8 - Biogeochemistry

Summary

The purpose of this Chapter is a short review of how biological processes affect the geochemical pathways that would prevail in the absence of life and how they contribute to the production of specific components that can occasionally form the bulk of some biological material such as oil. Expertise in biogeochemistry requires a strong background in biology and biochemistry and also some understanding of how biomass interacts as a whole with the mineral world. Background is well beyond the scope of the present book and we will hence try to restrict ourselves to the simplest of concepts.

The geological record

Oxidized rocks, limestones, cherts, and phosphates contain the biological materials with the most spectacular contribution to the geological record. Modern limestones are largely formed by the accumulation of carbonate tests of foraminifera and unicellular algae such as coccolithophores. Diatom frustules contribute massive amounts of silica to sediments at the bottom of the Southern oceans. The gigantic phosphorites of Africa represent fossil hard parts (teeth and bones) or their remobilization by diagenetic fluids: they are mined to produce fertilizers for agriculture. On the sea floor, these three types of rocks are often associated with each other in areas rich in nutrients, continental platforms, wind-driven upwellings of deep seawater such as next to the coasts of Morocco and Peru and the older seawater from the Southern oceans.