Introduction
Calcareous speleothems, which have advantages for precisely dating and high-resolution sampling, are becoming one of the best geological record carriers
for major climate changes (Burns et al., 2003; Cheng et al., 2009; Dykoski et
al., 2005; Genty et al., 2003; Fairchild et al., 2006; Wang et al., 2001,
2008; Qin et al., 1999; Yuan et al., 2004) and high-resolution reconstruction
of the paleoclimate and paleoenvironment (Committee on Surface Temperature
Reconstructions for the Last 2000 Years and National Research Council, 2006;
Fleitmann et al., 2003; Hou et al., 2003; McDermott et al., 2001; Paulsen et
al., 2003; Tan et al., 2003; Tan, 2007; Wang et al., 2005; Zhang et al.,
2008). In addition to the most widely used carbon (C) and oxygen (O) stable
isotopes and trace elements, laminae and the growth rate of stalagmites could
also be used as proxies for the paleoclimate environment. However, different
authors have very different climate and environment interpretations relative
to thickness of laminae based on different stalagmites from different
climatic regions. For instance, the stalagmite laminae were confirmed as
annual laminae in the earliest studies (Baker et al., 1993), the structure of
the laminae reflected the intensity of the ancient rainfall (Baker et al.,
1999), and there was a positive correlation between the growth rate of
stalagmites and precipitation (Brook et al., 1999). However, there was a
negative correlation between the growth rate of stalagmites and precipitation
(Proctor et al., 2000, 2002), there was a responsive relationship between the
growth rate of the stalagmites and the winter temperature (Frisia et al.,
2003), and the growth rate of the stalagmites was influenced by the
vegetation density on the top of the cave (Baldini et al., 2005). There was a
well-understood relationship between the speleothem growth rate and climate
(Baldini, 2010; Mariethoz et al., 2012). The situation is more complex in
humid and semi-humid regions because other factors such as drip rate,
atmospheric PCO2 in the cave and the seasonality of the climate
may also affect speleothem growth rates (Cai et al., 2011; Duan et al.,
2012). The investigation of stalagmite laminae in the middle reach of the
Yangtze River indicates that the thickness of stalagmite laminae may be
regarded as a substitute index for the summer monsoon intensity in East Asia
(Liu et al., 2005). There was a good response relationship between the
variations in the thickness of the laminae and the variations in rainfall
(Tan et al., 1997; Ban et al., 2005). There was a response relationship
between the growth rate of the stalagmites and the temperature in summer;
therefore, the thickness of the laminae may be regarded as a substitute index
for East Asia monsoon intensity (Tan et al., 2004). The δ18O record
of ZJD-21 indicates that δ18O in the stalagmite was influenced
mainly by the amount of rainfall and/or the summer / winter rainfall ratio,
with lower values corresponding to wetter conditions and/or more summer
monsoon rains (Kuo et al., 2011). The stalagmite δ18O record represented local–regional moisture
change and the monsoon rainfall (Li et al., 2011). The growth rate and the observed temperature have a
significant positive correlation (Tan et al., 2013).
The upper part of ky1 (from the top to a depth of 42.769, 0–42.769 mm)
consists of 678 continuous clearly transmitting annual laminae, because the
transmitting laminae of the stalagmite ky1 are very similar to the annual
laminae of Shihua Cave in Beijing and have all of the typical characteristics
of the latter laminae, which consist of so-called northern type laminae (Zhou
et al., 2010). There are clearly very thin opaque laminae between stalagmite
laminae, but the calcite laminae are thick and transmitting between the
stalagmite laminae (Tan et al., 1999, 2002). Because stalagmite ky1, with a
very short length, has no trace of any weathering, the stalagmite may have
stopped growing not long ago. Its deposition time may be the past several
centuries or one millennium, which has recorded the climatic–environmental
information of the Shandong Peninsula since the late MWP (Medieval Warm
Period), including the late MWP, the whole LIA (Little Ice Age) and the early
CWP (Current Warm Period) (Lamp, 1965, 1972; Matthews, 2005; Ogilvie and
Jónsson, 2001). In this research, on the basis of high-precision dating
with the U–230Th technique, we have observed and measured the thickness
of the laminae and dated all of the laminae in the upper part of stalagmite
ky1, obtained and researched the time series data on thickness of laminae and
compared these data with the time series data for both the oxygen (O) stable
isotope value and the drought–waterlog index, and we discussed the climatic and
environmental evolution of the coastal part of the warm temperate zone as
well as the East Asia monsoon area since the LIA, especially in the
transition periods of MWP/LIA and LIA/CWP.
