PAGES Magazine articles

Figure 1: Varved sediments of Lake Suigetsu, Honshu Island, Japan. The curve shows relative gray scale values as measured on the displayed image.

Figure 2: Photographic litho-stratigraphy of the SG06 core (modified after Nakagawa et al. 2012). Red shades show disturbed sediment sections. The coring team consisted of people from Newcastle University (UK), University of Tokyo, Osaka City University, Chiba University of Commerce, Naruto University of Education, Kyoto University (Japan), GeoForschungsZentrum Potsdam (Germany), and Royal Holloway University of London (UK).

Lake Suigetsu has a long history in Quaternary research. In 1991, a group of Japanese researchers, led by Prof. Yoshinori Yasuda of the International Research Center for Japanese Studies (IRCJS, Japan), recovered a couple of >10 m long piston cores from the lake and found that the lower parts of those cores were annually laminated (varved; Fig. 1). This marked the initiation of varve studies in Japan.

Two years later (1993), the same group conducted deep drilling of Lake Suigetsu and recovered a >73 m long sediment core that reached bedrock. The top ca. 40 m of the core was quasi-continuously varved. The earliest important scientific outcomes from this long core (SG93) were delivered by Hiroyuki Kitagawa of IRCJS and Hans van der Plicht of Groningen University (the Netherlands). They extended terrestrial radiocarbon calibration close to the limit of the method (ca. 50 ka) by combining varve counting and >300 radiocarbon measurements obtained on terrestrial leaf fossils extracted from the core (Kitagawa and van der Plicht 1998a, 1998b, 2000). Since then the lake has acquired international recognition as an environmental archive of high value (it is sometimes even referred to as “The Japanese Lake”) - especially by the radiocarbon community.

The internationally ratified IntCal radiocarbon calibration model for deriving calendar ages from radiocarbon data was first proposed in 1998 (Stuiver et al. 1998) and subsequently updated in 2004 and 2009 (Reimer et al. 2004, 2009). Ideally, radiocarbon calibration should be based on independently-dated terrestrial materials such as tree rings and terrestrial plant remains in lacustrine sediments (speleothem records, although terrestrial, require correction for unknown dead carbon fractions). However, the present IntCal model relies on marine archives, such as corals and Cariaco Basin sediments for the period beyond the continuous tree-ring record. Even in the latest version, the tree-ring record reaches only back to 12,550 cal. BP. This still leaves three quarters of the radiocarbon calibration based on marine data, including the assumption of a constant offset from the atmospheric radiocarbon concentration, known as the marine reservoir age. As a terrestrial archive, the Lake Suigetsu radiocarbon data are free from the marine reservoir age problem. However, the dataset has never become an integral part of IntCal, mainly because of a significant deviation of the Suigetsu 14C data from the marine-based records, which cast doubt on the SG93 varve chronology.

Early outputs from the SG93 core also included reconstruction and dating of paleoclimatic events. Based on the varve chronology and pollen analysis of >400 samples from the late glacial part of the core (ca. 16-10 ka), Nakagawa et al. (2003, 2005) argued that the late glacial climate in Japan exhibited similar millennial-scale oscillations to the North Atlantic but with centennial-scale offsets in timing. However, this was not widely accepted by the community, partly because of the issues with the SG93 varve chronology, and also because it was not in good agreement with East Asian speleothem records that were precisely U-Th dated (Wang et al. 2001; Shen et al. 2010).

More recently, Staff et al. (2010) re-analyzed the SG93 radiocarbon dataset by Bayesian statistical modeling techniques (Bronk Ramsey 2008, 2009) using IntCal09 as the matching target. This revealed that the gaps between core segments of SG93 were larger than initially supposed, and that the estimated gaps were roughly sufficient to account for the departure of the SG93 radiocarbon data from marine curves. In other words, it convincingly demonstrated that (i) the SG93 varve chronology was generally too young, and (ii) the main source of error was the discontinuity of the core rather than varve counting inaccuracy. This meant that the potential scientific value of the Lake Suigetsu varved sediment remained very high, if sampled in the form of continuous sediment cores.

Suigetsu returns!

Phase Two of the Lake Suigetsu study began in 2006, when funding was secured from the Natural Environment Research Council (NERC), UK to re-core the lake. An international team, consisting of researchers from the UK, Japan, and Germany, recovered a new master core (SG06) in summer 2006 from four parallel bore holes with sufficient overlap to provide a 100% continuous, 73.2 m long sediment profile covering the past ca. 150 ka (Nakagawa et al. 2012; Fig. 2). Building on the success of the SG06 coring, a further extended research group obtained additional funding from NERC and the German Research Foundation, along with other internal support from UK and Japanese institutions to analyze the core in detail.

