Precipitation δ18O Research Articles

Modeling calcification periods of <i>Cytheridella ilosvayi</i> from Florida based on isotopic signatures and hydrological data

Abstract. The isotopic signatures of ostracod shells are the result of the temperature and composition of their host water and the phenology and ecology of the target species. Investigations addressing the influence of site-specific environmental variations on the isotopic ranges of ostracod shells are still rare but can provide important information on habitat-dependent variations and may signify a seasonally restricted timing of calcification periods. Here we present isotopic signatures (δ18Oostr, δ13Costr) of living Cytheridella ilosvayi (Ostracoda) and physical, chemical, and isotopic (δD, δ18Owater, δ13CDIC) compositions of 14 freshwater habitats (rivers, lakes, canals, marshes, sinkholes) in South Florida from winter 2013 and summer 2014. We also present instrumental data of river temperatures and δ18O of precipitation (δ18Oprec) from this region. The physicochemical and isotopic compositions of the selected sites characterize the different habitats and show the influence of the source water, biological activity, and duration of exposure to the surface. Mean δ18Oostr and δ13Costr signatures of C. ilosvayi shells correlate well with the isotopic composition of their host waters. Within-sample variabilities in repeated isotopic measurements of single ostracod shells reflect habitat-dependent ranges. The similarly high range of ostracod δ18O in rivers and one marsh sample indicates that both temperature and δ18Oprec are responsible for their variation in the whole study area. Rivers and canals, which are predominantly influenced by the input and mixing of inorganic carbon from the catchment, show smaller δ13Costr ranges than the marsh dominated by local fluctuations in biological activities. Based on these observations, background data of water temperatures and δ18Oprec were used to calculate monthly δ18O variations in a theoretical calcite formed in rivers in Florida assuming a direct reaction on precipitation changes. The calculated values showed a high variation coupled with low mean values during the summer wet season, while during the winter dry season the variation remains small and mean values increased. Inferred configurations were used to approximate possible calcification periods of C. ilosvayi. For a plausible calcification period, mean values and ranges of δ18Oostr had to be equal to the theoretical calcite with a slight positive offset (vital effect). The applied model suggests a seasonal calcification period of C. ilosvayi in early spring that is probably coupled to the hydrologic cycle of Florida.

Does δ<sup>18</sup>O of O<sub>2</sub> record meridional shifts in tropical rainfall?

Abstract. Marine sediments, speleothems, paleo-lake elevations, and ice core methane and δ18O of O2 (δ18Oatm) records provide ample evidence for repeated abrupt meridional shifts in tropical rainfall belts throughout the last glacial cycle. To improve understanding of the impact of abrupt events on the global terrestrial biosphere, we present composite records of δ18Oatm and inferred changes in fractionation by the global terrestrial biosphere (ΔεLAND) from discrete gas measurements in the WAIS Divide (WD) and Siple Dome (SD) Antarctic ice cores. On the common WD timescale, it is evident that maxima in ΔεLAND are synchronous with or shortly follow small-amplitude WD CH4 peaks that occur within Heinrich stadials 1, 2, 4, and 5 – periods of low atmospheric CH4 concentrations. These local CH4 maxima have been suggested as markers of abrupt climate responses to Heinrich events. Based on our analysis of the modern seasonal cycle of gross primary productivity (GPP)-weighted δ18O of terrestrial precipitation (the source water for atmospheric O2 production), we propose a simple mechanism by which ΔεLAND tracks the centroid latitude of terrestrial oxygen production. As intense rainfall and oxygen production migrate northward, ΔεLAND should decrease due to the underlying meridional gradient in rainfall δ18O. A southward shift should increase ΔεLAND. Monsoon intensity also influences δ18O of precipitation, and although we cannot determine the relative contributions of the two mechanisms, both act in the same direction. Therefore, we suggest that abrupt increases in ΔεLAND unambiguously imply a southward shift of tropical rainfall. The exact magnitude of this shift, however, remains under-constrained by ΔεLAND.