Geological setting and sample description
Stalagmite ky1 was collected in AD 2008 from Kaiyuan Cave
(36∘24′32′′[U +2033] N, 118∘02′05′′[U +2033] E)
in western Shandong Peninsula, the coastal area of northern China (Figs. 1,
2). The cave is located in the northwest hilly area of Mount Lu in
Zibo City, Shandong Province, with an elevation of 175 m above sea level
(a.s.l.) (Fig. 2). As the largest peninsula in China, the Shandong Peninsula
is located between the Bohai Sea and the Yellow Sea, and in its western
region, the Middle Cambrian Zhangxia formation (mainly the oolitic shale,
shale in clip to thin-layer limestone, oolitic limestone, algal clot
limestone) and the Ordovician Badou formation and Gezhuang formation
(mainly the gray–dark gray thick layer of mud, wafer-thin limestone,
dolomitic limestone and marl) are widely distributed with a thickness of
24–238 m, including the lower section integrated with the Gezhuang Group
and the upper section disconformity in contact with the Carboniferous Benxi
formation (Shandong Provincial Bureau of Geology-Minerals, 1991), which are
the main components of the Mount Lu, Yi, and Meng
with the highest elevations (1108, 1031 and 1150 m, respectively).
According to field investigation, the landforms of the carbonate rocks in
mountainous caves are well developed, and there are many cave outcroppings on the
surface. Secondary carbonate sedimentary bodies are developing well with
typical morphological characteristics.
The map of Kaiyuan Cave. The black point is the location where we
collected the sample in the cave. The cave has an entrance and an exit and
consists of six small halls.
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Location of Kaiyuan Cave and Shandong Peninsula in monsoonal China.
KC: Kaiyuan Cave (36∘24′32′′[U +2033] N,
118∘02′05′′[U +2033] E). ISM: India summer monsoon; EASM: East
Asia summer monsoon. The dashed black thin line indicates the northwestern
boundary of the Asian summer monsoon. The dashed black lines with arrows
indicate the routes of the summer monsoon. The dashed black lines with arrows
on the left indicate the routes of the summer monsoon. The brown area is the
Qinghai–Tibet Plateau. The green and brown area is China, and the yellow area is the
other area.
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Kaiyuan Cave developed in the dolomite of the Ordovician
Zhifangzhuang formation with a total thickness of the strata of approximately 110 m. The
total length of the cave is 1280 m, the overall distribution is a
northwest–southeast strike with twists and turns, and the space width inside
the cave is generally 2 to 8 m and can be up to 30 m. At the top of the
cave, the surface of the bedrock is covered by soil with a general thickness
of 50–80 cm, and the thickest soil was more than 1.0 m. The soil types are
calcareous rocky soil and drab soil (Soil and Fertilizer Workstation of
Shandong Province, 1994). The area of Kaiyuan Cave is currently influenced by
both summer and winter monsoons with annual precipitation of ∼ 620 mm
and an annual mean temperature of ∼ 13∘C, and summer monsoons
prevail during July and August, contributing to half of the annual
precipitation (Fig. 3).
Monthly mean temperature (T) and precipitation (P) of Zibo
(1952–1980) at Zibo Station and Yiyuan (1958–2005) at the Yiyuan Station,
two meteorological stations close to the study site (Fig. 1).
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Analytical methods and data processing
Establishment of a timescale
The stalagmite ky1 is conical in shape and consists of very pure calcite
(Fig. 4). The polished surface of the stalagmite and observation of the
laminae by microscope show that stalagmite ky1 has no hiatus during the
growing process. The upper part (0–42.769 mm) comprises 678 laminae
overlain by continuous deposits. All laminae are typical transmitting annual
laminae. The stalagmite ky1 has 232Th concentrations ranging from
704.6 ± 5.1 to 1245.2 ± 5.0 ppt (Table 1), which was determined
at the High-precision Mass Spectrometry and Environment Change Laboratory
(HISPEC) of the National Taiwan University using high-precision dating with
the U–230Th technique (Shen et al., 2002).
Polished longitudinal cross section of stalagmite ky1.
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U-series isotopic results and ages for stalagmite ky1 from Kaiyuan
Cave–Shandong Peninsula–northern China.
