A brief introduction to... The Periglacial Environment



Great image of ice wedge polygons from physicalgeography.net



Well... not quite the post rate I'd expected after finishing Uni, but work and life take over and before you know it you've not posted anything in over a month! Before I get stuck in, I graduated last week with a 1^st, something I am especially happy with as I had resigned myself to a 2:1 following an uncomfortable feeling after an exam. In other news, I’ve not seen the sun in what feels like months as Britain drowns in the longest intermittent downpour I have ever been subject to. I’m also seriously considering further study/ research; my graduation reminded me how much I miss it already! Enough about me... If I remember rightly, I promised an introduction to the periglacial environment. When I say environment in this context, I mean the specific climatic and geomorphological conditions to allow for a specific suite of processes to operate.

First, (as I'm sure @Dawnitoes will delight in reading) we'll talk about the word "periglacial". The word literally means "near a glacier" and on its inception that is exactly what the word was used for. However, since the processes and features seen in these areas surrounding glaciers were identified in other cold areas in the absence of a glacier, the term eventually evolved. “Periglacial” is now the word used to describe a set of zonal processes that occur in cold environments due to the presence of ice and snow and the repeated freezing and thawing of water. Certain azonal processes can also exhibit distinctive characteristics in periglacial areas. I've visited permafrost before in a previous article (however not formally introduced you... how rude of me). Permafrost – an area of perennially frozen ground – is responsible for many of the processes and landforms associated with periglacial areas; however it is not the defining characteristic of a periglacial area. There is likely to be an article on permafrost in the near future.

So, we have rather hand-wavey, vague explanation of the factors controlling periglacial environments without explaining any processes or resultant landforms associated with places you may term “periglacial”. Or have we? I mentioned in the paragraph above about “the repeated freezing and thawing of water”, which is probably the most important process when we consider most specific periglacial processes and landforms.

In these periglacial environments, the temperature fluctuates about the freezing point of water often diurnally (daily) at the ground surface; and annually (with the seasons) for deeper freezing and thawing. Seasonal snow accumulation and subsequent melting and the movement of groundwater towards a freezing front are also important water processes. As a general statement, these water/ ice processes result in weathering processes acting upon bare rocks and transport processes acting upon sediments.

As I write this, I’m debating whether to name and dissect specific processes and resultant landforms, or whether they warrant their own “brief introductions”... I have an idea. I’ll recommend a reference for you to read (if you wish) and then follow this up, starting with a post on periglacial weathering processes and formations, I’ll formally introduce you to permafrost, finishing with sediment movement processes and distinctive landforms. Anything I don’t catch in either of these will more than likely land in a final summary post...

For your reading, I recommend you read the following book; a comprehensive summary of the processes and landforms typical of periglacial environments:

French, H. M. (2007). The Periglacial Environment. Thirdedition. John Wiley and Sons, Chichester.

A brief introduction to... Ice Core Records (Part 2)

bldgblog.blogspot.co.uk; Photo by Planet Taylor

I didn't keep you waiting too long for the follow up to ice cores pt1, did I? As promised, today we will discuss the cycles of climate change throughout the Quaternary and what we think the reasons for this are. We may need to go off on a tangent on this post too, but it's worth it.

So we've established that we can use ice cores as a record of regional temperature through the Quaternary. But regional temperature is subject to many different variables. If only we had information about global temperature change? Well, the last post mentioned that we'd concentrate on just the polar ice cores. Isotope ratios from ice cores in both Greenland and Antarctica have been compared and it seems that they correlate relatively well, implying that whatever is forcing the climate in the Northern Hemisphere is doing exactly the same thing in the Southern Hemisphere, a global climate forcing. Now for the promised tangent.

We mentioned solar forcing. Cyclic changes in received solar radiation (known as insolation) are caused by the rhythmic variation in the Earth's orbit and axial tilt. These are known as the Milankovitch cycles, named after Milutin Milankovitch. The eccentricity of the Earth's orbit (how elliptical it is) fluctuates on a 100,000 year cycle and affects both the total solar radiation received in a year  (due to the change in distance from the sun) and seasonality (difference in temperature between Summer and Winter). Obliquity is the angle of the tilt of the Earth's axis. This fluctuates between 22.1 and 24.5 degrees on a cycle of 42,000 years and is another control on seasonality. If the North Pole is further away from the sun during Southern Hemisphere Summer, Northern Hemisphere Winter is longer and colder than if it were closer to the sun. Do you follow? The third cycle is the Precession of the Equinoxes. This occurs on a cycle of 26,000 year cycle and is the cycle at which the Earth's axis rotates about an imaginary axis perpendicular to Earth's orbit. The precession is caused by the combined gravitational forces of the Sun and the Moon and is another control on seasonality. For a really great tutorial on Milankovitch Cycles check out this link, it's a great introduction to the concept (you do need flash player I think).

