Microbial Characteristics and Vegetation Changes as Recorded in Lipid Biomarker of Tianmushan Peat Bog

EARTH SCIENCE FRONTIERS Volume 15, Issue 4, July 2008 Online English edition of the Chinese language journal Cite this article as: Earth Science Frontiers, 2008, 15(4): 170–177.

RESEARCH PAPER

Microbial Characteristics and Vegetation Changes as Recorded in Lipid Biomarker of Tianmushan Peat Bog YANG Guifang1,2, , XIE Shucheng1,3, HUANG Junhua1,3, CHEN Zhongyuan4 1 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, Wuhan 430074, China 2 Key Laboratory of Lithosphere Tectonics and Lithoprobing Technology of Ministry of Education, China University of Geosciences, Beijing 100083, China 3 Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan 430074, China 4 State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China

Abstract:

Lipid biomarker and organic carbon isotope were presented for identifying the microbial characteristics and

paleovegetation changes of Tianmushan area, Zhejiang Province, China, since the Middle Holocene. Our results showed that organic carbon isotopic values (į13Corg) ranged from í26.51‰ to í30.44‰, with an average value of í28.49‰, evidently indicating the predominance of C3 plants in this area since the formation of peat. The results from lipid biomarkers denoted a mixed organic contribution from lower and higher plants. The small molecules dominated by C17 were mainly from algae and bacteria, whereas the heavy-molecular-weight homologues (>C21) were chiefly from higher plants. Lipid analysis of peat profile revealed the carbon number distributions of n-alkanes, varying from C15 to C33, having a distinct odd-over-even predominance and a unimodal distribution in partial samples peaking primarily at C29 in the woodlands. The paleovegetation changes in relation to relative organic contribution can hereby be clearly detected by combining variations of C27, C29 and C31. We also found that the contribution of organic matter was largely from the higher plants below 68 cm. In relatively deeper part of the peat profile, more inputs were probably from the algae and bacteria, whereas the organic compositions were primarily from high vegetation and largely reduced contribution from algae and bacteria in the upper peat section. Combined analysis of biomarker and organic carbon isotope signature consequently revealed three remarkable paleoenvironmental stages, including the early stable era between 4100 and 3200 a B.P., the middle changeable period from 3200 to 700 a B.P., and a relatively warm stage since 700 a B.P. In addition, our results were well in line with the pertinent research results in the study area, thereby complementing molecular data in paleoclimate studies and providing a useful basis for comparing global changes. Key Words: lipid biomarkers; peat; microbial characteristics; paleovegetation changes

1

Introduction

The preservation qualities, reasonable continuity, and high-accumulation rate characteristic of peat bogs make them unique in retaining plant fossils to record valuable information about past climate[1–4]. The development status of peat bog and changes of plant fossils preserved in the peat have been employed to deduce the climatic oscillations since the 19th century[2–8]. Over the recent decades, concerns regarding climatic evolution and environmental reconstruction have become to be linked to questions of variations in monsoon

control and contributive factors. Attempts have been made to utilize stable isotopic record[9,10], total organic carbon (TOC) concentration[11], pollen information[12], humification index[13], as well as plant macrofossil analysis[14], to adequately determine the dominant plant species, precipitation change, monsoon evolution, temperature oscillation, and relate these to environmental reconstruction studies, but among these, little attention has been given to the possible significance of microbial activities, considering the uncertain information derived from the indicators used. In this regard, lipid biomarker, when combined with organic carbon isotope, can aid the

Received date: 08-Jan-2008; Accepted date: 07-Mar-2008.

Corresponding author. E-mail: [email protected] Foundation item: Supported by the Open Research Program of State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences (No. GPMR200605), and Open Research Program of Laboratory of Earth Surface System, Hubei Province (No. LESS-45). Copyright © 2008, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B.V. All rights reserved.

