Bogs as archives of past climate change
Since approximately 12,000 years ago, when the major ice sheets of the last ice age began their slow but relentless decline in the face of rising temperatures, Northern Hemisphere high latitude peatlands (areas of decaying plants) have been steadily accumulating, often locking in a record of past climate and hydrological change as they go. It was in these Northern peatlands, particularly in the UK and north-west Europe, where peatland palaeoclimate research was established and began to grow in the 1980s and 1990s, though its roots lay in much earlier research. This diverse research field has now spread to the peatlands of the Southern Hemisphere and tropical regions and employs a range of techniques to address pressing questions about Holocene climate (the last 12,000 years).
Typically, it is rain-fed (ombrotrophic) bogs that are employed in palaeoclimate research. These bogs have a direct coupling with the atmosphere, receiving all their moisture and nutrient inputs from precipitation and are therefore naturally more sensitive to climate than other peatland types that may be influenced by groundwater flow or other complicating factors.
A range of proxy-methods are used to reconstruct past hydrological, and hence climate, change in cores extracted from rain-fed peat bogs. These have traditionally focused on estimating ‘bog surface wetness’ (BSW, which is essentially reflecting the water-table depth). A series of studies over the past 5-10 years have calibrated the recent peatland record against meteorological records in an effort to understand exactly what the BSW record is telling us. The conclusions were that BSW is primarily determined by the length and severity of the summer period when a bogs hydrological balance is in deficit. This in turn is closely linked to precipitation, with temperature, through its influence on evapotranspiration, playing a secondary role.
As with anything in science though, it’s probably not this simple. Bogs are dynamic systems, capable of a certain degree of hydrological self-regulation. It has been suggested that these internal (autogenic) responses can interfere with the BSW signal produced by our proxy-techniques. This is not a disaster, it just means we have to interpret our data more carefully. It also helps to have multiple strands of evidence, from multiple bogs in a region, so we can check for patterns that are common between different proxies and sites and therefore more likely to indicate a response to climate rather than internal bog dynamics.
Rain-fed bogs typically accumulate at a rate somewhere in the region of a centimetre every 10 years, allowing records showing a decadal pattern of change to be developed. A bog that therefore began accumulating 12,000 years ago might now be some 12 metres deep! By standing on the surface one of our bogs, we can core back through all that steadily accumulated peat and then, by studying samples down the length of that core back in the lab, we’re able to reconstruct how bog hydrology, which as we’ve seen is closely linked to climate, has changed over time.
Turning depth into age
If we combine the often rapid rates of accumulation with the huge potential peatlands have for providing an age context of past change, their value for palaeoclimatic research becomes even clearer.
Depending on the timescale we’re interested in, there are a number of different dating tools available to us. First and probably foremost is radiocarbon dating – a technique that estimates when organic matter once lived and was then deposited. The overwhelming majority of the material in a peat bog is organic and therefore we can obtain age estimates at almost any section of the core we like. This is a very different story to many other palaeoenvironmental archives, which can only employ the technique where dateable material is available – more of a challenge in, say, a lake record rich in mineral material.
A second technique, tephrochronology, provides us with age estimates of layers of volcanic ash. In theory, each ash layer corresponds to a specific volcanic eruption. We can identify precisely which one by analysing the chemical composition of the ash shards that make up the layer. Often, we know the age of these eruptions precisely, if they have occurred in recorded history, or at least approximately if we previously have dated their position in other peat or sediment archives. This technique is particularly useful because as well as adding additional age control to our cores, they also provide a tool for correlating records from different sites as volcanic eruptions can disperse ash widely.
Bugs, bryophytes and beyond…
So what methods do we actually use in peatland palaeoclimate studies? This is where the bugs (testate amoebae) and bryophytes (mosses, very often of the genus Sphagnum) come in. Testate amoebae are microscopic, single-celled organisms that live in the thin water films in and around moss leaves. They grow, or make, a shell called a ‘test’, hence the name. Though the live amoeba themselves are absent from all but the very surface samples, the shells can be preserved for many thousands of years in the low-oxygen and acidic environment of an rain-fed peat bog. Much like other palaeo-ecological methods, such as pollen analysis, the technique is based on the fact that different species prefer to live in different conditions (e.g. wetter or drier) on the bog surface. So by understanding which species prefer which conditions, by studying those alive today, then counting the proportion of different taxa in downcore samples we can recreate past hydrological conditions.
The bryophytes are what make up the majority of the peat itself. Sphagnum mosses are the dominant decaying plant material that forms peat, in both the Northern Hemisphere and in South America. In other regions of the world however, where peatland palaeoclimate research has been carried out, perhaps most notably in China and New Zealand, other mosses and vascular plants dominate. Again, different species of moss, as well as other plants such as heathers and grasses, prefer to grow in different conditions. Studying these vegetative remains in a peat core is known as plant macrofossil analysis. By combining this technique with testate amoeba analysis we begin to form what is known as a multi-proxy record. By testing different techniques against each other, it allows us to be more confident in our results if they are showing us the same thing. And if they are not, we can use our knowledge of plant and amoeba ecology to begin to work out why.
The techniques we use to study past climate change in peatlands has expanded and become steadily more quantitative over time. So alongside developments in the world of bugs and bryophytes, perhaps the most important step forward in recent years has been the emergence of stable isotope analysis. Isotopes are atoms of the same element that contain the same number of protons, but a different number of neutrons, making them ever so slightly different weights. Oxygen, for example, has three different isotopes known as oxygen-16, -17 and -18. The isotope signal in bogs comes from the rainfall that plants use to construct cellulose (for oxygen isotopes) or the carbon dioxide used in photosynthesis (for carbon isotopes). Cellulose is an organic compound that forms a plant’s cell structure. We can relate the isotope record to past climate because, under different climatic conditions, lighter or heavier oxygen isotopes are more common in rain and the diffusion of CO2 into plant tissue during photosynthesis is affected by the amount of water present around the moss leaves.
When multi-proxy records are carefully developed and analysed from well selected sites and cores, peatlands have the potential to provide valuable palaeoclimate records that can be used to test a range of important hypotheses on the evolution and forcing of Holocene climate. A multitude of ongoing research in regions as diverse as Europe, Canada, Patagonia, New Zealand and even the Antarctic Peninsula is testament to this fact.
Want to know more about bogs? Head over to Bogology.org.