M.J.L. Coolen

The formation of peak rings in large impact craters

Drilling into Chicxulubs formation The Chicxulub impact crater, known for its link to the demise of the dinosaurs, also provides an opportunity to study rocks from a large impact structure. Large impact craters have “peak rings” that define a complex crater morphology. Morgan et al. looked at rocks from a drilling expedition through the peak rings of the Chicxulub impact crater (see the Perspective by Barton). The drill cores have features consistent with a model that postulates that a single over-heightened central peak collapsed into the multiple-peak-ring structure. The validity of this model has implications for far-ranging subjects, from how giant impacts alter the climate on Earth to the morphology of crater-dominated planetary surfaces. Science, this issue p. 878; see also p. 836 Rock samples from IODP/ICDP Expedition 364 support the dynamic collapse model for the formation of the Chicxulub crater. Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

Seqenv : linking sequences to environments through text mining

Understanding the distribution of taxa and associated traits across different environments is one of the central questions in microbial ecology. High-throughput sequencing (HTS) studies are presently generating huge volumes of data to address this biogeographical topic. However, these studies are often focused on specific environment types or processes leading to the production of individual, unconnected datasets. The large amounts of legacy sequence data with associated metadata that exist can be harnessed to better place the genetic information found in these surveys into a wider environmental context. Here we introduce a software program, seqenv, to carry out precisely such a task. It automatically performs similarity searches of short sequences against the “nt” nucleotide database provided by NCBI and, out of every hit, extracts–if it is available–the textual metadata field. After collecting all the isolation sources from all the search results, we run a text mining algorithm to identify and parse words that are associated with the Environmental Ontology (EnvO) controlled vocabulary. This, in turn, enables us to determine both in which environments individual sequences or taxa have previously been observed and, by weighted summation of those results, to summarize complete samples. We present two demonstrative applications of seqenv to a survey of ammonia oxidizing archaea as well as to a plankton paleome dataset from the Black Sea. These demonstrate the ability of the tool to reveal novel patterns in HTS and its utility in the fields of environmental source tracking, paleontology, and studies of microbial biogeography. To install seqenv, go to: https://github.com/xapple/seqenv.

Coccolithovirus facilitation of carbon export in the North Atlantic

Marine phytoplankton account for approximately half of global primary productivity1, making their fate an important driver of the marine carbon cycle. Viruses are thought to recycle more than one-quarter of oceanic photosynthetically fixed organic carbon2, which can stimulate nutrient regeneration, primary production and upper ocean respiration2 via lytic infection and the ‘virus shunt’. Ultimately, this limits the trophic transfer of carbon and energy to both higher food webs and the deep ocean2. Using imagery taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite, along with a suite of diagnostic lipid- and gene-based molecular biomarkers, in situ optical sensors and sediment traps, we show that Coccolithovirus infections of mesoscale (~100u2009km) Emiliania huxleyi blooms in the North Atlantic are coupled with particle aggregation, high zooplankton grazing and greater downward vertical fluxes of both particulate organic and particulate inorganic carbon from the upper mixed layer. Our analyses captured blooms in different phases of infection (early, late and post) and revealed the highest export flux in ‘early-infected blooms’ with sinking particles being disproportionately enriched with infected cells and subsequently remineralized at depth in the mesopelagic. Our findings reveal viral infection as a previously unrecognized ecosystem process enhancing biological pump efficiency.Using a combination of remote-sensing technologies, lipidomics and gene-based biomarkers, the authors demonstrate a coupling between viral infection of an Emiliania huxleyi bloom and the export of organic and inorganic carbon from the photic zone.

