When the Arctic Ocean was filled with fresh water

The salinity of surface water at high latitudes in the North Atlantic is a critical factor in its sinking to draw warm, low-latitude water northwards in the Gulf Stream while contributing to the southwards flow of North Atlantic Deep Water along the ocean floor. One widely supported hypothesis for rapid cooling events, such as the Younger Dryas, is the shutdown of this thermohaline circulation (Review of thermohaline circulation, February 2002). That may happen when surface seawater at high latitudes is freshened and made less dense by rapid melting or break-up of continental ice sheets, or through the release of vast amounts of fresh water from glacially dammed lakes. The climatic decline leading to the last glacial maximum at around 20 ka was punctuated by irregular episodes known as Dansgaard-Oeschger and Heinrich Events that have been attributed to such hiccups in thermohaline processes. In this context, a whole new barrel of fish has been opened up by a geochemical study of the top few metres of sediments on the Arctic Ocean floor (Geibert, W. et al. 2021. Glacial episodes of a freshwater Arctic Ocean covered by a thick ice shelfNature, v. 590, p. 97–102; DOI: 10.1038/s41586-021-03186-y), particularly their content of an isotope of thorium (230Th).

Being radioactive (half-life ~75 ka), 230Th is useful in working out sediment deposition rates, especially as it is insoluble and adheres to dust grains. The isotope is a decay product of uranium, yet it not only forms on land from uranium in hard rocks, eventually to be transported into marine sediments, but from uranium dissolved in seawater too. Interestingly, the amount of uranium that can enter seawater in solution depends on water salinity. Fresh water, especially that locked up in glacial ice, has very low concentrations of uranium. Consequently, ordinary seawater adds additional 230Th to sediments whereas fresh water does not. An excess of the isotope in marine sediments signifies their deposition from salty water, but those deposited in fresh water carry no excess. In the course of analysing deep-sea cores from the floors of the Arctic Ocean and the northernmost part of the North Atlantic, Walter Geibert and colleagues at the Alfred Wegener Institute in Bremerhaven, and the University of Bremen, Germany revealed a series of sediment layers that were devoid of excess 230Th. This suggests that twice, probably in periods between 150 to 131 and 70 to 62 ka, water in the Arctic Ocean and the connected Nordic Sea was entirely fresh. In two cores the evidence suggests a third, restricted occurrence of fresh water fill at about 15 ka.

The most likely explanation is that the fresh-water episodes marked the development of major ice shelves, similar to those still present around Antarctic; i.e. floating or grounded ice of glacial origin (not sea ice). That had been anticipated, but not previously proved for the northern polar region. The outlets from the Arctic Ocean basin to the Pacific and North Atlantic Oceans are marked by barriers of shallow seabed. One is the Bering Straits, which became the Beringia land bridge that facilitated animal and human migrations from Siberia to North America when sea level fell as continental ice sheets grew. The other is the Greenland-Scotland Ridge formed by volcanism connected to the Icelandic hot spot as the North Atlantic opened. It is possible that the suggested ice shelves grounded on these ridges, to effectively dam and isolate the Arctic Ocean. Fresh water from melting land ice would ‘pond’ beneath the ice shelves, floating on denser salt water and eventually expelling it from much of the polar marine basin. A side effect of this would have been partially to accumulate and isolate the oxygen-isotope proportions that characterise snow and glacial ice. Remember that the light 16O isotope is preferentially extracted from sea water during evaporation, to become stored in glacial ice sheets so that the proportion of the heavier 18O increases in ocean water; δ18O is therefore an important proxy for glacial waxing and waning and thus the fluctuations of global sea level. Trapping a proportion of water of glacial origin in isolated Arctic Ocean water and ice shelves would explain discrepancies in the oxygen-isotope records of successive ice ages. Also, if the ice shelves periodically broke up, fresh water derived from them and ponded in the deepest Arctic Ocean basin could change the salinity of surface ocean water elsewhere – being lower density that fresh water would ‘float’.

The work of Geibert and colleagues may well result in a great deal of head scratching among palaeoclimatologists and perhaps new ideas on the dynamics of ice age climates.

See also: Hoffmann, S. 2021. The Arctic Ocean might have been filled with freshwater during ice ages. Nature, v. 590, p. 37-38; DOI: 10.1038/d41586-021-00208-7

