Possible and Demonstrated Connections Between Extraterrestrial Impacts and Habitats of Life

Of the wide range of physical environmental controls over recorded habitats of stromatolite-like, probable stromatolites and confirmed stromatolite colonies

Stilpnomelane

Fig. 8.8. Approximately 2.47-2.50-Ga Dales Gorge Iron Member S4 Macroband (DGS4) impact fallout unit. (A) stilpnomelane-dominated tuff and siltstone (SS) overlain by a 20-cm-thick microkrystite spherule unit (MKR), overlain by siderite chert (SC), Fortescue Falls; (B) Boulder of banded chert (CB) incorporated in the upper part of DGS4, resting in part on spherule-bearing stilpnomelane matrix (MKR) containing chert fragments. (C) Elongate and oval microkrystite spherules consisting of stilpnomelane (st) mantled by K-feldspar (Kf), plane polarized light; (D) scanning electron microscope image showing a K-feldspar rim bordering stilp-nomelane spherule (st). The K-feldspar (Kf), showing lower electron contrast, contains ilmenite microlites (Il) and submicron particles of NiS.

Fig. 8.8. Approximately 2.47-2.50-Ga Dales Gorge Iron Member S4 Macroband (DGS4) impact fallout unit. (A) stilpnomelane-dominated tuff and siltstone (SS) overlain by a 20-cm-thick microkrystite spherule unit (MKR), overlain by siderite chert (SC), Fortescue Falls; (B) Boulder of banded chert (CB) incorporated in the upper part of DGS4, resting in part on spherule-bearing stilpnomelane matrix (MKR) containing chert fragments. (C) Elongate and oval microkrystite spherules consisting of stilpnomelane (st) mantled by K-feldspar (Kf), plane polarized light; (D) scanning electron microscope image showing a K-feldspar rim bordering stilp-nomelane spherule (st). The K-feldspar (Kf), showing lower electron contrast, contains ilmenite microlites (Il) and submicron particles of NiS.

in the Pilbara Craton, volcanic and hydrothermal factors are the most manifest, whereas the role of asteroid and comet impacts is more difficult to evaluate due to (1) the paucity of the impact and palaeontological records; (2) the mm scale of microkrystite spherules and of microtektites renders these diagnostic entities difficult to recognize in the field; and (3) the effect of large impact in modifying tectonic environments and triggering volcanic and hydrothermal activity is inherently difficult to ascertain, in view of the preservation worldwide of only a small fraction of well-preserved and exposed Archaean supracrustal volcanic-sedimentary assemblages. However, several lines of evidence point to direct, indirect, and, in some instances, potentially critical controls of the environments in which Archaean and early Proterozoic microorganisms survived extraterrestrial impacts, as in the following examples from the Pilbara and Kaapvaal Cratons.

The appearance of undulating to dome-shaped stromatolite-like structures of possible, yet unproven, biogenic origin in the ~3.49-Ga Dresser Formation (Figs. 8.9 and 8.10 A), Pilbara Craton, intercalated with black chert, sili-cified arenite and carbonate (Buick et al., 1981; Dunlop and Buick, 1981; Grove et al., 1981) (Fig. 8.10 B), is characterized by its intimate intercala-

Pilbara Stratigraphic Collumn

Fig. 8.9. Schematic columnar stratigraphic section through the Warrawoona Group in the eastern part of the North Pole dome, between the Dresser Formation and the Strelley Pool Chert. Schematic stratigraphic column of the upper Warrawoona Group in the eastern part of the North Pole dome, Pilbara Craton, northwestern Australia, including the Dresser Formation, Mount Ada Basalt, Antarctic Creek Chert Member, Apex Basalt and Panorama Formation. Spherule-bearing Antarctic Creek Chert Member in the eastern North Pole dome area. TB, tholeiitic basalt; HMB, high-Mg basalt. Thicknesses in the left-hand column after Van Kranendonk (2000) and in the right-hand column based on the present study.

