Hydrobiologia 214: 333-340, 1991. J. P. Smith, P. G. Appleby. R. W. Battarbee, J. A. Dearing, R. Flower, E. Y. Haworth, F. Oldfield & P. E. O’Sullivan (eds), 333 Environmental History and Palaeolimnology. 0 1991 Kluwer Academic Publishers. Printed in Belgium.

Paleolimnology of Qilu Hu, Yunnan Province, China

Mark Brenner,’ Kathleen Dorsey,’ Song Xueliang,2 Wang Zuguan,2 Long Ruihua,’ Michael W. Binford, Thomas J. Whitmore’ & Allen M. Moore4 ’ Florida Museum of Natural History, Gainesville, FL 32611, USA; ’ Yunnan Institute of Geological Sciences, Baita Road, Kunming 65001 I, Yunnan, China; 3 Graduate School of Design, Harvard University, 48 Quincy Street, Cambridge, MA 02138, USA; 4Dept. of Biology, Western Carolina University, Cullowhee, NC 28733, USA

Key words: China, limnology, paleolimnology, Pleistocene, sediment geochemistry

Abstract

Qilu Hu is a large (A = 36.9 km2), shallow (z,,, = 6.8 m) lake that lies at an elevation of 1797 m above msl on the Yunnan Plateau, southern China. Lake waters are hard (Mg = 3.2 meq L- ‘, Ca = 1.3 meq L- ‘), fresh (conductivity = 380 PS cm- I), and productive (Secchi < 40 cm). An 1 l-m sediment core has a basal 14C age of 30960 k 860 B.P. Sediments between 11 m and 6 m are high in% dry weight, rich in clay components Al2O3, Fe,O,, K,O, MgO, and low in organic C (I 6.1%), carbonate-C (< 1.0 %), total N (< 3.2 mg g- ‘), and total S (I 1.7 mg g- ‘). Diatoms and pollen indicate open-water conditions between 9.0 m and 6.0 m (13420-l 1790 B.P.). Above 6.0 m, CaCO, and organic matter concentrations increase relative to elastics. The transition marks a change to shallow-water conditions as inferred from diatoms and pollen, and probably reflects a shift to drier climate. Uppermost (80-O cm) red clays were deposited rapidly, probably as a consequence of recent (decades to centuries) riparian disturbances (e.g. agriculture, lake-bottom reclamation, urban development). Dates assigned to events in the Qilu Hu profile are tentative because of potential hard-water-lake error.

Introduction

The goal of our joint U.S.-China research pro- gram is to explore the sedimentary histories of selected lakes on the subtropical, karsted Yunnan Plateau, China. We are using a multidisciplinary approach to examine historical ecosystem altera- tion induced by climatic change and shifts in human land use. We hope to learn about the environmental impact of agriculture, industry, and urban development on watersheds in southern China. The work in Yunnan extends the geographic scope of paleolimnological research conducted by the U.S. investigators in several

other subtropical and tropical karst regions (Binford et al., 1987). This paper reports pre- liminary results from an 1 l-m core taken in Qilu Hu, a lake located about 100 km south of Kunming, Yunnan’s capital (Fig. 1).

Kunming lies at about 2000 m above msl, 250 km north of Vietnam and Laos, and 400 km ENE of Burma. The area has a warm temperate climate and receives about 800 mm of rainfall annually. Like many karst regions, Yunnan’s landscape is pocked with numerous lakes. The larger basins are of tectonic origin, whereas many of the smaller lakes were formed by solution pro- cesses. Near Kunming, the vegetation is charac-

Fig. 1. Map of the Yunnan Lake District showing the loca- tion of Qilu Hu. Upper inset map shows the location of Yunnan Province in southern China. Lower inset map is a bathymetric map of Qilu Hu, redrawn from the July 1980 map of the Tonghai County Hydrological Bureau. Water depth at the core site was 4.5 m in June 1987 when cores were

collected.

terized as subtropical evergreen broadleaf forest, but varies with edaphic conditions (Li & Walker, 1986).

