Asian summer monsoon precipitation recorded by stalagmite oxygen isotopic composition in the western Loess Plateau during AD1875 ― 2003 and its linkage with ocean-atmosphere system

Based on 5 high-precision 230 Th dates and 103 stable oxygen isotope ratios ( δ 18 O) obtained from the top 16 mm of a stalagmite collected from Wanxiang Cave, Wudu, Gansu, variation of monsoonal precipitation in the modern Asian Monsoon (AM) marginal zone over the past 100 years was reconstructed. Comparison of the speleothem δ 18 O record with instrumental precipitation data at Wudu in the past 50 years indicates a high parallelism between the two curves, suggesting that the speleothem δ 18 O is a good proxy for the AM strength and associated precipitation, controlled by “amount effect” of the precipitation. Variation of the monsoonal precipitation during the past 100 years can be divided into three stages, increasing from AD 1875 to 1900, then decreasing from AD 1901 to 1946, and increasing again thereafter. This variation is quite similar to that of the Drought/Flooding index archived from Chinese historical documents. This speleothem-derived AM record shows a close association with the Pacific Decadal Oscillation (PDO) between AD 1875 and 1977, with higher monsoonal precipitation corresponding to cold PDO phase and vice versa at decadal timescale. The monsoonal precipitation variation is out of phase with the PDO after AD 1977, probably resulting from the decadal climate jump in the north Pacific occurring at around AD 1976/77. These results demonstrate a strong linkage between the AM and associated precipitation and the Pacific Ocean via ocean/atmosphere interaction. This relationship will aid to forecast future hydrological cycle for the AM monsoon region, and to improve forecasting potential of climatic model with observation data from cave.

The Asian Monsoon (AM) is an important part of global climate system and significantly influences the climate of the AM region via an interaction among the oceanland-atmosphere systems. Previous studies [1][2][3][4] have shown that the variation of ocean-atmosphere systems in the Pacific and Indian oceans at interdecadal and interannual timescales could lead to changes in the AM cir-culation and associated precipitation at the same timescales. For example, Krishnan and Sugi [1] found that there was a coherent negative correlation between interdecadal Pacific sea surface temperatures (SST) and the Indian monsoon rainfall during the last century. The warm phase of the Pacific Decadal Oscillation (PDO) corresponded with decreasing monsoonal precipitation and rising surface temperatures on the Indian subcontinent, while vice versa for the cool phase of the PDO. Precipitation associated with the AM circulation plays an important role in human liveliness and agricultural development for 60% of the world's population [5] . Therefore, additional research examining how AM precipitation varies on interannual to interdecadal timescales will contribute to a better understanding of how dramatic climatic fluctuations driven by anthropogenic greenhouse gases and aerosol will impact monsoonal precipitation. Currently instrumental data sets from most of China's monsoonal regions are too short (less than 60 years) to better understand and explore any temporal trends in monsoonal precipitation. Therefore in order to test current climate models it is necessary to develop precisely dated high-resolution proxies for monsoonal precipitation. Cave stalagmites have been widely used to study the AM because they are high sensitivity to climatic variation and can be precisely dated with U-series method [6][7][8][9][10][11][12][13][14] . Many significant results have been obtained on the variation and strength of AM [7][8][9][10][11]13,14] and the relationship between the AM and solar insolation on glacial/interglacial, millennial and centennial timescales using stalagmites [7][8][9]11,[13][14][15][16] . In particular, it was recently found that the stalagmite oxygen isotopic composition (δ 18 O) of stalagmite calcite could record interannual variation of the AM [17] and some historical severe famine events in tropics and subtropics [18] that corresponded to a weakening of the AM. Moreover, that the monsoonal precipitation was strongly linked to ocean-atmosphere systems in the Pacific Ocean and the North Atlantic Ocean, e.g., PDO, El Niño/Southern Oscillation (ENSO) [15] and North Atlantic Oscillation [19] .
Here we reported the δ 18 O record for the past 100 years reconstructed from the top part of a precisely dated stalagmite, WX42B, collected from Wanxiang Cave, Wudu, which is located in the transition zone between the Qinghai-Tibetan Plateau and Loess Plateau (western Loess Plateau). We compare this δ 18 O record with the instrumental precipitation data from the second half of 20th century and Chinese historical documents from the past 100 years, in order to investigate interdecadal varia-tions in monsoonal precipitation over the past 100 years and its linkage with ocean-atmosphere systems such as PDO.

