Laminae as Potential Biosignatures.

When and how life on Earth started is still an open question. Biochemical fingerprints stored in the ancient rock record indicate the presence of traces of life back to some of the oldest sedimentary rocks on the planet. The Earth has thus harboured life throughout most of its geologic history, and biological processes have contributed significantly to shaping the environmental conditions on the surface of the planet. Tracking the nature of ancient life using morphological, mineralogical, chemical and isotopic proxies in the rock record on Earth needs, however, to surmount a number of obstacles. Most important are the effects of post-depositional alteration of the sedimentary host rocks due to exposure to metamorphic temperatures and pressures and metasomatism during the protracted time before their present exposure. Diagenetic and metamorphic overprints may have resulted in recrystallisation of the original mineral assemblages and deformation of the original textural features in the sedimentary rocks, in many cases blurring the biologic signatures and jeopardizing the reliable interpretation of the nature of the lifeform.

Nitrogen isotopes and abundances in sedimentary rocks have become an important tool for reconstructing biogeochemical cycles in ancient ecosystems. There are two archives of nitrogen in the rock record, namely kerogen‐bound amines and silicate‐bound ammonium, and it is well documented that the isotopic ratios of these two archives can be offset from one another. This offset has been observed to increase with metamorphic grade, suggesting that it may be related to the bonding environment in differing nitrogen host phases and associated equilibrium isotope fractionation. However, theoretical bounds for this effect have not been established, and it remains possible that some isotopic offsets predate metamorphism. In support of this hypothesis, we report an unexpectedly large isotopic offset of 4–5‰ in siltstones of very low metamorphic grade from the late Mesoproterozoic Diabaig Formation in NW Scotland (1.0 Ga). Carbon to nitrogen ratios of bulk rocks are 2–3 times lower than in other Mesoproterozoic sections. The rocks also contain early‐formed phosphate concretions and display wrinkled surfaces on bedding planes, indicative of fossilised microbial mats. Collectively, these data are most parsimoniously interpreted as evidence of diagenetic ammonium release from microbial mats into porewaters, followed by partial oxidation to nitrite or nitrate at the sediment–water interface. This process would render residual ammonium in clays isotopically heavy, while the resulting nitrite or nitrate would be relatively lighter and captured in new biomass, leading to the observed isotopic divergence. The same diagenetic degradation pathway likely also liberated phosphate that was trapped within concretions. Diagenetic release of nutrients is known to occur in modern settings, and our data suggest that nitrogen isotopes may be a way to track this local sedimentary nutrient source in past environments. Lastly, we speculate that diagenetic nutrient recycling within Proterozoic microbial mats may have created a favourable niche for eukaryotic organisms in shallow waters.

The key to the understanding of the gross changes of climate throughout geological time indicated by the geological record in the various continents depends on finding a physical method of determining latitudes in the geological past. The palaeomagnetic method seems to be the most hopeful, but for this one must assume that the average geomagnetic field is a dipole along the axis of rotation. The palaeomagnetic evidence shows this to be true for late Tertiary times – the poles derived from the mean palaeomagnetic directions of Tertiary rocks cluster about the present geographical pole and are distinctly different from the present position of the magnetic pole. There are strong theoretical grounds for believing that it has been true throughout the earth's history. The theory of the irregular fluctuations in the length of the day indicates that the rotation of the core and mantle are only weakly coupled. The electro‐magnetic torque to which this coupling is ascribed is variable because of the short term changes in the geomagnetic field. These fluctuations appear to accumulate with time and so there is always a small differential angular velocity between core and mantle, probably of either sign. Therefore, to an observer fixed on the earth's crust, the earth's magnetic field is axisymmetrical when averaged over periods of the order of the secular variation, i.e., extremely small times geologically speaking.The palaeomagnetic surveys of the various continents show systematic differences which, it appears, cannot be explained even by assuming that the earth's magnetic field was non‐dipolar, though axial, in times earlier than the Tertiary. Continental drift, as well as polar wandering, must therefore be inferred.There is a prima facie case that the latitudes thus inferred for the various continents from palaeomagnetic observations do agree with the commonly held inferences concerning the past climates of the earth made by geologists from the study of evaporite deposits, red beds, zonation of fossils, and glacial deposits. A new important check has been provided by palaeowind observations. The inference from the palaeomagnetic studies of Europe and North America that these continents were in low latitude between the Devonian and Triassic is consistent with the wind directions inferred from dune sandstones in Britain and the western United States. The consistency of the directions in the western United States over large distances seems to show that the winds were not affected by local geography.It appears that an explanation of Continental drift involves the hypothesis of convection currents in the earth's mantle which requires that the mantle is fluid when subjected to small stresses existing for many millions of years.