Previous paleoenvironmental studies in Yun- nan have focused on the vegetational history of the region (Liu et al., 1986; Lin et al., 1986; Sun et al. ; Walker, 1986). Pollen diagrams from cores taken in the northern basin of Lake Dianchi (Caohai), near Kunming (Fig. l), yielded informa- tion on Pleistocene-Holocene climatic changes (Sunet al., 1986). Not surprisingly, prior to 10000 B.P., temperature on the Plateau was cooler than at present. Sun et al. (1986) suggest that Kunming was also wetter prior to 10000 B.P., contrary to the arid late Pleistocene conditions inferred for lowland subtropical and tropical sites in the western hemisphere (Watts, 1975; Bradbury et al., 1981; Deevey et al., 1983; Leyden, 1984). The authors conclude that the Caohai pollen record lacks strong evidence for anthropogenic vegetation disruption over the past 10000 years,

despite prehistoric settlement extending into the early Holocene. They speculate that Neolithic culture in southwest China may have developed and persisted in a forest context. Nevertheless, Li & Walker (1986) estimate that only about 10% of Yunnan Province is today covered by near-natural vegetation, most of it confined to alpine and subalpine areas.

Five lakes were cored by the U.S.-China team in June 1987 (Fig. 1). All five yielded short cores, l-3 m long. Long cores were unretrievable in four of the lakes (Cao Dian Hai, Yue Hu, Chang Hu, Qing Shui Hai) because their deeper deposits were indurated and probably non-lacustrine. Shallow basins Cao Dian Hai and Yue Hu pro- bably dried out in recent times, much as Dianchi was reported to have done nearly 200 years ago (Sun et al., 1986). Qilu Hu yielded an 11 m profile that contains a 30000-year record of sedimenta- tion (Deevey et al., 1988).

The study site

Qilu Hu lies at 24” 10’ N lat, 102” 45’ E long at an elevation of 1797 m (Fig. 1). The lake covers 36.9 km2 and the watershed: lake ratio is nearly 10 : 1. Qilu Hu has a z,,, of about 7 m and a Z of 4 m (Fig. I), but the lake level fluctuates and can vary by ~2 m annually. The lake volume is 1.486 x lo* m3 and water is supplied by rainfall (869 mm yr- ‘), ten intermittent rivers, and 39 springs. Water residence time is about two years. A sluice gate built in 1939 at the edge of a sinkhole regulates the lake level, and discharged water ulti- mately flows into the Nanpanjiang River. Qilu Hu waters are hard and the Ca : Mg ratio indicates local dolomitization (Ca = 1.3 Mg = 3.2 meq L-r: Moore et al 17::). &kk waters are fresh (cond = 380 PS cm-‘) and dense algal blooms, dominated by Microcystis sp., restrict Secchi disk depth to ~40 cm.

Recent excavations in Yunnan Province reveal a long history of hominid occupation. Middle Pleistocene Homo erectus teeth from Yuan-mou date to 0.5-0.6 x lo6 years (Chang, 1986). At Kunming and other sites in the province,

335

H. sapiens remains are associated with Paleolithic tools thought to be > 20000 years old (Li & Walker, 1986). Neolithic settlements were wide- spread by 4000 B.P. (Li & Walker, 1986), as was rice agriculture (Yen, 1987). Historical demogra- phics in Yunnan were influenced by the invasion and settlement of exogenous cultural groups (e.g. Mongols), and villages populated by cultural mi- norities persist to the present.

Large numbers of people occupied Tonghai County, near Qilu Hu, at least since the Ming Dynasty (A.D. 1368-1644). During that period, population in the county varied from 1.5 x lo4 to 3.5 x lo4 persons. At the beginning of the Qing Dynasty (A.D. 1644-1911), only 5.2 x lo3 people lived in the county, but the population grew to nearly 3.8 x lo4 under the reign of Emperor Qian Long. By A.D. 1984, the popula- tion of Tonghai County exceeded 2.1 x lo5 per- sons (Tonghai Office of Historical Records, 1985). Impacts of modern riparian settlement in- clude reclamation of near-shore lake bottom for agriculture, discharge of industrial eflluents, fishery development, and hydrologic control.

