Stalagmite δ13C Research Articles

Tropical vegetation productivity and atmospheric methane over the last 40,000 years from model simulations and stalagmites in Sulawesi, Indonesia

AbstractRecent research has shown the potential of speleothem δ13C to record a range of environmental processes. Here, we report on 230Th-dated stalagmite δ13C records for southwest Sulawesi, Indonesia, over the last 40,000 yr to investigate the relationship between tropical vegetation productivity and atmospheric methane concentrations. We demonstrate that the Sulawesi stalagmite δ13C record is driven by changes in vegetation productivity and soil respiration and explore the link between soil respiration and tropical methane emissions using HadCM3 and the Sheffield Dynamic Global Vegetation Model. The model indicates that changes in soil respiration are primarily driven by changes in temperature and CO2, in line with our interpretation of stalagmite δ13C. In turn, modelled methane emissions are driven by soil respiration, providing a mechanism that links methane to stalagmite δ13C. This relationship is particularly strong during the last glaciation, indicating a key role for the tropics in controlling atmospheric methane when emissions from high-latitude boreal wetlands were suppressed. With further investigation, the link between δ13C in stalagmites and tropical methane could provide a low-latitude proxy complementary to polar ice core records to improve our understanding of the glacial–interglacial methane budget.

Periodic hydroclimate variations during the first half of the Holocene in the Luoyang Basin: Evidence from the Tiancun paleolake sedimentary sequence

The periodicities of the climate change have long been explored by researchers not only because they reflect the inherent characteristics of the climate systems, but also because they are the foundations of climate-change predictability. This paper reports the periodicities of the hydroclimate variations reconstructed from a lacustrine sequence at Tiancun section (TC section) in the Luoyang Basin within the Central Plains of China and the sequence covers the first half of the Holocene. Our reconstruction shows that the hydroclimate variations at the TC section during the period from ∼10,135 and ∼ 5890 cal. yr BP were well corresponding with the effective soil moisture variations recorded by the stalagmite δ13C sequence at the Magou Cave. This corresponding relationship suggests that when the effective soil moisture increases (i.e., more negative δ13C values), the ratio of evaporation over precipitation in the paleolake at the TC section decreased, resulting in lower CaCO3 content. It also suggests that when the effective soil moisture increases, more inflowing water entered the paleolake, resulting in coarser sediments. Our reconstruction also shows that the hydroclimate variations or lake level variations in the Luoyang Basin were characterized by millennial-scale quasi-periodicities that were most likely paced by the changes in ITCZ (Inter-tropical Convergent Zone) position and also by alterations in ENSO-like phase. And, both (ITCZ and ENSO) were most likely regulated by the solar activity. Our further analysis shows that the hydroclimate in the Luoyang Basin also experienced centennial-scale variations. Specifically, the grain size data express the following cycles: 692-year, 538-year, 217-year, 170-year and 123-year, all at the 99% confidence level. And, the CaCO3 content data express the following cycles: 202-year, 184-year, 169-year and 85-year, all at the 99% confidence level. These cycles are approximately coincident with the widely-reported solar-activity cycles.

Imprints of Millennial-Scale Monsoonal Events during the MIS3 Revealed by Stalagmite δ13C Records in China

Regions located on the Chinese Loess Plateau are sensitive to changes in the Asian monsoon because they are on the edge of the monsoon region. Based on six 230Th experiments and 109 sets of stable isotope data of LH36 from Lianhua Cave, Yangquan City, Shanxi Province, we obtained a paleoclimate record with an average resolution of 120 years from 54.5 to 41.1 ka BP during the MIS3 on the Chinese Loess Plateau. Both the Hendy test and the replication test indicated an equilibrium fractionation of stable isotopes during the stalagmite deposition. Comparison with four other independently-dated, high-resolution stalagmite δ13C records between 29°N and 41°N in the Asian monsoon region shows that the stalagmite δ13C records from different caves have good reproducibility during the overlapped growth period. We suggest that speleothem δ13C effectively indicates soil CO2 production in the overlying area of the cave, reflecting changes in the cave’s external environment and in the Asian summer monsoon. Five millennial-scale Asian summer monsoon intensification events correspond to the Dansgaard–Oeschger 10–14 cycles recorded in the Greenland ice core within dating errors, and the weak monsoon processes are closely related to stadials in the North Atlantic. The spatial consistency of stalagmite δ13C records in China suggests that the Asian summer monsoon and the related regional ecological environment fluctuations sensitively respond to climate changes at northern high latitudes through sea-air coupling on the millennial timescale.

