Spatial variations of monsoonal rain in eastern China: Instrumental, historic and speleothem records

We select four caves and their nearby cities in the monsoonal region of China for studying the relationships among precipitation, temperature, summer monsoon intensity and stalagmite δ¹⁸O. The instrumental, historic and stalagmite δ¹⁸O records from these areas show strong spatial disparities on annual to decadal scales, so that climatic conditions in a single location cannot represent these of the entire eastern China. On time scales <500 years, stronger summer monsoon may lead to higher rainfall in some locations but not over eastern China. Correlation between the summer monsoon strength and precipitation is not only location-dependent but also changes with time. One may not use the paleoclimatic pattern of cold–dry and warm–wet on glacial/interglacial ages throughout all time scales for climatic conditions in the monsoonal region. On centennial to millennial scales, stalagmite δ¹⁸O variation trend from eastern China resemble solar irradiance with lighter δ¹⁸O corresponding to increased solar irradiance, and vice versa. The similar trends may reflect climatic feedbacks link to solar forcing to cause changes in the summer monsoon intensity and/or in monsoonal circulation. Changes in monsoonal circulation and intensity affect (1) summer rainfall intensity, (2) summer/winter precipitation ratio, or (3) ratio of moisture from Indian/Pacific oceans, or a combination of the three. Thus, a speleothem δ¹⁸O record may not be proper to be used as a proxy of paleo-precipitation amount, especially on short time scales. Based on the four stalagmite δ¹⁸O records during the last 2000 years, EASM strength decreased from AD 200 to AD 500, and from AD 1300 to AD 1600 (the 1st half of the Little Ice Age), whereas EASM strength increased from AD 1600 to AD 1900 (the 2nd half of the Little Ice Age). The EASM strength has weakened since early 1900’s.     

Seasonal variability of oxygen and hydrogen stable isotopes in precipitation and cave drip water at Guilin,southwest China

The interpretation of climatic information from stalagmites has traditionally been a complex research problem, with oxygen isotopes playing a particularly important role in global climate change studies. This study investigates the relationship between oxygen isotope composition of the atmospheric in precipitation and cave drip water at Panlong cave in southwest China on seasonal timescales of variability. Time series seasonal variability was derived from Panlong cave in Guilin by collecting daily precipitation samples for stable isotope analysis during 2012. Results indicate that δ¹⁸O of precipitation contains a clear seasonal variation whereby higher values are mainly distributed during winter and lower values during summer. Seasonal variations in water sources affect the precipitation δ¹⁸O values. Drip water δ¹⁸O also displayed a seasonal cycle which is attenuated relative to δ¹⁸O of precipitation. Drip water time series display seasonal cycle ranges from 1.5 to 3.5 ‰ relative to Vienna Standard Mean Ocean Water, which mainly follow the precipitation δ¹⁸O seasonal cycle. Seasonal variation in drip water δ¹⁸O supports interpretations of the stalagmite δ¹⁸O record as a paleoclimate proxy sensitive to the local environment. This monitoring experiment revealed that drip water must be transported through the epikarst in approximately 1.5 months during cold periods, and <0.5 months during warm periods. Different residence time percolation is mainly affected by the atmospheric precipitation amount, depending on whether soil moisture reaches saturation.     

Holocene climate variability in Europe: Evidence from δ18O,textural and extension-rate variations in three speleothems

