The mantle, CO₂and the giant Aptian chemogenic lacustrine carbonate factory of the South Atlantic: Some carbonates are made, not born

Abstract

During the Aptian (Cretaceous), in what is now the South Atlantic, the largest chemogenic (abiotic) carbonate factory so far identified in the Phanerozoic geological record developed as a vast hyper‐alkaline lake system. This covered at least 330 000 km2, producing carbonates, locally over 500 m thick, in what are now the offshore Santos and Campos basins (Brazil), and Kwanza Basin (Angola). Current evidence supports the view that almost all of this carbonate was chemogenic in origin, precipitated from hyper‐alkaline, shallow lake waters, probably by evaporation. This unit, best documented from offshore Brazil and known as the Barra Velha Formation (Santos Basin) and the Macabu Formation (Campos Basin), consists of just two basic carbonate components, millimetre to centimetre sized crystal shrubs and spherulites. These are commonly in situ but can also be reworked into a range of detrital facies. Demonstrable microbialites are generally rare. These carbonates are associated with Mg silicates (as clays) which had a profound influence not only on the textural development of the in situ carbonates, but also on their diagenesis. The dissolution of the clays produced much of the porosity in these limestones, which are the hosts for multi‐billion barrel oil fields. The source of the carbonate was most likely from metasomatic alteration of mafic rocks, such as continental flood basalts related to Atlantic opening, with some contribution from much older continental basement. Clear evidence that serpentinization of possible exhumed mantle is lacking but mantle CO2 is likely to have been a critical factor in determining the composition of the fluids from which the carbonates formed and the high alkalinities of the lake waters.

Paleoenvironmental Implications of Authigenic Magnesian Clay Formation Sequences in the Barra Velha Formation (Santos Basin, Brazil)

“…The distinction of microfacies was proposed by Netto [2] (Figure 2) and it is equivalent to that described by other authors in the Santos, Campos and Kwanza Basins [4,11,16,17,26,27].…”

“…Wright [27] discusses the sedimentological model from the identification of basic cyclothems occurring in the Barra Velha Fm [11], proposing that they would represent cycles of flooding and evaporation. In the first moment there would be inflow of fresh water, probably run-off water, which would decrease the salinity-alkalinity and would deepen the lake (expansion).…”

“…Some shrubs with good porosity show evidence that they probably had Mg-clays matrices. It is possible that this matrix would have been removed by wave action as a result of progressive shallowing [27]. Otherwise, shrubs would be nucleated above the wave base surface in a higher energy, shallower facies environment [14].…”

“…The causes, however, are still controversial. Some hypotheses have been invoked to explain a persistent high pH in a lake system: (i) input of mantle CO 2 into the lake waters [27,28,64], or (ii) reduced infiltration rates of lake waters into ground water aquifers [18]. In addition, it is suggested that the sources of Ca 2+ , Mg 2+ and SiO 2 , among others would come from the leaching and chemical weathering of basalts by phreatic and hydrothermal waters [65], and felsic rocks in a very extensive, shallow and endorheic lake [21].…”

The Limagne Basin: a journey through modern and fossil microbial deposits

“…Wright and Barnett, 2015;Herlinger et al, 2017;Souza et al, 2018;Lima and De Ros, 2019;Farias et al, 2019;Gomes et al, 2020) have considered these deposits as abiotic carbonate since they recognize some morphometric similarities with travertine deposits. Wright and Barnett (2015), Lima and De Ros (2019) and Wright (2021) assumed that the formation of shrubs is the combined result of changes in the lake water chemistry related to climate, abiotic precipitation resulting from CO2 loss by evaporation, magmatic CO2 input and hydrothermal activity. Although these authors admit that microbial mats were present in the lakes, they consider that the extreme alkalinity conditions reduced the level of metabolic activity to a point where biotic processes were minimized and thus the abiotic precipitation of carbonates predominated.…”