Ancient Earth Research Articles

Weak paleointensities from 1.6 Ga Greenland dykes: Further evidence for a billion-year period of paleomagnetic dipole low during the Paleoproterozoic

The Paleoproterozoic era is the longest in Earth's history, with significant changes hypothesised to have occurred in the deep Earth's physical and chemical conditions at this time. It has been suggested that the paleomagnetic field became weaker at this time (∼2.4 Ga) and remained weak for the next billion years. Paleomagnetism is intrinsically linked to, and is able to inform on, ancient deep Earth processes; a weak dipole strength sustained over this time period may have implications for both core and mantle evolution.We test this hypothesis here in a two-fold approach: (1) A paleointensity study on the widespread ca. 1.6 Ga diabase/dolerite Melville Bugt dyke swarm. The swarm extends along the west coast of Greenland for more than 1000 km and intruded over ∼13 million years, capturing polarity reversals of Earth's magnetic field. (2) A detailed statistical analysis on the long-term trend in average dipole moment from an improved paleointensity dataset (PINT.org) that has recently undergone a major update.Five of the Greenland dykes produce paleointensity results ranging from 1.4 μT to 5.1 μT (virtual dipole moment range 0.3–1.2 × 1022 Am2) during the mid-point of this extended period of ‘dipole low’. Our statistical study robustly confirms that this one-billion-year period was indeed associated with an anomalously weak dipole moment (2.7 × 1022 Am2) relative to 500-million-year intervals before and after, which were almost twice as strong. Sampling of more geographically diverse rocks from this time is needed to yield a clear picture of the long-term time evolution of the dipole moment.

Noble gases in shocked igneous rocks from the 380 Ma-old Siljan impact structure (Sweden): A search for paleo-atmospheric signatures

Finding direct records of the ancient Earth's atmosphere is key to understand the evolution of the entire planet. Nevertheless, such records are scarce, and only a few geological samples with preserved paleo-atmospheric signatures are available today. In this study, we analyzed noble gases contained in shocked granitic and volcanic rocks collected across the 380 Ma-old Siljan impact structure (in Sweden) to investigate the potential of shocked quartz crystals with planar deformation features (PDFs), often decorated by fluid inclusions, to preserve atmospheric signatures.Our results reveal that most of the investigated samples show relative elemental abundances of noble gases falling between those of air and meteoric water values. Atmospheric gases are present in the samples but excesses in radiogenic, nucleogenic, and fissiogenic noble gas isotopes highlight interactions of fluids with crustal rocks during the impact-generated hydrothermal circulation of fluids after the impact. Such fluid circulation event(s) could also have remobilized older fluids originally present in fluid inclusions preserved in the mineral lattices. Furthermore, results obtained on the two pure quartz fractions highlight a stronger atmospheric signature correlated with the presence of PDFs. The atmospheric component detected in this study could represent either a paleo-atmospheric signature trapped shortly after the Siljan impact event, or it may result from modern atmospheric contamination. Future studies of noble gases contained within shocked quartz crystals from other impact structures will unveil if such samples present advantages compared to other paleo-atmospheric proxies.

Hot and cold Earth through time

Reconstructing ancient Earth’s temperature reveals a global climate regulation system

Influence of preparative parameters on the morphology and reactivity of synthetic hexagonal birnessite

Birnessite is a phyllomangante mineral widely found in surficial environments. Its morphology not only affects its adsorption and oxidation properties but also indicate changes in the climatic conditions of ancient Earth. This study explored how preparative parameters affect the morphology and reactivity birnessite. Synthesized at boiling temperatures, birnessite nanoflowers measured 265 nm with a specific surface area (SSA) of 34.87 m2/g, while at 65 °C, they increase to 2251 nm with a similar SSA of 30.33 m2/g. Lowering KMnO4 or HCl concentrations increased nanoflower size. Reduced concentration of KMnO4 hindered Coulombic forces, fostering a parallel petal arrangement and a higher SSA (59.11 m2/g). Lower concentration of HCl led to perpendicular petals and a reduced SSA (12.33 m2/g). Decreased concentrations of both KMnO4 and HCl reduced initial δ-MnO2 concentration, allowing time for edge-to-edge assembly and nanoflake substrate formation. Subsequently, δ-MnO2 vertically assembled on substrates to form microwalls with SSA of 85.39 m2/g. Organic acids as capping agents disrupted this assembly. Adsorption tests for Cd2+ revealed microwalls achieved 7102 mmol/kg, surpassing original birnessite nanoflowers at 2114 mmol/kg. These results provide insights into the crystallization processes and reactivity of natural birnessite, as well as methods for the controllable synthesis of nanoflowers.

