Geology Reference
In-Depth Information
Plate 104 Salinity and Alkalinity Controls: The Satonda Model
Satonda is a small volcanic Indonesian island north of Sumbawa. The island is unique in that the crater lake is
filled with seawater with increased alkalinity. Because lake and ocean waters cannot communicate, the lake
water acquired higher alkalinity, pH and carbonate mineral saturation than in the open ocean. These conditions
exclude most marine macrobiota but favor reef-like structures composed of red algae, serpulids and foramin-
ifera, and biogenic mats built by in situ calcifying cyanobacteria and green algae.
Morphology and microstructure of these deposits are similar to certain types of microbialites that are com-
mon in the Late Precambrian and Early Phanerozoic oceans. The conditions exemplified by the Satonda case
study have been used to establish a new model focused on the role of alkalinity for calcification processes in
early Phanerozoic oceans (Kempe and Kazmierczak 1990).
The formation of carbonates was controlled by changes in salinity and alkalinity (Kempe and Kazmierczak
1993). Salinity changes are reflected by the composition of the ostracod fauna (Schudack and Reitner 1996). The
crater lake was originally filled with freshwater. Shortly after the lake was flooded with seawater. During this
phase marine organisms colonized the lake starting with serpulids. Large input of organic matter from the crater
walls caused intense oxygen consumption. As a result, the bottom water of the lake became anaerobic and
sulfate reduction led to the precipitation of carbonates. The lake became stratified into water bodies of different
salinity and high alkalinity. A significantly increased carbonate mineral supersaturation caused a in situ calcifi-
cation of cyanobacterial microbialites forming microstromatolites (-> 1) and to the lithification of red algal
crusts (-> 7).
1 Macroscopic view of microstromatolites exhibiting subcolumnar structures. The stromatolites were formed by cyanobac-
teria in association with non-calcifying green algae
Cladophora
in
a
w
ater depth of 13 m and waters with a salinity of
about 30 ‰.
2 In-situ calcification of cyanobacteria. The SEM view of the surface of a coccoid cyanobacterial mat shows the capsular
character of gelatinous sheats surrounding
Pleurocapsales
cells embedded within a microgranular Mg-calcite matrix
precipitated in vivo by the mat itself. The lower part of the photograph corresponds to a dark lamina of the microstromatolite,
the upper part to a fibrous aragonitic lamina (compare -> 4).
3 The internal fabrics of the microstromatolites are laminated (this figure) or cystose (-> 6). The light aragonite laminae
alternating with dark Mg-calcite cyanobacterial laminae overgrow filaments of non-calcifying siphonocladalean green
algae (empty molds).
4 The vertical section of the microstromatolitic structure shows sparry (light) laminae consisting of fibrous aragonite and
microcrystalline (dark) laminae composed of High-Mg calcite. The differences in the mineralogical composition are
interpreted by seasonally changing supersaturation in the lake. During the wet season the supersaturation is lowered due
to rain allowing the growth of the mats. The carbonate supersaturation rises during the dry season resulting in an in vivo
precipitation of High-Mg calcite at the surface mat. Decay of the mat below the permineralized surface triggered by
sulfate-reducing bacteria connected with increase in alkalinity and of the Mg/Ca ratio causes the post mortem precipita-
tion of aragonite instead of Mg-calcite. Similar alternations of thin microgranular and thicker fibrous laminae are com-
mon features of ancient fabrics interpreted as microbialites.
5 The upper part of the calcareous crust consists of intergrown corallinacean red algae and variously calcified pleurocapsalean
cyanobacteria. The lower part consists of squamariacean red algae.
6 The cystose microbialite structure reminds on a common Palaeozoic fossil (
Wetheredella,
see Pl. 51/9) whose systematic
position is not settled (Kazmierczak and Kempe 1992).
7 Parts of the reef framework consist of encrusting squamariacean red algae (
Peyssonelia
) and accumulations of dark
peloids of fecal and cyanobacterial origin. Present-day Squamariaceae grow as flattened crusts or petaloid forms on soft
substrates (e.g. circumlittoral soft-bottoms of the Mediterranean). The internal structure is characterized by closely packed
basal cells from which vertical rows of cells arise. Note the attached fans of aragonite (arrows). Fossil squamariaceans
show in thin sections a yellow-brown tint in contrast to corallinacean red algae (Pl. 54/1, 3) exhibiting a dark grey tint.
8 Green algal meadows with
Cladophora
occur in the upper parts of the lake down to 16 m. Red algal crusts are also found
in deeper positions. The photograph shows longitudinal and cross sections of algal filaments (arrows) and irregularly
shaped open voids. Compare the picture with the Tertiary algal structures shown in Pl. 130/1.
-> 1-8: Courtesy of S. Kempe (Darmstadt)
