Zhonghe Pang

The deep terrestrial subsurface is the largest reservoir of Earth’s freshwater resources as well as the largest habitat for prokaryotic life. However, the deep-subsurface microbiome, especially its spatial distribution across countries/continents, is still poorly understood. In this study, we compiled and compared 30 16S rRNA gene amplicon libraries from three deep fractured aquifers in different parts of the world (depth range of tens of meters to 2.4 km below surface) to understand the spatial distribution and functions of deep-subsurface microbial community, and to test for the presence of core taxa. The results revealed spatially heterogenous microbial community composition at both the local and the global scales, even at the phylum level. Environmental filtering was identified as an important driver of the microbial community structure of deep groundwaters. Despite the spatial heterogeneity, the three aquifers share a core microbiome at the genus level. Only one family, Comamonadaceae, was present in all the 30 samples analyzed. Several other families were also prevalent, including Hydrogenophilaceae, Omnitrophaceae, BSV26 (Candidatus Kryptonia), and an unclassified Thermodesulfovibrionia. FAPROTAX functional prediction indicated that chemoheterotrophic functions predominate, and the core microbial genera, together with the dominant genera, collectively govern the functional characteristics. Taken together, our findings provide new insights into the spatial heterogeneity and functional potential of deep-subsurface ecosystems across the globe.

The understanding of precipitation δ¹⁸O variation at the glacial scale has been controversial in paleoclimatology. Groundwater δ¹⁸O might help provide clarity as they are a widely distributed record of annual weighted mean precipitated δ¹⁸O. Here, groundwater δ¹⁸O during the last glacial maximum (LGM) and late Holocene (LH) are first illustrated throughout China based on 158 archived δ¹⁸O and ¹⁴C measurements. Between LGM and LH, the magnitude of changes in groundwater δ¹⁸O is higher in North China (NC > 2‰) than South China (SC < 0.5‰). This spatial inconsistency is similar to the seasonal precipitation δ¹⁸O variations in China caused by changes in atmospheric circulation patterns. Likewise, the difference of groundwater δ¹⁸O between LGM and LH in NC and SC is attributed to changes in the East Asian summer monsoon (EASM): during the LGM, the EASM was weaker in significantly affecting NC, whereas during the LH, the EASM was stronger and moved northward into NC. This illustrates that although the monsoon strengthens, the higher δ¹⁸O values in groundwater in NC are still found during the LH than that during the LGM. Furthermore, this spatial difference between NC and SC highlights the importance of moisture sources in affecting precipitation δ¹⁸O values, implying that the explanation for paleoclimate reconstruction based on stable isotope effects might need careful examination.

Dual isotopes of sulfate (δ³⁴SSO₄ and δ¹⁸OSO₄), along with isotopes in water and trace elements of geothermal waters, are systematically investigated to quantitatively elucidate sulfate sources and oxygen and sulfur isotopic behaviors during deep groundwater circulation and to constrain reservoir temperatures in the Jimo nonvolcanic geothermal system on the eastern coast of China. The results show that δ³⁴SSO₄ and δ¹⁸OSO₄ values in geothermal waters ranged from −21.0 to 5.7‰ and from 1.1 to 8.8‰, respectively. An increase in SO₄ concentrations (140–796 mg/L) with a systematic decrease in δ³⁴SSO₄ and δ¹⁸OSO₄ values was observed along the flow path from the central to eastern and western parts. The sulfate in the Middle Group was predominantly from atmospheric deposition, with sulfide oxidation contributions of 4 in the Eastern Group is derived from pyrite oxidation. In the Western Group, the oxidation of multiple metal sulfides contributed 43–66% of SO₄. Sulfate oxidation and mixing of shallow groundwater caused reservoir temperatures to be underestimated by 9 ± 6–14 ± 16% using silica and K-Mg geothermometers but overestimated by up to 52–62% using sulfate–water oxygen isotope geothermometers. The estimated average target reservoir temperature was 144 ± 8 °C, with geothermal waters circulating to depths of 3.6–4.6 km. This study offers new insights into the significant impact of sulfate-related processes on geothermometric estimates, a factor often overlooked when using aqueous geothermometers. It also provides valuable guidance for accurately estimating target geothermal reservoir temperatures and advancing exploration in nonvolcanic geothermal systems.

