Comparison Of The Characteristics,provenances,and Transport Mechanisms Of Modern Eolian Dust In The Northwest Pacific

The Pacific Ocean is located in the downwind direction of the East Asian winter monsoon?EAWM?,and is an important“sink”area of Asian dust.The eolian dust deposits in the Pacific record the evolution history of paleoclimate and paleoenvironment of the Asian continent.Modern dust samples were collected during the summer of 2014,the winter of 2015 and the spring of 2017 in the mid-latitude and low-latitude Northwest Pacific,respectively,which provide a good opportunity for the research of ocean eolian deposits.The mass fluxes and microscopic mineral characteristics of modern eolian dust particulates and the trace element compositions of the siliciclastic fractions of these samples collected from the Northwest Pacific,together with air mass backward trajectory analysis of dust particulates therein in different seasons,are compared to better constrain the provenances and transport dynamics of dust delivered to these regions.The Sr and Nd isotopic compositions of the siliciclastic fractions and the Nd isotopic compositions of planktonic foraminifera selected from sediment trap sediments collected in 2015 at water depths of 500 m and2800 m,are also investigated to quantify the eolian dust source to the West Pacific and to assess the influences of terrestrial inputs on the Nd isotopic compositions throughout the water column.There is no systematic difference in the mineral compositions between the eolian sediments from different regions and different seasons in the Northwest Pacific,thus the composition and morphology of minerals can only be used as a reference index of sediment sources.The mineral assemblages of the modern dust mainly consist of quartz,plagioclase and illite,with minor gypsum,smectite,kaolinite and chlorite.The grain size of the detrital minerals mostly varies from 2¹5?m,and few particles are larger than 20?m.The mineral particles are mainly subangular/subrounded,which is the typical characteristics of long-range transport by wind.These indicative minerals of dry and cold conditions suggest that the modern dust in this study are mainly derived from inland Asia,and the contribution of the volcanic island arc nearby is insignificant.As for the authigenic components,they may be generated by atmospheric processes over the marine-gas boundary layer.Eolian dust transported by the Westerlies from the Taklimakan Desert dominates the flux of dust in the mid-latitude Northwest Pacific during the spring of 2017,enhanced EAWM can also carry a small amount of dust from the East Asian deserts?EADs;e.g.,the Ordos Desert and the Badain Jaran Desert?to the mid-latitude Northwest Pacific in this season.Whereas,eolian dust in the low-latitude West Pacific is predominantly originated from the Ordos Desert,the contributions of the Taklimakan Desert and the Badain Jaran Desert are small.The EAWM is the dominant transport agent of dust to the low-latitude West Pacific.The provenance analysis results of the modern dust based on the Zr-Th-Sc compositions are consistent with that of core sediments in the geological past in both study regions,thus proving the index to be a reliable indicator of sediment provenances.The mass fluxes of modern dust are dominantly controlled by sampling locations and meteorological conditions.The influences of the precipitation and wind speed on the mass fluxes are relatively complex.During the precipitation period in the subsidence area,the settling process is promoted due to wet deposition,thus causing sharply reduced mass fluxes after the rainfall because of the effective wet deposition removal of fine particles.The dust fluxes show a delayed response to the wet deposition processes in the source areas as a result of long-time transport.Furthermore,there is a delay positive effect between the wind speed and the dust fluxes.The siliciclastic fractions isolated from sediment trap samples appear as a binary mixture of chemical weathering of volcanic matters from Luzon and eolian dust from Asian deserts.The provenance analysis demonstrates that dust is predominantly originated from the Ordos Desert?>80%?,while the contribution of sediments from the Taklimakan Desert,the Badain Jaran Desert and the Tengger Desert is generally less than 20%.The mass fluxes display seasonal variability,and are mainly controlled by seasonal precipitation and the EAWM intensity.No seasonal variability was observed among the?_Nd values of the planktonic foraminifera collected in 2015 at a water depth of 500 m in the West Pacific,indicating insignificant influence of seasonal changes in the terrigenous detrital input to the surface/intermediate water therein.However,the results at a water depth of 2800 m are obviously higher than the data collected at a water depth of 500 m,and tend to show seasonal variability characteristics.The adsorption of Mn coatings on the surface of the planktonic foraminifera during the settling process is considered to be the dominant factor account for the increase of the?_Nd values of the planktonic foraminifera collected in at a water depth of 2800 m.The enhanced boundary exchange process and submarine groundwater discharge have great potential to influence the Nd isotopic compositions of deepwater in the West Pacific.