Hydroclimatic variation over central China during the 4.2 ka event and its implications for cultural transformation

The 4.2 ka climate event, which marks the start of the late Holocene (Meghalayan), is linked to disruption to human cultures across a wide belt from the eastern Mediterranean to the Yangtze River Valley. Here we present a high-resolution stalagmite δ13C record for the middle Yangtze Valley from 4.4 to 3.0 ka BP, and identify two humid pulses and two dry episodes associated with the 4.2 ka event. Similar hydroclimate fluctuations associated with the 4.2 ka event also occur in the Chinese Loess Plateau and Central Plains of China, and may be at least partly responsible for the decline of the Neolithic culture in these regions. Although the 4.2 ka event marks the internationally recognised boundary between the middle and late Holocene, our stalagmite-based evidence shows that regime shift in hydrological variability occurred at ∼3.5 ka BP in the middle Yangtze Valley. Regime shift correlates with inflection points of temperature in the extratropical Northern Hemisphere and tropical El Niño Southern Oscillation (ENSO) systems, indicating teleconnections between local and global climate change at this time. Following regime shift, precipitation fluctuated with a larger amplitude. Two dry events and one humid rebound event correspond to the early, late, and middle phases of the Shang Dynasty, respectively. In summary, we propose that frequent climatic alternations and their increased amplitudes impacted late Holocene cultural evolution in China.

Stalagmite evidence of Last Glacial Maximum to early Holocene climate variability in southwestern Iran

ABSTRACTGlacial periods and their terminations are useful for assessing the full scale of natural climate variability in the diverse climate regions of West Asia (i.e. deserts, mountains, alluvial plains, coastal zones). In this study, we report the first stalagmite stable isotope (δ18O and δ13C) records from the southern Zagros Mountains in southwest Iran. The records partially span the period from the Last Glacial Maximum (LGM) to the early Holocene, 24.2–9.6 thousand years before the present. The southwest Iran stalagmite δ18O and δ13C records indicate that climate and environment in the area differed substantially between the LGM and early Holocene. High stalagmite δ13C values are found at the LGM (7‰ greater compared to the early Holocene), and are attributed to sparse vegetation and reduced soil bio‐productivity, and possibly a greater degree of prior calcite precipitation in the epikarst, as a result of a cold and dry climate. Stalagmite δ18O values are also high at the LGM (4‰ greater compared to the early Holocene), and are attributed to lower temperatures (larger water–calcite isotope fractionation) and higher δ18O values of the moisture sources (Mediterranean and Red Seas). Through the deglaciation, stalagmite δ18O, δ13C and/or growth features coincide with the North Atlantic Heinrich Stadial 1, the Bølling–Allerød warm period and the Younger Dryas cold event, supporting a relationship between southern Zagros climate and the North Atlantic millennial events.

Climatic implication of stalagmite δ13C in the middle reaches of the Yangtze River since the Last Glacial Maximum and coupling with δ18O

The relationship between monsoon circulation and regional hydrology in the Yangtze River Basin since the Last Glacial Maximum (LGM) remains unclear. In this study, we present speleothem (LS46) δ18O and δ13C records over the past 23.5 kyr from the Luoshui Cave in Central China. Stalagmite LS46 δ13C displayed a good similarity with variations in δDwax-derived monsoon precipitation from Lake TengchongQinghai (TCQH) and the extreme precipitation events identified by the IRMsoft-flux from Heshang Cave, thus indicating that speleothem calcite δ13C reflects the changes in regional hydrology. The ensemble empirical mode decomposition (EEMD) and wavelet spectrum analyses revealed that the regional hydrology was paced by ∼2000-year periodicity during the LGM and last deglaciation and ∼ 1000-year periodicity during the Holocene, respectively. Our results further indicated that the El Niño-Southern Oscillation (ENSO) plays an important role in regulating changes in the frequency and magnitude of regional hydrology in the Yangtze River Basin under different climatic conditions. Moreover, our results demonstrated that δ18O and δ13C were coupled on multiple timescales, thus potentially indicating that local hydrological variations in the middle reaches of the Yangtze River are regulated by large-scale monsoonal circulation. However, we also found that the δ18O and δ13C profiles decoupled during the last deglaciation, likely due to a decrease in evapotranspiration in combination with a decrease in monsoon intensity and temperature during this period. Consequently, the regional hydrological conditions did not experience any dramatic changes.

