Continental Recycling Research Articles

Seasonal Variations and Controls on Triple Oxygen and Hydrogen Isotopes in Precipitation—A Case Study From Monitoring in Southwest China

AbstractPrecipitation δ18O has offered valuable insights into the evolution of the Asian monsoon. Recent researches focusing on precipitation Δ′17O has enhanced our understanding by offering new perspectives beyond those of δ18O, revealing insights into vapor sources and continental recycling. Nevertheless, there remains a lack of interannual triple oxygen isotope data, particularly in the Asian monsoon region. In this study, we analyzed the triple oxygen isotopes and hydrogen isotopes in monthly precipitation samples collected from Chongqing in Southwest China between 2019 and 2022 A.D. Seasonal variations in δD, δ18O, δ17O, and d‐excess values were observed, with lower values during the rainy season and higher values during the dry season, highlighting the impact of changes in moisture sources and local meteorological conditions on seasonal shifts in δD, δ18O, and δ17O. While, mean Δ′17O values were higher in rainy season and lower in dry season. Notably, during rainy season, there is a negative correlation between monthly Δ′17O values and the RH of the vapor source area, as well as a positive correlation with d‐excess. Recalculated Δ′17O values based on RH of oceanic moisture source, are higher than the measured values for this period, indicating the contribution of terrigenous moisture to precipitation in SW China. Precipitation Δ′17O values provide a more precise reflection of changes in moisture source, continental recycling, and evapotranspiration processes that drive water cycling compared Integrating modeling works in future will facilitate the use of precipitation Δ′17O values to quantify the impact of different moisture source on precipitation.

Spatial and meteorological controls of stable water isotope dynamics of precipitation in Kashmir Valley, Western Himalaya, India

ABSTRACT In the Himalayas, the lives and livelihoods of millions of people are sustained by water resources primarily depending on the moisture brought by Western Disturbances and Indian Summer Monsoon. In the present study, a network of 12 precipitation stations was established across the Kashmir Valley to understand the spatial and meteorological factors controlling precipitation isotopes. Temperature and relative humidity are dominant meteorological factors, whereas altitude, proximity to forest canopy, land use/land cover, windward and leeward sides of the mountains are the main physical factors influencing precipitation isotopes. The study suggests that the Mediterranean Sea and nearby water bodies along with continental recycling are the dominant sources of moisture from October to May, while the Arabian Sea, Bay of Bengal and continental recycling are the main sources of moisture from June to September. However, some precipitation events from October to May collect moisture from the Arabian Sea and some precipitation events from June to September collect moisture from the Mediterranean Sea. The occasional passage of Western Disturbances in summer merging with the Indian Summer Monsoon yields heavy to very heavy precipitation. The study provides a better understanding of complex spatial and meteorological phenomena controlling precipitation isotopes across the Western Himalayas.

Evidence for a Paleogene boninitic arc following oceanic plateau-continent collision in the Western Cordillera of Colombia

The Caicedo Tuffs is a recently mapped Paleogene volcanic arc unit in the northern Andes, which provides insight into the magmatism of northwestern South America following its collision with the Caribbean-Colombian Oceanic Plateau (CCOP) in the latest Cretaceous. New regional geological mapping, igneous petrography, whole-rock and pyroxene geochemistry, and zircon U-Pb dating are provided for the northern sector of the eastern flank of the Western Cordillera of Colombia (Antioquia). These data reveal that the Caicedo Tuffs unit is predominantly composed of primary volcaniclastic deposits (tuff to breccia) that record mafic subaqueous volcanism and limited orogenic building shortly after collision of the CCOP. The geochemistry of these deposits includes major element characteristics and immobile trace element patterns that are typical of boninites. Zircons from these igneous rocks constrain a crystallization age of ca. 57 Ma, with meso-Proterozoic to Jurassic inherited zircons most likely reflecting continental recycling by turbidites in the subduction zone. The boninitic affinity of the Caicedo Tuffs and existing regional igneous and tectonic constraints can be explained by resuming of subduction and/or sinking of an older oceanic slab along northern South America following collision of the CCOP with South America. The boninitic, primary volcaniclastic deposits of the Caicedo Tuffs are crosscut by basaltic-gabbroic sills and dykes with oceanic plateau-like geochemical affinities, which lack evidence for hydrous melting and are compositionally similar to igneous rocks of the CCOP. These intrusions are interpreted to reflect decompression melting of plume-related mantle inherited during the collision of the CCOP with South America or slab detachment. The occurrence of oceanic plateau-like intrusions in a Paleocene volcanic arc casts doubt on the origin of other oceanic plateau sequences of the Western Cordillera, which are generally attributed to the Cretaceous CCOP. The Caicedo Tuffs is a unique boninite unit of which the detailed spatiotemporal extent and petrogenesis remain to be constrained, but that can provide valuable insight into tectono-magmatic processes during oceanic plateau-continent collision.