Time-series O isotope profiles for three U–Th dated stalagmites have revealed that for much of the Holocene, a site on the Atlantic seaboard (SW Ireland) exhibits first-order δ¹⁸O trends that are almost exactly out of phase with coupled δ¹⁸O curves from two southern European sites (SE France and NW Italy). In the Irish stalagmite (CC3 from Crag Cave, SW Ireland), low δ¹⁸O at 10,000 cal yr BP reflects cool conditions. By the early to mid-Holocene (9000–6000 cal yr BP) δ¹⁸O had increased, reflecting the onset of warmer conditions on the Atlantic seaboard. This shift to higher δ¹⁸O was accompanied by a marked increase in the stalagmite extension rate, reinforcing our interpretation that this was a period of relative warmth. Except for an episode of increased extension rate about 5500 yr ago, δ¹⁸O in the Crag stalagmite exhibits a gradual decrease, accompanied by declining extension rates between 7800 and 3500 cal yr BP, interpreted as a cooling trend. There is evidence for increases in both δ¹⁸O and stalagmite extension rate in the period from 3500 cal yr BP to the present day suggesting a return to warmer conditions on the Atlantic seaboard. In the stalagmite from NW Italy (ER76, Grotta di Ernesto, Trentino province) the early-Holocene (c. 9200-7800 cal yr BP) is characterised by high δ¹⁸O, probably indicative of warm and/or dry conditions. Exceptionally low δ¹⁸O from 7800 to 6900 cal yr BP at this site reflects a well-defined wet phase (Cerin wet phase). In the last three millennia, this stalagmite exhibits a shift to lower δ¹⁸O, interpreted as some combination of cooler and/or wetter conditions. Unlike the Irish stalagmite, the Italian sample does not show a correlation between δ¹⁸O and extension rate. Instead, its extension rate correlates roughly with δ¹³C, presumably reflecting a climate-driven vegetation change. In the early Holocene, δ¹⁸O in the French stalagmite (CL26, Grotte de Clamouse, Herault province, SE France) was low relative to its Holocene average. For much of the period since c. 3500 cal yr BP this stalagmite exhibits higher δ¹⁸O than in the early Holocene, suggesting warmer conditions. Like the Irish stalagmite, the French sample exhibits a well-defined correlation between δ¹⁸O and extension rate. Had drip-water availability been the dominant control on δ¹⁸O at this semi-arid site then higher δ¹⁸O would have been accompanied by lower, not higher extension rates. This suggests strongly that temperature rather than rainfall amount was the dominant control at this site. While conclusions regarding the patterns of climate variability on a continent scale must remain tentative because of the limited number of stalagmites studied we argue that early Holocene warm conditions on the Atlantic seaboard (Irish site) coincided with relatively cool conditions at the Clamouse site. By c. 3500 yr ago the pattern appears to have been reversed.     

Speleothem evidence for late Holocene climate variability and floods in Southern Greece

We present stable isotope data (δ¹⁸O, δ¹³C) from a detrital rich stalagmite from Kapsia Cave, the Peloponnese, Greece. The cave is rich in archeological remains and there are reasons to believe that flooding of the cave has directly affected humans using the cave. Using a combination of U–Th and ¹⁴C dating to constrain a site-specific correction factor for (²³²Th/²³⁸U) detrital molar ratio, a linear age model was constructed. The age model shows that the stalagmite grew during the period from ca. 950 BC to ca. AD 830. The stable oxygen record from Kapsia indicates cyclical changes of close to 500 yr in precipitation amount, with rapid shifts towards wetter conditions followed by slowly developing aridity. Superimposed on this signal, wetter conditions are inferred around 850, 700, 500 and 400–100 BC, and around AD 160–300 and AD 770; and driest conditions are inferred to have occurred around 450 BC, AD 100–150 and AD 650. Detrital horizons in the stalagmite indicate that three major floods took place in the cave at 500 BC, 70 BC and AD 450. The stable carbon isotope record reflects changes in biological activity being a result of both climate and human activities.     

Stable isotopes in a stalagmite from NW Sweden document environmental changes over the past 4000 years

