Abstract
Abstract. In this contribution we explore constraints on the fractions of deep water present in the Indian and Pacific oceans which originated in the northern Atlantic and in the Southern Ocean. Based on PO4* we show that if ventilated Antarctic shelf waters characterize the Southern contribution, then the proportions could be close to 50–50. If instead a Southern Ocean bottom water value is used, the Southern contribution is increased to 75 %. While this larger estimate may best characterize the volume of water entering the Indo-Pacific from the Southern Ocean, it contains a significant portion of entrained northern water. We also note that ventilation may be highly tracer dependent: for instance Southern Ocean waters may contribute only 35 % of the deep radiocarbon budget, even if their volumetric contribution is 75 %. In our estimation, the most promising approaches involve using CFC-11 to constrain the amount of deep water formed in the Southern Ocean. Finally, we highlight the broad utility of PO4* as a tracer of deep water masses, including descending plumes of Antarctic Bottom Water and large-scale patterns of deep ocean mixing, and as a tracer of the efficiency of the biological pump.
Highlights
CO2, they were referenced to that in mean ocean water
The attraction of PO∗4 as a water mass tracer is that the deep waters formed in the northern Atlantic range widely in temperature, all the contributors have PO∗4 values close to 0.7 (Fig. 1)
If the PO∗4 for deep waters formed in the Southern Ocean were known, the relative amounts of deep water produced in the two key source regions could be established
Summary
The difference between Southern Ocean water mass volume and tracer ventilation is pronounced in the deep radiocarbon budget. As NADW supplies about 130 mol 14C yr−1, this leaves about 70 mol 14C yr−1 to be supplied from the Southern Ocean (see Table 2). Ventilation of radiocarbon is dominated by the North Atlantic, even if the Southern Ocean contributes greater volume. This is due to 14C’s long equilibration time and the limited exchange time between Southern Ocean surface waters and the atmosphere. −70 ‰ 0.126 × 10−12 20 mol yr−1 220 mol yr−1 northern versus southern ventilation may depend on the tracer and process of interest
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