Late Paleocene paleoclimate recorded in fluid inclusion of halite in Subei basin, East China

Salt deposits are indicative of relatively extreme climate events. However, due to insufficient independent temperature proxies, paleotemperature records obtained from salt deposition are still lack. The Paleocene evaporite sequence deposited in the Hongze Depression of Subei Basin of eastern China provides an important terrestrial sediment record during this period. In this study we present total of 488 homogenization temperature (Th) data of halite fluid inclusions from drilling core with different stratigraphic depth after detailed petrological observation. The obtained Th ranged from 17.7 °C to 52.3 °C, with the mean Th value of 34.1 °C that in good agreement with the previous studies of climatic proxies. Our study shows that primary fluid inclusions of halite can serve as a robust tool to construct the ancient earth surface temperature.

A non-methanogenic archaeon within the order Methanocellales

Serpentinization, a geochemical process found on modern and ancient Earth, provides an ultra-reducing environment that can support microbial methanogenesis and acetogenesis. Several groups of archaea, such as the order Methanocellales, are characterized by their ability to produce methane. Here, we generate metagenomic sequences from serpentinized springs in The Cedars, California, and construct a circularized metagenome-assembled genome of a Methanocellales archaeon, termed Met12, that lacks essential methanogenesis genes. The genome includes genes for an acetyl-CoA pathway, but lacks genes encoding methanogenesis enzymes such as methyl-coenzyme M reductase, heterodisulfide reductases and hydrogenases. In situ transcriptomic analyses reveal high expression of a multi-heme c-type cytochrome, and heterologous expression of this protein in a model bacterium demonstrates that it is capable of accepting electrons. Our results suggest that Met12, within the order Methanocellales, is not a methanogen but a CO2-reducing, electron-fueled acetogen without electron bifurcation.

Speciation controls on Ni adsorption to birnessite and organo-birnessite

Nickel (Ni) is an essential micronutrient for phytoplankton. Its importance to both the modern and ancient Earth system has encouraged development of Ni and its isotopes as biogeochemical tracers. To interpret these signatures however, understanding of how Ni and its isotopes are recorded in marine archives is required. Here we simulate different inorganic and organic Ni species in seawater and investigate their adsorption behaviours to variably crystalline phyllomanganates and organo-mineral phyllomanganate. We conduct pH adsorption edge experiments to determine the binding affinity of the different Ni species to the minerals and then perform desorption experiments to operationally define Ni bonding strength. We also use thermodynamic surface complexation modelling to constrain Ni adsorption mechanisms. From the adsorption edges and stability constants generated from our modelling, the binding affinity increases in the order of Ni-formate+ (aq) < NiCl+ (aq) < Ni2+ (aq). From the desorption experiments, desorption at pH 8 is non quantitative and increases in the opposite order of Ni-formate+ (aq) > NiCl+ (aq) ∼ Ni2+ (aq). For the organo-mineral however, Ni desorption at pH 8 is non quantitative and similar for all three experiments, and is significantly higher compared to the variably crystalline phyllomanganates. Although both our adsorption and desorption experiments were performed over 48 h, it is possible that desorption is somewhat slower than adsorption such that a longer desorption period may result in further Ni loss to solution and thus greater adsorption reversibility. Taken together however, the Ni-formate+ (aq) and Ni organo-birnessite desorption experiments suggest that Ni bonding strength is decreased by the presence of organic carbon, compared to NiCl+ (aq) and Ni2+ (aq). Because bonding strength governs equilibrium stable isotope fractionation, we use our experimental findings to suggest how Ni speciation in seawater might influence Ni isotope behaviour during adsorption to phyllomanganate. We find that our suggestions are consistent with isotopic measurements from natural sediments. Although the balance of Ni adsorption versus incorporation during uptake to phyllomanganates may play a greater part in explaining the variation in the Ni isotope composition in Mn-rich sediments, Ni speciation and the presence of organics might increase the range of δ60Ni values measured in natural settings.