Sundqvist, H. S., Holmgren, K., Moberg, A., Spötl, C. & Mangini, A. 2009: Stable isotopes in a stalagmite from NW Sweden document environmental changes over the past 4000 years. Boreas, 10.1111/j.1502‐3885.2009.00099.x. ISSN 0300‐9483. This study of a 4000‐year‐old stalagmite from Korallgrottan in northwestern Sweden highlights the potentials and challenges when using stable isotopes in stalagmites as climate proxies, as well as the fact that the relationship between climate and proxy may change through time. Both the oxygen and the carbon isotopes display an overall trend of enrichment together with decreasing growth rates over the time period covered by the stalagmite, which is considered a generally cooling period according to current palaeoclimate understanding. The stable isotope records show enriched isotopic values during the, for Scandinavia, comparatively cold period AD 1300–1700 and depleted values during the warmer period AD 800–1000. The indication of a negative relationship between measured δ¹⁸O and surface temperature concurs with earlier reported stalagmite records from regions with a seasonal snow cover and is further supported by the fact that the stalagmite δ¹⁸O record shows general similarities with both regional and hemispheric temperature reconstructions available for the past 500 and 2000 years, respectively. Compared with a stable isotope record of lacustrine carbonates from northern Sweden, however, shifting correlations over time between the two records indicate that a local hydrological change may have taken place at Korallgrottan, or at the lake, compared with around 1000 years ago. The earlier part of the stalagmite δ¹⁸O might thus be influenced, to some extent, by another process than the later part, which means that a negative relationship between δ¹⁸O and surface temperature might not hold for the entire 4000‐year‐old record.     

Episodes of late Holocene aridity recorded by stalagmites from Devil's Icebox Cave, central Missouri, USA

Two stalagmites from Devil's Icebox Cave, central Missouri, display similar δ¹³C and δ¹⁸O values and trends during the late Holocene. Positive δ¹³C excursions at 3.5-2.6 ka and 1.2-0.9 ka are interpreted to reflect drier conditions. These elevated stalagmite δ¹³C values could have plausibly been driven by increasing C₄ plant abundances over the cave or an increased contribution of bedrock carbon, both of which could reflect decreased effective moisture. A lack of corresponding oxygen isotopic anomalies during these intervals suggests that neither mean annual temperature nor the seasonality of precipitation changed concomitantly with dryness. Both of the δ¹³C excursions identified in our stalagmite record are roughly coincident with dry intervals from a number of sites located across the Great Plains.     

Paleoclimatic implications of the spatial patterns of modern and LGM European land-snail shell δO

The oxygen isotopic composition of land-snail shells may provide insight into the source region and trajectory of precipitation. Last glacial maximum (LGM) gastropod shells were sampled from loess from Belgium to Serbia and modern land-snail shells both record δ¹⁸O values between 0‰ and − 5‰. There are significant differences in mean fossil shell δ¹⁸O between sites but not among genera at a single location. Therefore, we group δ¹⁸O values from different genera together to map the spatial distribution of δ¹⁸O in shell carbonate. Shell δ¹⁸O values reflect the spatial variation in the isotopic composition of precipitation and incorporate the snails' preferential sampling of precipitation during the warm season. Modern shell δ¹⁸O decreases in Europe along a N-S gradient from the North Sea inland toward the Alps. Modern observed data of isotopes in precipitation (GNIP) demonstrate a similar trend for low-altitude sites. LGM shell δ¹⁸O data show a different gradient with δ¹⁸O declining toward the ENE, implying a mid-Atlantic source due to increased sea ice and a possible southern displacement of the westerly jet stream. Balkan LGM samples show the influence of a Mediterranean source, with δ¹⁸O values decreasing northward.     

Analysis of the climate signal contained within δO and growth rate parameters in two Ethiopian stalagmites

We combine surface and cave climate monitoring with multiple stalagmite parameters to help understand and calibrate the climate records contained within stalagmites from a region with strong rainfall seasonality. Two actively growing stalagmites from Ethiopia were analysed in order to investigate the climate signal contained within δ¹⁸O and growth rate parameters. The δ¹⁸O and growth rate of the two stalagmites give different responses to surface climate due to variations in the climate signal transfer. Both stalagmites (Merc-1 and Asfa-3) have a climate response that is seasonal; however this signal is subsequently smoothed by the mixing of event and storage water within the aquifer. Merc-1 responds more to high frequency (‘event’) climate, due to a greater ratio of event to storage water in this sample, whereas Asfa-3 responds more to low frequency (‘storage’) climate. In addition, different parameters respond to different seasons. For example, stalagmite Asfa-3, from greater depth from the surface and with a slow drip rate, has a growth rate that responds to the amount of summer rain. In contrast, Merc-1, closer to the surface and with a faster drip rate, exhibits no clear response to surface climate, probably due to a more complex climate signal transfer. δ¹⁸O response varies with stalagmite due to the interplay between rainfall forcing factors (amount, seasonality) and disequilibrium kinetics, with opposing correlations between seasonal rainfall and δ¹⁸O between the samples. Our results demonstrate that analysis of seasonal climate forcing, and transfer functions reflecting the mixing of event and storage water, may be the most appropriate approach to develop of transfer functions appropriate for high-resolution, stalagmite climate reconstruction.     

