δ18O Variations Research Articles

Moisture Sources and Climatic Controls of Precipitation Stable Isotopes Over the Tibetan Plateau in Water‐Tagging Simulations

AbstractUnderstanding the climate controls of precipitation δ18O in the Tibetan Plateau (TP) is crucial for paleoclimate reconstructions from a wealth of regional archives. We use the outputs of iCAM5 model to quantify the different moisture contribution to local precipitation δ18O and to understand the climate controls of precipitation δ18O in the TP based on water‐tagging. The model shows some deficiencies in simulating the spatial and temporal characteristics of precipitation δ18O and the local climatic controls across the TP. Among all the tagged source regions, South Asia and the Indian Ocean contribute the most to the precipitation δ18O in the monsoon‐controlled domain, followed by the East Asia source region. The westerlies are identified as major moisture sources to the precipitation δ18O in the westerlies‐controlled domain. South Asia and the Indian Ocean also contribute substantially for the westerlies‐controlled domain. On interannual time scales, summer precipitation δ18O in the monsoon‐controlled domain is dominated by rainout processes occurring along the moisture transport pathway, indicating that precipitation δ18O variations here potentially record changes in the regional upstream convection. The δ18O signal can be altered by changes in the moisture source location, which implies that enhanced moisture delivery from remote source regions leads to more negative precipitation δ18O due to an increase in the rainout effect during transport. Our results have implications for the interpretation of past variations of archives with precipitation stable isotopes, such as ice cores, tree rings, lake sediments, and speleothems in the TP and surrounding regions.

Assessing the intercolony δ18O proxy calibration in a coral microatoll and its implication for ENSO reconstruction in the northern South China Sea

Massive Porites corals are widely used for paleoclimate reconstruction, however the potential of Porites coral microatoll δ18O as high-resolution archive of paleoclimate has only been recognized recently, and the systematic chemistry-climate relationship has yet to be well characterized in the western Pacific. In this study, we examined the reproducibility of microatoll δ18O records against two adjacent Porites corals from the Xisha Islands in the northern South China Sea (SCS) and evaluated the reliability of microatoll δ18O as a proxy for reconstructing regional climate and El Niño-Southern Oscillation (ENSO) variability. The seasonal to interannual variability in microatoll δ18O was primarily controlled by local sea surface temperature (SST), while the δ18O signal might be suppressed by sea surface salinity (SSS) variations on interannual timescale. Despite the overlapping coral δ18O records exhibited similar patterns of variability, the mean values were consistently offset by ~0.2‰ and the sensitivity of δ18O proxy to climate also varied across different coral colonies. The microatoll δ18O exhibited relatively high proxy-SST sensitivities and amplitude of the seasonal variabilities. These results suggested that intercolony δ18O variability was a significant source of uncertainty in coral-based paleoclimate reconstructions. Microatoll δ18O anomaly appeared to serve as a sensitive and relatively reliable proxy for ENSO variability, although the imprints of weak-to-moderate ENSO events could not be fully captured due to the complex relationship between the East Asian Monsoon and ENSO, as well as the local seawater salinity changes. This study further strengthened the evidence for microatoll as an alternate climate archive in the SCS and highlighted its potential in helping resolve poorly understood paleoclimate before instrumental observations.

Fire as a Major Factor in Dynamics of Tree-Growth and Stable δ13C and δ18O Variations in Larch in the Permafrost Zone