Organic matter accumulation as a convergence response to astronomical forcing: Insights from Milankovitch periods and sea-level fluctuations during the early Cambrian

The accumulation of organic matter (OM) in marine sediments is a vital part of the global carbon cycle today and on ancient Earth, and is driven by a cumulative shift of organic carbon flux through oceans to marine sediments. This shift is a complicated process that is known to be influenced by the biological pump, diagenesis, and thermal alteration, so is affected by oceanic conditions and palaeo-climate, but less attention has been paid to the triggers of OM accumulation. Here, a set of marine sedimentary rocks from the early Cambrian in the southwestern part of the Yangtze Platform in China was used to search for clues to the triggers of OM accumulation. The effect of astronomical forcing, which potentially is a major control on Earth's oceanic conditions, was assessed via frequency analysis based on a high-resolution log dataset from a borehole in Yunnan Province. An astronomical time scale was constructed based on a 405,000 year-tuned filter. Palaeo-oceanic sedimentary conditions and palaeo-climate, which were recorded in the sedimentary rocks, were determined through geochemical analyses of major, trace, and rare earth elements. The mutual influence of astronomical forcing and the early Earth oceanic sedimentary conditions and paleoclimate were examined though Pearson Correlation analyses to provide evidence for the intensity of alteration of sedimentary conditions and the forces impacting the shift of organic carbon in the marine ecosystem. The results show that two convergence responses of OM accumulation were associated with ca. 1.5 and 2.4 million year (Myr) Milankovitch cycles. Fluctuations in sea level establish a connection between Milankovitch forcings and changes in sedimentary conditions, thereby providing evidence for the triggering of a cumulative shift of OM flux to sediments, and the effect of astronomical forcing on the organic carbon cycle.

Late Cenomanian Plenus event in the Western Interior Seaway

The Late Cenomanian Plenus Cold Event is one of the most enigmatic paleoclimate episodes in Earth history with potential to inform understanding of global climate system variability under greenhouse warming conditions, as well as internal feedback pathways that modulate such variability. Following an interpreted massive addition of volcanic CO2 to the atmosphere and warming that led to a major ocean anoxic event (OAE2), there was a brief interval of cooling recorded in oxygen isotopes and biogeographic data. Here we present evidence that cooling was absent or muted within the Western Interior Seaway (WIS). Clumped isotope data from the basin suggest persistent extreme warmth during the Late Cenomanian, macroinvertebrate fossil assemblages do not record a decrease in temperature, and changes in other paleoceanographic proxies do not correlate temporally with Plenus interval signals from other locales. Using select proxy data to guide construction of GCM model simulations, we explore possible hypotheses to explain these observations. Our results suggest that the paleogeographic configuration of the basin and its gateways to adjoining oceans, which evolved in association with changing pCO2 and sea level, influenced winter sea ice formation at the northern aperture of the seaway, water mass circulation, salinity, temperature, and water column stratification. We propose that northward advection of warm Tethyan water muted expression of Plenus cooling in the seaway. Understanding the unique character of the Western Interior paleoceanographic record provides critical input for the development of robust models of ancient Earth System dynamics and should aid predictions of future climate system dynamics.

A rock record of H2 production in the ancient Earth

A rock record of H2 production in the ancient Earth

Intentional creation of carbon-rich dark earth soils in the Amazon.

Fertile soil known as Amazonian dark earth is central to the debate over the size and ecological impact of ancient human populations in the Amazon. Dark earth is typically associated with human occupation, but it is uncertain whether it was created intentionally. Dark earth may also be a substantial carbon sink, but its spatial extent and carbon inventory are unknown. We demonstrate spatial and compositional similarities between ancient and modern dark earth and document modern Indigenous practices that enrich soil, which we use to propose a model for the formation of ancient dark earth. This comparison suggests that ancient Amazonians managed soil to improve fertility and increase crop productivity. These practices also sequestered and stored carbon in the soil for centuries, and we show that some ancient sites contain as much carbon as the above-ground rainforest biomass. Our results demonstrate the intentional creation of dark earth and highlight the value of Indigenous knowledge for sustainable rainforest management.

