Oxygen And Radium Isotopes In The Bering Sea And The Western Arctic Ocean

The Arctic Ocean connects the Pacific to the Atlantic Ocean,and has an impact on global freshwater balance and climate change.In this study,oxygen isotopes and radium nuclides in seawater were determined during the 6th Chinese National Arctic Research Expedition in July-September,2014.The fractions of meteoric water and sea-ice melted water were calculated based on the mass balance of Salinity and ?18O with a three end-member mixing model.The distribution characteristics of freshwater components and radium isotopes,and their variations with the water mass property and current pathways were discussed.The transit time of river water from the Bering Shelf to the Canada Basin carried by the Pacific inflow was estimated based on the relationship between 228Ra/226Ra)A.R.and fraction of meteoric water.Besides,temporal variation of the freshwater components in the Bering Strait,Chukchi Sea and Canada Basin was depicted using the time-series data of ?18O from the 1st to 6th Chinese National Arctic Research Expedition(July-September,1999,2003,2008,2010,2012,2014).The factors controlling the temporal change of freshwater components were discussed.The major conclusions are as follows.1.The distribution of freshwater components in the Bering Sea and their controlling factors.The averaged fraction of meteoric water in the Bering shelf was higher than that in the Bering Basin,and the fraction of meteoric water in the western Bering shelf were lower than that in the eastern Bering shelf.The spatial variation of the meteoric water in the Bering Sea was regulated by the locations of the rivers and their discharges.The averaged fraction of sea-ice melted water in the basin was lower than that in the shelf,ascribing to seasonal production and melting of sea ice in the Bering shelf.The sea-ice melted water in the western Bering Shelf primarily derived from the outward transport from the Anadyr Gulf,while that in the eastern Bering Shelf mainly from the melting of local sea ices.The distribution of radium nuclides in the upper Bering Basin was affected by the Pacific surface water with high 226Ra and low 228Ra activities.The activity concentrations of radium nuclides in the Bering Shelf were affected by the mixing of several water masses,such as the low-Ra sea-ice melted water,the high-Ra meteoric water and the high-228Ra overlying water above sediments.Eight water masses with different characteristics were identified in the top 100 m water column of the sampling area in the Being Sea.Their levels of the fraction of meteoric water and 228Ra/226Ra)A.R.were closely related to the sources of water masses.In addition,a good positive linear relationship was observed between the fraction of meteoric water and that of sea-ice melted water among these water masses,which suggested that the enrichment of river component contributes to the formation of sea ice in winter,and increases the local sea-ice melted water in the next summer.2.The distribution of freshwater components in the western Arctic Ocean and their controlling factors.The averaged fraction of meteoric water in the Canada Basin or the Alaska coastal area was higher than that in the Chukchi Shelf or the central Chukchi Shelf,respectively,indicating the accumulation of river water by the Beaufort Gyre and the Alaska Coastal Current.The characteristics of the sea-ice melted water in the southern and northern waters of the pack-ice edge were different.The averaged fraction of sea-ice melted water showed a decrease from the south to the north,pointing to the effect of the solar radiation.The sea-ice melted water showed an accumulation in the marginal areas of pack-ice,which was ascribing to the northward transport of earlier sea-ice melted water by the Pacific inflow and the blocking effect of ice pack.The averaged negative fraction of sea-ice melted water showed an obvious gradient between the shelf and slope,suggesting the brine produced during sea ice formation in the shelf in winter would be transported to the basin along the isopycnic layer.Six water masses with different characteristics were identified in the sampling area in the western Arctic Ocean.Their levels of the fraction of meteoric water and 228Ra/226Ra)A.R.were closely related to the sources of different water masses.The property of the Alaska Coastal Water was related to the distance from the Alaska coast,while the property of the Bering Shelf Water was related to the pathways of the Pacific inflow.3.The transit time of river water carried by the Pacific inflow.Based on the linear relationships between 228Ra/226Ra)A.R.and fMW in the Bering Shelf,the Chukchi Shelf and the Canada Basin,the transit time for the Pacific inflow was constrained.The calculated transit time of river water from the Bering Shelf to the Chukchi Shelf was 3.2 ± 1.7 a,while that from the Chukchi Shelf to the Canada Basin was 11.4 ± 1.1 a.Thus the transit time of river water carried by the Pacific inflow entering the Canada Basin was estimated to be 14.6 ± 2.0 a.The spatial variation of the residence time in the Chukchi Shelf and the Canada Basin was mainly affected by the current intensity.4.Spatial and temporal variation of freshwater components in the Bering Strait and the western Arctic Ocean.The temporal variation of the total amount of meteoric water in the Bering Strait was related to total discharge of three major rivers in the Bering Sea last year,especially to the Yukon River discharge.The temporal variation of the total amount of meteoric water in the Chukchi Sea was affected by both the amount of meteoric water transported by the Pacific inflow and the sea ice condition in the Chukchi Sea.The temporal variation of the total amount of meteoric water in the Canada Basin was related to the summer AO index with a cyclical fluctuation on a time scale of about 10 a,which was resulted from the periodical variation of Beaufort Gyre driven by the atmospheric circulation.The temporal variation of the total amount of sea-ice melted water in the Bering Strait was related to the winter AO index.The temporal variation of the total amount of sea-ice melted water in the Chukchi Sea was related to the sea-ice extent anomalies in September in Arctic.The temporal variation of the total amount of sea-ice melted water in the Canada Basin was also related to the summer AO index.The temporal variation of the positive fraction of sea-ice melted water in the western Arctic Ocean was related to the sea-ice extent anomalies in September.Comparing to the temporal variation of the melting areas of sea-ice in September,the decreased positive fraction of sea-ice melted water suggested the increased export flux of sea-ice melted water from the Arctic to the Atlantic Ocean.The temporal variation of the negative fraction of sea-ice melted water in the western Arctic Ocean was related to the new areas of Arctic sea-ice in March.The total amount of freshwater in the Bering Strait was relatively stable,thus the temporal variation of the freshwater export flux to the Chukchi Sea was related to the variation of flow rate of the Pacific inflow,which also determined the total amount of freshwater in the Chukchi Sea.The temporal variation of the total amount of freshwater in the Canada Basin was related to several effect factors,the increasing trend of which during 2003-2012 probably resulted from the increased input flux of freshwater.