Cave site and subsampling
Stalagmite WX42B was collected from Wanxiang Cave (33°19′N, 105°00′E, 1200 m a.s.l.), located in the Wudu county, Gansu province of China, and situated in a low altitude of the transition zone between the eastern Qinghai-Tibetan Plateau and the western Loess Plateau. This site is near the northern limit of the modern AM and thus is very sensitive to the advance and retreat of the AM and is an important climatic geophysical site. The cave is capped with 30-250 m of Carboniferous limestone, which is overlain with ~10 m of Loess sediments. The surface soils support local natural vegetation that is composed mainly of C 3 forest-shrub plant, characteristic of warm-temperate zone. Inside the cave, water percolation from the surface has generated a great amount of modern carbonate stalagmite deposits [11,20,21] .
The WX42B is 116 mm in length and compactly crystallized calcite with white and some light gray stripes, without recrystallization. Field investigations showed that there was plenty of dripwater originating from the cave ceiling and water membrane on its top. Combined with 230 Th dating, it was confirmed that the stalagmite was active when it was collected in 2003. The WX42B stalagmite was halved lengthwise and subsamples were collected along the growth axis for O-C isotope analysis by scraping surface using a knife. To avoid cross-overlapping of subsamples, alternative subsamples were selected for analysis [17] . Four layers with clear growth lamina were selected to do the Hendy test. Subsamples for 230 Th-dating were collected from five layers, 0.6-0.8 mm in thickness, using a 0.3 mm medical dental drill. Collection of these subsamples were performed in the Ultra-clean Laboratory of the Department of Geology and Geophysics, University of Minnesota.

Analytical methods
Carbon dioxide for isotopic analysis was produced with the McCrea's phosphoric acid method [22] , i.e., calcite powder reacts with 100% phosphoric acid in a pretreatment system and the released carbon dioxide is analyzed using a Finnigan-Delta-Plus mass spectrometer housed in the Key Laboratory of Western China's Environ-  . It is obvious that the growth rate of the WX42B is relatively constant especially for the 1-13.2 mm section, and the average growth rate of 0.12 mm/a is relatively high. The WX42B has a very high uranium concentration with 238 U content ranging from 7454.3 to 10847.7×10 −9 and averaging ~9162.0×10 −9 , and a low detrital thorium ( 232 Th content is between (264-2235) ×10 −12 and averages ~867.4×10 −12 ). Therefore, extremely high-precision 230 Th dates were obtained with average dating error (2σ ) less than 1 year ( Table 1). The age model for WX42B was established by linearly interpolation between dated points ( Figure 1).

Modern cave carbonate-water isotopic system and the Hendy test
The prerequisite for the paleoclimatic application of speleothem δ 18 O is that speleothem calcite is deposited under isotopic equilibrium. The equilibrium condition requires that (1) Modern cave dripwater displays a δ 18 O value consistent with that of modern meteoric precipitation and (2) the speleothem calcite was precipitated in isotopic equilibrium with cave dripwater and there is no secondary alternation of speleothem carbonate. Previous studies have suggested that the Wanxiang cave dripwater and its δ 18 O is controlled by local meteoric precipitation and the speleothem carbonate was formed in isotopic equilibrium with the dripwater [11,20] . The WX42B is compactly crystallized and shows no sign of secondary alternation. In this study, four layers (A-D) at different distance from the top of the WX42B were selected and along each layer six subsamples were obtained to do the Hendy test [25] . Dense arc-like laminas are present in WX42B and the bands are blurry, so accurate subsampling may not be done strictly within individual layer. The variation of δ 18 O values of C layer is larger (about 0.3‰) than those of the other three layers (almost no more than 0.2‰). The result shows that δ 18    Finnigan Delta Plus MS and fifty-nine data measured by Kiel IV-MAT 253 MS. The high coherence suggests accurate measurements of the δ 18 O ratio by the two methods ( Figure 3). Results of the Hendy test (Figure 2) suggest an equilibrium deposition of carbonates in the WX42B. The δ 18 O of the WX42B depend mainly on cave temperature and the δ 18 O of meteoric precipitation [25,26] , and thus is appropriate for reconstruction of paleoclimate.

