This page authored by Steve Kershaw

Below is a summary from a recent paper, which provides a brief overview of the main features of the microbialites, and summarises the possible controls on their formation.

Reference: Kershaw, S., Crasquin, S., Li, Y., Collin, P-Y., Forel, M-B., Mu, X., Baud, A., Wang, Y., Xie, S., Maurer, F. and Guo, L. 2012.  Microbialites and global environmental change across the Permian-Triassic boundary: a synthesis. Geobiology, 10(1), 25–47.

ABSTRACT from Kershaw et al. (2012)

Permian–Triassic boundary microbialites (PTBMs) are thin (0.05–15 m) carbonates formed after the end-Permian mass extinction. They comprise Renalcis-group calcimicrobes, microbially mediated micrite, presumed inorganic micrite, calcite cement (some may be microbially influenced) and shelly faunas.

PTBMs are abundant in low-latitude shallow-marine carbonate shelves in central Tethyan continents but are rare in higher latitudes, likely inhibited by clastic supply on Pangaea margins.

PTBMs occupied broadly similar environments to Late Permian reefs in Tethys, but extended into deeper waters. Late Permian reefs are also rich in microbes (and cements), so post-extinction seawater carbonate saturation was likely similar to the Late Permian. However, PTBMs lack widespread abundant inorganic carbonate cement fans, so a previous interpretation that anoxic bicarbonate-rich water upwelled to rapidly increase carbonate saturation of shallow seawater, post-extinction, is problematic. Preliminary pyrite framboid evidence shows anoxia in PTBM facies, but interbedded shelly faunas indicate oxygenated water, perhaps there was short-term pulsing of normally saturated anoxic water from the oxygen-minimum zone to surface waters.

In Tethys, PTBMs show geographic variations: (i) in south China, PTBMs are mostly thrombolites in open shelf settings, largely recrystallised, with remnant structure of Renalcisgroup calcimicrobes; (ii) in south Turkey, in shallow waters, stromatolites and thrombolites, lacking calcimicrobes, are interbedded, likely depth-controlled; and (iii) in the Middle East, especially Iran, stromatolites and thrombolites (calcimicrobes uncommon) occur in different sites on open shelves, where controls are unclear.

Thus, PTBMs were under more complex control than previously portrayed, with local facies control playing a significant role in their structure and composition.

Some things to think about:

1. Many researchers focus on the importance of ocean acidification associated with mass extinctions and other biotic crises in Earth history. There is a mass of evidence used to support ideas of rising acidity associated with warming episodes, when CO2 levels were higher and the ocean capacity to absorb CO2 was being over-ridden. So there are numerous studies proposing ocean acidification associated with the end-Permian event.

    Therefore it is an interesting exercise to consider to what extent ocean acidification may have influenced microbialites after the end-Permian event. Certainly there is no clear increase in ocean saturation of carbonate after the extinction, because the microbialites generally have less carbonate cement fans than do the Permian reefs whose dead structures they replaced; that is good circumstantial evidence of acidification. But there is no directly provable evidence of seafloor dissolution affecting the latest Permian limestones, despite what you might read in the literature. But lack of evidence of seafloor dissolution does not mean that acidification did not happen, but that if it occurred, then pH did not fall sufficiently to dissolve carbonate off the sea floor.

    Also, we still have the question of why the microbialites were such thin deposits. The fact that in shallow waters there is a bed of grainstone between the extinction level and the first microbialites in many places suggests that the microbialites were depth/energy controlled, and may have been very thin because rapid sea-level rise outcompeted them in the first millenia of the Early Triassic. So if ocean acidification somehow was associated with microbialite growth, does the disappearance of microbialites relate to return to normal pH? I doubt it, but an interesting point is that dissolution is hinted at in the Dongwan site (shown in this atlas), but proving it was marine is another story. Also the fact that microbialites returned in some sites in three other occasions through the Early Triassic shows that controls on their growth were likely to be complex.

2. There is competing evidence of oxygenation of sediments in the microbialites. Pyrite framboids suggest that the microbialites grew in low-oxygen conditions, but abundant ostracods imply there was a ready supply of oxygen. There are alternative answers here; so how many can you think of?

  1. 3.Below are two graphics that relate to the end-Permian processes:

- The top graphic highlights two key possible causes of the end-Permian extinction, and there is a question in the green box, the answer to which seems obvious, but is it really?

- The bottom graphic draws attention to the potential of ocean overturn. To what extent does this relate to the current status of ocean oxidation?

(By the way, the volcano in the above photo is in Lanzarote.)