δ18O Signal Research Articles

Tracing water cycle in regulated basin using stable δ18O–δ2H isotopes: The Ebro river basin (Spain)

Variations in the stable-isotope compositions of O and H in a catchment's water balance are mainly caused by natural variations in the isotopic composition of rainfall, through mixing with pre-existing waters and evaporation. Stable isotopes were analysed in surface waters along the main course of the Ebro River (Spain), in its main tributaries and in some groundwaters as part of a year-long monthly survey at the outlet. Meteoric input data over the basin from five IAEA/GNIP stations surrounding the Ebro catchment showed that the mean weighted δ18O–δ2H rain input showing enriched values for four stations and a depleted value for the fifth. Most surface- and ground-waters collected in the Ebro catchment plot close to the global and local meteoric-water lines, reflecting a meteoric origin and a lack of significant evaporation or oxygen-isotope exchanges between water and the rock matrix. Although the main tributaries show large variations of δ18O and δ2H signatures (respectively −5.0 to −9.6‰ and −38.5 to −63.6‰) only the Guadalope river shows an evaporated signal. All tributaries have more depleted δ18O and δ2H values than the mean rainwater δ18O and δ2H signal for the Ebro catchment given by local rain monitoring stations. The depleted values in the Ebro tributaries originate from runoff from the Pyrenees, whereas normal Mediterranean rains yield mostly enriched values. The Ebro River itself shows δ18O and δ2H values that fall between those of its tributaries and no variation is related to the location along the river course. The river water at the outlet (Amposta) display a range of δ18O–δ2H values from respectively −7.2 to −8.4‰ and −47.5 to −53.9‰, though with a few enriched values. There is no link between river discharge and Cl content. Temporal δ18O–δ2H variations at the Ebro outlet reflect input of enriched water from Mediterranean rains and of depleted waters from Pyrenean runoff. The geochemical signal at the Ebro basin outlet was deconvolved through a conceptual scheme of the Ebro hydro system, highlighting the major role played by dam impoundments, not only on water quantity, but also on its quality.

Impact of Source Region on the δ18O Signal in Snow: A Case Study from Mount Wrangell, Alaska

Abstract The stable isotopic composition of water in ice cores is an important source of information on past climate variability. At its simplest level, the underlying assumption is that there is an empirical relationship between the normalized difference in the concentration for these stable isotopes and a specified local temperature at the ice core site. There are, however, nonlocal processes, such as a change in source region or a change in the atmospheric pathway, which can impact the stable isotope signal, thereby complicating its use as a proxy for temperature. In this paper, the importance of these nonlocal processes are investigated through the analysis of the synoptic-scale circulation during a snowfall event at the summit of Mount Wrangell (62°N, 144°W; 4300 m MSL) in south-central Alaska. During this event there was, over a 1-day period in which the local temperature was approximately constant, a change in δ18O that exceeded half that normally seen to occur in the region between summer and winter. As shall be shown, this arose from a change in the source region, from the subtropical eastern Pacific to northeastern Asia, for the snow that fell on Mount Wrangell during the event.

On the enigmatic similarity in Greenland δ18O between the Oldest and Younger Dryas

AbstractThe last deglaciation (20.0–10.0 kyr B.P.) was punctuated by two major cooling events affecting the Northern Hemisphere: the Oldest Dryas (OD; 18.0–14.7 kyr B.P.) and the Younger Dryas (YD; 12.8–11.5 kyr B.P.). Greenland ice core δ18O temperature reconstructions suggest that the YD was as cold as the OD, despite a 50 ppmv increase in atmospheric CO2, while modeling studies suggest that the YD was approximately 4–5°C warmer than the OD. This discrepancy has been surmised to result from changes in the origin of the water vapor delivered to Greenland; however, this hypothesis has not been hitherto tested. Here we use an atmospheric circulation model with an embedded moisture‐tracing module to investigate atmospheric processes that may have been responsible for the similar δ18O values during the OD and YD. Our results show that the summer‐to‐winter precipitation ratio over central Greenland in the OD is twice as high as in the YD experiment, which shifts the δ18O signal toward warmer (summer) temperatures (enriched δ18O values and it accounts for ~45% of the expected YD‐OD δ18O difference). A change in the inversion (cloud) temperature relationship between the two climate states further contributes (~20%) to altering the δ18O‐temperature‐relation model. Our experiments also show a 7% decrease of δ18O‐depleted precipitation from distant regions (e.g., the Pacific Ocean) in the OD, hence further contributing (15–20%) in masking the actual temperature difference. All together, these changes provide a physical explanation for the ostensible similarity in the ice core δ18O temperature reconstructions in Greenland during OD and YD.