Importance in Shifting Circulation Patterns for Dry Season Moisture Sources in the Brazilian Amazon

AbstractWater is redistributed from evaporation sources to precipitation sinks through atmospheric moisture transport. In the Brazilian Amazon, the spatial and temporal variability of dry season moisture sources for key agricultural regions has not been investigated. This study investigates moisture sources for dry season rainfall in the state of Rondônia in Brazil, especially during drought years. Using a precipitationshed framework, we quantified the variability of moisture contributions to rainfall in the state of Rondônia (Brazilian Amazon) and the influence of synoptic circulation patterns. Ocean evaporation accounts for 58% of mean dry season precipitation while continental recycling contributed 42%. During drought years, although forests maintain or increase evapotranspiration, the moisture contribution of both ocean and forests to dry season rainfall decreases due to the synoptic circulation changes, reducing the moisture transport into Rondônia.

Vehicle-based in situ observations of the water vapor isotopic composition across China: spatial and seasonal distributions and controls

Abstract. Stable water isotopes are natural tracers in the hydrological cycle and have been applied in hydrology, atmospheric science, ecology, and paleoclimatology. However, the factors controlling the isotopic distribution, both at spatial and temporal scales, are debated in low and middle latitude regions, due to the significant influence of large-scale atmospheric circulation and complex sources of water vapor. For the first time, we made in situ observations of near-surface vapor isotopes over a large region (over 10 000 km) across China in both pre-monsoon and monsoon seasons, using a newly designed vehicle-based vapor isotope monitoring system. Combined with daily and multiyear monthly mean outputs from the isotope-incorporated global spectral model (Iso-GSM) and infrared atmospheric sounding interferometer (IASI) satellite to calculate the relative contribution, we found that the observed spatial variations in both periods represent mainly seasonal mean spatial variations, but are influenced by more significant synoptic-scale variations during the monsoon period. The spatial variations of vapor δ18O are mainly controlled by Rayleigh distillation along air mass trajectories during the pre-monsoon period, but are significantly influenced by different moisture sources, continental recycling processes, and convection during moisture transport in the monsoon period. Thus, the North–South gradient observed during the pre-monsoon period is counteracted during the monsoon period. The seasonal variation of vapor δ18O reflects the influence of the summer monsoon convective precipitation in southern China and a dependence on temperature in the North. The spatial and seasonal variations in d-excess reflect the different moisture sources and the influence of continental recycling. Iso-GSM successfully captures the spatial distribution of vapor δ18O during the pre-monsoon period, but the performance is weaker during the monsoon period, maybe due to the underestimation of local or short-term high-frequency synoptic variations. These results provide an overview of the spatial distribution and seasonal variability of water isotopic composition in East Asia and their controlling factors, and they emphasize the need to interpret proxy records in the context of the regional system.

