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

J.W. Murray·B. Paul·J.P. Dunne·T. Chapin
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Abstract

Samples were collected during the 1992 US JGOFS EqPac Survey I and II cruises from 12??N to 12??S at 140??W in the central equatorial Pacific for water column profiles of dissolved, particulate and total 234Th, 210Pb and 210Po and total acid soluble stable Pb and sediment trap fluxes of 234Th, 210Pb and 210Po. 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. 234Th, 210Pb and 210Po 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 (??) for 234Th, 210Pb and 210Po 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 234Th relative to 238U was fairly uniformly distributed meridionally, though deficiencies were higher during Survey II when there was higher new production. Excess 210Pb relative to 226Ra was very asymmetrical with much higher excess values north of the equator. The distributions were similar for Surveys I and II. The deficiency of 210Po relative to 210Pb 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??N to 12??S. A mass balance for 210Pb was used to determine the atmospheric input of 210Pb. 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 210Pb north of the equator but there was a strong maximum at 2-3??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 210Po as a tracer for export of particulate organic matter during Survey I. Organic carbon and 210Po were highly correlated in suspended matter and sediment trap samples. Average values of organic carbon fluxes determined from the deficiencies of 210Po times the orgC/210Po ratio agreed well with those determined from the deficiencies of 234Th times the organic carbon/234Th ratio and 15N-new production, but had a much larger variability because of the more variable advection corrections. ?? 2005 Elsevier Ltd. All rights reserved.

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J.W. Murray, B. Paul, J.P. Dunne, T. Chapin. 2005. 234Th, 210Pb, 210Po and stable Pb in the central equatorial Pacific: Tracers for particle cycling. https://doi.org/10.1016/j.dsr.2005.06.016

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