in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
Causes and Timing of the Triassic-Jurassic Mass Extinction and
Subsequent Early Jurassic Recovery
380
M. Ruhl 1 , M.H.L. Deenen 2 , N.R. Bonis 1 , H.A. Abels 3 , W. Krijgsman 2 ,
W.M. Kürschner 1
1. Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, NL-3584, CD, Utrecht, The Netherlands
(E-mail: m.ruhl@uu.nl)
2. Paleomagnetic Laboratory, Utrecht University, Budapestlaan 17, NL-3584 CD Utrecht, The Netherlands
3. Department of Earth Sciences, Utrecht University, Budapestlaan 4, NL-3584 CD Utrecht, The Netherlands
The Triassic-Jurassic transition (~201.5 Ma) is
regarded as one of the five major mass extinction events
of the Phanerozoic, with a terrestrial ecosystem turn-
over and 50% loss in marine biodiversity. This event is
further marked by a negative excursion in δ 13 CTOC
records. It closely coincides with a period of extensive
volcanism in the Central Atlantic Magmatic Province
(CAMP), associated with the initial break-up of Pan-
gaea. A causal relationship is however still debated.
Here we present a concise chronostratigraphic frame-
work for the T–J boundary interval and establish
detailed trans-Atlantic and marine–continental correla-
tions, by integrating astrochronology, paleomagnetism,
basalt geochemistry and geobiology, in which the
end-Triassic mass extinction in the marine realm is
directly related to the onset of CAMP basalt deposition
in continental basins (Fig.1). Our results support the
hypotheses of Phanerozoic mass extinctions resulting
from emplacement of Large Igneous Provinces (LIPs)
and provide crucial time constraints for numerical
modelling of Triassic–Jurassic climate change and
global carbon-cycle perturbations.
We show that the negative carbon isotope ex-
cursion (CIE) in marine δ 13 CTOC records represents a
global carbon perturbation. This interpretation is based
on compound specific C-isotope data of long-chain
n-alkanes derived from waxes of land plants. It shows a
~8.5‰ negative excursion coincident with the extinc-
tion interval, indicating a strong 13 C depletion of the
end-Triassic atmosphere, within 5-10 kyr. Magnitude
and rate of this C-cycle disruption can only be
explained by the injection of ~12x10 3 Gt of isotopically
depleted carbon from the methane-hydrate reservoir.
Concurrent vegetation changes reflect strong warming
and an enhanced hydrological cycle. Hence the mass
extinction event at the T-J transition is, for the first time,
mechanistically linked to massive carbon release and
associated climate change.
Strongly reduced biodiversity during the end-
Triassic mass extinction in the marine realm is
succeeded by early Jurassic recovery, with origination
of ammonite species throughout the Hettangian, the
first stage of the Jurassic. Accurate timing of events is
however still poorly constrained. We present combined
field observations and physical and chemical proxy
records, covering the uppermost Triassic and lower
Jurassic marine successions of St Audrie’s Bay and
East Quantoxhead (UK). These data have been used to
construct a floating astronomical time-scale of ~2.5
Myr in length. This time-scale is based on the
recognition of meters thick cycles in limestone and
(black) shale predominance and concurrent variability
in physical and chemical proxy records. Three to five
individual black-shale beds occur within these meters-
scale sedimentary bundles and are interpreted to reflect
precession-controlled changes in monsoon intensity,
while the bundles are interpreted as ~100-kyr
eccentricity cycles. On the basis of these findings, we
propose an astronomically constrained duration of the
Hettangian stage of 1.8 Myr in the UK and unequal
duration of Hettangian ammonite zones (P. planorbis
zone: ~250 kyr; A. liasicus zone: ~750 kyr; S. angulata
zone: ~800 kyr). Within this astronomical framework,
the extinction interval and coinciding negative CIE
represent 1 to 2 precession cycles (~20-40 kyr). The
amount of time succeeding the end-Triassic negative
Carbon Isotope Excursion (CIE) and preceding the first
Jurassic ammonite occurrence (in the UK) is con-
strained to 6 climatic precession cycles (~120 kyr).
Cyclostratigraphic correlation to the astronomically-
tuned sedimentary record of the continental Newark
basin (USA) allows to locate the stratigraphic position
of the marine defined Triassic-Jurassic and Hettangian-
Sinemurian boundary in the continental realm.
Key words: Triassic-Jurassic mass extinction;
Early Jurassic recovery; Causes and timing
References:
Deenen, et al. A new chronology for the end-
Triassic mass extinction. Earth and Planetary Science
Letters, 2010, 291 (1-4): 113-125.