Varve counting is the crucial part of the whole project. We therefore employed both conventional counting by thin-section microscopy, and ultra-high resolution scanning (60 µm stepping) using an ItraxTM X-radiograph and XRF scanner (Marshall et al. 2012; Francus et al. 2009). The two parallel counting results were merged into a single age-depth model using newly developed statistical methods (Schlolaut et al. 2012; Marshall et al. 2012). The merged varve chronology of the SG06 core has already been established down to ca. 30 ka. Soon, it will be extended beyond the radiocarbon limit (>50 ka).

More than 600 radiocarbon measurements were made on terrestrial tree leaf fossils, manually extracted from SG06. The distribution of the radiocarbon dates shows good agreement with the terrestrial part of the IntCal09 model (0-12,550 cal. BP) supporting the continuity of the sediment and the accuracy of the merged varve chronology (Staff et al. 2011; Marshall et al. in press).

A notable tephra horizon (the ‘SG06-1288’, or ‘U-Oki’ tephra) provides an important marker layer in the early Holocene part of the SG06 core, which has been precisely dated by numerous radiocarbon dates and Bayesian wiggle matching to the terrestrial part of the IntCal09 calibration model (Staff et al. 2011; Bronk Ramsey 2009). Recently we conducted Ar/Ar dating of 34 sanidine crystals extracted from the proximal deposits of this tephra, and attained excellent Ar/Ar age determination with errors < 3%, in spite of operating close to the younger limit of the Ar/Ar technique (Smith et al. 2011). This suggests that we will be able to apply the same method for the deeper part of the core, reduce the cumulative counting error of the SG06 varve chronology, and significantly improve the accuracy of the terrestrial radiocarbon calibration dataset beyond the tree ring limit.

The Lake Suigetsu 2006 project has also yielded a range of paleoclimatological data that are almost ready to be published. Most of them are still waiting for the independent chronology to be finalized. However, Kossler et al. (2011) have already reported decadal to sub-decadal scale sequential changes in diatom flora, sedimentary structure and local vegetation during the Lateglacial transitions. Papers on event timings based on high-resolution pollen, biomarker, and XRF results will follow shortly.

Summary

Our new Lake Suigetsu project aims to (i) provide a key dataset for more reliable terrestrial radiocarbon calibration over the whole range of the method, and (ii) provide a reference record for the East Asian Monsoon region. After two decades of effort by three generations of researchers to pave the runway, research on the Lake Suigetsu varved sediment record is now finally taking off.

Selected references

Full reference list online under: http://pastglobalchanges.org/products/newsletters/ref2012_2.pdf

Kitagawa H, van der Plicht J (1998a) Science 279: 1187-1190

Nakagawa T et al. (2012) Quaternary Science Reviews 36: 164-176

Reimer PJ et al. (2009) Radiocarbon 51: 1111-1150

Smith VC et al. (2011) Quaternary Science Reviews 30: 2845-2850

Staff RA et al. (2011) Radiocarbon 53: 511-528

The impact of hydrologic regime shifts in the Asian Monsoon under changing global climate is significant, since the intensity of monsoon directly affects the agricultural production of the most densely populated regions in the world. Proxy climate records dating back to the pre-instrumental era are therefore essential for evaluating the roles of natural variability and anthropogenic impact on the Asian Monsoon system. Tree rings have often been used to reconstruct past climate variations with annual resolution, and extensive networks of tree-ring chronologies have been developed in North America and Europe. Unlike in higher latitudes, relatively few tree-ring chronologies were produced for tropical and subtropical Asia, since the lack of climatic seasonality hinders annual ring formation of most tree species in these regions. However, over the last couple of decades dendrochronologists have found increasing success in the low latitudes of Asia by selecting regions with more pronounced temperature or precipitation seasonality.

Tree-ring δ18O as a hydrologic proxy

A major recent advance has been the hydroclimatic reconstructions from rare and long-lived Fujian Cypress (Fokienia hodginsii) growing in Vietnam (Buckley et al. 2010; Sano et al. 2009). These records based on ring width measurement revealed that the region experienced decadal-scale droughts in the past, which have no analog in the instrumental period. Interestingly, these exceptional droughts coincided with periods of social unrest (Buckley et al. 2010). In addition to this progress, other recent studies indicated that oxygen isotope ratios of tree-ring cellulose might help improve our understanding of monsoon activity (Sano et al. 2012a,b ; Xu et al. 2011). In this brief note, we show some advantages of using tree-ring δ18O rather than ring width and wood density traditionally utilized in dendroclimatolgy.

