Search

THE MANY LIVES OF JAMES LOVELOCK Science, secrets and Gaia Theory JONATHAN WATTS 320pp. Canongate. 25 [pounds sterling]. James Lovelock, the inventor and architect of an influential view of Earth [...]

Calcium carbonate (CaCO3) biomineralizing organisms have played major roles in the history of life and the global carbon cycle during the past 541 Ma. Both marine diversification and mass extinctions reflect physiological responses to environmental changes through time. An integrated understanding of carbonate biomineralization is necessary to illuminate this evolutionary record and to understand how modern organisms will respond to 21st century global change. Biomineralization evolved independently but convergently across phyla, suggesting a unity of mechanism that transcends biological differences. In this review, we combine CaCO3 skeleton formation mechanisms with constraints from evolutionary history, omics, and a meta-analysis of isotopic data to develop a plausible model for CaCO3 biomineralization applicable to all phyla. The model provides a framework for understanding the environmental sensitivity of marine calcifiers, past mass extinctions, and resilience in 21st century acidifying oceans. Thus, it frames questions about the past, present, and future of CaCO3 biomineralizing organisms.

THE METEORITES Encounters with outer space and deep time HELEN GORDON 288pp. Profile. 20 [pounds sterling]. A pre-dynastic tomb in Lower Egypt preserves the remains of a long-departed youth, adorned [...]

Resolving how Earth surface redox conditions evolved through the Proterozoic Eon is fundamental to understanding how biogeochemical cycles have changed through time. The redox sensitivity of cerium relative to other rare earth elements and its uptake in carbonate minerals make the Ce anomaly (Ce/Ce*) a particularly useful proxy for capturing redox conditions in the local marine environment. Here, we report Ce/Ce* data in marine carbonate rocks through 3.5 billion years of Earth's history, focusing in particular on the mid-Proterozoic Eon (i.e., 1.8 - 0.8 Ga). To better understand the role of atmospheric oxygenation, we use Ce/Ce* data to estimate the partial pressure of atmospheric oxygen (pO2) through this time. Our thermodynamics-based modeling supports a major rise in atmospheric oxygen level in the aftermath of the Great Oxidation Event (~ 2.4 Ga), followed by invariant pO2 of about 1% of present atmospheric level through most of the Proterozoic Eon (2.4 to 0.65 Ga).

Crystallization by particle attachment (CPA) of amorphous precursors has been demonstrated in modern biomineralized skeletons across a broad phylogenetic range of animals. Precisely the same precursors, hydrated (ACC-H2O) and anhydrous calcium carbonate (ACC), have been observed spectromicroscopically in echinoderms, mollusks, and cnidarians, phyla drawn from the 3 major clades of eumetazoans. Scanning electron microscopy (SEM) here also shows evidence of CPA in tunicate chordates. This is surprising, as species in these clades have no common ancestor that formed a mineralized skeleton and appear to have evolved carbonate biomineralization independently millions of years after their late Neoproterozoic divergence. Here we correlate the occurrence of CPA from ACC precursor particles with nanoparticulate fabric and then use the latter to investigate the antiquity of the former. SEM images of early biominerals from Ediacaran and Cambrian shelly fossils show that these early calcifiers used attachment of ACC particles to form their biominerals. The convergent evolution of biomineral CPA may have been dictated by the same thermodynamics and kinetics as we observe today.

We present evidence of exceptional preservation in the nacre and prismatic layers of a 66 Ma bivalve shell using photoemission electron spectromicroscopy (PEEM). PEEM is a novel method to assess in situ the quality of mineralogical and organic preservation. The analysis is non-invasive and non-destructive, providing spatially explicit maps of microstructure, organics and mineral components, and the crystallographic orientation in mollusk shells. Comparison of a Late Cretaceous and a modern shell demonstrates that the 66 Ma shell (1) preserves original aragonite and calcite crystals in nacre and prismatic layers, respectively, (2) maintains nearly identical mineral microstructure and crystal orientations, and (3) preserves interprismatic proteins. Remarkably, interprismatic proteins are preserved with intact peptide bonds, and suggest an abundance of the amino acid glycine. These findings in a 66 Ma shell support the exceptional quality of organic preservation documented here, which may prove to be relatively common among fossil shells that preserve nacre. PEEM analysis is useful for understanding taphonomic processes influencing shell and molecular fossil preservation over geologic time scales, and contributes to our knowledge of molluscan physiology, biomineralization, evolution, and diagenesis.