Sample ID
1
2
3
Dist. from top (mm)
6.0
15.0
25.0
238U ppba
347.47 ± 0.63
434.45 ± 0.92
334.58 ± 0.61
232Th ppt
1245.2 ± 5.0
959.9 ± 4.9
704.6 ± 5.1
δ234Unmeasured
1457.9 ± 5.5
1341.2 ± 5.1
1320.3 ± 4.6
[230Th / 238U] activityc
0.00652 ± 0.00014
0.00732 ± 0.00011
0.01021 ± 0.00013
[230Th / 232Th] ppmd
30.0 ± 0.68
54.63 ± 0.89
79.9 ± 1.2
Age-uncorrected BPf
289.6 ± 6.5
341.4 ± 5.4
480.6 ± 6.3
Age-correctedc,e BPf
251.1 ± 20.3
316.4 ± 13.6
456.6 ± 13.6
Age-correctedc,e AD
1761.9 ± 20.3
1696.6 ± 13.6
1556.4 ± 13.6
δ234Uinitial correctedb
1458.9 ± 5.5
1342.4 ± 5.1
1322.1 ± 4.6
Chemistry was performed on 8 July 2013 with the analysis method of Shen et
al., 2003), and instrumental analysis on MC-ICP-MS (Shen et al., 2012).
Analytical errors are 2σ of the mean.
a [238U] = [235U] × 137.818 (±0.65 ‰) (Hiess et al., 2012); δ234U = ([234U / 238U]activity-1) × 1000.
b δ234Uinitial corrected was calculated based on
230Th age (T), i.e., δ234Uinitial=δ234Umeasured × eλ234×T, and T is the corrected age.
c [230Th / 238U]activity=1 - e-λ230T + (δ234Umeasured/1000)[λ230/(λ230-λ234)] (1-e-(λ230-λ234)T), where T is the age. Decay constants are 9.1705 × 10-6 for 230Th, 2.8221 × 10-6 for 234U
(Cheng et al., 2013, EPSL), and 1.55125 × 10-10 yr-1 for 238U
(Jaffey et al., 1971).
d The degree of detrital 230Th contamination is indicated by the
[230Th / 232Th] atomic ratio instead of the activity ratio.
e Age corrections for samples were calculated using an estimated atomic
230Th / 232Th ratio of 4 ± 2 ppm. Those are the values for a
material at secular equilibrium, with the crustal 232Th / 238U value
of 3.8. The errors are arbitrarily assumed to be 50 %.
f BP (Before Present), “present” in this table refers to AD 2013.
Because the stalagmite ky1 has no hiatus, the upper part (0–42.769 mm)
contains 678 clear and continuous laminae. These continuous and ongoing
laminae have clear and definite chronology meaning themselves. Therefore, based on high-precision dating with the
U–230Th technique, we used the method of counting annual laminae to
decide the sedimentation time of each of the laminae and the whole stalagmite
ky1 layer by layer and established the timescale of the stalagmite. In the
upper part (0–42.769 mm) of stalagmite ky1, we counted along the upward and
downward directions according to some laminae that had high-precision dating
results with the U–230Th technique, confirming times of formation of the
1st and 678th laminae first and then ensuring the age of each of
the laminae according to their positions.
Measurement of the thickness of the laminae
The stalagmite ky1 was first cut along the growth axis, and a slice was
selected from the profile of the stalagmite and then polished. Second, under
the LEIKA DMRX microscope (magnification of 200×, eyepiece of
10×, objective of 20×), we used transmission light to
observe characteristics of the laminae along the growth axis layer by layer.
Third, we measured the thickness of 678 laminae along three different paths
layer by layer, calculating the thickness of every one of the laminae on
average according to the three data points for each of the laminae. Fourth,
we dated every one of the laminae layer by layer and determined the time
series data for the thickness of the laminae of the stalagmite. Finally, we
contrasted the time series data and the δ18O ratio data series,
analyzed the paleoclimate environment characteristic of the different stages
and discussed the climatic–environmental meaning of the variations in the
thickness of the laminae.