Wildfires are one of the most important environmental factors controlling forest ecosystem physiology and the carbon balance in the permafrost zone of North Siberia. We investigated tree-ring width (TRW) and stable isotope chronologies in tree-ring cellulose (δ13CCell, δ18OCell) of Larix Gmelinii (Rupr.) Rupr. from a wet (WS) and a dry (DS) site. These sites are characterized by different fire histories (fire in 1852 at the wet and 1896 at the dry sites, respectively). TRW and δ18OCell are identified to be the most sensitive parameters in the changing tree growth conditions after fire. The differences in the soil seasonal thermal regime of sites after fires are shown in the relationship between the studied parameters. The δ13CCell values in tree rings from the two sites are positively correlated independently of the fire impact. This fact indicates that δ13CCell chronologies might be more adequate for climatic reconstruction in the region due to the climate signal consistency. Relationships of δ18OCell values between the two sites are still significantly positive 60 years after the fire impact. Dendroclimatic analysis indicates significant changes in tree-ring growth and isotopic ratio responses to climate due to the increased demand of water for trees during the post-fire period (deeper seasonal subsidence of permafrost).

Hydroclimate changes related to thermal state of the tropical Pacific in the northern coast of the South China Sea since ∼8000 cal yr B.P.

Under the complex influences including the monsoonal climates and tropical hydrological cycle, the features and forcing mechanisms of precipitation changes in the tropical monsoon regions remain controversial. The northern coast of the South China Sea (NCSCS), connecting the South China Sea (SCS) and the Pearl River Estuary (PRE), is a critical area providing reliable tropical precipitation records and probing the possible forcing mechanism of tropical precipitation, benefitted from its high deposition rate and hydroclimatic sensitivity. Here, δ18O variations of planktonic (δ18OG. ruber) and benthic foraminifera (δ18OC. lobatulus) were investigated respectively to reconstruct a high-resolution low-latitude precipitation record from the core 17NH-NC3 in the NCSCS. The results show a distinct δ18O difference between δ18OG. ruber and δ18OC. lobatulus, not only with respect to values, but also with respect to trends in some time intervals. The clear difference between the planktonic and benthic foraminifera (Δδ18Ob-p) illustrates the significant vertical salinity stratification. And the temporal trend of Δδ18Ob-p indicates the degree of salinity stratification variated since the mid-Holocene. We assume that the degree of stratification in the NCSCS was mainly controlled by tropical precipitation changes. Thus, the trend of Δδ18Ob-p values could indicate the temporal change of the tropical precipitation. The precipitation record of our research area is closely related to the tropical atmosphere–ocean dynamics stimulated by sea surface temperature (SST) changes of the tropical Pacific zone, analogous to the El Niño–Southern Oscillation (ENSO) events. During the mid-Holocene (from 8260 to 5180 cal yr B.P.), the sustainable higher Δδ18Ob-p values (>1.23‰) suggested a large amount of precipitation, pointing to a sustained state of La Niña-like, which is associated with lasted higher difference between Western SST and Eastern SST (W-E SST gradient) in the equatorial Pacific. Since 5180 cal yr B.P., the decreasing Δδ18Ob-p indicates less low-latitude precipitation, which can be ascribed to an El Niño-like mean state arising from decreased W-E SST gradient. Our study provides a new continuous high-resolution archive of low-latitude precipitation in the tropical monsoon region since 8260 cal yr B.P. And this record highlights that the thermal state of the tropical Pacific most likely adjusted the low-latitude precipitation since the mid-Holocene.

Influences of sea level changes and the South Asian Monsoon on southern Red Sea oceanography over the last 30 ka

AbstractThe southern Red Sea is a key region for investigating the effect of climate forces on a semi-closed basin. Detailed micropaleontological (planktic foraminifera) and isotopic (δ18Ο, δ13C) analyses along with reconstructions of sea surface temperature and salinity on a sediment core from the Farasan banks revealed the imprints of sea level changes and the South Asian Monsoon on the area. Comparison with surrounding records provided insights on the Red Sea-Gulf of Aden (GoA)-Northwestern Arabian Sea (NWArS) water exchange pattern over the last 30 ka. During glacial sea-level lowstand, flow of water from the GoA prevented hypersalinity in the southern Red Sea. Deglacial sea level rise improved water mass exchange between the NWArS, GoA and the entire Red Sea, resulting in relatively similar surface water conditions during the early Holocene when sea-level rise slowed. Thus, sea level change is the major driver of Red Sea δ18O variability. Southwest Monsoon (SWM), which was dominant during the late glacial and Early–Middle Holocene, enhanced surface productivity in the southern Red Sea. Northeast Monsoon (NEM) dominated around Heinrich stadial 1, as indicated by a nearly aplanktonic zone that was probably caused by restricted GoA inflow and low productivity.

