Fission Track Dating of Authigenic Quartz in Red Weathering Crusts of Carbonate Rocks in Guizhou Province

The Cenozoic evolution history of Guizhou Province, which is located on the southeastern flank of the Qinghai-Tibet Plateau, is unclear because of the lack of sedimentation records. The red weathering crusts widespread on the Yunnan-Guizhou Plateau may bear critical information about their evolution history. This work firstly determined the ages of four red weathering crusts in eastern, central and northern Guizhou. The material used in fission track dating is well-crystallized quartz occurring in many in-situ weathering crusts of carbonate rocks. The results showed that the fission track ages of quartz vary over a wide range from 1 to 25 Ma in the four profiles, significantly younger than the ages of the Triassic and Cambrian parent rocks. In combination with the evolution history of the regional geology during the period from 25 to 1 Ma, the ages of quartz can exclude the possibility that the origin of quartz has nothing to do with primary clastic minerals in parent rocks, authigenesis during diagenesis and hydrothermal precipitation or roplacement by volcanic activities. It is deduced that the well-crystallized quartz was precipitated from Si-rich weathering fluids during the weathering process of carbonate rocks. The recorded ages of quartz from the four profiles are consistent with the episodes of the planation surfaces on the Qinghai-Tibet Plateau, the forming stages of red soil in the tropics of South China, the tectonically stable periods in Guizhou, and the ages of weathering in other parts of the world during the Cenozoic era. That is to say, the ages of authigenic quartz dated by the fission track method are well feasible and credible.

Impact of animal manure addition on the weathering of agricultural lime in acidic soils＂ The agent of carbonate weathering

Fertilization and aglime(agricultural lime) application, as important agricultural activities in acid soil, exert an influence on the fluxes of carbon both between and within ecosystems. Animal manure added to soil can elevate the soil CO_2 and release organic acid due to microbial decomposition of the high organic matter content of animal manure. Additionally, the elevated CO_2 can accelerate carbonate weathering in alkaline soil, such as lime soil. However, in acidic soil, it is unclear whether the chemical weathering of additive aglime can be quickened by the elevated CO_2 due to animal manure addition. Thus, to ascertain the impact of animal manure addition on aglime weathering in acidic soil and to understand the weathering agent of aglime or underlying carbonate in the acidic soil profile, we established two contrasting profiles(control profile and manurial profile) in a cabbage-corn or capsicum-corn rotation in a field experiment site located in the Hua Xi district of Guiyang, China, and buried carbonate rock tablets at different depths of soil profiles to calculate the dissolution rate of carbonate rock by monitoring the weights of the tablets. The results indicated that soil CO_2 increased due to animal manure addition, but the rate of dissolution of the carbonate rock tablets was reduced, which was attributed to the increase in the p H in acidic soil after animal manure addition because the relationship between the dissolution rate of carbonate rock and soil p H indicated that the weathering rate of carbonate rock was controlled by pH and not by CO_2 in acidic soil. Thus, the contribution of H+ ions(mainly exchangeable acid) in acid soil as a weathering agent to the weathering of underlying carbonate(and/or aglime) may lead to the overestimation of the CO_2 consumption through chemical weathering at the regional/global scale using hydro-chemical methods.

The Response of Carbonate System to Watershed Urbanization Process in a Semi-Arid River

Different from rivers in humid areas,the variability of riverine CO_(2) system in arid areas is heavily impacted by anthropogenic disturbance with the increasing urbanization and water withdrawals.In this study,the water chemistry and the controls of carbonate system in an urbanized river(the Fenhe River)on the semi-arid Loess Plateau were analyzed.The water chemistry of the river water showed that the high dissolved inorganic carbon(DIC)concentration(about 37 mg L^(-1))in the upstream with a karst land type was mainly sourced from carbonate weathering involved by H_(2)CO_(3) and H_(2)SO_(4),resulting in an oversaturated partial pressure of CO_(2)(pCO_(2))(about 800μatm).In comparison,damming resulted in the widespread appearance of non-free flowing river segments,and aquatic photosynthesis dominated the DIC and pCO_(2) spatiality demonstrated by the enriched stable isotope of DIC(δ^(13)CDIC).Especially in the mid-downstream flowing through major cities in warm and low-runoff August,some river segments even acted as an atmospheric CO_(2) sink.The noteworthy is wastewater input leading to a sudden increase in DIC(>55 mg L^(-1))and pCO_(2)(>4500μatm)in the downstream of Taiyuan City,and in cold November the increased DIC even extended to the outlet of the river.Our results highlight the effects of aquatic production induced by damming and urban sewage input on riverine CO_(2) system in semi-arid areas,and reducing sewage discharge may mitigate CO_(2) emission from the rivers.