It is well known that tree-ring width and wood density are controlled not only by climate but also by endogenous disturbance pulses, such as competition with neighboring trees. To reduce such disturbances, climatically sensitive trees are sampled predominantly at the species ecological boundaries, such as near arid or cold forest limits. This spatial limitation can be overcome by using oxygen isotope ratios, since the isotopic ratio in the wood is not significantly affected by ecophysiological processes. Tree-ring δ18O is theoretically controlled by two climatic factors, δ18O of the source water and relative humidity (e.g. Robertson et al. 2001; Saurer et al. 1997), both of which are closely related to monsoon activity in South and Southeast Asia. Our preliminary analyses using samples from the Himalayas (Sano et al. 2010) and Laos (Xu et al. 2011) clearly show that tree-ring δ18O is more strongly correlated with precipitation, relative humidity, temperature and Palmer Drought Severity Index (PDSI) than is ring width or wood density. PDSI is a drought metric based upon a water balance model. Positive and negative values of the PDSI correspond respectively to wet and dry conditions (Palmer 1965; Dai et al. 2004).

Evidence of weakening monsoon over the Himalayas and Tibet for the last two centuries

Figure 1: Reconstructed June-September PDSI from tree-ring δ18O in the Nepal Himalayas. The thick curve represents a 30-year cubic spline. Figure modified from Sano et al. (2012a).

Tree-ring δ18O from the Nepal Himalayas accounts for 34% of the inter-annual variability of the PDSI in the monsoon season (July–September), and clearly shows a decreasing trend of precipitation/moisture over the last two centuries (Fig. 1). Evidence for a decrease in summer monsoon rainfall was also found in δ18O of tree rings from Tibet (Grießinger et al. 2011), δD of an ice core from Mt. Everest (Kaspari et al. 2007), snow accumulation of an ice core from Dasuopu, Tibet (Zhao and Moore 2006), and varve thickness of lake sediments in Tibet (Chu et al. 2011). The overall trends toward arid conditions indicate that summer monsoon has weakened across wide areas of the Himalayas and Tibet for at least the last couple of centuries.

Our reconstructed PDSI shows significant correlations with sea surface temperatures in the tropical Pacific and Indian Ocean, suggesting that the tropical oceans play a role in modulating hydroclimate in the Nepal Himalayas. A simulation model forced by observed sea surface temperature (SST) data reveals that a weakening trend of monsoon precipitation found in northern India and the eastern Tibetan Plateau over the latter half of the 20th century is deducible from the atmosphere’s response to an increase in SSTs over the tropical Pacific and Indian Ocean (Zhou et al. 2008). Therefore, the consistent warming found in reconstruction of tropical SSTs over the last two centuries (Wilson et al. 2006) might be responsible for the weakening monsoon in the Himalayas. In contrast, other proxy records from lower altitudes indicate a strengthening of the monsoon winds (Anderson et al. 2002) and precipitation (Wang et al. 2005). The increased north–south difference of monsoon activity is likely induced by a southward shift in the overall position of the monsoon trough.

Links between ENSO and tree-ring δ18O records from Southeast Asia

Figure 2: Comparison of inter-annual variations in tree-ring δ18O from Laos and the Multivariate ENSO index (MEI) for the period of 1951-2002. Figure modified from Xu et al. (2011).

In contrast to ring-width-based reconstructions from the Himalayas and Tibet, those from Southeast Asia are rare, since most trees grow in relatively warm and wet environments. To overcome this limitation, isotopic analyses have been conducted on samples from four cypress trees from Laos for the past 52 years (1951-2002). It turned out that contrary to the tree-ring widths data (Xu et al. 2011), the δ18O time series of the trees are significantly correlated with each other. Also, the δ18O chronology built from the four trees shows significant correlation with temperature (r = 0.64, p < 0.001), precipitation (r = –0.34, p < 0.05) and PDSI (r = –0.66, p < 0.001) in the monsoon season. In contrast, no significant correlation was found between any climatic variables and a ring-width chronology based on 15 trees, which includes the four trees utilized for the isotope measurement. Finally, the δ18O chronology is strongly correlated with ENSO-related indices, particularly with the Multivariate ENSO Index (MEI; Wolter and Timlin 2011) for the last 52 years (Fig. 2). To further explore the potential of isotope dendroclimatology, six more cypress trees from Vietnam have been subjected to δ18O analyses. Surprisingly, the δ18O chronology from Vietnam is closely correlated to that from Laos (Sano et al. 2012b), even though these sampling sites are around 150 km away from each other. Moreover, the teleconnected relationships between the δ18O chronology and ENSO-related indices are stable over the 135-year period of available instrumental data.

