Almost twenty-five years ago, in the late Autumn of 1990, I travelled to Orkney with my supervisor Keith Bennett to collect the first sediment cores for my PhD research project. Results from analysis of one of the core sequences have just been published in the journal Quaternary Science Reviews (Whittington et al. 2015 - can be accessed here if you have a subscription, or email/tweet me). This isn’t quite as bad as it sounds – I published my PhD findings from the site in 1994, focusing on what the pollen preserved in the sediments could tell us about the vegetation history of the islands since the last Ice Age (see here for Michelle's post on pollen analysis) – but rather reflects how long it can take to acquire data and write up collaborative research which is not supported by a single, comprehensive research grant, just by interested academics.
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| Location of Orkney in the North Atlantic (bad screengrab from google maps) |
Orkney is an archipelago situated to the north of mainland Scotland, and has a hyperoceanic climate (see map left: the google marker shows the location of Crudale Meadow, my study site, on the west of Mainland, the largest island). This means that the environment is very strongly affected by north Atlantic weather systems, and therefore that sediments accumulating in basins on the islands potentially contain a sensitive record of past climate oscillations. This latest paper focuses on the record of past climate, and particularly on abrupt changes in climate, recorded by sediments spanning the period from about 14000 years ago to about 8000 years ago. This covers the end of the last ice age and the beginning of the current warm period.
Crudale Meadow is today a valley mire, a low point in the landscape supporting a waterlogged plant community of reeds, sedges and other fen species. However, coring down into the sediments showed that this wasn’t always the case. Under about two metres of tough, fibrous peat full of the preserved stems of the reeds and sedges, we found a pale cream-coloured mud, containing obvious fragments of shells. This was lime mud, which forms in shallow lakes and ponds when the water flowing in is alkaline – it’s closely related to the flaky stuff that clogs up kettles in a hard-water area. In Orkney, most former and present lakes deposit acidic muds with minimal carbonate content, but in some places the till left behind by the retreating glaciers contains lumps of chalk, which dissolve in rainwater filtering through the soil, raise its pH, and lead to the formation of lime mud through chemical reactions within the lakes and ponds.
As a beginner pollen analyst, I was not very fond of those lime muds –they don’t preserve pollen as well as acidic lake muds – but for researchers looking to reconstruct the palaeoclimate of the North Atlantic they were actually very useful. This is because of a process called isotopic fractionation (click on this link for more information about isotopes and climate reconstruction). Oxygen naturally occurs in two forms, one lighter (oxygen-16) and one heavier (oxygen-18), and the ratio between these forms in the water that arrives through rainfall varies according to factors such as the source of the rain and the regional temperature. The carbonate part of the chemical structure of lime muds includes oxygen molecules from the lake water, and therefore locks in a record of the ratio of isotopes in the water. ‘Reading’ this record allows us to reconstruct past temperature, so when towards the end of my PhD I was asked to share my core with a larger team who had the capacity to carry out the analyses, I was interested to see what they’d find. We didn’t realise it would be quite such a long wait…
The figure below is redrawn from the article, and summarises the reconstructed temperature trends. Looking from left to right, we’re moving back in time. The dashed black line shows a smoothed long term trend and the jagged blue line shows the underlying data. The blue boxes identify periods which are statistically unusually cold, and seem to reflect short-lived climate changes.
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| Redrawn from Whittington et al. 2015. Oxygen isotope ratios from Crudale Meadow (see post for details) |
Most of these can be found elsewhere in the North Atlantic, in sediment records and even in the Greenland ice cap record, but one is apparently not yet described from other records, and the record is one of the best demonstrations of abrupt events early in the current warm period yet found in Britain. By looking at the remains of molluscs preserved in the alkaline sediments, at the pollen record, and at other sedimentological evidence, we are also able to explore the extent to which these shorter climate episodes affected life in and around the basin. Climate is the longer-term average of the weather, and in Orkney, you can often literally see changes in the weather blowing in across the sea. Locals say ‘if you don’t like the weather, wait half an hour’ or ‘there is one season a year, but four seasons a day’. Climate records like the one presented in this new paper show long term trends – the proportions of rain and sun, the mean of temperatures over multiple years, the natural variability of the complex and dynamic dance of atmosphere and ocean that creates climate and weather. Twenty five years is roughly equivalent to one sample in the results presented here, even if it seems a long time from a human perspective, or a publishing one!
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| View from Yesnaby, looking west (photo M. Farrell 2007) |
After a long, wet, muddy day in 1990 collecting sediment cores from Crudale, Keith and I drove a mile or so to the nearby cliffs at Yesnaby and ate a very late packed lunch watching the dusk settle over the rolling Atlantic, nothing but ocean between us and north America. At the time I had no idea that there was a record of 6000 years of the climate over that ocean in the core-box, just a hope that there was enough data for a chapter in a PhD. As I write this, a PhD student at Royal Holloway (Rhys Timms) is analysing a new core from another of my PhD sites, Quoyloo Meadow, using the remains of non-biting midges along with other methods to further investigate the climate changes at the end of the last ice age. I wonder what new techniques will have come along by 2040, another 25 years in the future?
Bunting, M.J 1994. Vegetation history of Orkney, Scotland: pollen records from two small basins in west Mainland. New Phytologist 128 771-792
Whittington, G., Edwards, K.J., Zanchetta, G., Keen, D.H., Bunting, M.J., Fallick, A.E. and Bryant, C.L. (2015). Lateglacial and early Holocene climates of the Atlantic margins of Continental Europe: stable isotope, mollusc and pollen studies from Orkney, Scotland. Quaternary Science Reviews 122, 112-130
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