CO clumping in speleothems: Observations from natural caves and precipitation experiments

The oxygen isotope composition of speleothems is an important proxy of continental paleoenvironments, because of its sensitivity to variations in cave temperature and drip water δ¹⁸O. Interpreting speleothem δ¹⁸O records in terms of absolute paleotemperatures and δ¹⁸O values of paleo-precipitation requires quantitative separation of the effects of these two parameters, and correcting for possible kinetic isotope fractionation associated with precipitation of calcite out of thermodynamic equilibrium. Carbonate clumped-isotope thermometry, based on measurements of Δ₄₇ (a geochemical variable reflecting the statistical overabundance of ¹³C¹⁸O bonds in CO₂ evolved from phosphoric acid digestion of carbonate minerals), potentially provides a method for absolute speleothem paleotemperature reconstructions independent of drip water composition. Application of this new technique to karst records is currently limited by the scarcity of published clumped-isotope studies of modern speleothems. The only modern stalagmite reported so far in the literature yielded a lower Δ₄₇ value than expected for equilibrium precipitation, possibly due to kinetic isotope fractionation.Here we report Δ₄₇ values measured in natural speleothems from various cave settings, in carbonate produced by cave precipitation experiments, and in synthetic stalagmite analogs precipitated in controlled laboratory conditions designed to mimic natural cave processes. All samples yield lower Δ₄₇ and heavier δ¹⁸O values than predicted by experimental calibrations of thermodynamic equilibrium in inorganic calcite. The amplitudes of these isotopic disequilibria vary between samples, but there is clear correlation between the amount of Δ₄₇ disequilibrium and that of δ¹⁸O. Even pool carbonates believed to offer excellent conditions for equilibrium precipitation of calcite display out-of-equilibrium δ¹⁸O and Δ₄₇ values, probably inherited from prior degassing within the cave system.In addition to these modern observations, clumped-isotope analyses of a flowstone from Villars cave (France) offer evidence that the amount of disequilibrium affecting Δ₄₇ in a single speleothem can experience large variations at time scales of 10 kyr. Application of clumped-isotope thermometry to speleothem records calls for an improved physical understanding of DIC fractionation processes in karst waters, and for the resolution of important issues regarding equilibrium calibration of Δ₄₇ in inorganic carbonates.     

Oxygen and carbon isotopic systematics of aragonite speleothems and water in Furong Cave, Chongqing, China