<inline-formula><mml:math display="inline"><mml:mrow><mml:msup><mml:mi mathvariant="italic">δ</mml:mi><mml:mn>18</mml:mn></mml:msup></mml:mrow></mml:math></inline-formula>O isotope test
First, perpendicular to the growth axis and along the position of 9.5 and
18.5 mm from the top, we collected four samples equally spaced at 20 mm
from the growth center that were used for the Hendy test. Second, along the
direction of growth, we collected a 4 mm depth ×5 mm
width × 75 mm length stone strip along growing axis, and scraped 330
samples using medical scalpel from top to bottom with a sampling density of
7–8 samples/mm (separation distance of 0.1296 mm on the average). From the
330 samples, we chose 175 samples to measure their δ18O ratios,
basically following the principle of an interval test to avoid the mixed
pollution between adjacent samples. Next, we confirmed the sedimentation time
according to their positions and formed the time series data for
δ18O ratios. The δ18O ratios were measured using an
automated individual carbonate reaction (Kiel) device coupled with a
Thermo-Fisher MAT 253 mass spectrometer at the State Key Laboratory of
Palaeobiology and Stratigraphy of the Nanjing Institute of Geology and
Palaeontology, Chinese Academy of Sciences. Each powdered sample
(∼ 0.08 to 0.1 mg of carbonate) was reacted with 103 %
H3PO4 at 90 ∘C to liberate sufficient CO2 for
isotopic analysis. The standard used is NBS-19, and one standard was analyzed
with every 10 samples. One sample out of 10 was duplicated to check the
replication. All isotope ratios are reported in per mil ( ‰)
deviations relative to the Vienna Peedee Belemnite (VPDB) standard in the
conventional manner. The standard deviation (1σ) for replicate
measurements on NBS-19 is ≤ 0.10 ‰.
Results and discussion
The thickness of the stalagmite laminae and the results of
dating
In the upper part (0–42.769 mm) of stalagmite ky1, the dating results for
ages corrected in Table 1 show that the three samples in the positions of 6, 15 and 25 mm are dated at AD 1761.9 ± 20.3,
1696.6 ± 13.6 and 1556.4 ± 13.6 respectively (Table 1).
Altogether, there are 221 laminae between the positions of 6 and 25 mm,
and their age intervals are 206 years according to the U–230Th dating
results. The difference in age between the laminae determined by counting and
by U–230Th dating is only 15 years. However, there are 109 laminae
between the positions of 6 and 15 mm, and their age intervals are 65
years according to the result of the U–230Th dating. There are 112
laminae between the positions of 15 and 25 mm, and their age intervals
are 141 years according to the results of U–230Th dating. If we use the
position of 6 mm as a datum for calculation, the ages of the 1st and
678th laminae are AD 1894 ± 20.3 and 1217 ± 20.3
respectively. If we use the position of 25 mm as a datum for calculation,
the ages of the 1st and 678th laminae are AD 1909 ± 13.6 and
AD 1232 ± 13.6 respectively. The age intervals are only different by 14 years. Finally, considering the error of the measurement of the thickness
of the laminae accumulating downward layer by layer, we chose the 133rd
of the laminae corresponding to the position of 6 mm as a datum to calculate
the age of the other laminae in the upper part of stalagmite ky1. The results
show that the deposition times of the 1st and 678th laminae are
AD 1894 ± 20.3 and 1217 ± 20.3 (the dating error is
±20.3 years, similar hereafter for the AD ages in this paper),
respectively; the ages of the other laminae were calculated by analogy. Thus,
we obtained the time series data for the thickness of the laminae of
stalagmite ky1 (Fig. 5).
The age model for stalagmite ky1 established by counting of laminae
and high-precision dating results with the U–230Th technique. This
figure is the photo of stalagmite ky1, and the age label was based on high-precision dating results with the U–230Th technique on the left. The
blue line is the high-precision dating results with the U–230Th
technique and the connecting lines. The red line is the age scale established
by this article. The ages of other laminae were determined by annual laminae
counting upward and downward based on the 133rd of the laminae
corresponding to the position of 6 mm. The date of which is AD 1762 ± 20.3
decided by high-precision dating results with the U–230Th technique.
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Characteristics of the shape of the laminae
Stalagmite ky1 obviously developed continuous transmitting laminae (Fig. 4).
