世界最高の解像度による白亜紀OAE2の古気候解析

フォーマット:

論文

責任表示:

長谷川, 卓 ; Hasegawa, Takashi

言語:

日本語

出版情報:

金沢大学自然科学研究科, 2009-03

著者名:

• 長谷川, 卓
• Hasegawa, Takashi

掲載情報:

平成19(2007)年度 科学研究費補助金 基盤研究(B) 研究成果報告書 = 2007 Fiscal Year Final Research Report

巻:

2006-2007

開始ページ:

15p.

バージョン:

author

概要:

白亜紀の海洋無酸素事変2(OAE2)は,世界で最も研究されているOAEであるが,本研究では前弧海盆堆積物である蝦夷層群堆積岩を用いて,世界最高の解像度での古環境解読を目指した.研究地において数千年の解像度で炭素同位体比(δ^<13>C)層序を構築し.OAE2研究の基準となっている英国の地層で認められるδ^<13>C変動区分を全て確認した.この成果によりOAE2期間中,欧米で最も深刻な酸素欠乏が記録され, CO2の大気海洋からの除 … 去による寒冷化が記録されている範囲を蝦夷層群中に特定することに成功した.その範囲(〜70m)の泥岩をエンジンカッターで連続採集し,1/1スケッチを取り,1/5柱状図を作成した.これにより再堆積,生物擾乱混入物を分析対象から除去し,また層序ギャップを評価し,数百年という超高解像度解析を実現した.欧米と調和的なδ^<13>C変動の中に,本研究特有のイベントが見られた(NEMOイベント).2.7‰に及ぶ負エクスカーションである. NEMOイベントを境として,下位では数百〜数千年程度の範囲ではδ^<13>Cは安定するが,上位ではばらつく.これは地球表層の炭素循環の変革を示唆している可能性があり,炭素循環の再構築とNEMOイベントが関連している可能性がある.この解像度と対比精度を土台として浮遊性微化石とバイオマーカーを用いた古環境の解読を進めた. OAE2の開始に伴い,浮遊性有孔虫全体が堆積物から姿を消す現象を2度に確認した.100km以上離れた2地域から確認しているので,露頭条件に左右されて化石の保存性が変わるために産出/不産出が交互するのではない.バイオマーカーでは,陸上植物由来分子の量比が変化しているので,背後の植物相に大きな変化が生じた可能性が示された.これら古環境分析の層序学的解像度は,δ^<13>Cの高い解像度には追いついていない.今後,層準を絞って解析を進める.<br />Cretaceous Oceanic Anoxic Event 2 (OAE2) is one of the most well studied oceanic anoxic event through the geologic history. Objective of this study is to decode paleoenvirorment in northwestern Pacific during the OAE2 with world-highest resolution.We firstly establish carbon isotope (δ^<13>C) stratigraphy in a resolution of several kyr through the Yezo Group sequence in the Tappu area (Tappu section) and compared it with world reference section in UK. All phases of δ^<13>C event through OAE2 observed in the reference section was found in the Tappu section. Based on this result, the interval that records strongest oxygen deficiency and associated cooling derived from CO_2 withdrawal from ocean-atmosphere system recognized in European sections was correlated and identified in the Tappu section. Then we performed detailed sampling through the interval (about 70m thick). Nearly continuous samples were cut out from river floor using engine cutter and full-scaled sketches were taken from the m, then 1/5 geologic column was compiled. Such description enables us to exclude material of re-deposition or considerable bioturbation when collecting sub-samples for analyses. Ultra-high resolution sampling was then successfully performed with time interval of several hundreds years.Carbon isotope stratigraphy of the Tappu section shows generally comparable pattern with that of the reference section in UK ; however, conspicuous 2.7‰ negative excursion that was peculiar to our section was observed and named as NEMO event. The sequence below the NEMO showsδ^<13>C pattern with little sample-to-sample variation whereas theδ^<13>C pattern above the NEMO apparently have larger variation within several adjacent samples. This potentially suggests re-establishment of surface earth carbon circulation (appearance of kyr-order fluctuation). NEMO may be a key event to elucidate such re-establishment of carbon circulation.Based on the resolution and the accuracy of correlation with the reference section, we analyzed plankton microfossils and biomarkers for paleoenvironmental reconstruction. Twice disappearance of entire planktonic foraminifera from sediments was observed associated with the early phase of OAE2. These biotic events were not resulted from secondary effect such as dissolution, for identical events were also observed in the Oyubari area, more than 100km away from the Tappu section. Biomarkers show a remarkable change in relative abundance of terrestrial molecules. It may suggest floral turnover over the provenance of the Yezo Group. At this moment, time-stratigraphic resolution of these paleoenvirormertal analyses has not caught up with the world-highest resolution ofδ^<13>C stratigraphy. We keep working through a strategy of zooming-in (selecting several inportant intervals as NEMO and perform concentrated analyses) to achieve ultra-high resolution analyses of the paleoenvironment during OAE2.<br />研究課題/領域番号:18340164, 研究期間(年度):2006-2007<br />出典：「世界最高の解像度による白亜紀OAE2の古気候解析」研究成果報告書　課題番号18340164 (KAKEN：科学研究費助成事業データベース（国立情報学研究所）)　　　本文データは著者版報告書より作成 続きを見る

URL:

http://hdl.handle.net/2297/00049418