Supernova at the start of the Pleistocene

This brief note takes up a thread begun in Can a supernova affect the Earth System? (August 2020). In February 2020 the brightness of Betelgeuse – the prominent red star at the top-left of the constellation Orion – dropped in a dramatic fashion. This led to media speculation that it was about to ‘go supernova’, but with the rise of COVID-19 beginning then, that seemed the least of our worries. In fact, astronomers already knew that the red star had dimmed many times before, on a roughly 6.4-year time scale. Betelgeuse is a variable star and by March 2020 it brightened once again: shock-horror over; back to the latter-day plague.

When stars more than ten-times the mass of the Sun run out of fuel for the nuclear fusion energy that keeps them ‘inflated’ they collapse. The vast amount of gravitational potential energy released by the collapse triggers a supernova and is sufficient to form all manner of exotic heavy isotopes by nucleosynthesis. Such an event radiates highly energetic and damaging gamma radiation, and flings off dust charged with a soup of exotic isotopes at very high speeds. The energy released could sum to the entire amount of light that our Sun has shone since it formed 4.6 billion years ago. If close enough, the dual ‘blast’ could have severe effects on Earth, and has been suggested to have caused the mass extinction at the end of the Ordovician Period.

Betelgeuse is about 700 light years away, massive enough to become a future supernova and its rapid consumption of nuclear fuel – it is only about 10 million years old – suggests it will do so within the next hundred thousand years. Nobody knows how close such an event needs to be to wreak havoc on the Earth system, so it is as well to check if there is evidence for such linked perturbations in the geological record. The isotope 60Fe occurs in manganese-rich crusts and nodules on the floor of the Pacific Ocean and also in some rocks from the Moon. It is radioactive with a half-life of about 2.6 million years, so it soon decays away and cannot have been a part of Earth’s original geochemistry or that of the Moon. Its presence may suggest accretion of debris from supernovas in the geologically recent past: possibly 20 in the last 10 Ma but with apparently no obvious extinctions. Yet that isotope of iron may also be produced by less-spectacular stellar processes, so may not be a useful guide.

There is, however, another short-lived radioactive isotope, of manganese (53Mn), which can only form under supernova conditions. It has been found in ocean-floor manganese-rich crusts by a German-Argentinian team of physicists  (Korschinek, G. et al. 2020. Supernova-produced 53Mn on Earth. Physical Review Letters, v. 125, article 031101; DOI: 10.1103/PhysRevLett.125.031101). They dated the crusts using another short-lived cosmogenic isotope produced when cosmic rays transform the atomic nuclei of oxygen and nitrogen to 10Be that ended up in the manganese-rich crusts along with any supernova-produced  53Mn and 60Fe. These were detected in parts of four crusts widely separated on the Pacific Ocean floor. The relative proportions of the two isotopes matched that predicted for nucleosynthesis in supernovas, so the team considers their joint presence to be a ‘smoking gun’ for such an event.

The 10Be in the supernova-affected parts of the crusts yielded an age of 2.58 ± 0.43 million years, which marks the start of the Pleistocene Epoch, the onset of glacial cycles in the Northern Hemisphere and the time of the earliest known members of the genus Homo. A remarkable coincidence? Possibly. Yet cosmic rays, many of which come from supernova relics, have been cited as a significant source of nucleation sites for cloud condensation. Clouds increase the planet’s reflectivity and thus act to to cool it. This has been a contentious issue in the debate about modern climate change, some refuting their significance on the basis of a lack of correlation between cloud-cover data and changes in the flux of cosmic rays over the last century. Yet, over the five millennia of recorded history there have been no records of supernovas with a magnitude that would suggest they were able to bathe the night sky in light akin to that of daytime. That may be the signature of one capable of affecting the Earth system. Thousands that warrant being dubbed a ‘very large new star’are recorded, but none that ‘turned night into day’. The hypothesis seems to have ‘legs’, but so too do others, such as the slow influence on oceanic circulation of the formation of the Isthmus of Panama and other parochial mechanisms of changing the transfer of energy around our planet

See also: Stellar explosion in Earth’s proximity, eons ago. (Science Daily; 30 September 2020.)

Explosive erosion in the Himalaya

As the Yalung-Tsangpo River on the northern flank of the Himalaya approaches  a bend the rotates its flow by almost 180 degrees to become the Brahmaputra it enters one of the world’s largest canyons. Over the 200 km length of the Tsangpo Gorge the river descends two kilometres between peaks that tower 7 km above sea level. Since the area is rising tectonically and as a result of the unloading that attends erosion, for the Tsangpo to have maintained its eastward flow it has been suggested that an average erosion rate of 3 to 5 km per million years was maintained continuously over the last 3 to 5 Ma. However, new information from the sediments downstream of the gorge suggests that much of the gorge’s depth was cut during a series of sudden episodes (Lang, K.A. et al. 2013. Erosion of the Tsangpo Gorge by megafloods, Eastern Himalaya. Geology, v. 41, p. 1003-1006).