Geochronological record of Cadomian exterior orogen reworking in bi-modal igneous protoliths of the Cabo Ortegal Allochthon (NW Iberia): the Cambrian onset of the Palaeozoic plate-tectonic cycle

ABSTRACT New geochemical data and radiometric ages of the protoliths of variably reworked metamafic (granulites and gabbros) and felsic rocks (orthogneisses) from the Cabo Ortegal allochthonous Complex are presented. Their igneous precursors were emplaced in ensialic environments and exhibit geochemical signatures transitional to tholeiitic character, probably originating in arc settings (with continental and oceanic components and minor crustal recycling). The new geochronological results (encompassing the Miaolingian and Furongian epochs of the Cambrian) link the aforementioned scenarios to a Cadomian magmatic arc. The 505Ma protoliths of calc-alkaline orthogneisses and mafic granulites would be the oldest in the sequence of intrusion events. The latter and the 498–486Ma protoliths of gabbros (emplaced in sedimentary host sequences later transformed into Variscan high-pressure and intermediate-pressure gneisses) can be considered as snapshots of the ongoing continental arc evolution. The 498–486Ma gabbro intrusives of these gneisses, together with the 505–503Ma orthogneisses, are ascribable to palaeo-geographically separate scenarios before the Variscan amalgamation of their country rocks. They would expand the initiation of rifting of the Gondwana continental margin to the Miaolingian Cambrian epoch. It is inferred that the Cadomian continental-arc (active at least since 750Ma) experienced in the 510–460Ma interval the consequences of a vanishing active subduction. Certain domains flanking the arc underwent forearc sedimentation and deformation, whereas others were affected by an increasing extension (rifting) and/or back-arc basin formation. The latter marked the initiation of the beginning of a Wilson Cycle, which ended in the late Palaeozoicwith the formation of the Variscan interior orogenic belt.

Investigating hydrometeorology of the Western Himalayas: Insights from stable isotopes of water and meteorological parameters

The Himalayas govern the hydrometeorology of the entire Indian subcontinent and feed 19 major rivers on which a large population is dependent. Despite its vast socio-economic relevance, there exist knowledge gaps in the detailed understanding of Himalayan hydrometeorology. The present study attempts to understand subtle hydrometeorological processes concerning precipitation in Western Himalaya (WH), which is further complex due to several mountain ranges in addition to the Great Himalaya and varied land cover. In this backdrop, oxygen and hydrogen isotopic analyses of daily precipitation samples collected from Jammu was done. Major processes and features identified are: (1) Raindrop re-evaporation is dominant, indicated by lower δ18O – δD regression slope (6.6 ± 0.2). (2) Maximum raindrop re-evaporation occurs during Western Disturbances (WD), evident from the low d-excess (<10‰) and high δ18O (>0‰) with low rainfall. (3) Contrary to expectations, negligible moisture is derived from the Mediterranean region (MR) through WD. In contrast, maximum moisture (~97%) is derived from local terrestrial sources, estimated through backward wind trajectories and confirmed by vertical wind velocity (ω) and OLR. (4) The Arabian Sea (AS) moisture contributes more (~11%) compared to the Bay of Bengal (BOB) moisture (~8%) during the monsoon. (5) Isotopically, the most depleted precipitation during July-Aug is associated with continental recycling from the Indo-Gangetic plains and not with the BOB moisture. (6) Isotopic enrichment in precipitation towards end of monsoon is attributed to raindrop re-evaporation, lesser rainout fraction and moisture derived from enriched local sources. (7) Annually, 87% of the moisture for precipitation is derived through continental recycling, and only 13% of the moisture is derived through marine sources (11% from AS and 2% from BOB). Moisture from MR (<0.1%) is negligible.

Recycling of continental crust in the southern North China Craton: Constraints from the Sr–Nd–Pb–Hf–O isotopic compositions of Early Cretaceous Funiushan granites