234U/238U in a boreal stream network — Relationship to hydrological events, groundwater and scale

Uranium isotopes (234U/238U) were measured in eight well-studied boreal streams of different sizes and characters and in seven deep groundwater wells (up to 11m deep) adjacent to these streams. The measurements covered an entire spring flood cycle, from winter baseflow conditions, through the peak flow and into the early summer, thereby capturing much of the hydrological dynamics in these systems. To deepen the interpretation of the spatiotemporal variability of the 234U/238U ratio a wide range of hydrochemical and geographical data were included in the analysis. Special attention was given to δ18O, the most widely used isotopic tracer for hydrological pathways and groundwater transit times, because high 234U/238U ratios have often traditionally been attributed to input of deep groundwater and long transit times. The results demonstrated that there was generally a gradual increase in 234U/238U ratios in groundwater with increasing catchment area, which based on δ18O data was caused by deeper groundwater pathways and longer residence times in larger catchments. A similar dependence on the catchment area was observed in the streams, which during baseflow conditions generally displayed higher 234U/238U ratios in the larger streams. This relationship remained as the peak flow arrived, although it coincided with a general decrease in the 234U/238U ratios. Simultaneously, the δ18O signal dropped, indicating a greater influence of meltwater and younger groundwater. When combined, this confirmed that the activation of shallower groundwater pathways in connection with the spring flood also activated shallower sources of uranium. In connection with the spring flood there was a strong, nearly 1:1 relationship between the streams and the adjacent groundwater wells in terms of 234U/238U ratios, suggesting that the Quaternary deposits were the main source for uranium. Despite large differences in uranium concentrations between different landscape types no effects could be seen on the isotope signature, indicating that the 234U/238U ratio is a reliable tracer for the primary source and that the fractionation after mobilisation probably is negligible. All in all, the results demonstrated that 234U/238U ratios and δ18O generally provide a consistent picture of the hydrological and biogeochemical functioning of the investigated catchment.

Mediterranean Outflow and surface water variability off southern Portugal during the early Pleistocene: A snapshot at Marine Isotope Stages 29 to 34 (1020–1135 ka)

Centennial-to-millennial scale records from IODP Site U1387, drilled during IODP Expedition 339 into the Faro Drift at 558m water depth, now allow evaluating the climatic history of the upper core of the Mediterranean Outflow (MOW) and of the surface waters in the northern Gulf of Cadiz during the early Pleistocene. This study focuses on the period from Marine Isotope Stages (MIS) 29 to 34, i.e. the interval surrounding extreme interglacial MIS 31. Conditions in the upper MOW reflect obliquity, precession and millennial-scale variations. The benthic δ18O signal follows obliquity with the exception of an additional, smaller δ18O peak that marks the MIS 32/31 transition. Insolation maxima (precession minima) led to poor ventilation and a sluggish upper MOW core, whereas insolation minima were associated with enhanced ventilation and often also increased bottom current velocity. Millennial-scale periods of colder sea-surface temperatures (SST) were associated with short-term maxima in flow velocity and better ventilation, reminiscent of conditions known from MIS 3. A prominent contourite layer, coinciding with insolation cycle 100, was formed during MIS 31 and represents one of the few contourites developing within an interglacial period. MIS 31 surface water conditions were characterized by an extended period (1065–1091ka) of warm SST, but SST were not much warmer than during MIS 33. Interglacial to glacial transitions experienced 2 to 3 stadial/interstadial cycles, just like their mid-to-late Pleistocene counterparts. Glacial MIS 30 and 32 recorded periods of extremely cold (<12°C) SST that in their climatic impact were comparable with the Heinrich events of the mid and late Pleistocene. Glacial MIS 34, on the other hand, was a relative warm glacial period off southern Portugal. Overall, surface water and MOW conditions at Site U1387 show a strong congruence with Mediterranean climate, whereas millennial-scale variations are closely linked to North Atlantic circulation changes.

