An Analogy Study Of Carbonate Deposits In The Qaidam Basin On The Evolution Of Martian Palaeoclimate And Habitable Environment

The main reason that human space exploration continues paying close attention to Mars is its potential for life and astrobiology.Water and carbon are important factors that affect the habitability of terrestrial planets,so the study on the distribution and evolution of water and carbon on Mars is the key to finding extraterrestrial life.Because carbonates are closely related to the cycle of water and carbon,even life activities,they have always been regarded as powerful tools to explore these basic relationships.However,for a long time,our knowledge of carbonates on the surface of Mars has been very limited.In an atmospheric environment where carbon dioxide accounts for up to96%,the orbit and the few in-situ explorations show very little carbonate distribution.A clear fact is that there is currently no large carbonate deposits as thick and continuous as on the Earth surface has been found on Mars,but there may be small areas or underground deposited carbonate units.However,the current Martian exploration has not been able to study the distribution and characteristics of small-scale carbonate outcrops on Mars globally,which restricts our understanding of ancient Martian climate evolution and carbon cycle.In comparative planetology,the Earth is the basis for the study and comparison of other planets.The establishment of an analgous environment on the Earth can help us infer the evolution history and development laws of terrestrial planets by analogy with similar geomorphology,mineral composition and depositional environment,which could also expand the understanding brought by planetary exploration data.The Qaidam Basin is located on the northern edge of the Qinghai-Tibet Plateau and has evolved to be the highest desert in the world.The cold,drought,high-UV radiation surface conditions,as well as the widespread distribution of evaporites and aeolian landforms are analogous with the surface characteristics of Mars,so it is considered one of the largest Martian analog environments.In this study,a set of carbonates in closed sedimentary basins ranging from piedmont diluvial and lakeside deposits to lake-hot spring deposits was selected as the research subjects.Carbonate nodules and carbonate ooids are not rare in the supergene environment.However,the response of the mineralogical and isotopic characteristics of carbonate nodules to paleoclimate changes under arid climate conditions is still unclear,and the formation of carbonate ooids and their role in the process of biogeochemical cycles have long been controversial.The pitted rocks were first reported in 1976 near the Viking 2 landing site in the northern plains of Mars.Although previous work documented some analogues in the Martian analog environment on Earth for these strangely shaped rocks,there is no convincing explanation obtained for their formation mechanism.By conducting detailed mineralogical,geochemical,chronological,and biogeochemical studies on the sedimentary facies such as carbonate nodules,carbonate ooids,and carbonate pitted rock,this research discussed deeply on the formation mechanism of these carbonate facies and assessed the possibility of similar paleoclimate and paleoenvironmental evolution on Mars,which could provide references for Mars exploration.The main results are as follows:(1)The formation and paleoclimate significance of carbonate nodulesIn the piedmont of Altun Mountain,western Qaidam Basin,aragonite associated with detrital minerals were first deposited through a flood event 250,000 years ago.The subsequent drought caused the sediment to crack and provided the growth nucleus for the formation of high-magnesium calcite during the flood events of the next 100,000years.The difference in element content(such as manganese),carbonate crystal morphology,and porosity are the internal factors that form the nodular structure,reflecting fluctuat geochemistry processes of the water environment.The continuous drought caused the?¹³C and?¹⁸O values to increase by 3‰and 6‰from the core to the edge of the nodules,respectively.Rayleigh distillation model suggested that 50%of the H₂O and 25%of the dissolved CO₂ had been removed during nodule growth in response to the drought.These results indicate that nodules formed in arid environments could serve as useful proxies of paleoclimate,which may provide new perspectives for studying the formation and paleoclimate significance of nodules in Gale Crater on Mars.(2)The formation of carbonate ooids and their astrobiological significanceIn this study,three carbonate ooids layers were found in the western Qaidam Basin,including one dolomite ooids layer formed 290,000 years ago and cemented by gypsum260,000 years ago;one aragonite ooids layer formed in 160,000 years ago and cemented by gypsum 110,000 years ago;one aragonite ooids layer formed in 30,000 years ago and cemented by halite 20,000 years ago.The elemental analysis and stable carbon and oxygen isotope analysis of these carbonate ooids revealed an arid salt lake sedimentary environment and did not support their biogenesis.We identified scytonemin,a sunscreen pigment from cyanobacteria in the extraction experiment of these ooids.Although the preservation of scytonemin is poor as the age of the ooids increases,effective signals can be detected for at least 290,000 years.This study shows that although ooids may be of inorganic origin,ooids cemented by evaporites have the potential to preserve organic matter in extreme environments.On the surface of Mars where evaporites deposits are widely distributed,the atmospheric carbon dioxide of ancient Mars may also be trapped in these evaporites in the form of carbonate ooids,and it is likely to preserve the key information about the evolution of ancient Martian life(if there is any).Future Mars explorations should consider carefully about these deposits on Mars.(3)The hot spring origin of carbonate pitted rock and and its astrobiological significanceThe carbonate pitted rocks found in the western Qaidam Basin in this study are very similar to the Martian pitted rocks in morphology,size and distribution.A group of authigenic dolomite,kaolinite,gypsum,and Opal-A combinations clearly indicate the existence of an ancient hot spring.As indicated by the isotopic results,these Qaidam pitted rocks are formed in the aqueous environment where hot spring water and lake water are mixed.Detailed scanning electron microscopic observations also found authigenic pyrite in these pitted rocks.Isotopic analysis showed that these pyrites were produced by sulfate-reducing bacteria under environmental conditions of approximately 25°C.By analogy,this study believes that the pitted rocks on Mars are likely to be the relics of an ancient hot spring,and that these pitted rocks can preserve biomarkers in an extremely dry and oxidized surface environment for a long time.Thus,pitted rock should be listed as the focus of Martian life detection.Ancient Mars has undoubtedly experienced an evolutionary stage where stable liquid water exists on the surface.Although the composition and thickness of the ancient atmosphere of Mars are still controversial,the carbonate on the surface of Mars are likely to exist in various microfacies.These microfacies can provide a certain amount of atmospheric carbon sequestration and can record the evolution history of paleoclimate.More importantly,if life had ever appeared on ancient Mars,these carbonate depositional environments are very likely to be places for life to be bred,so biosignatures can also be recorded in different forms.Today,when global outcrop-scale research on Mars has not yet been achieved,these research results are crucial to our understanding of Martian carbonate deposition and can provide significant scientific basis for Martian life detection and paleoclimate research.