Using oxygen isotopes to establish freshwater sources in Bedford Basin, Nova Scotia, a Northwestern Atlantic fjord

A weekly time-series of oxygen isotope (δ18O) measurements was collected over a 16-month period from near-surface (1 m) and near-bottom (60 m) waters of Bedford Basin, a coastal fjord adjacent to the Scotian Shelf, off eastern Canada. The time-series was complemented with δ18O measurements of local precipitation (rain and snow), river, and wastewater runoff. The isotopic composition of precipitation displayed strong seasonality with an average (volume-weighted) δ18O value of −5.39‰ (±0.96) for summer and a depleted value of −10.37‰ (±2.96) over winter. Winter precipitation exhibited more depleted and variable δ18O of solid precipitation relative to rainfall. The annual, amount-weighted average δ18O of Sackville River discharge (−6.49‰ ± 0.82) was not statistically different from precipitation (−7.24‰ ± 0.92), but exhibited less seasonal variation. Freshwater end-members (zero-salinity intercepts) estimated from annual and seasonal regressions of δ18O versus salinity (S) for Bedford Basin near-surface samples were consistent with the δ18O of summer precipitation and the annual, amount-weighted average for the Sackville River. However, the isotopically depleted signature of winter precipitation was not observed clearly in near-surface waters of Bedford Basin, which might reflect isotope enrichment during sublimation from accumulated snowfall prior to melting and discharge, or retention and mixing within the drainage basin. In near bottom waters, most of the δ18O-S variation (average freshwater end-member: 7.47‰ ± 2.17) could be explained by vertical mixing with near-surface waters (average freshwater end-member: −6.23‰ ± 0.34) and hence with locally-derived freshwater. However the near-bottom δ18O-S variation suggested an additional contribution of a freshwater end-member with a δ18O of −15.55‰ ± 2.3, consistent with a remotely-derived freshwater end-member identified previously for the Scotian Shelf. Residuals from a long-term regression of δ18O-S were generally within the range expected due to analytical uncertainty (±0.05); however near-surface waters exhibited seasonal variability of small amplitude, which was consistent with the timing and δ18O variability of local freshwater inputs.

Ambient Conditions and Feeding Strategy Influence δ18O of Milk Water in Cows (Bos taurus).

There are increasing concerns by consumers regarding agricultural product traceability and authenticity. Oxygen isotope composition (δ18O) has been used in this context based on the relationship between δ18O of animal products and annual precipitation. However, in dairy products this relationship is affected by the seasonality of δ18O in milk water which in turn depends on the feeding system used. We measured 608 milk samples from 28 farms with various feeding strategies in southern Germany throughout the year, investigating the influences of ambient conditions, drinking water source, and feeding strategies on seasonal variation of δ18O in milk water (δmilk). The mechanistic Munich-Kohn model reflecting these influences predicted the seasonal and farm-specific variation of δmilk well. The relationship between δ18O of precipitation and δmilk varied in different feeding strategies. The interplay of ambient conditions and feeding strategy on δmilk should thus be carefully considered when identifying the origin of milk.

ENSO variability reflected in precipitation oxygen isotopes across the Asian Summer Monsoon region

Oxygen isotope signals (δ18O) from paleo-archives are important proxies for past Asian Summer Monsoon (ASM) climate reconstruction. However, causes of interannual variation in the δ18O values of modern precipitation across the ASM region remain in argument. We report interannual δ18O variation in southern Tibetan Plateau precipitation based on long-term observations at Lhasa. These data, together with precipitation δ18O records from five Global Network of Isotopes in Precipitation (GNIP) stations and two ice core δ18O records, were used to define a regional metric of ASM precipitation δ18O (ASMOI). Back-trajectory analyses for rainy season precipitation events indicate that moisture sources vary little between years with relatively high and low δ18O values, a result that is consistent for the south (Lhasa), southeast (Bangkok), and east ASM regions (Hong Kong). In contrast, δ18O values at these three locations are significantly correlated with convection in the estimated source regions and along transport paths. These results suggest that upstream convection, rather than moisture source change, causes interannual variation in ASM precipitation δ18O values. Contrasting values of the ASMOI in El Niño and La Niña years reveal a positive isotope-El Niño Southern Oscillation (ENSO) response (e.g., high values corresponding to warm phases), which we interpret as a response to changes in regional convection. We show that the isotope-ENSO response is amplified at high elevation sites and during La Niña years. These findings should improve interpretations of paleo-δ18O data as a proxy for past ASM variation and provide new opportunities to use data from this region to study paleo-ENSO activity.