Interpretation of orbital scale variability in mid-latitude speleothem δ18O: Significance of growth rate controlled kinetic fractionation effects

Oxygen isotopes have been the most widely used climate indicator in stalagmites, applied to reconstruct past changes in rainfall δ18O and cave temperature. However, the δ18O signal in speleothems may also be influenced by variable kinetic fractionation effects, here conceived broadly as fractionation effects not arising from temperature variation. The regional reproducibility of speleothem δ18O signals has been proposed as a way to distinguish the δ18O variations arising directly from changes rainfall δ18O and cave temperature, from variations due to kinetic effects which may nonetheless be influenced by climate. Here, we compare isotopic records from 5 coeval stalagmites from two proximal caves in NW Spain covering the interval 140 to 70 ka, which experienced the same primary variations in temperature and rainfall δ18O, but exhibit a large range in growth rates and temporal trends in growth rate. Stalagmites growing at faster rates near 50 μm/yr have oxygen isotopic ratios over 1‰ more negative than coeval stalagmites with very slow (5 μm/yr) growth rates. Because growth rate variations also occur over time within any given stalagmite, the measured oxygen isotopic time series for a given stalagmite includes both climatic and kinetic components. Removal of the kinetic component of variation in each stalagmite, based on the dependence of the kinetic component on growth rate, is effective at distilling a common temporal evolution of among the oxygen isotopic records of the multiple stalagmites. However, this approach is limited by the quality of the age model. For time periods characterized by very slow growth and long durations between dates, the presence of crypto-hiatus may result in average growth rates which underestimate the instantaneous speleothem deposition rates and which therefore underestimate the magnitude of kinetic effects. The stacked growth rate-corrected speleothem δ18O is influenced by orbital scale variation in the cave temperature and the δ18O of the ocean moisture source, but also by temporally variable fractionation in the hydrological cycle. The most salient trend is increased hydrological fractionation during the GI-22 period, when warmer sea surface temperatures in the subtropical Atlantic moisture source region may have favored greater precipitation amounts.

Two modes of climatic control in the Holocene stalagmite record from the Japan Sea side of the Japanese islands

AbstractThe Holocene stalagmite FG01 collected at the Fukugaguchi Cave in Itoigawa, central Japan provides a unique high‐resolution record of the East Asian winter monsoon. Because of the climate conditions on the Japan Sea side of the Japanese islands, the volume of precipitation during the winter is strongly reflected in the stalagmite δ18O signal. Examination of the carbon isotopes and the Mg/Ca ratio of FG01 provided additional information on the Holocene climate in Itoigawa, which is characterized by two different modes separated at 6.4 ka. Dripwater composition and the correlation between the δ13C and Mg/Ca data of FG01 indicate the importance of prior calcite precipitation (PCP), a process that selectively eliminated 12C and calcium ions from infiltrating water from CO2 degassing and calcite precipitation. In an earlier period (10.0–6.4 ka), an increase in soil pCO2 associated with warming and wetting climate trends was a critical factor that enhanced PCP, and resulted in an increasing trend in the Mg/Ca and δ13C data and a negative correlation between the δ13C and δ18O profiles. A distinct peak in the δ13C age profile at 6.8 ka could be a response to an increase of approximately 10% in C4 plants in the recharge area. At 6.4 ka, the climate mode changed to another, and correlation between δ18O and δ13C became positive. In addition, a millennial‐scale variation in δ18O and pulsed changes in δ13C and Mg/Ca became distinct. Assuming that δ18O and PCP were controlled by moisture in the later period, the volume of precipitation was high during 6.0–5.2, 4.4–4.0, and 3.0–2.0 ka. In contrast, the driest interval in Itoigawa was during 0.2–0.4 ka, and broadly corresponds to the Little Ice Age.