Over the last decade, considerable effort has been devoted to reconstruct ENSO variability using tree rings and other proxy records that originate mainly from North America and the topical Pacific (e.g. D'Arrigo et al. 2005; Mann et al. 2000; Wilson et al. 2010). Although these records agree well during the twentieth century, there is relatively little agreement before this time. Wilson et al. (2010) point out that the teleconnected relationship between the tropical central/eastern Pacific and regions where proxy records are located does not seem to be stable. Since none of the previously published reconstructions of ENSO variability include data from mainland Southeast Asia, tree-ring δ18O is of great use to independently reconstruct ENSO, and to explore the spatial influence of ENSO before the instrumental period.

Selected references

Full reference list online under: http://pastglobalchanges.org/products/newsletters/ref2012_2.pdf

Buckley BM et al. (2010) PNAS 107: 6748-6752

Sano M, Ramesh R, Sheshshayee MS, Sukumar R (2012a) The Holocene 22(7): 809-817

Sano M, Xu C and Nakatsuka T (2012b) Journal of Geophysical Research 117, doi: 10.1029/2012JD017749

Wilson R et al. (2010) Journal of Quaternary Science 25(1): 62-78

Xu C, Sano M, Nakatsuka T (2011) Journal of Geophysical Research 116, doi: 10.1029/2011JD016694

Stalagmites as paleoclimate archives

Figure 1: The formation process of stalagmites.

Stable isotopic records of stalagmites are a powerful tool to reconstruct past climate change because stalagmites yield continuous terrestrial paleoclimate records and can be dated accurately by the U-Th dating method (e.g. McDermott et al. 2004; Fairchild et al. 2006). Figure 1 shows the formation process of stalagmites. In the soil and upper epikarst, water is enriched with pCO2 derived from plant respiration and organic matter decay (Fairchild et al. 2006). During its percolation in the carbonate bedrock, water dissolves carbonate components and eventually reaches supersaturation. When the water emerges into a cave as dripwater, carbon dioxide is degassed into cave air and calcium carbonate precipitates, forming a speleothem. Under the formation process described above, stalagmites record fluctuations of rainfall and/or temperature on the land surface as variation in their δ18O and δ13C composition.

Previous studies on stalagmites in Asia

Figure 2: Study sites of Asian stalagmites used to reconstruct rainfall variations over the last 2000 years. The red star shows the location of the Ciawitali Cave on West Java, Indonesia.

There are numerous studies based on δ18O and δ13C analyses of stalagmite investigating terrestrial climate changes on Milankovitch and millennial timescales (e.g. McDermott et al. 2001; Wang et al. 2001, 2008). In recent years, some stalagmite studies also focused on shorter timescales. Figure 2 shows stalagmite study sites in Asia targeting reconstructions of rainfall variation during the last 2000 years. Stalagmites collected in China, India and Thailand yield oxygen isotopic variations on subdecadal to subannual timescales (Zhang et al. 2008; Tan et al. 2009; Sinha et al. 2007, 2011; Cai et al. 2010). In these studies, oxygen isotopic variations in the stalagmites were generally interpreted as representing variability in monsoonal rainfall intensity, although the exact interpretation on the climatic causes for isotopic variations in precipitation were different at each study site (e.g. amount effect vs. source effect). While these studies provided important insight in the precipitation regime of southern and eastern Asia, little attention has been paid to the southeastern Asian tropical regions.

Our stalagmite study in Indonesia

The high insolation in the tropics is the driving force of poleward heat transport, and tropical climate anomalies, such as El Niño and La Niña events, also affect the rest of the world through teleconnection. Thus the tropics are a critical region to study the global climate system. However, instrumental meteorological data are very limited compared to Europe and eastern North America (Parker et al. 2000) and need to be supplemented by proxy records. In our study, we performed a systematic comparison between variations in precipitation amount and δ18O and δ13C measurements of an Indonesian stalagmite to reconstruct rainfall variability over the last 500 years.

