The fossil record of igneous rock

Abstract

A growing awareness of life in deep igneous crust expands our appreciation for life's distribution in the upper geosphere through time and space, and extends the known inhabitable realm of Earth and possibly beyond. For most of life's history, until plants colonized land in the Ordovician, the deep biosphere was the largest reservoir of living biomass. This suggests that deep crustal habitats played an important role in the evolution and development of the biosphere. Paradoxically, the paleo-perspective of deep life has been largely neglected in the exploration of the deep biosphere as well as in paleontology as a whole. Here, we review the collective understanding of the fossil record in igneous crust with the aim to highlight a rising research field with great potential for substantial findings and progress in the near future. We include new results that emphasize the importance of direct or indirect dating of fossils and introduction of new techniques into the field. Currently, an incoherent record of morphological fossils-and chemofossils stretching from present to~2.4 Ga implies the presence of an abundant and rich, yet largely unexplored, fossil record. Further investigations of deep paleo-environments will most certainly result in substantial insights into the distribution and development of biospheres throughout life's history, the early evolution of prokaryotes and eukaryotes, and Earth's early biogeochemical cycles. We emphasize the fossil record of igneous rock to give it the same status as the fossil record in sedimentary rocks, and to implement fossil investigations as standard procedures in future international drilling campaigns.

Biosignatures of ancient microbial life are present across the igneous crust of the Fennoscandian shield

“…Although no timing constraints have been presented for the Finnish biosignatures, they indicate that the eastern part of the shield has been colonized in similar manner as the investigated area of the current study. Taken together, the findings are complementary to previous scattered investigations of deep ancient and active microbial life from scientific boreholes and underground facilities on land [11][12][13][14]66,67,73 , and suggest that life in the continental igneous crust is not an anomaly in time and space. This supports the idea of large-scale long-term microbial activity in the continental igneous crust 9,16,17 .…”

“…The remnants of completely mineralized interconnected networks of filamentous structures with anastomosing and branching filaments occurring in spatial relation to bitumen in a sample from Lund (Fig. 6c-e) resemble in morphology, size, mineralization, and occurrence of previously described endolithic fungal fossils reported from both the Fennoscandian shield 31,66 and from the ocean floor 67 . Desiccation of biofilms can sometimes result in patterns of interconnected ridges with filamentous-like appearances.…”

“…However, the frequent anastomoses between branches (Fig. 6c, e) exclude a prokaryotic interpretation of the fossils and favors of a fungal interpretation 66,67 . The partly carbonaceous filamentous structures from Grängesberg (Fig.…”

“…It has been hypothesized that microbial activity in the continental igneous crust has been widespread during Earth's history 16,67 , but direct evidence for this omnipresence is scarce. Our approach using a comprehensive set of subsurface mineral samples collected from 33 mines across the Fennoscandian shield reveals omnipresent biosignatures of deep ancient life across this craton, particularly in the central part where most the samples in this study are from.…”

Carbon isotopic composition of Frutexites in subseafloor ultramafic rocks

“…By studying the deep subsurface fossil record, we may further investigate the evolution and distribution of early life on Earth and add to our understanding the interplay between the geosphere and biosphere through time. There is a growing literature of paleo-rock-hosted life from oceanic to terrestrial deep environments and an increased understanding of the biochemical energy flows within this biosphere (Ivarsson et al 2020).…”