Sunday, August 24, 2014

Geology Sonnet 6

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.

Other geosonnets: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28


See the previous posts in this series for background.

Note that I have edited the last line of this sonnet.

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