MARL FORMATION AT MALHAM TARN

Marl formation

Davis (1900) noted the important characteristics of typically marly lakes: Dissolved calcium and bicarbonate usually derived from dissolution of limestone. Davis argued against authogenic precipitation as the major cause of lacustrine calcification and observed that snail shells and other animal remains generally comprise of only a small part of the marl. Instead, aquatic plants, including angiosperms, cyanobacteria, and especially the alga Chara appeared to be the dominant calcifiers (from McConnaughey 1994).

In Malham Tarn the Chara globularis and Chara Vulgaris account for present day marl deposition.  Recent marl from Malham is almost entirely composed of fragments of calcified Chara.

McConnaughey et al (1994) describe how calcification induced from photosynthesis results from the following reaction:

 2HCO3- (arrow)  CH2O + O2 + CO32-                                                                                                                     (1)

which raises the ambient CO32- concentrations and therefore CaCO3 saturation levels.  McConnaughey et al. state that CO2 withdraw from water by photosynthesis is, alone, unlikely to result in CaCO3 supersaturation.  The amount of CO2 removal required would lead to pCO2 levels that would inhibit photosynthesis.

A mechanism for calcification unrelated to CO2 withdraw is described:  Chara displays pH polarization, with distinct acid and alkaline surfaces. Calcification occurs at the alkaline surfaces, where pH reaches as high as 10.5 (McConaughey and Falk 1991.)  Carbon uptake occurs at the acid surfaces, some carbon is translocated to the alkaline surfaces, where it apparently leaks from the cell as CO2 and precipitates externally as CaCO3 (McConnaughey 1991).  This precipitation of CaCO3 from CO2 generates two protons at the alkaline surfaces (reaction 2). The plant transports these to the acid surfaces where they protonate exogenous HCO3- to form CO2 (reaction 3).  The resulting high pericellular CO2 concentrations in the acidic zones speed CO2 uptake and photosynthesis.

Ca2+ + CO2 + H2O (arrow) CaCO3 + 2H+                                                                                                             (2)

 2H+ + 2HCO3 (arrow) 2CO2 + 2H2O                                                                                                                     (3)