The terrestrial sedimentary record provides a valuable archive of how ancient depositional systems responded to and recorded changes in Earth's atmosphere, biosphere, and geosphere. However, the record of these environmental changes in eolian sedimentary successions is poorly constrained and largely unquantified. Our study presents the first global-scale, quantitative investigation of the architecture of eolian systems through geological time via analysis of 55 case studies of eolian successions. Eolian deposits accumulating (1) under greenhouse conditions, (2) in the presence of vascular plants and grasses, and (3) in rapidly subsiding basins associated with the rifting of supercontinents are represented by significantly thicker eolian dune-set, sand-sheet, and interdune architectural elements. Pre-vegetation eolian systems are also associated with more frequent interactions with non-eolian environments. The interplay of these forcings has resulted in dune-set thicknesses that tend to be smallest and largest in Proterozoic and Mesozoic successions, respectively. In the Proterozoic, the absence of sediment-binding plant roots rendered eolian deposits susceptible to post-depositional wind deflation and reworking by fluvial systems, whereby highly mobile channels reworked contiguous eolian deposits. During the Mesozoic, humid greenhouse conditions (associated with relatively elevated water tables) and high rates of basin subsidence (associated with the breakup of Pangea) favored the rapid transfer of eolian sediment beneath the erosional baseline. The common presence of vegetation promoted accumulation of stabilizing eolian systems. These factors acted to limit post-depositional reworking. Eolian sedimentary deposits record a fingerprint of major environmental changes in Earth history: climate, continental configuration, tectonics, and land-plant evolution.

We propose a novel approach to identify the origin of pyrite grains and distinguish biologically influenced sedimentary pyrite using combined in situ sulfur isotope (δ34S) and trace element (TE) analyses. To classify and predict the origin of individual pyrite grains, we applied multiple machine-learning algorithms to coupled δ34S and TE data from pyrite grains formed from diverse sedimentary, hydrothermal, and metasomatic processes across geologic time. Our unsupervised classification algorithm, K-means++ cluster analysis, yielded six classes based on the formation environment of the pyrite: sedimentary, low temperature hydrothermal, medium temperature, polymetallic hydrothermal, high temperature, and large euhedral. We tested three supervised models (random forest [RF], Naïve Bayes, k-nearest neighbors), and RF outperformed the others in predicting pyrite formation type, achieving a precision (area under the ROC curve) of 0.979 ± 0.005 and an overall average class accuracy of 0.878 ± 0.005. Moreover, we found that coupling TE and δ34S data significantly improved the performance of the RF model compared with using either TE or δ34S data alone. Our data provide a novel framework for exploring sedimentary rocks that have undergone multiple hydrothermal, magmatic, and metamorphic alterations. Most significant, however, is the demonstrated potential for distinguishing between biogenic and abiotic pyrite in samples from early Earth. This approach could also be applied to the search for potential biosignatures in samples returned from Mars.

Fossilized lipids preserved in sedimentary rocks offer singular insights into the Earth's palaeobiology. These 'biomarkers' encode information pertaining to the oxygenation of the atmosphere and oceans, transitions in ocean plankton, the greening of continents, mass extinctions and climate change. Historically, biomarker interpretations relied on inventories of lipids present in extant microorganisms and counterparts in natural environments. However, progress has been impeded because only a small fraction of the Earth's microorganisms can be cultured, many environmentally significant microorganisms from the past no longer exist and there are gaping holes in knowledge concerning lipid biosynthesis. The revolution in genomics and bioinformatics has provided new tools to expand our understanding of lipid biomarkers, their biosynthetic pathways and distributions in nature. In this Review, we explore how preserved organic molecules provide a unique perspective on the history of the Earth's microbial life. We discuss how advances in molecular biology have helped elucidate biomarker origins and afforded more robust interpretations of fossil lipids and how the rock record provides vital calibration points for molecular clocks. Such studies are open to further exploitation with the expansion of sequenced microbial genomes in accessible databases.

