Pliocene-Pleistocene boundary: did Eltanin asteroid kickstart the ice ages?

Sediment deposits along shores of Antarctica, New Zealand and Chile suggest over 2 million years ago something big must have plunged somewhere in the middle of that triangle, creating a mega tsunami with hundreds of meters high waves engulfing coastal areas.

Eltanin asteroid Pleistocene ice ages
Possible spread of a megatsunami after the Eltanin asteroid strike in the South Pacific, right at the start of the Pleistocene. Can you imagine the splash – and would that somehow compare to this image?

Because that triangle would be Southern Pacific Ocean the supposed 2-kilometer-wide Eltanin asteroid has not left a nice and clear-cut impact crater on land for researchers to study. But Australian scientists now suggest it may have left other visible scar marks across the globe, even a hemisphere away, like the Great Lakes of North America, the moraine landscapes of for instance the Netherlands, Germany and Denmark and the endless loess deposits of China and Siberia – as they say the asteroid may have fast-forwarded an already cooling planet into the ice ages.

A lot of research into paleo climate change focuses on periods of climate warming, like the Paleocene-Eocene Thermal Maximum (PETM), the Pliocene warm climate – and even the mild interglacials of the (cool) Pleistocene – as studying these episodes in Earth’s history may provide important lessons* about climate sensitivity and ecological adaptivity.

The climate cooling lessons: Understanding the inverse relations between carbon/albedo and temperature

But what goes up must once come down and what at least we at Bits of Science have mostly ignored are the transitions from warm climates to cooler climates. Not only because we personally thought them less exciting, also because we thought that’s what they were intrinsically – less exciting – periods of many millions of years of slow biosphere recovery, slowly sequestered CO2 in slow, deep geological carbon cycles, and gradually setting temperatures.

In the case of the Pleistocene ice ages, we learned at school, astronomical cycles were at play, creating the so-called Milankovitch variables as forcers of climate cooling, also very slowly. But the decrease in (polar) insolation that is the actual cooling mechanism of the Milankovitch cycles would in turn itself have been too weak to have led to large-scale glaciations, had this setting of the Arctic Sun not been preceded by the very gradual late Pliocene climate cooling, which was mostly tectonically forced, so also very slowly.

[*) Episodes of ancient climate warming may be lessons, they never serve as analogues – the current climate warming is happening so much faster than anything that has been dug up from the fossil record.]

‘Why not redefine the Pliocene-Pleistocene boundary – I have a feeling it may turn out to be convenient one day…’

The climatic transition had been so slow, that Earth scientists have actually remained uncertain where to place the precise Pliocene-Pleistocene boundary. It took until 2009 before the International Union of Geological Sciences (IUGS) officially decided to shift it 782,000 years back in time, to a starting date of 2.588 million years before present.

Of course when you think anything about paleoclimatology could ever be dull, your science gut should perhaps warn you. All too often a lack of fascination is down to a lack of knowledge, isn’t it? If only you could zoom in to the human timescale and have full vision on the physical complexity of eras and worlds long buried, probably every little detail would turn out to be exciting too.

This we now for a fact from the final stages of the last ice age, which cold spell hick-ups like the Last Glacial Maximum and the Younger Dryas are relatively easy to study, as they are such recent history, evidence is still everywhere around us [heck, we even think the late-Holocene Little Ice Age was quite a dramatic climate cooling event, simply because we were able to witness the falling snowflakes with our own eyes].

Add an asteroid to The Day After Tomorrow to get a proper image of climate cooling drama

Now a group of scientists of the University of New South Wales tries to obtain a similarly detailed image of the prelude to the ice ages, by zooming in to the early days of the Pleistocene, closely examining and comparing flood sediments along the entire South Pacific – and correlating these to one single asteroid strike, that scientists now think may have hit Earth some 300,000 years earlier than previously suggested, exactly at the newly defined Pliocene-Pleistocene boundary…

Such hindsight reasoning may feel a bit opportunistic from a pure research standpoint [reminds of the Arctic warming cold winter hypothesis], but an Earth scientist can’t help to wonder if the now overlapping date of the asteroid strike and the P-P boundary is a mere coincidence. The Australians at least think it is not, they write in their publication in the Journal of Quaternary Science. If the asteroid was big enough to cause a Pacific megatsunami hundreds of meters high, it must have also been big enough to create an enormous plume of salty water and ocean floor sediments, kilometers high up in the atmosphere, perhaps leading tp large amounts of climate cooling aerosols, which could in turn have activated other negative climate feedbacks, like the polar sea ice albedo effect.

The researchers simply conclude they find geological evidence of a major impact right at the time that large-scale climatic transitions were underway – and although this does not necessarily link the two, a possible strong cooling effect is something for science to consider, since the Eltanin asteroid is the only known* deep-ocean impact – and although still unlikely, chances are always higher mankind will experience a similar strike somewhere in the world’s oceans than on land.

[*) Because plate tectonics is relatively efficient at recycling ocean floor – and of course because for practical reasons marine geological research is less intensive. (And if you recall that (supposed) dinosaur-killing asteroid  it fell on the Gulf of Mexico - today the area may be sea, and connected to the Atlantic Ocean, but geologically speaking  which today we are  that was a strike on the American continental shelf.)]

© Rolf Schuttenhelm | www.bitsofscience.org

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