On the potential roles of phosphorus in the early evolution of energy metabolism.

Energy metabolism in extant life is centered around phosphate and the energy-dense phosphoanhydride bonds of adenosine triphosphate (ATP), a deeply conserved and ancient bioenergetic system. Yet, ATP synthesis relies on numerous complex enzymes and has an autocatalytic requirement for ATP itself. This implies the existence of evolutionarily simpler bioenergetic pathways and potentially primordial alternatives to ATP. The centrality of phosphate in modern bioenergetics, coupled with the energetic properties of phosphorylated compounds, may suggest that primordial precursors to ATP also utilized phosphate in compounds such as pyrophosphate, acetyl phosphate and polyphosphate. However, bioavailable phosphate may have been notably scarce on the early Earth, raising doubts about the roles that phosphorylated molecules might have played in the early evolution of life. A largely overlooked phosphorus redox cycle on the ancient Earth might have provided phosphorus and energy, with reduced phosphorus compounds potentially playing a key role in the early evolution of energy metabolism. Here, we speculate on the biological phosphorus compounds that may have acted as primordial energy currencies, sources of environmental energy, or sources of phosphorus for the synthesis of phosphorylated energy currencies. This review encompasses discussions on the evolutionary history of modern bioenergetics, and specifically those pathways with primordial relevance, and the geochemistry of bioavailable phosphorus on the ancient Earth. We highlight the importance of phosphorus, not only in the form of phosphate, to early biology and suggest future directions of study that may improve our understanding of the early evolution of bioenergetics.

The hidden role of dissolved organic carbon in the biogeochemical cycle of carbon in modern redox-stratified lakes

Abstract. The dissolved organic carbon (DOC) reservoir plays a critical role in the C cycle of marine and freshwater environments because of its size and implication in many biogeochemical reactions. Although it is poorly constrained, its importance in ancient Earth's C cycles is also commonly invoked. Yet DOC is rarely quantified and characterized in modern stratified analogues. In this study, we investigated the DOC reservoirs of four redox-stratified alkaline crater lakes in Mexico. We analyzed the concentrations and isotopic compositions of DOC throughout the four water columns and compared them with existing data on dissolved inorganic and particulate organic C reservoirs (DIC and POC). The four lakes have high DOC concentrations with great variability between and within the lakes (averaging 2 ± 4 mM; 1 SD, n=28; i.e., from ∼ 15 to 160 times the amount of POC). The δ13CDOC signatures also span a broad range of values from −29.3 ‰ to −8.7 ‰ (with as much as 12.5 ‰ variation within a single lake). The prominent DOC peaks (up to 21 mM), together with their associated isotopic variability, are interpreted as reflecting oxygenic and/or anoxygenic primary productivity through the release of excess fixed carbon in three of the lakes (Alberca de los Espinos, La Preciosa, and Atexcac). By contrast, the variability of [DOC] and δ13CDOC in the case of Lake Alchichica is mainly explained by the partial degradation of organic matter and the accumulation of DOC in anoxic waters. The DOC records detailed metabolic functions such as active DIC-uptake and DIC-concentrating mechanisms, which cannot be inferred from DIC and POC analyses alone but which are critical to the understanding of carbon fluxes from the environment to the biomass. Extrapolating our results to the geological record, we suggest that anaerobic oxidation of DOC may have caused the very negative C isotope excursions in the Neoproterozoic. It is, however, unlikely that a large oceanic DOC reservoir could overweigh the entire oceanic DIC reservoir. This study demonstrates how the analysis of DOC in modern systems deepens our understanding of the C cycle in stratified environments and helps to set boundary conditions for the Earth's past oceans.

Elucidating the Mechanism of Freeze‐Thaw Driven Content Mixing between Protocells

AbstractModern cells rely on highly evolved protein networks to accomplish essential life functions, including the inheritance of information from parents to their offspring. In the absence of these sophisticated molecular machineries, alternatives were required for primitive protocells to proliferate and disseminate genetic material. Recurring environmental constraints on ancient earth, such as temperature cycles, are considered as prebiotically plausible driving forces capable of shuffling of protocellular contents, thereby boosting compositional complexity. Using confocal fluorescence microscopy, we show that temperature oscillations such as freezing‐thawing (FT) cycles promote efficient content mixing between giant unilamellar vesicles (GUVs) as model protocells. We shed light on the underlying exchange mechanism and demonstrate that transient periods of destabilized membranes enable the diffusion of cargo molecules across vesicle membranes. Furthermore, we determine essential parameters, such as membrane composition, and quantify their impact on the lateral transfer efficiency. Our work outlines a simple scenario revolving around inter‐protocellular communication environmentally driven by periodic freezing and melting of water.