How do these Milankovitch cycles relate to the fluctuations in the climate proxy records we see in ice cores? It is the current paradigm that these cycles in the relative position of the Earth to the Sun control the cycles in the Earth's climate from glacial conditions back to interglacial conditions.

The prevailing cycle in the climate proxy record is the 100,000 year glacial cycle. However, it has been noted by many researchers that the fluctuations in received solar radiation caused by the 100,000 year eccentricity cycles would not be enough to control the massive fluctuations in global air temperature seen between the interglacial and glacial climate extremes. Many summarise that it is probably the combined effect of the precession and obliquity cycles that control or trigger the 100,000 year climate cycle observed over the past 400-800,000 years.

There is however, a hitch. Work by Carl Wunsch analysing the temperature proxy record leads to the conclusion that most of the temperature variation over the past 400,000 years is indistinguishable from a stochastic (internally complex with unidentified feedback mechanisms) system and not a result of solar forcing. In my opinion, the jury is still out on this. Also, as highlighted in the work by Wunsch, an 800,000 year sample size isn't that big when you're only considering eight 100,000 year cycles.

So, make of that what you will! The best place to start reading about this is probably from the beginning, with the Hays et al. (1979) paper in Science, "Variations in the Earth's Orbit: Pacemaker of the Ice Ages". I know it talks about marine sediment cores, but the concept is exactly the same. It's definitely a great introduction to the concept of solar forcing of past climate.

There we have it, a brief introduction to ice core temperature proxies. Tomorrow I will discuss some of the other climate proxies we extract from ice cores: CO2, CH4 and dust flux, and what these tell us about the environments at the time.

A brief introduction to... Ice Core Records (Part 1)

Bad form, but source: wikipedia

As promised, for today and the next couple of days I will be writing about the ice core records of Greenland and Antarctica. I'll start today with an introduction to ice cores and discuss isotope signals within them. Tomorrow I hope to explain other proxies in the ice cores such as dust, CO2 and methane (CH4) and finish with a post about the controversies surrounding the conclusions based on proxies within the records, with the arguments against the validity of solar forcing (I'll get to that). So, I hope you enjoy exploring the ice core records. I've got some graphs I produced for my course to help explain the concepts, there are some garish colours in there (you have been warned).

Ice cores are simply a core of ice extracted from areas where annual accumulation of snow has resulted in a consistent record of winter precipitation over an extended period of time. These are obtainable from mountain glaciers and the polar ice caps, but I'm only going to focus on the ice caps here as these provide the longest records, and therefore a greater insight into past climates throughout the Quaternary. We know from a previous post that ice accumulation is the result of a greater snow accumulation than summer melt for any given year. Because of this annual accumulation - melt, annual layers are visible within an ice core, with clear layers signifying the melt season and cloudier layers indicating the winter accumulation. We also know that the ice will flow down slope from an ice centre in the form of glaciers. For the purpose of obtaining a complete record, it is very important to take ice cores from areas where lateral movement of ice does not occur, as this would distort or remove some of the record. Ice centres are perfect for this, however in polar regions, they are obviously extremely difficult to get to.

The three most well known ice cores from polar regions are the Vostok and Epica Dome C ice cores from the East Antarctic Ice Sheet, where ice is known to flow extremely slowly in comparison to the West Antarctic Ice Sheet; and the GRIP ice core chronology from Greenland.

I'm going to jump off on a tangent now and start talking about isotopes. Don't get scared, it's a simple concept based on the relationship between the ocean and temperature. As we know, snow is made of water, H20. The H part, hydrogen, normally consists of one proton and one electron. However, an isotope of hydrogen known as Deuterium exists, containing one proton, one electron and one neutron. This neutron effectively doubles the mass of the hydrogen molecule. Still with me? Deuterium is far less common than normal hydrogen on Earth, around 99.98% hydrogen to 0.015% deuterium. So imagine that the ocean is made up of mostly normal water, but a tiny fraction of this water is made with deuterium, and consequently slightly heavier than the rest of the water.