YANG Guifang et al. / Earth Science Frontiers, 2008, 15(4): 170–177

microbial activity elucidation in response to paleoenvironmental reconstruction. Lipid biomarker, regarded as an essential sort of organic matter and preserved in peat, may reveal diverse potentially informative biological features of the past period, thus making them the focus of interest for paleoenvironmental research for many years[3–6]. Organic carbon isotopic value (į13Corg) has been proved to be responsive to variation of paleovegetation[15,16]. It is a commonly held belief that higher organic carbon isotopic signature is usually associated with relatively higher temperature with a C4-dominated community. The more recent investigations, however, have revealed that fewer efforts were exerted to associate the lipid composition with variation of organic carbon isotopic value during the time of peat formation. The study appeared to be largely hampered by a lack of molecular elaboration specific to organic inputs and paleoecological evolution. The typical monsoon-dominant region, Tianmushan area, Zhejiang Province, China, was selected herein to reconstruct the microbial activity pattern and paleovegetation change as recorded in lipid biomarker and stable organic carbon isotope in response to paleoenvironmental variation. To achieve this, we have focused on lipid biomarker as well as organic carbon isotope to better discuss their potential use in microbial distribution preserved in peat mire. The high-resolution records can also be presented for the ecological response of vegetation, more or less synchronously to the shifts of monsoon climate. In addition, we have attempted to identify biomarker-based climate proxies from peat which can be more precisely utilized to trace the microbial activities and vegetation succession in relation to their environmental controls.

2

Study setting and analytical methods

The area studied (30°29ƍ58"N, 119°26ƍ27"E), situated in the northwest of Zhejiang Province, China, is an ideal place for addressing monsoon climate and global change. The average temperature per year in the study area is 14 °C and the highest monthly average temperature can reach approximately 26 °C in July. The region is featured by subtropical flora with a large variety of communities. The study section is located on the peak of Longwang Mountain. It is typical of a series of peat and clay layers. The sedimentation is successive and stable. In this study, we present a high-resolution analysis of the exceptionally well-dated 100 cm section from Tianmushan area. Forty-five samples of organic carbon isotope (į13Corg) were collected at 2 cm intervals. According to the variations of organic carbon isotope, 23 samples were chosen for lipid biomarker analysis. When testing organic carbon isotope, we heated the samples at 850 °C and measured the CO2 produced by using Finnigan MAT 251 mass spectrometer. The results were reported relative to the V-PDB standard. The measured precision of organic carbon isotope during the study was ± 0.2

× 10í3. The freeze-dried peat was ground to pass through a 0.5 mm sieve. Approximately 0.5 g peat was ultrasonic-extracted with dichloromethane/acetone for 20 minutes after adding a mixture of standards. The neutral fraction was further separated into hydrocarbon, aromatic, ketone/wax ester, alcohol/sterol, and polar fractions using column chromatography by sequential elution with hexane, hexane/dichloromethane, dichloromethane, dichloromethane/ methanol, and methanol. The alcohol/sterol fraction was adducted with saturated urea in methanol solution to separate n-alkanes from cyclic compounds. GC-MS analysis of the fractions were performed using a Hewlett-Packard 5973 mass spectrometer, interfaced directly with a 6890 gas chromatography equipped with a HP-5MS fused silica capillary column (30 m × 0.25 mm i.d.; film thickness 0.25 ȝm). The initial GC temperature program was 70 °C, and temperature was increased from 70 to 280 °C at 3 °C/min, held at 280 °C for 15 minutes; the carrier gas was helium; ionization energy of the mass spectrometer set at 70 eV; and the scan range was from 50 to 550 m/z. Chronologies were developed by 14C dating 4 samples from the study of Yin[17]. On the basis of the mean sedimentation rate, chronological sequence of this section can be adequately constructed by intercalation.

3 Distributions of n-alkanes and characteristics of ƣ13Corg values A large variety of compounds were identified in lipid extracts including normal alkanes, long-chain terpenoid, fatty acid, and polycyclic aromatic hydrocarbons. Previous work indicates that n-alkanes were particularly responsive to significance of origin of organic matter owing to its resistance to the decomposition of microbe. In this case, it will be primarily discussed with regard to its high potential to microbial activity records and vegetation diversification[18]. Normal alkanes commonly have a wide distribution in response to various kinds of plants and organisms, thus making them particularly sensitive to variations in biological origin in the light of their types of distribution, carbon number extension, and maximal carbon number. It is well recognized that the lower molecular weight alkanes, ranging from C13 to C21, is mainly of bacterial origin, whereas the presence of C17, C18 or C19 points to a contribution from the algae, fungi, and other microorganisms. Usually higher molecular weight alkanes are absent and odd-and-even carbon predominance is not distinct[19]. Whereas the longer chain homologues (usually >C21) might originate from higher plants normally with a unimodal distribution at C27, C29 or C31, indicating a high odd-and-even carbon predominance[20]. Consequently, the bimodal distribution often represents a mixed origin. Modern molecular organic geochemical studies discovered that C27 and

