The impact of the freeze-thaw process on the active layer is reflected in the changed subsurface flow (SSF) process in cold alpine regions. Identifying sources and pathways of SSF in the freeze-thaw process is critical but difficult, and the related dominant factors and mechanisms are still unknown. In this paper, the effective identification and analysis of SSF are promoted based on field sampling data from the thawing (June) to freezing (September) period of 2022 in the Qinghai Lake basin on the northeastern Qinghai-Tibetan Plateau. By the proposed method with a high sampling frequency and refined sampling spatial scale, the sources and pathways of SSF are clearly identified. The results are as follows: (1) The soil temperature is considered the most fundamental factor affecting the SSF pathways, it influences water infiltration to the deep layer and the effect is extended to the saprolite and weathered bedrock layers. (2) Thawing promotes water to infiltrating into deep layer. 30 cm soil water contributes the most to SSF (2 %-86 %) in the thawing period, while the contribution difference of the water from the 30 cm, 60 cm, and 90 cm layers is small (ranging from 32 %-33 %, 24 %-26 %, and 32 %-35 %, respectively) in the thawed period. (3) Meanwhile, the soil water from different slope positions contribute differently to SSF, and the SSF from deep soil layer is transit in prolong paths and depths. It is caused by the outof-sync water transit process in the hillslope. With continuing climate warming, we propose that the differences in the water sources of SSF across soil layers may decrease, while the differences in the transit processes of SSF across soil layers may increase.

Despite the extensive research conducted on plant-soil-water interactions, the understanding of the role of plant water sources in different plant successional stages remains limited. In this study, we employed a combination of water isotopes (delta 2H and delta 18O) and leaf delta 13C to investigate water use patterns and leaf water use efficiency (WUE) during the growing season (May to September 2021) in Hailuogou glacier forefronts in China. Our findings revealed that surface soil water and soil nutrient gradually increased during primary succession. Dominant plant species exhibited a preference for upper soil water uptake during the peak leaf out period (June to August), while they relied more on lower soil water sources during the post-leaf out period (May) or senescence (September to October). Furthermore, plants in late successional stages showed higher rates of water uptake from uppermost soil layers. Notably, there was a significant positive correlation between the percentage of water uptake by plants and available soil water content in middle and late stages. Additionally, our results indicated a gradual decrease in WUE with progression through succession, with shallow soil moisture utilization negatively impacting overall WUE across all succession stages. Path analysis further highlighted that surface soil moisture (0- 20 cm) and middle layer nutrient availability (20- 50 cm) played crucial roles in determining WUE. Overall, this researchemphasizes the critical influence of water source selection on plant succession dynamics while elucidating un- derlying mechanisms linking succession with plant water consumption.

Clumped isotope paleothermometry using pedogenic carbonates is a powerful tool for investigating past climate changes. However, location-specific seasonal patterns of precipitation and soil moisture cause systematic biases in the temperatures they record, hampering comparison of data across large areas or differing climate states. To account for biases, more systematic studies of carbonate forming processes are needed. We measured modern soil temperatures within the San Luis Valley of the Rocky Mountains and compared them to paleotemperatures determined using clumped isotopes. For Holocene-age samples, clumped isotope results indicate carbonate accumulated at a range of temperatures with site averages similar to the annual mean. Paleotemperatures for late Pleistocene-age samples (ranging 19-72 ka in age) yielded site averages only 2 degrees C lower, despite evidence that annual temperatures during glacial periods were 5-9 degrees C colder than modern. We use a 1D numerical model of soil physics to support the idea that differences in hydrologic conditions in interglacial versus glacial periods promote differences in the seasonal distribution of soil carbonate accumulation. Model simulations of modern (Holocene) conditions suggest that soil drying under low soil pCO2 favors year-round carbonate accumulation in this region but peaking during post-monsoon soil drying. During a glacial simulation with lowered temperatures and added snowpack, more carbonate accumulation shifted to the summer season. These experiments show that changing hydrologic regimes could change the seasonality of carbonate accumulation, which in this study blunts the use of clumped isotopes to quantify glacial-interglacial temperature changes. This highlights the importance of understanding seasonal biases of climate proxies for accurate paleoenvironmental reconstruction. Reconstructing the amount of temperature change associated with past climate changes for individual regions is important for understanding their climate vulnerability. Carbonate minerals developed naturally in desert soils record past temperatures in the numbers of their rare isotopes, called clumped isotopes. However, the temperature recorded in soil minerals is linked to the time of year they form, which varies greatly from winter to summer, so understanding the timing is key to interpreting past climate. We measured underground temperatures in the southern Rocky Mountains, compared them to mineral temperatures from young soils, and found that they record mean annual soil temperature. In contrast, temperatures recorded by soil minerals during the last ice age were only 2 degrees C colder than young soil temperatures, despite evidence that ice age air temperatures were 5-9 degrees C colder. We performed numerical modeling to predict the seasonal timing of soil carbonate accumulation under interglacial and glacial climate states and found that carbonate likely forms year-round during interglacial states but forms during the summertime under glacial conditions due to delayed melting of snow under colder temperatures. This lowers the difference between glacial and interglacial temperatures, which is important to account for when quantifying past climate change for the region. Clumped isotope temperatures for soil carbonate are biased to different seasons in different regions and time periods depending on climate In the San Luis Valley, USA, monitoring, modeling, and isotope results suggest carbonate accumulation throughout the year in the Holocene In the glacial late Pleistocene, clumped isotopes and soil modeling indicate longer snow cover shifted carbonate accumulation to the summer

