Abstract

Orbital forcing is one of the main drivers of climate change. The orbital cyclicity recorded in sediments provides the high resolution and robust astronomical time scale for geologic records and hence a clue to understanding the dynamics of Earth surface system. Numerous studies have examined cyclic bedding and its relation to astronomical cycles, primarily on carbonate sequences. In contrast, few reports have searched for orbital cycles on siliceous sequence such as bedded chert.

Bedded chert consists of centimeter-scale rhythmic alternations of chert and shale beds,which are mainly composed of biogenic silica and terrigenous material, respectively. Numerous hypotheses have been proposed for the origin of bedded chert and its sedimentary rhythms. Although previous studies have hypothesized that the origin of bedded chert is related to astronomical cycles, conclusive proof remains elusive. In addition, the meaning of paleoceanographic and paleoclimatologic changes recorded in sedimentary rhythms of bedded chert was still debated.

This thesis explores the nature and ultimate origin of the sedimentary rhythm of bedded chert. For this purpose, I first reconstructed a continuous sequence of the Lower Triassic to Lower Jurassic bedded chert in the Inuyama area, central Japan. To test the possibility that the sedimentary rhythm of bedded chert has distinct cyclicities, I measured individual chert and shale bed thickness with bed by bed throughout the sequence. The average duration of a chert-shale couplet based on biostratigraphy is 19 kyr similar to that of the precession cycle during the Triassic to Jurassic. Spectral analysis of a bed number series of chert bed thickness reveals the cyclicities of ca. 1,500, 400, 200, 100, 20, 5, and 2 beds. Then I converted the bed number series to a time series, assuming that the 20 bed cycle represents a 405 kyr eccentricity cycle, because this cycle is originated from gravitational interaction between Jupiter and Venus, and is highly stable and constant frequency due to the great mass of Jupiter. Spectral analysis of the time series revealed distinct periodicities of ca. 30,000, 8,000, 4,000, 2000, 100 and 40 kyr, in addition to 405 kyr. Besides 30,000, 8,000, 4,000, 2,000 kyr, these periodicities agree well with the 120, 95, and 37 kyr periodicities for eccentricity and obliquity cycles during the Triassic. The ca. 4,000 kyr and 2,000 kyr periodicities are correlated to present-day long-term eccentricity cycles of 4,800 and 2,400 kyr evolved by the chaotic behavior of solar planets. Collectively, these similarities in the periodicities of dominant cycles and their hierarchy in chert bed thickness with those of astronomical cycles strongly support the hypothesis that the sedimentary rhythm of bedded chert is paced by astronomical cycles.

Using one chert-shale couplet and ca. 20 bed bundle as approximately 19 kyr and constant and stable 405 kyr cycle of periodicity, I constructed the cyclostratigraphic age model for the Lower Triassic to Lower Jurassic bedded chert sequence. This age model is anchored by the end-Triassic mass extinction (ETE) zone based on the biostratigraphic correlation between the Inuyama area and shallow marine sequences in Peru, where the 201.36 ± 0.13 Ma of U-Pb age had measured within the ETE zone.

For the reconstructed sequence, I examined the meaning of paleoceanographic and paleoclimatologic changes recorded in sedimentary rhythms of bedded chert on timescales of obliquity and eccentricity cycles based on the estimation of the biogenic silica and terrigenous burial rates observed in the bedded chert. To accomplish this objective, I conducted the major element chemical analysis of individual chert and shale beds on the continuous sequence with bed-by-bed resolution. The biogenic silica and terrigenous contents were estimated, assuming the terrigenous material in the bedded chert as constant composition with the lowest SiO2 content of shale samples analyzed, which is consistent with composition of the modern terrigenous material accumulated in the pelagic ocean. I calculated weights of biogenic silica and terrigenous material accumulated as a chert-shale couplet per unit area, which are well correlated with the chert bed thickness (r = 0.96) and shale bed thickness (r = 0.90), respectively. Based on these clear correlations, I regarded the chert bed thickness as an approximate measure of biogenic silica burial rate during one precession cycle, and reconstructed the variation in the biogenic silica and terrigenous burial rates during the Early Triassic to Early Jurassic for the bedded chert sequence in the Inuyama area. The spectral analyses of time series of shale bed thickness also show periodicities similar to the obliquity and eccentricity cycles, suggesting that terrigenous burial rate changed in pace with the astronomical cycles. The range of terrigenous burial rate changes is consistent with that of the modern eolian dust burial rate in equatorial pelagic ocean. Thus, orbitally controlled eolian dust burial rate in the pelagic ocean would be the possible cause of the changes in the terrigenous burial rate for the Lower Triassic to Lower Jurassic bedded chert in obliquity and eccentricity cycles.