Analysis of a database of ∼600,000 detrital zircon ages constrained by the depositional ages of the respective sedimentary units reveals a dynamic evolution of zircon age peaks through time. This analysis demonstrates that zircon age peaks from ancient sedimentary successions are often out of phase with the detrital zircon record obtained from modern sediments. The growth and diminishment of the zircon age peaks through time implies the presence of continental crust whose age is not proportionately represented in the modern record, and therefore that the current crustal archive is biased. However, when the detrital zircon record is viewed in terms of its evolution through time, that is taken as a time-lapse view of continental growth, it appears there never was a time in Earth history without an associated zircon age peak. The analysis of detrital zircon age peaks presented herein also reveals an evolution that can be broadly divided into three temporal groupings that broadly correspond with phases of Earth9s tectonic evolution, namely pre-supercontinent continental growth (pre-2.1 Ga), Earth9s middle age (2.1–0.8 Ga), and post-onset of modern-day plate tectonics (post-0.8 Ga). These three groupings each display increasing degrees of zircon age diversification with time, and are a likely result of a net increase of preserved continental crust through time. The presence of these three tectonic states in multiple geologic proxies (detrital zircon ages, changing styles of metamorphism, paleogeography/supercontinents) suggests that while the growth of the continental crust is continuous, the tectonic processes that shape the long-term preservation of the crust have evolved over geologic time.

Better understanding the conditions of formation of authigenic Mg‐silicates and their reactivity is key to interpret the palaeoenvironmental message carried by the sedimentary record and evaluate the effect of reverse weathering, a process involved in long‐term climate evolution. Microbialites from most alkaline crater lakes in Mexico contain Mg‐silicates except those in Lake Alchichica, where concentration of orthosilicic acid is low (<26 μm). This study investigated the first metre of sediments in Lake Alchichica in order to check how their mineralogy compared with that of shoreline microbialites. The mineralogy and chemistry of the sediment column were determined, together with the pore water chemistry, providing insights on the processes occurring during early diagenesis. Below ca 3 cm in depth, diatom frustules are progressively pseudomorphized into Al‐poor Mg‐silicates with a composition corresponding to stevensite. This diagenetic process is massive and the resulting silicate represents between 30 and 53 wt.% of the sediment content at all depths. This observation questions the possibility to infer lake palaeochemistry from the presence/absence of Mg‐silicates in the sedimentary record. Moreover, it allowed refinement of the conditions under which Mg‐silicates authigenesis occurs: the saturation of the solution should be higher or equal to the solubility of a Mg‐silicate phase close to that of ‘amorphous sepiolite’. Although the solubility of authigenic silicates is a key parameter of reverse weathering modelling during geological times, it is still debated. In this study, a solubility constant deduced from a natural system is proposed that should be considered when modelling the formation of Mg‐silicates in a natural environment. The proportion of reverse weathering associated with this solubility constant could be higher than previously predicted based on experiments and thus have a greater impact on climate stability over geological timescales.

Epeirogeny or eustasy? Paleozoic–Mesozoic vertical motion of the North American continental interior from thermochronometry and implications for mantle dynamics

Do sedimentary rocks record mainly average, continuous, day-to-day processes or relatively rare, large-magnitude ones separated by long nondepositional intervals? Subtle legacies from Lyellian uniformitarianism may still impose a subconscious abhorrence of unique events, discontinuities, and large deviations from magnitudes. Where repeated sharp changes of sedimentation are inescapable, periodic cycles are commonly invoked to preserve uniform, orderly variations from some supposed norm. The sedimentary record rarely reflects such uniformity, however, as sedimentologists have gradually realized. Magnitude versus frequency of processes has long been debated in geomorphology, but has received less attention in sedimentology. Recurrence interval, recovery time, and preservation potential are critical factors for evaluating significance for the sedimentary record. Large-magnitude processes, which represent positive deviations from the norm and are rare on the human time scale, must be significant over geologic time. But how significant? Could not everyday processes have obliterated much of the evidence? Flood deposits have relatively low preservation potential because they lie above base level. Marine gravel layers dispersed by abnormal waves have greater preservation potential because most ordinary processes are not competent to modify them. Sandy or shelly deposits formed by la ge waves and displaying either hummocky cross-stratification or graded bedding have a moderate preservation potential, especially if too thick for burrowing animals to homogenize them. Turbidites, which provide exceptional records of episodicity, have excellent preservation potential because they lie well below base level. Many bedding planes are important records of episodicity, too; some are surfaces of erosion, others of nondeposition. These represent negative deviations from average process magnitudes. The sedimentological importance of rare events is difficult to End_Page 564------------------------------ assess because the record of such events may be very subtle. This is especially true if a deposit has been thoroughly bioturbated or if the record of an event is simply an erosional surface. It has now become possible to evaluate quantitatively ancient episodic sedimentation using modern-process rates as well as refined biostratigraphic and isotopic dating. I predict that such evaluation will necessitate revision of our favorite depositional models, which have become so important for exploration as well as for research. End_of_Article - Last_Page 565------------

Lithifying microbial mats in Lagoa Vermelha, Brazil: Modern Precambrian relics?