To understand oxygen and carbon stable isotopic characteristics of aragonite stalagmites and evaluate their applicability to paleoclimate, the isotopic compositions of active and fossil aragonite speleothems and water samples from an in situ multi-year (October 2005-July 2010) monitoring program in Furong Cave located in Chongqing of China have been examined. The observations during October 2005-June 2007 show that the meteoric water is well mixed in the overlying 300-500-m bedrock aquifer, reflected by relatively constant δ¹⁸O, ±0.11-0.14‰ (1σ), of drip waters in the cave, which represents the annual status of rainfall water. Active cave aragonite speleothems are at oxygen isotopic equilibrium with drip water and their δ¹⁸O values capture the surface-water oxygen isotopic signal. Aragonite-to-calcite transformation since the last glaciation is not noticeable in Furong stalagmites. Our multi-year field experiment approves that aragonite stalagmite δ¹⁸O records in this cave are suitable for paleoclimate reconstruction. With high U, 0.5-7.2 ppm, and low Th, 20-1270 ppt, the Furong aragonite stalagmites provide very precise chronology (as good as ±20s yrs (2σ)) of the climatic variations since the last deglaciation. The synchroneity of Chinese stalagmite δ¹⁸O records at the transition into the Bølling-Allerød (t-BA) and the Younger Dryas from Furong, Hulu and Dongge Caves supports the fidelity of the reconstructed East Asian monsoon evolution. However, the Furong record shows that the cold Older Dryas (OD) occurred at 14.0 thousand years ago, agreeing with Greenland ice core δ¹⁸O records but ∼200 yrs younger than that in the Hulu record. The OD age discrepancy between Chinese caves can be attributable to different regionally climatic/environmental conditions or chronological uncertainty of stalagmite proxy records, which is limited by changes in growth rate and subsampling intervals in absolute dating. Seasonal dissolved inorganic carbon δ¹³C variations of 2-3‰ in the drip water and 5-7‰ in the pool and spring waters are likely attributed to variable degrees of CO₂ degassing in winter and summer. The variable δ¹³C values of active deposits from −11‰ to 0‰ could be caused by kinetically mediated CO₂ degassing processes. The complicated nature of pre-deposition kinetic isotopic fractionation processes for carbon isotopes in speleothems at Furong Cave require further study before they can be interpreted in a paleoclimatic or paleoenvironmental context.     

Deuterium and oxygen 18 in precipitation and atmospheric moisture in the upper Urumqi River Basin, eastern Tianshan Mountains

The contribution of stable isotopes in meteorological, climatological and hydrological research is well known. This study analyzed the deuterium and oxygen 18 contents (δD and δ¹⁸O) of precipitation in event-based samples at three stations (Glacier No. 1, Zongkong, Houxia) along the upper Urumqi River Basin from May 2006 to August 2007. The δ¹⁸O in precipitation revealed a wide range and a distinct seasonal variation at all three stations, with enriched values occurring in summer and depleted values in winter. A statistically significant positive correlation was observed between the δ¹⁸O and δD and local surface air temperature, and better linear relationship existed between δ¹⁸O and air temperature than that of δD. This suggests that paleoclimatic archives relating to precipitation δ¹⁸O and δD can be useful for qualitative temperature reconstruction. The d-excess in precipitation also exhibited a seasonal variability. Based on NCEP/NCAR reanalysis data, three-dimensional isentropic back-trajectories in HYSPLIT model were employed to determine the moisture source for each precipitation event. Results indicate a dominant effect of westerly air masses in summer and the integrated influence of westerly and polar air masses in winter, and d-excess can be used as a sensitive tracer of the moisture transport history.     

Modeling speleothem δC variability in a central Sierra Nevada cave using C and Sr/Sr

Carbon isotopes in speleothems can vary in response to a number of complex processes active in cave systems that are both directly and indirectly related to climate. Progressing downward from the soil zone overlying the cave, these processes include soil respiration, fluid-rock interaction in the host limestone, degassing of CO₂ and precipitation of calcite upflow from the speleothem drip site, and calcite precipitation at the drip site. Here we develop a new approach to independently constrain the roles of water-rock interaction and soil processes in controlling stalagmite δ¹³C. This approach uses the dead carbon proportion (dcp) estimated from coupled ¹⁴C and ²³⁰Th/U measurements, in conjunction with Sr isotope analyses on stalagmite calcite from a central Sierra Nevada foothills cave in California, a region characterized by a highly seasonal Mediterranean-type climate, to determine the roles of water-rock interaction and soil processes in determining stalagmite δ¹³C. Increases in stalagmite dcp between 16.5 and 8.8 ka are coincident with decreased δ¹³C, indicating a varying yet substantial contribution from the soil organic matter (SOM) reservoir, likely due to significantly increased average age of SOM in the soil veneer above the cave during wet climatic intervals.We use geochemical and isotope mixing models to estimate the host-carbonate contribution throughout the δ¹³C time series and determine the degree of degassing and calcite precipitation that occurred prior to precipitation of stalagmite calcite. The degree of degassing and prior calcite precipitation we calculate varies systematically with other climate indicators, with less degassing and prior calcite precipitation occurring during wetter climatic intervals and more during drier intervals. Modeled δ¹³C values and degassing calculations suggest that some degree of prior calcite precipitation is necessary at all time intervals to explain measured stalagmite δ¹³C values, even during relatively wet intervals. These results illustrate the importance of constraining degassing and prior calcite precipitation in the interpretation of speleothem δ¹³C records, particularly those from caves that formed in seasonal semi-arid to arid environments.     