Together with paleogeographic distribution of bedded chert compiled from previous studies, the biogenic silica burial rate in the global ocean in the form of bedded chert was a half to several times higher than the biogenic silica burial rate in the modern ocean (DeMaster et al., 2004). This result suggests that bedded chert was the major sink of dissolved silica in the ocean at least during the Early Triassic to Early Jurassic.

If I can assume that the biogenic silica burial rate in the bedded chert in the Inuyama area varied in a manner similar to that in the global ocean on timescales longer than 19 kyr, it is possible to use chert bed thickness of Inuyama bedded chert as a potential measure for global silicate weathering intensity. Based on this idea, the biogenic silica burial rate in the form of bedded chert can be regarded as a measure of the global silicate weathering intensity during Early Triassic to Early Jurassic.

In order to validate this hypothesis, variation in the biogenic silica burial rate of bedded chert in the Inuyama area was compared with the variation in 87Sr/86Sr isotopic ratio of sea water during the Early Triassic to Early Jurassic. Assuming the constant hydrothermal flux, and the constant isotopic values of present hydrothermal source and continental source, the phase and relative amplitude of variation in 87Sr/86Sr isotopic ratio agree well with those of the variation in the biogenic silica burial rate of bedded chert in the Inuyama area filtered with 30 myr window. An exception is during the Early Jurassic, which can be explained by the increased hydrothermal flux in association with the concomitant rapid opening of the Tethys ocean. These results support the idea that the biogenic silica burial rate in the form of bedded chert as a measure of the global silicate weathering flux.

To further explore the meaning of paleoclimatologic changes recorded in the sedimentary rhythms of bedded chert, I examine the major controlling factor of the global silicate weathering intensity during the Late Triassic to Early Jurassic. Because more than 70 % of the global silicate weathering occurred in low latitude region of Pangea during the Late Triassic to Early Jurassic, according to the results of the global biogeochemical cycle model, and because the regional silicate weathering rates would be depend on the runoff and temperature based on the modern observations, precipitation variation driven by orbitally-forced summer monsoon was the potential controlling factor of the global silicate weathering intensity. To test this possibility, I examined the phase relationship between the chert bed thickness variation of Inuyama bedded chert and the lake level changes in the Newark basin in the northern low latitude region of Pangea, which is considered to reflect the summer monsoon intensity in the low latitude region. Using the end-Triassic mass extinction zone as the high resolution tie point for their sequences, the ca. 100 kyr, 405 kyr, 1,800 kyr, and 3,600 kyr cycles of the lake level changes in the Newark basin are nearly in-phase with those cycles of chert bed thickness variation in the Inuyama area. Although the provincialism of terrestrial palynoflora has been pointed out, the synchronism of the ETE zone within 150 kyr has been confirmed based on the U-Pb ages of the ETE zones of the lacustrine sequence in the Newark basin and the shallow marine sequence in Peru, where could be correlated with the ETE zone of the bedded chert sequence in the Inuyama area by radiolarian and conodont biostratigraphy. According to this U-Pb age correlation, in-phase prelationships of 1,800 kyr, and 3,600 kyr cycles between the lake level changes in the Newark basin and chert bed thickness variation of bedded chert sequence in the Inuyama area, although 100 kyr and 405 kyr cycles are within the error of 150 kyr. If the biogenic silica burial rate in the form of bedded chert was a measure of the global silicate weathering flux, the nearly in-phase relationships between the lake level changes in the Newark basin and chert bed thickness variation of Inuyama bedded chert support the possibility that the major controlling factor of the variation in the global silicate weathering was the precipitation in low latitude region of Pangea driven by the orbitally controlled summer monsoon on timescales of at least 1,800 kyr and 3,600 kyr eccentricity cycles.

The biogenic silica burial rate in the form of bedded chert will provide new insights on the nature of the global silicate weathering intensity over the orbital timescale before Mesozoic. These findings will contribute to the decoding the dynamics of the global silica cycle in the geologic time.

