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Paleoarchean Trace Fossils in Altered Volcanic Glass

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Paleoarchean Trace Fossils in Altered Volcanic Glass Paleoarchean trace fossils in altered volcanic glass Hubert Staudigela,1, Harald Furnesb,c, and Maarten DeWitc,d aScripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0225; bDepartment of Earth Science and cCentre for Geobiology, University of Bergen, 5007 Bergen, Norway; and dAfrica Earth Observatory Network and Earth Stewardship Science Research Institute, Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa Edited by Norman H. Sleep, Stanford University, Stanford, CA, and approved March 31, 2015 (received for review November 3, 2014) Microbial corrosion textures in volcanic glass from Cenozoic details of microbial drilling into volcanic glass remain to be ex- seafloor basalts and the corresponding titanite replacement micro- plored (2), biogenicity appears to be the widely accepted explana- textures in metamorphosed Paleoarchean pillow lavas have been tion for the origin of granular and tubular alteration textures in interpreted as evidence for a deep biosphere dating back in time Cenozoic volcanic glass (6). through the earliest periods of preserved life on earth. This inter- Putative bioalteration textures were found in Paleoarchean pretation has been recently challenged for Paleoarchean titanite re- greenstone belts, in particular the upper sequences of the Hoog- placement textures based on textural and geochronological data genoeg Complex of the BGB in South Africa (3, 4) (Fig. 1 B and E from pillow lavas in the Hooggenoeg Complex of the Barberton and Fig. S5) and the Euro Basalts of the PC in Australia (5) (Fig. 1F Greenstone Belt in South Africa. We use this controversy to explore and Fig. S6). However, these putative trace fossils in greenstones the strengths and weaknesses of arguments made in support or are preserved not as cavities but as mineral fillings, especially by rejection of the biogenicity interpretation of bioalteration trace titanite, and they are recognizable by their remarkable textural fossils in Cenozoic basalt glasses and their putative equivalents in similarities with Cenozoic glass bioalteration, including their Paleoarchean greenstones. Our analysis suggests that biogenicity association with external glass surfaces and cracks. Such titanite cannot be taken for granted for all titanite-based textures in textures have been the subject of a petrographic, geochemical, metamorphosed basalt glass, but a cautious and critical evaluation and geochronological study of a drill core in the Hooggenoeg of evidence suggests that biogenicity remains the most likely inter- Complex of the BGB, but they were interpreted as metamorphic pretation for previously described titanite microtextures in Paleo- features without a biogenic precursor (6). archean pillow lavas. In this paper, we will take the opportunity to explore weaknesses and strengths of various lines of evidence cited in support or re- biogeosciences | early life | Paleoarchean | astrobiology | ichnofossil jection of biogenicity for purported biotextures in Cenozoic vol- canic glass and in Paleoarchean greenstones. Our discussion will icrobial corrosion textures in volcanic glass are well stud- address but not be limited to the four main arguments extended i ied in Cenozoic seafloor basalts, they are distinct from the recently by Grosch and McLoughlin (6): ( ) the apparent lack of M ii well-understood abiotic alteration textures, and they are con- chemical biomarkers in their samples, ( ) the difficulties in sidered an indication of an active deep oceanic biosphere down finding the most unique signature fossils (spiraling or annulated iii to at least 500 m below the seafloor (Fig. 1 A–C and Figs. S1–S4) tubes), ( ) the apparent differences in tube diameter size distri- (1, 2). Such corrosion textures were proposed to be preserved in butions between titanite replacement textures and precursor fossils, iv the form of analogous titanite textures in chloritized volcanic glass and ( ) the substantial age difference between crustal ages and in greenschist metamorphosed glass from pillow basalts of the ca. apparent titanite replacement ages. We will show that many of ’ 3.5 Ga Barberton Greenstone Belt (BGB, South Africa) (3, 4) Grosch and McLoughlin s arguments are flawed or ill-supported, (Fig. 1 D–F and Fig. S5) and the Pilbara Craton (PC/Australia) and none of them carry much weight in ruling out biogenicity of the ’ (Fig. S6) (5), suggesting the presence of a deep oceanic biosphere textures studied. However, we concur with Grosch and McLoughlin s at this early time in the evolution of life on earth. Recently, the (6) assessment that many of the textures in their study site are biogenicity interpretation of such