Ammonite zones at the base of the Jurassic are different in duration

Sinemurian

Hettangian Rhaetian

Hesselbo, S.P., Robinson, S.A., Surlyk, F., Piasecki, S., 2002. Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbations: A link to initiation of massive volcanism? Geology, v. 30, no. 3, p. 251-254.

Gradstein, Ogg & Smith, 2004. A Geologic Time Scale. Published by: Cambridge University Press

Schaltegger, U., Guex, J., Bartolini, A., Schoene, B. and Ovtcharova, M., 2007. Precise U-Page constraints for end-Triassic mass extinction, its correlation to volcanism and Hettangian post-extinction recovery. Earth and Planetary Science Letters, In Press Olsen, P.E. and Kent, D.V., 1996. Milankovitch climate forcing in the tropics of Pangaea during the Late Triassic. Palaeogeography, Palaeoclimatology, Palaeoecology, 122(1-4): 1-26.

Conclusions

High resolution proxy records (end-Triassic to first stage of Jurassic) show eccentricity, obliquity and precession forcing Duration of the Hettangian is reduced from 3.1Ma to 1.2Ma

Ammonite zones at the base of the Jurassic are different in duration

We suggest either an extended duration of volcanic activity or a different cause for the main CIE

ASTRONOMICAL TIME CONSTRAINTS FOR THE TRIASSIC- JURASSIC TRANSITION PERIOD: preliminary results

Ruhl ¹ , M.; Deenen ² , M; Krijgsman ² , W.; Kürschner ¹ , W.M. Universiteit Utrecht

1 Palaeoecology, Institute of Environmental Biology, Faculty of Science, Utrecht University

Laboratory of Palaeobotany and Palynology, Budapestlaan 4, NL-3584 CD Utrecht, the Netherlands

2 Paleomagnetic Laboratory Fort Hoofddijk, Faculty of Geosciences, Utrecht University

Introduction

The end-Triassic is one of the “Big 5” major mass extinction events of the Phanerozoic. This period and the transition to the Jurassic, is marked by major faunal extinction events in both the marine and terrestrial realms, major floral turn-overs, large perturbations of the global carbon cycle and the deposition of the Central Atlantic Magmatic Province (CAMP), the largest igneous province on earth. Timing and dura- tion of events during this important interval in earth history are subject to major debate. The duration of the Hettangian, the first stage of the Jurassic, was previously reported to be 3.1Ma (Gradstein et al., 2004). Schaltegger et al. (2007) reduced this to 2.05Ma based on U-Pb dating of volcanic ash layers close to the Hettangian stage boundaries (Utcubamba valley, northern Peru). Insights in sedimentation rate, duration of c-isotopic events, biozones etc. for this period is critical in understanding recovery processes after the major ecological distur- bance of the end-Triassic.

Results

We present high resolution δ ¹³ C _org , CaCO ₃ (weight %), TOC (%), N (%) and magnetic susceptibility records from a 120m long Triassic- Jurassic boundary record from St. Audrie’s Bay (UK). Bandwith filters of these proxy records, based on Blackman-Tukey power spectra, show periodic forcing of the data with an average period of ~578cm, ~313 & ~230cm and ~163 & ~130cm, representing 100kyr eccentricity, obliquity and precession forcing respectively. In addition, a stacked proxy-curve also records 400kyr eccentricity forcing.

Discussion

Based on this floating astronomical time-scale, we suggest a length of the Hettangian of ~1.2Ma. Ammonite zones in the Hettangian, in contrary to common view, are different in duration, with the first zone less than half the length of the second zone. The short initial Carbon Isotope Excursion (CIE) and long main CIE are suggested to be related to major volcanic activity in the transition from the Triassic to the Jurassic (Hesselbo et al., 2002). Astronomical forcing of lacustrine sediments in the Newark basin (eastern US) suggest a period of 600kyr for the duration of the volcanic activity (Olsen et al., 1996). A minimal duration of the main CIE of 1.4Ma suggests either a longer volcanic activity or an other cause for the main CIE.

Triassic Jurassic bound

ary

0 200

400 600

800 800 600 400 200 0

δ

C

[‰]

Magnetic susceptibility

CaCO

[%] TOC [%] N [%]

10 20 30 40 50 60 70 80 -31 -30 -29 -28 -27 -26 -25 -24

0.0 0.1 0.2 0.3 0.4 0.5 0 1 2 3 4 5 6 7 8 9 10

Stacked-curve

RhaetianHettangianSinemurian

Psiloceras planorbis

Alsatites liasicusSchlotheimia angulata

0 24 68 1012 14 1618 2022 2426 2830 3234 3638 4042 4446 4850 5254 56 5860 6264 66 6870 7274 7678 8082 8486 8890 9294 9698 100102 104106 108 110112 114116 118120

Triassic Jurassic

-2 -1 0 1 2 -0.06 -0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06

-0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04

-0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 0.05

100 kyr Eccentricity Obliquity Precession 100 kyr Eccentricity Obliquity Precession 100 kyr Eccentricity Obliquity Precession 100 kyr Eccentricity Obliquity Precession 400 kyr Eccentricity 100 kyr Eccentricity Obliquity Precession

Peru

1.48 - 2.62 Ma

Ashbed L-19

(199.53 +/- 0.29)

Ashbed 86

(201.58 +/- 0.28)

St. Audrie’s Bay (UK)

(this study)

0 200

400 600

800 800 600 400 200 0

Extinction interval Recovery interval

100kyr 900kyr 800kyr 700kyr 600kyr 500kyr 400kyr 300kyr

200kyr 1000kyr 1500kyr 1400kyr 1300kyr 1200kyr 1000kyr 1100kyr

main CIE

initial CIE

Period in cm Period in cm Period in cm Period in cm