Modelling δC and δO in the solution layer on stalagmite surfaces

We derive equations describing the evolution of the carbon and oxygen isotope composition of the bicarbonate in a calcite precipitating solution on the surface of a stalagmite using a classical Rayleigh approach. The combined effects of calcite precipitation, degassing of CO₂ and the buffering effect of the water reservoir are taken into account. Whereas δ¹³C shows a progressive increase to a final constant value, δ¹⁸O shows an initial isotopic enrichment, which exponentially decays due to the buffering effect of the water reservoir. The calculated evolution is significantly different for both carbon and oxygen isotopes than derived in a recent paper [Dreybrodt W. (2008) Evolution of the isotopic composition of carbon and oxygen in a calcite precipitating H₂O-CO₂-CaCO₃ solution and the related isotopic composition of calcite in stalagmites. Geochim. Cosmochim. Acta72, 4712-4724.].Furthermore, we discuss the isotopic evolution of the bicarbonate in the solution for long residence times on the stalagmite surface, i.e., for t→∞. The equilibrium isotope ratio of the bicarbonate is then determined by isotopic exchange between the cave atmosphere and the bicarbonate in the solution and can be calculated by equilibrium isotope fractionation. For strongly ventilated caves exchange with the cave atmosphere will result in higher δ¹³C and δ¹⁸O values than those observed in a pure Rayleigh distillation scenario, for sparsely ventilated caves it will result in lower δ¹³C and δ¹⁸O values.     

Climate deterioration in the Eastern Mediterranean as revealed by ion microprobe analysis of a speleothem that grew from 2.2 to 0.9 ka in Soreq Cave, Israel

Analysis of oxygen isotope ratios (δ¹⁸O) by ion microprobe resolves a sub-annual climate record for the Eastern Mediterranean from a Soreq Cave stalagmite that grew between 2.2 and 0.9 ka. In contrast to conventional drill-sampling methods that yield a total variation of 1.0‰ in δ¹⁸O_calcite values across our sample, the methods described here reveal up to 2.15‰ variation within single annual growth bands. Values of δ¹⁸O measured by ion microprobe vary in a regular saw-tooth pattern that correlates with annual, fluorescent growth banding where calcite grades from light to dark fluorescence. Modern records of precipitation and of cave dripwater indicate that variable δ¹⁸O_calcite values record regular seasonal differences in δ¹⁸O_rainfall modified by mixing in the vadose zone. Large differences in δ¹⁸O values measured across a single band (i.e., between the dark and light fluorescent calcite, or Δ¹⁸O_dark-light) are interpreted to indicate wetter years, while smaller differences represent drier years. Oxygen isotopes record: 1) month-scale growth increments, 2) changes in Δ¹⁸O_dark-light that represent seasonality, 3) a systematic, long-term decrease in maximum Δ¹⁸O_dark-light values, and 4) an overall increase in average δ¹⁸O_calcite values through time. These results suggest a drying of regional climate that coincides with the decline of the Roman and Byzantine Empires in the Levant region.     

