Detection of Remarkably Low Isotopic Ratio of Iron in Anthropogenic Aerosols and Evaluation of its Contribution to the Surface Ocean

Abstract:

It has been reported that phytoplankton growth in the High Nutrient-Low Chlorophyll (HNLC) regions is limited by dissolved iron (DFe) concentration (e.g., Martin and Fitzwater, 1988). Aerosol is known as one of the dominant sources of DFe to the ocean and classified into two origins such as anthropogenic and natural. A series of recent studies showed that Fe in anthropogenic aerosols is more soluble than that in natural aerosols (Takahashi et al., 2013) and has lower isotopic ratio (Mead et al., 2013). However, the difference between Fe isotopic ratio　(δ⁵⁶Fe: [(⁵⁶Fe/⁵⁴Fe)_sample/(⁵⁶Fe/⁵⁴Fe)_IRMM-14]-1) of two origins reported in Mead et al. (2013) is not so large compared with the standard deviation. Therefore, the aim of this study is to determine Fe species and δ⁵⁶Fe in anthropogenic aerosols more accurately and to evaluate its contribution to the ocean surface.

 Iron species were determined by X-ray absorption fine structure (XAFS) analysis, while δ⁵⁶Fe in size-fractionated aerosols were measured by MC-ICP-MS (NEPTUNE Plus) after chemical separation using anion exchange resin.

 Dominant Fe species in the samples were, ferrihydrite, hematite, and biotite. It was also revealed that coarse particles contained a larger amount of biotite and that fine particles contained a larger amount of hematite, which suggested that anthropogenic aerosols were emitted during combustion processes. In addition, results of Fe isotopic ratio analysis suggested that δ⁵⁶Fe of coarse particles were around +0.25‰, whereas that of fine particles were -0.5 ∼ -2‰, which was lower than the δ⁵⁶Fe in anthropogenic aerosol by Mead et al. (2013). The size-fractionated sampling made it possible to determine the δ⁵⁶Fe in anthropogenic aerosol. Soluble component in fine particles extracted by simulated rain water also showed much lower δ⁵⁶Fe (δ⁵⁶Fe = -3.9±0.12‰), suggesting that anthropogenic Fe has much lower isotopic ratio. The remarkably low δ⁵⁶Fe may be caused by the anthropogenic combustion process. The δ⁵⁶Fe in anthropogenic aerosols measured here is important to model the budget of iron in the surface ocean.