Hydrothermal vent fluid-seawater mixing and the origins of Archean iron formation

Precambrian iron formation provides valuable windows onto ancient marine environments, but exactly how these Fe- and Si-rich chemical sedimentary rocks formed have remained a matter of debate. Traditional models envisage Fe(III)-oxide formation through chemical and/or biological oxidation of soluble Fe2+, but sedimentological and textural data show that, throughout much of Archean stratigraphy, the finest-grained Fe(III)-oxides, previously interpreted to reflect primary components, were largely derived through the oxidation of the Fe(II)-silicate mineral greenalite. These observations have formed the basis for an alternative model where Fe2+ and SiO2(aq) combined to precipitate greenalite upon the mixing of hydrothermal fluids with ambient seawater. All models for iron formation genesis invoke hydrothermal alteration of seafloor basalt as the principal source of soluble Fe2+, and its subsequent transport in an anoxic deep ocean. However, the processes and products associated with the venting of near-axis hydrothermal fluids into Precambrian seawater, and implications for Archean iron formation, have not been investigated.In order to test models for Archean iron formation genesis, we used reaction path models to investigate the interactions between ancient hydrothermal vent fluids and anoxic seawater. We assembled available thermodynamic data and executed reaction path models to examine: (1) the equilibration between anoxic SO4-free seawater and basalt/gabbro at subseafloor hydrothermal conditions (400oC and 400-500 bar), (2) the cooling and decompression of these fluids during their ascent through the oceanic crust, and (3) their subsequent mixing with anoxic SO4-free seawater. Our results confirm previous suggestions that the effective lack of SO4 in Archean seawater would have allowed fayalite-magnetite-pyrrhotite-quartz equilibria to buffer fluid chemistry, in turn leading to reaction zone fluids characterised by high Fe/H2S ratios. Cooling and decompression of these reducing fluids upon ascent, and further reaction with mafic wall rock, leads to significant Fe-chlorite and quartz precipitation with little to no sulfide precipitation. As these vent fluids mix with cold seawater directly, or with conductively-heated seawater, mineral assemblages are dominated by pyrite, greenalite, and/or siderite. These results were obtained with no constraints imposed by Archean iron formation, aside from the exclusion of minnesotaite, a demonstrably secondary mineral.The quantity of hydrothermally-derived Fe2+ partitioned into Fe(II)-bearing minerals upon mixing (96-99% of total aqueous Fe), the relative proportion of greenalite precipitated, and its further concentration via plume-driven transport (by virtue of its low density), together imply that the mixing between hydrothermal vent fluids and anoxic seawater may have served as a principal mechanism for generating a large proportion of the mineral mass now preserved as Archean iron formation. These results reconcile long-standing sedimentological and geochemical observations indicating that Archean banded iron formations record a strong hydrothermal influence, and suggest that these unique deposits have the potential to constrain the nature of seafloor-hosted hydrothermal systems on the ancient Earth, and their chemical interactions with seawater.

Anhydrite solubility enhanced by CaO in silicate melts: Implications for sulfur cycling in subduction zones