This paper reports new zircon U–Pb ages, whole-rock geochemistry, and Sr–Nd–Pb–Hf–O isotopic compositions for the Funiushan (FNS) pluton of the southern North China Craton (NCC). These data and statistical analyses of previously reported compositions of late Mesozoic granites from the southern NCC are used to constrain late Mesozoic continental recycling and geodynamic processes. The FNS pluton includes coexisting early Early Cretaceous (140 Ma), late Early Cretaceous (130–116 Ma) and highly evolved granites (HEGs; 126–125 Ma). The early Early Cretaceous granites have high Sr contents (332–566 ppm), high Sr/Y (53–138) and (La/Yb)N (29–52) ratios, and low MgO contents (0.15–0.48 wt%). They have relatively low (87Sr/86Sr)i values (0.7051–0.7054), relatively high εNd(t) values (−4.11 to −3.96) and variable εHf(t) values (−22.9 to −0.53). They are also characterized by high initial 206Pb/204Pb values (17.893–17.975) and relatively low δ18O values (5.34‰–7.48‰) as well as Neoproterozoic (851–734 Ma) inherited zircons, suggesting that they have an adakitic affinity formed by recycling of subducted continental crust of the Yangtze Craton (YC) with input from a mantle component. In contrast, the late Early Cretaceous granites have relatively high (87Sr/86Sr)i values (0.7077–0.7085), low εNd(t) values (−14.2 to −12.0), relatively high initial 206Pb/204Pb values (17.398–17.497), εHf(t) values of −23.7 to −10.7, and δ18O values of 5.76‰–6.55‰. These data, and the Neoproterozoic zircon ages (716–702 Ma), indicate that the primary magmas formed from recycled YC crust with a minor contribution from the NCC basement. The late Early Cretaceous HEGs have relatively high εNd(t) values (−9.33 to −8.70), high initial 206Pb/204Pb values (17.656–17.885), intermediate εHf(t) values (−12.3 to −6.18), and relatively low δ18O values (5.22‰–6.72‰), indicating that they formed from subducted YC and mantle material. Previous work on granites of the southern NCC has established two phases of granite formation. Late Jurassic to early Early Cretaceous I-type granites (160–130 Ma) occur throughout the southern NCC. These are adakitic and formed from thickened lower continental crust (LCC). Late Early Cretaceous A-type granites (130–110 Ma) formed from thinned LCC. The differences between Late Jurassic to early Early Cretaceous granites and late Early Cretaceous granites reflect a transformation in tectonic regime from thickening to thinning, which is attributed to rollback of the subducting Paleo-Pacific Plate.

Deep seismic reflection insights into syn-Rodinian crustal recycling

Owing to most tectonic events in Precambrian have been modified by intensive post-Precambrian overprinting, the question how exactly crustal recycling operated on earlier Earth remains an open one. Here, we report a SE-striking, 175 km-long deep seismic reflection profile that traverses the Sichuan basin, China. The image shows a well-preserved syn-Rodinian orogen resting beneath flat lying sedimentary cover of the basin. Relative to the counterparts of modern Earth, this orogen shows a structurally complex Moho geometry composed of multiple layers of basaltic sills, exhibiting extensive crustal extraction. The outlined crustal architecture further depicts that growth of the upper plate arose dominantly through tectonothermal processes, while coeval ductile underthrusting and overthrusting drove contemporaneous crustal shortening of the lower plate. Together with accretionary stacking above, these processes coevally generated a crustal thickness to ~ 30 km. This well-preserved syn-Rodinian orogen provides direct evidence for understanding contemporaneous continental recycling that assembled the Rodinia supercontinent. Plain language summaryNear-vertical, deep seismic reflection profiling is an effective technique for mapping complicated subsurface structures. In this study, the obtained high-resolution deep seismic reflection image captured a non-modified syn-Rodinian orogenic belt resting beneath flay lying sedimentary cover of the Sichuan basin, western Yangtze block, China. Structural interpretation and associated tectonic implications tell us: (1) the Yangtze block was actually a tectonic collage of microcontinents accreted during Neoproterozoic Rodinian assembly, (2) contemporaneous crustal recycling is featured with extensive crustal extraction, and (3) crustal growth was dominantly driven by tectonothermal events in the upper plate of the convergent zone and crustal shortening in a way of coeval underthrusting and overthrusting in the lower plate, a scenario that has shown no big difference with the counterparts of modern Earth. This well-preserved Neoproterozoic orogenic belt provides direct evidence for helping understand the continental recycling that assembled the Rodinia Supercontinent.