Fig. 8.9. Schematic columnar stratigraphic section through the Warrawoona Group in the eastern part of the North Pole dome, between the Dresser Formation and the Strelley Pool Chert. Schematic stratigraphic column of the upper Warrawoona Group in the eastern part of the North Pole dome, Pilbara Craton, northwestern Australia, including the Dresser Formation, Mount Ada Basalt, Antarctic Creek Chert Member, Apex Basalt and Panorama Formation. Spherule-bearing Antarctic Creek Chert Member in the eastern North Pole dome area. TB, tholeiitic basalt; HMB, high-Mg basalt. Thicknesses in the left-hand column after Van Kranendonk (2000) and in the right-hand column based on the present study.

tion with and injection by stratiform barite, which also occurs in barite-chert veins (T-chert) underlying these units. These commonly brecciated veins contain similar assemblages as the stromatolite-bearing units and are considered hydrothermal feeders of the latter (Farmer, 2000). Barium is typically highly enriched in felsic volcanics of the —3.49-3.43-Ga Warrawoona Group, for example in the Duffer Formation (Glikson and Hickman, 1981), representing late hydrothermal activity in felsic volcanic centers. Mafic volcanic units that overlie the Dresser Formation - the Mount Ada Basalt and Apex Basalt (Van Kranendonk, 2000), comprise many kilometer-thick sequences of pillowed basalt, suggesting rapid subaqueous foundering of the sea bed, allowing no foothold for stromatolite development. Intervals between volcanic activity and corresponding subsidence are represented by numerous chert units, signifying mainly colloidal silica precipitation accompanied by minor sedimentary fluxes. At least two of these intercalations, located between the Mount Ada Basalt and the Apex Basalt, includes the 3.47-Ga impact fallout microkrystite spherules described above (Fig. 8.3). The impact fallout unit is succeeded by a unique jaspilite chert unit correlated with the Marble Bar Chert - a distinct stratigraphic marker in the eastern Pilbara.

Fig. 8.10A,B. Pilbara stromatolite-like structures, probable stromatolites, and possible microorganisms. (A) 3.49-Ga carbonate-chert stromatolite-like structure intercalated with barite, Dresser Formation, North Pole dome, central Pilbara; (B) carbonate-chert stromatolite-like unit intercalated with barite, Dresser Formation;

Fig. 8.10A,B. Pilbara stromatolite-like structures, probable stromatolites, and possible microorganisms. (A) 3.49-Ga carbonate-chert stromatolite-like structure intercalated with barite, Dresser Formation, North Pole dome, central Pilbara; (B) carbonate-chert stromatolite-like unit intercalated with barite, Dresser Formation;

Stromatolite-like structures reappear in the wake of a major felsic volcanic phase represented by the 3.434-3.426-Ga Panorama Formation, and are located in the unconformably overlying Strelley Pool Chert, associated with chert, arenite and barite (Fig. 8.10.C,D). The biogenicity of these stromatolites remains the subject of current debate (Hoffman et al., 1999; Brasier et al., 2004).

Fig. 8.10C,D. (C) 3.43-Ga Strelley Pool Chert silicified carbonate stromatolite-like structures - planar bedding plane view; (D) Strelley Pool Chert stromatolite-like structures - cross-sectional view, showing branching morphologies.

The fundamental change in crustal environment associated with the 3.26-3.225-Ga impact cluster is succeeded in the Pilbara by an assemblage of tur-bidites, felsic volcanics, and black shales that contain bundles of tubular microbial remains reminiscent of Archaeon Pyridictium (Rieger et al., 1995) from marine hydrothermal environments (Fig. 8.10.G) (M.V. Glikson and L. Duck, personal communication, 2004). It follows that, despite the intense bombardment during the ~3.2-Ga period, extremophile and thermophile microorganisms survived, probably in deep fractures and faults of the "deep hot biosphere" of Gold (1999).

Stromatolite colonies associated with felsic tuffs of the ~2.73-Ga Tumbiana Formation display evolved morphologies (Fig. 8.10 H), where cabbage-like individual bioherms separated by well-defined debris-filled collars can reach meter-scale dimensions. Yet larger stromatolites up to several tens of meter

Fig. 8.10E. (E) Strelley Pool Chert stromatolite-like structures, cross-sectional view.

Fig. 8.10F. (F) Strelley Pool Chert carbonate with silicified veins and possible molds of anhydrite or barite.

