12:00 〜 12:15
[MGI28-11] ICDP-BASEプロジェクトで得られた南アフリカ・32億年前ムーディーズ層群中の縞状鉄鉱層の特異的特徴：隕石衝突の証拠とシアノバクテリアの活動との関係
キーワード：ICDP、ムーディーズ層群、隕石衝突、シアノバクテリア
Flourishment of early cyanobacteria changed the surface environments of Earth from anoxic to oxic at Archean age. On the other hand, metabolisms of cyanobacteria require manganese oxide and other oxidized molecules, which are difficult to form in anoxic environments. This rises a question how early cyanobacteria acquired oxidation species in the strictly anoxic worlds.
ICDP-BASE project successfully drilled 3.2 Ga sedimentary rocks of Moodies Group in Barberton Greenstone Belt, S. Africa. Recovered core samples contain banded iron formation deposited at 3.2 Ga coastal environments. This banded iron formation is mainly made of hematite/quartz layers. Barite (BaSO4), which is also considered as an oxidation specie, was also found in hematite/quartz layers. On the other hand, siderite (FeCO3) layers, which only formed in strictly reduced environments, underlay hematite/quartz layers. Such mineralogical change with time indicates that local sedimentary environments radically changed from anoxic to oxic.
More surprisingly, Iridium-rich layers are found in the hematite/quartz layers by micro-XRF analyses of the same BIF samples. Sandstones associated with BIFs also contain Ir-rich micro grains. Those Ir-rich materials are most likely derived from meteorite, which impacted before BIF deposition.
It is known that meteorite impacts can split water into hydrogen and oxygen, followed by oxide formations. Direct connection of meteorite impact and BIF deposition is still uncertain. But here I propose the following hypothesis:(1) temporal and local oxide-rich environments were created by meteorite impacts, (2) such oxides activated early cyanobacteria, and (3) oxygen produced from cyanobacteria were responsible for hematite-rich BIF formation even at 3.2 Ga.
ICDP-BASE project successfully drilled 3.2 Ga sedimentary rocks of Moodies Group in Barberton Greenstone Belt, S. Africa. Recovered core samples contain banded iron formation deposited at 3.2 Ga coastal environments. This banded iron formation is mainly made of hematite/quartz layers. Barite (BaSO4), which is also considered as an oxidation specie, was also found in hematite/quartz layers. On the other hand, siderite (FeCO3) layers, which only formed in strictly reduced environments, underlay hematite/quartz layers. Such mineralogical change with time indicates that local sedimentary environments radically changed from anoxic to oxic.
More surprisingly, Iridium-rich layers are found in the hematite/quartz layers by micro-XRF analyses of the same BIF samples. Sandstones associated with BIFs also contain Ir-rich micro grains. Those Ir-rich materials are most likely derived from meteorite, which impacted before BIF deposition.
It is known that meteorite impacts can split water into hydrogen and oxygen, followed by oxide formations. Direct connection of meteorite impact and BIF deposition is still uncertain. But here I propose the following hypothesis:(1) temporal and local oxide-rich environments were created by meteorite impacts, (2) such oxides activated early cyanobacteria, and (3) oxygen produced from cyanobacteria were responsible for hematite-rich BIF formation even at 3.2 Ga.