The Dolomite Problem - Peeking Under The Hood

A recent paper in Geology addresses one of the most enduring problems in sedimentary geology- the origin of the mineral dolomite. One can extend their findings to answering the origin of massive dolomite carbonate sequences that recur throughout earth history. Massive dolomite means that most of the rock is made up of the mineral dolomite and such dolomite strata can be hundreds of feet thick, representing deposition over millions of years.  The paper  examines one specific mechanism of sedimentary dolomite formation. Microbial activity in shallow water depths has long been suspected to induce dolomite precipitation and the paper brings out the specifics of the bacterial micro-environment that might aid dolomite to precipitate directly from sea water as pore filling cement or replace existing calcite or aragonite.

Such microbial ecosystems exist today in very restricted settings such as hypersaline lakes and supratidal flats and whether such microbial induced dolomite can explain the thick dolomite sequences which were deposited in more varied environments is a question that still needs more attention.

David Bressan on Scientific American blog has written two posts on the history of research on dolomites and in reference to the paper in Geology on the question of the microbial origin of dolomite using the Triassic Dolomite sequences of the Alps as an example. A considerable part of this sequence is made up of thinly laminated strata. The laminae have been interpreted as structures arising from sediment being trapped or precipitated between bacterial sheets. Based on these microtextures there is case being built up that much of the Triassic dolomite, especially that deposited in earlier phase in the Norian stage is likely to be of microbial origin i.e. induced by microbial activity.

First a few SEM images that show a spatial link between microbial filaments and incipient dolomite crystals.


The first image below show results from a dolomite culture experiment.

 Source: McKenzie J.A. and Vasconcelos, C.  2009

You can see a development of incipient dolomite associated with suphate reducing bacterial filaments to progressively large dumbbell shaped dolomite crystals.

In the second image below A B and C are globular nanobacteria encrusted by dolomite crystals growing within bacterial mats in a hypersaline lagoon (Lagoa Vermelha, Brazil). D is a globular structure interpreted to be bacteria fossilized in the Triassic dolomite sequences.

  Source: McKenzie J.A. and Vasconcelos, C.  2009

So, such globular and dumbbell shaped dolomite crystals along with sedimentary macro-structures like fine laminae may be indicators of microbial dolomite in the ancient rock record.

Davids' post summarizes the Triassic dolomites quite well. I want to expand on this a bit by including the wider dolomite record of the Phanerozoic. The assumption for a long time was that dolomite abundance as a percentage of carbonate rock increases with age. That might be true if the Phanerozoic is compared with the Precambrian. But some analysis (Given and Wilkinson 1987) along with some others shown in the figure to the left (compiled in McKenzie J.A. and Vasconcelos, C.  2009) shows that within the Phanerozoic dolomite abundance varies episodically . Massive dolomite is more abundant during eustatic sea level highs in the lower Ordovician/lower Silurian and Cretaceous.

These sea level cycles which last for tens of millions of years also coincide with changes in ocean chemistry. Sea level highs are characterized by lower Mg/Ca, higher pCO2 (more dissolved CO2 in oceans) and calcite saturation i.e. the tendency to precipitate low Mg calcite as an inorganic cement. The Triassic too has abundant massive dolomite but is something of an oddball time. Unlike other periods, in the Triassic, most of the massive dolomitization occurred during a eustatic sea level fall in the Norian stage lasting about 9 million years.

There has been plenty of debate on why dolomite abundance coincides with these eustatic sea level cycles and secular variations in sea water chemistry. For example, the correspondence of dolomite with low sea water Mg/Ca is puzzling since dolomite formation is favored at high Mg/Ca values as seen in experimental set ups and its natural occurrence today in hypersaline high Mg/Ca lagoons and lakes. A lower Mg/Ca value during sea level highs which are periods of increased sea floor spreading has been explained by the uptake of Mg in hydrothermal reactions at mid oceanic ridges. A clever counter argument is that it is the massive dolomite formation that is causing a lowering of sea water Mg/Ca. That argument may be more strongly applied to the Triassic. Sea floor spreading rates were slow but yet the estimated sea water chemistry indicate a much lower Mg/Ca than expected. Perhaps dolomite formation was taking up the excess Mg?

High pCO2 and calcite saturation state are thought not to inhibit dolomite formation. But there is no evidence that they favor its formation either. Much work, both experimental and modeling (summarized here), is slowly concluding that there is no neat link between any of the above sea water chemical parameters and dolomite abundance. That leaves some yet unidentified cause and the attention shifted towards a microbial influence.

What could be conditions that favor a spread of microbiota across the sea floor? Curve E in the figure above is a clue. All periods of dolomite abundance are periods of relative lower atmospheric oxygen possibly leading to mild ocean anoxia i.e. the oceans were less oxygenated during these times. Anaerobic bacteria can thrive in these conditions. These bacteria remove via sulphate reducing reactions dissolved sea water sulphate which is a hinderance to dolomite formation, thus creating favorable conditions on the sea floor for dolomite to either precipitate as carbonate mud or as pore filling cement or as an early replacement of calcite and aragonite skeletons.

But there are dolomites and dolomites. How widespread is the association between microbiota and dolomite during other phases of dolomite formation in the Phanerozoic? As my sedimentary geology professor once remarked - the dolomite debate rages on decade after decade. I won't bet on it not lasting for a few more.