Environmental controls on stable isotopes of precipitation in Lanzhou, China: An enhanced network at city scale

Stable hydrogen and oxygen isotopes in precipitation are very sensitive to environmental changes, and can record evolution of water cycle. The Lanzhou city in northwestern China is jointly influenced by the monsoon and westerlies, which is considered as a vital platform to investigate the moisture regime for this region. Since 2011, an observation network of stable isotopes in precipitation was established across the city, and four stations were included in the network. In 2013, six more sampling stations were added, and the enhanced network might provide more meaningful information on spatial incoherence and synoptic process. This study focused on the variations of stable isotopes (δ18O and δD) in precipitation and the environmental controls based on the 1432 samples in this enhanced network from April 2011 to October 2014. The results showed that the precipitation isotopes had great spatial diversity, and the neighboring stations may present large difference in δD and δ18O. Based on the observation at ten sampling sites, an isoscape in precipitation was calculated, and the method is useful to produce isoscape for small domains. The temperature effect and amount effect was reconsidered based on the dataset. Taking meteorological parameters (temperature, precipitation amount, relative humidity, water vapor pressure and dew point temperature) as variables in a multi-linear regression, the result of coefficients for these meteorological parameters were calculated. Some cases were also involved in this study, and the isotopic characteristics during one event or continuous days were used to understand the environmental controls on precipitation isotopes.

Toward more accurate temperature reconstructions based on oxygen isotopes of subfossil chironomid head‐capsules in Australia

AbstractThis study investigates the potential of applying stable oxygen isotopes (δ18O) from head capsules (HCs) of subfossil chironomids (subfamily Tanypodinae) to reconstruct past temperature changes from south‐eastern Australia. The study reports δ18O results from Tanypodinae HCs in nine lakes. The relationship between δ18O values of Tanypodinae HCs in lakes and summer (February) air temperature is robust (r = 0.84) supporting its potential to be applied as a temperature proxy in the Australian region. The comparison of these results with the δ18O values measured on Chironomus spp. HCs from the same lakes reveals differences between the two groups. δ18O values of Tanypodinae HCs have a stronger correlation with the δ18O of lake water, δ18O of precipitation and air temperature as compared with Chironomus δ18O values. This suggests that Tanypodinae HCs are superior targets to Chironomus spp. for temperature reconstructions. Our data indicate that the δ18O of Tanypodinae HCs could perform as well as the assemblage based chironomid transfer function from the Australian mainland. We recommend analysis of HCs from additional lakes to develop a more robust calibration curve relating Tanypodinae HC δ18O to temperature.

Interannual trends in stable oxygen isotope composition in precipitation of China during 1979–2007: Spatial incoherence

According to the simulations of isotope-equipped general circulation models (GCMs) compiled by the Stable Water Isotope Intercomparison Group Phase2 (SWING2), the interannual variability and trend of precipitation isotopic composition in China during 1979–2007 are studied. Six simulations of isotope-enabled GCMs are involved, and monthly series of stable oxygen isotopes in precipitation for each grid are applied to calculate the linear trends from 1979 to 2007. Most isotope-equipped GCM simulations show an increasing trend in δ18O, while the GISS-E (NCEP) result shows a decreasing trend. In most areas of north China and northwest China, the GCM-simulations positively correlate with surface temperature. The correlation coefficients between oxygen isotope composition and surface air temperature generally increase from coast to inland. The correlation between observed and simulated δ18O in precipitation of northwest China simulated by LMDZ (free) is best (r = 0.73) among different subregions and isotope-equipped GCMs. The comparison of GCMs simulations and BW model all show the low value region of δ18O is in the Tibetan Plateau and the high value in south China. China's meteoric water line simulated using isotope-enabled GCMs (slopes range from 7.63 to 8.26, and intercepts range from 9.25 to 11.92) is similar to the global meteoric water line, and all the correlation coefficients are statistically significant at the 0.01 level.