A 500-year seasonally resolved δ&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O and δ&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C, layer thickness and calcite aspect record from a speleothem deposited in the Han-sur-Lesse cave, Belgium

Abstract. Speleothem δ18O and δ13C signals enable climate reconstructions at high resolution. However, scarce decadal and seasonally resolved speleothem records are often difficult to interpret in terms of climate due to the multitude of factors that affect the proxy signals. In this paper, a fast-growing (up to 2 mm yr−1) seasonally laminated speleothem from the Han-sur-Lesse cave (Belgium) is analyzed for its δ18O and δ13C values, layer thickness and changes in calcite aspect. The studied record covers the period between AD 2001 and 1479 as indicated by layer counting and confirmed by 20 U / Th ages. The Proserpine proxies are seasonally biased and document drier (and colder) winters on multidecadal scales. Higher δ13C signals reflect increased prior calcite precipitation (PCP) and lower soil activity during drier (and colder) winters. Thinner layers and darker calcite relate to slower growth and exist during drier (and colder) winter periods. Exceptionally dry (and cold) winter periods occur from 1565 to 1610, at 1730, from 1770 to 1800, from 1810 to 1860, and from 1880 to 1895 and correspond to exceptionally cold periods in historical and instrumental records as well as European winter temperature reconstructions. More relative climate variations, during which the four measured proxies vary independently and display lower amplitude variations, occur between 1479 and 1565, between 1610 and 1730, and between 1730 and 1770. The winters during the first and last periods are interpreted as relatively wetter (and warmer) and correspond to warmer periods in historical data and in winter temperature reconstructions in Europe. The winters in the period between 1610 and 1730 are interpreted as relatively drier (and cooler) and correspond to generally colder conditions in Europe. Interpretation of the seasonal variations in δ18O and δ13C signals differs from that on a decadal and multidecadal scale. Seasonal δ18O variations reflect cave air temperature variations and suggest a 2.5 °C seasonality in cave air temperature during the two relatively wetter (and warmer) winter periods (1479–1565 and 1730–1770), which corresponds to the cave air temperature seasonality observed today. Between 1610 and 1730, the δ18O values suggest a 1.5 °C seasonality in cave air temperature, indicating colder summer temperatures during this drier (and cooler) interval. The δ13C seasonality is driven by PCP and suggests generally lower PCP seasonal effects between 1479 and 1810 compared to today. A short interval of increased PCP seasonality occurs between 1600 and 1660, and reflects increased PCP in summer due to decreased winter recharge.

Southwest Atlantic water mass evolution during the last deglaciation

AbstractThe rise in atmospheric CO2 during Heinrich Stadial 1 (HS1; 14.5–17.5 kyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that wind‐driven upwelling in the Southern Ocean can liberate 13C‐depleted carbon from the abyss, causing atmospheric CO2 to increase and the δ13C of CO2 to decrease. One prediction of the Southern Ocean hypothesis is that water mass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of 12 cores from the Brazil Margin. We show that records below 2300 m remained 13C‐depleted until 15 kyr B.P. or later, indicating that the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ18O results are consistent with abyssal South Atlantic isolation until 15 kyr B.P., in contrast to shallower sites. The depth dependent timing of the δ18O signal suggests that correcting δ18O for ice volume is problematic on glacial terminations. New data from 2700 to 3000 m show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid‐depth δ13C minima were most likely driven by an abrupt drop in δ13C of northern component water. Low δ13C at the Brazil Margin also coincided with an ~80‰ decrease in Δ14C. Our results are consistent with a weakening of the Atlantic meridional overturning circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO2 during HS1.

A 3000-yr annually laminated stalagmite record of the Last Glacial Maximum from Hulu Cave, China

A high-resolution, annual layer-counted and 230Th-dated multi-proxy record is constructed from a stalagmite in Hulu Cave, China. These proxies, including δ18O, annual layer thickness (ALT), gray level (GL) and Sr/Ca, cover a time span of ~3000yr from 21 to 24ka. The physical proxies (ALT and GL) and the geochemical index (Sr/Ca), all primarily reflecting karst hydrological processes, vary in concert and their coherence is supported by wavelet analyses. Variations in the δ18O data agree with fluctuations in the ALT and Sr/Ca records on multi-decadal to centennial scales, suggesting that the Hulu δ18O signal is strongly associated with varying local rainfall amounts on short timescales. A monsoon failure event at ~22.2ka correlates with a decrease in tropical rainfall, a reduction in global CH4 and an ice-rafted event in the North Atlantic. This correlation highlights roles of the Asian monsoon and tropical hydrological cycle in modulating global CH4, because the high-latitude emission was inhibited during the Last Glacial Maximum (LGM). Spectral analysis of the δ18O record displays peaks at periodicities of 139, 59, 53, 43, 30, 23 and 19–15yr. The absence of typical centennial solar cycles may be related to muted changes in ocean circulation during the LGM.

