Role of Source, Mineralogy, and Organic Complexation on Lability and Fe Isotopic Composition of Terrestrial Fe sources to the Gulf of Alaska

Iron (Fe) is a key trace nutrient supporting marine primary
production,
and its deposition in the surface ocean can impact multiple biogeochemical
cycles. Understanding Fe cycling in the subarctic is key for tracking
the fate of particulate-bound sources of oceans in a changing climate.
Recently, Fe isotope ratios have been proposed as a potential tool
to trace sources of Fe to the marine environment. Here, we investigate
the Fe isotopic composition of terrestrial sources of Fe including
glacial sediment, loess, volcanic ash, and wildfire aerosols, all
from Alaska. Results show that the δ56Fe values of
glaciofluvial silt, glacial dissolved load, volcanic ash, and wildfire
aerosols fall in a restricted range of δ56Fe values
from −0.02 to +0.12‰, in contrast to the broader range
of Fe isotopic compositions observed in loess, −0.50 to +0.13‰.
The Fe isotopic composition of the dissolved load of glacial meltwater
was consistently lighter compared to its particulate counterpart.
The ‘aging’ (exposure to environmental conditions) of
volcanic ash did not significantly fractionate the Fe isotopic composition.
The Fe isotopic composition of wildfire aerosols collected during
an active fire season in Alaska in the summer of 2019 was not significantly
fractionated from those of the average upper continental crust composition.
We find that the δ56Fe values of loess (<5 μm
fraction) were more negative (−0.32 to +0.05‰) with
respect to all samples measured here, had the highest proportion of
easily reducible Fe (5.9–59.6%), and were correlated with the
degree of chemical weathering and organic matter content. Transmission
electron spectroscopy measurements indicate an accumulation of amorphous
Fe phases in the loess. Our results indicate that Fe isotopes can
be related to Fe lability when in the presence of organic matter and
that higher organic matter content is associated with a distinctly
more negative Fe isotope signature likely due to Fe-organic complexation.