My top 10 picks for historical geology research of the year. There is an inevitable overlap with palaeontology, so I’ve included only those papers that can only be considered pure historical geology, so stuff about the origin of life, mass extinctions, the history of climate, that kind of thing. The master list contains 29 papers. [OA] indicates open access papers.
By the way, when you’re done here, check out the 55th Carnival of Evolution, up at Genome Engineering!
Near the end of this post is this diagram showing the relative contributions of climate change and hunting to the megafaunal extinctions of the Pleistocene. Many have asked me how we know that humans could have had such a large impact, especially in Australia. This paper presents the answer, as well as confirmation that it is indeed humans that caused the extinctions there, and not drying of the climate. First of all, there’s the incredible coincidence that the faunas declined and went extinct shortly after human arrival on Australia, 45 ka. But it’s the evidence compiled by Rule et al. that’s the most compelling. They got a core going back 130000 years from a volcanic lake in Queensland, and measured the amounts of Sporomiella spores, charcoal, pollen, and of various plant types. Sporomiella is a fungus that grows in the dung of megaherbivores, and so is indicative of their presence. 130-41 ka, Sporomiella numbers are very high, plants indicate a tropical rainforest setting with little charcoal (no wildfires). Then there was a sudden shift at 41 ka: Sporomiella levels plummet, grassy-type pollen becomes dominant, Eucalyptus forests become the landscape type, and a lot fo charcoal gets deposited. Looking at this in high resolution, Rule et al. observed that the Sporomiella decline was the first change – i.e. it was the megaherbivores that died out first, before the habitat change. This means that climate change was not the cause of their extinction. With the herbivores gone, the habitat changed, and Rule et al. suggest that the change in plant type led to the wildfire-friendly habitats that caused the deposition of lots of charcoal, although it’s just as likely that human-caused fires were the culprit there too. So the case is more or less closed: we did it. Hooray!
I’m not a fan of the RNA world model, but it does have as much evidence, and is as plausible as, the metabolism-first model that I support (see here for summaries of the two models). This paper provides some excellent evidence for the RNA world model. It’s part of a larger research direction these days to create self-replicating RNA systems in the lab. Past studies have managed to make ribozymes (the catalytic components of RNA) that amplify themselves and compete against each other. In this study, Lehmann et al. depart from that and manage to create networks of ribozymes that aren’r selfish, but instead cooperate – they still amplify themselves, but not in a way as to compete with each other. The end result is that these cooperative network are much more efficient than the lone wolves. The implication for the RNA World is that this is what might have been the key trigger: there may have been a soup of competing RNAs, but then a specific set of cooperative ones arose and obliterated the individuals; this research tells us that this is possible, and not just speculation.
The Hettangian is the first stage of the Jurassic, coming right after the massive end-Triassic extinction. In this paper, Richoz et al. report finding extraordinary amounts of isorenieratane in their German and Luxembourgian shales from the start of the stage, as shown in (e). The significance is that isorenieratane is a biomarker for green sulphur bacteria, and finding such an abundance of it means that the sediments that later became shales were deposited in an anoxic ocean floor with very high hydrogen sulphide concentrations (the environment that green sulphur bacteria thrive in). This then gives us some more insights into the extinction that occurred at the end of the Triassic and the recovery from it – specifically, the strange disconnect between marine and terrestrial recoveries. The first dinosaurs radiated as soon as the earliest Hettangian, but it took 10 million years for the shallow seas to regain their pre-extinction diversities. We now know why.
Even if the Chixculub asteroid hadn’t come, there were several biotic changes happening in the Cretaceous – just look at the wackiness of Cretaceous ammonites. Terrestrial ecosystems were no exception, and this paper provides evidence that the changes occurring there made the extinction more effective. The changes occurred in the Maastrichtian, the last stage of the Cretaceous, and were of varying nature: tectonic changes led to increasing provinciality and isolation; changes in dinosaur species diversity; and functional shifts in ecologies. What they led to was an increased risk of extinction, setting the threshold for secondary extinction lower than in the Campanian (the stage before the Maastrichtian), and thus exacerbated the effect of the extinction event (although, to be fair, they were doomed under the circumstances anyway).
The P-T extinction isn’t only the most severe mass extinction to have ever hit the Earth, it’s also a classic example for a delayed recovery from an extinction event. See the last section of this post for details. The results of this study change the anrrative somewhat, since Song et al. find that the brunt of the extinction happened in the early Triassic – when the recovery was supposedly being delayed. There was an earlier extinction pulse 180000 years earlier at the end of the Permian. This just emphasises the need to look at such seemingly sudden events in very high resolution – extinction events are never, ever instantaneous.
That said, the delayed recovery was a true phenomenon, even with this extra extinction pulse. See Delayed recovery of non-marine tetrapods after the end-Permian mass extinction tracks global carbon cycle [OA] for a terrestrial vertebrate example, with the link to the carbon cycle.