Chicxulub and the Exploration of Large Peak-Ring Impact Craters through Scientific Drilling

The Chicxulub crater is the only well-preserved peak-ring crater on Earth and linked, famously, to the K-T or K-Pg mass extinction event. For the first time, geologists have drilled into the peak ring of that crater in the International Ocean Discovery Program and International Continental Scientific Drilling Program (IODP-ICDP) Expedition 364. The Chicxulub impact event, the environmental calamity it produced, and the paleobiological consequences are among the most captivating topics being discussed in the geologic community. Here we focus attention on the geological processes that shaped the ~200-km-wide impact crater responsible for that discussion and the expedition’s first year results.

Palaeobiology of red and white blood cell-like structures, collagen and cholesterol in an ichthyosaur bone

Carbonate concretions are known to contain well-preserved fossils and soft tissues. Recently, biomolecules (e.g. cholesterol) and molecular fossils (biomarkers) were also discovered in a 380 million-year-old concretion, revealing their importance in exceptional preservation of biosignatures. Here, we used a range of microanalytical techniques, biomarkers and compound specific isotope analyses to report the presence of red and white blood cell-like structures as well as platelet-like structures, collagen and cholesterol in an ichthyosaur bone encapsulated in a carbonate concretion from the Early Jurassic (~182.7u2009Ma). The red blood cell-like structures are four to five times smaller than those identified in modern organisms. Transmission electron microscopy (TEM) analysis revealed that the red blood cell-like structures are organic in composition. We propose that the small size of the blood cell-like structures results from an evolutionary adaptation to the prolonged low oxygen atmospheric levels prevailing during the 70u2009Ma when ichthyosaurs thrived. The δ13C of the ichthyosaur bone cholesterol indicates that it largely derives from a higher level in the food chain and is consistent with a fish and cephalopod diet. The combined findings above demonstrate that carbonate concretions create isolated environments that promote exceptional preservation of fragile tissues and biomolecules.

Metagenomics of pigmented and cholesterol gallstones: the putative role of bacteria

There is growing evidence for bacteria playing a role in the pathogenesis and formation of pigmented gallstones from humans. These studies mainly involved cultivation of gallstone-associated bacteria and 16S rRNA profiling, providing an indirect link between processes involved in gallstone formation by the bacteria in-situ. Here, we provide functional metagenomic evidence of a range of genes involved in bile stress response, biofilm formation, and anaerobic energy metabolism by Gram-negative Klebsiella in pigmented gallstones from a 76-year-old male patient. Klebsiella was also present in one cholesterol-type stone in a 30-year-old female patient who had additional cholesterol gallstones characterised by Gram-positive bacteria. Pigmented stones further revealed a predominance of genes involved in carbohydrate metabolism, whilst cholesterol stones indicated a profile dominanted by protein metabolism possibly reflecting known chemical differences between Gram-negative and Gram-positive biofilm matrices. Archaeal genes were not detected. Complementary carbon and hydrogen isotopic analyses of cholesterol within the patients’ stones revealed homogeneity, suggesting a common diet or cholesterol biosynthesis pathway that has little influence on microbial composition. This pilot study provides a framework to study microbial processes that play a potential role in gallstone formation across markedly different types of stones and patient backgrounds.

Light regulation of coccolithophore host–virus interactions

Viruses that infect photoautotrophs have a fundamental relationship with light, given the need for host resources. We investigated the role of light on Coccolithovirus (EhV) infection of the globally distributed coccolithophore, Emiliania huxleyi. Light was required for EhV adsorption, and viral production was highest when host cultures were maintained in continuous light or at irradiance levels of 150-300xa0μmolxa0m-2 xa0s-1 . During the early stages of infection, photosynthetic electron transport remained high, while RuBisCO expression decreased concomitant with an induction of the pentose phosphate pathway, the primary source of de novo nucleotides. A mathematical model developed and fitted to the laboratory data supported the hypothesis that EhV replication was controlled by a trade-off between host nucleotide recycling and de novo synthesis, and that photoperiod and photon flux could toggle this switch. Laboratory results supported field observations that light was the most robust driver of EhV replication within E.xa0huxleyi populations collected across a 2000 nautical mile transect in the North Atlantic. Collectively, these findings demonstrate that light can drive host-virus interactions through a mechanistic interplay between host metabolic processes, which serve to structure infection and phytoplankton mortality in the upper ocean.