First, we analyzed a stalagmite collected in the Ciawitali Cave in western Java and compared its δ18O and δ13C records with available instrumental rainfall records back to the 1950s. Negative correlations were statistically significant (r2=0.50 for δ18>O; r2=0.85 for δ13C; Watanabe et al. 2010) and suggest that isotopic ratios of stalagmites are useful proxies for reconstructing rainfall amounts in the region. To account for the time lag of the water percolation from the land surface into the cave, we measured the percolation time of dripwater in the Ciawitali Cave by the 3H-3He dating method (Yamada et al. 2008). We obtained a percolation time of ca. 13 years. Taking this into account, the negative correlation disappears between the isotopic data and the instrumental rainfall record. This suggests that the δ18O and δ13C values of the stalagmite are immediately influenced by the climatic conditions outside the cave, probably via the piston-flow mechanism of groundwater and in-situ CO2 degassing described by Watanabe et al. (2010).

In addition, we measured δ18O and δ13C values over the last 500 years (1440-2006 AD) and found that δ18O and δ13C varied from -7.7 to -5.4‰ and from -14.1 to -11‰, respectively. The δ18O and δ13C variations show synchronous changes throughout the entire period. Enriched 18O signatures around 1600, 1800 and 1990 AD suggest episodes with drier conditions. These episodes coincide with evidence of droughts documented in lake sediments of East Java (Rodysill et al. 2011), and hence are likely associated with regional hydrologic anomalies caused by ENSO dynamics.

Further δ18O and δ13C measurements are in progress to reconstruct rainfall anomalies in the Asian tropics during the past millennium, especially during the Medieval Climate Anomaly and Little Ice Age.

Selected references

Full reference list online under: http://pastglobalchanges.org/products/newsletters/ref2012_2.pdf

Cai B et al. (2010) Journal of Geophysical Research 115, doi: 10.1029/2009JD013378

Sinha A et al. (2011) Geophysical Research Letters 38, doi: 10.1029/2011GL047713

Watanabe Y et al. (2010) Palaeogeography, Palaeoclimatology, Palaeoecology 293: 90-97

Yamada M (2008) Geophysical Research Letters 35, doi: 10.1029/2008GL033237

Zhang P (2008) Science 322: 940-942

The glacier as a water resource

Figure 1: A) Location of the Heihe River, northwest China. Red rectangle indicates the area shown in panel B. B) The catchments of upper stream basins. Stations where several decades worth of hydrological and meteorological data are available are indicated (black circles), as is the July 1st Glacier, where glaciological observations were conducted for the period 2002-2005. Blue dots denote glaciers in the basin delineated from satellite images. Each grid square is equivalent to the grid of the used APHRODITE precipitation dataset (0.5°; Yatagai et al. 2009).

As glaciers continue to retreat due to the on-going effects of global climate change, the availability of water during summer within river systems is expected to decline (e.g. IPCC 2007). Consequently, there has been considerable interest in gaining a better understanding of the contribution of glacial meltwater to river runoff in a number of catchments (e.g. Immerzeel et al. 2010; Kaser et al. 2010; Huss 2011). A mutual conclusion in these studies is that fluctuation of glacier meltwater significantly affects the river runoff if the river flows into arid terrain because water supply from precipitation will for a large part not attain the lower reaches. These studies tend to focus on evaluating the present-day contribution of meltwater to river runoff, and projections of likely future levels. In contrast, there has been less interest in the reconstruction of long-term historical runoff patterns at high temporal resolutions (e.g. annual) in glaciated catchments. One explanation for this may be that while paleoclimatic studies use several established proxies to reconstruct climate indices such as temperature, precipitation, or flood and drought events, finding an appropriate proxy for river runoff remains challenging. In this paper, we use models and climate reconstruction data to estimate the long-term (past 2000 years) runoff of the Heihe River in an arid basin in northwestern China (Fig. 1), where precipitation in the surrounding mountain ranges plays a significant role in providing water for the desert and oasis cities.

Reconstructing glacial runoff

Figure 2: Climate and runoff records for the Heihe Basin covering the past two millennia. A) Annual mean temperature anomaly rel. to AD 1966-1995 (Yang et al. 2002), and annual precipitation (Zhang et al. 2003). B) Glacier area and contribution of glacier runoff to total runoff (Sakai et al. 2012). The present and LIA extents are shown by the black dot and line, respectively. C) Total annual catchment runoff, annual glacier runoff (Sakai et al. 2012), and trend of glacier extent (see equation in the main text). D) Five-year moving average of river runoff and drought events recorded in historical documents. Five prolonged drought periods (a-e) are highlighted (Sakai et al. 2012). Thick lines denote drought-free periods, which are discussed in the main text. Green shading denotes trends of agricultural activity in the region as retrieved from historical documents (Inoue et al. 2007; Nakawo 2011).

