| Geogenic high arsenic(As)groundwater occurs mostly in flat,low-lying river floodplains,deltas,and fluvial-lacustrine plains of Late Pleistocene-Holocene sedimentary aquifers under reducing geochemical conditions.Iron minerals and organic matter in sediments are the key factors controlling As participating in sediments and groundwater,which are closely related to paleoclimate changes.Therefore,identifying the types and abundance of iron minerals and organic matter driven by paleoclimate changes is a prerequisite and basis for studying As enrichment in aquifer media.Considering these knowledge gaps,the study takes the Datong Basin as the study area,reconstructs the framework of depositional evolution since the Pleistocene,and analyzes the main mechanisms driving the evolution of depositional environments.On this basis,we discuss the effects of paleoclimate changes on the type and abundance of iron minerals and organic matter,and finally,reveal the enrichment mechanism of As in the aquifer media driven by paleoclimate changes.The major advances of this study are as follows:(1)The high-As aquifers in the Datong Basin are mainly distributed in the shallow and middle aquifers,which are spatially heterogeneous.Both iron mineral reduction and sulfate reduction jointly control As enrichment in groundwater,with organic matter as the main driver.Dissolved organic matter in groundwater includes labile dissolved organic matter,oxidized quinones-like compounds,and reduced quinones-like compounds.Dissolved organic matter controls As release and transport in two ways:(a)the degradation of labile dissolved organic matter provides electrons;and(b)electron shuttling between oxidized and reduced quinones-like compounds accelerates electron transfer.(2)Paleoclimate changes are the main driving force for the evolution of the paleolake environment in the Datong Basin since the Pleistocene.Sedimentary records from the SZ core were used to reconstruct paleoclimate changes.The sedimentary phases are alternation of lakeside and shallow lake facies,shallow lake facies,Alternation of lakeside and shallow lake facies,shallow lake facies,alternation of lakeside and river facies,and river facies,representing the paleolake expansion,shrinkage,and extinction.Our results showed that paleoclimate changes and neotectonic movement control the environmental evolution of paleolake.The climate proxies of SZ core sediments show alternating changes in warm-humid and cold-dry climates.The shrinkage and expansion of paleolake correspond to cold-dry and warm-humid climates.The uplift of the Tibetan Plateau causes the prevalence of cold-arid climates.In addition,periods of active volcanic activity correspond to the transition periods from paleolake expansion to shrinkage,indicating that Datong volcanism is one of the important driving forces for the evolution of paleolake.As a result,paleoclimate changes and volcanism in Datong drive the evolution of depositional environment in the Datong Basin since the Pleistocene.However,paleoclimate changes are the most fundamental driving factor of the evolution of the depositional environment.(3)Paleoclimate changes cause changes in iron minerals and organic matter composition in the Datong Basin.Most of sediments in the Datong Basin originate from the siliceous sedimentary rocks,and weathering controls the spatial distribution of As and Fe in sediments.Clay mineral bound Fe and crystalline Fe are the main Fe species.With the paleoclimate changes,the account of clay mineral bound Fe and strongly crystalline Fe changes in opposite trends,indicating that they are highly sensitive to paleoclimate changes.Sedimentary organic matter in the Datong Basin is governed by the source and degradation.Sedimentary organic matter is dominated by terrestrial C3 plant sources.Sedimentary organic matter reactivity is sensitive to paleoclimate changes,and the contents of hydrogen-rich and hydrogen-poor organic compounds show opposite trends with paleoclimate changes.Under cold-arid climates,strong physical erosion increases the supply of terrestrial derived debris,which lead to the increasing Fe and As accumulation in sediments.At this time,favorable clay minerals during cold-arid climates(e.g.,illite,chlorite,and illite-smectite mixed layer)enhance the accumulation of Fe-rich clay minerals and then As accumulation.Hydrogen-rich organic compounds and aliphatic and saturated compounds with lower nominal oxidation state of carbon(NOSC)are significantly enhanced during cold-dry climates.During warm-humid climates,stronger chemical weathering favors crystalline Fe accumulation.Hydrogen-poor organic compounds as well as polycyclic aromatics and polyphenols with lower NOSC are enriched during warm-humid climates.(4)Sedimentary As accumulation is controlled by the Fe minerals and organic matter composition.Clay mineral bound Fe and crystalline Fe are the main carriers of As in sediments.The degradation of sedimentary organic matter is related to the reductive dissolution of terminal electron donors,which further affects the behavior of As.The energy gained from the decomposition of aliphatic and saturated compounds with low NOSC values approaches the energy required to sustain dissimilatory Fe reduction,leading to sedimentary As release into groundwater.Thermodynamically favorable organic compounds(higher NOSC values)are preferentially degraded by microorganisms to provide sufficient energy to sustain sulfate reduction,favoring sedimentary As sequestration.(5)As contents of sediments recorded in the Datong Basin have a good response to paleoclimate changes.Under cold-dry climates,clay mineral bound Fe dominates.High NOSC organic compounds are degraded to drive sulfate reduction.Under warm-humid climates,strongly crystalline Fe dominates.Low NOSC organic compounds are preferentially degraded and converted to high NOSC organic compounds,which drives the Fe mineral reduction.Thus,cold-dry climates favor As enrichment in sediment,and warm-humid climates promote As release from sediments,which leads to As enrichment in groundwater.In summary,sedimentary As enrichment in the Datong Basin is controlled by paleoclimate changes.Cold-arid climates are beneficial to As accumulation,while warmhumid climates are unfavorable to As accumulation.The response of the Fe-C-S system to paleoclimate changes further influences As enrichment in aquifer media.The innovations of this thesis are:(1)to reveal how paleoclimate changes drive changes in iron mineral phases,organic matter composition,and sulfur species;(2)systematically investigate the effects of iron mineral phases,organic matter composition,and sulfur species on As enrichment in sediments.This study develops a conceptual model of how the response of sedimentary iron mineral,sulfur species,and organic matter composition driven by paleoclimate changes influences As enrichment.This model expands the theory of the genesis of geogenic contaminated groundwater and provides a theoretical basis for predicting the effects of future climate change on As contamination. |
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