Genesis of the Xiarihamu Magmatic Ni-Co Sulfide Deposit in the East Kunlun Orogen, Northern Tibetan Plateau: In Situ Oxygen Isotope and Geochemical Perspectives

Abstract Convergent margins are commonly deemed to have poor potential to form giant magmatic Ni-Cu-(platinum group element) sulfide deposits, due to the possibly low lithospheric geothermal gradient and the small basaltic magma fluxes. This view, however, is challenged by the recent discovery of the Xiarihamu Ni-Co deposit in the East Kunlun orogen (northern Tibetan Plateau), which contains ~157 million metric tons (Mt) of sulfide ores at 0.65% Ni, 0.013% Co, and 0.14% Cu, making it the world’s 20th largest magmatic sulfide deposit. To investigate the mantle source nature, magmatic processes, and their genetic linkage to Ni-Co mineralization in orogenic belts, we presented new in situ orthopyroxene oxygen isotope and pyroxene trace element data from the mafic-ultramafic cumulate rocks at Xiarihamu. Our results show that orthopyroxene crystals have much higher δ18O (6.40–8.58‰) than those in unmodified mantle peridotite (5.72 ± 0.14‰). Orthopyroxene grains in the primitive harzburgite cumulates have the lowest δ18O values (6.40–7.71‰) and the highest Mg# contents (88.7–83.9). From the harzburgite to the evolved lithologies (e.g., websterite and gabbronorite), the δ18O values of orthopyroxene increase, whereas the Mg# values, Ni contents, and heavy/light rare earth element (HREE/LREE) ratios of orthopyroxene decrease, implying crustal contamination during magma fractionation. The δ18O variations and sharp increases in V/Sc and LREE/HREE ratios from the primitive orthopyroxene crystals (with the highest Ni and Mg#) to the evolved ones (with low Ni and Mg#) indicate that crustal contamination played a critical role in sulfide saturation. The absences of any trend in orthopyroxene δ18O values versus Mg# contents, Ni concentrations, and La/Yb ratios toward unmodified mantle values suggest an enriched mantle source. On the mid-ocean ridge basalt (pyroxene)-normalized multielement plots, both orthopyroxene, clinopyroxene, and calculated parental magma show LREE and large ion lithophile element (LILE) enrichments and depletions in Nb-(Ta), Sr, and Zr-(Hf). The predicted primitive magma trace element compositions and the high-δ18O characteristics of the high-Mg# orthopyroxene grains suggest that the Xiarihamu primitive magma was partly originated from the pyroxenite mantle metasomatized by subducted slab-derived melts and fluids. Given the lower solidus temperature of clinopyroxene relative to olivine, pyroxenites in the mantle wedge may generate voluminous high-Ni magmas during partial melting. The high-Ni magmas are likely conducive to magmatic Ni-Cu sulfide mineralization in orogenic belts.

Surface water isoscapes (δ18O and δ2H) reveal dual effects of damming and drought on the Yangtze River water cycles

