Re-Evaluating the Age of Deep Biosphere Fossils in the Lockne Impact Structure

Abstract

Impact-generated hydrothermal systems have been suggested as favourable environments for deep microbial ecosystems on Earth, and possibly beyond. Fossil evidence from a handful of impact craters worldwide have been used to support this notion. However, as always with mineralized remains of microorganisms in crystalline rock, certain time constraints with respect to the ecosystems and their subsequent fossilization are difficult to obtain. Here we re-evaluate previously described fungal fossils from the Lockne crater (458 Ma), Sweden. Based on in-situ Rb/Sr dating of secondary calcite-albite-feldspar (356.6 ± 6.7 Ma) we conclude that the fungal colonization took place at least 100 Myr after the impact event, thus long after the impact-induced hydrothermal activity ceased. We also present microscale stable isotope data of 13C-enriched calcite suggesting the presence of methanogens contemporary with the fungi. Thus, the Lockne fungi fossils are not, as previously thought, related to the impact event, but nevertheless have colonized fractures that may have been formed or were reactivated by the impact. Instead, the Lockne fossils show similar features as recent findings of ancient microbial remains elsewhere in the fractured Swedish Precambrian basement and may thus represent a more general feature in this scarcely explored habitat than previously known.

A Cryptic Alternative for the Evolution of Hyphae

“…[ 52,53,54 ] In continental crystalline basement fungal fossils ranging back to Devonian age (≈400 Ma) have been described from various sites in the Swedish igneous basement. [ 43,55–58 ] Thus, hyphal fungi were present at great depths in the oceanic and continental crust contemporaneous with the presumed evolution of hyphae as plants and fungi colonized the terrestrial realm. [ 18,57 ]…”

“…[ 43,55–58 ] Thus, hyphal fungi were present at great depths in the oceanic and continental crust contemporaneous with the presumed evolution of hyphae as plants and fungi colonized the terrestrial realm. [ 18,57 ]…”

“…[ 43,44,60 ] In subseafloor basalts, fossil fungi are syngenetic with prokaryotes associated with iron‐ and manganese oxidation, whereas in continental basement, fossil fungi have been associated with bacterial sulfate reduction and methanogenesis. [ 56,57,61 ] A possibility is that hyphal growth into open pore space is a strategy, not only to reach open space, but also to act as substrate for prokaryotic growth, enabling subsequent decomposition of the prokaryotic biomass by the fungal host. Thus, hyphal interaction with other organisms plays an important role in the subsurface realm.…”

“…An intimate symbiotic‐like fungal‐prokaryotic partnership can be seen throughout the Phanerozoic and is established in the deep biosphere realm already at 400 Ma. [ 18,43,55–58 ] Such an interaction is probably a requirement for eukaryotic colonization of deep oligotrophic environments. If we assume a scenario where deep biosphere fungi predates the terrestrial diversification of fungi, early fungal/hyphal‐prokaryote interactions could be seen as preadaptations to the subsequent connection to land plants.…”

“…[ 71,72 ] Develop techniques for more accurate dating of fossils in igneous crust. Radioisotopic dating of secondary minerals associated with fossils have enhanced the accuracy [ 56,57 ] but dating of individual fossils would be preferable. However, such analyses rely on many factors, and certain requirements need to be fulfilled such as: 1) suitable mineralization of the organisms (e.g., U‐rich minerals for U‐Pb dating or pairs of Rb‐ and Sr‐rich minerals for Rb/Sr dating); 2) closed system conditions to exclude mobilization of U; 3) the mineral assemblage having stayed below closure temperature for the isotope system; 4) absence in the precipitate of radiogenic lead, which could have been remobilized during previous transport; 5) control of mineral zonation.…”