Fig. 8.10G. (G) Tubular bundles of microbial remains in black shale of the ~3.24-Ga Pincunah Formation, Sulphur Springs Group (L. Duck and M. Glikson, personal communication, 2004), reminiscent of the Archaeon Pyridictium reported by Rieger et al. (1995) from marine hydrothermal environments. Microbial remains were isolated from organic matter concentrated from black shales of the Pincunah Formation, Sulphur Springs, central Pilbara Craton, Western Australia.

Fig. 8.10G. (G) Tubular bundles of microbial remains in black shale of the ~3.24-Ga Pincunah Formation, Sulphur Springs Group (L. Duck and M. Glikson, personal communication, 2004), reminiscent of the Archaeon Pyridictium reported by Rieger et al. (1995) from marine hydrothermal environments. Microbial remains were isolated from organic matter concentrated from black shales of the Pincunah Formation, Sulphur Springs, central Pilbara Craton, Western Australia.

across occur in the ~2.56-2.54-Ga Carawine Dolomite, where the stromatolites are located stratigraphically higher than the tsunami-generated impact spherule-bearing megabreccia (SBMB) (Fig. 8.7).

Direct potential relations between impacts and biogenic relations occur between the >2.63-Ga Jeerinah Impact Layer and the immediately overlying banded iron formation of the Marra Mamba Iron Member (Fig. 8.5). In so far as a bacterial origin may be attributed to banded iron formations (Morris, 1993; Konhauser et al., 2002), the location of banded iron formation and ferruginous sediments above impact fallout units, also observed in the Antarctic Chert Member and above the ~3.24-Ga impact cluster, may signify Fe enrichment of the hydrosphere following volcanic and hydrothermal input triggered by the impacts.

The increasing diverse nature of marine microplankton observed from the latest Proterozoic (Vendian), allowing detailed speciation, and preservation of amino acids and noble gas species in unmetamorphosed Phanerozoic sediments, opens the way for possible cometary contributions (not yet observed in older sequences). Examples are the identification of extinction/radiation

Fig. 8.10H,I. (H) 2.73-Ga carbonate stromatolites of the Tumbiana Formation, Meentheena area; (I) 2.56-2.54-Ga stromatolites in the Carawine Dolomite, Woodie Woodie area, eastern Hamersley Basin.

relations across the a. 580 Ma Acraman impact boundary in South Australia and Western Australia (Grey, 2004) and a number of major impact related, or possibly impact related, extinctions during the Phanerozoic, including the K-T boundary (Alvarez, 1986). The discovery of racemic AIB (a-amino isobutyric acid) and Isovaline halo associated with K-T boundary horizon at Stevns Klint (Zhao and Bada, 1989; Zahnle and Grinspoon, 1990) (Fig. 8.11 A) and anomalous extraterrestrial 3He flux displayed by the late Eocene section at Massignano, Italy (Farley et al., 1998), allows identification

Fig. 8.11. Cometary components associated with impact boundaries. (A) Relations between AIB (a-amino isobutyric acid) and Ir across the K-T boundary at Stevns Klint (from Zhao and Bada, 1989; Zahnle and Grinspoon, 1990); (B) Extraterrestrial 3 He flux (1012 cc STP cm 2 ka 1) displayed by the late Eocene section at Massignano, Macertata, Italy (from Farley et al., 1998).

Fig. 8.11. Cometary components associated with impact boundaries. (A) Relations between AIB (a-amino isobutyric acid) and Ir across the K-T boundary at Stevns Klint (from Zhao and Bada, 1989; Zahnle and Grinspoon, 1990); (B) Extraterrestrial 3 He flux (1012 cc STP cm 2 ka 1) displayed by the late Eocene section at Massignano, Macertata, Italy (from Farley et al., 1998).

of cometary components and their discrimination from Ir-rich impact ejecta (Fig. 8.11 B). Another component proposed as of extraterrestrial origin may be fullerenes (Poreda and Becker, 2003). Future extension of these and new methods to least-altered Precambrian sediments may increase knowledge on the role of asteroid and comet impacts on the early Earth.

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