Spatiotemporal variation of stable isotopic composition in precipitation: Post‐condensational effects in a humid area

AbstractIn the present study, a 2‐year dataset on δ18O and δ2H in precipitation is used to investigate hydrometeorologic controls on the isotopic compositions in a temperate maritime climate. Data was collected in Denmark along a transect of Six sampling stations across a landscape with a small topographic gradient and predominant westerly winds. Data showed the local meteoric water line for this region is expressed by the equation δ2H = 7.4δ18O + 5.4‰. A significant trend correlating enriched isotopic values to humidities around 70% during dry season and more depleted isotopic values to humidities around 90% during wet season was derived from the dataset. Temperature was found to only influence the isotopic composition in a secondary way, whereas no significant relationship was obtained for precipitation amount and evapotranspiration. It is suggested that subcloud post‐condensation exchange strongly influences the isotopic composition at the study site. A simple model of evaporation on falling rain was applied with the aim to reproduce observational data and show the potential influence of changing humidity conditions on precipitation compositions. The rather simple model approach did not fully explain the observational data, but it highlights the drastic isotopic changes from a falling raindrop that potentially can occur due to its release into a dryer atmosphere. This study shows that regional conditions and especially humidity can alter the isotopic composition in precipitation substantially even in regions without major topographic and hydrometeorologic gradients.

The possibility of obtaining ultra-low-δ18O signature of precipitation near equatorial latitudes during the Snowball Earth glaciation episodes

Hydrothermally-altered rocks with ultra-low-δ18O and δD were discovered in Karelia, Russia, over 500km along the Belomorian Belt of the Baltic Shield. The ages of low δ18O and δD values are 2.41Ga and 2.29Ga, corresponding to the first and third Paleoproterozoic Snowball earth glacial episodes. Paleogeographic reconstruction suggests low latitudes for Baltica throughout Paleoproterozoic ice ages, requiring an effective distillation of water isotopes over short transport distances. Here we summarize how to use hydrothermally altered rocks to infer δ18O value of altering meteoric waters, then use an idealized General Circulation Model to test climate states with variable amount of ice cover and to check the hydrologic possibility of ultra-low-δ18O precipitation at low latitudes. Our model results suggest that the low δ18O values in precipitation is best achieved if the area of open water is quite narrow around the equator between latitudes 5°S and 5°N, or the Jormungand climate state. The δ18O of precipitation is high (>−15‰) over open water where high-δ18O vapor from ocean mixes with low-δ18O atmospheric water vapor. As such, a position of about 5–10° north or south of the equator is needed in order to achieve low δ18O of precipitation from an open water source. Our results suggest a slushball climate state with wide areas of open water is less likely to produce ultra-low-δ18O rocks for the low latitude position of Baltica.

Holocene temperatures and isotopes of precipitation in Northwest Greenland recorded in lacustrine organic materials

Reconstructions of Holocene lake water isotopic composition based upon subfossil aquatic organic material offer new insights into Arctic climate. We present quantitative estimates of warmth during the Holocene Thermal Maximum in northwest Greenland, inferred from oxygen isotopes of chironomid head capsules and aquatic moss preserved in lake sediments. δ18O values of chironomids from surface sediments of multiple Greenland lakes indicate that these subfossil remains record the δ18O values of the lake water in which they grow. Our lake water δ18O reconstruction is supported by downcore agreement with δ18O values in aquatic moss and chironomid remains. δ18O of both organic materials from Secret Lake decrease after 4 ka (ka = thousands of years ago) by 3‰ into the Neoglacial. We argue that lake water at Secret Lake primarily reflects precipitation δ18O values, which is strongly correlated with air temperature in NW Greenland, and that this signal is biased towards summer and early autumn conditions. Other factors may have influenced Secret Lake δ18O values through the Holocene, including evaporation of lake water and changing seasonality and source of precipitation. The maximum early Holocene summer and early autumn-biased temperature anomaly at Secret Lake is 2.5–4 °C warmer than present from 7.7 (the beginning of our record) to ∼6 ka. The maximum late Holocene cold anomaly (which includes the Little Ice Age) is 1.5–3 °C colder than present. These ranges of possible temperature anomalies reflect uncertainty in the δ18O – temperature relationship for precipitation at the study site through the Holocene.