We calculated river runoff components using an energy-mass balance model for glacier runoff (Fujita and Ageta 2000; Fujita et al. 2007) and a simple hydrological model for runoff from glacier-free terrain (Sakai et al. 2010). As meteorological variable input for the models, we used temperature and precipitation, and estimated relative humidity and solar radiation from precipitation (Matsuda et al. 2006; Sakai et al. 2010). Temperature and precipitation input data for the past 2000 years were obtained from tree-ring and ice-core studies (Yang et al. 2002; Zhang et al. 2003; Fig. 2A). Statistical downscaling was performed using meteorological data from neighboring stations (Fig. 1B). The spatial distribution and lapse rate were established by reference to gridded datasets (Kalnay et al. 1996; Yatagai et al. 2009) and are assumed to have remained constant through time. Accuracies of the models were tested against observed hydrological and glaciological data in the same region (Sakai et al. 2009, 2010). Changes in the size of the glacier are significant over the long term, as water runoff is significantly affected by changes in the extent of the ice cover. A simple but plausible glacier fluctuation model was established using an area-volume relationship for the glacier (Chen and Ohmura 1990; Liu et al. 2003). Glaciers in the catchment were delineated from a Landsat ETM+ image (Fig. 1B), and were classified into five classes based on the area they cover, as changes in glacier size vary according to their geometry (van de Wal and Wild 2001; Raper and Braithwaite 2006).

Controls on glacial runoff

We validated our calculated results against the known extent of glaciation in the Little Ice Age (LIA), which was determined from the positions of moraines on the satellite image and the present extent (Fig. 2B). River runoff levels reconstructed throughout the last 2000 years correlate closely with precipitation (r = 0.997, p<0.001; Fig. 2A and 2C), due to the small proportion of the catchment that was glaciated (1.3±0.2%). The average contribution from glacier runoff (1.8%) is slightly larger than might be expected based on the proportion of the glaciated catchment (1.3%), due to the effect of altitude on precipitation (it increases with altitude), which allows the glaciers to provide relatively more water than their extent would indicate.

We defined the trend of glacier extent (GT, km2 yr-1) by the following equation,

GT(t)=(GA(t+1)−GA(t−1))/2,

where GA and t denote glacier extent and time, respectively. Glacier runoff, and its contribution to the total river runoff, both showed a strong negative correlation with the trend of glacier extent (r = –0.741 and r = –0.925 respectively, both p<0.001, Fig. 2C). This implies that the glaciers have supplied excess meltwater to the river by reducing their ice storage during periods of glacier shrinkage. In contrast, when the glaciers were expanding during colder or more humid periods, storage of ice mass in the body of the glacier increased, and glacier runoff and its relative contribution to total runoff decreased accordingly. These results suggest that variations in glacier runoff and its contribution to river runoff do not always relate directly to glacier extent, but are also influenced by the nature of glacier response to changing environmental conditions.

Impact of human activities

Many historical documents covering the last 800 years are available for the Heihe River catchment, and allow us to compare our reconstructed river runoff levels with contemporary accounts (Fig. 2D). We use a record of drought events and highlight five of the most significant drought periods, which continued intermittently for periods of several years (labeled a-e), and a record of relative agricultural activity in the middle reach of the river (Inoue et al. 2007; Nakawo 2011). River runoff levels during drought events a, c, and e were relatively small; consequently, we suggest that these droughts were climate driven. In contrast, reconstructed river runoff during drought events b and d were relatively large. Therefore, these drought events may be attributable to human activity rather than climatic factors, and might have been caused for example, by excessive extraction of river water for irrigation. Indeed, in this region, the pioneering phase of agricultural land-use had already begun in the 13th century. In figure 2D, three relatively long drought-free periods are marked with thick lines. During the first period (AD 1355-1425), agricultural land-use had declined; therefore, human-induced droughts were unlikely, as there was a plentiful supply of natural water available (high runoff) to meet demand. The latter two periods (AD 1825-1855 and AD 1885-1925) coincide with the Dongan Revolt, during which agricultural land was left uncultivated.

Our results suggest that longer drought-free periods in the Heihe River catchment only occurred during phases with relatively low human impact and high river runoff (Sakai et al. 2012).

