228Ra and 226Ra in the Kara and Laptev seas

Abstract

The surface water in the Transpolar Drift in the Arctic Ocean has a strong signature of 228 Ra. In an earlier study of 228 Ra in the open Arctic we showed that the major 228 Ra source had to be in the Siberian shelf seas, but only a single shelf station was published so far. Here we investigate the sources of this signal on the Siberian shelves by measurements of 228 Ra and 226 Ra in surface waters of the Kara and Laptev Sea, including the Ob, Yenisey and Lena estuaries.In the Ob and Lena rivers we found an indication for a very strong and unexpected removal of both isotopes in the early stage of estuarine mixing, presumably related to flocculation of organic-rich material. Whereas 226 Ra behaves conservatively on the shelf, the distribution of 228 Ra is governed by large inputs on the shelves, although sources are highly variable. In the Kara Sea the maximum activity was found in the Baydaratskaya Bay, where tidal resonance and low freshwater supply favour 228 Ra accumulation. The Laptev Sea is a stronger source for 228 Ra than the Kara Sea. Since a large part of Kara Sea water flows through the Laptev Sea, the 228 Ra signal in the Transpolar Drift can be described as originating on the Laptev shelf.The combined freshwater inputs from the Eurasian shelves thus produce a common radium signature with a 228 Ra/ 226 Ra activity ratio of 4.0 at 20% river water. The radium signals of the individual Siberian rivers and shelves cannot be separated, but their signal is significantly different from the signal produced on the Canadian shelf (Smith et al., in press). In this respect, the radium tracers add to the information given by Barium. Moreover, with the 5.8 year half-life of 228 Ra, they have the potential to serve as a tracer for the age of a water mass since its contact with the shelves. r

Age characteristics of a shelf‐break eddy in the western Arctic and implications for shelf‐basin exchange

“…Elapsed time‐since‐residence on the shelf (T) is derived from the simple radioactive decay equation where λ = decay constant of 228 Ra (year −1 ), R = observed 228 Ra/ 226 Ra ratio, and Ro is the initial 228 Ra/ 226 Ra ratio prior to aging. Without knowing the exact source location of the eddy, an initial 228 Ra/ 226 Ra ratio cannot be precisely assigned and variability of the source either spatially or temporally introduces uncertainty [e.g., Rutgers van der Loeff et al , 2003]. We can, however, estimate the age by comparing shelf values of the 228 Ra/ 226 Ra ratio with that of the eddy.…”

“…This procedure is replicated over a period of several weeks to determine the 228 Th activity of the sample. On land, the fibers were sealed in plastic petrie dishes then counted by gamma spectrometry using established procedures to determine the 228 Ra/ 226 Ra ratios [e.g., Michel et al , 1981; Rutgers van der Loeff et al , 1995, 2003; Kadko and Muench , 2005].…”

Shelf‐Basin Interactions and Water Mass Residence Times in the Western Arctic Ocean: Insights Provided by Radium Isotopes

“…Because of the shorter half‐life of 228 Ra, this isotope regenerates more quickly than 226 Ra in sediments. This can result in 228 Ra/ 226 Ra activity ratios greater than 1 over continental shelves; ratios as high as 3.9 have been observed over the Laptev Shelf (Rutgers van der Loeff et al, ). After water leaves the shelf, residence times in the surface and halocline are long enough (~3–10 years; Östlund, ; Östlund & Hut, ; Schlosser et al, ) that the 228 Ra/ 226 Ra activity ratio will decrease due to 228 Ra decay.…”

228Ra, 226Ra, 224Ra and 223Ra in potential sources and sinks of land-derived material in the German Bight of the North Sea: implications for the use of radium as a tracer

“…The salinity was in the range of 15-18 psu at high tide and 6-12 psu at low tide. Ra to the ocean (Moore 1981), prominent examples being the Arctic and the Yellow Sea (Rutgers van der Loeff et al 1995Loeff et al , 2003Kim et al 2005). The North Sea as a whole resembles these larger examples, while the Wadden Sea, with its extensive intertidal flats, constitutes a distinct situation within the North Sea, with exceptionally strong water and sediment dynamics.…”