Geographic origins of a War of 1812 skeletal sample integrating oxygen and strontium isotopes with GIS-based multi-criteria evaluation analysis

The oxygen and strontium isotope composition of human bones and teeth are used to investigate human geographic origins and mobility in the archaeological record. We measured the oxygen (18O/16O) and strontium (87Sr/86Sr) isotope ratios in 14 teeth and 22 bone sections from 19th century soldiers who died in the Battle of Stoney Creek, Ontario, and were buried in a commingled mass grave at Smith's Knoll. To spatially locate the tooth isotope data we generated overlay maps of the eastern United States and Great Britain based on regional 87Sr/86Sr and meteoric precipitation δ18O variation. To do this, we employed a new method to visualize the isotopic information using GIS-based Multi-Criteria Evaluation analysis (GIS-MCE), a computational algorithm yet to be applied to stable isotope migration research. Our data confirm historic information concerning the areas from which American and British soldiers originated or were recruited, namely from the mid- to northerly latitude United States and Great Britain. This approach revealed the probable identity of at least one British soldier, and possibly one American soldier within the Smith's Knoll assemblage, making it the first archaeological study to provide substantial evidence for the identity of individuals interred in a mass grave. Our analysis highlights a new way to integrate isotopic and geophysical data through GIS-MCE technology to locate the most likely place of origin of individuals in a skeletal sample.

A tentative study of the relationship between annual δ18O & δD variations of precipitation and atmospheric circulations—A case from Southwest China

In this study, we present the 6-year δ18O and δD records of precipitation from Chongqing, southwest China. Based on these data, the local meteoric water line (LMWL) has been set up as: δD = 8.33δ18O+19.42. It is demonstrated that in El Niño scenarios, more vapor from closer moisture source (Western Pacific) was transported to south China, resulted in heavier stable isotopes of precipitation in southwest China, while in La Niña scenarios, the situations were just on the contrary. In addition, there is a positive correlation between the δ18O of precipitation in southwest China and the Northern Atlantic Oscillation Index (NAOI) when the NAO is in the positive phase (typically in boreal winter-half year). At the interannual timescale, the δ18O of precipitation in southwest China is more negative in strong NAO years than that in weak NAO years. We speculate that the north-south position migration of the westerlies and the route changes of vapor transport are correlated with the δ18O changes of precipitation in southwest China under the climate change scenario around the North Atlantic.

Lake evolution and hydroclimate variation at Lake Qinghai (China) over the past 32 ka inferred from ostracods and their stable isotope composition

We used ostracod species assemblages and their δ18O values in a 32-m sediment core from Lake Qinghai, China, along with information from cores collected at other sites in the lake, to infer lake evolution and hydroclimate changes since the last glacial. Dominant ostracod species Ilyocypris bradyi and its low δ18O values showed that Lake Qinghai was small in size or even consisted of several playa lakes, and the 1F core site could have even been in a wetland setting, under cold and dry climate conditions before 15.0 ka. Presence of Limnocythere inopinata with low δ18O values, and absence of I. bradyi after 15.0 ka, indicate the lake area increased or that the playas merged. The decrease or disappearance of ostracods with high δ18O values showed that the lake shrunk under dry climate from 12.0 to 11.6 ka. After 11.6 ka, hydroclimate shifts inferred from ostracod species changes (Eucypris mareotica and L. inopinata) and their δ18O values were as follows: (1) 11.6–7.4 ka—larger, but still small lake area with greater moisture availability under primarily dry climate conditions, (2) 7.4 to 3.2 ka—increasing lake level under a warmer and wetter climate, and (3) 3.2 ka to present—stable, large, brackish lake. The low ratio of lake water volume to runoff, and close proximity of the core site to freshwater input from the river mouth would have resulted in relatively lower ostracod δ18O values when Lake Qinghai was small in area during the interval from 32.0 to 15.0 ka. Lower ostracod δ18O values during interstadials and throughout the entire Last Glacial Maximum and early deglacial (ca. 24.0–16.0 ka) were caused by a greater contribution of seasonal meltwater from ice or snow and low incoming precipitation δ18O values related to cold climate conditions in the region at that time.

How does sea ice influence <i>δ</i><sup>18</sup>O of Arctic precipitation?