A high-resolution fluid inclusion δ18O record from a stalagmite in SW France: modern calibration and comparison with multiple proxies

The δ18O of speleothem fluid inclusions can provide information about local climate and the atmospheric circulation if the fate of oxygen isotopes from precipitation to the stalagmite is well understood. At Villars cave, SW France, the present-day relationship between the δ18O of precipitation and the δ18O of cave drip water has been determined by previous monitoring studies. In this article, we examine the link between drip water and fluid inclusions in speleothems from the same cave, and investigate whether the fossil water in the stalagmites provides a record of the isotopic composition of precipitation.Replicated fluid inclusion isotope measurements (δD and δ18O) from two modern calcite samples, which had precipitated under the monitored stalactites, are in agreement with the corresponding contemporaneous drip waters. The drip water, in turn, represents average pluri-annual precipitation. This gives strong evidence that the isotopic composition of the infiltrating water is preserved in the pores of speleothem calcite, thus providing a record of average precipitation δ18O at the site.Furthermore, we measured isotope ratios in fluid inclusions in the stalagmite vil-stm1, a 2300 year old sample with a high growth rate (up to 0.6 mm yr−1) and a high water content, which allows a high-resolution reconstruction of the drip water isotopic composition covering the last two millennia. The stalagmite is well dated by 13 U–Th ages and the counting of annual growth layers. The fluid inclusion δ18O varies by 2‰, with high δ18O values in Medieval times, and low values during the Little Ice Age, indicating a temperature influence on precipitation δ18O. However, temperature change alone cannot account for the full range of variability observed.Other proxies measured in the same stalagmite indicate that the cave environment has undergone substantial changes during this period: Stable isotopes and trace element concentrations in calcite show large variations, with amplitudes similar to those found in other Villars speleothems during large climatic shifts such as Dansgaard–Oeschger events or glacial–interglacial transitions. In the context of the last 2000 years, the proxy variability cannot be attributed to climate alone, but is likely influenced by anthropogenic deforestation and the associated changes in carbon dynamics and hydrology.These changes might also have an influence on the isotope ratios in drip water relative to precipitation, e.g. through an altered evaporation–transpiration ratio. Nevertheless, despite some common trends in all proxy records, the marked shifts in the other proxies are not seen in the fluid inclusion stable isotopes, which gives evidence that the relationship between precipitation and drip water δ18O in the past was similar to present-day conditions, and not much affected by the anthropogenic impact. A replication of the fluid inclusion isotope record using stalagmites from the same region will be necessary to determine whether local cave processes or large-scale temperature and atmospheric circulation dominate the fluid inclusion δ18O signal, and in how far the fluid inclusions can provide a direct record of precipitation δ18O.

Extreme snow metamorphism in the Allan Hills, Antarctica, as an analogue for glacial conditions with implications for stable isotope composition

AbstractUnderstanding physical processes in near-zero accumulation areas can help us to better understand polar ice-core records, particularly during periods when accumulation rates were lower than today. We report measurements from a 5 m firn core from the Allan Hills, Antarctica, which include physical properties using computer tomography, stable isotope ratios δD and δ18O, and 210Pb activity. The core shows a highly metamorphosed firn with homogeneous and stable structure, but with discrete layers near the surface. The observed firn structure is caused by a combination of unique depositional and post-depositional processes. The irregular δD and δ18O signal does not follow the stratigraphic sequence and implies post-depositional modification caused by microscopic pressure gradients in the firn that can result from either forced ventilation over rough surfaces in the presence of wind or alternating temperature-gradients between the firn and the atmosphere. Our results also indicate impact snow deposition under high winds and with a high initial density and air exchange between the atmosphere and the snowpack.210Pb activity below 0.3 m falls below the detection limit, implying that most of the core is more than 100 years old. We conclude that the Allan Hills record provides a unique opportunity to investigate important processes that would have affected ice-core records from glacial periods.