English: Map of the Yarlung Tsangpo River wate...
The Yarlung Tsangpo River watershed which drains the north slope of the Himalayas. (credit: Wikipedia)

It has become clear from a series of mountainside terraces that during the Pleistocene glaciers and debris from them often blocked the narrow valleys through which the river flowed along the northern flank of the Himalaya. Each blockage would have impounded enormous lakes upstream of the Tsangpo Gorge, containing up to 800 km3 of water. Failure of the natural dams would have unleashed equally spectacular floods. The researchers from the University of Washington in Seattle examined the valley downstream of the gorge, to find unconsolidated sediments as much as 150 m above the present channel. They have similar grain size distributions to flood deposits laid down some 30 m above the channel by a flood unleashed in 2000 by the failure of a temporary dam caused by a landslide. The difference is that the higher level deposits are densely vegetated and have well-developed soils: they are almost certainly relics of far larger floods in the distant past from the lakes betrayed by the terraces above the Tsangpo Gorge.

By measuring the age of zircons found in the megaflood deposits using the U/Pb methods the team  have been able to show that the sediments were derived mainly from 500 Ma crystalline basement in the Tsangpo Gorge itself rather than from the younger terranes in Tibet. There are four such deposits at separate elevations above the modern river below the gorge. Like the 2000 AD flood deposit, each terrace is capped by landslide debris suggesting that flooding and associated erosion destabilised the steep slopes so characteristic of the region. Because the valleys are so narrow (<200 m at the bottom), each flood would have been extremely deep, flows being of the order of a million cubic metres per second. The huge power would have been capable of moving blocks up to 18 m across with 1 m boulders being carried in suspension. It has been estimated that each of the floods would have been capable of removing material that would otherwise have taken up to 4000 years to erode at present rates of flow.

Arctic climate in the run-up to the Great Ice Age

Around 3.6 Ma ago a large extraterrestrial projectile slammed into the far north-east of Siberia forming crater 16 km across. The depression soon filled with water to form Lake El’gygytgyn, on whose bed sediments have accumulated up to the present. A major impact close to the supposed start of Northern Hemisphere glacial conditions was a tempting target for coring: possibly two birds with one stone as the lowest sediments would probably be impact debris and boreal lake sediments of this age are as rare as hens’ teeth. The sedimentary record of Lake El’gygytgyn has proved to be a climate-change treasure trove (Brigham-Grette, J and 15 others 2013. Pliocene warmth, polar amplification, and stepped Pleistocene cooling recorded in NE Arctic Russia. Science, v. 340, p. 1421-1426).

El'gygytgyn, Russia, is a impact crater with a...
Lake El’gygytgyn impact crater. (credit: Wikipedia)

The team of US, Russian, German and Swedish scientists discovered that the sedimentary record was complete over a depth of 318 m and so promised a high resolution climate record. The striking feature of the sediments is that they show cyclical variation between five different facies, four of which are laminated and so preserve intricate records of varying weathering and sediment delivery to the lake. The sediments also contain pollens and diatom fossils, and yield good magnetic polarity data. The last show up periods of reversed geomagnetic polarity, which provide age calibration independent of relative correlation with marine isotope records.

A host of climate-related proxies, including pollen from diverse tree and shrub genera, variations in silica due to changes in diatom populations and organic carbon content in the cyclically  changing sedimentary facies are correlated with global climate records based on marine-sediment stable isotope. These records reveal intricate oscillations between cool mixed forest, cool coniferous forest, taiga  and cold deciduous forest, with occasional frigid tundra conditions through the mid- to late Pliocene. Compared with modern conditions NE Siberia was much warmer and wetter at the start of the record. Around the start of the Pleistocene sudden declines to cooler and drier conditions appear, although until 2.2 Ma ago average summer conditions seem to have been higher that at present, despite evidence from marine proxies of the onset of glacial-interglacial cycles in the Northern Hemisphere.

In detail, Lake El’gygytgyn revealed some surprises including rapid onset of a lengthy cold-dry spell of tundra conditions between 3.31 to 3.28 Ma. The first signs that the lake was perennially frozen appear around 2.6 Ma, well before evidence for the first continental glaciation in North America, presaged by signs around 2.7 Ma that winters consistently became colder than present ones. Overall the lake record presents a picture of a stepped shift in climate in the run-up to the Great Ice Age. Lake El’gygytgyn seems set to become the standard against which other, more patchy records around the Arctic Ocean are matched and correlated. Indeed it is the longest and most detailed record of climate for the Earth’s land surface, compared with 120 and 800 ka for the Greenland and Antarctic ice-caps.

Modelling their findings against likely atmospheric CO2 levels the authors provide grist to the media mill which focuses on how the late Pliocene may be a model for a future warm Earth if emissions are not curtailed, with visions of dense polar forests