Methanogens, also known as methanogenic archaea, are among the most ancient and widespread microorganisms, despite their particular requirements for growth. These oxygen-sensitive microorganisms have impacted climate and biogeochemical cycles throughout Earth’s history, although their specific roles in the long-term carbon cycle remain little explored. Methanogens evolved early during Earth’s history, likely during the Archaean Eon, in layered benthic microbial communities called microbial mats. These ancient mats, when lithified, form microbialites that represent some of the earliest evidence of life in the fossil record dating back > 3.5 Gy. Contemporary microbial mats experience a wide range of fluctuating conditions, including dramatic diel shifts in oxygen, sulfide, redox, temperature, salinity and pH. Methanogens are an integral part of marine and freshwater microbial mats and have been identified in the oxic zone of these sedimentary ecosystems; however, their adaptations to apparently unfavorable conditions and their role in long-term CO2 sequestration through precipitation of carbonate are unclear. Furthermore, the importance and coevolution of methanogens and microbial mats may explain the global role these organisms had on Earth’s major climate events during the Archean and Proterozoic eons, notably in the ending of icehouse periods and recovery of mats following mass extinctions – often in conditions with low or no oxygen. In addition to an important role in the evolution of our planet, methanogens may also produce biosignatures that are relevant for astrobiology research [and space exploration]. This review will discuss the diversity, physiology, and ecology of methanogens in order to clarify their role in biogeochemical processes through geologic time.

Unfortunately, there are only a few surviving fragments concerning one of the most interesting areas of enquiry opened up by the young Montesquieu. In the Nouveau Mercure dated 1 January 1719 – and, soon afterwards, in the Journal des Savants – we find the prospectus of a Projet d’une histoire physique de la terre ancienne et moderne. We know from indirect sources that the thirty-year-old academic and member of parliament for Bordeaux devoted a lot of time and energy to this project, only to consign all his work to the flames, for reasons which are still unknown. The prospectus was an appeal to the international scientific community to embark on a clearly outlined and collective naturalistic-geographical project. The aim was to cover the history of the earth’s geological and climatic development as well as the history of the changes and transformations brought about by human activity, with a careful balance between the two factors.

Sedimentary rocks form at low temperatures and pressures at the surface of Earth owing to deposition by water, wind, or ice. By contrast, igneous and metamorphic rocks form mainly below Earth's surface where temperatures and pressures may be orders of magnitude higher than those at the surface, although volcanic rocks eventually cool at the surface. These fundamental differences in the origin of rocks lead to differences in physical and chemical characteristics that distinguish one kind of rock from another. Sedimentary rocks are characterized particularly by the presence of layers, although layers are also present in some volcanic and metamorphic rocks, and by distinctive textures and structures. Many sedimentary rocks are also distinguished from igneous and metamorphic rocks by their mineral and chemical compositions and fossil content.

The Quaternary ice ages were paced by astronomical cycles with periodicities of 20–100 k.y. (Milankovitch cycles). These cycles have been present throughout earth history. The Quaternary fossil record, marine and terrestrial, near to and remote from centers of glaciation, shows that communities of plants and animals are temporary, lasting only a few thousand years at the most. Response of populations to the climatic changes of Quaternary Milankovitch cycles can be taken as typical of the way populations have behaved throughout earth history. Milankovitch cycles thus force an instability of climate and other aspects of the biotic and abiotic environment on time scales much less than typical species durations (1–30 m.y.). Any microevolutionary change that accumulates on a time scale of thousands of years is likely to be lost as communities are reorganized following climatic changes. A four-tier hierarchy of time scales for evolutionary processes can be constructed as follows: ecological time (thousands of years), Milankovitch cycles (20–100 k.y.), geological time (millions of years), mass extinctions (approximately 26 m.y.). “Ecological time” and “geological time” are defined temporally as the intervals between events of the second and fourth tiers, respectively. Gould's (1985) “paradox of the first tier” can be resolved, at least in part, through the undoing of Darwinian natural selection at the first tier by Milankovitch cycles at the second tier.