Carbon Flux at Malham Tarn

Project outline

This Ph.D Project is supported by a CASE/NERC studentship jointly supervised by Dr Pentecost, Kings College London and Dr. Spiro, NERC Isotope Geoscience Laboratory, Keyworth.

Drs Pentecost and Spiro have published over 100 research papers over the past 25 years.

Other related research interests.

I am producing a stable C and O profile for fossil Chara fragments and calcified fruits down a core collected from the Tarn in Summer 1996. I hope this may be of use as a paleoenvironmental record.

Previous work.

Pentecost (1984) investigated seasonal Ca²⁺ uptake at Malham Tarn as a result of photosynthetic calcification of Chara. As much as half the DIC is removed in the productive summer months.

Pentacost and Spiro undertook a preliminary study of tends of stable carbon and oxygen isotope ratios with depth and season.  A pronounced summer uptake of light carbon (¹²C) from the tarn water by photosynthesis and calcification was noted. The isotope depth profile was shown to be well mixed with a concentration of heavy carbon (¹³C) at the tarn bed identified. The Ph.D project shall develop upon these findings by identifying and quantifying the sources of carbon, the processes and associated isotopic fractionation affecting carbon in Malham Tarn.

Jones et. al. (1996) assayed the carbonate on present day calcified Chara fruits (“Gyrogonites”) at Malham Tarn identifying a large range of d¹³Ccalcite reflecting either fluctuations in ¹³C Dissolved Carbonate (DC) over the short period of fruit biomineralization of variations between locations within the lake. The implications for use of d¹⁸0calcite and d13Ccalcite as paleoenvironmental and paleoecolocical where discussed. An detailed understanding of the carbon and oxygen isotope dynamics in the tarn may be a prerequisite to any interpretations from the Chara gyrogonite fossil record.

The relationship between isotopic composition of calcite encrustation on Chara and that of the surrounding water has not been studied sufficiently. I intend to investigate this relationship, through field work and sampling from the Natural History Museum Chara collection, and assess/develop its utility in paleoenvironmental/paleoclimatic studies. 

Sampling at Malham Tarn.

This regular sampling would not be possible without the co-operation of staff at the field centre, I am grateful for their invaluable assistance and happy to acknowledge this.

Sampling on a 3-5 weekly basis is carried out on visits to Malham by myself and Allan Pentecost.  Samples are taken from the points mentioned above; the east springs and various points within the Tarn and along the water courses associated with the Tarn.  These samples are analysed for: pH; CO₂/bicarbonate; calcium alkalinity; stable isotope ratios; suspended solid concentration.

Over summer, weekly Secchi disk depths are recorded at a tarn site marked with red and white buoys (see map). ¹⁴C fixation; DIC concentration; stable carbon and oxygen isotope ratio for DIC; phytoplankton diversity and abundance studies are undertaken at 2-3 week intervals at this site.

Carbon species in the Malham Tarn ecosystem.

1. 1)  dissolved inorganic carbon (DIC = CO²(aq) + HCO^3- + CO₃^2-) mostly as bicarbonate at Malham;
2. 2)  dissolved/colloidal organic carbon; particulate organic carbon (phytoplankton and material from erosion of Tarn Moss);
3. 3)  particulate inorganic carbon (mineral grains and coating on lime secreting plankton);
4. 4)  phytobenthos organic carbon; phytobentos inorganic carbon (CaCO₃ coating Chara and Rivularia)

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