Under the microscope, first, the thickness of the laminae is rather
changeable. The maximum thickness is more than 800 µm, and the minimum
thickness is less than 15 µm (Fig. 6a). Because the variations in the
thickness of the laminae may correspond to the climatic environmental changes
when the laminae were growing, the potential value of these transmitting
laminae for reconstructing the paleoclimate environment is illustrated (Genty
et al., 1996; Baker et al., 1999; Tan et al., 2004; Ban et al., 2005;
Y. H. Liu et al., 2005; Zhang et al., 2008; Muangsong et al., 2014; D. B. Liu
et al., 2015). Second, most of the boundaries of the laminae are straight,
but some laminae are obviously curved (Fig. 6b). When we analyzed the
climatic–environmental meaning of the thickness of the stalagmite laminae, we
acquired the laminae thickness values of the same laminae in different paths
and calculated their average values along multiple paths to determine the
substituted index information for climatic–environmental change that had
statistical significance. Third, colors in some of the boundaries of the
transmitting laminae are obviously deeper (Fig. 6c). These laminae have a
special structure similar to supra-annual laminae. This special structure
may indicate that climatic–environmental changes not only have seasonal
changes but also have multi-interannual changes. Fourth, the light
transmission of some transmitting laminae is obviously different from the
light transmission of adjacent laminae: the color is deeper, and there are
dark spots (Fig. 6a, d). Whether these dark laminae have some mineralogy and
geochemistry characteristics different from other transmitting laminae and
what their climatic–environmental significance may be, these dark laminae may
need further and special research in the future.
The characteristics of the transmitting laminae in the upper part of
stalagmite ky1 show that the thickness of the laminae has obvious
variations. The boundary was curved, and the color near the boundary was
deeper because of the dark transmitting laminae. The thickness of the
laminae shows obvious variations (a), the curve of the boundary of
transmitting laminae (b), the color variations of the boundary of
transmitting laminae, the arrows indicating the darker boundaries, the
boundaries in the middle were obviously whiter (c), dark transmitting
laminae (d) (the arrows indicated in the figure).
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Variations in the thickness of the laminae
The range of variation in the thickness of the 678 laminae of stalagmite ky1
(upper part) is 13.03–872.8 µm. The age determined for the
maximum thickness (872.8 µm) of the laminae is AD 1551. The age
determined for the minimum thickness (13.03 µm) of the laminae is
AD 1245 and the average value for all laminae is 63.08 µm
(Fig. 7a). In the 678 years from AD 1217 to 1894, the thickness of the
laminae from stalagmite ky1 have obvious stages of variation. Stalagmite ky1
had undergone the transition from low values to high values and again to low
values, and both the thickness of the laminae and the fluctuating degree of
variation in the thickness of the laminae had obvious stages of variation
(Fig. 7a). AD 1217–1471 was the low-value period of thickness of the
laminae with an average value of 46.08 µm. Then, the period from
AD 1217 to 1372 was a relatively low-fluctuation period. The period from
AD 1372 to 1471 was a period of relatively high fluctuation. The two periods
above presented the trend of rising first and then falling. AD 1471–1744 was a period of high value and high fluctuation in the thickness of
the laminae, with the average value of 88.8307 µm. This period
could be divided into three secondary high-value–high-fluctuation periods:
AD 1471–1548, 1548–1637 and 1637–1744. Every period shows the trend of
increasing first and then decreasing. The average values for the thickness of
the laminae were 82.2027, 82.5491 and 98.8252 µm, successively.
From AD 1744 to 1894, there was a period of relatively low values of the
thickness of the laminae, with a group of peak values appearing in
approximately AD 1776 with an average value of 45.1164 µm. The
period from AD 1217 to 1372 was a period of relatively low fluctuation. The
period from AD 1744 to 1831 was a period of relatively high fluctuation. The
two periods above present the trend of rising first and then falling. The
period from AD 1831 to 1880 was a period of relatively high fluctuation,
without a trend of obviously rising or falling. The period of rising was
short from AD 1880 to 1894.
The year of formation and the thickness data series of the 678
laminae in the upper part (0–42.769 mm) of stalagmite ky1 (a), the
cumulative departure curve (b) and the δ18O ratio data
series for 172 samples (c). The thicknesses of the laminae formed in
AD 1551 and 1646 were up to 872.818 and 820.423 µm, respectively,
much higher than other laminae. The cumulative departure curve (b)
is drawn by drought–waterlog indices on the basis of the Yearly Charts of
Dryness/Wetness in China for the Last 500-Year Period (Chinese Academy of
Meteorological Sciences of the China Meteorological Administration, 1981).
The curve has a rising trend representing less precipitation and the climate
becoming drier, and the curve has a declining trend representing more
precipitation and the climate becoming waterlogged.