A speleothem perspective on millennial-scale climate change in south-eastern Europe during the last glacial

High-resolution stable O isotope data from two U-Th dated speleothems from Romania, one from Tausoare cave in N Romania spanning the last 68 ka, and one from Ascunsa cave in S Romania spanning the interval between 47 ka and 30 ka BP record a regional N-S climate contrast in SE Europe during marine isotope stage 3. The Tausoare data-set records Heinrich-stadial type cold events (low δ18O) throughout MIS 3. While the magnitude of speleothem δ18O variability in N Romania is much less pronounced in comparison to Sofular cave in NW Turkey, the pacing is generally similar. That suggests a common response to Atlantic millennial scale climate variability as observed in the Greenland record, but without the magnification of the δ18O signal observed in NW Turkey due to a local Black Sea source effect. The short time-interval recorded in Ascunsa cave (S Romania), however, shows virtually no similarity in δ18O with the corresponding section at Tausoare in N Romania, but instead resembles the record from Soreq cave (Israel).

Monitoring of a fast-growing speleothem site from the Han-sur-Lesse cave, Belgium, indicates equilibrium deposition of the seasonal δ&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O and δ&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C signals in the calcite

Abstract. Speleothems provide paleoclimate information on multimillennial to decadal scales in the Holocene. However, seasonal or even monthly resolved records remain scarce. Such records require fast-growing stalagmites and a good understanding of the proxy system on very short timescales. The Proserpine stalagmite from the Han-sur-Less cave (Belgium) displays well-defined/clearly visible darker and lighter seasonal layers of 0.5 to 2 mm thickness per single layer, which allows a measuring resolution at a monthly scale. Through a regular cave monitoring, we acquired a good understanding of how δ18O and δ13C signals in modern calcite reflect climate variations on the seasonal scale. From December to June, outside temperatures are cold, inducing low cave air and water temperature, and bio-productivity in the soil is limited, leading to lower pCO2 and higher δ13C values of the CO2 in the cave air. From June to December, the measured factors display an opposite behavior. The absence of epikarst water recharge between May and October increases prior calcite precipitation (PCP) in the vadose zone, causing drip water to display increasing pH and δ13C values over the summer months. Water recharge of the epikarst in winter diminishes the effect of PCP and as a result the pH and δ13C of the drip water gradually decrease. The δ18O and δ13C signals of fresh calcite precipitated on glass slabs also vary seasonally and are both reflecting equilibrium conditions. Lowest δ18O values occur during the summer, when the δ13C values are high. The δ18O values of the calcite display seasonal variations due to changes in the cave air and water temperature. The δ13C values reflect the seasonal variation of the δ13CDIC of the drip water, which is affected by the intensity of PCP. This same anticorrelation of the δ18O versus the δ13C signals is seen in the monthly resolved speleothem record that covers the period between 1976 and 1985 AD. Dark layers display lower δ18O and higher δ13C values. The cave system varies seasonally in response to the activity of the vegetation cover and outside air temperature between a "summer mode" lasting from June to December and a "winter mode" from December to June. The low δ18O and high δ13C values of the darker speleothem layers indicate that they are formed during summer, while light layers are formed during winter. The darker the color of a layer, the more compact its calcite structure is, and the more negative its δ18O signal and the more positive its δ13C signal are. Darker layers deposited from summer drip water affected by PCP are suggested to contain lower Ca2+ concentration. If indeed the calcite saturation represents the main factor driving the Proserpine growth rate, the dark layers should grow slower than the white layers.

Probabilistic sequence alignment of stratigraphic records

AbstractThe assessment of age uncertainty in stratigraphically aligned records is a pressing need in paleoceanographic research. The alignment of ocean sediment cores is used to develop mutually consistent age models for climate proxies and is often based on the δ18O of calcite from benthic foraminifera, which records a global ice volume and deep water temperature signal. To date, δ18O alignment has been performed by manual, qualitative comparison or by deterministic algorithms. Here we present a hidden Markov model (HMM) probabilistic algorithm to find 95% confidence bands for δ18O alignment. This model considers the probability of every possible alignment based on its fit to the δ18O data and transition probabilities for sedimentation rate changes obtained from radiocarbon‐based estimates for 37 cores. Uncertainty is assessed using a stochastic back trace recursion to sample alignments in exact proportion to their probability. We applied the algorithm to align 35 late Pleistocene records to a global benthic δ18O stack and found that the mean width of 95% confidence intervals varies between 3 and 23 kyr depending on the resolution and noisiness of the record's δ18O signal. Confidence bands within individual cores also vary greatly, ranging from ~0 to &gt;40 kyr. These alignment uncertainty estimates will allow researchers to examine the robustness of their conclusions, including the statistical evaluation of lead‐lag relationships between events observed in different cores.