Abstract. This study investigates how variations in Arctic sea ice and sea surface conditions influence δ18O of present-day Arctic precipitation. This is done using the model isoCAM3, an isotope-equipped version of the National Center for Atmospheric Research Community Atmosphere Model version 3. Four sensitivity experiments and one control simulation are performed with prescribed sea surface temperature (SST) and sea ice. Each of the four experiments simulates the atmospheric and isotopic response to Arctic oceanic conditions for selected years after the beginning of the satellite era in 1979. Changes in sea ice extent and SSTs have different impacts in Greenland and the rest of the Arctic. The simulated changes in central Arctic sea ice do not influence δ18O of Greenland precipitation, only anomalies of Baffin Bay sea ice. However, this does not exclude the fact that simulations based on other sea ice and sea surface temperature distributions might yield changes in the δ18O of precipitation in Greenland. For the Arctic, δ18O of precipitation and water vapour is sensitive to local changes in sea ice and sea surface temperature and the changes in water vapour are surface based. Reduced sea ice extent yields more enriched isotope values, whereas increased sea ice extent yields more depleted isotope values. The distribution of the sea ice and sea surface conditions is found to be essential for the spatial distribution of the simulated changes in δ18O.

Characteristics of Stable Isotope in Precipitation and Its Relationship with ENSO in Shanghai

A total of 70 precipitation samples were collected from August 2014 to August 2015 in Shanghai and the oxygen and hydrogen isotope of precipitation were analyzed. The relationship between the δ18O and deuterium excess of precipitation and ENSO events was addressed. The results showed that δD and δ18O of precipitation in Shanghai were higher in winter and spring than in summer and fall. The slope of meteoric water line with these data was a little smaller than the global meteoric water line. The main reason could be the unbalanced secondary evaporation in the process of precipitation. The correlation between the δ18O in precipitation and temperature and precipitation varied in different time scales. On a seasonal scale, this work revealed that there was no significant temperature effect in winter, but significant precipitation effect was found in the annual scales, which was obviously affected by the atmospheric circulation. δ18O values and deuterium excess in precipitation recorded the La Niña and El Niño conversion process. During the La Niña event, the δ18O and d values were more negative than those in the El Niño event.

Controls over spatial and seasonal variations on isotopic composition of the precipitation along the central and eastern portion of Brazil

ABSTRACT Based on Global Network Isotopes in Precipitation (GNIP) isotopic data set, a review of the spatial and temporal variability of δ18O and δ2H in precipitation was conducted throughout central and eastern Brazil, indicating that dynamic interactions between Intertropical and South Atlantic Convergence Zones, Amazon rainforest, and Atlantic Ocean determine the variations on the isotopic composition of precipitation over this area. Despite the seasonality and latitude effects observed, a fair correlation with precipitation amount was found. In addition, Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) air mass back trajectories were used to quantify the factors controlling daily variability in stable isotopes in precipitation. Through a linear multiple regression analysis, it was observed that temporal variations were consistent with the meteorological parameters derived from HYSPLIT, particularly precipitation amount along the trajectory and mix depth, but are not dependent on vapour residence time in the atmosphere. These findings also indicate the importance of convective systems to control the isotopic composition of precipitation in tropical and subtropical regions.

A monsoon-related 174-year relative humidity record from tree-ring δ18O in the Yaoshan region, eastern central China

The authors present a time series of tree-ring cellulose stable oxygen isotope ratios (δ18O) constructed using a numerical mixing method, from five Pinus tabulaeformis from Yaoshan, eastern central China. The time series is synchronous with the observed April-to-September precipitation δ18O time series records from surrounding stations, implying that the tree-ring δ18O signals reflect Asian summer monsoon-related precipitation. Moreover, significant correlations were detected between the tree-ring δ18O records and central-eastern Pacific sea surface temperature (SSTs)/Asian summer monsoon indices. This suggests that the Yaoshan tree-ring δ18O time series offers a promising means to study past Asian summer monsoons and ENSO variability. The time series is significantly correlated with the mean relative humidity from April to September (RHAS) (r=−0.638, n=52, p<0.0001) over the Yaoshan region. The authors assign a transfer function to the reconstruct the RHAS time series from 1835 to 2008. The mean value of RHAS over the period 1870–1890 is the lowest observed value during the past 174years, implying severe drought at that time. From 1930 to 2008, an obvious drying trend in the Yaoshan region is observed. Particularly from 1985 to 2008, frequent, extremely dry years occurred, with few extremely wet years. The drying trend is consistent with decreasing Asian summer monsoon precipitation over the Loess Plateau over the past 60years. The acidification implied by this data extends well beyond the vicinity of Yaoshan and may be expected to impact the entire monsoonal region in northern China. A multi-taper spectral analysis and an ensemble empirical mode decomposition EEMD of the time series indicate 14.6–12.4 and 6.4–2.3 year cycles in the reconstructed RHAS time series during the past 174years, which correspond with solar cycles and ENSO cycles, respectively.