Resolving controversies surrounding carbon sinks from carbonate weathering

The importance of carbonate weathering carbon sinks(CCSs)is almost equal to that of vegetation photosynthesis in the global carbon cycle.However,CCSs have become controversial in formulating carbon neutral policies to deal with global climate problems in various countries,since the carbonate dissolution is reversible.In order to address these controversies,we reviewed recent advances in understanding CCSs and examined the outstanding controversies surrounding them.We have analyzed the five controversies,revealing the existence of CCSs,quantifying their magnitude,clarifying their spatiotemporal pattern,and documenting how they have increased and how they evolved under the background of global change.By addressing these five controversies,we help to bring clarity to the role of CCSs in the carbon cycle of global terrestrial ecosystems.

Sulfate sources constrained by sulfur and oxygen isotopic compositions in the upper reaches of the Xijiang River,China

While it is critical to accurately understand the sources and transformation of sulfate based on time-series analysis, there are limited studies on temporal variation of sulfate in rivers and on rock weathering by sulfuric acids.We conducted a monthly sampling campaign in the Beipan, Nanpan, and Hongshui Rivers over the course of one hydrological year. This study examined seasonal variations in riverine sulfate impacted by the monsoon climate in the upper reaches of the Xijiang River basin. In general, the SO_4^(2-) contents in these rivers dropped from relatively high levels to low values during the high-flow season, in response to increasing discharge. The sulfate was generally enriched in heavy isotopes during the low-flow season compared to the high-flow season. The calculated results indicate that the riverine sulfate was mainly derived from sulfide oxidation, but that evaporite dissolution could be an important source during the low-flow season, based on isotopic evidence. Mine drainage is likely an important source of sulfate to these rivers during the high-flow season due to contributions from fast surface flow, which responds to frequent heavy rain in monsoonal climate regions. Arelatively high proportion of HCO_3^- was found to be derived from rock weathering by sulfuric acid during the high-flow season when compared to that observed during the low-flow season. The results suggest that approximately one quarter of the HCO_3^- in the Hongshui River originated from carbonate weathering by sulfuric acid derived from the oxidation of sulfide. Such information on the specific dual isotopic characteristics of riverine sulfate throughout a hydrological year can provide unique evidence for understanding the temporal variability of sulfate concentrations and weathering processes in rivers.

Determining nitrogen fate by hydrological pathways and impact on carbonate weathering in an agricultural karst watershed

