Alexander Thomas Brasier

Hot-springs have fascinated geologists including James Hutton for centuries. Here we synthesize data collected over timescales from centuries to days to reveal rules governing hot-springs, including where and when they occur. Data reveal waters dominantly sourced from the surface (either meteoric or seawater) and circulated through faults and fractures systems. Heat comes from magmatic sources (active plate margins) or from the geothermal gradient. Silica-precipitating springs tend to be hotter than carbonate-precipitating springs, and spring vent temperatures are generally near-constant over historical timescales. An exploration of the geological record reveals a few additional known cases of ancient hot-springs formed around impact craters. Given the number of active hot-spring sites, and decades of astrobiologically driven research, the geological record of hot-springs is more populous than commonly reported, but still sparse. One Archean example, three reported Paleoproterozoic examples and one Ediacaran example are known in the Precambrian. Eight Paleozoic and at least ten Mesozoic examples have also been reported. This contrasts with thousands of known Recent examples, implying that many ancient deposits have not been preserved, while many others are yet to be found. Guidelines emerging from this synthesis will hopefully assist in the search for more ancient hot-spring Lagerstätten sites.

Microbially mediated early diagenetic pyrite formation in the immediate vicinity of organic material has been the favored mechanism by which to explain widespread preservation of soft-bodied organisms in late Ediacaran sedimentary successions, but an alternative rapid silicification model has been proposed for macrofossil preservation in sandstones of the Ediacara Member in South Australia. We here provide petrological evidence from Nilpena National Heritage Site and Ediacara Conservation Park to demonstrate the presence of grain-coating iron oxides, framboidal hematite, and clay minerals along Ediacara Member sandstone bedding planes, including fossil-bearing bed soles. Scanning electron microscope (SEM), cathodoluminescence microscopy (CL), and petrographic data reveal that framboids and grain coatings, which we interpret as oxidized pyrite, formed before the precipitation of silica cements. In conjunction with geochemical and taphonomic considerations, our data suggest that anactualistically high concentrations of silica need not be invoked to explain Ediacara Member fossil preservation: We conclude that the pyritic death mask model remains compelling.

In many basins, Upper Cambrian carbonate successions display intervals with a positive carbon isotope excursion (CIE) of up to +5‰. In North America, this marks the boundary between the Sauk II–III super-sequences. A Steptoean positive carbon isotope excursion (SPICE) locality previously identified in the Port au Port peninsula, western Newfoundland, has been revisited and an additional potential SPICE locality found. In both locations, a CIE is found to be associated with a prominent bioherm and sandstone layer within a sequence of carbonate rocks. At March Point columnar stromatolites occur, whereas at Felix Cove thrombolites can be seen. In the latter, the sandstone immediately overlies the thrombolites coincident with the CIE, whereas at March Point a dolomitized grainstone occurs above the stromatolites. The sandstone at this locality post-dates the CIE. Although lower than the SPICE in some localities, a positive CIE is present in both sections: March Point (+1.1‰) and Felix Cove (+1.8‰). Additionally, δ¹³C_org rises from −30.0‰ to −22.0‰ at March Point and from −27‰ to −24.0‰ at Felix Cove and, in accordance with previously published work, we suggest that this could be the SPICE. Comparison of the stratigraphy and petrography between the two localities suggest that both depositional and diagenetic factors could have influenced the nature of the interpreted SPICE in Newfoundland. It is also possible that the local carbon isotopic signature may have been influenced by a semi-restricted depositional and early diagenetic environment related to the paleogeographic configuration rather than the global marine excursion.

Lacustrine carbonate chimneys are striking, metre‐scale constructions. If these were bioinfluenced constructions, they could be priority targets in the search for early and extraterrestrial microbial life. However, there are questions over whether such chimneys are built on a geobiological framework or are solely abiotic geomorphological features produced by mixing of lake and spring waters. Here, we use correlative microscopy to show that microbes were living around Pleistocene Mono Lake carbonate chimneys during their growth. A plausible interpretation, in line with some recent works by others on other lacustrine carbonates, is that benthic cyanobacteria and their associated extracellular organic material (EOM) formed tubular biofilms around rising sublacustrine spring vent waters, binding calcium ions and trapping and binding detrital silicate sediment. Decay of these biofilms would locally have increased calcium and carbonate ion activity, inducing calcite precipitation on and around the biofilms. Early manganese carbonate mineralisation was directly associated with cell walls, potentially related to microbial activity though the precise mechanism remains to be elucidated. Much of the calcite crystal growth was likely abiotic, and no strong evidence for either authigenic silicate growth or a clay mineral precursor framework was observed. Nevertheless, it seems likely that the biofilms provided initial sites for calcite nucleation and encouraged the primary organised crystal growth. We suggest that the nano‐, micro‐ and macroscale fabrics of these Pleistocene Mono Lake chimneys were affected by the presence of centimetre‐thick tubular and vertically stacked calcifying microbial mats. Such carbonate chimneys represent a promising macroscale target in the exploration for ancient or extraterrestrial life.

The main source rock (SR) of the Aquitaine Basin, the most important oil province in France, corresponds to organic-rich marine limestones of Upper Kimmeridgian age (Lons Formation in the Béarn area and Parnac Formation in the Quercy area). In order to better characterize their depositional environment, in particular the conditions of accumulation and preservation of organic-matter, a sedimentological, micropaleontological and geochemical study of the Crayssac section (Quercy) has been performed. Organic-rich sediments are argillaceous limestones (65 to 99% CaCO₃) organized in repetitive beds of up to 1-m thickness. Their total organic matter content reaches up to 15 wt.%, and in this SR kerogen is type of II and immature. Microfauna content, the lack of barrier facies and the control of the wave action base over the depositional environments reflect deposition in an open marine type homoclinal ramp. Strong similarities with Kimmeridgian organic-rich limestones of the Middle East (Hanifa Formation) suggest that the Parnac Formation could act as an analogue of this prolific SR.