HOLOCENE DOLOMITE IS A BIOLOGICAL PRECIPITATE AND IS ABUNDANT IN CRUSTOSE CORALLINE ALGAE

For over a century geologists have been faced with a problem with respect to dolomite. How is it that a mineral and sink for magnesium in the rock record is so rare in younger rocks? This observation seemed to fly in the face of a uniformitarian paradigm. We have found that dolomite is abundant in some coralline algal facies on modern coral reefs. We observed that both dolomite and magnesite are common precipitates within the cells of the crustose coralline algae (CCA) Hydrolithon onkodies. These CCAs are abundant, covering approximately 25% of the reef area in the Indo-Pacific region, dominantly on the windward margins. These CCA facies are, in contrast, relatively rare in Caribbean reefs. The bulk magnesium content of the mixture of high magnesium calcite, dolomite and magnesite is 32 mole % Mg.

Given what we know about Holocene neritic carbonate distributions and abundances we can make an estimate of the Late Holocene magnesium flux into these CCAs. If the total neritic carbonate flux is 12 x 10¹² moles/year and 20% of global reefs are covered with CCAs today (after discounting the Caribbean). This gives 2.4 x 10¹² moles/year of CCA carbonate, 32% of which is potentially magnesium on a mole % basis.

Recognizing the favored environment of deposition for this syn-sedimentary dolomite is important for interpreting the occurrence of early dolomite in the rock record. If dolomite precipitating organisms prefer environments close to sea level with high wave action it will help in the modeling of dolomite occurrence relative to sea level change. For example, if CCAs are more common on ‘keep up’ reefs relative to ‘catch up’ reefs we might anticipate that this form of dolomite would be less common during times of rapid sea level change as opposed to times of relatively stable sea level.