Identifying the nitrogen(N)fate is complicated and a great challenge in karst watersheds because of the co-existence of natural pools and anthropogenic sources.The objective of the study was to use stable isotopic composition of dual-isotope(δ^(15)NNitrate and δ^(18)O_(Nitrate))and LOADEST model approaches to trace N sources,pathways in karst watershed.The study was conducted in the Houzhai watershed,which is a typical agricultural karst watershed from July 2016 to August 2018,to reveal the N fate and the coupled carbon(C)-N processes occurring in the riverine-watershed with agricultural activities.We found that the wet deposition of total nitrogen(TN)flux was 33.50 kg hm^(-2)·a^(-1)and dissolved nitrogen(DN)flux was 21.66 kg hm^(-2)·a^(-1).The DN runoff loss was 2.10×10^(5)kg·a^(-1)and the loss of DN during the wet season accounted for 95.4%over a year.In the wet season,NO_(3)^(-)-N daily efflux was 977.62±516.66 kg ha^(-1)·day^(-1)and 248.77±57.83 kg ha^(-1)·day^(-1)in the dry season.The NH_(4)^(+)-N efflux was 29.17±10.50 kg ha^(-1)·day^(-1)and 4.42±3.07 kg ha^(-1)·day^(-1)in the wet and dry seasons,respectively.The main form output load of N was NO_(3)^(-)-N which was more than 30 times as much as NH_(4)^(+)-N output loss.The NO_(3)^(-)N caused by rainfall contributed11.82%-53.61%to the export load.Nitrate from soil contributed over 94%of the N to Houzhai river caused by N leaching.In addition,manure and farmland soil were the main sources of groundwater in the Houzhai watersheds,the contribution rates were 25.9%and 22.5%.The chemical N fertilizers affected carbonate weathering strongly,and the HCO_(3)^(-) flux caused by nitrifi-cation due to N fertilizers application in soil accounted for 23.5%of the entire watershed.This study suggested that carbonate weathering may be influenced by nitrogen nitrification in the karst watershed.

Significance of the carbon sink produced by H_2O–carbonate–CO_2–aquatic phototroph interaction on land

One of the most important questions in the science of global change is how to balance the atmospheric CO2 budget. There is a large terrestrial missing carbon sink amounting to about one billion tonnes of carbon per annum. The locations, magnitudes, variations, and mechanisms responsible for this terrestrial missing carbon sink are uncertain and the focus of much continuing debate. Although the positive feedback between global change and silicate chemical weathering is used in geochemical models of atmospheric CO2, this feedback is believed to operate over a long timescale and is therefore generally left out of the current discussion of human impact upon the carbon budget. Here, we show, by synthesizing recent findings in rock weathering research and studies into biological carbon pump effects in surface aquatic ecosystems, that the carbon sink produced by carbonate weathering based on the H2O- carbonate-CO2-aquatic phototroph interaction on land not only totals half a billion tonnes per annum, but also displays a significant increasing trend under the influence of global warming and land use change; thus, it needs to be included in the global carbon budget.

Global and regional controls on carbon-sulfur isotope cycling during SPICE event in south China

The positive S-isotopic excursion of carbonate-associated sulfate(δ34S_(CAS))is generally in phase with the Steptoean positive carbon isotope excursion(SPICE),which may reflect widespread,global,transient increases in the burial of organic carbon and pyrite sulfate in sediments deposited under large-scale anoxic and sulphidic conditions.However,carbon-sulfur isotope cycling of the global SPICE event,which may be controlled by global and regional events,is still poorly understood,especially in south China.Therefore,theδ13CPDB,δ18OPDBδ34S_(CAS),total carbon(TC),total organic carbon(TOC)and total sulfate(TS)of Cambrian carbonate of Waergang section of Hunan Province were analyzed to unravel global and regional controls on carbon-sulfur cycling during SPICE event in south China.Theδ34S_(CAS)values in the onset and rising limb are not obviously higher than that in the preceding SPICE,meanwhile sulfate(δ34S_(CAS))isotope values increase slightly with increasingδ13CPDB in rising limb and near peak of SPICE(130–160 m).The sulfate(δ34S_(CAS))isotope values gradually decrease from 48.6‰to 18‰in the peak part of SPICE and even increase from 18%to 38.5%in the descending limb of SPICE.The abnormal asynchronous C−S isotope excursion during SPICE event in the south China was mainly controlled by the global events including sea level change and marine sulfate reduction,and it was also influenced by regional events such as enhanced siliciclastic provenance input(sulfate),weathering of a carbonate platform and sedimentary environment.Sedimentary environment and lithology are not the main reason for global SPICE event but influence theδ13CPDB excursion-amplitude of SPICE.Sea level eustacy and carbonate platform weathering probably made a major contribution to theδ13CPDB excursion during the SPICE,in particularly,near peak of SPICE.Besides,the trilobite extinctions,anoxia,organic-matter burial and siliciclastic provenance input also play an important role in the onset,early and late stage of SPICE event.