MINERALOGICAL CONTROLS ON NUTRIENT (N AND P) SUPPLY IN THE DEEP BIOSPHERE

In order to sustain life within the deep biosphere and in extraterrestrial systems, it is essential to identify possible sources of key nutrients and to understand their behaviour. At the Earth's surface, the nitrogen cycle is dominated by interaction between the hydrological cycle and surface biota, with N ultimately sourced from the atmosphere. The conventional N cycle cannot function at great depths, or in systems with greatly different atmospheric compositions.

Within deep groundwaters from oil and coal fields, ammonium occurs at concentrations up to 1000 mg/l NH₄⁺, increasingly with total salinity¹. Deep brines derived from fissures intersected by a 1km deep borehole in granite (Weardale, UK) have NH₄⁺ contents of 10 - 12 mg/L. In both cases, ammonium in solution is controlled by ion exchange (K⁺- NH₄⁺) with layer silicates. The higher ammonium contents for the oil and coal field waters reflect inputs from organic sources, absent in the case of the granite.

Observations from biodegraded oils show that organic N compounds resist biodegradation. This paradoxically suggests that the microbial communities responsible for biodegradation source a proportion of their N from elsewhere; aqueous ammonium is a readily available source.

A mineralogical source for N is provided by K-Al silicates. Igneous feldspars contain up to 1000 mg/kg NH₄⁺ which becomes available through alteration, when it partitions between layer silicate and fluid according to ion exchange equilibria. As ammonium is consumed by a microbial system, it is replenished from the layer mineral source to maintain chemical equilibrium with the pore solution.

Alteration of K feldspars, as primary igneous minerals, also provides a source of the vital nutrient phosphorus. Natural K feldspars typically contain 0.2 - 0.5% P₂O₅, reaching 1% in highly evolved granitic rocks. On alteration, P is released to solution, where it becomes available as a nutrient for microbial systems. The predominance of feldspar within the Earth's crust, and the importance of feldspar alteration as part of the Earth system process, suggests that provision of key nutrients N and P may relate closely to feldspar diagenesis. If so, this nutrient source can be expected in extraterrestrial environments where feldspars and water coexist.

¹Manning DAC & Hutcheon IE 2004 App Geochem 19 1495-1503.