Questioning the biogenicity of titanite mineral trace fossils in Archean pillow lavas

Abstract

Staudigel et al.(1) compare early Archean titanite microtextures to recent microtubules in Cenozoic volcanic seafloor glass to support a biogenic origin. However, given the 3.5 billion years of Earth history since eruption of the Archean lavas, many geological processes have affected these rocks, complicating the simple case for trace fossils. Using hollow and partially mineralized microtextures in modern seafloor basalt as an analog for argued microbial alteration of Archean glass is, in our opinion, a weak line of argument and an overextrapolated interpretation in support of biogenicity. The many assumptions required in their proposed bioalteration model are not supported by microbiological experiments or geological observations. For example, Staudigel et al.(1) require that hollow microbial tunnels are filled in by some process forming titanite, but when and how this occurs is not substantiated. The authors also contradict earlier work by abandoning organic carbon linings to the microtextures as evidence in support of biogenicity. Staudigel et al. provide no new data to support a biogenic origin, and we highlight that they have further complicated their lines of argument.In our article (2), we indicated that Staudigel and his colleagues (1, 3) failed to document the geological and metamorphic context of their candidate trace fossils, including a major approximately 2.9-Ga mafic intrusion directly adjacent to their sample site. Extensive petrographic studies show that the pillow lavas were thermally affected by the intrusion, producing titanite microtextures of various morphologies (2), unlike the fine titanite dustings observed in pillow lavas regionally (4). In their images, all taken from earlier publications, Staudigel et al. (1) confuse hornfelsic titanite formed during thermal metamorphism for apparent igneous "varioles" of undescribed mineralogy. The authors refer to "delicate" textures in limited samples from an approximately 15-m section and make titanite width arguments based on this highly biased sample set, while also adding caveats to the size distribution of recent microtubules used for comparison. Rather, we provide statistical analysis of all titanite microtexture size and shape variation from across the 183-m drill core section spanning the intrusion and the original sampling site of refs. 1 and 3 to propose a metamorphic origin.No new data are provided by Staudigel et al. (1) to support their suggestion that preexisting titanite was reset at 2.9 Ga; neither do they provide criteria that separate their proposed "signature" microtextures from the continuum of morphologies we report. In Fig. 1 We reiterate that the titanite microfilaments are abiotic and not syngenetic to a 3.5-Ga subseafloor model and that Staudigel et al. (1) provide no new data to suggest otherwise. We reassert that the biological origin of the titanite microtextures be treated with caution (2), and that the search for the earliest traces of life must consider all abiotic scenarios before invoking biological processes. Eugene...

Deciphering Biosignatures in Planetary Contexts

“…In many cases, the original mineral or biological content of a depositional event is not preserved. Determining the extent and timing of preservational effects is not trivial and requires extensive comparison with natural systems and laboratory experiments (Grosch and McLoughlin, 2014, 2015). Unfortunately, there is often incomplete information about depositional and subsequent diagenetic conditions, and it is not always possible to accurately and experimentally model diagenetic processes.…”

“…Taken collectively, these textures can provide clues about paleoenvironments and the types of communities that inhabited them. However, physicochemical changes to sedimentary deposits over geologic time can also result in the formation of similar abiotic textures in ancient rocks ( e.g ., Grosch and McLoughlin, 2015; Davies et al , 2016). For example, mudcracks can resemble burrows in a cross-sectional view, or clotted paleosol development can resemble burrow textures.…”

Deconstructing Earth’s oldest ichnofossil record from the Pilbara Craton, West Australia: Implications for seeking life in the Archean subseafloor

“…So where do we now stand concerning the evidence for life in the Archean subseafloor? Regarding the Barberton microtextures, an abiotic relatively high-temperature metamorphic model was proposed, involving the growth of later metamorphic titanite in the contact aureole of a diabase sill (Grosch and McLoughlin 2015). This differs from the mechanism proposed here, involving low-temperature seafloor-hydrothermal metamorphism with the spontaneous nucleation and growth of titanite +/anatase in the shallow subseafloor of the Euro Basalt.…”

“…A previous study by Banerjee et al (2007) obtained a radiometric age estimate from the Euro Basalt titanites of 2.93 Ga, implying a c. 400 Ma time difference between the age of pillow lava eruption and titanite mineralization, this requires that the hollow or partially mineralized microtunnels were not destroyed by subsequent hydrothermalseafloor and burial metamorphic events, which we suggest is unlikely. It has also been shown by Grosch and McLoughlin (2015) the challenges of dating amorphous low-grade Archean titanites, particularly given their high common lead content, and sensitivities to the lead correction chosen, and we therefore have concerns as to the reliability of the 2.930 Ga age estimate obtained for the Euro Basalt titanites. In short, we reject a model for younger metamorphic titanite mineral growth preserving seafloor Archean microbial microtunnels given the timing relationships required, and in addition to the petrographic and ultrastructural data presented above.…”

A New Frontier for Palaeobiology: Earth's Vast Deep Biosphere

“…Many workers have also regarded microtubular and granular textures in fractured or vesicular pillow basalt glass as ancient microbial “trace fossils.” These common alteration textures probably begin to form near the seafloor shortly after lava is erupted, but continue to develop in the subseafloor realm in association with fresh burial‐induced fractures, and so may record the activity of the deep biosphere . However, there are reasonable doubts about the biogenicity of some examples and the most ancient specimens have been reinterpreted as nonbiological artefacts of metamorphism …”