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Isorenieratane: a biomarker for photic zone euxinia?

The Basics

Isorenieratane is an aryl isoprenoid (i.e. contains at least one aromatic ring and an isoprenoid chain). Isorenieratane is diaromatic (two aromatic rings), contains 40 carbons and is synthesised by Chlorobiaceae (a type of brown-pigmented green sulfur bacteria). It also has a 2,3,6 trimethyl substitution pattern if your into that sort of stuff.

Isorenieratene.

In modern settings, Chlorobiaceae are found in stratified lakes and silled fjords. We do not find them in the open ocean*. What I find interesting is that Chlorobiaceae require both light and hydrogen sulfide to survive. This means that Chlorobiaceae are anoxygenic phototrophs. They do not need oxygen.

Therefore, Chlorobiaceae = Isorenieratene = sulfidic (or euxinic) conditions.

Application to the past

Why would a palaeoclimatologist care about this weird compound? Let’s use the Cretaceous (145-65Ma) as an example. The Cretaceous was warm. Very warm. The tropics were warm (>34°C) and the poles were warm (>20°C). In summary, it was warm.

During the Cretaceous, there were at least five oceanic anoxic events (OAEs). These events were associated with widespread ocean anoxia and massive organic carbon burial. During these events, we find lots of Isorenieratane. Therefore, Cretaceous OAEs were assosicated with periods of photic zone euxinia. And I think that is pretty neat.

Caveats

If you find Isorenieratene it does not always mean photic zone euxinia. For example, Isorenieratene can be found in microbial mats. Other problems include the potential formation of β-Isorenieratene from β-carotene, a photosynthetic pigment that requires light and oxygen (note: not hydrogen sulfide) and the difficulty of getting compound specific carbon isotope measurements (δ13C).

Summary

  • Isorenieratene is a photosynthetic pigment which is made by green sulfur bacteria.
  • These bacteria require light and hyrogen sulfide.
  • If we find Isorenieratene in ancient sediments, we have strong evidence for photic zone euxinia.

———-

* except for the Black Sea which is crazy anyway

References: see Brocks et al. (2008: GCA), Summons and Powell (1986, Nature) and Koopmans et al. (1996) for more detail. Most of this blog is based upon the aforementioned source material.

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