CHEMICAL LEACHING METHODS AND MEASUREMENTS OF MARINE LABILE PARTICULATE FE

Dissolved phase Fe has been considered the most biologically accessible form of iron, however the particulate phase may contain important, labile reservoirs of Fe that may also be available to phytoplankton. Isotopic analysis of natural particles may help elucidate the biogeochemical cycling of Fe, although a leaching method which accesses bioavailable Fe needs to be found.

33 chemical leaches were performed on a marine sediment reference material, MESS-3. The combinations included four different acids (25% acetic acid, 0.01M HCl, 0.5M HCl, 0.1M H₂SO₄ at pH2), various redox conditions (0.02M hydroxylamine hydrochloride or 0.02M H₂O₂), three temperatures (25°C, 60°C, 90°C), and three time points (10 minutes, 2 hours, 24 hours). Leached [Fe] varied from 1mg/g to 35mg/g, with longer treatment times, stronger acids, and hotter temperatures associated with an increase in leached Fe. δ⁵⁶Fe in these leaches varied from -1.0‰ to +0.2‰. Interestingly, regardless of leaching method used, there was a very similar relationship between the [Fe] leached and the δ⁵⁶Fe of this iron. Isotopically lighter δ⁵⁶Fe values were associated with smaller [Fe] whereas isotopically heavier δ⁵⁶Fe values were associated with larger [Fe]. Two hypotheses could explain these data. Either, the particles may contain pools of isotopically light Fe that are easily accessed early in dissolution, or isotopically light Fe may be preferentially leached due to a kinetic isotope effect during dissolution.

To explore the first hypothesis, we modeled dissolution of Fe from particles assuming two separate pools, labile and refractory. The model produces a good fit to the data assuming 3mg/g of a labile Fe pool with δ⁵⁶Fe = -0.9‰ and a refractory Fe pool with δ⁵⁶Fe = +0.1‰.

If the second hypothesis is true, and there is a kinetic isotope effect during dissolution, the similar relationship between amount of Fe leached and δ⁵⁶Fe for both organic and mineral acids suggests that Fe is leached from particles via proton-promoted dissolution.

These leaches will be used to treat sediment trap material from the Cariaco Basin to further investigate the relationship between δ⁵⁶Fe and the labile, bioavailable fraction of iron particles. A leaching technique will be chosen and applied to filtered particle samples from portions of the US GEOTRACES A10 (North Atlantic) transect.