Onset of summer monsoon in Northeast India is preceded by enhanced transpiration

Variations in isotopic composition of water vapor in the atmosphere is an important indicator of the processes within the hydrological cycle. Isotopic signature of water vapor and precipitation can be helpful in partitioning evaporation and transpiration fluxes. It is well known that transpiration from forested regions supplies a significant amount of vapor to the atmosphere in monsoon and post-monsoon seasons. Here, we utilize observations from Tropospheric Emission Spectrometer (TES), Atmospheric Infra-Red Sounder (AIRS) and simulation models to ascertain that transpiration is dominant in the forests of Northeast India (NE) during pre-monsoon season. Our results show an increase in δD of 78.0 ± 7.1‰ and in specific humidity of 3.1 ± 0.2 g kg−1 during the pre-monsoon months of April-May compared to January-February. In the monsoon months of July-August, δD reduces by 53.0 ± 6.5‰ albeit the specific humidity increases by 3.4 ± 0.2 g kg−1. Using joint observations of specific humidity and isotope ratio in lower troposphere, we discern the moisture sources over NE India in pre-monsoon and monsoon seasons and posit the role of transpiration in continental recycling during pre-monsoon season.

Seasonal Patterns of Water Cycling in a Deep, Continental Mountain Valley Inferred From Stable Water Vapor Isotopes

AbstractThe origin of atmospheric moisture is difficult to determine from meteorological measurements alone. Stable water vapor isotopes can be used to trace the atmospheric history of water vapor, and therefore are a powerful tool for understanding water cycle processes across a range of spatial and temporal scales. We present subhourly measurements of summer and winter vapor isotope ratios from a deep mountain valley in northwestern Wyoming. Vapor isotopes are paired with local meteorological measurements, and relationships between local vapor isotopic compositions and large‐scale air transport are explored using atmospheric back trajectories and a Lagrangian moisture source diagnostic. Isotopic compositions vary on diurnal to seasonal timescales, and to first order, track changes in local specific humidity. Local vapor isotopic composition varies with air transport pathway in both seasons, but cannot be uniquely mapped to a source region due to continental recycling of vapor. Diurnal variability in d‐excess is substantial in summer, but not in winter. This seasonal contrast is likely driven by local surface and near‐surface processes including evapotranspiration and boundary layer mixing, which partially overprint the source isotope ratios of the regional source during the summer and casts doubt on the use of d‐excess over continental interiors as a tracer of evaporative conditions over the ocean. Continued monitoring of the isotopic composition of near‐surface vapor in continental settings may help detect changes in regional moisture convergence and the local evapotranspiration flux, as well as assess meso‐to‐regional scale responses to hydrologic variability.

Pervasive Eclogitization Due to Brittle Deformation and Rehydration of Subducted Basement: Effects on Continental Recycling?

AbstractThe buoyancy of continental crust opposes its subduction to mantle depths, except where mineral reactions substantially increase rock density. Sluggish kinetics limit such densification, especially in dry rocks, unless deformation and hydrous fluids intervene. Here we document how hydrous fluids in the subduction channel invaded lower crustal granulites at 50–60 km depth through a dense network of probably seismically induced fractures. We combine analyses of textures and mineral composition with thermodynamic modeling to reconstruct repeated stages of interaction, with pulses of high‐pressure (HP) fluid at 650–670°C, rehydrating the initially dry rocks to micaschists. SIMS oxygen isotopic data of quartz indicate fluids of crustal composition. HP growth rims in allanite and zircon show uniform U‐Th‐Pb ages of ∼65 Ma and indicate that hydration occurred during subduction, at eclogite facies conditions. Based on this case study in the Sesia Zone (Western Italian Alps), we conclude that continental crust, and in particular deep basement fragments, during subduction can behave as substantial fluid sinks, not sources. Density modeling indicates a bifurcation in continental recycling: Chiefly mafic crust, once it is eclogitized to &gt;60%, are prone to end up in a subduction graveyard, such as is tomographically evident beneath the Alps at ∼550 km depth. By contrast, dominantly felsic HP fragments and mafic granulites remain positively buoyant and tend be incorporated into an orogen and be exhumed with it. Felsic and intermediate lithotypes remain positively buoyant even where deformation and fluid percolation allowed them to equilibrate at HP.

