Middle Archean volcano-hydrothermal sequence: Bacterial microfossil-bearing 3.2 Ga Dixon Island Formation, coastal Pilbara terrane, Australia

Abstract

The 3.2 Ga Dixon Island Formation in theCleaverville Group of the coastal Pilbara terrane, Australia, is one of the most complete and best-preserved examples of middle Archean oceanic stratigraphy and contains possible microbial material. Field observations and geochemical evidence suggest that this formation contains a low-temperature hydrothermal vent system with a biogenic microbial colony from the Archean ocean. The Dixon Island Formation is ~350 m thick and consists of the Rhyolite Tuff, Black Chert, and Varicolored Chert Members, in ascending order. The Rhyolite Tuff Member contains many vein swarms, such as quartz and black chert veins, and highly altered rhyolite tuff layers, which are identifi ed as an underground bypass zone for circulating hydrothermal fl uid. Many black chert vein swarms in the Rhyolite Tuff Member imply intensive low-temperature hydrothermal activity during deposition of the Black Chert Member, which is 10-15 m thick. The Black Chert Member is composed of massive black chert, laminated black chert, dark-greenish siliceous shale and tuffaceous laminated chert, which are mainly composed of very fi ne quartz. Abundant pseudomorphs of silica after aragonite, barite, and gypsum, and a distinctly continuous, stromatolite-like biomat layer (10-20 cm thick), are preserved within the laminated black chert bed. The stromatolite-like biomat bed is formed of fi ne iron or iron-coated quartz pisolite within fi ne-grained silica. The absence of detrital sediment of continental origin and the many vein injections imply that this sedimentary facies represents a pelagic hydrothermal environment at ~500-2000 m paleodepth, and may have been on the slope of an immature island arc. Microbial material has been preserved well in the lower part of Black Chert Member. The massive black chert has carbonaceous peloids (0.3-2.0 mm in diameter), which are similar to those in the black chert veins. The massive black chert contains spiral-, rod, and dendrite-shaped bacterial material. The total organic carbon (TOC) value of massive black chert in the lower part of the Black Chert Member is higher (TOC = 0.15-0.45%) than that of the overlying laminated chert section (TOC = 0.02-0.15%) and the black chert vein (TOC = 0.1-0.13%), and the carbon isotope (δ 13 C) values of this lithology (−33‰ to ~-27‰) are also lighter than for the black chert veins (-29‰ to ~-26‰) and the laminated black chert in the upper part of the Black Chert Member and the Varicolored Chert Member (−27‰ to ~-13‰). This evidence suggests that the carbonaceous grains and bacteria-shaped material in the lower part of the Black Chert Member are of biogenic origin and were formed close to a lowtemperature hydrothermal vent system. The microbial colony may have been rapidly fossilized by silicifi cation related to hydrothermal activity. Laminated black chert in the upper part of the Black Chert and the Varicolored Chert Members may have formed by cyanobacterial sedimentation from the ocean surface.

Isotopic fractionation of nitrogen and carbon in Paleoarchean cherts from Pilbara craton, Western Australia: Origin of 15N-depleted nitrogen

“…2a). It has been suggested that Dixon Island Formation represents a pelagic hydrothermal environment at $500-2000 m depth, and may have been on the slope of an immature island arc (Kiyokawa et al, 2006). The lower part of the Black Chert Member contains carbonaceous peloids and spiral-, rod-, and dendrite-shaped bacteriomorphs with d 13 C values from À27& to À32&.…”

“…The C isotopic signature suggests that the carbonaceous grains and bacteria-shaped material are of biogenic origin. They were possibly formed close to a low-temperature hydrothermal vent system promoting silicification and preservation of the organic material (Kiyokawa et al, 2006).…”

“…This member consists of, in ascending order, a $350 mthick rhyolite tuff, and black and varicolored chert members (Kiyokawa et al, 2006;Fig. 2a).…”

“…With this in mind, we analyzed a new selected subset of kerogen-rich Paleoarchean cherts (with ages from 3.2 to 3.5 Ga) from different formations of Pilbara Craton, Australia, where independent studies suggested the occurrence of chemical and isotopic signatures of an ancient microbial activity. These are the Dresser Formation at North Pole Ueno et al, 2004Ueno et al, , 2006, the Apex Basalt at the ''Schopf locality", Chinaman Creek (Schopf, 1993;Brasier et al, 2002) and the Dixon Island Formation (Kiyokawa et al, 2006). We also analyzed a barite, coeval of the chert veins of the Dresser Formation at North Pole and a dolomitic stromatolite from the Strelley Pool Chert Formation, also from North Pole.…”

Accretion, Trapping and Binding of Sediment in Archean Stromatolites—Morphological Expression of the Antiquity of Life

“…As a result of the recent questioning of the authenticity of the Earth's oldest evidence of life (Brasier et al 2004(Brasier et al , 2006, irrefutable proof for the involvement of life in the formation of Earth's oldest, Archean stromatolites has been collected (Allwood et al 2006;Kiyokawa et al 2006;Noffke et al 2006aNoffke et al , 2006bTice and Lowe 2006;Ueno et al 2006aUeno et al , 2006bLowe and Tice 2007;Schopf et al 2007a;Sugitani et al 2007). Additional evidence has been presented that the Earth's oldest (3.46 to 3.45 Ga) microfossils from the Apex Basalt chert (Schopf 1993), from the Barberton Mountain Land (Walsh 1992) and from the Dresser Formation (Ueno et al 2001) are indeed cellularly preserved remains of Archean microbial life.…”