During the colder periods (ie. glacial maxima), there is less energy to evaporate seawater into the atmosphere. Because it takes more energy to evaporate the heavier deuterium water the clouds, and consequently the snow that lands on the ice caps, are made up of a higher ratio of normal water to deuterium water (more than 99.98% to less than 0.015%). Therefore, the ratio of deuterium to normal hydrogen in the snow for that year is less than normal. The opposite is true when temperatures are warmer, with more energy, a greater proportion of deuterium can be evaporated and is deposited on the ice caps as snow. So for warmer years, the deuterium to normal hydrogen ratio is greater than colder years.

Ok, so we're back onto ice cores again. As I mentioned earlier, each years' snow deposition is recorded in a thin band of cloudy ice. This is made up of the ratio of deuterium to hydrogen that was deposited as snow all those years into the past. When we compare these with previous years, we can see that the ratio fluctuates year on year, indicating regional temperature increases and decreases back through time. The Epica Dome C ice core goes back an astonishing 800,000 years and the data extracted from this ice core (and numerous others) is available for free to download as a spreadsheet at ncdc.noaa.gov/paleo/data.html. I find it amazing that the average Joe Public (like me) can download this data and manipulate it, graph it and make their own decisions about the climates of the past. The graph below is exactly that. It shows the deuterium ratio (delta D) for the Epica Dome C ice core. The X axis is age, youngest at the left and oldest at the right, with the deuterium concentration on the Y axis. As you can see, a pattern emerges. Sharp warming events are followed by a gradual cooling to glacial conditions.

Click image for full size

So, you've been introduced to the ice core records and the concept of isotope ratios. The use of oxygen isotope ratios are used in the same way (18O / 16O), but all you really need to know is that water made with oxygen 18 is heavier than the normal water made with oxygen 16, and the ratio of these two are used to indicate temperature in exactly the same way. Tomorrow, I'll discuss the cycles of temperature changes, how they relate to the amount of sunlight the Earth receives. On Thursday we will discuss the other measurable properties of ice cores including CO2, CH4 and dust concentrations, and what they are likely to mean.

A brief introduction to... Glaciation

Glen Nevis. www.thewalkingzone.co.uk

Back again! Today's brief intro is on the evolution of landscapes through the transition from interglacial to glacial. I'll try not to focus too much on global temperature changes or the causes of these, more on a hypothetical landscape. And we're off...

A gradual drop in temperature will result in periglacial conditions and the onset of permafrost. Periglacial processes occur on the surface and subsurface, resulting in patterned ground features, pingos, palsas (in wetlands), sorting of sediments and frost action processes on exposed rocks. 

Precipitation in winter months results in a build up of snow cover, with melting occurring during the summer months. Seasonality is considered one of the primary controls on glaciation. If the net snow accumulation is positive for an extended period of time, snow cover thickness increases. Pressure from the snow overburden causes the transition of snow into firn, the point where the pore spaces between ice crystals become enclosed (more on this in a further post), trapping the air at its present concentration within the firn. Under increasing pressure, firn becomes ice.

Once overburden reaches a critical point, the ice spreads out in all directions from this "ice cap". Just as a river will find the path of least resistance, ice will flow in a direction which is easiest, this could be a relict river valley or any natural depression in the land. The erosive power of a massive body of moving ice is huge. Imagine the base of the glacier includes fragments of rock like giant sand-paper. Through time, this ice carves the classic glacial troughs seen throughout the Yorkshire Dales, Lake District, Snowdonia and Glen Nevis (Ben Nevis is an amazing place to hike; see the picture above and follow the link for more). This is probably the most awe inspiring period to imagine; millions of tonnes of ice grinding away the rock it is forced over by more ice being produced at the ice cap it originated from.

The Last Glacial Maximum (LGM) was between 12 and 14 thousand years ago depending on where you were at the time. It is known as the Loch Lomond Stadial or the Younger Dryas, and was preceded by an interglacial, just as every other glacial within the Quaternary.

Just as these periods capture my fascination due to the pure scale of landscape change, I find the transition from glacial to interglacial, known as the paraglacial period (coined by Colin K Ballantyne), is by far the most interesting. Landscapes just go crazy! This will be covered in my next post, so make sure to check back for that!

I hope you found this interesting. Please have a go at the feedback below, let me know what you think and what you'd like to read about, I'd be grateful for the ideas!