Structural similarity between Termination III and I

The sequence of abrupt climate variability during Termination III (T-III) is still controversial due to the scarcity of well-dated and high-resolved paleoclimate records. Here, we present 230Th-dated stalagmite δ13C and δ18O data covering T-III from northern China for the first time, with an average resolution of ∼20 years. T-III was featured by two Weak Monsoon Intervals (WMIs), separated by a millennial-scale interstadial. The earlier and later WMI was labeled as Mystery Interval-III (MI-III) and Younger Dryas-III (YD-III), respectively, and the interstadial as Bølling-Allerød-III (BA-III). The transition of MI-III/BA-III, BA-III/YD-III and YD-III/PI (Penultimate Interglacial), constrained by lamination counting, lasted 52, 87 and 68 years, respectively. According to 230Th dating, the duration of the climate variability is 3253 ± 2835, 1373 ± 2294 and 1266 ± 2309 years, respectively. If the KLS-20 δ13C record was aligned to Sanbao cave record within chronology uncertainties, the best estimates of these durations are ∼2300, ∼2100 and ∼4000 years, respectively. Additionally, two significant centennial-scale climate rebounds within YD-III were identified in our records, comparable to the three short-lived climate excursions within YD observed in stalagmite BW-1 δ13C and δ18O records from the same cave and NGRIP δ18O record. The close coupling of abrupt climate oscillations during T-III and T-I highlights the structural similarity between the two deglaciations, both of which are characterized by multi-interruption of WMI. It is probably due to the relatively low amplitude and rate of northern summer insolation rise during T-III and T-I, compared with T-II and T-IV.

Highly resolved δ13C and trace element ratios of precisely dated stalagmite from northwestern China: Hydroclimate reconstruction during the last two millennia

Compared with the widely used stable oxygen isotope (δ18O), the climate response of the stable carbon isotope (δ13C) of speleothems in monsoonal China has received less attention, mainly because of its relatively complex controlling factors. Nevertheless, an increasing number of studies are suggesting that δ13C can represent hydroclimatic changes. Here, we present highly resolved continuous δ13C and trace element ratio records of a speleothem from Wanxiang Cave, northwestern China, spanning the last two millennia. Our results show that δ13C values oscillated around the mean value on centennial scales, indicating that the overlying vegetation types and soil humidity did not change significantly. High consistency was found between detrended δ13C and δ18O (δ13Cd and δ18Od) records, suggesting that soil moisture balance and vegetation density related to the regional hydrological cycle is an important factor controlling δ13C variations on multidecadal-centennial scales. The regional hydrological cycle is mainly controlled by summer precipitation induced by the Asian summer monsoon (ASM) and the stable prior calcite precipitation (PCP), which was inferred from trace element ratio (Mg/Ca, Sr/Ca, Ba/Ca) records. The δ13C and trace element ratio records, consistent with δ18O throughout the considered periods, are not only closely coupled with the intensity of the El Niño-Southern Oscillation and north-south shift of the Intertropical Convergence Zone but also respond to solar activity and temperature variations in the Northern Hemisphere. However, both δ13C and trace element ratios exhibited a persistent significantly decreasing trend in the last 150 years. Furthermore, the variation of δ18O during the Current Warm Period (AD. 1850–2000) suggests that anthropogenic emissions of atmospheric CO2 and aerosols rather than ASM caused an unusual increase of regional precipitation and depletion of atmospheric δ13C, which induced increased infiltration of atmospheric CO2 and available water into soil pores and caves without any obvious PCP effect. The abnormal changes in our speleothem δ13C are analogous to the oceanic 13C Suess effect, suggesting that stalagmite δ13C record is greatly affected by atmospheric CO2 concentration under global warming.

