The distributions of, and relationship between, 3He and nitrate in eddies

Abstract

Abstract We present and discuss the distribution of 3 He and its relationship to nutrients in two eddies (cyclone C1 and anticyclone A4) with a view towards examining eddy-related mechanisms whereby nutrients are transported from the upper 200–300 m into the euphotic zone of the Sargasso Sea. The different behavior of these tracers in the euphotic zone results in changes in their distributions and relationships that may provide important clues as to the nature of physical and biological processes involved. The cyclonic eddy (C1) is characterized by substantial 3 He excesses within the euphotic zone. The distribution of this excess 3 He is strongly suggestive of both past and recent ongoing deep-water injection into the euphotic zone. Crude mass balance calculations suggest that an average of approximately 1.4±0.7 mol m −2 of nitrate has been introduced into the euphotic zone of eddy C1, consistent with the integrated apparent oxygen utilization anomaly in the aphotic zone below. The 3 He–NO 3 relationship within the eddy deviates substantially from the linear thermocline trend, suggestive of incomplete drawdown of nutrients and/or substantial mixing between euphotic and aphotic zone waters. Anticyclone (A4) displays a simpler 3 He–NO 3 relationship, but is relatively impoverished in euphotic zone excess 3 He. We suggest that because of the relatively strong upwelling and lateral divergence of water the residence time of upwelled 3 He is relatively short within the euphotic zone of this eddy. An estimate of the recently upwelled nutrient inventory, based on the excess 3 He observed in A4's lower euphotic zone, is stoichiometrically consistent with the oxygen maximum observed in the euphotic zone.