Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmpp31c1511i&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #PP31C-1511
Astronomy and Astrophysics
Astronomy
4802 Anoxic Environments (0404, 1803, 4834, 4902), 4910 Astronomical Forcing, 4924 Geochemical Tracers, 4934 Insolation Forcing, 4946 Milankovitch Theory
Scientific paper
The Permo-Triassic mass extinction is known as the largest mass extinction event in the Phanerozoic, and the Early to Middle Triassic period is characterized by the interval of delayed biotic and environmental recovery. One of the main cause of the delayed recovery is considered to have been "glong-lasted anoxia"h of deep ocean (Superanoxia; Isozaki, 1997). However, the reality of long-lasting "ganoxia"h, its maintaining mechanism, and details of the recovery process during the middle Triassic are largely unknown. In this study, we established high-resolution and continuous reconstruction of the middle Triassic bedded chert sequence that deposited in the middle of Panthalassa, and explored the possibility of the Milankovitch cycle origin of the sedimentary rhythms observed in the bedded chert sequence so as to establish the cyclostratigraphic framework for the sequence. We also examined the temporal change in the redox condition of the bottom water based on the lamina preservation index (LPI) and abundance of redox sensitive trace elements in order to reconstruct the detailed recovery process from "glong-lasted anoxia"h. The middle Triassic pelagic sequence in Inuyama area consists of bedded chert with periodical change in color. The sequence is divided into lower gray chert (lower Anisian), lower red chert (lower-middle Anisian), upper grey chert (middle-upper Anisian), and upper red chert (upper Anisian-Ladinian) units, in ascending order. The radiolarian biostratigraphy has been already established for the sequence by Sugiyama (1997). We examined cyclicities of chert and shale bed thicknesses using wavelet and spectral analyses. Previous studies suggested the possibility of Milankovitch cycle origin for the sedimentary rhythms of bedded chert. This idea was based on the observation that one chert-shale couplet represent about 20 ky duration, corresponding to precession cycle (e.g. Hori et al., 1993). However, agreement with just one periodicity of the Milankovitch cycle is insufficient as a proof for Milankovitch origin. So, wavelet analysis was conducted to demonstrate the dominant periodicities (with respect to bed numbers) for variations of the chert and shale thicknesses based on an assumption that one chert-shale couplet correspond to ca. 20 ky precession cycle. The results of spectrum analyses of chert and shale bed thickness revealed ca. 20 beds and 100 beds cyclicities that corresponds to 0.4 Myr and 2.4 Myr eccentricity cycles, respectively. In addition, ca. 20 beds cycle changes its length between 17 and 25 beds with the periodicity of about 100 beds. The amplitude of ca. 20 bed cycle is also modulated with the periodicity of ca. 100 beds. Similar types of modulations are observed in insolation curve (La04; Laskar et al., 2004). These results support the idea that the cyclicities observed in middle Triassic bedded chert sequence are controlled by Milankovitch cycle. We also reconstructed the bottom-water redox condition by the LPI based on the observation of the soft-X ray radiographs and by the trace metal abundance measured by the X-ray fluorescence. We will discuss the relationship between longer Milankovitch cycles and the bottom-water oxygenation changes in middle Triassic super-ocean Panthalassa.
Ikeda Maho
Sakuma Hiroshi
Tada Ryuji
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