Field methods

A coring site was established at the east end of Qilu Hu (Fig. 1). Two mud-water interface cores were taken with a 4-cm diameter piston corer. The two profiles were designated cores 15-VI-87-1 and 15-VI-87-2. They were 100 cm and 99 cm long, respectively, and similar in gross stratigra- phy. Cores were extruded and sectioned in the field at 2 cm intervals to 10 cm, at 1 cm intervals between 10 and 30 cm, and at 4 cm intervals from 30 cm to the base of the section. A long core (16-VI-87) was obtained with a steel-barrel square-rod corer (Wright et al., 1984) beginning 0.5 m below the mud-water interface.

Laboratory methods

Percent dry weight was measured by weight loss on drying at 105 “C (HAkanson & Jansson,

1983). Total C was measured with a Coulometrics, Inc. Model 5011 coulometer (Huffman, 1977), and a System 120 preparation line set at 950 “C. Inorganic (carbonate) C was determined by coulometry using a System 140 prep line. CO, was evolved with 2N HClO,. Organic C was figured as total C minus inorganic C. Total S was measured with a Coulometrics, Inc. Model 3200 coulometer and 3220 prep line using prep line temperatures of 1050 “C and 850 ‘C. Total N and P were measured by autoanalyzer following digestion (Parkinson &Allen, 1975). CaO, Al,O,, Fe203, K,O, MgO, Na,O, Mn, Pb, and Zn were determined by X-ray fluorescence with an Ameri- can Research Laboratories, Inc. Model 8680 XRF.

Results

Chronology and sediment accumulation rates

Radiocarbon dates were provided by three labo- ratories: Guiyang, University of Pittsburgh, and the NSF-Arizona Accelerator Facility (Table 1). Low organic C content throughout the core re- quired broad-interval stratigraphic sampling for 14C analysis. The ages of the bottom two samples, PITT-0217 and GC-87045, are in reverse strati- graphic order. The great antiquity and very low organic C content of these deep deposits (Fig. 3) may place them beyond the reliable dating capa- bility of the equipment.

T&L 1. Uncalibrated radiocarbon dates from Qilu Hu Core 16-VI-87. The organic fraction of bulk sediments was dated following pretreatment to remove carbonates.

Depth (m) Material Lab-sample Radiocarbon age (yr B.P.)

2.8 1-2.99 Bulk sediment PITT-02 14 5,450 + 40 5.81-5.99 Bulk sediment PITT-02 15 11,790 f 70 6.60 Mollusc shell AA-3069 3,570 + 65 7.36-7.39 Wood AA-3070 10,740 + 120 9.01-9.19 Bulk sediment PITT-0216 13,420 f 200

10.06-10.27 Bulk sediment GC-87045 > 40,000 10.81-10.99 Bulk sediment PITT-021 7 30,960 +_ 860

336

The PITT dates on bulk sediment are all in stratigraphic order, but are subject to possible hard-water-lake error (Deevey & Stuiver, 1964). Additional dating anomalies may be a conse- quence of colluviation, if some lake sediment is redeposited soil of unknown isotopic content. The wood sample from 7.36-7.39 m is younger than the bulk sediment sample at 5.81-5.99 m (Table l), suggesting that dates run on the organic fraction of bulk sediment are artificially old. Assuming that the magnitude of dating error is similar in PITT samples 0215 and 0216, and that net sediment accumulation was constant between 5.90 m and 9.10 m, a bulk sediment sample at 7.38 m would be 12 540 years old, or about 1800 years older than wood from the same level. The shell date at 6.60 m (3 570 f 65 B.P.) is out of sequence and probably too young.