The water regimes and biogeochemical cycles are remarkably affected by river damming and drought around the world. However, the dual effects on river water cycles have not yet attracted sufficient attention along the Yangtze River because of the relative consistency of annual water regimes. This study first explored the altered water cycle associated with damming and drought using surface water isoscapes (δ18O and δ2H) of river and lake water along the Yangtze River during the flooding seasons of 2016 and 2018, combined with other historical isotopic data from 2003 to 2013. The results show that river water isotopes are markedly varied on both spatial and temporal scales. The relatively more enriched river water representing δ18O values were observed from 2003 to 2007 compared with those in 2018, which were possibly caused by increased evaporative fractionation under abnormal drought conditions in the Yangtze River. Spatially, river water δ18O values exhibited a positive trend from upstream to the lower reaches because of the contrasting recharged sources, complex mixing, and evaporation from upstream to the lower reaches of the Yangtze River. This trend was related to variations in surface water temperature and water evaporation during the flooding seasons evidenced by a significant correlation between river isotopes and river water temperature. Moreover, the large variability in deuterium-excess (d-excess) and δ18O in the downstream could be inherited from the upstream because of short residence time and incomplete mixing in the Three Gorges Reservoir (TGR) during the initial stage of the Three Gorges Dam (TGD) operation (2003–2007). In contrast, the relatively lower variability of d-excess and δ18O was found in the downstream area due to the complete mixing in the TGR, causing the homogenization of reservoir water during the normal operation stage after 2010 (e.g., 2013 and 2018). Drought stimulates evaporative fractionation on river water and result in more negative d-excess values especially in 2006. The dual effects of river damming and drought remarkably changed the longitudinal hydrological connectivity and water cycle dynamics by prolonging the residence time and enhancing the water mixing, which should be thoroughly considered for water resource management, especially for regions with heavily-regulated rivers.

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.

Late Holocene increase of winter precipitation in mid-continental North America from a seasonally resolved speleothem record

Abstract Subannual climate reconstructions of the Holocene are rare despite the ability of such records to provide a better understanding of the underlying factors that drive subannual climate variability. We used specialized confocal laser fluorescent microscope imaging and automated secondary ion mass spectrometry microanalysis to resolve a seasonal oxygen isotope (δ18O) record of a late Holocene–aged (2.7–2.1 ka) speleothem from mid-continental North America. We did this by measuring intra-band δ18O variability (Δ18O) within 117 annual bands over a 600 yr span of the late Holocene. We interpret a change in Δ18O values after 2.4 ± 0.1 ka to reflect an increase in the amount of winter precipitation. Our study produced direct measurements of past seasonality, offers new insights into shifting seasonal precipitation patterns that occurred during the late Holocene in central North America, and adds a new tool for understanding the complex precipitation and temperature histories of this region.

Global carbon isotopic events in a Campanian–Maastrichtian deltaic succession (Tremp-Graus Basin, Spain) and multiproxy stratigraphy for high sedimentation rate environments

The Campanian to Maastrichtian sedimentary succession of the Tremp-Graus Basin (NE Spain) is characterized by the offshore-to-prodeltaic upper Vallcarga Fm giving way to prograding deltaic units (the Aren Fm), which are overlain by transitional facies (the Grey Unit) that correspond to the basal part of the Tremp Group. This work aims to improve the stratigraphical scheme of these deposits using a new dataset of δ13Ccarb and δ13Corg, δ18O and TOC variations through time, supported by a new biostratigraphical analysis and compiling previous biostratigraphical and paleomagnetic data. The comparison of isotopic signals from marine reference sections where δ13Ccarb is well calibrated in the Tethys realm allows recognition of global isotopic events that can be used as correlative marker events. Significant high-frequency δ13C variations defined in the δ13Ccarb are identified in the studied succession despite the different sedimentary setting and some can be related to global geochemical events such as the Late Campanian Event (LCE) and the Campanian–Maastrichtian Boundary Event (CMBE), in good agreement with the magnetic chrons at the base of the Tremp Group, and with some new biostratigraphic constraints. From multiple geochemical and petrographic analysis of the organic matter we suggest that the geochemical signal corresponds to global changes, but at higher resolution, sedimentary dynamics are also reflected in the geochemical signals, particularly inputs of surface water in the deltaic signal.

