Instant Attraction: Clay Authigenesis in Fossil Fungal Biofilms

Abstract

Clay authigenesis associated with the activity of microorganisms is an important process for biofilm preservation and may provide clues to the formation of biominerals on the ancient Earth. Fossilization of fungal biofilms attached to vesicles or cracks in igneous rock, is characterized by fungal-induced clay mineralization and can be tracked in deep rock and deep time, from late Paleoproterozoic (2.4 Ga), to the present. Here we briefly review the current data on clay mineralization by fossil fungal biofilms from oceanic and continental subsurface igneous rock. The aim of this study was to compare the nature of subsurface fungal clays from different igneous settings to evaluate the importance of host rock and ambient redox conditions for clay speciation related to fossil microorganisms. Our study suggests that the most common type of authigenic clay associated with pristine fossil fungal biofilms in both oxic (basaltic) and anoxic (granitic) settings are montmorillonite-like smectites and confirms a significant role of fungal biofilms in the cycling of elements between host rock, ocean and secondary precipitates. The presence of life in the deep subsurface may thus prove more significant than host rock geochemistry in directing the precipitation of authigenic clays in the igneous crust, the extent of which remains to be fully understood.

The fossil record of igneous rock

“…As colonies age, they begin to fill the entire pore space and eventually become mineralized by clays and/or iron oxides (Ivarsson et al, 2015a;Sallstedt et al, 2019). Microbial growth and subsequent mineralization is thus a major factor in the secondary filling of voids along with abiotic precipitation of zeolites and carbonates.…”

“…In fact, clay authigenesis seems to play a ubiquitous part in biofilm formation and preservation, in the deep crust as well as in the shallower surface realm (e.g., Ferris et al, 1986Ferris et al, , 1987 ; Konhauser and Urrutia, 1999). In the deep igneous crust of both land (Drake et al, 2017a(Drake et al, , 2017b and oceans (e.g., Ivarsson et al 2013a, 2015cBengtson et al, 2014), fungal biofilms in particular seem to be preserved with authigenic clay minerals suggesting an important link between the biological cell and clay species such as Al, Si, Fe, and Mg. To evaluate the effect of host rock geochemistry on the precipitating clay minerals associated with fossil microbial biofilms, Sallstedt et al (2019) investigated several sites differing in ambient redox degree, and suggested that swelling smectites of montmorillonite type dominate the fossil related clay mineralogy at both oxic seamounts and deep anoxic continental granite environments. This, in turn, suggests that life's presence at depth in the crust has a greater effect than previously known on the geochemistry of secondary alteration products like clay minerals and that biology, in fact, may play a large role in the precipitation of authigenic secondary minerals in the igneous subsurface (Sallstedt et al, 2019).…”

“…In a cavity in a Paleoproterozoic quartz vein at 740 m depth at Laxemar, Sweden, fungal hyphae that formed extensive mycelia covering zeolites and carbonates have been detected (Drake and Ivarsson, 2018;Drake et al, 2017b). The hyphae are of unknown age and were partly mineralized as clays (Sallstedt et al, 2019), partly as carbonaceous material like the Lockne fossils (Ivarsson et al, 2013b). Associated pyrites were analyzed by secondary ion mass spectrometry and were shown to have isotope signatures (significantly 34 S-depleted; δ 34 S down to −53.3‰) specific for microbial sulfate reduction (MSR).…”

“…Fossilization in general is influenced by the geochemical conditions of the environment, and subsurface fossils are no exception. The similarity in mineralization and fossilization between geologically and geochemically quite disparate settings such as mafic, ultra-mafic and felsic granite provinces suggests a complex view influenced by the present biogeochemistry (Ivarsson et al, 2015a(Ivarsson et al, , 2015b(Ivarsson et al, , 2015c(Ivarsson et al, , 2015d(Ivarsson et al, , 2018a(Ivarsson et al, , 2018bDrake et al, 2017bDrake et al, , 2018aSallstedt et al, 2019). The chemical composition of the host rocks influences the chemical composition of the circulating fluids, which in turn gives rise to fossilization.…”

“…Depending on the preservation and diagenesis, various amounts of organic molecules can remain. Fungal fossils in deep environments are exclusively fossilized by montmorillonite-type clays and minor amounts of Fe-oxides (Ivarsson et al, 2015a(Ivarsson et al, , 2015b(Ivarsson et al, , 2015cSallstedt et al, 2019). Fungal clay authigenesis appears to be consistent through all morphological growth structures, environments and ages (Sallstedt et al, 2019).…”