Water vapour source identification for daily rain events at Ahmedabad in semi‐arid western India: wind trajectory analyses

ABSTRACTThe Arabian Sea (AS), the Bay of Bengal (BOB) and continental recycling are three principal vapour sources identified for the rain events at Ahmedabad located in semi‐arid western India. In the present study, the 48 h backward wind trajectories, at 6 h intervals, converging at an altitude of 1.5 km above the ground, for 120 daily rain events during the monsoon seasons of 2005–2008, have been examined using HYsplit Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Temperature, relative humidity, altitude and rainfall along each trajectory were studied at hourly intervals. The temperature and relative humidity data were used to compute the absolute humidity (g m−3) of the air parcel at its location at hourly intervals. The relationship between the height of the air parcel, its relative humidity and rainfall along each trajectory was used to identify the vapour pick‐up region. The region around the maximum value of absolute humidity for each trajectory was designated as the dominant vapour pick‐up region for each rain event at Ahmedabad. In this manner, principal vapour source identification was unambiguously possible for 93.3% of all the 120 rain events. The fraction of total events and the fractions of total rainfall attributable to the three principal vapour sources, respectively, are: AS, 68.3 and 62.1%; BOB, 2.5 and 1.5%; recycled, 22.5 and 24.5%; mixed, 6.7 and 11.8%. Most of the rain events with recycled vapour contribution occur during the latter part of the monsoon season, namely during September.

Continuous measurements of atmospheric water vapour isotopes in western Siberia (Kourovka)

Abstract. The isotopic composition of atmospheric water vapour at the land surface has been continuously monitored at the Kourovka astronomical observatory in western Siberia (57.037° N, 59.547° E; 300 m a.s.l.) since April 2012. These measurements provide the first record of δD, δ18O and d-excess in this region. Air was sampled at 8 m height within a forest clearing. Measurements were made with a wavelength-scanned cavity ring-down spectroscopy analyzer (Picarro L2130-i). Specific improvements of the measurement system and calibration protocol have been made to ensure reliable measurements at low humidity during winter. The isotopic measurements conducted till August 2013 exhibit a clear seasonal cycle with maximum δD and δ18O values in summer and minimum values in winter. In addition, considerable synoptic timescale variability of isotopic composition was observed with typical variations of 50–100‰ for δD, 10–15‰ for δ18O and 2–8‰ for d-excess. The strong correlations between δD and local meteorological parameters (logarithm of humidity and temperature) are explored, with a lack of dependency in summer that points to the importance of continental recycling and local evapotranspiration. The overall correlation between δD and temperature is associated with a slope of 3‰ °C−1. Large d-excess diurnal variability was observed during summer with up to 30‰ decrease during the night and the minima manifested shortly after sunrise. Two dominant diurnal cycle patterns for d-excess differing by the magnitude of the d-excess decrease (21‰ and 7‰) and associated patterns for meteorological observations have been determined. The total uncertainty of the isotopic measurements was quantified as 1.4–11.2‰ for δD, 0.23–1.84‰ for δ18O and 2.3–18.5‰ for d-excess depending on the humidity.

Developing a western Siberia reference site for tropospheric water vapour isotopologue observations obtained by different techniques (in situ and remote sensing)

Abstract. Water stable isotopologues provide integrated tracers of the atmospheric water cycle, affected by changes in air mass origin, non-convective and convective processes and continental recycling. Novel remote sensing and in situ measuring techniques have recently offered opportunities for monitoring atmospheric water vapour isotopic composition. Recently developed infrared laser spectrometers allow for continuous in situ measurements of surface water vapour δDv and δ18Ov. So far, very few intercomparisons of measurements conducted using different techniques have been achieved at a given location, due to difficulties intrinsic to the comparison of integrated with local measurements. Nudged simulations conducted with high-resolution isotopically enabled general circulation models (GCMs) provide a consistent framework for comparison with the different types of observations. Here, we compare simulations conducted with the ECHAM5-wiso model with two types of water vapour isotopic data obtained during summer 2012 at the forest site of Kourovka, western Siberia: hourly ground-based FTIR total atmospheric columnar δDv amounts, and in situ hourly Picarro δDv measurements. There is an excellent correlation between observed and predicted δDv at surface while the comparison between water column values derived from the model compares well with FTIR estimates.

