Publication Date:
2021-03-29
Description:
The crater lake of the small volcanic island Satonda, Indonesia,
is unique for its red-algal microbial reefs thriving in marinederived
water of increased alkalinity. The lake is a potential analogue
for ancient oceans sustaining microbialites under open-marine conditions.
Current reef surfaces are dominated by living red algae covered
by non-calcified biofilms with scattered cyanobacteria and diatoms. Minor
CaCO3 precipitates are restricted to the seasonally flooded reef
tops, which develop biofilms up to 500 mm thick dominated by the
cyanobacteria Pleurocapsa, Calothrix, Phormidium, and Hyella. Microcrystalline
aragonite patches form within the biofilm mucilage, and
fibrous aragonite cements grow in exopolymer-poor spaces such as the
inside of dead, lysed green algal cells, and reef framework voids. Cementation
of lysed hadromerid sponge resting bodies results in the
formation of ‘‘Wetheredella-like’’ structures.
Hydrochemistry data and model calculations indicate that CO2 degassing
after seasonal mixis can shift the carbonate equilibrium to
cause CaCO3 precipitation. Increased concentrations of dissolved inorganic
carbon limit the ability of autotrophic biofilm microorganisms
to shift the carbonate equilibrium. Therefore, photosynthesis-induced
cyanobacterial calcification does not occur. Instead, passive, diffusioncontrolled
EPS-mediated permineralization of biofilm mucus at contact
with the considerably supersaturated open lake water takes place. In
contrast to extreme soda lakes, the release of Ca21 from aerobic degradation
of extracellular polymeric substances does not support CaCO3
precipitation in Satonda because the simultaneously released CO2 is
insufficiently buffered.
Subfossil reef parts comprise green algal tufts encrusted by microstromatolites
with layers of fibrous aragonite and an amorphous, unidentified
Mg–Si phase. The microstromatolites probably formed when
Lake Satonda evolved from seawater to Ca21-depleted raised-alkalinity
conditions because of sulfate reduction in bottom sediments and pronounced
seasonality with deep mixing events and strong CO2 degassing.
The latter effect caused rapid growth of fibrous aragonite, while
Mg–Si layers replaced the initially Mg-calcite-impregnated biofilms.
This could be explained by dissolution of siliceous diatoms and sponge
spicules at high pH, followed by Mg-calcite dissolution and Mg-silica
precipitation at low pH due to heterotrophic activity within the entombed
biofilms.
Keywords:
Paläontologie: Allgemeines
;
551
;
VU 000
;
38.20
;
38.2
Language:
English
Type:
article
,
publishedVersion
Format:
application/pdf
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