We estimated bulk sediment accumulation rates by assigning the uncorrected 14C age of each PITT sample to the midpoint of the dated sample interval (Table 2). Net sediment accumulation rate near the bottom of the section (10.9-g. 1 m) is slow, only 0.01 cm yr- ’ (8.0 mg cmw2 yr- ‘). Moving upward through the core, the sediment accumulation rate in the next interval (9.1-5.9 m) is more than ten times greater, 0.196 cm yr- ’ (111.7mgcm-*yr-‘). Between 11790B.P. and 5450 B.P. (5.9-2.9m), the sedimentation rate declined relative to the interval below, dropping to 0.047 cm yr-’ (18.0 mg cmd2 yr-‘). Net sedi- ment accumulation rate for the topmost section of the core (2.9-0.0 m: 0.053 cm yr- ‘, 19.7 mg cme2 yr - ‘) is similar to the value computed for the interval below.

Bulk sediment accumulation rates were cal- culated by an alternative method, using the wood

date at 7.36-7.39 m (10,740 B.P.), and the age at the base of the section (30960 B.P.). Prior to 10740 B.P., sediment accumulated slowly, at a rate of 0.17 cm yr-’ (12.4mg cmV2 yr-‘), whereas after that date the rate increased to 0.69 cm yr- ’ (28.3 mg cm-* yr- ‘).

Sediment geochemistry

Dry density (p) in the Qilu Hu core ranges from 0.17 to 0.87 g dry cme3 wet sediment. High values were recorded near the base of the core (Table 2) where predominantly inorganic sedi- ments are highly compacted. Percent dry weight parallels dry density and varies from a high of 57.3% (9.4 m) to a low of 12.6% at the mud surface (Figs. 2 & 3). Highest percent dry weight was measured in the most inorganic deposits, below 6.0 m, and in clayey sediments near the top of the section, between 80 and 15 cm.

Organic C concentrations are low (I 6.1%) in the bottom 6 m of the core, but rise sharply, averaging > 11 y0 between 5.0 m and 1.0 m depth (Fig. 2). Organic C content declines to < 5% in the clay-rich deposits above 80 cm (Fig. 3). Inorganic C concentrations are also low (< 1%) below 6.0 m depth, but rise to a maximum value of 8.2% at 2.0 m (Fig. 2). If all inorganic C is bound in CaCO,, carbonates account for 59-68x of the dry weight from 1.0 m to 2.0 m depth. Inorganic C content drops precipitously between 90 and 80 cm, and remains low to the top of the core (Fig. 3). Total N was measured on widely spaced samples, but shows similar trends to organic C. N values are low (0.15-0.32x) below 6 m in the core, but rise as high as 1.03 %

Table 2. Sediment accumulation rates in Qilu Hu computed from PITT radiocarbon rates.

Depth Interval interval thickness (cm) (cm)

O-290 290 290-590 300 590-910 320 910-1090 180

Time interval (years)

5450 6340 1630

17540

Accumulation rate [depth] (cm yr- ‘)

0.053 0.047 0.196 0.010

Mean dry density (g dry cmm3 wet)

0.37 0.38 0.57 0.78

Accumulation rate [mass] [g cme2 yr-‘)

0.0197 0.0180 0.1119 0.0080

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QILU HU Long Core 16-W-87

Dry Weight org*n,c c I”c.,g.“lc c TOM N C/N Tom s P cao A’2o3 % % b % mo n-1 ma a-l $I %

Pb 2” ps a-’ PO 9-l

10203040 50 150

I

% 123

(:

t

i

i

i

1

i i :

;

c

Fig. 2. Geochemistry of Qilu Hu long core 16-VI-87. Percent dry weight, organic C, inorganic C, total N, S, and P were measured at the Florida Museum of Natural History. CaO, Al,O,, Fe,O,, K,O, MgO, Na,O, Mn, Pb, and Zn were analyzed at Hubei

Geological Laboratory. Concentrations are expressed on a dry weight basis.

in the organic section of the profile (5.0-1.0 m) 18 in the core (Figs. 2 & 3). The mean ratio is 15.7 before falling again in the clayey upper 80 cm in the organic sediments (5.0-0.8 m), but only (Figs 2 & 3). Organic C and total N concentra- 10.4 in the basal 6 m of the section, and 10.3 in tions are highly correlated (r = 0.98, P <: 0.01). the uppermost 80 cm of the profile. Total S con- Organic C : total N weight ratios range from 7 to centrations vary from 0.2 to 8.0 mg g- r over the