Rapid recovery of life at ground zero of the end-Cretaceous mass extinction

The Cretaceous/Palaeogene mass extinction eradicated 76% of species on Earth1,2. It was caused by the impact of an asteroid3,4 on the Yucatán carbonate platform in the southern Gulf of Mexico 66 million years ago5, forming the Chicxulub impact crater6,7. After the mass extinction, the recovery of the global marine ecosystem—measured as primary productivity—was geographically heterogeneous8; export production in the Gulf of Mexico and North Atlantic–western Tethys was slower than in most other regions8–11, taking 300 thousand years (kyr) to return to levels similar to those of the Late Cretaceous period. Delayed recovery of marine productivity closer to the crater implies an impact-related environmental control, such as toxic metal poisoning12, on recovery times. If no such geographic pattern exists, the best explanation for the observed heterogeneity is a combination of ecological factors—trophic interactions13, species incumbency and competitive exclusion by opportunists14—and ‘chance’8,15,16. The question of whether the post-impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems. If there is a relationship between the distance from the impact and the recovery of marine productivity, we would expect recovery rates to be slowest in the crater itself. Here we present a record of foraminifera, calcareous nannoplankton, trace fossils and elemental abundance data from within the Chicxulub crater, dated to approximately the first 200 kyr of the Palaeocene. We show that life reappeared in the basin just years after the impact and a high-productivity ecosystem was established within 30 kyr, which indicates that proximity to the impact did not delay recovery and that there was therefore no impact-related environmental control on recovery. Ecological processes probably controlled the recovery of productivity after the Cretaceous/Palaeogene mass extinction and are therefore likely to be important for the response of the ocean ecosystem to other rapid extinction events.Micro- and nannofossil, trace fossil and geochemical evidence from the Chicxulub impact crater demonstrates that proximity to the asteroid impact site did not determine rates of recovery of marine ecosystems after the end-Cretaceous mass extinction.

Neoglacial Climate Anomalies and the Harappan Metamorphosis

Climate exerted constraints on the growth and decline of past human societies but our knowledge of temporal and spatial climatic patterns is often too restricted to address causal connections. At a global scale, the inter-hemispheric thermal balance provides an emergent framework for understanding regional Holocene climate variability. As the thermal balance adjusted to gradual changes in the seasonality of insolation, the Intertropical Convergence Zone migrated southward accompanied by a weakening of the Indian summer monsoon. Superimposed on this trend, anomalies such as the Little Ice Age point to asymmetric changes in the extratropics of either hemisphere. Here we present a reconstruction of the Indian winter monsoon in the Arabian Sea for the last 6000 years based on paleobiological records in sediments from the continental margin of Pakistan at two levels of ecological complexity: sedimentary ancient DNA reflecting water column environmental states and planktonic foraminifers sensitive to winter conditions. We show that strong winter monsoons between ca. 4500 and 3000 years ago occurred during a period characterized by a series of weak interhemispheric temperature contrast intervals, which we identify as the early neoglacial anomalies (ENA). The strong winter monsoons during ENA were accompanied by changes in wind and precipitation patterns that are particularly evident across the eastern Northern Hemisphere and tropics. This coordinated climate reorganization may have helped trigger the metamorphosis of the urban Harappan civilization into a rural society through a push–pull migration from summer flood-deficient river valleys to the Himalayan piedmont plains with augmented winter rains. The decline in the winter monsoon between 3300 and 3000 years ago at the end of ENA could have played a role in the demise of the rural late Harappans during that time as the first Iron Age culture established itself on the Ghaggar-Hakra interfluve. Finally, we speculate that time-transgressive land cover changes due to aridification of the tropics may have led to a generalized instability of the global climate during ENA at the transition from the warmer Holocene thermal maximum to the cooler Neoglacial.