Publication Date:
2019-07-17
Description:
The partitioning of Fe in sediments and soils has
traditionally been studied by applying sequential leaching
methods. These are based on reductive dissolution and exploit
differences in dissolution rates between different reactive Fe
(oxyhydr)oxide minerals. We used lab-made ferrihydrite,
goethite, hematite and magnetite spiked with 58Fe and leached
two-mineral mixtures with both phases abundant in excess of
the methods dissolution capacity. Leaching was performed
with 1) hydroxylamine-HCl and 2) Na-dithionite as the
reactive agent. Following Poulton & Canfield (2005) [1], the
first dissolution is designed to selectively leach the most
reactive Fe-phases, ferrihydrite and lepidocrocite, whereas the
second dissolution is designed to leach goethite and hematite.
Magnetite would then be dissolved in a third dissolution step
with oxalic acid.
First results show that the hydroxylamine-HCl method for
ferrihydrite dissolves only insignificant amounts of goethite
and hematite. However, magnetite-Fe constitutes about 10%
of the total dissolved Fe. The Na-dithionite dissolved Fe from
goethite-magnetite and hematite-magnetite mixtures contain
about 30% of magnetite-Fe.
We applied selective sequential leaching and Fe isotope
analysis to fine-grained marine sediments from a depocenter
in the North Sea, which contain abundant reactive Fe
(oxyhydr)oxides and show evidence for Fe sulfide formation
within the upper 10 cm. Fe isotopes of the hydroxylamine-HCl
leach targeting ferrihydrite shows a downcore increase of
!56Fe typical for sediments undergoing microbial reductive Fe
dissolution, whereas Fe isotopes of the Na-dithionite leach
(goethite and hematite) and oxalic acid leach (magnetite) are
identical and show no downcore variation in !56Fe. This
means, that only the most reactive Fe phases participate in the
Fe redox cycle in this location. The similar isotopic
composition of goethite + hematite and magnetite suggests a
detrital source, which is not utilized possibly due to the
abundant ferrihydrite and lepidocrocite present.
[1] Poulton & Canfield (2005), Chemical Geology 214, 209–
221
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
notRev
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