A new stalagmite oxygen isotope record over the last 1350 years: Insights into spatial variation in Asian summer monsoon and temperature forcing

Climate reconstructions over the last millennium are essential to estimate natural and anthropogenic forcing. Here we present a stalagmite δ18O record from southwestern China, which characterizes changes in the Asian Summer Monsoon (ASM) precipitation over the past 1350 years. Wet climate is clearly identified during the Dark Age Cold Period (DACP) and Little Ice Age (LIA); dry climate is clearly identified during the Medieval Warm Period (MWP) and Current Warm Period (CWP). These precipitation oscillations, except that over the CWP, are remarkably similar to, but strongly antiphase with, those in northwestern China. Over the CWP, our δ18O record, similar to other stalagmite records in East Asia, also exhibits a gradually increasing trend. This trend has been assumed to be caused by anthropogenic temperature warming. Here a better correlation is identified between our record and atmospheric temperature change during the preindustrial and industrial eras. This correlation provides solid evidence for the consistent modulation of the temperature on the ASM precipitation without and with the human-induced forcing. However, this modulation may act in a different manner during the industrial era, which results in the same δ18O variation across East Asia. In sum, our record provides a clear spatial variation of the ASM precipitation and highlights persistent temperature modulation on the ASM without and with anthropogenic emissions.

Stable isotopic variations (δD and δ18O) in a mountainous river with rapidly changing altitude: Insight into the hydrological processes and rainout in the basin

AbstractHydrological processes play an important role in stable isotopic compositional variations of the river water. The present study includes spatial distributions of stable hydrogen and oxygen isotopes (δD and δ18O) of a Himalayan River (the Teesta) to trace various hydrological processes and their associated roles on isotopic fractionation of river water. The δ18O compositions of the water samples varies between −12.2‰ and −5.4‰ (mean: −8.01‰ ± 1.81‰) and that of δD between −95.1‰ and −38.5‰ (mean: −57.6‰ ± 15.6‰). The observed compositional variation in δ18O, δD and d values of river water is mainly governed through snowmelt input, evaporation, and recycled moisture contribution. The variation of δ18O and δD values point towards an early melting of snow in the basin and a significant contribution of snowmelt to the river water. The orographic induced precipitation front appears to control the hydrological regimes in a distinct manner as reflected in the trend between d values and elevation. The rain shadow affected northern reaches of the basin are characterized with dry & arid climate triggering evaporation whereas, the windward southern side witness temperature influenced evaporation. The study suggests that the differential degree of recycled moisture contribution to the local precipitation is also a key component in bringing the stable isotopic variation in the river water. This varied magnitude of moisture recycling could result from uneven distribution of evapotranspiration and surface water evaporation. Results found that a rapid change in the altitude of a river basin can give rise to different hydrometeorological conditions, thereby resulting in simultaneous operation of almost all the major hydrological processes and a complex isotopic compositional variation in the river water. Also, the rainout percentage in the Teesta basin is found to be higher owing to this rapid rate of elevation increase.

Isotopic Characterization of Rainwater for the Development of a Local Meteoric Water Line in an Arid Climate: The Case of the Wadi Ziz Watershed (South-Eastern Morocco)