Extreme events can be a major driver of evolutionary change over geological and contemporary timescales. Outstanding examples are evolutionary diversification following mass extinctions caused by extreme volcanism or asteroid impact. The evolution of organisms in contemporary time is typically viewed as a gradual and incremental process that results from genetic change, environmental perturbation or both. However, contemporary environments occasionally experience strong perturbations such as heat waves, floods, hurricanes, droughts and pest outbreaks. These extreme events set up strong selection pressures on organisms, and are small-scale analogues of the dramatic changes documented in the fossil record. Because extreme events are rare, almost by definition, they are difficult to study. So far most attention has been given to their ecological rather than to their evolutionary consequences. We review several case studies of contemporary evolution in response to two types of extreme environmental perturbations, episodic (pulse) or prolonged (press). Evolution is most likely to occur when extreme events alter community composition. We encourage investigators to be prepared for evolutionary change in response to rare events during long-term field studies.This article is part of the themed issue 'Behavioural, ecological and evolutionary responses to extreme climatic events'.

Low‐lying islands in tropical regions are vulnerable to near‐term sea‐level rise and hurricane‐induced flooding, with substantial human impact. These risks motivate researchers to elucidate the processes and timescales involved in the formation, growth and stabilization of coastlines through the study of Holocene shoreline dynamics. Little Ambergris Cay (Turks and Caicos Islands) is a low‐lying carbonate island that provides a case study in the nucleation and growth of such islands. This study investigates the sedimentology and radiocarbon chronology of the island's lithified sediments to develop a model for its history. The island's lithified rim encloses a tidal swamp populated by microbial mats and mangroves. Preliminary radiocarbon data supported a long‐standing inference that the island is Holocene in age. This study integrates petrographic, sedimentological and new radiocarbon data to quantify the age of the island and develop a model for its evolution. Results indicate that the ages of most lithified sediments on the island are <1000 cal yr bp, and the generation and lithification of carbonate sediment in this system supports coastline growth of at least 5 cm/year. The lithification of anthropogenic detritus was documented, consistent with other evidence that in recent centuries the lithified rim has grown by rates up to tens of centimetres per year. A unit of mid‐Holocene age was identified and correlated with a similar unit of early transgressive aeolianite described from San Salvador, The Bahamas. It is proposed that this antecedent feature played an important role in the nucleation and formation of the modern island. Results extend an established Bahamian stratigraphic framework to the south‐western extreme of the Lucayan archipelago, and highlight the dynamism of carbonate shorelines, which should inform forward‐looking mitigation strategies to increase coastal resiliency to sea‐level rise. These results inform interpretation of the palaeoenvironmental record of carbonate environments, underscoring their geologically rapid pace of lithification. [ABSTRACT FROM AUTHOR]

Silicified peritidal carbonates of the Tonian Draken Formation, Spitsbergen, contain highly diverse and well‐preserved microfossil assemblages dominated by filamentous microbial mats, but also including diverse benthic and/or allochthonous (possibly planktonic) microorganisms. Here, we characterize eight morphospecies in focused ion beam (FIB) ultrathin sections using transmission electron microscopy (TEM) and X‐ray absorption near‐edge structure (XANES) spectromicroscopy. Raman and XANES spectroscopies show the highly aromatic molecular structure of preserved organic matter. Despite this apparently poor molecular preservation, nano‐quartz crystallization allowed for the preservation of various ultrastructures distinguished in TEM. In some filamentous microfossils (Siphonophycus) as well as in all cyanobacterial coccoids, extracellular polysaccharide sheaths appear as bands of dispersed organic nanoparticles. Synodophycus microfossils, made up of pluricellular colonies of coccoids, contain organic walls similar to the F‐layers of pleurocapsalean cyanobacteria. In some fossils, internal content occurs as particulate organic matter, forming dense networks throughout ghosts of the intracellular space (e.g., in Salome svalbardensis filaments), or scarce granules (in some Chroococcales). In some chroococcalean microfossils (Gloeodiniopsis mikros, and also possibly Polybessurus), we find layered internal contents that are more continuous than nanoparticulate bands defining the sheaths, and with a shape that can be contracted, folded, or invaginated. We interpret these internal layers as the remains of cell envelope substructures and/or photosynthetic membranes thickened by additional cellular material. Some Myxococccoides show a thick (up to ~ 0.9 μm) wall ultrastructure displaying organic pillars that is best reconciled with a eukaryotic affinity. Finally, a large spheroid with ruptured wall, of uncertain affinity, displays a bi‐layered envelope. Altogether, our nanoscale investigations provide unprecedented insights into the taphonomy and taxonomy of this well‐preserved assemblage, which can help to assess the nature of organic microstructures in older rocks. [ABSTRACT FROM AUTHOR]