Past or future?

In the studied catchment, glacier extent is too small for glacier runoff to significantly affect the total runoff of Heihe River. Fluctuations in river runoff are primarily controlled by precipitation. Consequently, patterns of past precipitation levels reconstructed from ice cores and tree rings can be used to reconstruct changes of water availability in this region. However, in a more glaciated catchment, river runoff can fluctuate in a more complicated way and the influence of the glaciers is more important. It is generally believed that glacier shrinkage will lead to a shortage of water resources under a warming climate; however, our study suggests that the trend of glacier extent, advancing or retreating, is, on a shorter term, a more significant control on glacier runoff and its contribution to the river system.

Forecasting changes to the extent of glaciers, and the impact on river runoff is a societal need under the present warming climate. Current river-runoff level projections contain numerous uncertainties caused by changing environmental conditions and the application of differing general circulation and regional models. We have shown that our river runoff reconstruction based on paleoclimatological data combined with historical documents from the region, can be used to discriminate the runoff types and attribute the nature of drought events in the arid Heihe River Basin.

Selected references

Full reference list online under: http://pastglobalchanges.org/products/newsletters/ref2012_2.pdf

Fujita K, Ohta T, Ageta Y (2007) Hydrological Processes 21(21): 2882-2891

Immerzeel WW, van Beek LPH, Bierkens MFP (2010) Science 328: 1382-1385

Sakai A, Fujita K, Nakawo M, Yao TD (2009) Hydrological Processes 23(4): 585-596

Sakai A et al. (2010) Hydrological Processes 24(19): 2798-2806

Sakai A et al. (2012) Water History 4(2): 177-196

Figure 1: Map of the northwestern Pacific showing the locations of the Ishigaki and Ogasawara Islands. The typical winter East Asian sea-level atmospheric pressure distribution (isolines) and the path of the Kuroshio current (red arrow) are also shown. Northerly winds (East Asian winter monsoon) blowing from the Siberian High influence the climate around Japan in winter (blue arrows). Modified from Mishima et al. (2010).

Only about 100 years or less of observational data for the ocean environment in the subtropical northwestern Pacific exist. Geochemical tracers, such as stable oxygen isotopes (δ18O), strontium/calcium (Sr/Ca) and uranium/calcium (U/Ca) ratios in skeletons of massive corals are excellent paleoclimatic and paleoceanographic proxies to extend our knowledge of the ocean environment beyond the instrumental record. Recently, a coral record going back to 1873 AD from Chichijima in the Ogasawara Islands (Felis et al. 2009) and a 165-year long record from Ishigaki Island, southern Ryukyus (Mishima et al. 2010) have become available (Fig. 1). These coral records reveal several coupled ocean-atmosphere phenomena during the early 20th century in the northwestern subtropical Pacific.

Abrupt freshening event recorded in the Ogasawara coral

The Ogasawara Islands (27°N, 142°E), approximately 1000 km south of Tokyo, Japan, are exposed to the open ocean environment. Felis et al. (2010) reported that the winter Sr/Ca and U/Ca paleotemperature records from an annually banded coral were significantly correlated with the instrumental winter Pacific Decadal Oscillation (PDO) index over the last century. The PDO is an indicator of North Pacific sea surface temperature (SST) anomalies poleward of 20°N.

Figure 2: Time series of (A) annual coral δ18O (SST) data, (B) observed winter (December–February average) SST at Ishigaki Island, and (C) residual coral δ18O records (Δδ18O) in the Ogasawara Islands, based on both δ18O and Sr/Ca. VPDB refers to the Vienna PeeDee Belemnite standard. The coral-based sea surface salinity (SSS) anomaly was calculated from the regional δ18O seawater–salinity relationship (0.42‰ per 1 psu; Felis et al. 2009). The yellow bars correspond to the abrupt cooling (1900-1905) inferred at Ishigaki Island and the abrupt freshening (1905-1910) in the Ogasawara Islands.

Felis et al. (2009) also reconstructed annually resolved sea surface salinity (SSS) variations since 1873 from the δ18O record and identified an abrupt shift toward fresher surface ocean conditions between 1905 and 1910 (Fig. 2C). They concluded that the Ogasawara freshening could not be explained by precipitation changes but resulted from a combination of atmospheric and oceanic convection processes.

