25th International Meeting on Organic Geochemistry IMOG 2011

P-208
Alkenone producers during late Oligocene-early Miocene
revisited
Julien Plancq 1 , Vincent Grossi 1 , Jorijntje Henderiks 2 , Laurent Simon 1 , Emanuela Mattioli 1
1 CNRS - Université de Lyon, Villeurbanne, France, 2 Uppsala University, Uppsala, Sweden (corresponding
author:vincent.grossi@univ-lyon1.fr)
Since the nineties, alkenones have been widely
used as proxies for (paleo)oceanographic
reconstructions. In modern oceans, these long-chain
lipids are specifically produced by a few species of
coccolithophores, namely Emiliania huxleyi and
Gephyrocapsa oceanica. It is commonly assumed
that the most probable Cenozoic alkenone producers
are to be found within the family Noelaerhabdacaea,
notably the genera Reticulofenestra and
Dictyococcites (grouped as reticulofenestrids). To
date, the contribution of reticulofenestrids species to
alkenone production has never been clearly
demonstrated and has received little attention.
This study investigates ancient alkenone
producers at Deep Sea Drilling Project Site 516
during the late Oligocene-early Miocene. Studies by
Pagani et al. [1, 2] and Henderiks and Pagani [3]
showed the simultaneous presence of nannofossils
and alkenones at this site, but neither alkenone
concentrations nor absolute abundances of coccoliths
were determined.
A comparison between nannofossil speciesspecific
absolute abundances and alkenone content
in the same sedimentary samples shows, for the first
time, that the species Cyclicargolithus floridanus was
a major alkenone producer at the studied site. This
interpretation is supported by statistical analyses
(linear regressions) which show that only absolute
and relative abundances of the genus Cyclicargolithus
produce a significant and positive correlation with the
total alkenone content. However, Cyclicargolithus
abundances explain 40 % of the total variance of
alkenone concentration, suggesting that other taxa
also contributed to the alkenone production at DSDP
Site 516. Among the reticulofenestrids, the
contribution by the large Dictyococcites seems
relatively significant, whereas that of Reticulofenestra
appears questionable. This study thus suggests that,
contrary to common assumptions, Reticulofenestra
was not the most important alkenone producer
throughout the studied time interval.
On the other hand, the distribution of the different
alkenone isomers (MeC37:2, EtC38:2, and MeC38:2)
remains unchanged across the sedimentary interval
considered, which may imply that there was no
species-specific bias in alkenone molecular ratios
within the Noelaerhabdaceae Family during the late
Oligocene-early Miocene.
Our results challenge the use of alkenone-based
proxies in sediments pre-dating the first occurrence of
modern producers and underscore the importance to
carefully evaluate the likely alkenone producers in
(paleo)oceanographic studies. The implications of our
findings, especially concerning the reconstruction of
ancient partial pressure of CO2 (pCO2), will be
discussed.
References
[1] Pagani, M., Freeman K.H., Arthur M.A. (2000a),
Geochimica et Cosmochimica Acta, 64, 37-49.
[2] Pagani, M., Arthur M.A., Freeman K.H. (2000b),
Paleoceanography, 15, 486-496.
[3] Henderiks, J., Pagani M. (2007), Paleoceanography, 22,
PA3202.
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