“Dancing continents and frozen oceans: reading Earth’s diary in natural stone tablets”
Presentation Abstract:
In school I was taught that continents and oceans are primordial. Mountain ranges border continents due to stresses generated where continents and oceans meet. Passed over was the fact that the mountain ranges of Eurasia are all in the continental interior. Then marine geology confirmed that the ocean basins are opening and closing, causing continental rearrangement over geological time. Eurasia is an aggregate of six once-autonomous continents, the collided margins of which track the interior mountain ranges—the Alps, Urals, Himalayas and others. What happened before the present oceans existed? Geological and geophysical surveys of my generation showed that North America is also an aggregate of six paleocontinents, which coalesced long before Eurasia. Its interior mountains ranges are worn down, but their structure is still embedded in the Precambrian crust. The longer-term perspective revealed that the dance of the continents is like musical chairs. Every 700 million years on average, nearly every continent was swept together for a time, before they dispersed again only to reunite later in a different arrangement. The last such ‘supercontinent’ was centered on Africa and the next one will be centered on East Asia, when the Pacific basin closes. Continents are like human populations—arriving from various places at different times with diverse histories.
In 1984, the Conservative Party swept to power in Canada and regional surveys were defunded. I left the government that raised me and took a university job, switching the locus of my field studies from the Arctic to Africa, and its focus from tectonics to climates. The paradox of late Precambrian glacial deposits at sea level in the paleo-tropics had been known since the 1930’s. Numerical climate models from 1969 onward lapsed abruptly into global panglacial states when polar sea-ice extent reached a tipping point in the mid-latitudes. In 1981, a small planetary science group postulated that prolonged volcanic CO2 emissions in the absence of sinks could reverse a panglacial state, making it geologically acceptable. Against well-intended advice, I was drawn to the self-reversing ‘snowball earth’ hypothesis because it made testable predictions. A snowball earth should begin and end abruptly. It should be long-lived because of the enormous CO2 forcing required to counter the reflective surface. It should be followed by the hottest-ever climate because the ice would melt far faster than the CO2 excess could be consumed. Since 2015, each of these predictions has been shown to be true. The challenge now is to explain why such events occurred when they did—not once, but twice in tandem not long before the Paleozoic radiations of animals and plants.
How did the surface biosphere survive if the oceans were 99.9% perpetually dark for 60 million years? The land was mostly buried by ice sheets with equatorial dry valleys of bare permafrost and ice-covered lakes. It is reasonable to assume that diverse microbial biomes had adapted to polar and alpine glacial environments long before the late Precambrian. At snowball onsets, they would have migrated in step with the ice margins to the equatorial zone of net sublimation, where lake ice was thin and dust clumped on bare ice generated meltwater and supplied nutrients. When each snowball finally ended, those cold-adapted lineages restocked the oceans. This scenario predicts a polar−alpine ancestry for the extant surface biosphere, which can be tested by molecular phylogenomic analysis of living organisms.
Prof. Paul F. Hoffman will be introduced Prof. Ian Eisenman of Scripps Institution of Oceanography, UC San Diego.
Paul F. Hoffman is the 2024 Kyoto Prize Laureate in Basic Sciences. He is an adjunct Professor at University of Victoria, who has conducted groundbreaking research in the “Snowball Earth” (global freezing) hypothesis and plate tectonics occurring in the first half of the Earth's 4.6 billion year history. A Ph.D. graduate of Johns Hopkins University, Hoffman served the Geological Survey of his native Canada for 24 years followed by teaching at Harvard and related research in Sub-Saharan Africa. He has geologically demonstrated the occurrence of the postulated global freeze, so-called "Snowball Earth", which drove the rapid diversification of animals in the Cambrian period approximately 520 million years ago. In 1992 Hoffman received the Geological Association of Canada’s highest honor, the Logan Medal, and in 2011 was awarded the Penrose Medal from the Geological Society of America.