![]()
Variations in the <inline-formula><mml:math display="inline"><mml:mrow><mml:msup><mml:mi mathvariant="italic">δ</mml:mi><mml:mn>18</mml:mn></mml:msup></mml:mrow></mml:math></inline-formula>O ratio
The variation range of δ18O ratios in the 172 samples above is
-6.247 to -8.599 ‰, with the maximum value (-6.247 ‰)
appearing in AD 1603 and the minimum value (-8.599 ‰) appearing in
AD 1460. The value of all of the samples is -7.674 ‰ on average
(Fig. 7c). In the 678 years from AD 1217 to 1894, δ18O ratios had
obvious stages of variation. The ratios had undergone a transition from low
values to high values and again to low values, and both the δ18O
ratios and the degree of fluctuation of δ18O ratios had obvious
stages of variation (Fig. 7c). From AD 1217 to 1480, there was a period of
low values of δ18O ratios with an average value of
-8.104 ‰. The period from AD 1217 to 1384 was a period of
relatively low fluctuation. This period had a trend of decreasing slowly. The
period from AD 1384 to 1480 was a period of relatively high fluctuation, and
this period showed the trend of rising first and then falling. AD 1480–1746 was a period of high value and high fluctuation with an average value of
-7.301 ‰. This period could be divided into three secondary high-value–high-fluctuation periods: AD 1480–1542, 1542–1633 and 1633–1746.
Every secondary period had the trend of increasing first and then decreasing
or decreasing first and then increasing. The inflection points appeared in
the ages of AD 1498, 1603 and 1663, respectively. The average values of the
δ18O ratios were -7.393, -6.953 and -7.513 ‰,
successively. AD 1764–1894 was a low-value period with an average
value of -8.199 ‰. The period from AD 1746 to 1831 was a period of
relatively high fluctuation. This period showed a trend of rising first and
then falling. The period from AD 1831 to 1880 was a period of relatively low
fluctuation and did not have a trend of obviously rising or falling. There
was a short rising period from AD 1880 to 1894.
Drought–waterlog index variations
To show the relationship between the variations in the thickness of the
laminae, the δ18O ratios and the changes in climate, we calculated
cumulative departure values for the drought–waterlog index in the area of
Kaiyuan Cave from AD 1470 to 1894. The data source was the Yearly Charts of
Dryness/Wetness in China for the Last 500-year Period. The charts are
compiled by the Chinese Academy of Meteorological Sciences of the China
Meteorological Administration according to extensive Chinese historical
literature and published by the China Cartographic Publishing House (Chinese
Academy of Meteorological Sciences of the China Meteorological
Administration, 1981). In the charts, the degree of drought–waterlog is
represented by the drought–waterlog index, which has five values including 1,
2, 3, 4 and 5 – with 1 representing the waterlog and 5 representing drought –
and its distribution is represented through the index isolines. On the basis
of Yearly Charts of Dryness/Wetness in China for the Last 500-Year Period,
we acquired the drought–waterlog indices for the area near Kaiyuan Cave
according to its geographical coordinates, and we checked the
drought–waterlog indices again referring to the local chronicles. We drew a
cumulative departure curve from AD 1470 to 1894 with a rising trend
representing the changes associated with becoming dryer and a declining trend
representing the change associated with becoming waterlogged (Fig. 7b). Based
on the cumulative departure curve, there was a period of less precipitation
in this area from AD 1480 to 1744. This period started with the transition of
MWP/LIA and ends with the transition of LIA/CWP. The primary fluctuations of
this period correspond to the curve of the thickness of the laminae.
(Fig. 7b). The high-value–high-fluctuation period of the thickness of
stalagmite ky1 laminae above occurred under the background of drought and
less precipitation. However, there is a correlation between the
δ18O ratios of stalagmite ky1 and the change in the summer monsoon
intensity and precipitation (Cheng et al., 2009). So, there is a correlation
between the summer monsoon intensity and precipitation and the growth of
stalagmites: the weaker summer monsoon intensity together with less
precipitation may be of benefit to the growth of stalagmites during LIA.