The δ18O and δ13C records in an aragonite stalagmite from Furong Cave,Chongqing, China: A-2000-year record of monsoonal climate

A 7-cm long aragonite stalagmite, FR0510-1, from Furong Cave, Chongqing, was dated by ²¹⁰Pb and ²³⁰Th methods, revealing a-2000-year record of climate history under the influence of the East Asian Monsoon. The FR0510-1 record resembles Dongge Cave DA record on 10–100-year scales, but quite different from the Wanxiang Cave WX42B record, indicating that while stalagmite δ¹⁸O record represents local/regional moisture change, spatial variability of the monsoonal rainfall over eastern China must take into account. During the past 2000 years, climate in Chongqing was relatively wet in the intervals of 50 BC–AD 250, AD 1150–1450 and AD 1600–1950, and relatively dry during the periods of AD 250–1150 and AD 1450–1600. Dry conditions were prevailing over the Medieval Warm Period, whereas wet climates were dominant during the most time of the Little Ice Age in Chongqing area.     

A paleoclimate record of the last 17,600 years in stalagmites from the B7 cave,Sauerland, Germany

Petrographical and geochemical parameters of stalagmites from the B7 cave in Iserlohn–Letmathe (Northern Rhenish Massif, NW Germany) record Late- and postglacial climate changes (temperature and/or precipitation). Fabrics and microfacies of the stalagmite profiles lead to a differentiation of four hierarchies of rhythms. Clastic layers in the stalagmites are caused by flooding events and are time markers. Twenty-four TIMS Th/U-age-dates provide a time calibration of stalagmite growth phases. One stalagmite reveals an early growth period between 17.6 and 16.7 ka BP. Between 9.6 and 5.5 ka BP (Atlantic episode of the Holocene) the growth rate of the stalagmites was higher than before and after this time, with dominant light-porous microfacial laminae and high δ¹⁸O and δ¹³C values representing partly kinetic fractionation effects. This part of the Holocene is interpreted as a mainly warm episode with frequent interruptions of dripping. Within the past 4 ka the profiles with predominant dark compact facies reveal low isotopic values which may be interpreted as a temperature proxy record. The stalagmite records resemble records from an Irish stalagmite. Correlation with the Δ¹⁴C record from European tree rings suggests that colder periods in the North Atlantic were accompanied by drier winters in central Europe.     

The Medieval Warm Period and the Little Ice Age from a sediment record of Lake Ebinur,northwest China

Ma, L., Wu, J., Yu, H., Zeng, H. & Abuduwaili, J. 2011: The Medieval Warm Period and the Little Ice Age from a sediment record of Lake Ebinur, northwest China. Boreas, Vol. 40, pp. 518–524. 10.1111/j.1502‐3885.2010.00200.x. ISSN 0300‐9483. Lake Ebinur, Xinjiang, northwest China, is a closed‐basin, shallow lake that responds rapidly to changes in the ratio of precipitation to evaporation (P/E). A sediment record spanning the last 1500 years was obtained from the lake. We used δ¹⁸O and δ¹³C in bulk carbonate, and δ¹³C of organic matter in the lake sediments to infer environmental changes in the Ebinur region during the Medieval Warm Period (MWP) and the Little Ice Age (LIA). Decreased δ¹⁸O values of carbonate largely reflect an enhanced P/E ratio within the basin and a higher lake level. Bulk carbonates with higher δ¹³C values are deposited during periods when lake‐water pH is high, while lower δ¹³C values reflect a lower pH in the water column. δ¹³C in organic matter is associated with the amount of precipitation. The results indicate that the Ebinur region experienced a dry MWP and a wet LIA, although the MWP and LIA were warm and cold periods, respectively, as expected. Furthermore, the MWP and LIA were hydrologically complex and cannot be characterized as uniformly wet or dry. Peak wet periods are recorded in the sediment core around AD 1000, 1400 and 1700, and a dry event also occurred in the period of temperature change within the LIA (cold to warm around AD 1500). A comparison of the Lake Ebinur data with proxy records for the strength of the Siberian High and climate proxy indicators suggests that precipitation in the Ebinur region was a consequence, in part, of an enhanced Siberian High during the LIA.     