Different response of stalagmite δ18O and δ13C to millennial-scale events during the last glacial, evidenced from Huangjin Cave, northern China

An anti-phased relationship between Greenland Dansgaard-Oeschger (DO) and Antarctic temperature was revealed by ice core records, and the “bipolar see-saw” mechanism has since been proposed in explanation for the interhemispheric thermal redistribution. However, limited by chronology uncertainties of ice cores, particularly the ice age-gas age differences, the exact phase relationship and triggers for millennial-scale events are still in debate. Searching for proxies that could reflect both boreal and austral signals in one geological archive is therefore, a potential solution for the phase dilemma. Here, high-resolution paleoclimatic records from 55.2 to 36.5 ka BP were reconstructed using 21 230Th/U dates and 647 sets of δ18O and δ13C data by one stalagmite HJ1 from Huangjin Cave, Hebei Province, northern China. Robust millennial-scale fluctuations are found in δ18O and δ13C time series, corresponding to DO events 8 to 14 and Heinrich 4 and 5 events within dating errors. Discrepancies exist in the structures and onsets in δ18O and δ13C records; that is, abrupt DO onsets in δ13C data and gradual onsets in δ18O. Timing of DO onsets in δ13C are prior to (e.g. DO12 and DO14) or synchronous with (e.g. DO8 to DO11) those in δ18O. Comparison of HJ1 δ18O and bipolar ice core records suggests that the East Asian summer monsoon (EASM) is influenced by both hemispheres, but better mimics the Antarctic temperature. This suggests that the EASM is controlled by the cross equatorial airflows, and further indicates an important role of the Atlantic Meridional Overturning Circulation (AMOC) in modulating the inter-hemispheric heat gradient. However, our δ13C profile strongly resembles with the Greenland δ18O record on the millennial timescale, in terms of the “sawtooth” structure and the abrupt transition. This indicates that northern high-latitude climate could modulate hydro-thermal conditions in northern China, possibly via the mid-latitude westerly jet and/or the Silk Road teleconnection, which influences the vegetation/biomass changes and thereby controls stalagmite δ13C values. Therefore, HJ1 δ18O receives climatic signals from the Southern Hemisphere (SH) to a larger extent, while δ13C captures largely the changes in the Northern Hemisphere (NH). Furthermore, comparison between calcite and ice core records shows that during DO14, temperature increases in the NH leads the SH cooling by ∼120 years. However, during short-term DO events, the SH may start cooling ∼150 years before the NH warming. These findings demonstrate two possibilities for the NH-SH phase relationship which are dependent on the trigger location of the AMOC recovery and the “bipolar see-saw” mode is likely controlled by the oceanic processes.

The transport mechanism of carbon isotopes based on 10 years of cave monitoring: Implications for paleoclimate reconstruction

Cave monitoring is crucial to unraveling the factors controlling carbon isotope compositions and their transportation in karst systems. Here, we report a 10-year (2010 ~ 2019) cave monitoring of carbon isotopes from the plants, soil, cave water, and speleothems in Jiguan Cave, Central China, located on the Chinese north–south divide in an area that is sensitive to the Asian monsoon. The results show that the cave ventilation and soil air CO2 controlled by the local temperature and humidity conditions are responsible for the seasonal variability in the δ13CDIC values. The variation in the δ13CDIC values and trace element ratios of the cave water are sensitive to the local hydrological conditions. Although there is the lag time from precipitation to cave water in drought year, the δ13CDIC and trace element ratios (Mg/Ca, Ba/Ca and Sr/Ca) of cave water are strongly coupled and reflect the extreme drought due to the influence of prior calcite precipitation (PCP) and water–rock interaction. The 10-year monitoring period covered two complete El Niño Southern Oscillation (ENSO) cycles. During the two strong El Niño years (2010 and 2015), the cave drip water δ13CDIC values were negative but the δ18O values were positive. This opposite trend is mainly due to their different controlling mechanisms. The rainfall amount and moisture sources are responsible for the δ13CDIC and δ18O values of the drip water, respectively. Throughout the karst cave system, the carbon isotopes in Jiguan Cave are continuously enriched when migrating from the vegetation to the soil to the cave drip water to the modern speleothems. Comparisons with the stalagmite δ13C records for the nearby Jiguan Cave suggest that stalagmite carbon isotopes may reflect the regional precipitation changes induced by the ENSO on the decadal scale. Overall, we demonstrate that the δ13C is a potentially reliable proxy that records the changes in the regional environment, especially precipitation changes, and is also a sensitive indictor that indirectly reflects ENSO.

