Tag: paleoclimate

The song of ice and diatoms

In a previous post on this lethargic blog, I briefly touched the Antarctic extinction, the mass extinction of the fauna and flora inhabiting a whole continent, reduced to a desert of ice.

As is too often happens, I worried myself only about the macroscopic biota, and lazily forgot the microscopic lifeforms. Luckly, scientists are smarter than I am. Eveline Pinseel and coworkers have now described, in a paper on Science Advances, what happened to some of the most iconic microbial taxons – diatoms – that inhabited Antarctica (I have to thank Sylvie Coyaud for bringing this to my attention). Diatoms are special for many reasons, but mainly as being an example of microorganism with incredibly beautiful and unique shells, that readily fossilize and can be classified into prehistoric species and genera, something that it is hardly possible with many other unicellular beings (a notable exception are foraminiferans).

Extant Southern hemisphere diatoms and their extinct relatives, from Eveline et al. 2021

The study shows that in the Miocene (14-15 millions of years ago) Antarctic lakes had a rich diatom flora, with unique species so far unknown to science, but strongly related to those now present in New Zealand, Tasmania; and was overall not unlike the flora present in the contemporary Arctic. Temperatures, now hovering around -12° C, were around +5 °C at the time.

Then, the ice came. In the Pleistocene, about 150.000 years ago, the climate of Antarctica was much similar to today and, correspondingly, diatoms were much less diverse than in the Miocene, but still more diverse than today. The last glacial period was the final hammer for the Antarctic diatom flora: more diatom species were wiped out, not unlike the mammoths, or were confined to sub-Antarctic realms. The diatoms of today’s Antarctica land are a relict of what was once a diverse flora, adapted to some of the harshest conditions on Earth.

According to the authors

Although there are multiple records of regional extinctions and species turnover of lacustrine diatoms in Quaternary paleorecords of the Northern (60) and Southern (61) Hemisphere, the scale of the extinction of diatoms in continental Antarctica since the mid-Miocene is, both at the species- and genus-level, beyond anything reported in the literature thus far.

In other words, the Antarctic extinction was also the most profound mass extinction of diatoms known stodayo far. Their fate was most probably shared by large parts of the Antarctic microflora, of which we cannot say anything only because they didn’t leave shells to fossilize. Mass extinctions ,therefore, events reshape biodiversity at all levels. This is not news per se, we know that for example the end-Cretaceous extinction led to the extinction of numerous foraminiferan species. But it is a somber reminder of how much biodiversity has been lost forever, how much is going lost now, without us even noticing, without us being even able to interpret what such a loss would mean for other lifeforms.

The paper is: Eveline Pinseel et al. “Extinction of austral diatoms in response to large-scale climate dynamics in Antarctica”, Science Advances, Volume 7, Issue 38, 15 September 2021, https://doi.org/10.1126/sciadv.abh3233

Baked Alaska, or a more complex Permian extinction

The Permian extinction lies in the twilight of deep time: not so remote we cannot fathom what was going on, not close enough to get a clear view. Getting a reasonable idea of what went on during the K/T event was no easy exercise, despite the (now) obvious clues such as a giant crater buried in the north Atlantic or an iridium peak at the K/T boundary. The Permian extinction, that happened almost 200 million years earlier and had no obvious silver bullet, is a much harder headache. The narrative I bought and published in my book was that of a relatively sudden global warming triggered by massive volcanism such as the Siberian Traps, but there are devilish details. Now Zhicai Zhu et al. report on Scientific Reports evidence of an abrupt change of regime from meandering to braided rivers and aeolian deposits in China. Such rapid changes in hydrology are also documented from other sites such as the Karoo Basin in Africa or Russia, but their interpretation was (and is) unclear. According to Zhu et al.:

The synchronous dramatic negative excursion in δ13C and δ18O in the uppermost Sunjiagou Formation provide reliable evidence for reduced weathering, coolness, aridification, and anoxia.

The keyword here is coolness. What coolness? Isn’t the end-Permian extinction event a period of catastrophic warming? A possible scenario is laid on:

Our study indicates a relatively cool temperature across the PTB, which was supported by some previous studies74,75,76,77 though it is different from most views that indicate a rapid increase in palaeotemperature across the PTB. However, in models for the outcomes of a massive volcanic eruption, such as that of the Siberian Traps, release of massive volumes sulphur dioxide when mixed with atmospheric water may produce a transient cooling phase before the warming, driven by CO2, methane and water vapour. Such cooling can be localised around the volcanic source, or can spread worldwide and last for 1–2 years78. Whether the conflicting findings of either global warming or cooling following the PTB eruptions can be explained by these differing consequences of the eruption, perhaps acting in sequence, or whether these differing temperature changes reflect latitudinal or regional regional effects cannot at this stage be determined.

It makes sense. A sudden warming might be catastrophic, but imagine a sudden cooling followed by sudden warming. That would really kick a biosphere off: the few that were cold-adapted enough to survive a bout of icy temperatures find then themselves at the mercy of a spike of heat. No wonder only very few beings would survive such climatic swings. We still have no idea if this is true or not, but we must be wary of using the past extinctions are strict proxies for our present crisis. Every extinction event teaches us lessons, but is also unique, no less than the species they wipe off the Earth.

The paper is: Zhu, Z., Liu, Y., Kuang, H. et al. Altered fluvial patterns in North China indicate rapid climate change linked to the Permian-Triassic mass extinction. Sci Rep 9, 16818 (2019) doi:10.1038/s41598-019-53321-z