Abstract
Iron speciation is one of the most mature and widely
applied proxies to understand ancient oxygen levels and
redox conditions in past aqueous environments. The iron
speciation proxy estimates proportions of different reactive
iron species in fine-grained sedimentary rocks which are
mapped to redox conditions based on empirical calibrations
from modern sediments. The proxy is based on a standardized
extraction technique of sequentially applying acetate,
hydroxlamine-HCl (in some cases), dithionite, and oxalate
solutions to a powdered sample in order to dissolve iron
phases and quantify the amount of iron carried by carbonates,
“easily reducible” oxyhyroxides, ferric iron (oxyhyr)oxides,
and magnetite, respectively. Although tested on pure
minerals, fundamental technique development checks on
whether this sequential extraction process accurately
dissolves the targeted minerals in natural sediments and
sedimentary rocks have not been performed. Our study
applied rock magnetic experiments to sediment and shale
samples dating from the Holocene to 1.5 Ga with diverse total
iron contents, different proportions of iron from each iron
speciation extraction, and distinct redox interpretations. Our
approach was to characterize bulk powders as well as residues
taken after each sequential extraction step in order to identify
and quantify the ferromagnetic minerals that were dissolved.
These data reveal distinct changes associated with the
sequential extractions and the leaching of different magnetic
minerals. In sedimentary rock samples, the acetate extraction
at least partially dissolved siderite and paramagnetic phases,
possibly other iron-bearing carbonates. The dithionite
extraction appears to be the most robust in removing the
targeted mineralogy as the data show it to effectively
solubilize nearly all of the goethite. However, magnetic
quantification of magnetite was orders of magnitude less than
the iron measured in the oxalate extraction, suggesting that
this step also dissolved iron-bearing silicates or remaining
iron-bearing carbonates. Understanding this disparity is vital
for holistic multi-proxy interpretation of past oxygen levels
and communication between disciplines.
applied proxies to understand ancient oxygen levels and
redox conditions in past aqueous environments. The iron
speciation proxy estimates proportions of different reactive
iron species in fine-grained sedimentary rocks which are
mapped to redox conditions based on empirical calibrations
from modern sediments. The proxy is based on a standardized
extraction technique of sequentially applying acetate,
hydroxlamine-HCl (in some cases), dithionite, and oxalate
solutions to a powdered sample in order to dissolve iron
phases and quantify the amount of iron carried by carbonates,
“easily reducible” oxyhyroxides, ferric iron (oxyhyr)oxides,
and magnetite, respectively. Although tested on pure
minerals, fundamental technique development checks on
whether this sequential extraction process accurately
dissolves the targeted minerals in natural sediments and
sedimentary rocks have not been performed. Our study
applied rock magnetic experiments to sediment and shale
samples dating from the Holocene to 1.5 Ga with diverse total
iron contents, different proportions of iron from each iron
speciation extraction, and distinct redox interpretations. Our
approach was to characterize bulk powders as well as residues
taken after each sequential extraction step in order to identify
and quantify the ferromagnetic minerals that were dissolved.
These data reveal distinct changes associated with the
sequential extractions and the leaching of different magnetic
minerals. In sedimentary rock samples, the acetate extraction
at least partially dissolved siderite and paramagnetic phases,
possibly other iron-bearing carbonates. The dithionite
extraction appears to be the most robust in removing the
targeted mineralogy as the data show it to effectively
solubilize nearly all of the goethite. However, magnetic
quantification of magnetite was orders of magnitude less than
the iron measured in the oxalate extraction, suggesting that
this step also dissolved iron-bearing silicates or remaining
iron-bearing carbonates. Understanding this disparity is vital
for holistic multi-proxy interpretation of past oxygen levels
and communication between disciplines.
| Original language | English |
|---|---|
| Publication status | Published - 17 Aug 2018 |
| Event | Goldschmidt 2018 - Boston, United States Duration: 12 Aug 2018 → … |
Conference
| Conference | Goldschmidt 2018 |
|---|---|
| Country/Territory | United States |
| City | Boston |
| Period | 12/08/18 → … |