Fossil evidence of past sea ice loss – wissenschaft.de

What climate change could soon bring us, a study shows, also happened during the last ice age: in the so-called interglacial period 129,000 to 115,000 years ago, the Arctic Ocean was ice-free in summer. This is evidenced by microfossils of plankton species that are normally found in areas free of sea ice in sediment cores from the Arctic region. Scientists say further study of the last interglacial could thus contribute to a deeper understanding of climate dynamics in a world without sea ice in the Arctic.

Climate change is showing itself everywhere on Earth – but it’s especially severe in the far north. Strong warming in the Arctic is reflected, among other things, in a significant decrease in sea ice cover: records of summer melting of ice in the Arctic Ocean are increasing. According to projections, sea ice could temporarily disappear completely in late summer around 2050. Not only is this a strong sign of global warming, this loss of sea ice will also have serious consequences. Because Arctic sea ice also plays an important role in the climate system by sealing in water and reflecting light. Thus, diminishing sea ice cover can trigger feedback processes that again have a significant impact on regional and global climate.

When was the Arctic Ocean free of ice?

For further climate predictions and models, information on the effects of the recent disappearance of sea ice in the Arctic will be important. But when did that happen? The questioning look is directed at the last interglacial period – the Ice Age between 129,000 to 115,000 years ago. According to previous studies, the temperatures at that time were the same or slightly higher than they are today. However, it is unclear to what extent this led to a seasonal loss of sea ice cover in the Arctic Ocean at that time. Researchers led by Flor Vermsen of Stockholm University have addressed this question.

Their findings are based on examining microfossils in a series of sediment cores. They come from locations close to the poles, which even today lie directly under the thickest parts of the Arctic sea ice. In the data layers of these samples, scientists examined the microscopic shells of some representatives of zooplankton – the so-called foraminifera. There are many species of these organisms that live in the open waters of some marine areas. The species in question can be identified by the characteristics of their small calcareous shells.

Freedom from ice in the mirror of microfossils

As reported by the scientists, they found the microfossils of the foraminiferal species Turborotalita quinqueloba in core drilling layers from the last ice age. However, it is not found in younger deposits. As the researchers explain, it is a species now found only in the mostly seasonal, ice-free waters of the Atlantic Ocean. Apparently, similar conditions also existed at the sites where the samples were taken during the interglacial period. At that time, small creatures were able to migrate from the southern regions to the Arctic Sea region. In other words: the scientists concluded that the results show that the area was not covered in sea ice for an extended period of time during the summer months.

Accordingly, during the last interglacial period, the process that is also evident today was apparently very advanced: the so-called “atlantification” of the Arctic Ocean. Researchers say the discovery that the Arctic Ocean was seasonally ice-free at that time is particularly worrisome because it may have been slightly warmer than it is today. According to them, their study now demonstrates the importance of the last ice age for climate research: “Our results show that this epoch is a suitable counterpart for the study of the seasonally ice-free Arctic Ocean, which is expected to occur in this century,” the researchers write.

So the study must now follow further research, Vermas says: “In order to understand the conditions in this Arctic devoid of seasonal sea ice during the last ice age, investigations that provide information on sea surface temperatures and other parameters in it must now be carried out.” In addition, targeted studies of climate and oceanography are required for this period, ”says the scientist.

Source: Stockholm University, specialized article: Nature Geoscience, doi: 10.1038/s41561-023-01227-x

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