QILU HU Mud-Water Interface Cores

<------------------ ,5+,+7-* ----------------~-, <------------------ t5-VI-87-l l -----------------* Dry Weight organic c I”olgwIc c TOM N C/N Total s P cao Al203 Fe203 ‘(20 WJ

% % % 10 203040

ma 0-l ma a-’ % % 0

5,015 - - 123 i -2 10 20 1020 5 1 10

Na20 Yll PO zn

mg g-1 m&l g-1 PO g-1 JJg g-1 I 2 0.51.01.5 2040 50 150 -f-

I 1 t I

1

J

/

-

Fig. 3. Geochemistry of Qilu Hu mud-water interface cores 15-VI-87-1 and 15-VI-87-2. *Core 1 was analyzed at the Hubei Geological Laboratory. Core 2 was analyzed at the Florida Museum of Natural History. Open circles in Core 1 denote values

obtained in the topmost section of long core 16-W-87 (see Fig. 2). Concentrations are expressed on a dry weight basis.

338

length of the section (Figs. 2 & 3), and are highly correlated with organic C concentrations (r = 0.88, P < 0.01). Reduced inorganic S was undetectable. Total P content ranges from 0.3 1 to 2.23 mg g- ’ (Figs. 2 & 3) and is uncorrelated (P > 0.05) with both organic C and inorganic C.

Plots of CaO (X-ray fluorescence) and in- organic C (coulometry) concentrations show nearly identical trends (Figs. 2 & 3). CaO : inorga- nic C weight ratios between 5 m and 1 m depth are close to 4.67, the expected value if all Ca and inorganic C is bound in CaCO,. Above 1 m and below 5 m, Ca : inorganic C ratios are generally > 4.67, suggesting that some calcium is present in non-carbonate form. Al,O,, Fe203, and K,O show similar concentration profiles (Figs. 2 & 3), and are probably associated with siliceous clas- tics. The clay components are inversely related to organic and inorganic C content. MgO is highly correlated with A&O, (r = 0.88, P < O.Ol), sug- gesting that most Mg is associated with the clay, rather than the carbonate fraction of the sediment. Na,O shows weak, but significant negative cor- relation with A&O, (- 0.40, P < O.Ol), and is uncorrelated with organic C (P > 0.05). Mn con- centration is positively correlated with organic C (r = 0.59, P < O.Ol), and even more so when the outlier Mn value at 8.0 m is removed (r = 0.82). Both Pb and Zn are associated with the inorganic, clay component of the sediment, as they are highly correlated with Al,O, (r = 0.91, P < 0.01 and r = 0.96, P < 0.01, respectively).

Discussion

The Qilu Hu section is divisible into three prin- cipal stratigraphic zones based on lithology and geochemistry. The basal zone (11.0-6.0 m) is dominated by gray clay and has low organic C and carbonate content. In the middle zone of the core between 6.0 m and 1.0 m, sediments are richer in organic C and carbonates. Above 1.0 m, organic C and carbonates decline and are replaced by iron-rich, red clays. The general stra- tigraphy of the Qilu Hu section correlates well with core DZ 18 from Caohai (Sun et al., 1986)

although organic matter attains higher concentra- tions in Caohai than in Qilu Hu.

The lithologic shift at 6.0 m in the Qilu Hu core is probably the consequence of a drop in water level that was caused by a climate change. The timing of the event is difficult to establish because 14C dates are subject to hard-water-lake error and some are out of stratigraphic order. Jumbled radiocarbon dates were also a problem at Caohai (Sun et al., 1986). Preliminary diatom data show that the assemblage between 9.0 m and 6.0 m depth in the Qilu Hu core is dominated by plank- tonic centrics (e.g. Cyclotellu rudiosu [ Grunow] Lem., Stephunodiscus rot& [Katz.] Hendey, Cy- clostephunos dubius [ Fricke] Round, Aulucoseiru grunulutu [Ehr.] Simonsen. These taxa indicate open-water conditions. Periphytic diatoms (Frug- iluriu pinnutu Ehr. pinnatu, F. pinnuta var. luncettulu [ Schum.] Hust., Opephoru murtyi HCrib. var. murtyi, Pinnuluriu sp., Stuurosiru sp. Q) are prevalent between 5 m and 1 m depth in the core and suggest shallow lake conditions. The inferred reduction in water level is supported by palynological evidence. Pollen of Alismu is very rare below 5.8 m, but becomes abundant (grains cm-‘) between 5.8 and 3.4 m. The emergent plant inhabits swamps and shallow waters. If we accept the 14C age on terrestrial wood at 7.36-7.39 m, the lake level reduction detected at about 6.0 m depth occurred after 10740 B.P.

