Hydrogen isotopic ratios of plant wax n-alkanes in a peat bog deposited in northeast China during the last 16 kyr

Abstract

Based on paleoclimatic reconstructions using various proxies, the Holocene Climate Optimum (10.5–6 ka) has been characterized as a warmer and wetter period in most of East Asia. The summer monsoons associated with the East Asian Monsoon evidently intensified and extended further inland from the Pacific Ocean, a source region of moisture. A notable exception to this general pattern exists in northeast China, where less wet conditions are recorded. We determined molecular compositions of individual plant wax hydrocarbons and their hydrogen isotope compositions (δD values) in a radiocarbon-dated peat core recovered from the Hani marsh in Jilin Province (China) and confirmed that the temperature-dependent effective precipitation in northeast China decreased during the Holocene Climate Optimum. A combination of P_aq, an indicator of the relative contribution of aquatic to terrestrial plants, and the difference in δD between low (C₂₃, C₂₅ and C₂₇) and high molecular weight (C₃₁) n-alkanes in the Hani peat bog indicates a dramatic change in vegetation from the deglaciation to the Holocene. No significant differences were observed between the δD values of low and high molecular weight n-alkanes with relatively high δD values and low P_aq during the early Holocene, indicating that all n-alkanes were produced by evapotranspiration-sensitive terrestrial plants during that time. However, lower δD values of mid-chain n-alkanes (C₂₃, C₂₅ and C₂₇) relative to the long chain n-alkane (C₃₁), together with higher P_aq values during the deglaciation (14–11 ka), suggest an increase in the contribution of aquatic plants and a higher water level during the period. The study demonstrates that northeast China was under a markedly wetter climate condition during the late deglaciation. For the 16 kyr record in the Hani peat sequence, we infer that moisture delivery by the East Asian Monsoon was relatively invariable in northeast China, but increased evaporation during the warmer Holocene Climate Optimum reduced the effective precipitation, defined by the balance between precipitation and evaporation.

Introduction

Peat deposits are important archives for reconstructing continental paleoenvironmental histories (cf. Blackford, 2000). The moist environments in which peat forms encourage growth of vascular plants and retard decomposition of their remains to produce thick accumulations of organic-rich material that can be used for high resolution paleoenvironmental reconstructions. The balance among precipitation, evapotranspiration and outflow determines the water depth in a particular bog, which in turn controls the kind and abundance of bog plants and their preservation as peat. Climate changes modify this balance by altering the precipitation of water and its temperature-sensitive evaporation. These changes can be recorded in the organic geochemical and paleoclimatic proxies of the peat layers that were deposited at different stages in the bog history.

Peat bogs are extensively distributed in northeast China. Some have been studied for the cimate-influenced Holocene evolution of biological environments (Liu, 1989, Hong et al., 2001, Hong et al., 2003). These and other studies confirm that northeast China has been sensitive to postglacial climate changes (Zhou et al., 2001, Jiang et al., 2006). The region lies on a steep precipitation gradient between sub-polar Siberia and the Sea of Japan. The climate in East Asia is associated with the development of the East Asian Monsoon. The winter component of this system consists of the transport of cold and dry air masses that originate from the Siberian high pressure and their subsequent outflow towards the Pacific Ocean. Summertime heating in North Asia creates low pressure over Siberia. Warm and moist air masses are then transported towards Siberia from the Pacific Ocean, providing the precipitation in northeast Asia. The consequence is annual alternations between dry winters and wet summers.

Paleoenvironmental proxies such as pollen, plant macrofossil and humification indices as well as carbon and oxygen isotopic compositions of cellulose isolated from peat cores have been used to reconstruct the historical postglacial and Holocene evolution of climate in various parts of the world (Barber et al., 1994, Blackford, 1995, Pendall et al., 2001, Hong et al., 2001, Hong et al., 2003, Zhou et al., 2004, Xu et al., 2006). Analysis of biomarker lipids from peat samples have also been used for reconstruction of paleovegetation and associated paleoclimate history (Ficken et al., 1998, Nott et al., 2000, Baas et al., 2000, Xie and Evershed, 2002, Xie et al., 2000, Xie et al., 2004, Pancost et al., 2002, Pancost et al., 2003, Disnar et al., 2005, Zhou et al., 2005, Nichols et al., 2006, Zheng et al., 2007, Hou et al., 2007). Most investigators have interpreted their proxy evidence of paleoenvironmental changes in terms of precipitation-induced variations in past bog water levels. In particular, the amount of precipitation delivered to the peat-forming plants during the summer season has been postulated to be the major paleoclimate factor recorded in peat sequences (Charman et al., 2004, Charman, 2007). The water levels in peat bogs can also be controlled by the effective precipitation – the difference between precipitation and evaporation (Barber et al., 2000, Barber and Langdon, 2007). This parameter is largely determined by summer temperatures. Changes in water levels owing to changes in either precipitation or temperature are clearly important to paleoclimatic reconstruction derived from peat sequences.

A novel approach for reconstructing changes in hydrological cycles is to measure hydrogen isotopic compositions (δD values) of individual biomarkers representative of submerged peat-forming plants that are protected from evaporation and of emergent and land plants that are sensitive to temperature-influenced evapotranspiration (Xie et al., 2000, Xie et al., 2004). Here, we present the hydrogen isotope compositions of plant wax n-alkanes from a radiocarbon-dated peat core in northeast China. The results allow us to infer a less moist Holocene Climate Optimum in the region, in contrast to most parts of East Asia. We discuss the significance of this finding in terms of the local evolution of the East Asian Monsoon system over the past 16 cal kyr.