Stable oxygen isotope differences between the areas to the north and south of Qinling Mountains in China reveal different moisture sources

ABSTRACTδ18O variability in daily precipitation at two stations (Lanzhou and Zhangye) north of the Qinling Mountains at the eastern margin of the Tibetan Plateau, China, and monthly precipitation δ18O at four stations to the south of the Qinling Mountains were examined. The data show that the δ18O composition of precipitation south of the Qinling Mountains is influenced strongly by the prevailing westerlies during winter and by the summer Asian monsoon during its most active periods. To the north, the westerlies prevail in winter, and δ18O trends coincide strongly with temperature. In summer, the Qinling Mountains block or weaken the Asian monsoon, which crosses them only during the most active periods. In these periods, the stations to the north experience relatively depleted δ18O values (&lt;−7‰). During weak monsoon periods, however, most of the δ18O values in summer precipitation at Lanzhou and Zhangye are relatively enriched (&gt;−5‰, and even as high as +9.2‰). A positive northward weighted average δ18O gradient of summer precipitation with increasing distance from the coast suggests the influence of the prevailing westerlies and of continental recycling of moisture. Both the frequency of these heavily depleted δ18O values and their duration decrease from south to north, reaching minimums at Zhangye, which may have been in the monsoonal tail region during the sampling period. The data reveal that the Qinling Mountains act as an important climatic divide along the eastern margin of the Tibetan Plateau. The effects of the Asian summer monsoon and of different moisture sources should be considered for paleoclimatic studies.

Role of continental recycling in intraseasonal variations of continental moisture as deduced from model simulations and water vapor isotopic measurements

Climate models suggest an important role for land‐atmosphere feedbacks on climate, but exhibit a large dispersion in the simulation of this role. We focus here on the role of continental recycling in the intraseasonal variability of continental moisture, and we explore the possibility of using water isotopic measurements to observationally constrain this role. Based on water tagging, we design a diagnostic, named D1, to estimate the role of continental recycling on the intraseasonal variability of continental moisture simulated by the general circulation model LMDZ. In coastal regions, the intraseasonal variability of continental moisture is mainly driven by the variability in oceanic moisture convergence. More inland, the role of continental recycling becomes important. The simulation of this role is sensitive to model parameters modulating evapotranspiration. Then we show that δD in the low‐level water vapor is a good tracer for continental recycling, due to the enriched signature of transpiration. Over tropical land regions, the intraseasonal relationship between δD and precipitable water, named D1_iso, is a good observational proxy for D1. We test the possibility of using D1_iso for model evaluation using two satellite data sets: GOSAT and TES. LMDZ captures well the spatial patterns of D1_iso, but underestimates its values. However, a more accurate description of how atmospheric processes affect the isotopic composition of water vapor is necessary before concluding with certitude that LMDZ underestimates the role of continental recycling.