Climate controls on rainfall isotopes and their effects on cave drip water and speleothem growth: the case of Molinos cave (Teruel, NE Spain)

The interpretation of stable isotopes in speleothems in terms of past temperature variability or precipitation rates requires a comprehensive understanding of the climatic factors and processes that influence the δ18O signal in the way through the atmosphere to the cave, where carbonate precipitates acquiring its final isotopic composition. This study presents for the first time in the Iberia Peninsula an integrated analysis of the isotopic composition of rainfall (δ18Op) during 2010–2012 years and, through a detailed monitoring survey, the transference of the primary isotopic signal throughout the soil and epikarst into the Molinos cave (Teruel, NE Spain). Both air temperature and amount of precipitation have an important effect on δ18Op values, clearly imprinting a seasonal variability modulated by an amount effect when rainfall events are more frequent or intense. Air mass history and atmospheric circulation influences are considered through the study of weather types, synoptic-scale climate patterns and large-scale atmospheric circulation indexes (North Atlantic Oscillation and Western Mediterranean Oscillation) revealing a dominant source effect on δ18Op values in this region where tropical North Atlantic and Western Mediterranean are the two moisture source regions. A delay of 2–3 months occurs between the dripwater oxygen isotopic composition (δ18Od) respect to δ18Op values as a consequence of large residence time in the epikarst. Limited calcite precipitates are found from winter to spring when δ18Od values are less negative and dripwater rates are constant. This study suggests that NE Iberian δ18Ocalcite proxy records are best interpreted as reflecting a combination of parameters, not just paleotemperature or paleorainfall and, if extending present-day situation towards the recent past, a biased signal towards winter values should be expected in Molinos speleothem records.

Magnitude and temporal evolution of Dansgaard–Oeschger event 8 abrupt temperature change inferred from nitrogen and argon isotopes in GISP2 ice using a new least-squares inversion

Polar temperature is often inferred from water isotopes in ice cores. However, non-temperature effects on δ18O are important during the abrupt events of the last glacial period, such as changes in the seasonality of precipitation, the northward movement of the storm track, and the increase in accumulation. These effects complicate the interpretation of δ18O as a temperature proxy.Here, we present an independent surface temperature reconstruction, which allows us to test the relationship between δ18Oice and temperature, during Dansgaard–Oeschger event 8, 38.2 thousand yrs ago using new δ15N and δ40Ar data from the GISP2 ice core in Greenland. This temperature reconstruction relies on a new inversion of inert gas isotope data using generalized least-squares, and includes a robust uncertainty estimation.We find that both temperature and δ18O increased in two steps of 20 and 140 yrs, with an overall amplitude of 11.80±1.8 °C between the stadial and interstadial centennial-mean temperature. The coefficient α=dδO18/dT changes with each time-segment, which shows that non-temperature sources of fractionation have a significant contribution to the δ18O signal. When measured on century-averaged values, we find that α=dδO18/dT=0.32±0.06‰/°C, which is similar to the glacial/Holocene value of 0.328‰/°C.

Model evaluation of the coherence of a common source water oxygen isotopic signal recorded by tree-ring cellulose and speleothem calcite

Oxygen isotopic compositions (δ18O) of tree rings and speleothems inherit δ18O of precipitation (δ18Oppt) and are used to reconstruct past variations in the amount of precipitation or temperature. Integration of δ18O records of both the proxies for a multiproxy climate reconstruction necessitates understanding of the extent to which the initial δ18Oppt is modified in both the proxies before getting preserved in them. A forward modeling approach, assuming both the proxies have access to a common source water δ18O signal, is employed to assess the expected maximum correlation between the δ18O records of the two proxies on interannual time scale and the factors influencing the correlation. Variations in the leaf temperature, relative humidity, δ18O of atmospheric water vapor introduce variations in δ18O of tree-ring cellulose, while the variations in the cave temperature and oxygen isotopic equilibrium/disequilibrium conditions between calcite and drip water influence δ18O of speleothem calcite. Variation introduced by these factors in δ18O of individual proxies as compared to the variation in δ18Oppt is crucial in deciding the correlation between δ18O records of the two proxies. When the former is higher than or comparable with the latter, finding one to one correspondence between the δ18O records of the two proxies could be challenging. Modeling exercise revealed a favorable climatic condition at Jagdalpur (19.08°N, 82.03°E), a potential tropical location for tree ring and speleothem-based climate reconstruction, for inheriting a common source water δ18O signal in both the proxies. δ18O records of tree ring and annually resolved speleothem from Jagdalpur could be integrated for the multiproxy climate reconstruction.