The mid-Cretaceous is well-known as one of the more severe prolonged greenhouse climate phases the Earth went through. Studying and modelling it provides a useful analog for modern global warming. This paper presents a thorough model of the mid-Cretaceous climate, with a special focus on vegetation responses and feedbacks. The findings are as expected: forests expand towards the poles, leading to lower albedo and thus reinforcing the warming. On the other hand, in temperate and tropical zones, there is a decrease in forest cover, leading to more cooling. There were also some novel results. The ocean surface gets warmer due to more atmospheric vapour leading to more longwave radiation being retained. And there is more precipitation, leading to more freshwater input, leading to a screwing up in the circulation patterns, leading to less heat transfer to the poles. This later stuff isn’t too applicable for us today, since our continental distributions and thus circulation patterns are different, but they do provide an indication of just how complex climate dynamics are.
For more on the mid-Cretaceous greenhouse, see Orbital control on carbon cycle and oceanography in the mid-Cretaceous greenhouse. The oceanographical aspect going to the late Cretaceous is summarised in Evolution of middle to Late Cretaceous oceans—A 55 m.y. record of Earth’s temperature and carbon cycle.
For information on the forests of the Cretaceous, see Cretaceous forest composition and productivity inferred from a global fossil wood database.
Mass extinctions are typically ranked by number of extinct species. It works fine, but similar to how metrics other than species richness are used in modern ecology, more insights can be gained from using other indices. For example, phylogenetic diversity, or, as proposed in this paper, resultant ecological changes. The Permian-Triassic and Cretaceous extinctions led to a restructuring of terrestrial and marine ecosystems to accommodate the new dominant organisms. On the other hand, the end-Ordovician extinction resulted in a lot of species going extinct, but the replacements were more or less the same, ecologically. Different actors, same roles – there was very little ecological impact. Hence, by this metric, the end-Ordovician extinction ranks pretty low, and another extinction event comes to take its place: the Serpukhovian extinction (mid-Carboniferous, ~326 Ma), with 31% of species going extinct, mostly ammonites, crinoids, conodonts, and fish. However, the ecological impact was fairly significant, with the glaciation that caused it leading a much decreased recovery rate.
In #6, I linked to my post containing the summary of the P-T extinction, where it’s made clear that the whole ordeal started with global warming. This paper gives a high-resolution look at the temperature changes, by looking at the isotopes, with the mainr esult being an increase in surface water temperature of 8°C in the low latitudes right before the end of the Permian, so at the start of the first large extinction phase (see commentary to #6).
To see how complex palaeoclimate of the time can be, see Climatic and biotic upheavals following the end-Permian mass extinction.
By looking at the foraminifera across the K-T boundary, Alegret et al. find that, like in the terrestrial ecosystems, there wasn’t any drastic ecological collapse in the oceans as a result of the K-T extinction. Of course, there was a drop in productivity at impact time, but ocean primary productivity remained more or less stable. The authors suggest that ocean acidification was the main extinction driver in the oceans, something which doesn’t contradict what we observe from other periods of time nor from today.
This one gets first place for being extremely interesting. It’s a typical “where could have the first cells originated?” paper, but it usesd a wide data range to make its case, looking at both phylogenetics and geochemistry. Their argument is basically as follows. All modern cells require large amounts of specific ions. Modern cells have pumps to get these ions from the environment, but the primordial cell-like constructs didn’t have them, so true cells must have originated in a place where these ions are highly-concentrated. Geochemistry tells us that the primordial oceans and geothermal vents in them don’t fulfil the criteria. However, ponds within geothermal fields tick all the right boxes, therefore that’s the more likely place where cells originated. As always with such papers, it’s a war of evidenced speculation. The origin of life/cells near geothermal vents is explained in my origin of life post, where you’ll see the major evidences for it (ancestral hyperthermophily, chemical concentrations) are also fulfiled by this hypothesis. So, basically, believe whatever you want, they’re both as viable as each other at this time :) One very interesting implication of this new hypothesis though is that cells invaded the oceans secondarily. I find it appealing because it provides a convenient explanation (or maybe it’s a dreadful just-so story?) for the origin of genetic repair, mechanisms of which are found all over the kingdoms of life, since a terrestrial origin origin of life and cells means they were exposed to UV and ionising radiation since “birth”, meaning they had to evolve those mechanisms ASAP.
In Open Questions on the Origin of Life at Anoxic Geothermal Fields, the authors deal with all the critiques that have been thrown at them, so read it as an addendum.
The hypothesis hinges on the biogeochemical analysis the authors did. Magnesium didn’t play much of a role, but in The significance of Mg in prebiotic geochemistry [OA], Holm explains how magnesium was important in the origin of life.