Samples were collected from 12°N to12°S at 140°W on the R/V Thompson during EqPac Survey I (TT007; February-March, 1992) and Survey II (TT011; August-September, 1992). Stations were occupied at 12°N, 9°N, 7°N, 5°N, 3°N, 2°N, 1°N, Equator, 1°S, 2°S, 3°S, 5°S and 12°S for 1.5 to 2.5 day periods. The hydrographic and nutrient data for these cruises was presented by Murray et al. (1995).

          Discrete water column samples were collected from the surface to 250m using a CTD-Rosette equipped with 30-l Go-Flo bottles. Samples for total and dissolved isotopes (~20 l) were processed on deck within 8 hours after collection. Samples (~50 to 80 l) for dissolved/particulate partitioning were filtered through either 142mm diameter 0.50 mm Nuclepore or 0.45 mm Millipore filters before processing. Samples for stable Pb were collected using a Moss Landing Marine Lab (MLML) trace metal clean rosette built for the EqPac study (Sanderson et al., 1995). The flux of particles was sampled using drifting sediment traps of the PIT design (Knauer et al, 1979). Traps were deployed at 5 - 7 depths from 50 - 250m. The details of trap deployment and ²³⁴Th and organic carbon sample processing were given in Murray et al. (1996).

                ²³⁴Th, ²¹⁰Pb and ²¹⁰Po were analyzed by conventional alpha and beta counting techniques and the same water and particulate samples. For dissolved and total water samples the water was acidified to pH 1.5 with conc. HCl, spiked with yield tracers (²³⁰Th, ²⁰⁹Po and stable Pb) and FeCl₃, equilibrated for at least 6 hours and neutralized to pH  9 with NH₄OH. The Fe(OH)₃ precipitate was removed and dissolved in 9N HCl. The isotopes were separated using a series of anion exchange columns (AG 1x8, 100-200 mesh)(Anderson and Fleer, 1982; Coale and Bruland, 1985). Th was eluted with 9N HCl, Pb with 8N HNO₃ and Po with 0.1N HNO₃. All the ²³⁴Th chemistry and beta counting for the dissolved and total water column samples were completed at sea. The suspended and sediment trap particulate samples were analyzed for ²³⁴Th in the lab at UW as was the chemistry and alpha counting for all ²¹⁰Pb and ²¹⁰Po samples.

                Stable Pb was determined using a flow injection method with inductively coupled plasma mass spectrometry (ICP-MS) detection (Chapin and Murray, submitted). Sample preconcentration was performed with a micro column packed with 8-hydroxyquinoline (8-HQ) immobilized on Fractogel (EM Science) by the technique of Landing et al. (1986). The detection limit (3 sigma blank) for Pb was 3.9 pM. Accuracy was verified by analysis of standard reference materials (CASS-2 and NASS-3) and by intercalibration with Moss Landing Marine Lab (MLML) using a graphite furnace atomic absorption spectrophotomery (GFAAS) method (K. Coale, personal communication).

                Water column (4-6 l) and sediment trap samples for particulate carbon were filtered through pre-combusted 25 mm Whatman GF/F filters. Particulate carbon analyses were conducted using a Leeman Labs CEC 440 CHN analyzer as described by Murray et al. (1996).

                ²³⁸U (dpm l^-1) was calculated using the ²³⁸U/salinity relationship of Chen et al. (1986) which is:

                                                ²³⁸U = 0.0686 x s_t

                ²²⁶Ra (dpm/100 liter) was determined at the same stations by Ku et al (1995). We used this data and the EqPac Si data from the same samples (Murray et al., 1995) to determine the linear Ra-Si relationship in the upper 300m of:

[Ra] = 0.082 [Si] + 8.238            ( r² = 0.741)

Similar relationships were determined previously by Nozaki et al (1998) in the Pacific using the GEOSECS data of Chung and Craig (1980) and by Ku and Lin (1976) in the Antarctic Ocean. We used this equation and the Si data from the bottle casts to calculate ²²⁶Ra.

REFERENCES

Anderson R.F. and Fleer A.P. (1982) Determination of natural activities of thorium and plutonium in marine particle matter. Analytical Chemistry 54, 1142-1147.

Chapin T.P. and J.W. Murray (submitted) On-line preconcentration and interference removal for the analysis of trace metals in seawater by flow injection inductively coupled plasma mass spectrometry. Analytical Chemistry

Coale K.H. and Bruland K.W. (1985) 234Th:238U disequilibria within the California Current. Limnology and Oceanography, 30, 22-33.

Ku T.-L. and M.-C. Lin (1976) ²²⁶Ra distribution in the Antarctic Ocean. Earth and Planetary Science Letters, 32, 236-248.

Ku T.-L., S. Luo, M. Kusakabe and J.K.B. Bishop (1995) ²²⁸Ra-derived nutrient budgets in the upper equatorial Pacific and the role of "new" silicate in limiting productivity. Deep-Sea Research II, 42, 479-498.

Murray J. W., E. Johnson and C. Garside (1995) A U.S. JGOFS Process Study in the equatorial Pacific (EqPac): Introduction. Deep-Sea Res. 42, 275-293

Murray J.W., J. Young, J. Newton, J. Dunne, T. Chapin, B. Paul and J.J. McCarthy (1996) Export flux of particulate organic carbon from the central equatorial Pacific determined using a combined drifting trap-²³⁴Th approach. Deep-Sea Res., 41, 1095-1132

Nozaki Y., F. Dobashi, Y. Kato and Y. Yamamoto (1998) Distribution of Ra isotopes and the ²¹⁰Pb and ²¹⁰Po balance in surface seawaters of the mid-Northern hemisphere. Deep-Sea Research, 45, 1263-1284.

Sanderson M.P., C.N. Hunter, S.E. Fitzwater, R.M. Gordon and R.T. Barber (1995) Primary productivity and trace-metal contamination measurements from a clean rosette system versus ultra-clean Go-Flo bottles. Deep-Sea Research II, 42, 431-440.