For any hydrological or hydrogeological system, the arrival of new rains is the input signal to the system. This isotopic signature of precipitation is of major interest in understanding the recharge processes of the aquifer system. On the scale of a given basin, staged stations at different altitudes and spread out in space allow this input signal to be well characterized and to draw the local meteoric water line. In south-eastern Morocco, specifically, in the Errachidia region, several chemical and isotopic studies of the waters of the various aquifers have been carried out. In the absence of a local meteoric water line, these studies were based on the use of the global meteoric water line (GMWL). Thus, the objective of this work is the isotopic characterization and the elaboration of the local meteoric water line of the rainwater of the Ziz watershed. This characterization of the input signal in the study area is based on 41 measurements of stable isotopes (δ18O and δ2H) relating to the precipitations collected during the period from December 2019 to November 2020 in four staged stations at different altitudes and spread over the space from upstream to downstream of the watershed. The linear relationship of δ2H as a function of δ18O describes the local meteoric water line (LMWL) by equation δ2H = 7.5 ± 0.3 δ18O + 4.6 ± 1.7; R2 = 0.93. This equation displays evaporation confirmed by the arrival of continental currents in an arid environment. The variation in precipitation δ18O as a function of the sampling altitudes for the rains highlighted the relationship δ18O = −0.0026 ∗ Z − 1.67, with R2 = 0.93, which means an altitudinal gradient of −0.26‰ per 100 m of altitude. In this regard, the development of the local meteoric water line and the determination of the altitudinal gradient for the first time in this arid to semi-arid region of the watershed will be of great use to researchers and water resource managers; for example, to help determine the groundwater recharge areas, determine the exchanges between surface water and groundwater, and analyze many other hydrological problems.

Middle Pleistocene paleoclimate and paleoenvironment of central Italy and their relationship with hominin migrations and evolution

To investigate the paleoclimate and paleoenvironment of central Italy during the Middle Pleistocene, we analyzed the carbon and oxygen stable isotope composition of the carbonate component of sequential enamel samples from twenty-four rhinoceros (Stephanorhinus spp.) teeth. The samples come from two key archeological and paleontological sites located in the Molise region: Isernia La Pineta (ILP; ∼600 ka) and Guado San Nicola (GSN; ∼400 ka).Carbon isotope ratios of enamel from both ILP and GSN indicate feeding in a pure C3 ecosystem that consisted predominantly of woodlands and/or mesic C3 grasslands with the possible sparse presence of closed canopy forests at or near ILP ∼600 ka. The average (± 1 σ) enamel δ13C for ILP (−13.6 ± 0.6‰) is lower than that for GSN (−12.1 ± 0.4‰), suggesting higher mean annual precipitation and lesser aridity ∼600 vs. ∼400 ka. Average intra-tooth variability in enamel δ13C from both sites is low (∼1‰), likely indicating seasonally uniform diets, plant carbon isotope compositions, and precipitation amounts. However, the low intra-tooth variability in enamel δ13C may also reflect amplitude attenuation of the isotopic signal of the plants into tooth enamel.With respect to oxygen isotopes, the average (± 1 σ) enamel δ18O for ILP (24.2 ± 0.7‰) is slightly lower than that for GSN (25.0 ± 0.7‰), likely reflecting the higher elevation of ILP as well as lesser aridity and/or slightly lower mean annual temperature in the region ∼600 vs. ∼400 ka. The δ18O values of meteoric precipitation (∼-7‰) calculated from enamel compositions are indistinguishable from the modern values, suggesting that mean annual temperatures were broadly similar to today (∼13°C). Both sites exhibit a moderate average intra-tooth variability in enamel δ18O (∼2‰), likely indicating a temperature seasonality similar or slightly decreased relative to today. The temperate climate and the increase in aridity that occurred sometime between ∼600 and ∼400 ka were potentially important factors for the human colonization of the Italian peninsula and for the cultural and behavioral evolution of the early hominins during the Middle Pleistocene.

The Seasonally Altered Atmosphere Moisture Circulations With Rainfall and Rainfall Isotopes in Southwest China

To interpret the climatic signals of precipitation/speleothem δ18O, it is critical to identify the importance of the factors affecting the precipitation δ18O. This study presents new stable isotope data for precipitation δ18O and δD in the site of Shenqi cave, southwest China, from November-2015 to October-2016 (the “Super-El Niño” event), to investigate the regional-scale climate forcing on precipitation δ18O. The precipitation δ18O, δD and d-excess have an obvious seasonality, relatively low values in the wet season and high in the dry season. The further analysis of seasonally altered LMWL and moisture circulations suggested that changes in atmosphere moisture circulations would be the key factor underlying the precipitation/speleothem δ18O fluctuations in our study area at least on seasonal timescales. Combined with the seasonal-monthly variations of the IsoGSM δ18O, GPCP/CRU rainfall and NCEP/NCAR moisture fluxes, we detected that the super-El Niño of 2016 have changed the distributions of monthly rainfall in wet season through the Western Pacific Subtropical High, but not mainly the precipitation isotopic compositions and moisture circulations in our study area.

