Does adsorption of dissolved organic carbon and thorium onto membrane filters affect the carbon to thorium ratios, a primary parameter in estimating export carbon flux?

Abstract

The particulate organic carbon (POC) to 234Th ratio, or POC/234Th, is crucial to constrain the 234Th-derived downward export flux. Marine particles for 234Th based export studies are typically collected via two sampling modes: filtration of Niskin or GO-FLO bottle water samples and filtration of in situ pumped samples. Large discrepancies have been frequently observed between these bottle and pump filtration methods and attributed to the difference in adsorption of dissolved material. We noted however, that simultaneous measurements of carbon and 234Th have not been made to evaluate such a putative effect on the POC/234Th ratio. In this study, we adopted a “two filters in-line” approach method with one stacked on another and we measured both carbon and 234Th to compare their adsorption behavior in bottle and pump sampling modes. We proposed that the 234Th and carbon on the second filter (filter-B) might be attributable to direct adsorption from the dissolved phase (including the partial capture from the submicron particles) and/or breakdown of particles initially collected on the first filter (filter-A). The relative importance of these two processes differed between the two filtration modes. For bottle sampling, we observed significantly positive correlation (R2=0.54) between the 234Th recorded on the second filter and dissolved 234Th activities, suggesting that adsorption was the dominant contributor to the second filter. The overestimation due to adsorption (filter-B/filter-A) is estimated to be 22±7% for POC and 25±17% for particulate 234Th. The effect of adsorption on the POC/234Th, on the other hand, would be negligible since the adsorption-corrected and -uncorrected POC/234Th on filter-A were similar. For the large volume pumping mode, the absorbed 234Th on the filters appeared to reach the saturation state as evidenced by the fact that the amount of particulate 234Th (dpmcm−2) on filter-B was independent of the filtered sample volume. We justified that the saturation amount should be >0.21dpmcm−2 which was the highest unsaturated value from the bottle sample, but <0.31dpmcm−2 which was the upper value on filter-B taken from pump samples. The calculated adsorption still dominated for 234Th on filter-B and particle breakdown might be important in the euphotic zone. The collected POC on filter-B tended to increase with the filtration volume which implied that DOC adsorption remained under-saturated. Similar to bottle filtration, the corrected POC/234Th was comparable to the uncorrected POC/234Th on filter-A. We thus conclude that adsorption did not appear to influence the POC/234Th ratio in either sampling mode. We suggested that the positive offset in POC/234Th between bottle and pump sampling, if any, cannot be caused by adsorption. Other mechanisms such as zooplankton invasion and/or artificial particle formation need to be examined in order to resolve the discrepancy.