Climatic–environmental meanings of variations in the thickness of the
laminae
Because the difference in homologous thickness stages of the laminae and
δ18O ratios ranges from 2 years to 14 years,
the error of the dating technique is ±20 years (the time series data
from Sect. 4.1), and the resolution of the δ18O sample is 3.9
years, we may conclude that the two synchronize with time variation; i.e., the low-value period and the high-value period of the δ18O ratios
correspond to the low-value period and the high-value period of the thickness
of the stalagmite laminae. The low-fluctuation period and the high-fluctuation period for the δ18O ratios correspond to the low-fluctuation period and high-fluctuation period of thickness of stalagmite
laminae (Fig. 7a, c). The analysis result for the δ18O variations
shows that δ18O ratios for the four samples are -7.506,
-7.753, -7.981 and -7.691 ‰ for the samples that are
collected at a 9.5 mm distance from the top of the stalagmite and the 5, 10,
15 and 20 mm distance from the axis of growth, respectively. The δ18O ratios for the four samples that are collected at an 18.5 mm
distance from the top of the stalagmite are -6.571, -6.671, -6.540 and
-6.542 ‰. At 5, 10, 15 and 20 mm distances from the axis of
growth, respectively, and the δ18O ratios are similar for the same
laminae (Table 2). Hence, the Hendy test carried out for ky1 indicates that
calcite in ky1 should be deposited under isotopic equilibrium conditions. The
possibility of the dynamic fractionation of the calcite in the sedimentary
process is small; therefore, the stalagmite δ18O mainly reflects
the original external climate signal (Hendy, 1971). Therefore, the stalagmite
δ18O can be used to collect and reconstruct the information on
climate change (Tan et al., 2009, 2013; Kuo et al., 2011; Li et al., 2011;
Liu et al., 2015).
The results of the Hendy tests conducted along two growth laminae of
ky1 at depths of 9.5 and 18.5 mm individually, which indicate that calcite
in ky1 was deposited under isotopic equilibrium conditions according to the
Hendy test rules (Hendy, 1971).
Distance from
Distance from the
the top
Center of growth
Sample number
mm
mm
δ18O/ ‰
KY1-9/10-5
9.5
5.0
-7.506
KY1-9/10-10
10.0
-7.753
KY1-9/10-15
15.0
-7.981
KY1-9/10-20
20.0
-7.691
KY1-18/19-5
18.5
5.0
-6.571
KY1-18/19-10
10.0
-6.671
KY1-18/19-15
15.0
-6.540
KY1-18/19-20
20.0
-6.542
The obvious synchronization relationship between the variations in the
thickness of the laminae and the δ18O ratios variations in
stalagmite ky1 shows a close relationship between the variations in the
deposition rate of the stalagmite and climate change (Fig. 7). Because
Kaiyuan Cave is located in a warm temperate zone influenced by the East Asia
monsoon, its rainy season coincides with high temperatures. The
precipitation, carried by the summer monsoon from the low latitudes of the
Pacific Ocean, is concentrated in summer. However, when the winter monsoon from
the interior Asian continent at a high latitude prevails, rarely is there
precipitation. In this research, we interpreted the climatic meanings of the
stalagmite ky1 δ18O ratios, based on the relationship between the
cumulative departure of the drought–waterlog index and the curves of the
δ18O ratios. In consideration of characteristics of contemporary warm temperate
weather, also referring to the assumption of the Asia monsoon intensity by
Cheng et al. (2009) and the precipitation assumed by Zhang et al. (2008) about the climatic meanings of stalagmite δ18O records, with
lower δ18O ratios representing a stronger summer monsoon and higher
δ18O ratios representing a weaker summer monsoon, the
δ18O ratios are anti-correlative with precipitation (Fig. 7). There
was a strong summer monsoon and more-precipitation period from AD 1217 to 1480, a weak summer monsoon and less-precipitation period from AD 1480 to 1746
and a strong summer monsoon and more-precipitation period again from AD 1746 to
1894. The degree of fluctuation of the summer monsoon intensity and
precipitation is not the same or similar in different periods. As a whole,
the degree of fluctuation was lower when the summer monsoon was stronger and
the precipitation was more. The degree of fluctuation was higher when the
summer monsoon was weaker and the precipitation was less. The period from
AD 1217 to 1480 can be divided into one low-fluctuation period and one
high-fluctuation period. The period from AD 1480 to 1746 can be divided
into three high-fluctuation periods. The period from AD 1746 to 1894
included a high-fluctuation period, a low-fluctuation period and a
weaker and less fluctuation period, successively.