The Ishigaki coral record and the East Asian winter monsoon

Ishigaki Island (24°N, 124°E) is in the marginal East China Sea (Fig. 1), where the ocean environment reflects complex influences from the Kuroshio Current and the East Asian monsoon. Although δ18O can record changes in both SST and SSS, at Ishigaki, seasonal SSS changes are very small, whereas seasonal SST are well reflected by the δ18O values. Winter δ18O and observed SST, measured at Ishigaki Port, showed a significant linear correlation (Fig. 2A, B), which suggests that the winter δ18O data from this coral sample is a good proxy for reconstructing past climate change. SST reconstructions can be used to investigate the relationship between the East Asian winter monsoon (EAWM) and the El Niño–Southern Oscillation (ENSO). Before 1988/1989, the coral-based and instrumental winter SSTs were related to the EAWM, whereas after that winter they were more highly correlated to ENSO, reflecting a regional change in the climate regime (Tsunoda et al. 2008).

The Ishigaki δ18O coral record revealed abrupt cooling during 1900-1905 (Fig. 2A). This cold event was also seen in the coral Sr/Ca data, and its timing is consistent with Japanese observations of exceptionally low temperatures in the winter of 1902. The presence of a strong Siberian High coupled with a strong EAWM and weak westerly winds, possibly accompanied by active heat convection in the tropics, might account for the cold winter at this time (Mishima et al. 2010).

Unique observations of coupled early 20th century climate events in the northwestern Pacific

Interestingly, the abrupt cooling at Ishigaki Island occurred several years before the abrupt freshening in the Ogasawara Islands inferred from the coral isotope and trace element variations (Felis et al. 2009). The North Pacific High, which dominates the North Pacific Ocean, affects SSS; a strong North Pacific High is associated with high salinity. Strong westerly winds tend to enhance the North Pacific High because of its great height, approaching the height of the jet stream. Conversely, the freshening at Ogasawara suggests weak westerly winds, possibly associated with a weakened Kuroshio recirculation gyre, which flows from south of Honshu, Japan’s main island, toward the Ogasawara Islands. In contrast, the Kuroshio's path from Taiwan to Ishigaki Island is relatively stable. Thus Ishigaki tends to be always under the influence of the saline Kuroshio and is thus unlikely to experience freshening such as that inferred at Ogasawara.

The decrease in SST indicated in the Ishigaki coral δ18O record was not observed in the Ogasawara coral record. A strong EAWM, which brings cold winds to the Japanese archipelago, is not likely to affect more pelagic environments such as the Ogasawara Islands. The approximately five-year time lag between the cold SSTs at Ishigaki and the freshening in the Ogasawara Islands is consistent with the findings of Deser et al. (1999), who reported that changes in the Kuroshio and associated North Pacific Ocean circulation lagged westerly wind changes by four to five years. By this mechanism, the early 20th century cooling recorded by the Ishigaki coral may be related to the surface ocean freshening record in the Ogasawara coral. Thus, these records are unique evidences of coupled ocean-atmosphere phenomena during the first decade of the 20th century in the subtropical northwestern Pacific.

Future research

The reliability of paleoclimate reconstructions could be improved by applying the dual proxy approach, utilizing element ratios such as Sr/Ca along with oxygen isotope data, to the Ishigaki coral record. This approach would allow independent reconstruction of SST and SSS variation and make it possible to determine whether cold events around Ishigaki Island are associated with successive regional climate regime shifts. Coral-based reconstructions are now being extended to different time windows, including the Little Ice Age, the mid-Holocene warm period (Yokoyama et al. 2011), the last glacial maximum (Mishima et al. 2009), and the mid-Pliocene Warming Period (Watanabe et al. 2011). Also, the investigation of Porites coral tsunami boulders is a promising approach for long-term paleoclimate reconstruction (Suzuki et al. 2008; Araoka et al. 2010). The robustness of Porites coral records has been tested in a series of laboratory culture experiments to provide practical guidelines for the interpretation of coral climate proxies (Suzuki et al. 2005; Omata et al. 2008, Hayashi et al. in press).

Selected references

Full reference list online under: http://pastglobalchanges.org/products/newsletters/ref2012_2.pdf

Felis T et al. (2009) Geology 37: 527-530

Mishima M et al. (2010) Geophysical Research Letters 37, doi: 10.1029/2010GL043451

Suzuki A, Hibino K, Iwase A, Kawahata H (2005) Geochimica et Cosmochimica Acta 69: 4453-4462

Watanabe T et al. (2011) Nature 471: 209-211

Yokoyama Y et al. (2011) Geophysical Research Letters 38, doi: 10.1029/2010GL044231