Continental recycling and true continental growth

Continental crust is very important for evolution of life because most bioessential elements are supplied from continent to ocean. In addition, the distribution of continent affects climate because continents have much higher albedo than ocean, equivalent to cloud. Conventional views suggest that continental crust is gradually growing through the geologic time and that most continental crust was formed in the Phanerozoic and late Proterozoic. However, the thermal evolution of the Earth implies that much amounts of continental crust should be formed in the early Earth. This is “Continental crust paradox”. Continental crust comprises granitoid, accretionary complex, and sedimentary and metamorphic rocks. The latter three components originate from erosion of continental crust because the accretionary and metamorphic complexes mainly consist of clastic materials. Granitoid has two components: a juvenile component through slab-melting and a recycling component by remelting of continental materials. Namely, only the juvenile component contributes to net continental growth. The remains originate from recycling of continental crust. Continental recycling has three components: intracrustal recycling, crustal reworking, and crust–mantle recycling, respectively. The estimate of continental growth is highly varied. Thermal history implied the rapid growth in the early Earth, whereas the present distribution of continental crust suggests the slow growth. The former regards continental recycling as important whereas the latter regarded as insignificant, suggesting that the variation of estimate for the continental growth is due to involvement of continental recycling. We estimated erosion rate of continental crust and calculated secular changes of continental formation and destruction to fit four conditions: present distribution of continental crust (no continental recycling), geochronology of zircons (intracontinental recycling), Hf isotope ratios of zircons (crustal reworking) and secular change of mantle temperature. The calculation suggests some important insights. (1) The distribution of continental crust around at 2.7 Ga is equivalent to the modern amounts. (2) Especially, the distribution of continental crust from 2.7 to 1.6 Ga was much larger than at present, and the sizes of the total continental crust around 2.4, 1.7, and 0.8 Ga became maximum. The distribution of continental crust has been decreasing since then. More amounts of continental crust were formed at higher mantle temperatures at 2.7, 1.9, and 0.9 Ga, and more amounts were destructed after then. As a result, the mantle overturns led to both the abrupt continental formation and destruction, and extinguished older continental crust. The timing of large distribution of continental crust apparently corresponds to the timing of icehouse periods in Precambrian.

What do moisture recycling estimates tell us? Exploring the extreme case of non-evaporating continents

Abstract. Moisture recycling estimates are diagnostic measures that could ideally be used to deduce the response of precipitation to modified land-evaporation. Recycling estimates are based on moisture-budget considerations in which water is treated as a passive tracer. But in reality water is a thermodynamically active component of the atmosphere. Accordingly, recycling estimates are applicable to deduce the response to a perturbation only if other mechanisms by which evaporation affects climate do not dominate the response – a condition that has not received sufficient attention in the literature. In our analysis of what moisture recycling estimates tell us, we discuss two such additional mechanisms that result from water's active role. These are (I) local coupling, by which precipitation is affected locally via the thermal structure of the atmosphere, and (II) the atmospheric circulation, by which precipitation is affected on a large spatial scale. We perform two global climate model experiments: One with and another without continental evaporation. By this extreme perturbation we test the predictive utility of a certain type of recycling measure, the "continental recycling ratio". Moreover, by such a strong perturbation the whole spectrum of possible responses shows up simultaneously, giving us the opportunity to discuss all concurrent mechanisms jointly. The response to this extreme perturbation largely disagrees with the hypothesis that moisture recycling is the dominant mechanism. Instead, most of the response can be attributed to changes in the atmospheric circulation, while the contributions to the response by moisture recycling as well as local coupling, though noticeable, are smaller. By our case study it is not possible to give a general answer to the question posed in the title, but it demonstrates that recycling estimates do not necessarily mirror the consequences of land-use change for precipitation.

Understanding the Sahelian water budget through the isotopic composition of water vapor and precipitation

The goal of this paper is to investigate the added value of water isotopic measurements to estimate the relative influence of large‐scale dynamics, convection, and land surface recycling on the Sahelian water budget. To this aim, we use isotope data in the lower tropospheric water vapor measured by the SCIAMACHY and TES satellite instruments and in situ precipitation data from the Global Network for Isotopes in Precipitation and collected during the African Monsoon Multidisciplinary Analysis field campaign, together with water‐tagging experiments with the Laboratoire de Météorologie Dynamique general circulation model (LMDZ) fitted with isotopes. We show that some isotopic biases in LMDZ reveal the misrepresentation of dehydrating processes that would be undetected without isotopic measurements. In dry regions, the vapor isotopic composition is primarily controlled by the intensity of the air dehydration. In addition, it may also keep some memory of dehydration pathways that is erased in the humidity distribution, namely the relative contribution of dehydration in the tropical upper troposphere versus midlatitudes. In wet regions, vapor and rain isotope compositions are primarily controlled by changes in convection, through rain reevaporation and through the progressive depletion of the vapor by convective mixing along air mass trajectories. Gradients in vapor isotope composition along air mass trajectories may help estimate continental recycling intensity, provided that we could quantify the effect of convection on the isotopic composition of water vapor.