Comparison of the WX42B δ 18 O record with instrumental precipitation data
Stalagmite δ 18 O records from the AM region mainly reflect the variations of δ 18 O values of monsoonal precipitation on glacial-interglacial, millennial to centennial timescales [7][8][9]11,13,14,27] . Some recent studies [12,17,28,29] suggested that there is an apparent negative correlation between stalagmite δ 18 O values and rainfall amount on shorter timescale, i.e., mainly influenced by "rainfall amount" effect. Johnson and Ingram's investigation [30] showed a large spatial and temporal variability in the stable isotope systematics of modern precipitation in China, and the degree of the δ 18 O variation influenced by rainfall amount was gradually reduced and that by temperature was gradually increased from monsoonal to non-monsoonal region. The modern stalagmite δ 18 O of Wanxiang Cave, a site close to the northern limit of modern summer monsoon, is possibly influenced both by meteoric precipitation δ 18 O and temperature. Johnson and Ingram's investigation [30] suggested further that stalagmite δ 18 O values from Wanxiang Cave mainly inherit the characteristics of local meteoric precipitation. The temperature dependent calcite-water fractionation of δ 18 O is negative (−0.23‰/℃) [31] and cancel the temperature effect of rainfall δ 18 O (0.24‰/ ) ℃ [32] . In order to further ascertain the climatic and environmental implications of the δ 18 Figure 4(a)). Moreover, in order to eliminate the year to year deviation between 230 Th dates and instrumental dates during the recent 50 years, a five-year running smoothing was conducted on both the δ 18 O data and instrumental data and a much higher correlation (the correlation coefficient is −0.64, significant at the 99% confidence level, Figure 4(b)) was found between the two records. These show that the WX42B δ 18 O record is significantly affected by the amount of precipitation conveyed through the AM, and therefore can be used to indicate the variation of the AM and associated precipitation in shorter timescale. Global investigation [33][34][35][36] suggested that stable isotope of precipitation shows a significant "amount effect" in monsoonal region at middle and low latitudes, i.e., stable isotope ratio of precipitation correlated negatively with rainfall amount. The δ 18 O value of precipitation in the most parts of China influenced by the AM circulation is lower in summer than in spring [37] . In Wanxiang Cave site, the δ 18 O w values of cave dripwater, collected in different seasons from 1999 to 2002, range from −9.6‰ to −8.7‰ (VSMOW) with the mean value of −9.1% (VSMOW) in the summer, relatively enriched in 16 O, and from −9.4‰ to −8.0‰ (VSMOW) with the mean dripwater δ 18 O w of −8.8‰ (VSMOW) in the spring, relatively enriched in 18 O. These indicate that modern precipitation in Wanxiang Cave site derives still from the monsoonal circulation. The stable isotope studies on meteoric precipitation in high-elevation section of the western Qilian Mountain in 2002 also presented that shifts in precipitation δ 18 O are influenced by rainfall amount originated from the monsoonal circulation rather than by temperature [38] .

Comparison between the WX42B δ 18 O record and Chinese historical documents from AD 1875 to AD 2003
The δ 18   During weakening periods of the AM, the monsoonal precipitation decreases and serious famine will occur. On the contrary, floods will appear during periods of stronger summer monsoon anomaly [5] . Therefore, the comparing the WX42B δ 18 O record ( Figure 5(a)) with the Drought/Flooding sequence ( Figure 5(b)) in Wudu archived by Chinese historical documents [39][40][41] over the past 100 years illustrates that the monsoonal precipitation is consistent with the Drought/Flooding index (Figure 5(a) and 5(b)), and the three typical monsoonal variation periods can also be reflected by Drought/ Flooding index as well. In the increasing period of monsoonal precipitation between AD 1875 and AD 1900, 46.1% of the years had flood or sub-flood event, and only 11.5% of the years experienced drought or subdrought; while during decreasing period of the monsoonal precipitation between AD 1901-1946, years experiencing drought and sub-drought amount to 33 % with frequent crop failures and widespread famines; and during the secondary enhanced period of the monsoonal precipitation between AD 1947-2003, years experi-encing sub-drought and drought are almost the same as those experiencing sub-flood and flood.
Within age errors due to 230 Th dating method and sampling of subsamples for 230 Th dating (about 5 years), decreasing or increasing monsoonal precipitation correlates well with famine or flooding events recorded by Chinese historical documents despite some small deviations between the two records. Especially some dramatic drought (AD 1878, 900, 1926-1930, 1936-1938, 1955, 1970-1971and AD 1996 and flooding events (AD 1879, 1889, 1917, 1947, 1962, 1984and AD 1990 in historical documents of Wudu [40] are well reflected in the WX42B δ 18 O record during AD 1875 to 2003.