Lumbini isa world heritage site located in the southern plains region of Nepal, and is regarded as a potential site for evaluating transboundary air pollution due to its proximity to the border with India. In this study, 82 aerosol samples were collected between April 2013 and July 2014 to investigate the levels of particulate-bound mercury (PBM) and the corresponding seasonality, sources, and influencing factors. The PBM concentration in total suspended particulate (TSP) matter ranged from 6.8 pg m-3 to 351.7 pg m-3 (mean of 99.7 +/- 92.6 pg m-3), which exceeded the ranges reported for remote and rural sites worldwide. The Hg content (PBM/TSP) ranged from 68.2 ng g-1 to 1744.8 ng g-1 (mean of 446.9 +/- 312.7 ng g-1), indicating anthropogenic enrichment. The PBM levels were higher in the dry season (i.e., winter and the pre-monsoon period) than in the wet season (i.e., the monsoon period). In addition, the d202Hg signature indicated that waste/coal burning and traffic were the major sources of Hg in Lumbini during the pre-monsoon period. Meanwhile, precipitation occurring during photochemical processes in the atmosphere may have been responsible for the observed D199Hg values in the aerosol samples obtained during the monsoon period. The PBM concentration was influenced mostly by the resuspension of polluted dust during dry periods and crop residue burning during the post-monsoon period. The estimated PBM deposition flux at Lumbini was 15.7 lg m-2 yr-1. This study provides a reference dataset of atmospheric PBM over a year, which can be useful for understanding the geochemical cycling of Hg in this region of limited data. (c) 2021 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

In arid regions, the stable hydrogen and oxygen isotopic composition in raindrops is often modified by sub-cloud secondary evaporation when they descend from cloud base to ground through the unsaturated air. As a result of kinetic fractionation, the slope and intercept of the delta H-2-delta O-18 correlation equation decrease. The variation of deuterium excess from cloud base to the ground is often used to quantitatively evaluate the influence of secondary evaporation effect on isotopes in precipitation. Based on the event-based precipitation samples collected at Urumqi Glacier No. 1, eastern Tianshan during four-year observation, the existence and impact of secondary evaporation effects were analyzed by the methods of isotope-evaporation model. Under high air temperature, small raindrop diameter and precipitation amount, and low relative humidity conditions, the remaining rate of raindrops is small and the change of deuterium excess is large relatively, and the slope and intercept of delta H-2-delta O-18 correlation equation are much lower than those of Global Meteoric Water Line, which mean that the influence secondary evaporation on precipitation enhanced. While on the conditions of low air temperature, high relative humidity, heavy rainfall, and large raindrop diameter, the change of deuterium excess is small relatively and the remaining rate of raindrops is large, and the slope and intercept of delta H-2-delta O-18 correlation equation increase, the secondary evaporation is weakened. The isotope-evaporation model described a good linear correlation between changes of deuterium excess and evaporation proportion with the slope of 0.90%/%, which indicated that an increase of 1% in evaporation may result in a decrease of deuterium excess about 0.90%.

