25th International Meeting on Organic Geochemistry IMOG 2011

P-480
The 3 P’s* and the Albian oceanic anoxic event (OAE) 1b
(*palaeoclimate, palaeoenvironment & palaeooceanography)
Isabel Urbat
Fugro Robertson Limited, Llandudno, North Wales, United Kingdom (corresponding
author:isabel.urbat@fugro-robertson.com)
The Cretaceous OAE 1b is one of the major
perturbations of the global carbon cycle. It is
documented in Albian black shale deposits from the
Atlantic regions as well as in regions of the Western
Tethys. In the here presented study, high-resolution
geochemical profiles from two palaeogeographically
distinct locations (Mazagan Plateau DSDP Site 545
and the Vocontian Basin) are established. The
Mazagan Plateau was located in the lower Albian
tropical climate belt offshore Morocco and the
Vocontian Basin was situated in the subtropical Albian
climate belt in the western Tethys region.
High-resolution geochemical profiles show, that the
perturbation of the carbon cycle occurred
synchronously in the oceanic- and atmospheric
carbon reservoirs. This is indicated by the stable
isotopic composition of biomarkers for terrestrial
higher plant waxes and marine algal lipids. OAE 1b is
associated with a pronounced negative shift in various
carbon isotope reservoirs. Negative shifts in the
carbon isotopes are also observed for the Toarcian
OAE and the early Aptian OAE 1a and are thought to
result from the catastrophic release of methane from
marine gas hydrates. Climate proxies indicate a shift
towards more humid climate conditions. The effects of
this major perturbation of the climate had also
consequences for the biota. At the Mazagan Plateau
the organic matter production increased even though
the upwelling weakened probably due to an increased
influx of nutrients from the adjacent African Continent.
The bioproduction of the Vocontian Basin shifted
towards bacterial dominated communities.
Discussing the trigger mechanisms leading to the
abrupt onset and termination of OAE 1b is another
important feature of this work. The magnitude of the
isotope excursion at the Mazagan Plateau remains
constant over several ten thousand years. By
contrast, a single methane blast would strongly affect
the atmospheric signal, but the concentration of light
carbon dioxide would than constantly exhaust. Hence,
the Mazagan record rather suggests a constant
supply of 13 C-depleted CO2 during the entire black
shale interval. Therefore, a combination of moderate
scale methane pulses supplemented by continuous
methane emission at elevated levels over a longer
time interval is indicated. The two pulses encompass
the total trigger period of OAE 1b, estimated to be
about 25 ka. The results calculated with a coupled
atmosphere–land–ocean–sediment model presented
for the Mazagan Plateau reveal similarities to the
Vocontian Basin record. Nevertheless, both climate
systems reacted differently due to the perturbation at
the onset of OAE 1b. These differences can be
explained by a shifting Inter Tropical Convergence
Zone (ITCZ). Modelled net flux rates, using an
Atmospheric General Circulation Model (AGCM),
highlight precipitation and evaporation rates for the
studied sections during the Cretaceous and highlight
strong regional differences dependent on
palaeolatitudes. A moderate ITCZ shift during OAE 1b
explains the beginning humid climate conditions
coupled to alleviative tradewinds at the Mazagan
Plateau and also the enhanced climate contrasts at
the Vocontian Basin with a general trend towards
higher aridity. In summary, the OAE 1b is recorded at
several distinctive settings reflecting specific climatic
and environmental conditions. These are still
identifiable due to characteristic C-isotope curves and
temperature proxies. Furthermore, the isochronous
widespread deposition and a comparison to literature
data indicate that OAE 1b was a global rather than a
supraregional event.
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