Malham Tarn: an introduction |
In the past, carbon flux models have proved to give valuable insights into the dynamics of carbon in freshwater ecosystems and can be an important lake management tool.
At a recent meeting of the "Terrestrial Initiative for Global Geological Research," the urgent need for "detailed work on the relationship between climatic variables and the stable isotope geochemistry of lake-waters" was emphasized. Paleoclimatic/paleoenvironmental interpretations from fossil marls may be tested in light of information provided by the study, new proxy methods may become apparent.
The Tarn surface area is about 153 acres, the average depth is 2.4m and the maximum depth is 4.4m (bathometry map ).
The Malham Tarn catchment area covers about 6 km. Average rainfall is 1542.5 mm per year. The retention time (vol./discharge) of the tarn is 11 wks., G.Fryer (1991)
Thermal stratification is rare/transient. The Tarn is frozen
for most of the winter, in the summer water temperature reaches as high
as 20 o C.
The inflow to the Tarn consists of a small stream entering at the north-western corner and to a lesser extent the small springs that issue from close to the limestone/shale boundary at the base of the limestone scar ("Great. Close") on the eastern shore. There is one outflow: at the southern end of the Tarn ("Tarn Foot"). The outflow stream flows for only a short distance- depending on outflow strength- before sinking into the limestone. (see map)
To the west is a raised bog called "Tarn Moss" a cliff has formed as a result of erosion by the Tarn water subsequent to the raising to the water level in 1771. Small beds of Carex rostrata (sedge) grow in the NW corner and the sheltered east bay. The north and north-east shores are of limestone with glacial drift covering and variable sized boulders and pebbles of limestone.
The Tarn lies largely over Silurian slates covered with thick glacial drift and marl deposits. Surrounding the Tarn is a karstic limestone landscape of predominately Carboniferous age (see geological section (available soon!))
The Tarn provides a rare habitat- being in an upland limestone districts and having a western peat shore- which supports diverse and some rare species. For example: there are two rare species of benthic harpacticoid copepods: Bryocamptus rhaeticus and Motatia mrazeki. The former only known in the UK from N. Wales and a few sites in Scotland, the latter known from Scotland and Malham only. Also present is the caddis fly Agreypnetes crassicornis which is unknown elsewhere in Britain. Nine of the twenty two molluscs species found in the Tarn are at their highest altitude in Britain. G.Fryer (1991)
Limestone deposition occurred, over what is now the Malham area, in a deep part of the Carboniferous sea.
The tarn basin was dammed by a gravely moraine during the retreat of
ice at the end of the last (Devensian) Glaciation c10,000 yrs. ago.
Prior to the silting up and mire development in the west, Malham Tarn
was originally twice it
Deforestation of the surrounding area and the prevention of regrowth by grazing has occurred since the Iron Age.
In 1971 the embankment and sluice gate at the outflow were added by Lord Ribblesdale in order to raise the Tarn level. This has resulted in erosion of Tarn Moss.
Piggot & Piggot (1959)
cored Tarn Moss to show a sedimentary sequence with moraine at the
base overlain by glacial clays, with varves at the top and bottom, thought
to represent the Drays cold phases ether side of the stable Allerod interstadial.
This in turn is overlain by local sandy material, then a thick sequence
of Charaphyte marl and, ultimately, peat.
Chara calcification studies at Malham
- The relationship between water carbonate and Chara growth
Pictures of Malham area
Carbon Cycle
Pietro Coletta
Pietro Coletta. 20th Feb. 2002
Pietro.Coletta@kcl.ac.uk