Century-scale climatic oscillations during the Last Glacial Maximum revealed by stalagmite isotopic records from Longfugong Cave, China

The isotopic records of two 230Th-dated stalagmites from Longfugong Cave, Mt. Shennongjia, central China, reveal a highly resolved Asian summer monsoon (ASM) history from 29.5 to 14.5 ka BP. The two δ18O records are consistent with other Chinese cave δ18O records, suggesting that our new records are of regional significance. Due to the reservoir mixing of seepage water in the epikarst system, the stalagmite δ18O signals in this cave are more severely muted than those in other caves. Here, we observed that the amplitude of the δ13C variability is approximately twice that of the δ18O signal and can be used as a sensitive proxy of East Asian monsoon climates. First, the two stalagmite δ13C records are consistent with each other on decadal to centennial timescales. Second, there are high correlation coefficients between the δ13C and δ18O signals over shorter timescales. Third, from 25.2 to 19.6 ka BP, six peaks in the δ13C records were identified and are coincident with corresponding warming events in the Greenland ice-core δ18O records, suggesting that strong coupling occurred between the high- and low-latitude northern climates on centennial timescales. Therefore, we propose that atmospheric transmission mechanisms probably played an important role in linking the ASM and Greenland climates on short timescales. A shift in westerly winds associated with variability in the extent of temperature in the North Atlantic likely influenced changes in hydrological and thermal conditions at the cave site, leading to changes in vegetation cover and soil CO2 concentrations above the cave. Therefore, the calcite δ13C records probably had faster and more sensitive responses to rapid climate shifts compared with their related δ18O records.

Variation of the Asian summer monsoon since the last glacial-interglacial recorded in a stalagmite from southwest China

A number of studies proposed a close link between the climate changes in the Asian summer monsoon (ASM), the Southern Hemisphere (SH), and high latitudes of the Northern Hemisphere (NH). But the mechanisms is still an open question. Although speleothems δ18O records have displayed excellent values in the reconstruction of paleoclimate change at centennial - millennial to orbital timescales, the significance of speleothems δ18O and δ13C is still debatable. Here, a high-precise 230Th dated stalagmite from southwestern China was used to reconstruct the changes of ASM and regional hydrological conditions since the last interglacial (3.6–118.1 ka BP) by the coupled δ18O and δ13C. We found: (1) The EASM and ISM are synchronous changing on the orbital timescale, responding to the change of North Hemisphere summer insolation (NHSI) and high latitude climate change. There is a teleconnection between Antarctic temperature change and ASM through the Mascarene High and the Somali Jet. (2) During MIS 5d, the stalagmite δ18O in Yangzi Cave is nearly 1.5‰ higher than that in MIS 2, the maximum of last glacial. This should be attributed to the different isotopic compositions in moisture source of ISM and EASM, and the dynamic changes of monsoon circulations. (3) From MIS 3 to MIS 2, the ASM showed a “stepped” pattern in weakening, consistent with the decreasing NHSI and increasing global ice volume. (4) The changes in stalagmite δ13C indicated the changes of local hydrological conditions and closely correlated with climate changes revealed by stalagmite δ18O. The effective humidity, which determined by temperature, evaporation, and precipitation, dominated the regional hydrological conditions. The change of effective humidity recorded in stalagmite δ13C by influencing surface biomass and soil CO2-production.

Inter-relationship and environmental significance of stalagmite δ13C and δ18O records from Zhenzhu Cave, north China, over the last 130 ka

There is a vigorous and ongoing debate about the interpretation of the stalagmite δ18O record from east China as an indicator of East Asian summer monsoon (EASM) intensity. This is mainly because of the inconsistent spatial variation of precipitation amount in east China, but the generally consistent variation of precipitation δ18O, on various timescales. However, a direct comparison of a proxy record of precipitation amount with the δ18O record from the same stalagmite has rarely been reported, especially in north China. Here, we present precisely-dated high-resolution stalagmite δ13C and δ18O records for the past 130 ka (1 ka =1000 years) from Zhenzhu (ZZ) Cave in north China. On multi-millennial to orbital timescales, the δ13C and δ18O records exhibit a high degree of in-phase co-variation; however, there are differences in the trend and amplitude of change between various marine isotopic stages (MIS): specifically, MIS5e, MIS5c, MIS5a, MIS3 and MIS1. Modern monitoring results of two years from ZZ Cave, together with a comparison of the δ13C record from the cave with a pedogenic carbonate δ13C record from the western Chinese Loess Plateau (CLP), demonstrate that the δ13C record is a sensitive and reliable indicator of EASM rainfall amount. Considering that variations in both the amount and δ18O of precipitation can be modulated by the atmospheric circulation system, we conclude that the amplitude and trend of the stalagmite δ18O variations from north China are not an absolute indicator of EASM intensity, but rather an indicator of changes in the phase of the EASM. We suggest that stalagmite δ18O records combined with other unambiguous proxy indicators from stalagmites can provide a more comprehensive history of changes of the EASM.