Variability in effective moisture inferred from inclusion fluid δ18O and δ2H values in a central Sierra Nevada stalagmite (CA)

The oxygen isotopic composition of stalagmites is widely used to infer regional changes in terrestrial surface temperatures and precipitation dynamics. The stalagmite δ18O values, however, record the influence of multiple environmental conditions (e.g., temperature, precipitation source and amount) as well as in-cave physicochemical processes and possible disequilibrium precipitation effects. The δ18O and δ2H values of fluids entombed in stalagmites as inclusions have the potential to be robust proxies of paleo-precipitation δ18O and δ2H. Here we analyze the inclusion-fluid δ18O and δ2H values for a stalagmite from a central Sierra Nevada foothill cave, McLean's Cave, to reconstruct changes in effective moisture (precipitation – evaporation) in the region over the last deglaciation (20–13 ka). The results demonstrate high variability in inclusion-fluid δ18O and δ2H values and further suggest that the δ18O and δ2H values have several intervals driven by disequilibrium dis oxygen isotopic fractionation. These findings demonstrate that effective moisture was likely lower during past warm periods in comparison to some colder periods, consistent with other stalagmite calcite-based paleoclimate records from the southwestern United States.

Tracking breastfeeding and weaning practices in ancient populations by combining carbon, nitrogen and oxygen stable isotopes from multiple non-adult tissues.

This paper explores the potential of combining different isotope systems from different tissues to improve resolution when reconstructing breastfeeding and weaning practices (BWP) in archaeology. Additionally, we tested whether changes in diet can be detected in deciduous teeth. Rib collagen samples from 22 infants/children from the archaeological site of Bacuranao I (Mayabeque, Cuba) were processed for nitrogen (δ15N) and carbon (δ13Cco) stable isotopes and assessed using a Bayesian model (WARN). In addition, enamel of 48 teeth from 30 infants/children were analyzed for oxygen (δ18Oen) and carbon (δ13Cen) stable isotopes. Data revealed that the timing of weaning cannot be characterized precisely by analyzing either δ18O or δ15N. While a depletion in both δ15N and δ13Cco is only evident after one year, the WARN model suggested that the weaning process started at around 3 months and ended around 1.7 years. Most teeth were enriched in δ18Oen compared to deciduous incisors, suggesting a breastfeeding signal. However, a high variability in δ18O was found between similar teeth from the same individuals. Higher enrichment in δ18Oen, and variability, was observed in tissues formed during the first six months of life. A δ13C enrichment of 1.0‰ was observed among deciduous teeth and ribs. While most individuals enriched in δ15N showed enrichment in δ13C, the δ18O values were more variable. Our data suggests that stable isotopes of deciduous teeth, especially δ13Cen, can be used to detect changes in diet during the weaning process. It is also possible that the δ18O enrichment observed in M1 is influenced by the effects of cooking techniques on weaning foods. The combination of multiple isotope systems and tissues overcome some of the limitations posed by single tissue approaches.

