^226Ra evidence for the ecosystem shift over the past 40 years in the North Pacific Subtropical Gyre

Surface seawater was collected for ~(226)Ra measurement in the North Pacific Subtropical Gyre from July to October, 1999 and October to December, 2003. Combined with the historical data reported for this sea area, a declined trend of surface ~(226)Ra concentrations was observed since 1960s, indicating the ecosystem shift in response to global warming. On one side, the enhanced stratification of the upper water column resulting from global warming reduced the ~(226)Ra input from the depth, on the other, the temporal increase of biological production resulting from the climate-related ecosystem structure change strengthened the ~(226)Ra removal from the surface ocean. Both the physical and biological processes resulted in the decrease of surface ~(226)Ra concentrations in the North Pacific Subtropical Gyre. The temporal trend of surface ~(226)Ra concentrations was consistent with the trends of chlorophyll a, silicate, phosphate and primary production previously reported. This study provided ~(226)Ra evidence for the ecosystem shift under global change.     

The mean residence time of river water in the Canada Basin

Seawater was collected from the western Arctic Ocean for measurements of 18О, ^226Ra and ^228Ra. The fractions of river runoff and sea ice melt-water in water samples were estimated by using δ^18О-S-PО^＊ tracer system. The mean residence time of river water in the Canada Basin was calculated based on the relationship between ^228Ra/^226Ra）A.R. and the fractions of river runoff in the shelf and deep ocean. Our results showed that the river runoff fractions in the Canada Basin were significantly higher than those in the shelf regions, suggesting that the Canada Basin is a major storage region for Arctic river water. ^228Ra activity concentrations in the Chukchi shelf and the Beaufort shelf ranged from 0.16 to 1.22 Bq/m^3, lower than those reported for shelves in the low and middle latitudes, indicating the effect of sea ice melt-water. A good positive linear relationship was observed between ^228Ra/^226Ra）A.R. and the fraction of river runoff for shelf waters, while the ^228Ra/^226Ra）A.R. in the Canada Basin was located below this re-gressive line. The low ^228Ra/^226Ra）A.R. in the Canada Basin was ascribed to ^228Ra decay during shelf wa-ters transporting to the deep ocean. The residence time of 5.0-11.0 a was estimated for the river water in the Canada Basin, which determined the time response of surface freshening in the North Atlantic to the river runoff into the Arctic Ocean.     

Export and remineralization of POM in the Southern Ocean and the South China Sea estimated from 210Po/210Pb disequilibria

Disequilibria between ^210Po and ^210Pb in the upper water and their potential applications as a proxy of particle export and remineralization were examined in the Southern Ocean （station IV3） and the South China Sea （NS44）. ^210po was deficit in surface waters but excessive relative to ^210Pb in subsurface waters. Good positive correlation between ^210Po and particulate organic carbon （POC） indicated deficits and excess of ^210Po resulted from particulate organic matter （POM） export and remJneralization respectively, which was also supported by the decreased δ^13C and increased δ^15N downwards as a result of particle remineralization. On the basis of ^210Po/^210Pb box-model, POC export flux out of the surface waters were 1.2 mmol C. m^-2. d^-1 and 2.3 mmol C. m^-2. d^-1 for station NS44 and IV3, respectively. In the subsurface waters, remineralization fluxes of ^210Po were 0.062 Bq. m^-2.d^-1 and 0.566 Bq.m^-2.d^-1 for station NS44 and IV3 along with the recycle efficiency of 52±26% and 119±52%, respectively. Remineralized fluxes of POM derived from ^210Po and exported POC were 0.6 mmol C.m^-2.d^-1 and 2.7 mmol C. m^-2. d^-1 for NS44 and IV3. This study suggested that ^210Po was a powerful tracer of particle export and remineralization.     

Export and remineralization of POM in the Southern Ocean and the South China Sea estimated from ~(210)Po/~(210)Pb disequilibria

Disequilibria between 210Po and 210Pb in the upper water and their potential applications as a proxy of particle export and remineralization were examined in the Southern Ocean (station IV3) and the South China Sea (NS44). 210Po was deficit in surface waters but excessive relative to 210Pb in subsurface waters. Good positive correlation between 210Po and particulate organic carbon (POC) indicated deficits and excess of 210Po resulted from particulate organic matter (POM) export and remineralization respecti...