The rural population in the Dry Zone of Sri Lanka is largely affected by Chronic Kidney Disease of Unknown etiology (CKDu). According to the multidisciplinary research carried out so far, quality of groundwater is considered one of the possible causative factors for CKDu. Therefore, assessment of the quality of groundwater being used for drinking and its evolution mechanism is the key to identifying the linkage between CKDu and drinking water. This study aimed to perform a detailed investigation on groundwater sources using isotopic, chemical, and hydrogeological methods in the CKDu-endemic (site A) and the control area (sedimentary formation—site B) in the Malwathu Oya basin and the control areas in the Malala Oya basin (site C) selected for a systematic comparison. Our investigation shows that elevated levels of TDS, magnesium, and fluoride in the shallow groundwater affected by climatic, geochemical, and hydrogeological processes may contribute to the CKDu in the Dry Zone of Sri Lanka. All the groundwater samples analysed have exceeded the hardness threshold. Prominent Mg hardness proportion together with excess F⁻ in the CKDu endemic area may produce nephrotoxic MgF₂ complexes that may trigger renal damage. In contrast, NaF complexes in the CKDu control area leads to reduction of F⁻ toxicity in the human body. Elevated F⁻ and Mg²⁺ are found in site A, low F⁻ and high Mg²⁺ in site B, and either combinations of low F⁻ and low Mg²⁺, high F⁻ and low Mg²⁺, or low F⁻ with high Mg²⁺ in site C. TDS, hardness, Mg²⁺, Na⁺, and F⁻ are formed with different mechanisms in the three selected areas. The primary process that regulates the evolution of groundwater types and contents in sites A and C is the weathering of silicates. Similarly, in site A, carbonate dissolution and reverse ion exchange are quite strong. Cation exchange and evaporite dissolution are more pronounced in site C. Shallow groundwaters are evapo-concentrated, hence their quality deteriorates more significantly than the deep groundwater in the CKDu endemic area. Dilution decreases the ion content in site A while evaporite dissolution increases it in site C after the rainy season. Evaporation and seawater mixing affect the quality of groundwater in site B. It is also found that a statistically significant difference exists in the F⁻/Na⁺, F⁻/Mg²⁺, and F⁻/Ca²⁺ between the endemic and control areas. Intensive rock weathering combined with desorption has added excess F⁻ to the groundwater in site A, while cation exchange and fluorite dissolution are contributing factors in site C.

The rural population in the Dry Zone of Sri Lanka is largely affected by Chronic Kidney Disease of Unknown etiology (CKDu). According to the multidisciplinary research carried out so far, quality of groundwater was considered one of the possible causative factors for CKDu. Therefore, the assessment of water quality variations and evolution mechanism of water quality parameters of drinking water sources is important for identifying any relationships between CKDu and the drinking water quality. The current study aimed to perform a detailed hydrogeochemical investigation using isotopic and chemical methods on selected groundwater sources in the CKDu-endemic, non-endemic and control areas. TDS, hardness, Mg2+, Na+ and F- are the main parameters that cause water quality deterioration but, their derived mechanism in these three regions are completely different. The excess concentrations of F- and Mg2+ in the CKDu endemic area, low F- and high Mg2+in the CKDu non-endemic sedimentary formation, and either combinations of low F- and low Mg2+, high F- and low Mg2+ or low F- with high Mg2+ in the control area are the distinct features. The primary processes that regulate the evolution of groundwater types in CKDu endemic and control area is the weathering of silicates. Similarly, in the CKDu endemic area, carbonate dissolution and reverse ion exchange are prominent. Cation exchange and evaporite dissolution are more pronounced in the control area. Shallow groundwaters are evapo-concentrated hence it further deteriorates quality when compared to the deep groundwater with non-mixed, non-evaporated, and higher elevation recharge effect (from 100 m altitude) in the CKDu endemic area. Dilution decreases the ion content in the CKDu endemic area while evaporite dissolution further increase it in the control area after the rainy season. Evaporation, and seawater mixing affected the quality of groundwater in the non-endemic sedimentary formation. A statistically significant difference of F- ionic ratio with Na+, Mg2+, Ca2+ in the endemic and control areas. Intense rock weathering combined with the desorption added excess F- to the groundwater in the CKDu endemic area while the cation exchange and fluorite dissolution is contributing factor in CKDu control area. All the groundwater samples analyzed have exceeded the moderate hardness threshold. However, prominent Mg hardness proportion together with excess F- in the CKDu endemic area may produce nephrotoxic MgF2 complexes that may trigger renal damage. In contrast, produced NaF complexes in the CKDu control area leads to reduction of F- toxicity in the human body. Therefore, increased levels of magnesium, fluoride and TDS in the shallow groundwater, along with geochemical, hydrogeological and climatic factors, may contribute to the CKDu in the Dry Zone of Sri Lanka.

This paper aims to update our understanding of the carbon cycle in the Himalayas, the most intense collisional orogeny globally, by providing new insight into its impact on Cenozoic climate cooling through the use of isotopic variations in both organic and inorganic carbon and an isotopic mass balance model. Our results from 20 selected hot springs show that the relative contributions of dissolved carbon from the mantle, metamorphic decarbonization, aqueous dissolution, and soil organic matter are approximately 2%, 82%, 6%, and 10%, respectively. Approximately 87% ± 5% of CO₂ generated in the deep crust precipitates as calcite, while approximately 5.5% ± 1% of this carbon is converted to biomass through microbial chemosynthesis at depths less than 2 km. Our random forests approach yielded a metamorphic carbon flux from the entire Himalayan orogenic belt of approximately 2.7 ∼ 4.5 × 10¹² mol/yr. The minor CO₂ released into the atmosphere (2.5 ∼ 4.2 × 10¹¹ mol/yr) is comparable to the carbon consumption driven by Himalayan weathering. This paper provides new insights into deep carbon cycling, notably that approximately 93% of deeply sourced carbon is trapped in the shallow crust, rendering orogenic processes carbon neutral and possibly acting as one of the major triggers of long‐term climate cooling in the Cenozoic.