Qilu Hu’s hard waters and the low organic C content of deep deposits make i4C dating of bulk sediments unreliable. Computed sediment accu- mulation rates based on bulk sediments are thus considered preliminary, and subject to error. Slow sediment accumulation between 30 960 and 13420 B.P. (8.0 mg cm-’ yr- ‘) may, in part, be a consequence of hiatuses in deposition. At some levels below 9.1 m in the core, gritty deposits contain low numbers of pollen grains and are devoid of diatoms. A high sedimentation rate was computed for the interval between 13420 and 11790 B.P. (9.1-5.9m: 111.9mg cme2 yr-‘), and corresponds to the period of high water level in the lake. This wet event detected in Qilu Hu may coincide with the moist period recorded in Caohai that is said to have occurred before 10 000

339

B.P. (Sun et al., 1986). Because i4C dates on bulk sediments may be older than their true age, we cannot say with certainty that the wet event occurred in the late Glacial, Net accumulation rates computed for the time spans between 11790 B.P. and 5540 B.P. (18.Omg cm-* yr-l, 0.47 mm yr- ‘) and between 5540 B.P. and present (19.7 mg cm- ’ yr- ‘, 0.53 mm yr- ‘) are nearly identical, and not unlike the Holocene (10000-l 500 B.P.) rate of 0.38 mm yr- ’ mea- sured at Caohai (Sun et al., 1986).

Sediments above 80 cm in Qilu Hu are clay-rich and differ from the underlying organic and car- bonaceous deposits. The change may be asso- ciated with lake level fluctuation, but probably is a consequence of land clearance and lake bottom reclamation for agriculture. If we assume con- stant sediment accumulation since 5450 B.P. (2.9 m), the shift to clay deposition at 80 cm occurred about 1500 years ago. Nevertheless, several lines of evidence indicate that clay deposi- tion began more recently, perhaps associated with the ten-fold increase in riparian human popula- tion since the turn of the century. The major change in sediment geochemistry suggests that the assumption of constant deposition is dubious. Furthermore, there is some evidence that the clays were deposted rapidly. Pollen concentrations are low at most levels above 80 cm, and diatoms are very scarce or absent in the clays. We attempted to determine whether Pb and Zn increases in the topmost 80 cm postdate industrial development. Throughout the Qilu Hu profile, the metals are correlated with the clay fraction (Al,O,) of the sediment. We looked at the Pb: Al,O, and Zn : Al,O, ratios in the uppermost clays (O-80 cm) and in the basal deposits (9.0-l 1.0 m) to see whether there might be a surplus of lead and zinc in the recent muds. Pb : Al,O, ratios are 38 y0 higher in recent muds than in deep deposits. Zn : Al,O, ratios in the topmost clays are 30% higher than those computed for the basal sedi- ments. The excess heavy metals in the recent sedi- ments suggest an anthropogenic (industrial?) source, but the timing of their increase must await 210Pb dating of the core.

Acknowledgements

This work was supported by NSF-INT grant 8802793 to E.S. Deevey, and supplemental funding from NSF and University of Florida to M. Brenner. Mr. Robert Dorion supported fieldwork in 1987. Financial assistance was pro- vided by grants from NSF-China and the Yunnan Provincial Commission of Science and Techno- logy to Yunnan University and the Yunnan Institute of Geological Sciences. Radiocarbon dates were kindly provided by Dr. Robert Stuckenrath and Ms. Qiao Yulou. We thank Bill DeBusk for assistance with geochemical analyses.

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