Can Trends In Carbonate Marine Cements Be Recognized In Sequences?

The June issue of Journal of Sedimentary Research has a paper by Vionette De Choudens-Sánchez and Luis A. González which studies controls of precipitation of aragonite and high Mg calcite.

Calcite and Aragonite Precipitation Under Controlled Instantaneous Supersaturation: Elucidating the Role of CaCO3 Saturation State and Mg/Ca Ratio on Calcium Carbonate Polymorphism

Marine inorganic calcium carbonate precipitates occur in three varieties or polymorphs, low Mg calcite, high Mg calcite and aragonite. This study deals with High Mg calcite and aragonite and indicates that the calcite lattice structure is affected by varying Mg/Ca ratios and its precipitation rates slow down with increasing Mg/Ca ratios. It can maintain a particular rate of precipitation only by increasing the CO3 saturation state of water. The aragonite lattice is not hindered by the presence of Mg and as a result aragonite might well be the dominant inorganic precipitate at higher Mg/Ca ratio of seawater.

I don't have access to the full paper so this is not a review but I since I have been working with carbonate cements I ended up thinking about the practical problems of recognizing cement mineralogy in ancient carbonates.

Let's take a step back. The three calcite polymorphs don't occur with equal abundance through geological time. Sea water chemistry has fluctuated from calcite seas where low Mg calcite has been the stable inorganic precipitate to aragonite seas where increased Mg/Ca sea water ratio resulted in aragonite and high Mg calcite becoming more abundant. These trends apply only to inorganic precipitates. Organisms that use one of the three polymorphs to build skeletons don't switch between the varieties if sea water composition changes. There are very conservative about which latticeware they drape themselves in.

If Mg/Ca ratios play a role in controlling rates of CaCO3 polymorphs, can we recognize trends in abundance of aragonite vs high Mg calcite in carbonate sequences. I am thinking of scenarios which could produce such trends. Carbonate sediments accumulate as thick piles often hundreds to thousands of metres thick. These piles of sediments are not homogeneous mixtures of sediment but are layered and there is a particular architecture to this layering which reflects the rise and fall of sea level. An initial rise in sea level will produce environments of deposition with considerable water depths and good circulation of sea-water throughout the depositional system. At this point in the history of the platform - known as transgressive systems tracts- tidal flats have just about nucleated along shorelines, reefs have begun aggrading but have not developed a distinct profile. The Mg/Ca ratios and saturation levels of sea-water would promote both aragonite and high Mg calcite cements in varying subenvironments.

But later as sea-level rise peaks and then starts to fall - known as highstand systems tracts followed by the beginnings of a lowstand systems tract - the distribution of facies in this deposition systems has changed. Tidal flats prograde over large expanses of the platform. Reefs have a topography that acts as a barrier to currents and produces quiet back lagoon environments with restricted circulation. Overall in this mature stage of sequence development there are larger pockets of evaporative environments and very high Mg/Ca ratio of sea-water. Does that lead to more aragonite precipitation as high Mg/Ca ratios becomes a kinetic barrier to calcite precipitation?

This was my speculation but I haven't come across studies which try to assess the relative abundance of these two carbonate polymorphs across any one platform and sequence history. Maybe no one is interested but a more practical reason is the extreme difficulty in identifying and quantifying ancient marine cements.

Both aragonite and Mg calcite are unstable in fresh water. Sooner or later carbonate sequences are exposed to meteoric diagenesis and these two carbonate polymorphs are altered to low Mg calcite. The change is often fabric disruptive, meaning even the original distinct morphology of marine cements - acicular, bladed, elongated - is lost.

This is where a second study reported in the latest issue of Sedimentology gives hope to researchers interested in understanding ancient marine cement distributions. This piece of research compares the rare earth element (REE) composition of unaltered aragonite coral skeletons with the REE composition of their altered counterpart i.e. low Mg calcite in the same reef. So what has happened is that a Pleistocene reef has been incompletely altered to low Mg calcite giving researchers an opportunity of understanding how REE composition changes as aragonite alters to calcite.

And they find that unlike minor elements like Sr, Ba, U which are lost from the aragonite lattice and are not incorporated in the newly forming calcite, the REE are conservative and are retained in almost the same proportions in the new diagenetic calcite as they existed in the precursor aragonite.

The study does not include original high Mg calcite so what has this to do with trying to quantify ancient ratios of aragonite to high Mg calcite in carbonate sequence?

I would be willing to put up a small wager that there is a systematic difference in the amount of REE the aragonite lattice can incorporate versus what a high Mg calcite lattice can. Aragonite is orthorhomic and generally accepts more easily large ions (ion size similar or larger than Calcium) like the REE in its lattice. Calcite cannot readily accept such large ion sizes in its trigonal lattice. Since the original REE distribution may be preserved in later diagenetic products, it may just be possible to tease out the original relative proportions of aragonite vs high Mg calcite cements in ancient sequences.

As an aside the use of rare earth element geochemistry has a long tradition in sedimentary geology, but it has been used in clastic sedimentary geology for provenance analysis much before its use in diagenetic studies. REE patterns in crustal rocks from different tectonic provinces are distinct. So mature craton versus forearc versus recycled orogen may be differentiated by REE composition of their detritus. Their use in carbonate diagenesis is a relatively recent development and given the complicated diagenetic histories of ancient carbonates, a very welcome addition to the carbonate geologists toolkit.