научная статья по теме HUMIFICATION DEGREE AS A PROXY CLIMATIC RECORD SINCE THE LAST DEGLACIATION DERIVED FROM A LIMNOLOGICAL SEQUENCE IN SOUTH CHINA Геология

Текст научной статьи на тему «HUMIFICATION DEGREE AS A PROXY CLIMATIC RECORD SINCE THE LAST DEGLACIATION DERIVED FROM A LIMNOLOGICAL SEQUENCE IN SOUTH CHINA»

ГЕОХИМИЯ, 2011, № 4, с. 427-434

КРАТКИЕ СООБЩЕНИЯ

HUMIFICATION DEGREE AS A PROXY CLIMATIC RECORD SINCE THE LAST DEGLACIATION DERIVED FROM A LIMNOLOGICAL

SEQUENCE IN SOUTH CHINA

© 2011 Wei Zhong*, Qiaohong Ma*, Jibin Xue*, Yanming Zheng*, Ying Cai*, Yangjun Ou*, Xuefeng Yu**

*School of Geography Science, South China Normal University, Guangzhou 510631, China **State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS, Xi'an 710075, China

Email: DL06@scnu.edu.cn Поступила в редакцию 06.03.2009 г.

Keywords: swamp Dahu; humification degree, the Last Deglaciation, East Asian monsoon, South China.

Humification, in general, is a proxy to describe the degree of decomposition ofplant remains. Since the process of decomposition of died plants involves both the local climate and other conditions, so humification can be used to indicate variations of sedimentary environment or paleoclimate. A number of researches have demonstrated that the humification (generally expressed as humification degree, HD) ofpeat is a useful proxy indicator of past climatic changes [1—7], because peat humifica-tion is strongly influenced by microorganism activity, conditions of wetness and temperature, pH values of the soil and the type of peat-forming plants, among which the conditions of wetness and temperature are the most important ones [8]. Up to now, almost all the relevant studies concerning the climatic implication of the HD mainly concentrate on the peat deposits, few on the lacustrine or limnological sediments (here refers to sedimentary sequence that is dominated by alternations between the lacustrine and the marshy sediments). However, a recent study points out that for the sediments ofLake Barkol in the extremely arid northeastern Xinjiang, NW China, the HD record can also provide very useful information on Holocene climatic changes [9]. So we think the HD record of limnological sediments has great potentiality for indicating past climatic changes as well. In the paper, we try to test the using of the HD record of a limnological sequence derived in Swamp Dahu to indicate the past climatic changes in South China.

Swamp Dahu (24°45.3'N~24°45.7'N, 115°2.1'E~ 115°2.3'E), developed in a small closed intermontane basin in the eastern Nanling Mountains, is located about 2 km southwest of the Dingnan county town in Jiangxi Province (Fig. 1). This swamp is mainly fed by atmospheric precipitation and some small streamlets. Detrital materials deposited in the swamp are considered to have been derived from the surrounding areas and transported by streamlets or surface runoff. The study area is the watershed between the Yangtze River and Zhujiang River systems and lies in the southern boundary of the mid-

subtropical zone. The East Asian (EA) monsoon climate as well as zonal climatic shifts strongly influences the study region [10]. Mean annual precipitation is about 1600 mm and mean annual temperature is 17.8°C. Modern vegetation here consists primarily of subtropical species of evergreen broadleaf trees with Castanopsis and varieties of Lithocarpus and Cyclobalanopsis as the dominant assemblages and Schima, Eurya, Michelia, Symplocus and Ilex as the companion assemblages [10]. Granite bedrock formed a near-surface aquiclude that trapped water and established favorable conditions for deposition of limnological sediments in the swamp.

A paleoclimatic history during the last 18000 yr in the swamp based on pollen results has been reported by Zhou et al. [10] and Xiao et al. [12]. Their researches demonstrated that owing to geographical location, Swamp Dahu has the potential to provide a high-quality record of the EA monsoon climate that should improve our understanding of the process of monsoon variation and the dynamic linkage ofAsian monsoon variations with global climate changes. In December 2006, we recovered several sediment cores using an Eijkelkamp sampling core device, of which a 350-cm-long core (named core K02, 24°45'43"N~115°02'25"E) was chosen for paleoclimatic study.

LITHOLOGY AND CHRONOLOGY OF THE SEDIMENT CORE

The core K02 was split, photographed and described on the spot. Sediments of the upper 20 cm of the core were reworked by the modern human's activity. Core sediments are characterized by alternations between lacustrine sediments and marshy sediments. The lacustrine sediments are primarily composed ofgray or grayish green silt or silty clay, while the marshy sediments are dominated by herb-rich peat or organic-rich dark gray or black silt or clay (gyttja). Sedimentary features ofthe core are shown in Fig. 2.

80°

100°

120°

140°

E

115°02.1'E

115°02.3'E

Winter monsoon

^-JWyJet

Westerly

N

50°

40° 30° 20°

10°

100 m J

Legend Swamp Dahu

Core^ite

Outline c

vally <<rrrtf,

Fig. l. The locality of Swamp Dahu and its climatic background (modified after Peng et al [12].