Bedrock geology controls on catchment storage, mixing, and release: A comparative analysis of 16 nested catchments

AbstractThe bedrock controls on catchment mixing, storage, and release have been actively studied in recent years. However, it has been difficult to find neighbouring catchments with sufficiently different and clean expressions of geology to do comparative analysis. Here, we present new data for 16 nested catchments (0.45 to 410 km2) in the Alzette River basin (Luxembourg) that span a range of clean and mixed expressions of schists, phyllites, sandstones, and quartzites to quantify the relationships between bedrock permeability and metrics of water storage and release. We examined 9 years' worth of precipitation and discharge data, and 6 years of fortnightly stable isotope data in streamflow, to explore how bedrock permeability controls (a) streamflow regime metrics, (b) catchment storage, and (c) isotope response and catchment mean transit time (MTT). We used annual and winter precipitation–run‐off ratios, as well as average summer and winter precipitation–run‐off ratios to characterise the streamflow regime in our 16 study catchments. Catchment storage was then used as a metric for catchment comparison. Water mixing potential of 11 catchments was quantified via the standard deviation in streamflow δD (σδD) and the amplitude ratio (AS/AP) of annual cycles of δ18O in streamflow and precipitation. Catchment MTT values were estimated via both stable isotope signature damping and hydraulic turnover calculations. In our 16 nested catchments, the variance in ratios of summer versus winter average run‐off was best explained by bedrock permeability. Whereas active storage (defined here as a measure of the observed maximum interannual variability in catchment storage) ranged from 107 to 373 mm, total catchment storage (defined as the maximum catchment storage connected to the stream network) extended up to ~1700 mm (±200 mm). Catchment bedrock permeability was strongly correlated with mixing proxies of σδD in streamflow and δ18O AS/AP ratios. Catchment MTT values ranged from 0.5 to 2 years, based on stable isotope signature damping, and from 0.5 to 10 years, based on hydraulic turnover.

Analysing the origin of rain- and subsurface water in seasonal wetlands of north-central Namibia

We investigated the origins of rain- and subsurface waters of north-central Namibia’s seasonal wetlands, which are critical to the region’s water and food security. The region includes the southern part of the Cuvelai system seasonal wetlands (CSSWs) of the Cuvelai Basin, a transboundary river basin covering southern Angola and northern Namibia. We analysed stable water isotopes (SWIs) of hydrogen (HDO) and oxygen (H218O) in rainwater, surface water and shallow groundwater. Rainwater samples were collected during every rainfall event of the rainy season from October 2013 to April 2014. The isotopic ratios of HDO (δD) and oxygen H218O (δ18O) were analysed in each rainwater sample and then used to derive the annual mean value of (δD, δ18O) in precipitation weighted by each rainfall volume. Using delta diagrams (plotting δD vs. δ18O), we showed that the annual mean value was a good indicator for determining the origins of subsurface waters in the CSSWs. To confirm the origins of rainwater and to explain the variations in isotopic ratios, we conducted atmospheric water budget analysis using Tropical Rainfall Measuring Mission (TRMM) multi-satellite precipitation analysis (TMPA) data and ERA-Interim atmospheric reanalysis data. The results showed that around three-fourths of rainwater was derived from recycled water at local–regional scales. Satellite-observed outgoing longwave radiation (OLR) and complementary satellite data from MODerate-resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Scanning Radiometer (AMSR) series implied that the isotopic ratios in rainwater were affected by evaporation of raindrops falling from convective clouds. Consequently, integrated SWI analysis of rain-, surface and subsurface waters, together with the atmospheric water budget analysis, revealed that shallow groundwater of small wetlands in this region was very likely to be recharged from surface waters originating from local rainfall, which was temporarily pooled in small wetlands. This was also supported by tritium (3H) counting of the current rain- and subsurface waters in the region. We highly recommend that shallow groundwater not be pumped intensively to conserve surface and subsurface waters, both of which are important water resources in the region.