The capacity of slab-derived melt in mobilizing sulfur from the slab to the mantle wedge is essential for understanding global S cycling in subduction zones of the present or ancient Earth. The promotive role of the CaO constituent in the silicate melts, provided chiefly by subducting sediments, in anhydrite dissolution in silicate melts has been noted before. Nevertheless, the dependence of SCAS, sulfur concentration in the melt at anhydrite saturation, on melt CaO concentration and the underlying dissolution mechanism have yet to be well constrained. Using a piston-cylinder apparatus, we carried out five series of phase equilibrium experiments between anhydrite and felsic melts to obtain constraints on the effect of melt CaO concentration, water concentration, temperature (T), and Ca/(Ca+Mg) ratios on SCAS at 1–3 GPa and 1050–1350 °C. Electron probe microanalyses of quenched products confirmed that the dissolved sulfur was predominantly present as SO42−. The experimental results show that SCAS increased significantly with increasing melt CaO and Ca/(Ca+Mg) ratios at a fixed degree of silicate melt polymerization (NBO/T = 0.43 ± 0.01). The SCAS also increased with increasing T and dissolved water concentration, and the effect of water was non-linear. Previous SCAS models could not capture well enough our new experimental data, therefore a new SCAS parameterization was developed using previous and our new experimental data. With our new results and model, we find that slab melts with high CaO concentration could efficiently transport S from the slab to the mantle wedge in the Archean. Slab melts in modern hot subduction zones can carry more than 1800 ppm sulfur, raising S concentration in the mantle wedge to the lower end of the observed level by metasomatism. In modern intermediate or colder subduction zones, however, slab melt is not an effective agent of sulfur transfer even at CaO concentration >10 wt%.

The origin of life: RNA and protein co-evolution on the ancient Earth.

How life emerged from simple non-life chemicals on the ancient Earth is one of the greatest mysteries in biology. The gene expression system of extant life is based on the interdependence between multiple molecular species (DNA, RNA, and proteins). While DNA is mainly used as genetic material and proteins as functional molecules in modern biology, RNA serves as both genetic material and enzymes (ribozymes). Thus, the evolution of life may have begun with the birth of a ribozyme that replicated itself (the RNA world hypothesis), and proteins and DNA joined later. However, the complete self-replication of ribozymes from monomeric substrates has not yet been demonstrated experimentally, due to their limited activity and stability. In contrast, peptides are more chemically stable and are considered to have existed on the ancient Earth, leading to the hypothesis of RNA-peptide co-evolution from the very beginning. Our group and collaborators recently demonstrated that (1) peptides with both hydrophobic and cationic moieties (e.g., KKVVVVVV) form β-amyloid aggregates that adsorb RNA and enhance RNA synthesis by an artificial RNA polymerase ribozyme and (2) a simple peptide with only seven amino acid types (especially rich in valine and lysine) can fold into the ancient β-barrel conserved in various enzymes, including the core of cellularRNA polymerases. These findings, together with recent reports from other groups, suggest that simple prebiotic peptides could have supported the ancient RNA-based replication system, gradually folded into RNA-binding proteins, and eventually evolved into complex proteins like RNA polymerase.

Frontiers of Carbonate Clumped Isotope Thermometry

Carbonate minerals contain stable isotopes of carbon and oxygen with different masses whose abundances and bond arrangement are governed by thermodynamics. The clumped isotopic value Δi is a measure of the temperature-dependent preference of heavy C and O isotopes to clump, or bond with or near each other, rather than with light isotopes in the carbonate phase. Carbonate clumped isotope thermometry uses Δi values measured by mass spectrometry (Δ47, Δ48) or laser spectroscopy (Δ638) to reconstruct mineral growth temperature in surface and subsurface environments independent of parent water isotopic composition. Two decades of analytical and theoretical development have produced a mature temperature proxy that can estimate carbonate formation temperatures from 0.5 to 1,100°C, with up to 1–2°C external precision (2 standard error of the mean). Alteration of primary environmental temperatures by fluid-mediated and solid-state reactions and/or Δi values that reflect nonequilibrium isotopic fractionations reveal diagenetic history and/or mineralization processes. Carbonate clumped isotope thermometry has contributed significantly to geological and biological sciences, and it is poised to advance understanding of Earth's climate system, crustal processes, and growth environments of carbonate minerals. ▪ Clumped heavy isotopes in carbonate minerals record robust temperatures and fluid compositions of ancient Earth surface and subsurface environments. ▪ Mature analytical methods enable carbonate clumped Δ47, Δ48, and Δ638 measurements to address diverse questions in geological and biological sciences. ▪ These methods are poised to advance marine and terrestrial paleoenvironment and paleoclimate, tectonics, deformation, hydrothermal, and mineralization studies.

