Nobody studies fucking iodine.
The halogen too rare for us to care,
But iodate to carbonate’s inclined
So we might have a useful proxy there.
This IO3 requires oxygen,
And thus does not exist in reduced seas.
Its presence in old carbonates means then
Ozone and oxygen were in the breeze.
Archean carbonates do not have I,
But it appears when O first graced the air.
And thus another tool is forged, whereby
Our planet’s past can be unearthed to share.
This gas we breathe controls
the biosphere.
We’d like to know
what made it first appear.
Dalton S. Hardisty, Zunli Lu,Noah J. Planavsky, Andrey Bekker, Pascal Philippot, Xiaoli Zhou and Timothy W.Lyons (2014) An iodine record of Paleoproterozoic surface ocean oxygenation.
Geology 42 619-622.
Abstract
Constraining oxygen levels in the early Precambrian surface
ocean has been a longstanding goal, but efforts have been challenged by the
availability of suitable proxies. Here we present a novel approach, iodine
geochemistry, which broadens our perspective by providing constraints on
shallow, carbonate-dominated marine settings. Iodate (IO3–)
persists exclusively in oxic waters and is the sole iodine species incorporated
into carbonate minerals, allowing iodine-to-calcium ratios (I/Ca) in shallow
carbonates to be used as a paleoredox indicator. Our data from a series of
Mesoarchean through Paleoproterozoic carbonates deposited under shallow-marine
conditions reveal a progressive surface ocean oxygenation in the early
Paleoproterozoic. These data seem to indicate that a largely anoxic surface
ocean extended throughout the Archean until the Great Oxidation Event (GOE) at
ca. 2.4 Ga, implying that previous inferences of pre-GOE oxygen production may
reflect oxygen oases, transient oxidation events, or oxygen levels below those
required for IO3– accumulation. The data suggest
formation and persistence of IO3– and, consequently,
surface ocean oxygen concentrations of at least 1 μM during the GOE. Following
the initial rise of oxygen, carbonate-associated iodine in globally extensive
carbonate units deposited during the Lomagundi positive carbon isotope
excursion at ca. 2.22–2.1 Ga suggests a widespread aerobic iodine cycle beyond
that operating prior to the event, synchronous with high relative rates of
organic carbon burial and apparent expansion of oxidative conditions.
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