YANG Guifang et al. / Earth Science Frontiers, 2008, 15(4): 170–177

Fig. 1

GC/MS mass chromatograms of n-alkanes in peat profile

a: Unimodal distribution pattern; b: Bimodal distribution characteristics

Fig. 2 Distributions of n-alkanes in peat profile of Tianmushan area

C29 are of value as biomarkers for woody plants as compared with C31 and C33 for grassy vegetation[21,22]. Analysis of peat horizons showed a dominance of n-alkanes ranging from C15 to C33, having a unimodal distribution in partial samples with a maximum at C29 (Fig. 1). A distinct odd-over-even carbon number distribution was observed above C21 throughout the profile (Fig. 1). The most striking feature of carbon number pattern was the relative abundance of the C17 component, with the most abundant component being the C17 and C29 homologues, particularly the latter, notably displaying the vegetation indicative function. ACL values varied between 26.38 and 29.65, with a mean value of 28.22. The ACL values gradually increased in relative abundance in lower part of the

section, presented in Fig. 2. Peat horizons from 30 to 68 cm showed a relatively fluctuated distribution, with minimal ACL value at 38 cm, whereas those above 30 cm exhibited a stable period, with higher ACL values in relative abundance with an oscillation at the depth of 9 cm. The CPI ranged from 4.76 to 8.77, with an average value of 6.49 (referring to Fig. 2), reflecting the consistent trends with the value of (C15-21)/(C22-33) in the peat section. Accordingly, three remarkable periods could be recognized through the profile. Comparatively lower (C15-21)/(C22-33) and higher CPI values appeared in the bottom of profile (below 68 cm), which is followed by a waving period from 30 to 68 cm, with a maximal (C15-21)/(C22-33) and minimal CPI at 38 cm, whereas a declining trend of CPI value and relatively lower (C15-21)/(C22-33) values were observed in the upper part of peat profile. Figure 2 also showed that the į13Corg values of the bulk peat sampled from the profile mainly ranged from í26.51‰ to í30.44‰, with an average value of í28.49‰. At the beginning, the values of į13Corg had a slight fluctuation between approximately í29.00‰ and í28.00‰. Values in the interval from 68 to 30 cm were comparatively higher, with their distributions varying from 27.24‰ to 29.04‰. A dramatic trend toward more depleted values (í27.60‰ to í30.40‰) was seen above 42 cm. Beginning at 30 cm, the į13Corg values changed promptly, reaching the maximum at 18 and 9 cm, respectively. Interestingly, the variations of the organic carbon isotope with depth were well in accordance with the aforesaid indicators of lipid biomarkers.

4

Microbial features and vegetation changes

4.1

Microbial features of the peat profile

The n-alkane compositions of bimodal maximum at C29 and

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C17 at all levels presented the mixed inputs from higher plants plus bacteria and algae. The progressive shifts in n-alkane distribution were detected to illuminate the evolution of plant species, equally indicating the impacts from the microorganisms. This typical model of n-alkanes with maximum at C29 was found throughout the profile to present the main organic matter input from woody plants in this area, whereas the occurrence of short-chain homologues was of bacterial contribution. The comparatively lower (C15-21)/(C22-33) versus higher CPI values in the deeper section (below 68 cm) revealed the predominance of higher plants. Variations of (C15-21)/(C22-33) and CPI clearly indicated that the climate during this period was more favorable for the higher plants but was not favorable to the bacteria and algae to a certain extent. High-frequency changes of CPI and (C15-21)/(C22-33) values from 30 to 68 cm represented the decrease in inputs from higher plant and increase in algous and bacterial contributions. The higher values of (C15-21)/(C22-33) in this part might relate to the moderate climate which favored the development of animals and plants. It was for this reason that their bodies were then buried under the ground or decomposed by the microbes after death, supplying the nourishment for the survival of microorganisms. The following lower CPI as well as lower (C15-21)/(C22-33) values in the upper peat profile implied the elevated higher plant supply and largely reduced contributions from algae and bacteria. The climate at that time allowed for the blooming of most of the higher species. Slight decrease in CPI values at the depth of 9 cm was assumed to be due to inputs from both long-chain and short-chain biological compounds, affecting distributions in source organisms at the time of their growth. One possible explanation for the slight variations of CPI was closely linked to microbial actions. The detailed analysis has been implemented previously by Huang[23]. The results indicated that higher CPI values commonly occurred at the depth of 10–25 cm and decreased downward in the modern soil in the highlands of England. The changes were related to the microbial activities and chemical decomposition. The decrease in the CPI values with depth in our study has the similar mechanism in the variations of n-alkanes. The prime cause may be illustrated as the result of the microbial activity. Algae were capable of producing n-alkanes ranging in chain length from C14 to C32, usually without odd-and-even carbon predominance. Most photosynthetic bacteria also contained predominantly C14-C20 hydrocarbons, whereas many non-photosynthetic bacteria were reported to have C26-C30 hydrocarbons. So the input of organic matter by microorganisms could lead to a decrease in the CPI value, as seen clearly in our present profile. Previous work reported that various organic matters can be derived from the diversified plant remains, animal residues, and microbes[24]. The abrupt increase in relative abundance of low-carbon-weight n-alkanes down the profile was determined

