25th International Meeting on Organic Geochemistry IMOG 2011

P-194
Palaeotemperatures and palaeoenvironmental changes during
OAE 1a at Shatsky Rise
Simon Brassell
Indiana University, Bloomington, United States of America (corresponding author:simon@indiana.edu)
Validation of the temperature dependence in the
natural distributions of select molecular components,
such as alkenones [1] and glycerol dialkyl glycerol
tetraethers (GDGTs; [2]), has proven the potential and
utility of biomarkers as palaeotemperature proxies
and demonstrated that such measures can contribute
to the description of ancient climate change. In
addition, stratigraphic variations in the abundances or
compositions of other geochemical properties, both
molecular and elemental, can provide evidence of
biological responses to temperature fluctuations and
trends and their relationship to palaeoenvironmental
conditions, notably the levels of oxygenation (oxic,
dysoxic, anoxic, euxinic) of the depositional setting.
The potential influence on biomarker abundances
of changes in ocean temperatures accompanying
stratification and anoxicity is illustrated by the
observed higher concentrations of isorenieratane and
chlorobactane in sediments corresponding to the
Cenomanian-Turonian oceanic anoxic event (OAE2)
during warmer episodes [3]. It is therefore pertinent to
examine whether similar phenomena are features of
other OAE [4]. The presence of two cooler intervals
based on TEX86 measurements [5] within the organicrich
sediment sequence from Shatsky Rise (ODP Site
1207) that corresponds to OAE1a (early Aptian; [6])
affords the opportunity for such assessment.
The major variations in biomarkers and other
geochemical proxies observed through this OAE1a
interval include: (i) decreasing concentrations of 2methylhopanes
derived from cyanobacteria [7,8] with
increasing temperature (Fig. 1), (ii) a correspondence
of the lycopane index [9] with Mo concentrations [10],
which both reflect anoxic conditions that appear more
prevalent during warmer intervals, and (iii) fluctuations
in V concentrations that reflect temperature trends,
and correspond closely with organic C contents,
except where Corg >30%. These results suggest that
changes in ocean temperatures during OAE exert an
influence on surface ocean conditions that govern
seasonal productivity and stratification, but remain
independent from the primary controls on anoxia.
References
[1] Brassell et al., Nature, 320, 129-133 (1986)
OAE1a
ODP Site 1207
Shatsky Rise
Fig. 1: Plot of values for methylhopane index [7]
versus TEX86 temperatures
[2] Schouten et al., Earth Planetary Sci. Letts., 204,
265-274 (2002)
[3] van Bentum et al., Palaeogeog., Palaeoclimatol.,
Palaeoecol., 280, 489-498 (2009)
[4] Jenkyns, Geochem., Geophys., Geosystems, 11
10.1029/2009GC002788 (2010)
[5] Dumitrescu et al., Geology, 34, 833-836 (2006)
[6] Dumitrescu & Brassell, Palaeogeog., Palaeoclimatol.,
Palaeoecol., 235, 168-191 (2006)
[7] Summons et al., Nature, 400, 554-557 (1999)
[8] Dumitrescu & Brassell, Org. Geochem., 36,
1002-1022 (2005)
[9] Sinninghe Damsté et al., Earth Planetary Sci.
Letts., 209, 215-226 (2003)
[10] Lyons et al., Ann. Rev. Earth Planetary Sci., 37,
507-534 (2009)
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