Depth/cm Age/Cal yr BP 0 4000 8000 12000 16000

50 100 150 200 250 300

350

Date Lith. (cal yr BP)

i— 1617

5090

7320 8150 8828

9866 11066 11850 13 448

14019 15105

15931

Legend

Grayish green Sand and silt

Dark gray or black gyttja

Grayish green or brown silt or silty clay

Herb-rich peat

Coarse sand and gravel

Fig. 2. Lithology and relationship between age and depth of core K02 in Swamp Dahu.

Twelve organic-rich bulk samples were collected for radiocarbon dating using the conventional method. Radiocarbon ages were calculated with a half-life of 5568 yr (Table, Fig. 3). All 14C dates were calibrated to the calendar ages using Calib 5.0 program [13]. The chronological sequence for core K02 is established by linear interpolation between the two adjacent dated points. The bottom age of the core was determined at ca. 16000 cal yr B.P.

ANALYTICAL TECHNIQUES

Core sediments were sampled at 1 cm intervals for analyses of humification degree (HD) and organic matter content (OM). Measurements of organic carbon isotope (S13Corg) and bulk dry density (DD) were performed on samples collected at 2 cm intervals. Concentrations of chemical elements rubidium (Rb) and strontium (Sr) were analyzed at 5 cm intervals.

Radiocarbon dating results of core K02 in Swamp Dahu

Field code Lab. code Depth (cm) 14C age(yr B.P) Calib. Age (2a, cal yr B.P)

JXDN-01-01 07-119 5-10 1711 ±64 1420-1815

JXDN-01-03 07-121 33-38 4468 ± 80 4874-5307

JXDN-01-05 07-123 78-83 6421 ± 76 7174-7466

JXDN-01-06 07-124 110-115 7319 ±89 7972-8329

JXDN-01-07 07-125 131-136 7961 ± 106 8544-9113

JXDN-01-09 07-127 175-180 8555 ± 122 9621-10112

JXDN-01-10 07-128 200-205 9742 ± 102 10746-11387

JXDN-01-11 07-147 220-225 10180 ± 108 11348-12352

JXDN-01-12 07-148 235-240 11565 ± 109 13225-13672

JXDN-01-13 07-149 270-275 12160 ±91 13780-14259

JXDN-01-14 07-150 285-290 12798 ± 126 14642-15568

JXDN-01-15 07-151 343-348 13410 ± 155 15386-16477

To determine the HD, samples were sieyed through 60 ^mand were uniformly mixed. The 0.10 g sample was put in a beaker, and 100 mL of 0.1 mol/L NaOH were added. Then it was heated and boiled for one hour in order to extract humic acid. After cooling down, the solution was filtered through Whatman Qualitative 1 filter paper and diluted. The absorbance of the diluted solution was measured at the wavelength of 520 nm using a UV-1901 spectrophotometer. Before measurement of the diluted solution, the absorbance of distilled water was measured and was used as the standard value (0%). The absorbance of the diluted solution was expressed as relative to the standard value. The absorbance were used to express the HD, higher absorbance value indicate enhanced HD and vice versa.

Samples for 813Corg measurement were acid-washed with 5% HC1 to remove possible trace amounts of carbonate carbon, rinsed with de-ionized water and oven dried at 50°C. Then, about 20 mg of each sample was combusted in an excess of oxygen at 1020°C using an elemental analyzer. The value of produced CO2 were then measured using a Finnigan MAT-253 Mass Spectrometer. The analytical precision is 0.1%o. Results are given in the 8 notation relative to the international PDB standard in per mille (%) [14].

DD was measured as the weight of the dry mass per unit volume [15]. Dry mass was determined by oven drying at 50°C until constant weight was achieved.

Depth/ cm

Lith.

0 1 1617—

50 - 5090—

7320—

100 - 8150—

150 - 8828—

9866—

200 - 11066—

11850—

250 - 13448—

14019__

300 - 15105—

350 - 15931—

Age

(Cal. yr)

HD (abs, %) 0 20 40 60 80 100

OM (%)

10 20 30 40

--^tJ^

I-• I I ■■ * * '

32 -31 -30 -29 -28 -27

513Corg (%)

0 0.15 0.35 0.45 0.60 0.75 DD (g/cm3)

Fig. 3. Variations of HD, S13Cor„, OM, DD records and Rb/Sr ratios in core K02 in Swamp Dahu.

org

OM were determined using the potassium dichro-mate-vitriol oxidation titration technique with an uncertainty of 0.2%.

Chemical elements rubidium (Rb) and strontium (Sr) were analyzed using a VP-320 X-ray fluorescence (XRF) spectrometer, after sample being ground to fine powder <38 ^m. The error is less than 1 ppm.

PALEOCLIMATIC IMPLICATION OF THE HUMIFICATION DEGREE

Although some experimental studies have revealed a nonlinear influence of water and heat conditions on the decomposition intensity of plant organism [5], this kind ofexplanation can not be simply used to ascertain the pa-leoclimatic indication of the HD in core K02.

For the core K02, lithological variation suggests that the appearance ofthe marshy sediments characterized by peat or gyttja generally represent shrinkage ofwater body in the swamp, suggesting relatively drier climates, whereas the lacustrine sediments indicate expansion of water body, implying wetter conditions.

It is often considered that changes in temperature and primary productivity are the most important factors controlling the OM in the lacustrine sediments. We infer that the input of organic matters into Swamp Dahu primarily originates from the terrestrial and near shore aquatic plants around this swamp, because OM generally appear ver

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