by the change in organic inputs from different origins. The low-carbon-weight homologues were largely substituted by the higher plants below 68 cm due to the reduced relative contribution of organic inputs with depth. The major contribution was probably from the lower carbon weight fungi, algae, and non-photosynthetic bacteria in the middle of the profile, whereas the surface organic matter might originate from the higher plants. The results of former studies strongly confirmed that microbial characteristics weakened down the peat profile[25–27]. Our current study showed that the remarkable decrease in relative abundance below 68 cm might hint the contributions from both higher plants and lower species, particularly the latter, which showed an evident decrease. An increasing trend occurred with depth above 68 cm, cumulatively suggesting the increased contributions from fungi and bacteria. The active decomposition of bacteria and fungi in hot-moist climate considerably changed the n-alkanes, depleting the high-carbon number components. 4.2 Paleovegetation changes documented by lipid biomarkers and į13Corg values The predominance of C29 member identified in the long-chain n-alkanes was indicative of the presence of the woody plants in the peat deposit, though variations of paleovegetation can hereby be classified into 3 remarkable stages according to the changes of ACL values. The distributions of lipid biomarkers in the deepest section (below 68 cm) revealed the predominance of woody vegetation as followed by the mixed contributions mainly from grassy vegetation in most samples. Analysis of ACL variations reflected certain intensive vegetation changes over this time. The climate in the late period should be more optimal for the development of grassy plants. The higher values of ACL between 40 and 68 cm elucidated the continuous input of grassy vegetation. Particular case was seen in the extreme low ACL values at 38 cm, actually signifying the less grassy inputs and more woody plant supply. In the upper 30 cm, higher ACL might be the result of the more grassy input instead of the contribution from woody plants. A minor variation at 9 cm might be referred to the warm-wet climate in relation to the appreciable blooming of the woody species. The factors which could restrict the ACL value may include the change in vegetation since the different vegetation types have different maximum carbon number. The ACL value can be directly related to the climate change. In warmer tropical climates, land plants are postulated to biosynthesize longer chain compounds for their waxy coatings, whereas in cooler temperate regions somewhat shorter chain compounds are produced. The ACL values of plants in warm areas would consequently be higher than those of plants in cool regions. So the increase of the ACL value may be related to global warming, also facilitating the indirect control of the change in

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ACL via the change in vegetation. Typically, the elevated ACL values observed within the profile may be indicative of a comparatively warm period. The į13Corg values are closely related to the organic inputs, mainly from accumulation of decomposed plant remains. The į13Corg values of the bulk peat mainly ranging from í26.51‰ to í30.44‰, with an average value of í28.49‰, primarily indicate the dominance of C3 plants in association with moderate climate. The remarkable variation of 3.93‰ occurred in the 100 cm peat profile as compared with that of 2.00‰ in Guilin area[28]. This might indicate different water-heat condition and microbial control. The comparatively lower į13Corg values were observed in the entire peat profile, though higher values can be revealed in the upper 30 cm. We argued that the higher values were much likely influenced by the change in climate, resulting in the enrichment of į13Corg from the large variety of breakdowns of plant remains. Similar studies in Southwest China also depicted that decomposition of microbes preferably accumulate 13C when utilize the organic matter from high plants, leading to the increasing of į13Corg values[29]. Several researches have favorably testified the appearance of higher į13Corg values in the middle parts of the peat section. The largely reduction in the number of C3 plants and increase in the number of C4 plants in this period had led to higher values between 30 and 68 cm. The relatively lower and stable į13Corg values were presented in the lower profile (below 68 cm), showing the coexistence of C3 and C4 plants. Dramatically, the trend of changes in į13Corg values downward in the profile occurred to correlate well with that in vegetation through time, as shown by lipid biomarker