Large meteorite impacts must have strongly affected the habitability of the early Earth. Rocks of the Archean Eon record at least 16 major impact events, involving bolides larger than 10 km in diameter. These impacts probably had severe, albeit temporary, consequences for surface environments. However, their effect on early life is not well understood. Here, we analyze the sedimentology, petrography, and carbon isotope geochemistry of sedimentary rocks across the S2 impact event (37 to 58 km carbonaceous chondrite) forming part of the 3.26 Ga Fig Tree Group, South Africa, to evaluate its environmental effects and biological consequences. The impact initiated 1) a giant tsunami that mixed Fe2+-rich deep waters into the Fe2+-poor shallow waters and washed debris into coastal areas, 2) heating that caused partial evaporation of surface ocean waters and likely a short-term increase in weathering and erosion on land, and 3) injection of P from vaporization of the S2 bolide. Strata immediately above the S2 impact event contain abundant siderites, which are associated with organic matter and exhibit light and variable δ13Ccarb values. This is consistent with microbial iron cycling in the wake of the impact event. Thus, the S2 impact likely had regional, if not global, positive and negative effects on life. The tsunami, atmospheric heating, and darkness would likely have decimated phototrophic microbes in the shallow water column. However, the biosphere likely recovered rapidly, and, in the medium term, the increase in nutrients and iron likely facilitated microbial blooms, especially of iron-cycling microbes. [ABSTRACT FROM AUTHOR]

Nacre, the iridescent outer lining of pearls and inner lining of many mollusk shells, is composed of periodic, parallel, organic sheets alternating with aragonite (CaCO3) tablet layers. Nacre tablet thickness (TT) generates both nacre's iridescence and its remarkable resistance to fracture. Despite extensive studies on how nacre forms, the mechanisms controlling TT remain unknown, even though they determine the most conspicuous of nacre's characteristics, visible even to the naked eye. Thermodynamics predicts that temperature (T) will affect both physical and chemical components of biomineralized skeletons. The chemical composition of biominerals is well-established to record environmental parameters, and has therefore been extensively used in paleoclimate studies. The physical structure, however, has been hypothesized but never directly demonstrated to depend on the environment. Here we observe that the physical TT in nacre from modern and fossil shallow-water shells of the bivalves Pinna and Atrina correlates with T as measured by the carbonate clumped isotope thermometer. Based on the observed TT vs. T correlation, we anticipate that TT will be used as a paleothermometer, useful to estimate paleotemperature in shallow-water paleoenvironments. Here we successfully test the proposed new nacre TT thermometer on two Jurassic Pinna shells. The increase of TT with T is consistent with greater aragonite growth rate at higher T, and with greater metabolic rate at higher T. Thus, it reveals a complex, T-dependent biophysical mechanism for nacre formation.

The Proterozoic-Cambrian transition records the appearance of essentially all animal body plans (phyla), yet to date no single hypothesis adequately explains both the timing of the event and the evident increase in diversity and disparity. Ecological triggers focused on escalatory predator-prey "arms races" can explain the evolutionary pattern but not its timing, whereas environmental triggers, particularly ocean/atmosphere oxygenation, do the reverse. Using modern oxygen minimum zones as an analog for Proterozoic oceans, we explore the effect of low oxygen levels on the feeding ecology of polychaetes, the dominant macrofaunal animals in deep-sea sediments. Here we show that low oxygen is clearly linked to low proportions of carnivores in a community and low diversity of carnivorous taxa, whereas higher oxygen levels support more complex food webs. The recognition of a physiological control on carnivory therefore links environmental triggers and ecological drivers, providing an integrated explanation for both the pattern and timing of Cambrian animal radiation.