Rainfall variability and vegetation recovery in rocky desertification areas recorded in recently-deposited stalagmites from Guilin, South China

An actively growing stalagmite (sample DY-2) from Maomaotou Big Cave in Guilin, south China was dated using a combined approach of 230Th/U, 210Pb and AMS 14C dating techniques. The post-bomb 14C curve shows that the DY-2 was deposited during 1964–2009 CE and a fast growth rate of ∼2.25 mm/a was determined for this 9.3 cm-long stalagmite. The 210Pb dating of the top 7 mm part of another stalagmite sample PL4 from Panlong Cave located in the same area as the Maomaotou Big Cave constrained a depositional interval during 1830–2009 CE with a much slower growth rate of ∼0.04 mm/a. The δ18O variations of DY-2 and PL4 are highly comparable on decadal time-scales within age uncertainties, and both reveal a good coherence with local instrumental rainfall and historic dry-wet index records. These two δ18O records suggest that rainfall variability in the region of Guilin is modulated by Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillation (ENSO), as the latter can affect the East Asian Summer Monsoon (EASM) and Western Pacific Subtropical High (WPSH). On annual-to-decadal scales, the increase of EASM intensity and WPSH as well as the shift of negative (cold) PDO and La Niña phases would enhance the summer rainfall in the Guilin region, as revealed by the lower stalagmite δ18O values. While weakening EASM and WPSH with positive (warm) PDO and El Niño phases tend to lead to a dry climate in this region of south China. In addition, the stalagmite δ13C and Dead Carbon Proportion (DCP) records show gradually decreasing trends over the last several decades. According to the findings of synchronous increase in both soil pCO2 and dissolved inorganic carbon (DIC) concentrations in seepage waters from the area of Guilin, the variation of stalagmite δ13C and DCP may reflect a history of vegetation recovery in such a rocky desertification area like Guilin, where the area of rocky desertification has been reducing since the 1990s due to afforestation policies and measures.

Millennial-scale Asian monsoon variability during the late Marine Isotope Stage 6 from Hulu Cave, China

AbstractA precisely 230Th-dated stalagmite δ13C profile from Hulu Cave, China, is presented to characterize the frequency and pattern of millennial-scale Asian monsoon (AM) variability from 160.6 to 132.5 ka. Evidence for an antiphased relationship of the δ13C and δ18O on the millennial scale suggests that the δ13C is indicative of the local hydrological cycle associated with changes in AM strength. Owing to the δ13C responding to AM changes more sensitively than the δ18O, we could identify 15 strong AM events that correlate to cold intervals recorded in Antarctic ice cores within 230Th dating uncertainty. This result supports a dynamic link of AM strength and southern hemispheric climates via the cross-equatorial airflows. Power spectrum analysis shows a predominant periodicity of 1.5–2.5 ka for the δ13C profile, similar to the Dansgaard-Oeschger frequency during the last glacial period. Moreover, the AM events are characterized by rapid transitions at the onset, suggesting that the observed millennial-scale AM variability is likely forced by northern high-latitude climates via north–south shifts of the Intertropical Convergence Zone associated with the bipolar seesaw mechanism. As evidence for a common mechanism for ice age terminations, a strong AM event (~134 ka) surrounding Termination II is analogous to the Bølling-Allerød warming interval.

Timing and duration of the East Asian summer monsoon maximum during the Holocene based on stalagmite data from North China