titanite textures was rejected indeed abiotic or at least ambiguous in terms of a potential biotic based on a geochemical, textural, and geochronological study of a 180-m drilled core section through pillow lavas of the Hooggenoeg Significance Complex of the BGB (6). This controversy affords an opportunity to critically evaluate biogenicity in terms of what is known about The dawn of sustainable life on earth is preserved in the form of bioalteration of volcanic glass and how these trace fossils may be fossil or chemical evidence in ancient rock sequences, such as preserved and be recognized in greenschist metamorphosed the Barberton Greenstone Belt in South Africa. Studies of sed- volcanic glass. imentary rocks offered a glimpse at life at the earth’ssurface, Biotextures are widely found as dissolution features on the and trace fossils in pillow lavas offered evidence for a potential surfaces of volcanic glass in Cenozoic seafloor volcanics and may deep biosphere back in time to the Paleoarchean. Recent data take on two major forms, micrometer-sized spherical cavities or cast doubt on the biogenicity of these putative trace fossils, tubules that extend into the glass up to about 100 μm (2, 7, 8). rejecting their potential in exploring a deep biosphere. We Such glass dissolution cavities, in particular their more complex discuss biogenicity of Cenozoic and Archean examples of such signature expressions such as bifurcating, annulated, decorated, putative biocorrosion textures and conclude that microbial ori- or coiled tubes (9), are considered impossible to have been gin remains the best explanation for the textures described caused by abiotic dissolution. Well-documented cases of micro- previously in these Paleoarchean rocks (e.g., >3.4 Ga). bial drilling into soil feldspars (10) and mollusk shells (11) offer analogs that give precedence for biogenicity of such dissolution Author contributions: H.S., H.F., and M.D. designed research, performed research, and features (2). Microbial dissolution of glass is likely to be caused wrote the paper. or at least aided by the excretion of organic acids in the contact The authors declare no conflict of interest. area of colonizing microbes or their cell extensions such as fungal This article is a PNAS Direct Submission. hyphae (2). Reasons for microbial boring into glass remain un- Freely available online through the PNAS open access option. certain, but there are several potentially chemical energy-producing 1To whom correspondence should be addressed. Email: [email protected]. weathering reactions of volcanic glass that would become available This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. in an otherwise largely oligotrophic environment. Although many 1073/pnas.1421052112/-/DCSupplemental. 6892–6897 | PNAS | June 2, 2015 | vol. 112 | no. 22 www.pnas.org/cgi/doi/10.1073/pnas.1421052112 Downloaded by guest on October 1, 2021 occurs mostly as metamorphic mineral growths in the chloritized glass, but it also commonly replaces varioles, a common quench texture in the pillow lavas of the Hooggenoeg Complex of the BGB (12). Clearly identifiable titanite-based biotextures are much rarer than the broadly occurring titanite in these metamorphic rocks. Why would this common metamorphic mineral crystallize in and around bioalteration cavities in basaltic glass? It is well known that alteration of basaltic glass leads to the passive ac- cumulation of Ti in its alteration products such as palagonite (13) and such accumulation has been found in the walls of mi- crotubules as well (14, 15). These observations may be combined into a two-stage model for the origin of titanite bioalteration replacement textures (16). The first stage involves the Ti en- richment in the walls of the microbial excavations in volcanic glass, and the second stage is marked by the titanite crystalliza- tion stage during greenschist metamorphism in and around these zones of Ti preenrichment. A chemical mass balance requires that these titanites also have to draw additional Ti from the surrounding glass as it is transformed to chlorite. Grosch and McLoughlin (6) offer a different concept for titanite crystallization, drawing from a retrograde metamorphic reaction that involves the formation of titanite (and actinolite) from the breakdown of clinopyroxene, ilmenite, and quartz. Grosch and McLoughlin argue that this process occurred dur- ing retrograde cooling from >600 °C to 350 °C subsequent to contact metamorphism associated with the intrusion of an 84- m-thick diorite dike 18–19 m away from the analyzed samples (figure S3B in ref. 6). Grosch and McLoughlin propose that this process (figure S3B in ref. 6) begins with a complexly shaped Fig. 1. Biotextures in fresh Cenozoic basalt glass and titanite microtextures ilmenite texture that resembles
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