Surficial Evolution Processes In Dajiuhu And Jemez Critical Zones And Their Responses To Variation Of Enrironmental Factors

As the carrier of human genesis,evolution,and prosperity,Earth's Critical Zone provides most resources for maintaining human activities.Multidisciplinary Earth and environmental research on Critical Zone evolution is considered as very important work in the 21^st century.Couplings between physical,chemical,and biological processes within Critical Zone enable elemental cycling and energy transfer continue.The formation and evolution of Critical Zone are controlled by climatic conditions,as different climates lead to different water availability,heat,and plant cover,which exert important influences on Earth's surface processes such as physical erosion,chemical weathering,mineral transformation,dry-wet cycle,organic carbon storage,soil respiration,and carbon cycling.It's necessary to include consideration of climatic factor when study the formation and evolution of Critical Zone,while most previous studies didn't focus this important factor.This limitation can be resulted from subjective recognition and objective research methods.A thorough understanding of correlation and interaction different components within Critical Zone,as well as their responses to climatic changes is central to Critical Zone science,which is important for an accurate prediction of Critical Zone evolution and for maintaining Earth's environment and human sustainability.In order to obtain a better understanding on the responses of Earth's surface processes?in particular chemical weathering?to variation of environmental factors in representative different climatic zones,we investigated two Critical Zones—Dajiuhu of western Hubei Province?northern subtropical montane climate?and Jemez River Basin of New Mexico,southwestern US?semiarid montane climate?.The Dajiuhu research is focused on clay mineralogic and geochemical compositions and their changes of samples from different microtopographic sites and elevations,to decipher the influences of microtopography-and elevation-induced environmental changes on a variety of Earth's surface processes.For Jemez research,we collocated a series of sensors at different soil horizons to monitor soil temperature,moisture,O₂ and CO₂partial pressures,and porewater geochemistry,in order to explore correlations and couplings among a series of soil physical?including moisture and temperature changes?,chemical?including elemental migration,primary mineral dissolution,secondary mineral precipitation,and mineral-organic matter complexation?,and biological?including soil respiration?processes,and their responses to climate changes,in addition to mineralogic and geochemical research on soil solids.Clay minerals in Dajiuhu soils are complex,being dominated by various interstratified clays together with relatively“primary”clay minerals.Evidence provided by rare-earth element?REE?patterns,immobile element ratios,clay-mineral compositions,and particle-size distributions suggests that these soils have rather uniform parent materials.Our research suggests that microtopography and elevation exert important influences on mineral transformation,soil weathering,elemental migration,and organic retention.Weathering intensity scale that is closely associated with Si content can more precisely reflect weathering condition of the studied samples compared to common indices such as CIA.Most soils at convergent sites with poor drainage contain more smectitic clays?interstratified illite/smectite and chlorite/smectite?and less vermiculitic clays?hydroxyl-interlayered vermiculite and interstratified illite/hydroxyl-interlayered vermiculite?,and exhibit weaker chemical weathering and fewer elemental losses than those from non-convergent sites.The diversity of clay types can be ascribed to the complexity and heterogeneity,in particular of p H and hydrology,in these soil environments.Given acidic soil condition and absence of discrete smectite,we suggest that smectitic clay minerals were resulted from solid state transformation under acidic leaching condition rather than dissolution and precipitation.Well drained hillslope and bulge sites were more likely to formed vermiculitic clays due to frequent dry-wet cycling formed under monsoonal climate.Soil organic carbon?SOC?content generally increases at higher elevations,which is attributable to lower temperatures and a consequent reduction of microbial remineralization,and under more reducing soil water conditions.SOC retention is closely associated with soil redox status,and more reducing environments are more favourable for SOC accumulation.Soils from the Jemez Critical Zone have experienced weak-intermediate weathering,characterized by release of Ca and Na during plagioclase decomposition.This weathering stage has not experienced K release during mica decomposition and Si release during late stage.Temporal variation in soil temperature and moisture within the three soil pits is generally similar.Variation ranges for soil p CO₂ and p O₂ are obviously different,and vertical variations in soil p CO₂ show similar trends:increase with depth,reach maximum at 30 cm and then decrease,opposite to the trend of p O₂.Rapid increase of soil moisture would increase soil CO₂ diffusivity,resulting in rapid increase of soil CO₂ efflux.The responses of soil respiration to variation of environmental factors are different at different timescales.Plotting soil CO₂ efflux with soil temperature would generate hysteresis loops.The size and rolling direction of hysteresis loops are different in different seasonal backgrounds.The hysteresis loops are closely associated with variation ranges in soil moisture,temperature,precipitation.In addition,temporal variation of soil respiration is different at different soil depths.Time-series of soil porewater compositions and total charges show that both season and soil depths exert important controls on soil pore-water chemistry.Saturation indices of different minerals show similar distributions,with positive and negative values distributing symmetrically with respect to zero line.In most cases,Fe?oxy?hydroxides are more likely to precipitate than Al?oxy?hydroxides.Dynamics of porewater DOM also shows notable differences at different soil depths and in different seasons.The concentration of DOM with increasing depth did not follow a typical decresing trend,but rather a decreasing-increasing trend.This can be mainly attributed to FI of DOM increases with depth,showing an opposite trend to SUVA₂₅₄?SUVA₂₈₀.Vertical transport of DOM-polyvalent metals/secondary minerals,as well as variation in microbial activity at different depth can significantly influence the depth distribution.At a seasonal scale,concentration and composition of DOM at different depths show different couplings with other important biogeochemical parameters,suggesting that DOM dynamics at different depths is modulated by a variety of factors.During the water sufficient period,much CO₂ produced from soil respiration reacted with minerals to form HCO₃^-,which were subsequently released from soil system,rather than emitted to the atmosphere,and mineral weathering inhibited CO₂efflux from soil.There is a lag between variations in soil gas partial pressure and pore-water chemistry.This time lag represents a coupling between soil respiration and chemical weathering,i.e.,CO₂ produced from soil respiration promoted mineral weathering.This can explain why soil respiration remained relatively stable before summer monsoon precipitation,and why the balance was broken after summer monsoon precipitation.This thesis synthetically studied the correlations and interactions among a series of Earth's surface processes and their responses to climate changes within typical Critical Zones in different climates.We propose that the formation and evolution of Critical Zone as well as the correlations and couplings are strongly controlled by climatic background,and that these correlations and interactions can exert important influences on climate change and carbon cycling.This study bears important implications for Critical Zone science,pedogenesis,formation and transformation of mineral,soil respiration,terrestrial carbon cycling,and global climate change.