We present a continuous C-O isotope series that shows the detailed variability of East Asian summer monsoon (EASM) since 11.0 ka BP. The series is based on two stalagmites, namely, DSY1 and LM2, which were, respectively, obtained from Dongshiya and Laomu caves. The δ18O profiles of stalagmite excurse negatively in early Holocene and gradually become positive after around 6.9 ka BP, tracking the change in Northern Hemisphere summer insolation. Moreover, the ‘early-Holocene maximum’ supported by stalagmite δ18O records differs from the ‘mid-Holocene maximum’ indicated by geological archives, such as lake sediments and loess. This difference may be caused by different definition indicators of monsoon intensity. Stalagmite δ18O is relative to EASM intensity, but irrelative to precipitation in the East Asian monsoon region. The time intervals of EASM maximum and Holocene climatic optimum are desynchronized, which is confirmed by the variation in the stalagmite δ13C series. Stalagmite δ13C and δ18O have different variation tendencies. The tendency of δ13C in early mid-Holocene was generally light, but it was discontinuity and disrupted by rapid positive shift between 8.2 and 7.7 ka BP. We conclude that a rapid shift of about 8 ka BP is a turning point, before and after which δ13C acquires different controlling factors. Stalagmite δ13C showed no signs of positive excurse in late Holocene but it exhibited another characteristic, namely, millennial time scale oscillations. The few changes in stalagmite δ13C is attributed to weakened insolation during summer in the northern hemisphere, which leads to low evaporation rate, thereby modulating effective humidity change. The edge of the seasonal monsoonal front in northern China during monsoon recession is sensitive to the rain belt and causes the δ13C of the stalagmite to fluctuate greatly. This phenomenon shows that the climate in the study area is unstable in the late Holocene

Abrupt monsoonal shifts over the precessional cycles documented in Yongxing Cave in China during the antepenultimate glacial period

Precession is the major orbital control of monsoon intensity, but the monsoonal response to the transition between precessional cycles has yet to be explored. Here, we present an Asian Summer Monsoon history that spans 327.7–280.5 ka BP based on a 230Th-dated and centennially resolved multi-proxy speleothem record from Yongxing Cave, central China. The δ18O profile, a proxy of monsoon intensity, follows the summer insolation changes, with three abrupt shifts occurring at precessional transitions. These shifts are supported by the stalagmite δ13C, grey-level and petrography analysis, indicating a coherent link between the karst processes and monsoon changes. Evidence from the coeval rapid changes in atmospheric CH4 and forest evolution records within dating errors suggests a wider regional signal of monsoon changes. The weak monsoon intervals are temporally consistent with the ice-rafted debris layers in the North Atlantic, indicating a strong coupling of the high- and low-latitude climate systems. We speculate that the abrupt monsoonal shifts are initiated by the south–north shifts of the Intertropical Convergence Zone associated with the Atlantic Meridional Overturning Circulation and further amplified by land cover. In addition, the rates and magnitudes of insolation changes modify monsoon shifts, thus indicating the abrupt monsoonal shift as a result of an interaction of orbital and millennial forcing.

Strong coupling of centennial-scale changes of Asian monsoon and soil processes derived from stalagmite δ18O and δ13C records, southern China

The paleoclimate application of speleothem δ13C is influenced by site-specific processes. Here we present four stalagmite δ13C records from two caves in southern China, covering early and late Marine Isotope Stage (MIS) 3 and the Holocene, to investigate the spatio-temporal pattern of calcite δ13C changes and the relationship with Asian monsoon (AM) variability. In each growth period, precessional- to millennial-scale changes are clear in the δ18O record. In contrast, millennial variability is absent in the δ13C record, which characterizes persistent centennial oscillations. However, centennial-scale δ18O variations agree well with those of δ13C, with a larger amplitude in δ13C changes (about twice that of δ18O). This suggests that soil humidity balance associated with regional hydrological circulations is important for these centennial δ13C changes, although evaporation-related kinetic fractionation can induce concurrent enrichments in δ18O and δ13C. In frequency, the detrended δ18O and δ13C records are coupled at a periodicity of about 300 yr during the last glacial period and 150 yr during the Holocene. Those centennial-scale δ13C variations are generally consistent with Greenland temperature variability, indicating a climate response over broad regions. Thus, strong co-variation of δ18O and δ13C records should have a climatic origin, even if it is amplified by kinetic effects.

Trace metal and carbon isotopic variations in cave dripwater and stalagmite geochemistry from northern Borneo

