²²²Rn, ²²⁶Ra, and ²²⁸Ra as tracers for the evolution of warm core rings

Abstract

Naturally occurring levels of Ra-228, Ra-226, and Rn-222 were measured in and around warm core rings to enhance interpretation provided by conventional conservative tracers. Surplus Rn-222 was found in warm core ring 82B, thereby indicating penetration of rapidly advecting shelf water (transit time < 16 days). An in situ pumping system has been developed to rapidly extract Ra-228 from seawater; it attaches to the 30-L rosette allowing simultaneous collection of 12 x 30-L samples and continuous data from the CTD. Extraction is fast (12 min/sample) and efficient (90% radium removal onto Mn fiber) for each of six 1000-L samples collected per cast. This instrument may also simplify large-volume sampling for other trace species in seawater. Coincidence counting (β- γ) has been applied to analysis of oceanic Ra-228, providing precise, low-level determinations shortly after sampling with relatively high counting efficiency (5.3%), low background (0.0054 cpm), reasonable ease of analytical processing, and intermediate capital equipment cost ($12,000). Results show, Ra-228 activity within ring 82B increased with ring age, marking infiltration of surrounding waters. Also Ra-228 must have been lost from ring 82B before it was 2 months old, an interpretation supported by physical evolution models of ring 82B and distributions of particulate Mn and Ra-228. Because Ra-226 is not similarly lost, Ra-228 could not have been scavenged (e. g., via barite formation). Particulate Mn is the only species found to correlate with Ra-228. Source regions of both Ra-228 and particulate Mn are linked (reducing sediments along continental margins); there significant Ra-228 adsorbs to Mn-rich particles and is advected rapidly offshore into the ring. Loss of both species in young ring 82B is attributed to their incorporation into rapidly sinking fecal material. Later, enhanced zooplankton activity indirectly facilitated solubilization of adsorbed Ra-228; particles subsequently lost were deficient in Ra-228. Like warm core rings, the Gulf Stream loses Ra-228 at a rate 60 times faster than explainable by radioactive decay, without corresponding loss of Ra-226 or particulate Ba. Evaluation of counting methods reveals a fivefold sensitivity jump is feasible for Ra-228. More manageable sampling and better precision are now within our grasp.