234Th, 210Pb, 210Po and stable Pb in the central equatorial Pacific:: Tracers for particle cycling

Samples were collected during the 1992 US JGOFS EqPac Survey I and II cruises from 12 degrees N to 12 degrees S at 140 degrees W in the central equatorial Pacific for water column profiles of dissolved, particulate and total Th-234, Pb-210 and Po-210 and total acid soluble stable Pb and sediment trap fluxes of Th-234, Pb-210 and Po-210. Survey I occurred in February/March with moderate El Nino conditions while Survey II was conducted in September/October when there was a well developed cold-tongue. Th-234, Pb-210 and Po-210 are all particle reactive yet they partition differently between dissolved and particulate phases. Fractionation factors (the ratios of the distribution coefficients) show that the selectivity for suspended and sediment trap particles follows Th > Po > Pb. Scavenging residence times (T) for Th-234, Pb-210 and Po-210 ranged from 25 to 100 d, 3 to 8 years and 100 to 500 d, respectively. These particle reactive tracers have very different distributions in the water column, which reflect differences in their sources and sinks. The deficiency of 234 Th relative to U-238 was fairly uniformly distributed meridionally, though deficiencies were higher during Survey 11 when there was higher new production. Excess Pb-210 relative to Ra-226 was very asymmetrical with much higher excess values north of the equator. The distributions were similar for Surveys I and II. The deficiency of Po-210 relative to Pb-210 had a symmetrical distribution about the equator for both Survey I and II but the deficiencies were larger during Survey I when upwelling was smaller. Stable Pb was generally higher at the surface than at 250 m and there was no meridional trend from 12 degrees N to 12 degrees S. A mass balance for Pb-210 was used to determine the atmospheric input of Pb-210. The average values for Surveys I and II were 0.12 and 0.32 dpm cm(-2) year(-1), respectively. There was no general increase in atmospheric input of Pb-210 north of the equator but there was a strong maximum at 2-3 degrees N during Survey I coincident with the location of the intertropical convergence zone (ITCZ), suggesting a large role for wet deposition. A mass balance for stable Pb was used to determine the atmospheric input of stable Pb. Results ranged from 110 to 140 pmol cm(-2) year(-1). This flux was low in the southern hemisphere and increased steadily north of the equator. We evaluated use of Po-210 as a tracer for export of particulate organic matter during Survey I. Organic carbon and Po-210 Were highly correlated in suspended matter and sediment trap samples. Average values of organic carbon fluxes determined from the deficiencies of Po-210 times the orgC/Po-210 ratio agreed well with those determined from the deficiencies of Th-234 times the organic carbon/Th-234 ratio and N-15-new production, but had a much larger variability because of the more variable advection corrections. (c) 2005 Elsevier Ltd. All rights reserved.