Southeastern United States Hydroclimate During Holocene Abrupt Climate Events: Evidence From New Stalagmite Isotopic Records From Alabama

AbstractWe present new high‐resolution absolute‐dated stalagmite δ18O and δ13C records from Alabama, southeastern United States (SE US), spanning the last 12 thousand years (ka). A local relationship between annual rainfall amount and its amount‐weighed δ18O composition exists on interannual timescales, driven mostly by an amount effect during summer and spring seasons. Based on a novel quantitative interpretation of modern rainfall isotopic data, stalagmite δ18O variability is interpreted to reflect the relative contribution of summer and spring precipitation combined, relative to combined fall and winter precipitation. Precipitation amount in the SE US increases during an interval of 500 yr coeval with the Younger Dryas and during the 8.2 ka and Little Ice Age abrupt cooling events. High precipitation during these events reflects enhancement of spring and summer precipitation by 90% combined, relative to today's conditions, while the contribution of fall and winter rainfall remained unchanged or decreased slightly. Results from this study support model simulation results that suggest increased precipitation in the SE US during Atlantic Meridional Overturning Circulation (AMOC) slowdown/shutdown. In association with Northern Hemisphere mid‐latitude cooling from the Early to mid‐Holocene, annual precipitation in the SE US decreases, a pattern distinctive from that observed during abrupt cooling events related to AMOC shifts. Long‐term hydroclimate change in the SE US is likely sensitive to summer insolation reduction as inferred to have occurred in other tropical and subtropical regions. The dynamical link between AMOC shutdown and precipitation variability in the SE US remains to be examined.

Variability in <i>Neogloboquadrina pachyderma</i> stable isotope ratios from isothermal conditions: implications for individual foraminifera analysis

Abstract. Individual foraminifera analysis (IFA) holds promise to reconstruct seasonal to interannual oceanographic variability. Even though planktonic foraminifera are reliable recorders of environmental conditions on a population level, whether they also are on the level of individuals is unknown. Yet, one of the main assumptions underlying IFA is that each specimen records ocean conditions with negligible noise. Here we test this assumption using stable isotope data measured on groups of four shells of Neogloboquadrina pachyderma from a 16–19 d resolution sediment trap time series from the subpolar North Atlantic. We find a within-sample variability of 0.11 ‰ and 0.10 ‰ for δ18O and δ13C respectively that shows no seasonal pattern and exceeds water column variability in spring when conditions are homogeneous down to hundreds of metres. We assess the possible effect of life cycle characteristics and delay due to settling on foraminifera δ18O variability with simulations using temperature and δ18Oseawater as input. These simulations indicate that the observed δ18O variability can only partially be explained by environmental variability. Individual N. pachyderma are thus imperfect recorders of temperature and δ18Oseawater. Based on these simulations, we estimate the excess noise on δ18O to be 0.11±0.06 ‰. The origin and nature of the recording imprecision require further work, but our analyses highlight the need to take such excess noise into account when interpreting the geochemical variability among individual foraminifera.

Small Size Gastropod Fauna from the Matli Geothermal Spring, Bhagirathi Valley, Garhwal Himalaya, Uttarakhand: Ecological Implications

Abstract The occurrence of freshwater molluscs in the geothermal springs is reported by numerous workers in different parts of the world. However, reports of such occurrences in the Indian subcontinent are either scanty or are not available in the published literature. In this study, a swarm of small size gastropod community have been encountered in the vicinity of Matli geothermal spring, which is located on the right bank of the downstream Bhagirathi river in the Garhwal Himalaya in Uttarakhand, India. Gastropod samples were collected from 3 locations near the spring and water samples were collected from the spring and the river. The samples were identified as belonging to the species Lymnaea acuminata Lamarck, 1822 according to their form and habitat. The abundance, size and thickness of the snails were higher near the spring than near the Bhagirathi river. The δ18Oshell and δ13Cshell values of all the bulk samples range from – 8.2 to – 7.8‰ (VPDB) and from – 9.5‰ (VPDB) to – 6.5‰ (VPDB), respectively. Since the temperature of the geothermal spring remains constant irrespective of seasons, no significant intra-shell variation in δ18O is observed, while intra–shell δ13C values show shell shell significant variation reflecting season change. Lymnaea accuminata is originally a freshwater gastropod which is found all along the Ganga-Brahmaputra watersheds. This species prefers a habitat near the geothermal spring owing to the abundance of nutrients in spring water and other physiological factors.