Increased permafrost temperatures have been reported in the circum-Arctic, and widespread degradation of permafrost peatlands has occurred in recent decades. The timing of permafrost aggradation in these ecosystems could have implications for the soil carbon lability upon thawing, and an increased understanding of the permafrost history is therefore needed to better project future carbon feedbacks. In this study, we have conducted high-resolution plant macrofossil and geochemical analyses and accelerator mass spectrometry radiocarbon dating of active layer cores from four permafrost peatlands in northern Sweden and Norway. In the mid-Holocene, all four sites were wet fens, and at least three of them remained permafrost-free until a shift in vegetation toward bog species was recorded around 800 to 400 cal. BP, suggesting permafrost aggradation during the Little Ice Age. At one site, Karlebotn, the plant macrofossil record also indicated a period of dry bog conditions between 3300 and 2900 cal. BP, followed by a rapid shift toward species growing in waterlogged fens or open pools, suggesting that permafrost possibly was present around 3000 cal. BP but thawed and was replaced by thermokarst.

Permafrost degradation is altering biogeochemical processes throughout the Arctic. Thaw-induced changes in organic matter transformations and mineral weathering reactions are impacting fluxes of inorganic carbon (IC) and alkalinity (ALK) in Arctic rivers. However, the net impact of these changing fluxes on the concentration of carbon dioxide in the atmosphere (pCO(2)) is relatively unconstrained. Resolving this uncertainty is important as thaw-driven changes in the fluxes of IC and ALK could produce feedbacks in the global carbon cycle. Enhanced production of sulfuric acid through sulfide oxidation is particularly poorly quantified despite its potential to remove ALK from the ocean-atmosphere system and increase pCO(2), producing a positive feedback leading to more warming and permafrost degradation. In this work, we quantified weathering in the Koyukuk River, a major tributary of the Yukon River draining discontinuous permafrost in central Alaska, based on water and sediment samples collected near the village of Huslia in summer 2018. Using measurements of major ion abundances and sulfate (SO42-) sulfur (S-34/S-32) and oxygen (O-18/O-16) isotope ratios, we employed the MEANDIR inversion model to quantify the relative importance of a suite of weathering processes and their net impact on pCO(2). Calculations found that approximately 80% of SO42- in mainstem samples derived from sulfide oxidation with the remainder from evaporite dissolution. Moreover, S-34/S-32 ratios, C-13/C-12 ratios of dissolved IC, and sulfur X-ray absorption spectra of mainstem, secondary channel, and floodplain pore fluid and sediment samples revealed modest degrees of microbial sulfate reduction within the floodplain. Weathering fluxes of ALK and IC result in lower values of pCO(2) over timescales shorter than carbonate compensation (similar to 10(4) yr) and, for mainstem samples, higher values of pCO(2) over timescales longer than carbonate compensation but shorter than the residence time of marine SO42- (similar to 10(7) yr). Furthermore, the absolute concentrations of SO42- and Mg2+ in the Koyukuk River, as well as the ratios of SO42- and Mg2+ to other dissolved weathering products, have increased over the past 50 years. Through analogy to similar trends in the Yukon River, we interpret these changes as reflecting enhanced sulfide oxidation due to ongoing exposure of previously frozen sediment and changes in the contributions of shallow and deep flow paths to the active channel. Overall, these findings confirm that sulfide oxidation is a substantial outcome of permafrost degradation and that the sulfur cycle responds to permafrost thaw with a timescale-dependent feedback on warming.

