 2 ka, a temporal pattern mostly owing to the delayed effect of long-term decay of previously accumulated peat C. NCB from tropical and southern peatlands represents much smaller terms, mostly less than 10 TgC/yr. Northern peatlands represent about 90% of global total peatland C pool of 612 GtC and >90% of global peatland NCB. Our bottom-up global peatland synthesis indicates a decrease in rates of peatland area expansion and reduced CH 4 emissions during the late Holocene, thus lending support for an anthropogenic source of late-Holocene CH 4 rise. The C balance analysis of global peatland data indicates a cumulative net C uptake of 272 GtC in the early Holocene (11–7 ka), 151 GtC at 7–4 ka, and 116 GtC after 4 ka. The large cumulative fluxes and significant variations throughout the Holocene could greatly contribute to the observed atmospheric CO 2 and δ 13 CO 2 patterns derived from Antarctic ice cores. Thus, global mass-balance calculations or climate–carbon cycle simulations have to consider these large net C uptake terms from global peatlands and their variations over the Holocene.
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Macrofossil
Ombrotrophic
Sphagnum
Testate amoebae
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