Dataset: Activity of actinium-227 from water samples collected on Leg 1 (Seattle, WA to Hilo, HI) of the US GEOTRACES Pacific Meridional Transect (PMT) cruise (GP15, RR1814) on R/V Roger Revelle from September to October 2018

227Ac Methods for Geotraces GP15

Note about figures: The following methods are also described in the attached PDF named "Methods for Geotraces GP15 227Ac Analysis" (see Supplemental Files). Figure numbers refer to figures in the PDF.

Overview: The decay chain for 235U is shown in Fig. 1. This illustrates both how 227Ac is formed in the ocean, and also the decay scheme of its progeny that are used for its measurement. McLane Pumps were deployed at various depths on the GEOTRACES GP15 cruise aboard the R/V Revelle. Typically, a total of about 1500 liters (L) of water was pumped through two filter paths for the collection of particulates, with about half going through a QMA glass Fiber filter (1-micrometer (µm)) and the other half passing through a stack of filters with the smallest a PES Supor800 filter (0.8 µm). The effluent from the filters was combined and directed to a set of commercial acetate or acrylic cartridges that had been impregnated with MnO2. The approximate flow rate was 6 L/minute during the pre-set pump time of 4 hours. Ac, Ra, and Th in seawater were sorbed as water was pumped through them, although at this high flow rate, the sorption efficiency was less than 100% for each of these isotopes. Presumably, 231Pa, the 227Ac parent was also sorbed because it is very particle reactive. Analyses of Ra on these cartridges have been made by collaborating groups (M. Charette at WHOI and W. Moore at U. So. Carolina).

Two cartridges were used in series (A and B), allowing the 227Ac extraction efficiency from seawater at high pump rates to be calculated (see below). After extracting 227Ac in the field, it was analyzed in the lab. A stream of He passed through these fibers to transport the 219Rn progeny from 227Ac decay into a Radecc counter (Moore and Arnold, 1996). Radecc has circuitry that can distinguish decays of 219Rn, 220RRn, and 222Rn by opening windows following detection of an alpha decay. If a second decay is observed in these windows produced by a short-lived Po daughter, its arrival time can be used to distinguish which Rn may have produced the initial signal.

Considerable time was spent on confirming the accuracy of the Radecc method, including preparation of new standards for 227Ac and evaluating standard stability, effects of geometry, flow rate used for analysis, and "crosstalk" that has been observed for Radecc channels measuring 219Rn and 220Rn (descendants of 227Ac and 228Ra). Sections below show details of data reduction and calibration.