Rachael Avery in the room where the collected sediment cores are stored
Rachael Avery in the room where the collected sediment cores are stored. Photo: Private

One of the most critical uncertainties in the behavior of continental ice sheets is the response time in relation to climate forcing and the rates of ice margin or grounding line change. While the observational record of contemporary ice-sheet change is a few decades long at best, the geological record of former ice-sheet demise offers an opportunity to assess rates of ice-margin retreat (i.e. ice sheet mass loss) over several thousand years of climate change after a glacial maximum.

Records with annual to seasonal precision

Sediments from within and in front of the ice sheet, transported by glacial meltwater and deposited in proglacial lake basins, can provide such records with annual to seasonal precision in the form of laminated sediments, also called varves. Varved proglacial sediments give an unparalleled means to not only reconstruct the annual pattern of ice sheet decay, but also to draw conclusions about the climate in former times.

Gap in the map

The Swedish Varve Chronology is an unparalleled tool for linking the deglacial history of Sweden with associated palaeo‐environmental change at an annual time scale, and it forms part of Sweden's cultural heritage. A full deglacial chronology connected to the present day does not yet exist though. A notable gap is in the most southeastern part of Sweden, where few varved records are successfully connected to reconstruct ice‐margin retreat.

Eyeing that gap, geologist and Bolin Centre scientist Rachael Avery and colleagues set out to fill it.
“By using new offshore samples, we have been able to provide a credible answer to an over 100 years old problem. We have extended the existing south coast chronology to the troublesome east coast,” she explains.

With Stockholm University’s research vessel Electra, they managed to obtain 10‐cm‐diameter varved cores from eleven of the 13 selected coring sites in Kalmarsund, the strait between mainland Sweden and Öland. “Researchers last century didn’t have access to offshore cores, but they have been key to integrating the varve chronologies in this difficult area. These offshore cores are an invaluable resource,” says Rachael Avery.

Faster off shore retreat rates

In the study, legacy varve records collected since the early 1900s also have been revisited, and combined with new terrestrial and offshore cores. The new Skåne‐Småland chronology covers the subaqueous–terrestrial transition of the retreating ice sheet and spans 725 varve years of deglaciation. It reveals that the retreat rates in the offshore sector were three‐ to fivefold faster than where the margin was close to or above the palaeo‐shoreline. This means a retreat rate of 160-490 meters per year offshore, compared to 60-100 meters per year onshore. The results are shown in the figure 10.
“These results give insights into the dynamics of the Fennoscandian Ice Sheet as it transitioned from water-terminating to terrestrial terminating during its retreat, says Rachael Avery.

graphs showing varve sites and the ice retreat.
The figure shows varve sites and the ice retreat.

The Swedish Varve Chronology – a part of Sweden's cultural heritage

The ‘father of the field’ of the Swedish Varve Chronology was Gerard De Geer, a Swedish scientist. Some of his original data was used in the study mentioned in the text. The pioneering work of De Geer can be seen on the Department of Physical Geography’s website (in Swedish). The continuation of De Geer’s work was carried on by Lars Brunnberg and Barbara Wohlfarth and forms an important part of the legacy and work done at Stockholm University.

The study was financed by KAW, Stockholm University and Bolin Centre for Climate Research, Research Area 5.

Read the article “A 725‐year integrated offshore terrestrial varve chronology for southeastern Sweden suggests rapid ice retreat ~15 ka BP” by Bolin Centre researchers Rachael S. Avery, Sarah L. Greenwood, Frederik Schenk, Björn M. Morén, David I. Armstrong Mckay, Lars Brunnberg and Barbara Wohlfarth, published in Boreas 17 November 2020. https://onlinelibrary.wiley.com/doi/10.1111/bor.12490

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