地球軌道要素の準周期的変動は、地球への日射量の季節・緯度変化を生み出し、表層環境変動を支配する重要な要因の一つである.このような軌道要素の準周期的変動周期に起因する気候変動は、しばしば地層の堆積リズムとして記録されており、天体力学的計算に基づいて、堆積記録に細かな時間目盛を入れる事が可能である。これまで、石灰岩については堆積リズムと天文学的周期との詳細な対応が解析された例があるが、層状チャートについては、未だ十分に解明されていない.本研究は、日本に産する層状チャートを対象にその堆積リズムと天文学的周期性との対応の解明を目指すものである。

本論文は5章からなる。第一章は序論であり、層状チャートの堆積リズムとその天文学的周期の関係、および層状チャートのグローバルシリカ循環における意義について先行研究をレビューし、本研究の目的を述べ、さらに本論文の構成を記述している。第二章では、美濃帯犬山地域に分布する下部三畳系-下部ジュラ系遠洋性層状チャートについて確立した完全連続層序を記載し、層状チャートの堆積リズムが天文学的周期に起因した可能性を検証している。本章では、先ず、天文学的周期を記録した堆積リズムの重要性、層状チャートの堆積リズムの形成メカニズムとその起源について、これまでの研究をレビューして問題点を抽出し、本研究の目的を述べている。次に、西南日本美濃帯犬山地域に産する過去の遠洋深海堆積物としての層状チャートの地質学的背景を説明し、野外調査の手法および岩型の定義の記述を行っている。さらに、複数のセクションを検討した結果、下部三畳系-下部ジュラ系層状チャートの連続的な岩相層序を復元したことを述べている。さらに、珪質部・泥質岩からなる1組の地層の平均堆積期間を推定し、それらの層厚変化が階層性をもつ天文学的周期に対応することを述べている。また、層状チャートの堆積リズムに基づいてサイクル層序の確立を試みている。

第三章では、層状チャートがもつ堆積リズムの形成メカニズムについて議論している。本章の目的を述べ、試料採取地点、分析手法、および解析手法について記述している。分析結果に基づき、生物源シリカと陸源砕屑物の濃度および両者の埋没速度変動の復元、そして、それらの周期解析を試みた上で、識別された卓越周期毎に変動振幅を記述している。これらの結果に基づき下部三畳系-下部ジュラ系の層状チャートの堆積リズムの成因を以下のように議論している。即ち、層状チャート中の陸源砕屑物の埋没速度は天文学的周期に連動する風成塵放出量変動に支配されること、またその大きな生物源シリカ埋没速度(現在の全球生物源シリカの埋没速度の半分から数倍程度)はチャート堆積が過去の海洋中の溶存シリカの主要シンクであった可能性を指摘している。また、層状チャートの生物源シリカ埋没速度の変動は、陸上でのケイ酸塩風化速度変動,とくに夏モンスーン強度変動に依存しておきた可能性を述べている。

第四章では、上述の「チャートの生物源シリカ埋没速度変動は夏モンスーン強度変動に依存する」と言う仮説を検証するために、同時代の超大陸パンゲア低緯度域に位置した北米東岸のNewark 超層群の湖水位変動と層状チャートの生物源シリカ埋没速度変動の関係について議論している。両地層の層序対比に基づき、前者の湖水位変動と後者の生物源シリカ埋没速度変動の位相関係について記述し、夏モンスーン強度変動と生物源シリカ埋没速度変動が連動した結果、層状チャートの生物源シリカ埋没速度が変動したと考察している。

第五章では、本論文の結果をまとめている。

本委員会は、論文提出者に対し、平成24 年1 月24 日に学位論文の内容および関連事項について口頭試験を行なった。本研究によって、層状チャートの堆積リズムが地球軌道要素の準周期的変動に起因したことが確認され、そのメカニズムとして、天文学的周期に連動した夏モンスーン強度変動に伴って、陸域での風化速度の変動および海洋へのシリカ供給量の変動がおこり、層状チャートの堆積リズムとして記録された可能性が指摘されたことは地質学における重要な知見であると判断し、審査委員全員一致で合格と判定した。

なお、本研究の第二章は多田隆治、佐久間広展との、第三章は多田隆治、第四章は多田隆治、Paul Olsen との共同研究であるが、いずれも論文提出者が主体となって調査と結果の解析を行なったもので、論文提出者の寄与が十分であると判断される。