25th International Meeting on Organic Geochemistry IMOG 2011

P-468
Carbon isotopic composition of Thaumarchaeotal ether-bound
biphytanes during the early Eocene carbon isotope excursions
Petra Schoon 1 , Appy Sluijs 2 , Henk Brinkhuis 2 , Claus Heilmann-Clausen 3 , Bo Pagh
Schultz 4 , Jaap Sinninghe Damsté 1 , Stefan Schouten 1
1 NIOZ Royal Netherlands Institute for Sea Research, Department of Organic Biogeochemistry, 't Horntje,
Texel, Netherlands, 2 Biomarine Sciences, Institute of Environmental Biology, Faculty of Science, Utrecht
University, Utrecht, Netherlands, 3 Aarhus Universitet, Geologisk Institut, Aarhus, Denmark, 4 Fur Museum,
Fur, Denmark (corresponding author:petra.schoon@nioz.nl)
Marine non-thermophilic archaea (recently
reclassified within the new phylum Thaumarchaeota
[1]) are important organisms among present day
marine microbes. Their membrane lipids, analyzed as
glycerol dialkyl glycerol tetraethers (GDGTs) by
means of HPLC, or as biphytane moieties after
laboratory ether-bond cleavage with HI/LIAlH4, are
ubiquitous, and are found in recent and ancient
sediments [2]. Crenarchaeol is thought to be a
specific GDGT for Thaumarchaeota and consists of
two biphytanes, one with two cyclopentyl moieties and
the other with an additional cyclohexyl moiety (c and
d; Fig. 1A). Stable carbon isotopic measurements of
these bihytanes have revealed a remarkably
consistent � 13 C value of ca. 20-22 ‰ [2]. As archaea
utilize dissolved bicarbonate as carbon source [3],
variations in � 13 C of the biphytanes may thus reflect
variations in � 13 C of DIC. Indeed, this can probably
explain the enrichment in biphytane 13 C content
during the Cretaceous anoxic event 1b [3]. However,
the controlling factors on the stable carbon isotopic
composition of crenarchaeol are not well known.
Here we report the results of � 13 C analyses of
biphytanes from crenarchaeol, and other GDGTs, in
sediments deposited during two Eocene
hyperthermals. The Paleocene-Eocene Thermal
Maximum (PETM; ~55 Ma) and Eocene Thermal
Maximum 2 (ETM2; ~53 Ma) are geological short
episodes of extreme global warming and
environmental change. Furthermore, both events are
characterized by a negative carbon isotope excursion
(CIE) of several ‰, induced by the injection of 13 Cdepleted
carbon in the ocean-atmosphere system.
However, among the various carbon reservoirs, such
as marine and terrestrial carbonates, the magnitude
of the CIEs vary, making it difficult to constrain the
actual CIE and thereby the amount of carbon that was
involved during these hyperthermals. Our � 13 C results
of sediments deposited in the Arctic Ocean showed
that at maximum of the CIE of ETM2, crenarchaeol
was ~2.5 ‰ more negative than prior to the event.
However, this � 13 C negative shift of the biphytanes
coincided with peak concentrations of derivatives of
the specific biomarker isorenieratene and during
cooler sea water temperatures inferred from TEX86.
This may imply that the Thaumarchaeota responded
to the arising euxinic conditions at this location by
migrating to below the chemocline. It is therefore not
certain whether this 2.5 ‰ truly reflects the CIE in
DIC. In addition, changing climate and oceanography
may have altered the � 13 C of DIC differently in this
relatively restricted basin than in the global ocean.
Currently we are analyzing the � 13 C composition of
GDGTs in sediments deposited during the PETM, at
three off-shore sections in Denmark. This may
provide the first continuous � 13 C DIC record across an
Eocene hyperthermal and may provide knowledge on
the CIE in the DIC pool during the PETM.
Fig. 1. Structure of acyclic and cyclic biphytanes (A) and
their distribution (B) in a sediment deposited during ETM2
with their corresponding � 13 C values.
References
[1] Spang et al. (2010), Trends Microbiol 18, pp. 331–340.
[2] Schouten et al. (1998), Org. Geochem. 29, pp. 1305-
1319.
[3] Kuypers et al. (2001), Science 293, pp. 92-94.
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