in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
Stepwise Atmospheric Carbon Isotope Excursion During the Early
Jurassic Oceanic Anoxic Event
Stephen P. Hesselbo 1 , Grzegorz Pieñkowski 2
1. Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
(E-mail: Stephen.hesselbo@earth.ox.ac.uk )
2. Polish Geological Institute – National Research Institute, Rakowiecka 4, PL-00-975 Warszawa, Poland
(E-mail: grzegorz.pienkowski@pgi.gov.pl )
During the Mesozoic (250-64 Myr ago) geo-
logically short intervals of about 0.5 Myr were subject
to particularly severe environmental changes, including
high sea-surface temperature and very low oxygen
content of marine water. These so-called Oceanic
Anoxic Events, or OAEs, occurred simultaneously with
profound disturbance to the carbon cycle. A giant Early
Toarcian (~182 Myr ago) negative carbon-isotope
excursion, possibly the largest such anomaly in whole
Phanerozoic, has been described from marine materials,
and high-resolution datasets in marine sections have
shown that the shift to light carbon-isotope values occur
as a series of stratigraphically abrupt steps towards
lighter isotopic values. (Hermoso et al., 2009; Jenkyns
et al., 2001; Kemp et al., 2005). However, what is still
poorly known about this event is its manifestation in
non-marine and marginal marine environments. Al-
though the prominent negative carbon-isotope anomaly,
has been described from terrestrial organic matter in
fully marine deposits (Hesselbo et al., 2000; Hesselbo
et al., 2007), the same excursion towards light isotopic
values has until now been documented from only a
single marginal marine site, on the island of Bornholm,
Denmark (Hesselbo et al., 2000; Hesselbo et al., 2007;
McElwain et al., 2005) and, furthermore, the stepwise
character of the excursion has not hitherto be identified
from non-marine materials.
Fig.1 Parkoszowice core, showing Lower Toarcian Ciechocinek Formation (mudstones, heteroliths and subordinate sandstones)
and major sedimentary and geochemical events
Here we present new carbon-isotope data from
terrestrial organic matter (phytoclast separates), col-
lected from 5 fully cored boreholes (Fig. 1) and one
outcrop through a Toarcian coastal and marginal
marine sedimentary succession (Ciechocinek Forma-
tion) in the Polish Basin, a setting where hinterland
climate and sea-level change are particularly well
recorded. A comprehensive sequence stratigraphy and
inferred relative sea-level history has previously been
developed for the Early Jurassic succession in the
Polish Basin, based on sedimentological and palaeon-
tological study of boreholes and outcrops (Pieñkowski,
2004). Early Toarcian strata are assigned to a single
unconformity-bounded depositional sequence, which
has been objectively subdivided into five para-
sequences (Pieñkowski, 2004). The principal new
dataset constitutes >400 carbon-isotope analyses of
phytoclasts separated manually from palynological
preparations. The organic content consists mainly of
phytoclasts (wood fragments, cuticle) and spores and is
thus made up of material that originates almost entirely
in the terrestrial environment. All profiles analyzed
show the same well-developed excursion towards
exceptionally light isotopic values through the Early
Toarcian. The results show that the shift to light
carbon-isotope values in atmospheric carbon dioxide
also occurred in a series of major steps that match those
identified from marine materials, where they have
previously been identified with 100 kyr eccentricity
forcing of climate change. The new data are particularly
343