Stable isotopic evidence for Middle Pleistocene environmental change from a loess‐paleosol sequence: Kärlich, Germany

Loess‐paleosol sequences are important terrestrial archives of palaeoenvironmental change. Such sequences are rich in pedogenic carbonate, the oxygen and carbon isotopic values of which can provide important palaeoenvironmental information. Although some studies have pioneered the use of O and C isotopes in loess‐paleosol sequences, they are not routinely used as palaeoclimate proxies. In this study we analysed the sedimentology, micromorphology, geochronology and isotopic geochemistry of a Middle Pleistocene loess‐paleosol section, located at Kärlich, Germany. The section studied correlates with the Elsterian glacial (MIS 12) and Holsteinian interglacial (MIS 11). Embedded tephra layers yielded 40Ar/39Ar ages of 466±3 ka, 447±1 ka and 361±3 ka. The sedimentology and micromorphology of the sequence record a shift from accretionary loess accumulation (MIS 12) to prolonged pedogenesis at a stable land surface (MIS 11). Soil carbonate δ18O values record an enrichment of ∼3‰ during the accumulation of the loess, reaching peak values comparable with those found in the MIS 11 soil. The δ18O signal is interpreted as reflecting temperature, highlighting the potential of δ18O analysis of European loess soil carbonates as a means of reconstructing palaeotemperature history.

Stable isotope (δ18O and δ13C) sclerochronology of Callovian (Middle Jurassic) bivalves (Gryphaea (Bilobissa) dilobotes) and belemnites (Cylindroteuthis puzosiana) from the Peterborough Member of the Oxford Clay Formation (Cambridgeshire, England): Evidence of palaeoclimate, water depth and belemnite behaviour

Incremental δ18O and δ13C signals were obtained from three well-preserved specimens of Cylindroteuthis puzosiana and from three well-preserved specimens of Gryphaea (Bilobissa) dilobotes from the Peterborough Member of the Oxford Clay Formation (Cambridgeshire, England). Through-ontogeny (sclerochronological) δ18O data from G. (B.) dilobotes appear to faithfully record seasonal temperature variations in benthic Callovian waters of the study area, which range from c. 14°C to c. 17°C (arithmetic mean temperature c. 15°C). Water depth is estimated to have been in the region of c. 50m, based upon comparisons between these data, previously published non-incremental sea surface δ18O values, and a modern analogue situation. Productivity in Callovian waters was comparable with that in modern seas, based upon δ13C data from G. (B.) dilobotes, with 13C depletion occurring during warmer periods, possibly related to an interaction between plankton blooms and intra-annual variations in mixing across a thermocline. Incremental δ18O data from C. puzosiana provide temperature minima of c. 11°C for all specimens but with maxima varying between c. 14°C and c. 16°C for different individuals (arithmetic mean values c. 13°C). Temperatures for late ontogeny, when the C. puzosiana individuals must have been living close to the study site and hence the analysed specimens of G. (B.) dilobotes, are closely comparable to those indicated by the latter. However, for significant portions of ontogeny C. puzosiana experienced temperatures between c. 2°C and c. 3°C cooler than the winter minimum as recorded by co-occurring G. (B.) dilobotes. Comparisons with modern seas suggest that descent to a depth of c. 1000m would be necessary to explain such cool minimum temperatures. This can be discounted due to the lack of deep waters locally and due to estimates of the depth tolerance of belemnites. The most likely cause of cool δ18O signals from C. puzosiana is a cosmopolitan lifestyle including migration to more northerly latitudes. Mean δ13C values from C. puzosiana are comparable with those from G. (B.) dilobotes. However, the incrementally acquired data are highly variable and probably influenced by metabolic effects. The probable identification of migratory behaviour in C. puzosiana calls into question the reliability of some belemnite species as place-specific palaeoenvironmental archives and highlights the benefits of adopting a sclerochronological approach.