Geodetic Joint Inversion Of The Mass Changes Beneath The Antarctic Ice Sheet

The mass change beneath the Antarctic ice sheet(AIS)refers to the change caused by the complex transport of basal meltwater generated by the combined effects of basal heat flow,friction heating and basal pressure melting.The presence of the basal water helps to lubricate the Antarctic basement and affects the velocity of the overlying ice flow.The migration of the basal water affects the Antarctic basal physical environment,and the discharge of the basal water at the grounding line will thermally erodes the base of the ice shelf,thereby affecting the stability of the grounding line and changing the direction of the ice flow.Furthermore,the migration of the basal water can also lead to the transportation of basal solute and microorganisms,thus affecting the basal chemical and biological environment beneath the AIS.Overall,the study of the mass change beneath AIS contributes to further understanding the ice sheet dynamics and basal environment of AIS,analyzing the drivers of the mass changes,as well as predicting future mass changes trends.However,the continental direct observation on the basal mass changes by traditional method is rarely available since it is covered by a thick ice layer,so the studies of the mass changes beneath AIS are mostly limited to local observations or numerical simulations.To date,there are three main types of methods to study the basal mass changes beneath the AIS:altimetry method,numerical simulation and hybrid method.The principle of the altimetry method is to utilizing satellite altimetry observations to infer the water changes in ’active’ subglacial lakes,by examining the Antarctic surface height variations due to the inflow and outflow of subglacial lake water.While this method is only applicable to subglacial lakes with obvious filldischarge cycle,and the observation is usually limited to local regions.The principle of numerical simulation is to calculate basal hydraulic potential gradient and simulate the motion of basal water or the distribution of basal water pathway by utilizing the ice thickness and height bottom.This method is performed mainly based on numerical models in combination with other constrains such as mass conservation and physical parameters such as surface accumulation,et al.Theoretically,it is possible to simulate the continental mass changes beneath AIS,while the large uncertainties remain in the simulation results as lacking sufficient observational constraints.The hybrid approach utilizes multiple observations combined with a numerical model of the ice sheet to calculate the basal mass changes beneath the AIS.However,the error of the hybrid result varies largely in different regions as the non-uniform distribution of the field observations.To address the problems of limited coverage and large uncertainties in the current studies on the basal mass changes of AIS,this study proposes the geodetic joint inversion method to detect the basal mass changes of AIS,which successfully separates the basal mass changes signals by the unified combination of multi-source satellite observation data and related models.In the joint inversion process,the simulated basal pathway is added as additional constraints in order to improve the resolution of the inversion results.After that,the basal water storage variations beneath the AIS is calculated by combining the basal mass changes and basal melting data.The reliable accuracy analyses of the inversion result are presented based on the error propagation theory.In addition,this study identifies the effect of non-mass changes on the calculation of mass changes through gravimetry method in regions with confined aquifers,and proposes a corresponding correction method.The main contributions are shown as follows:(1)This study proposed a geodetic joint inversion method to obtain the spatial distribution of the basal mass changes beneath AIS during 2003-2009.For the current situation that the basal mass changes is difficult to be accurately observed on a large scale,this study make a detailed analysis of the composition of the mass changes within AIS regions and its observability,and for the first time proposes a geodetic jointinversion method that combines multi-source geodetic data such as satellite gravimetry,satellite altimetry and GPS observations data,and related models such as GIA model and firn densification model to detect the basal mass change beneath AIS during 20032009.The results show that the basal mass changes obtained using different input data have high consistency in spatial distribution,which prove the feasibility of the proposed method.In AIS region,obvious basal mass increases are mainly located in Palmer Land in Antarctic Peninsula Ice Sheet(APIS),the Ellsworth Mountain and Amundsen coast in West Antarctic Ice Sheet(WAIS),Adelie,Clarie,Wilhelm II Coast in East Antarctic Ice Sheet(EAIS),while basal mass decreases are mainly situated in north region of AP,the Budd Coast,the upstream of Lambert ice flow,Princess Martha Coast region and their extending inland regions in EAIS.The average basal mass change rate within AIS region is-24 Gt/yr,which accounts for about 31%of the total mass balance of AIS.(2)This study presented an error assessment method based on the error propagation theory of gravity cylinder model to estimate the error of the basal mass changes beneath AIS.For the problem that the usage of various input dataset in the joint inversion process that make it difficult to make error assessment,this study presents a simple error assessment method based on the error propagation theory of gravity cylinder model.And based on this error assessment method,the spatial distribution of the error of the basal mass changes is obtained.The larger errors are mainly located in APIS,while the smaller errors are mainly situated in APIS,Amundsen Coast region in WAIS,and Wilkes Land regions in EAIS.The total error of the basal mass changes within AIS is±35 Gt/yr.(3)Combining the AIS’s basal mass change results and basal melting rate data,the basal water storage change rate beneath AIS is calculated as+46±35Gt/yr,with the obvious increasing rate mainly located in Amundsen Coast region in WAIS and Wilkes Land in EAIS.(4)This study points out the defect of utilizing GRACE method to estimate the Antarctic mass changes in confined aquifer region,and proposed the corresponding correction method.For the problem of the inconsistency between the mass changes obtained by GRACE observation and other geodetic method,this study proposed the influence of non-mass changes on the mass changes estimated through GRACE method in confined aquifer regions.Taking the AIS region as an example,a detailed simulation was performed to analyze how the non-mass changes affect the mass changes result through GRACE method,and the corresponding correction method was presented.After that,the actual calculations were carried out based on the actual observation data in AIS region,and the results show that the using conventional GRACE method underestimates the AIS mass loss by about 14%,the consistency between the mass changes estimated from GRACE method and satellite altimetry method has increased by 40%.