Abstract
Chlorofluorocarbon (CFC)-12 and -11 (CF 2Cl 2 and CFCl 3) measurements were made in seawater on the Ross Sea continental shelf and adjacent slope region in 1984. Concentrations of CFC-12 and CFC-11 in Ross Sea continental shelf water averaged nearly half that of saturated surface water. Circumpolar Deep Water within 50 km of the Ross Sea continental shelf also contained measureable CFC-12 and CFC-11, but an order of magnitude less than shelf waters. CFC-12 and CFC-11 concentrations in the deep water overlying the continental slope increased with depth, indicating recent ventilation and bottom water formation near the continental shelf. Several water masses on the continental shelf that are commonly distinguished on the basis of temperature and salinity characteristics also varied in CFC content, and thus in their modes and rates of ventilation and renewal. A time-dependent model reproducing the 1984 sub-surface shelf water CFC concentrations demonstrates the relative importance of mixed layer entrainment, gas exchange through leads in the winter sea ice field, and mixing with source waters derived from the Circumpolar Deep Water. Model results show that mixed layer entrainment at the beginning of winter is the dominant process ventilating sub-surface shelf water in the eastern Ross Sea, while western shelf waters also require significant gas exchange during winter. Shelf water residence time can only be constrained by the model and available data to 10 years, but most probable values are ∼2.5 years in the eastern Ross Sea and ∼4 years in the western Ross Sea. CFC concentrations in water which has circulated beneath and has been modified by the Ross Ice Shelf indicate that the transformation time from High Salinity Shelf Water to Ice Shelf Water can be as short as 3.5 years. With reasonable oxygen consumption and nutrient regeneration rates, the balance of ventilation and mixing processes that reproduced observed shelf water CFC-12 concentrations also can account for observed oxygen and nitrate levels, if the western surface waters are undersaturated by ∼10% at entrainment.
Highlights
CflLOROFLUOROCARBONS(CFC)-12 (CF:CI2) and CFC-I[ (CFCI3) are anthropogenic, time-dependent tracers potentially useful in determining the processes and rates of deep water ventilation and bottom water formation in the Antarctic
The plots are contoured with CFC-12 isopleths, and the shaded regions show the suite of wind speeds and Warm Core (WMCO) water exchange rates which bracket the observed 1984 Low Salinity Shelf Water (LSSW) and High Salinity Shelf Water (HSSW) CFC-12 concentrations (1.25 _ 0.10 and 1.60 +_0.11 pmol I- t, respectively)
Additional CFC input by winter gas exchange must be balanced by mixing with the CFC-depleted WMCO
Summary
Circumpolar Deep Water within 50 km of the Ross Sea continental shelf contained measureable CFC-12 and CFC-11, but an order of magnitude less than shelf waters. A time-dependent model reproducing the 1984 sub-surface shelf water CFC concentrations demonstrates the relative importance of mixed layer entrainment, gas exchange through leads in the winter sea ice field, and mixing with source waters derived from the Circumpolar Deep W;iter. Model results show that mixed layer entrainment at the beginning of winter is the dominant process ventilating sub-surface shelf water in the eastern Ross Sea. while western shelf waters reqt,irc signific.'mt gas exchange during winter. With reasonable oxygen consumption and nutrient regeneration rates, the balance of ventilation anti mixing processes that reproduce observed shelf water CFC-12 concentrations c:m account fl~robserved oxygen anti nitrate levels, if the western surface waters are undersaturated by ~ I11% at entrainment
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