According to the thickness of the laminae and the δ18O record
of stalagmite ky1, the thickness of the laminae and both summer monsoon
intensity and precipitation have a negative correlation. The higher-value
period of the thickness of the laminae corresponds to weaker summer
monsoon and less precipitation, and the lower value corresponds to stronger
summer monsoon and more precipitation. The thickness of the laminae and the
degree of fluctuation of the summer monsoon intensity and precipitation have a
positive correlation. The period of the higher values for the thickness of
the laminae corresponds to a high degree of fluctuation of the summer monsoon intensity and precipitation, and a lower value corresponds to a low
degree of fluctuation in the summer monsoon and precipitation. Therefore,
Kaiyuan Cave, in the coastal area both of a warm temperate zone and the East
Asia monsoon area, demonstrates that the variations in the thickness of the
laminae are not only relative to the summer monsoon intensity and precipitation
but also relative to their degree of fluctuation. This is because karstic water
cycles faster and residence time is shorter in the fracture of rock. The
dissolution was insufficient and weak; therefore, the deposition rate and
the thickness of the laminae from the stalagmite were low in the period with
more precipitation. However, in the period of less precipitation, the
karstic water cycled slower, the residence time was longer in the
fracture of the rock, and the dissolution was sufficient and strong; therefore,
the deposition rate and the thickness of the laminae of the stalagmite were
high. However, karstic water would be reduced or dry up if the period of
less precipitation lasted for a long time. The period of less precipitation
is also bad for water dissolution and growth of the stalagmite laminae.
Under the background of weaker summer monsoons and less precipitation, the
degree of fluctuation of the summer monsoon intensity and precipitation becomes
higher, beneficial to increasing the average value of the thickness of the
laminae of the stalagmite, but the degree of fluctuation also becomes
higher. Because of the degree of fluctuation of the summer monsoon intensity and precipitation reflecting the degree of climatic stabilization,
according to both the thickness of the laminae and the δ18O
record of stalagmite ky1 from the Kaiyuan Cave, the climate change between
MWP and LIA in the coastal area of both a warm temperate zone and the East
Asia monsoon area, in addition to less precipitation and a lower
temperature, also shows that the degree of climatic stability obviously
decreased.
Conclusions
The upper part of stalagmite ky1 (0–42.769 mm) clearly consists of 678
continuously transmitting annual laminae. The time of deposition ranges from
AD 1217 ± 20 to 1894 ± 20; therefore, the laminae contain
the climatic–environmental change information for the late MWP, the whole LIA
and the early CWP. The analysis shows that both the variations in the
thickness of the laminae themselves and the fluctuating degree of variation
in the thickness of the laminae of stalagmite ky1 have obviously staged
characteristics from AD 1217 to 1894. Both the variations in the
thickness of the laminae themselves and the fluctuating degree of variation
in the thickness of the laminae of stalagmite ky1 had undergone the
transition from low values to high values and again to low values,
synchronized with the contemporaneous variations in the δ18O ratios
and the degree of fluctuation of the δ18O ratios. According to the
comparison among the thicknesses of the laminae, the drought–waterlog index
and the synchronous δ18O ratios of stalagmite ky1, the thickness of
the laminae and the summer monsoon intensity and precipitation have a negative
correlation. The higher-value periods of the thickness of the laminae
correspond to weaker summer monsoon and less precipitation, and low-value periods
correspond to stronger summer monsoon and more precipitation. The thickness of
the laminae and the degree of fluctuation of the summer monsoon intensity and precipitation have a positive correlation. The higher-value periods
of thickness of the laminae correspond to a high degree of fluctuation of
summer monsoon intensity and precipitation, and the lower-value periods
correspond to a low degree of fluctuation in the summer
monsoon and precipitation. Therefore, Kaiyuan Cave, in the coastal area both of a
warm temperate zone and the East Asia monsoon area, with the relationship
between the variations in thickness of the laminae and climate change, in
addition to the effects of climate factor variations such as temperature and
precipitation on the thickness of the laminae, also reflects closely the
degree of fluctuation of the summer monsoon intensity and the degree of
climatic stability. On the whole, there was a period of stronger summer
monsoons from AD 1217 to 1470. The climatic stability was high from AD 1217
to 1370 first and was reduced from AD 1370 to 1470. From AD 1470
to 1740, there was a period of weaker summer monsoon and lower degree of
stability that could be divided into three secondary periods with a trend of
stronger first and then weaker or weaker first and then stronger divided by
AD 1550 and 1640. Since AD 1640, the summer monsoon has again entered a
strong period. The degree of stability was high from AD 1740 to 1830,
and the degree of stability was reduced from AD 1830 to 1880. The summer
monsoon became weaker for a short time since AD 1880.
The conclusions of this research can enrich the knowledge about the
climatic–environmental meaning of the thickness of the laminae of a
stalagmite, contribute to the comprehension of the specific manifestation
of the MWP and LIA in the coastal area both of a warm temperate zone and the
East Asia monsoon area of northern China, especially the transition time of
MWP/LIA and the lasting time and the climatic characteristics
of the LIA, and deepen the research into the climate change in the
Asian summer monsoon area based on the secondary carbonate record in the
karst cave.