We investigate stalagmite trace metal ratios and carbon isotopic composition (δ13C) as potential paleoclimate proxies by comparing cave dripwaters, stalagmites, and bedrock composition from Gunung Mulu and Gunung Buda National Parks in northern Borneo, a tropical rainforest karst site. Three year long, biweekly time series of dripwater Mg/Ca, Sr/Ca, and δ13C from several drips at our site are not correlated with rainfall variability, indicative of a relatively weak relationship between hydroclimate and dripwater geochemistry at our site. However, combining all of the dripwater geochemical data gathered over four field trips to our site (N > 300 samples), we find that drips with highly variable Mg[Sr]/Ca have relatively invariable δ18O values close to the mean. We hypothesize that increased residence times translate into reduced variance in dripwater δ18O through mixing in the epikarst as well as increased Mg[Sr]/Ca values through increased calcite precipitation in the epikarst. Mg/Ca, Sr/Ca, and δ13C time series from three overlapping stalagmites that grew over the last 27 kyrs are characterized by strong centennial‐scale variations, and bear little resemblance to previously published, well‐reproduced δ18O time series from the same stalagmites. The only shared signal among the three stalagmites' geochemical time series is a relative decrease of 1‰ in δ13C from the Last Glacial Maximum to the Holocene, consistent with a transition from savannah (C4) to rainforest (C3) conditions documented in nearby records. Taken together, our study indicates that stalagmite Mg[Sr]/Ca ratios are poor indicators of hydroclimate conditions at our site, while stalagmite δ13C exhibits some reproducible signals on glacial‐interglacial timescales.

A Last Glacial Maximum through middle Holocene stalagmite record of coastal Western Australia climate

Stable isotope profiles of 230Th-dated stalagmites from cave C126, Cape Range Peninsula, Western Australia, provide the first high-resolution, continental paleoclimate record spanning the Last Glacial Maximum, deglaciation, and early to middle Holocene from the Indian Ocean sector of Australia. Today, rainfall at Cape Range is sparse, highly variable, and is divided more or less equally between winter and summer rains, with winter precipitation linked to northwest cloud bands and cold fronts derived from the southern mid- to high-latitudes, and summer precipitation due primarily to tropical cyclone activity. Influences of the Indo-Australian summer monsoon at Cape Range are minimal as this region lies south of the modern monsoon margin. The interaction of these atmospheric systems helps shape the environment at Cape Range, and thus C126 stalagmite-based paleoclimatic reconstructions should reflect variability in moisture source driven by changing ocean and atmospheric conditions.The C126 record reveals slow stalagmite growth and isotopically heavy oxygen isotope values during the Last Glacial Maximum, followed by increased growth rates and decreased oxygen isotopic ratios at 19 ka, reaching a δ18O minimum from 17.5 to 16.0 ka, coincident with Heinrich Stadial 1. The origin of this oxygen isotopic shift may reflect enhanced moisture and lower oxygen isotopic ratios due to amount effect-driven changes in rainfall δ18O values from an increase in rainfall derived from tropical cyclones or changes in northwest cloud band activity, although the controls on both systems are poorly constrained for this time period. Alternatively, lower C126 stalagmite δ18O values may have been driven by more frequent or more intense frontal systems associated with southerly-derived moisture sources, possibly in relation to meridional shifts in positioning of the southern westerlies which have been linked to southern Australia megalake highstands at this time. Finally, we also consider the possibility of contributions of tropical moisture derived from the Indo-Australian summer monsoon. The Intertropical Convergence Zone and associated monsoon trough shifted southward during Heinrich events and other periods of high northern latitude cooling, and although clearly weakened during glacial periods, rainfall with low δ18O values associated with the monsoon today suggests that even small contributions from this moisture source could have accounted for some of the observed oxygen isotopic decrease. Despite a pronounced isotopic excursion coincident with Heinrich Stadial 1, no identifiable anomaly is associated with Heinrich Stadial 2.The Holocene is also characterized by overall low δ18O values and rapid growth rates, with decreasing oxygen isotopic values during the earliest Holocene and at ∼6.5 ka, roughly coincident with southern Australia megalake highstands. The origins of these stalagmite oxygen isotopic shifts do not appear to reflect increases in mean annual temperature but are tied here largely to changes in the δ18O values of precipitation and may reflect a more southerly influence of the Indo-Australian summer monsoon at this time.C126 stalagmite carbon isotopic ratios offer an important complement to the oxygen isotopic time series. Stalagmite δ13C values averaged −5‰ during the Last Glacial Maximum and early deglaciation, and reached a plateau during the oxygen isotopic minimum at 17.5 ka. However, δ13C values decreased sharply to −12‰ between 11 and 8 ka, a shift interpreted to reflect increases in plant density in response to the onset of interglacial conditions. Stalagmite δ13C values at 6 ka are lower than expected for the modern C4-dominated vegetation and thin soils of Cape Range, suggesting that a more C3-rich environment was present during elevated rainfall conditions of the early and middle Holocene. The Cape Range stalagmite time series thus reveals for the first time the millennial-scale sensitivity of the moisture source variations in northwestern Australia, a result that has implications for precipitation dynamics across much of the continent.