Post by Dan Schillereff
Puzzled faces are often returned when people learn that my doctoral research involves analysing mud extracted from the bottom of a lake. Common questions include why? How? What does it look like? My interest in lake sediments was developed through my Undergraduate degree at Liverpool and addressing these queries is now an important part of my PhD. In this post I will briefly outline some answers from a personal perspective.
Why? Lake sediment records have contributed hugely to our understanding of past environmental changes around the globe. Where a lake is fed by a river which drains the local catchment, sediment is transported through the fluvial system and deposited at the lake bottom. These sediments can be characterised using numerous parameters measured in the field or using advanced laboratory techniques and fluctuations in these measurements may indicate a change in local climate, land-use or vegetation cover. My research focuses on recovering sediment sequences from lakes in the English Lake District (Bassenthwaite and Brotherswater) which should contain records of extreme flood events which have occurred in the catchment in the past. If the sedimentological signature of discrete flood layers within a long sediment sequence can be deciphered, counting the number of such layers can provide insight into flood frequency and a relationship between grain size and river discharge may provide data on flood magnitude. Our sediment cores cover many centuries and, in some cases, millennia. These datasets will therefore be invaluable to river managers and policy makers who need a better context in which to place the current spate of extreme flood disasters witnessed in recent years and to develop more effective mitigation scenarios of future flood risk.
How? Many techniques have been developed to extract the soft sediments lying at a lake bottom. Preserving the internal structure of the sediment sequence is crucial, however the high water content of the mud means methods must be used which recover the lake sediments with minimal disturbance. For extracting sequences potentially many meters thick, we use a Russian-style sediment corer, which consists of a semi-cylindrical metal case with a rotating cover plate, with an optional manual hammer system that can be employed. At Liverpool, we have constructed a raft which enables coring to take place on a solid, well-anchored platform. The corer is lowered through the water column, with metal support rods attached as required to reach the desired depth. If the sediment proves difficult to penetrate, the hammer system is attached to the vertical supporting rods and a weight is lifted and dropped to drive the corer further into the basal sediments. When the desired depth is reached, the support rods are rotated clockwise, the cover carves through the sediment column and an undisturbed sediment sequence is captured within the casing. The corer is lifted to the surface, lain flat and the rotating motion is reversed, revealing a lovely stratigraphic sequence upon the cover. Each core drive (which can be 0.5, 1.0 or 1.5 m in length) is encased in PVC drainpipe and transported to the lab here in Geography.
What does it look like? In my short academic career, I have observed numerous textures, colours and smells associated with mud extracted from different lake environments. Examining the factors which generate such characteristics is far beyond the scope of a blog post, but certainly these are subjects which feature prominently in the Physical Geography Undergraduate curriculum and the MSc Environment and Climate Change course.