Fig. 3

distributions. This correlation possibly indicated the chief control of climate and environment. Significantly, the variations of the lipid biomarkers showed a strong correlation with those of the lithology (Fig. 3). For instance, the interface of brown-black mire peat at the depth of 68 cm can well match with the key point of global climatic change[30]. Furthermore, another two points of lipid biomarkers can be exactly correlated to the end of the Holocene Megathermal and Little Ice Age[30]. The regular fluctuations of lipid biomarkers might be responsive to microbial activities, replacement of the vegetation species, and change in climate.

5

Conclusions

The į13Corg values were incorporated into lipid biomarkers to demonstrate the variations of microbial activities. The consistent changes of these indicators presumably indicated the domination of microbial nature. The results of this study demonstrated that the lower part of the peat section below 68 cm was characterized by reduced vegetation inputs from both higher and lower plants. In relatively deeper parts of the profile (from 30 to 68 cm), more contributions originated from algae and bacteria, whereas the organic inputs were primarily from higher vegetation and partially from minor microorganisms in the upper layer. A good identity in all aspects between lipid biomarkers and į13Corg signatures was helpful to achieve a vegetation-based reconstruction of local paleoclimate evolution since 4 ka B. P. in the study region. We concluded that three particular periods could be distinguished as follows: (1) Before 3200 a B.P., all indices were relatively steady,

Paleoenvironment variation of the Tianmushan area since 4 ka B.P.

YANG Guifang et al. / Earth Science Frontiers, 2008, 15(4): 170–177

showing the stability of climate. The moderate į13Corg as well as the lipid biomarker distributions revealed that the climate during the time was noticeably moderate, with a cool-dry trend toward the late period. Evidence from spore reported the decline of tropical and subtropical vegetation accompanied by the occurrence of ferns[17]. Variations in vegetation were potentially linked to the changeable stage from 4000 to 3000 a B. P. and similar phenomena could also be discovered in other regions[9,31–34]. (2) Between 3200 and 700 a B.P., the climate of this period was of warm-cold alternations, as revealed by the CPI, ACL and (C15-21)/(C22-33) values. The higher values of į13Corg also made it clear that the climate over the time was warm and moist. Particular attention should be given to the dramatic warm oscillations that occurred in 2700–2000 a B.P. and 1400–1000 a B.P., exactly corresponding to the warm periods as documented in the previous studies[35]. By contrast, two major cold periods, appeared in 2000–1400 a B.P. and 1000–700 a B.P., can particularly match the two historical cold events[35]. (3) After 700 a B.P., the tremendous changes of higher į13Corg and CPI values depicted another warm epoch, with dominance of C4 plant and presence of typical forest vegetation. These changes, evidently verified by the distributions of lipid biomarkers, were presumably controlled by the climatic variations. However, the temperature decreased remarkably in the later part of this stage, clearly suggesting that the vegetation transformed into grassland. Significantly, several research results had been consolidated in our present study. The comparative research revealed that a strong consistence was detected to highlight the climatic and environmental evolution since the middle Holocene[17,35]. Of most importance were couples of semi-millennial events, which were well in accordance with those of Southern China Sea, though identification of driving mechanisms still remains a challenge for some events through this period. In addition, the distributions of C3 and C4 plants were tightly associated with the longitude, latitude as well as the height level[36]. The study area we chose was exactly situated in the transition zone of precipitation, temperature, and responsive height, enabling the effective recording of the microbial characteristics and paleovegetation changes. With regards to the potential traits of the lipid biomarkers in documenting the variations of rainfall and temperature, our results undoubtedly can enrich their in-depth implication to paleoclimate and paleoecology, thus providing essential basis for the paleoenvironmental evolution.

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