Oxygen isotopes from Chinese caves: records not of monsoon rainfall but of circulation regime

Oxygen isotope variations in Chinese stalagmites have been widely interpreted as a record of the amount of East Asian summer monsoonal rainfall. This interpretation infers decreasing monsoonal rainfall from the mid‐Holocene and large, dipolar rainfall oscillations within glaciations. However, the speleothem δ¹⁸O variations conflict with independent palaeoclimate proxies (cave δ¹³C, loess/palaeosol magnetic properties, δ¹³C alkanes), which indicate no systematic decline in rainfall from the mid‐Holocene, and no glacial rainfall maxima. Using mass balance calculations (which incorporate seasonality effects in both δ¹⁸O concentration and amount of precipitation), we demonstrate that the cave δ¹⁸O variations cannot be accounted for by summer rainfall changes, or rainfall seasonality or winter cooling, but instead reflect changes in moisture source. A possible driver of the δ¹⁸O variations in Chinese stalagmites is precessional forcing of inter‐hemispheric temperature gradients, and resultant shifts in the position and intensity of the subtropical pressure cells. Through such forcing, Indian monsoon‐sourced δ¹⁸O may have dominated at times of high boreal summer insolation, and local Pacific‐sourced moisture at low insolation. Suppression of summer monsoonal rainfall during glacial stages may reflect diminished sea and land surface temperatures and the radiative impacts of increased regional dust fluxes. Copyright © 2012 John Wiley & Sons, Ltd.     

Evidence of rainfall variations in Southern Brazil from trace element ratios (Mg/Ca and Sr/Ca) in a Late Pleistocene stalagmite

Trace element ratios in the Bt2 stalagmite from Botuverá cave, Southern Brazil, are explored as a proxy for changes in the local rainfall recharge during the last 116 ky. BP Mg/Ca and Sr/Ca ratios, measured with an electron microprobe, are significantly positively correlated with one another throughout the entire record, and vary in a way that is very consistent with variations of δ¹⁸O in the same speleothem during the last glacial period. We suggest that prior calcite precipitation in the vadose zone of the cave system is the main factor affecting the incorporation of Mg and Sr into calcite of the stalagmite. This interpretation is supported by trace element correlation patterns and by results from a hydrochemistry study performed in a cave located in the same region and in a similar environmental setting. Therefore, we conclude that higher (lower) Mg/Ca and Sr/Ca values are associated with lower (higher) levels of recharge into the karstic aquifer, as such conditions lead to an increase (decrease) in the volume of calcite precipitated in the unsaturated zone above the cave during dry (wet) climate periods.Trace element variations point to generally dryer (wetter) conditions during lower (high) phases of summer insolation in the southern hemisphere. These periods coincide with decreased (increased) activity of the South American summer monsoon, as revealed by δ¹⁸O stalagmite records. In addition trace element variations show that rather wet conditions persisted throughout most of the last glacial period from approximately 70 to 17 ky BP. We suggest that during this period the glacial boundary conditions, especially ice volume buildup in the northern hemisphere, played an important role for monsoon rainfall intensification in the region.     

Climatic and hydrologic variability during the past millennium in the eastern Rocky Mountains and northern Great Plains of western Canada

Modelling of tree-ring δ¹³C and δ¹⁸O data from the Columbia Icefield area in the eastern Rocky Mountains of western Canada provides fuller understanding of climatic and hydrologic variability over the past 1000 yr in this region, based on reconstruction of changes in growth season atmospheric relative humidity (RH_grs), winter temperature (T_win) and the precipitation δ¹⁸O-T_win relation. The Little Ice Age (~ AD 1530s-1890s) is marked by low RH_grs and T_win and a δ¹⁸O-T_win relation offset from that of the present, reflecting enhanced meridional circulation and persistent influence of Arctic air masses. Independent proxy hydrologic evidence suggests that snowmelt sustained relatively abundant streamflow at this time in rivers draining the eastern Rockies. In contrast, the early millennium was marked by higher RH_grs and T_win and a δ¹⁸O-T_win relation like that of the 20th century, consistent with pervasive influence of Pacific air masses because of strong zonal circulation. Especially mild conditions prevailed during the “Medieval Climate Anomaly” ~ AD 1100-1250, corresponding with evidence for reduced discharge in rivers draining the eastern Rockies and extensive hydrological drought in neighbouring western USA.