Total and Particulate Thorium-234 Results From GEOTRACES Intercalibration Cruises

Particulate and total water-column Thorium-234 is extensively used to understand the distribution and export flux of carbon and many other trace elements and isotopes in the ocean. With the aim to understand and quantify the variability between different labs when measuring Thorium-234 in the ocean, samples were collected and distributed to 15 labs worldwide during Atlantic and Pacific GEOTRACES inter-calibration cruises. Samples were collected from open ocean stations at BATS (Atlantic) and SaFe (Pacific), a shelf station in Atlantic near Norfolk, Virginia and from the shallow anoxic Santa Barbara basin. In order to collect and distribute large volume particulate samples from the same depth and time, a rosette that can hold eight McLane pumps was build and deployed during the cruises.
Preliminary results from BATS shallow station indicate a good agreement between labs with a standard deviation of 13% for Quartz filters (0.8 um) and 25% for Supor filters (0.45um). This variability includes an inter-filter variability of 10% and 17% for Quartz and Supor filters respectively. Additional experiments were also conducted during both the cruises to understand how other factors impact particle collection such as pump types, flow rates, volume filtered, filter types and nominal pore sizes. Total water samples with high and low Thorium activities were collected during the Pacific cruise from 4000m at Safe station and from surface water at Santa Barbara basin. Samples were acidified and homogenized before they were sub-sampled and distributed to different labs. Inter-calibration results for total Thorium-234 will be presented as well as new particulate data from the Pacific cruise.