Enlarge /. There was an ice sheet on Spitsbergen over 14,000 years ago.
The rapid rise in global temperature over the past century is almost certainly unprecedented in recent Earth's history, but our current sea level rise rate has fiercer competition. About 14,650 years ago, when the last ice thawed, sea level made a remarkable jump of 12 meters or more – in less than 400 years. It is an event known to scientists as Meltwater Pulse 1A.
It was not easy to find out where all the water came from. It was certainly the result of glacial ice melting (and not some kind of biblical flood of heaven), but models of past ice sheet changes haven't quite added up.
A new study by Jo Brendryen at the University of Bergen shows in an interesting way that the largely overlooked melting of the Eurasian ice sheet may only explain things.
Looking for a date
It really depends on carbon dating – measuring the amount of radioactive carbon-14 in something to calculate its age. Carbon dating works somewhat differently from other types of radiometric dating because carbon-14 is constantly generated by reactions in the Earth's atmosphere. Everything that absorbs carbon atoms – including living things – therefore has a similar amount to the atmosphere. As soon as it stops this uptake, carbon-14 gradually breaks down and becomes nitrogen-14. The older the thing is, the less carbon-14 remains in it.
One complication is that the production rate of carbon-14 in the atmosphere varies slightly over time. Because this technique can only be used in the past for around 50,000 years or so, scientists have been able to measure things of known age (such as tree rings) to check and calibrate for carbon 14 levels.
But there is a second complication in the ocean. Carbon-14 from the atmosphere easily gets into surface water, but deep water is isolated. That said, it could have been hundreds of years since carbon in deep water last saw the atmosphere. If a small baby organism absorbs this carbon, it will look hundreds of years older than it is in terms of carbon dating.
So just subtract the right number when working with deep sea samples, and it's fixed, right? Well, the problem is that the “right number” depends on the ocean circulation at that place and at this time. In paleoceanography, this generally leads to a lot of crying and grinding of teeth.
Back to the ice sheets. During the last ice age, an ice sheet once stretched across Scandinavia and the Barents Sea. The record of its shrinkage is based on the carbon dating of sediment cores on the ocean floor, determining the times when the ice retreated and life returned to one place. The reconstructions have shown that before the start of Meltwater Pulse 1A, the ice here had essentially melted, which gave it a clear alibi. However, the age dated with carbon assumed that the carbon 14 lag in the deep sea in this region was constant over time and conformed to the modern pattern.
In this new study, the researchers carefully examined this assumption. They turned to an apparently improbable source – a cave in China. There is actually a pretty good correlation between ocean circulation, temperatures in the North Atlantic and Asian monsoon rains linked by a series of climatic dominoes. Cave records have excellent timelines with annual shifts and radiometric uranium dating.
By lining up the shaky movements in the cave records and sediment records of the Norwegian sea, the researchers do not have to guess the unknown carbon 14 lag in the deep sea. Instead, they can calculate this delay and its changes to provide a new calibration for paleoclimate ocean floor recordings in this region.
This shifts the reconstructed timing of the melt of the Eurasian ice sheet. Instead of showing that the local ice melted before Meltwater Pulse 1A even started, they see a big ice loss during this event. Earlier reconstructions gave the Eurasian ice sheet a credit for perhaps one meter of the 12 or more meter sea level rise that took place at that time. This study increases this contribution to about five meters – plus another meter or so in the following century.
Enlarge /. Previous expansion of the ice sheet with ages in thousands of years.
It's obvious to find out which giant block of ice melted thousands of years ago. But there are valuable clues. When an ice sheet melts, the rise in sea level does not rise evenly worldwide. The attraction of a massive ice sheet actually pulls seawater there and raises the sea level near the ice sheet a little. When the ice sheet shrinks, its attraction relaxes so that the sea level can drop right next to the ice sheet, even if it rises elsewhere. And the records of changes in sea level at various locations actually agree that the Eurasian ice sheet is a major source: locations in Norway and Finland show a decrease in sea level during this period of global high-speed sea level rise.
A realignment of the history of the Eurasian ice sheet would make it much easier to understand where 12-meter sea levels come from, but it also raises some interesting questions. For example, such a massive flow of fresh water into the Norwegian Sea could be expected to refresh the critically important south-north conveyor belt flow in the Atlantic, but records show that it was actually quite strong at that time. And how exactly did this part of the Eurasian ice sheet collapse so quickly?
This question about the past is of interest to our future. The portion of the Eurasian ice sheet in question spanned topographic lows in contact with the ocean, making it prone to rapid collapse. The same applies today to the ice sheet of western Antarctica – the largest wildcard for the future rise in sea levels. Every ice sheet is different and local details are important, but an equally rapid collapse of the ice in Antarctica would be a worst-case scenario.
Nature Geoscience, 2020. DOI: 10.1038 / s41561-020-0567-4 (About DOIs).