Linkage of the AM precipitation to the oceanatmosphere system from the Pacific Ocean during the past 100 years
The AM system (including the East Asian Monsoon and Indian Monsoon) were driven by thermal contrast between ocean and continent, so it should be closely linked to the Pacific Ocean and the Indian Ocean circulation.
In order to explore the relationship between the monsoonal precipitation and the PDO, the WX42B δ 18  record is compared systematically with the PDO index [42] during 1900-2003 AD ( Figure 6). The two records parallel each other well with a higher correlation coefficient of 0.71 (significant at the 99.00% confidence level) before AD 1977 after 5-year running average. Two cold phases indicated by the PDO index during (I) AD 1900-1924/25 and (III) AD 1947-1976/77, respectively, are in accordance with two periods of increasing monsoonal precipitation, and one warm phase during (II) AD 1925-1946/47 is consistent with decreasing monsoonal precipitation. This correspondance of the monsoonal precipitation with the PDO indicates an increasing surface temperature contrast between middle Northwest Pacific Ocean and middle to high latitude of the East Asia continent associated with abnormally low sea surface temperature (SST) in the former region and the relationship between exceptionally high SST in the North Pacific region and cold SST in the tropical mideast Pacific [43] , respectively. These demonstrates that the oscillation of ocean-atmosphere systems of Pacific Ocean on timescale of five years or longer (including the decadal) may lead to persistent anomaly of the East Asian monsoon for several years [44] , and result in de-cadal climate variations of the Pacific Ocean and its surrounding regions including China. This is consistent with modern observation on the Pacific [45,46] . Moreover, a notable decadal climate jump occured at around AD 1976/77 in the north Pacific Ocean (IV in Figure 6), a rapid change of PDO from cold phase into warm phase which is not consistent with the monsoonal precipitation variations illustrated by the WX42B δ 18 O record. This inconsistence may result from a transition of dominant factor controlling the East Asian summer monsoon from SST anomaly in the North Pacific prior to AD 1976 to SST anomaly in the equator mid-east Pacific Ocean thereafter, a so-called decadal shift of the key zone of interannual SST anomalies [43] .

Conclusions
The WX42B δ 18 O record from Wanxiang Cave sensitively recorded changes of the AM strength and associated monsoonal precipitation in the AM marginal zone on short timescales.
Over the past 100 years, the AM strengthened and associated monsoonal precipitation increased during AD 1875-1900 and AD 1947-2003, respectively, and relatively weakened during AD 1901 -1946. The WX42B δ 18 O record correlates negatively to local precipitation over the past 50 years. Comparison between the AM variation recorded by the WX42B δ 18 O data and Drought/Flooding index reconstructed from Chinese historical documents indicates a good coherence between the two records within 230 Th dating errors, with more frequent flooding events corresponding to stronger AM and higher monsoonal precipitation and drought events to reduced AM and monsoonal precipitation.
Variation of the AM strength recorded by the WX42B δ 18 O data suggests that it was closely related to the PDO during the past 100 years with a warm PDO phase corresponding to decreasing monsoonal precipitation and vice versa on interdecadal timescale. However, The monsoonal precipitation variation became out-of-phase to the PDO after AD 1977, which may be a reflection of the decadal climate jump in the North Pacific at around AD 1976/77. All of these results indicate that variations of the AM strength and associated monsoonal precipitation are closely linked to the Pacific Ocean via an interaction ship between ocean and atmosphere systems. This relationship will aid to forecast future hydrological cycle for the AM region, and to improve forecasting potential of climatic model with observation data from cave.
The authors would like to thank Zhang P Y and Xu Q Z, Cao J X and Dai Z B for their help in fieldwork and experimental analysis. Especially we thank two anonymous referees for their constructive suggestions on the manuscript.