Tropical Arctic

Tropical Arctic asks us to imagine the Arctic as a lush, green paradise (think Miami) through the story of an expedition to Greenland and the subsequent analysis of the fossils collected there by a team including an artist, botanist, and paleobotanist. The expedition team was specifically interested in a mass extinction event that occurred during a period of catastrophic climate change 200 million years ago between the Triassic and Jurassic periods. The team targeted particular fossil beds to help them find the answers they were looking for. They collected more than a metric ton of fossils and used the data to put together a picture of what the ancient Arctic ecosystem looked like by determining which plants were dominant and which were rare both before and after the mass extinction. They then used the fossils to create three-dimensional models to increase their understanding of the plant characteristics. This provided insight into which plants were resilient in the face of change and which characteristics might have contributed to that resilience. The team coupled this evidence with laboratory evidence on extant plants that measured their response to conditions similar to ancient Earth’s.What was a dense forest canopy in the Triassic transitioned to lower-growing plants that had to survive harsh atmospheric conditions in addition to frequent fire in the early Jurassic. When conditions stabilized, those plants able to survive could thrive. The team hypothesized that the slow rate of change of Earth’s conditions contributed to the plants’ ability to migrate, essentially germinating where conditions were favorable. Certain species were better adapted for conditions on one side of the boundary. The book ends with lessons for the future, pinpointing the magnitude of temperature change associated with a reduction in biodiversity and loss of species and what it might mean for Earth’s future.While the photographs and illustrations are beautiful, I found the story hard to follow. At the end of the book, I wished some of the information presented in Chapter 4 had been presented earlier. I was less motivated to learn about the expedition and all of the fossils collected because I didn’t know the story of the conditions on Earth that led to and followed the mass extinction. Had that context come first, it would have provided framing for the expedition, sparking curiosity about what the team found. I found myself wanting a map or a timeline to help me track Earth’s history both spatially and temporally.It may be beyond the scope of this book, but I was left wondering about the conditions in the Arctic that allowed all that plant life to grow in the first place. How does the 40,000-year cyclical change in the Earth’s tilt play into the story of energy and matter in the Arctic? Does that explain why it was lush there once but cold and icy there now? And how will the anthropogenic changes we have made to Earth’s system interact with that natural cycle?Today, many high school biology teachers are tasked with integrating Earth and space science standards into their curricula. Using historical events that caused changes to Earth’s system as phenomena can help students understand the flow of energy and matter and provide a context for understanding why the mechanisms of photosynthesis and cellular respiration are so important. Tropical Arctic is a beautiful text and illuminates a part of Earth’s history that I knew little about. On its own, it is missing necessary pieces to spark the interest of someone who lacks expertise in the field, but it would be a worthwhile complement to other texts on the topic.

An isolated chemolithoautotrophic ecosystem deduced from environmental isotopes: Ayyalon cave (Israel)

The stable isotopes composition of chemolithoautotrophic cave ecosystems is known to differ from epigenic caves. Here we show that in addition, dead carbon (devoid of 14C), is utilized and transferred throughout this ecosystem, rendering it unsuitable for radiocarbon dating. The connectivity of the Ayyalon Cave ecosystem with the surface is studied, along with its sources of energy and carbon, as well as the interconnections between its constituents. We use isotopic evidence to show that its ancient resilient ecosystem is based on an underground food web depending on rich biomass production by chemolithoautotrophic nutrient supplies, detached from surface photosynthesis. Carbon isotopic values indicate that: (1) the microbial biota use bicarbonate from the groundwater (23.34 pMC [% of modern carbon]) rather than the atmospheric CO2 above the water (71.36 pMC); (2) the depleted 14C signal is transferred through the entire ecosystem, indicating that the ecosystem is well-adapted and based on the cave biofilm which is in turn based on groundwater-dissolved inorganic carbon. Incubation of Ayyalon biofilm with 14C-labelled bicarbonate indicates uptake of the radio-labeled bicarbonate by sulfur-oxidizing proteobacteria Beggiatoa, suggesting that these sulfur-oxidizing microorganisms use the water-dissolved inorganic carbon for chemolithoautotrophic carbon fixation. Organic matter in the cave is much lighter in its stable nitrogen and carbon isotopes compared with respective surface values, as expected in chemolithoautotrophic systems. This evidence may be applicative to subsurface voids of ancient Earth environments and extraterrestrial systems.