This study reports day-night and seasonal variations of aqueous brown carbon (BrCaq) and constituent humic-like substances (HULIS) (neutral and acidic HULIS: HULIS-n and HULIS-a) from the eastern Indo-Gangetic Plain (IGP) of India during 2019-2020. This is followed by the application of the receptor model positive matrix factorization (PMF) for optical source apportionment of BrCaq and the use of stable isotopic ratios (813C and 815N) to understand atmospheric processing. Nighttime BrCaq absorption and mass absorption efficiencies (MAE) were enhanced by 40-150 % and 50-190 %, respectively, compared to the daytime across seasons, possibly as a combined effect from daytime photobleaching, dark-phase secondary formation, and increased nighttime emissions. MAE250 nm/MAE365 nm (i.e., E2/E3) ratios and Angstrom Exponents revealed that BrCaq and HULIS-n were relatively more aromatic and conjugated during the biomass burning-dominated periods while BrCaq and HULIS-a were comprised mostly of nonconjugated aliphatic structures from secondary processes during the photochemistry-dominated summer. The relative radiative forcing of BrCaq with respect to elemental carbon (EC) was 10-12 % in the post-monsoon and winter in the 300-400 nm range. Optical source apportionment using PMF revealed that BrCaq absorption at 300, 365 and 420 nm wavelengths in the eastern IGP is mostly from biomass burning (60-75 %), followed by combined marine and fossil fuel-derived sources (24-31 %), and secondary processes (up to 10 %). Source-specific MAEs at 365 nm were estimated to be the highest for the combined marine and fossil fuel source (1.34 m2 g-1) followed by biomass burning (0.78 m2 g-1) and secondary processing (0.13 m2 g-1). Finally, 813C and 815N isotopic analysis confirmed the importance of summertime photochemistry and wintertime NO3--dominated chemistry in constraining BrC characteristics. Overall, the quantitative apportionment of BrCaq sources and processing reported here can be expected to lead to targeted source-specific measurements and a better understanding of BrC climate forcing in the future.

A total of 256 water samples were collected from the river, precipitation, and permafrost active layer in a typical small alpine catchment during the ablation periods in 2020 and 2021. The results indicated that every water body was alkaline, and the TDS and EC concentrations were in the following order: precipitation Ca2+ & AP; Mg2+ and Na+ + K+ > Mg2+ > Ca2+, respectively; the anion concentration showed the order of SO42 � > Cl- > NO3 . The results revealed that permafrost and river water had similar geochemical compositions. Similar & delta;2H and & delta;18O values were also observed between river and permafrost water. Additionally, the water chemistry of rivers and permafrost revealed that the chemical weathering of carbonate and silicate rocks is an important source of riverine solutes; however, silicate weathering played a more crucial role. Both hydrochemistry and stable isotopes collectively indicated that there was a close hydraulic connectivity between the water content in river and permafrost active layer in the small alpine catchment. Based on the end-member mixing analysis model, the water in permafrost active layer and precipitation accounted for 62% and 38% of the runoff, respectively, indicating that it was dominated by permafrost during the ablation period. The warming and hu-midification of climate tend to facilitate permafrost degradation. Thus, studying the transformation of different water bodies in alpine regions is imperative to provide water resource security and sustainable development in alpine regions.

The Arctic soil communities play a vital role in stabilizing and decomposing soil carbon, which affects the global carbon cycling. Studying the food web structure is critical for understanding biotic interactions and the functioning of these ecosystems. Here, we studied the trophic relationships of (microscopic) soil biota of two different Arctic spots in Ny-angstrom lesund, Svalbard, within a natural soil moisture gradient by combining DNA analysis with stable isotopes as trophic tracers. The results of our study suggested that the soil moisture strongly influenced the diversity of soil biota, with the wetter soil, having a higher organic matter content, hosting a more diverse community. Based on a Bayesian mixing model, the community of wet soil formed a more complex food web, in which bacterivorous and detritivorous pathways were important in supplying carbon and energy to the upper trophic levels. In contrast, the drier soil showed a less diverse community, lower trophic complexity, with the green food web (via unicellular green algae and gatherer organisms) playing a more important role in channelling energy to higher trophic levels. These findings are important to better understand the soil communities inhabiting the Arctic, and for predicting how the ecosystem will respond to the forthcoming changes in precipitation regimes. Wetter soils, with a higher organic matter content, host more diverse soil biota and support more complex food webs, in which bacterivorous and detritivorous pathways are relevant in supplying energy.