Peat: Core blimey!

Sphagnum austinii

Highly magnified cells of Sphagnum austinii

14 October 2011 by Matt Amesbury

Peat bogs hold evidence of past climate change, and scientists are using this archive to track changes in North Atlantic climate systems stretching back many thousands of years. Matt Amesbury describes some of the insights he and his colleagues are gaining from them.

I am slowly sinking into a bog in far northern Newfoundland, and the slushy peat threatens to overtop my wellies a little more with every hard-fought drive of the corer. We have spent several days touring this remote region searching for bogs deep enough and old enough to tell us about the dramatic changes that occurred here some 8,200 years ago.

Around then, part of the final remnants of the vast Laurentide Ice Sheet which covered much of North America during the last ice age burst dramatically into the Labrador Sea, near the bogs we are studying.

This caused an abrupt freshening of sea water that had a major effect on ocean circulation and the transport of heat from the equator into the North Atlantic. We're very interested to see how terrestrial environments responded to this, because there's a chance something similar might happen in the future if the ice sheet over Greenland continues melting at its current rate.

Two men taking a core sample from a peat bog

Coring under way in Nova Scotia

Important surface and deep ocean currents pass through this area, and we're also interested in how these, and the atmospheric and climate systems associated with them, have changed since the influence of the ice sheet waned around 7,000 years ago.

These ocean currents include the North Atlantic Drift - part of the Gulf Stream. This brings heat from the equator and keeps the UK's climate much warmer than equivalent latitudes in Canada, which are chilled by the Labrador current flowing from the frigid north alongside Greenland.

Crossing the Cabot Strait to Newfoundland after a hard week of coring in the heat of Nova Scotia, that distinct drop in temperature was very welcome, but with the heat transfer of these ocean currents predicted to decline in response to increasing greenhouse gas concentrations in the atmosphere, the future impact on our climate is uncertain.

So how do peat bogs help? The bogs we study are acidic, largely oxygen-free environments that grow upwards over time because dead plant remains do not decay fully, and are gradually overgrown by new plant communities.

The growth of plants and animals on the surface is highly sensitive to changes in precipitation and temperature, which cause the position of the water table, at or near the bog surface, to fluctuate over time.

So if the climate gets wetter or cooler, the plants change in response. If it gets drier or warmer, they change again. As the bog grows upwards, it preserves a record of all of these changes and we can now stand on the surface and take a core back through all the accumulated layers to reconstruct the history of the climate. Which brings us nicely back to my sinking wellies.

In the cores we retrieve, we study the plants that make up the peat (mainly mosses of the genus Sphagnum) and microscopic, single-celled organisms called testate amoebae. These amoebae grow a shell, or test, that is readily preserved in the peat sediments, and we can use it to identify different species.

The real work - the hours, days and months of painstaking analysis - begins when we get the cores back to the lab. We take samples at regular intervals down the core, stretching further into the past the deeper we go.

We can then examine the fine details of moss leaves, other plants and testate amoebae under a microscope and build up a picture of how conditions on the bog have changed over time. We know this relates to past climate because we study ombrotrophic, or 'rain-fed', bogs, which receive all their water and nutrients from the atmosphere.

Magnified view of an amoebae

The testate amoebae Nebela carinata

We can also look at changes in the ratios between different oxygen and carbon isotopes (lighter and heavier versions of these atoms) preserved in the cellulose of the moss leaves.

In an ombrotrophic bog, we know that the water used by the moss to construct the cellulose must have come from precipitation, and because Sphagnum mosses have a very simple biology, there is a direct link back to past climate.

What have we found so far? We have just completed our second field season and are flush with material to work on. The records of past climate change that we will end up with each contain hundreds of samples and each one may take up to a few hours to analyse.

Multiply that by three methods - looking at the plants, testate amoebae and the isotopes - and you have one very busy research team!

So far, both plant and testate amoebae records from Nova Scotia and Newfoundland suggest a series of coherent shifts in climate. The base of our 865cm core from Petite Bog in Nova Scotia has also just been dated to around 13,500 years, much older than we expected - but more work is needed before we fully appreciate what these results are telling us.

One exciting early result has been the discovery of tiny (about 0·02mm wide) shards of volcanic ash from as far away as Alaska in our cores. These tephra shards are common in north-west Europe but are rarely found in the western North Atlantic, up to 6,000km from their source.

By analysing their chemical make-up, we can tell not only which volcanic region they came from, but the exact volcano and eruption event. This helps us to build a chronology for the shifts in climate we identify.

So what's next? We'll continue to build up more detailed records of past climate change that will allow us to address our research questions on climate variability in this region over the past 10,000 years or so, and about how terrestrial environments responded.

An important part of the project is to develop good chronologies, so that we can be as confident as possible that we know when the shifts we're seeing actually occurred, so hunting down those tephra layers and dating our cores by other methods will be a priority. Check back in a future issue of Planet Earth to see how we got on!

Dr Matt Amesbury is an associate research fellow in geography at the University of Exeter.

The lead researcher on this project is Dr Paul Hughes at the University of Southampton.