Focus on glaciation…and avoid physics envy

About 1.3 billion years ago two small black holes, each weighing in at about 30 solar masses, ran into one another and fused. At that time Earthly life forms had neither mouths nor anuses, nor even a nervous system, and they were not much bigger than a sand grain. The distant collision involved  rapid acceleration of considerable masses. A century ago Albert Einstein predicted that the movement of any matter in the universe should perturb space-time in a wave-like form that travels at the same speed as light. Well, he was right for, at 9:50:45 universal time on 14 September 2015, four exquisitely engineered mirrors deployed in the two set-ups of a Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington states in the US minutely shuddered, first in the Deep South and 0.007 seconds later in the Pacific Northwest. The signal lasted 0.25 seconds and, when rendered as sound, comprised a sort of chirrup starting at 35 Hz and rising to 250 Hz before an abrupt end. Five months later, and silent internationally shared theoretical verification, the story was released to the back slapping, stamping and pawing the air that we have come to expect from clever, ambitious and persuasive people who have spent a great deal of our money and have something to show for it. So now we know that the universe is probably throbbing – albeit very, very, very quietly – with gravitational waves generated by every single motion that has taken place in the whole of ‘recorded’ history since the Big Bang. Indeed, it is claimed, LIGO-like machines may one day detect the big wave itself if, that is, it hasn’t already passed through the solar system. Recall, 13.7 billion years ago the Big Bang didn’t take much longer than this comparatively mundane collision at 1.3 Ga . Physicists are going to have a lot to ponder on now they have a lever to get yet greater funds. To put all this in perspective, the detected chirrup had been traveling for 1.3 Ga, and so too must the actual place in the universe where it took place: I guess we will never know where it is now or what damage or otherwise may have been visited upon planetary systems in its vicinity, if indeed it had even the slightest recognisable geological or ecological consequence.

So, onto the mundane world of glaciology and climate change.

Tibet is the third greatest repository of glacial ice on the surface of the Earth’s continents. It is the focus of one of the planet’s greatest climatic system, the South Asian. While much of the Plateau hasn’t borne glaciers continuously throughout even the last glacial cycle, it is becoming clear that its western margin has remained cold enough to retain ice throughout an even longer period. In the Kunlun mountains is a 200 km2 ice cap known as the Guliya. At the start of detailed glacial stratigraphic ventures in 1990s, focused mainly on Greenland and Antarctica, analysis of a core from the Guliya ice cap yielded dates extending back to 130 ka, before the start if the last interglacial. This section lies above ice that at the time could not be dated reliably other than to show that it may be older than about 750 ka. This stemmed from its lack of the radioactive 36Cl formed, similarly to 14C, by cosmic-ray interactions with stable 35Cl in atmospheric salt aerosols: such cosmogenic chlorine can be used for radiometric dating of ice younger than 750 ka.

A News Feature in the 29 January issue of Science (Qiu, J. 2016. Tibet’s primeval ice. Science, v. 351, p. 436-439) focused on the preliminary results of an expedition, led by Yao Tandong of the Institute of Tibetan Plateau Research, Beijing and Lonnie Thompson of Ohio State University, Columbus, to drill a further five ice cores at Guliya in September 2015, one of which penetrated ove 300 m of glacial ice. It is now possible to date ice layers back to a million years using argon isotopes. Combined with stable isotope and other measurements through the cores, the dating should provide a huge amount of new information on the evolution of the monsoon, which is currently understood only vaguely. Such information would sharpen models of how the monsoon system works and even hint at how it might change during a period of anthropogenic warming. An estimated 1.4 billion people – a fifth of humanity – who live in the Indian subcontinent, China and SE Asia depend for their food-production on the monsoon.

With less humanitarian urgency but equally fascinating is the discovery that, as well as sea-ice, the central Arctic Ocean once hosted vast ice shelves during the last-but-one glacial episode (Jakobsson, M. and 24 others 2016. Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciations. Nature Communications, v. 7, doi:10.1038/ncomms10365. Clues emerged from multibeam sonar bathymetry that created detailed images of topography on the floor of the Arctic Ocean. These revealed sets of parallel ridges on the shallowest parts of the polar basin, thought to have formed when moving ice shelves grounded. The depths of the grooved areas indicate ice thicknesses up to and exceeding 1 km. The grooves look very similar to the large-scale lineaments that formed on the surface of the Canadian Shield when the Laurentide ice sheet ground its way from zones of glacial accumulation. Grounding of an ice shelf would have resulted in its thickening in the upflow direction as a result of plastic deformation of the ice, tending to lock the flow and direct ice escape over the deeper parts of the Arctic basin.

Antarctic Ice Shelf
Antarctic Ice Shelf (credit: Wikipedia)

Back-tracking the grooves defines the ice shelf’s source regions in the northern Canadian islands, north Scandinavia and the lowlands of eastern Siberia as well as regional flow patterns and the extent of floating continental ice. The last is a major surprise: at over 4 million km2 it was four times larger than all modern Antarctic ice shelves. The ice moved to ‘escape’ to the North Atlantic Ocean through the Fram Strait between East Greenland and Svalbard (Spitzbergen). Dating sediment stratigraphy in the grooved areas using magnetic and fossil data shows that the ice shelves existed between 160 and 140 ka during the penultimate glacial maximum. For such a mass of glacial ice to be expelled into the Arctic Ocean implies that a great deal more snow fell on its fringes then than during the last glacial maximum. Another possibility is that the huge mass of floating ice regulated the salinity and density of the upper Atlantic in a different way from the periodic iceberg ‘armadas’ that characterized the last glacial epoch and help account for a whole number of sudden warming and cooling events.

Domack, E. 2016. A great Arctic ice shelf. Nature, v. 530, p. 163-164.