Developing a tool for evaluating the role of seasonal sea ice in deep-water formation
The primary study sites are two semi-enclosed, seasonally ice-covered sea-lagoons on the Okhotsk Sea coast of Hokkaido in northern Japan, which are treated as "natural laboratories". By assuming that the lagoons are closed systems and using measurements of noble gas concentrations througho...
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| Format: | Thesis |
| Language: | English |
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University of Southampton
2002
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| Online Access: | https://eprints.soton.ac.uk/464903/ https://eprints.soton.ac.uk/464903/1/891226.pdf |
| Summary: | The primary study sites are two semi-enclosed, seasonally ice-covered sea-lagoons on the Okhotsk Sea coast of Hokkaido in northern Japan, which are treated as "natural laboratories". By assuming that the lagoons are closed systems and using measurements of noble gas concentrations throughout the ice season, it was possible to estimate the bulk partition coefficients of the gases (ratio of solubility of gas in each phase) between ice-sea and seawater. Similarly, laboratory experiments were carried out whereby a tank of artificial seawater was frozen under controlled conditions and the noble gas concentration changes monitored. Combining the results from both sets of experiments indicates the partitioning between sea-ice and seawater is 1.12 ± 0.15, 0.66 ± 014 and 0.08 ± 0.12 for helium, neon and argon respectively. Therefore, helium is slightly more soluble in sea-ice than in seawater and will become depleted in the residual seawater during sea-ice formation. Conversely, argon is barely soluble in sea-ice, due to its large atomic radius, and will accumulate in the seawater beneath the ice. This noble gas 'ice signature' can theoretically be used to identify waters formed via brine rejection. The Japan/East Sea (JES) is used as a case study to determine whether the noble gas signals observed in the shallow sea lagoons and laboratory experiments have a practical use in the wider marine environment. In situ deep-water formation is known to occur in the JES and the northern sector is seasonally ice covered, making it an ideal location for this study. Noble gas measurements are presented alongside oxygen isotope data, a more traditional tracer of sea-ice formation. The range of oxygen isotope measurements in the subsurface waters of the JES indicates that two deep-water formation mechanisms were active in the past. These are 1) convection driven by brine rejection and 2) convection driven by extreme cooling of waters from south of the sub-polar front. |
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