to present-day ocean, very likely due to the pH in the archaic “soda ocean” and the
likewise high level of phosphate. Phosphate, if available, resulted in a rapid formation of Ca-phosphate [Ca 3 (PO 4 ) 2 ] that is insoluble at <100 mM (Einsele 2000).
As generally accepted, the Precambrian oceans were richer in dissolved silica in
comparison to the present-day marine environment (Simonson 1985; Siever 1992;
Kazmierczak et al. 2004). However, in order to judge/assess the level of silica, an
estimation of the pH in Precambrian oceans must be given. The solubility of silica
depends strongly on the pH of the aqueous system; at pH 8, the solubility is lowest
(2 mM) and a strong increase is seen above pH 9 (>10 mM); Iler (1979), Morey and
Rowe (1964). The pH of the present-day ocean varies around 7.5–8.4 and the level
of silica is the range of 10–180 mM in the deep sea and in the coastal regions only
less than 3 mM (Maldonado et al. 1999). The higher concentration of silica (1 mM)
in the Precambrian oceans had been enhanced due to the increased pH (Kempe and
Degens 1985; Siever 1992). These authors proposed that during the Proterozoic, the
pH shifted from 10 to 8.5 allowing a “soda ocean” to occur. Hence the dominant
components in the “soda ocean” were Na 2 CO 3 and NaHCO 3 as well as high
concentrations of Fe
2+ . While only a portion of that silica was immobilized and
disappeared from the ocean through pore water diffusion, the biogenetic consumption of silica was negligible. The consequence was an increase of dissolved silica in
the ocean close to (super)saturation levels (around 2 mM). Because silica can
persist for months at pH 7–8 (Morey and Rowe 1964) and silica is nontoxic as
amorphous material at those levels (Bramm et al. 1980), it provided the starting
material for the formation of a poly(silicate)-based skeleton and hence could be
fixed by biogenic processes (Simonson 1985), especially by sponges. As can be
deduced from present-day sponges, some of which live in a high pH/alkaline milieu
as the Lake Chagytai (Siberia; Wiens et al. 2009), these animals could have coped
with those extreme physical conditions. There is no solid evidence that the diatoms
that are the dominant silica consuming and precipitation organisms in the presentday oceans, existed already in the Proterozoic (Sims et al. 2006), most likely they
evolved in the Mesozoic oceans (230–70 Ma). In contrast to silica, the level of Ca
2+
was much lower than at present. During the Vendian Period, the pCO 2 level in the
atmosphere rose, an event that had been correlated with the lowering of the ocean’s
Mg
2+ and increase of Ca
2+ levels (Tucker 1992).
9.2.2 Emergence of the Animal Organic Hard Skeletons
Two major processes during the Proterozoic provided the basis for the evolution of
a skeleton. First, accumulation/rise of atmospheric oxygen and second the availability of dissolved silica in the ocean. These two parameters gave the basis for the
organic (oxygen) and the inorganic (silica) ancient metazoan skeletons. At the
beginning of the Proterozoic Eon (2,500 Ma), the atmospheric oxygen level was
low or almost not existent, compared to the present atmospheric level (PAL).
Following the staging of (Walker 1978/79; Kasting et al. 1992), at Stage I
9 The Unique Invention of the Siliceous Sponges
255
likewise high level of phosphate. Phosphate, if available, resulted in a rapid formation of Ca-phosphate [Ca 3 (PO 4 ) 2 ] that is insoluble at <100 mM (Einsele 2000).
As generally accepted, the Precambrian oceans were richer in dissolved silica in
comparison to the present-day marine environment (Simonson 1985; Siever 1992;
Kazmierczak et al. 2004). However, in order to judge/assess the level of silica, an
estimation of the pH in Precambrian oceans must be given. The solubility of silica
depends strongly on the pH of the aqueous system; at pH 8, the solubility is lowest
(2 mM) and a strong increase is seen above pH 9 (>10 mM); Iler (1979), Morey and
Rowe (1964). The pH of the present-day ocean varies around 7.5–8.4 and the level
of silica is the range of 10–180 mM in the deep sea and in the coastal regions only
less than 3 mM (Maldonado et al. 1999). The higher concentration of silica (1 mM)
in the Precambrian oceans had been enhanced due to the increased pH (Kempe and
Degens 1985; Siever 1992). These authors proposed that during the Proterozoic, the
pH shifted from 10 to 8.5 allowing a “soda ocean” to occur. Hence the dominant
components in the “soda ocean” were Na 2 CO 3 and NaHCO 3 as well as high
concentrations of Fe
2+ . While only a portion of that silica was immobilized and
disappeared from the ocean through pore water diffusion, the biogenetic consumption of silica was negligible. The consequence was an increase of dissolved silica in
the ocean close to (super)saturation levels (around 2 mM). Because silica can
persist for months at pH 7–8 (Morey and Rowe 1964) and silica is nontoxic as
amorphous material at those levels (Bramm et al. 1980), it provided the starting
material for the formation of a poly(silicate)-based skeleton and hence could be
fixed by biogenic processes (Simonson 1985), especially by sponges. As can be
deduced from present-day sponges, some of which live in a high pH/alkaline milieu
as the Lake Chagytai (Siberia; Wiens et al. 2009), these animals could have coped
with those extreme physical conditions. There is no solid evidence that the diatoms
that are the dominant silica consuming and precipitation organisms in the presentday oceans, existed already in the Proterozoic (Sims et al. 2006), most likely they
evolved in the Mesozoic oceans (230–70 Ma). In contrast to silica, the level of Ca
2+
was much lower than at present. During the Vendian Period, the pCO 2 level in the
atmosphere rose, an event that had been correlated with the lowering of the ocean’s
Mg
2+ and increase of Ca
2+ levels (Tucker 1992).
9.2.2 Emergence of the Animal Organic Hard Skeletons
Two major processes during the Proterozoic provided the basis for the evolution of
a skeleton. First, accumulation/rise of atmospheric oxygen and second the availability of dissolved silica in the ocean. These two parameters gave the basis for the
organic (oxygen) and the inorganic (silica) ancient metazoan skeletons. At the
beginning of the Proterozoic Eon (2,500 Ma), the atmospheric oxygen level was
low or almost not existent, compared to the present atmospheric level (PAL).
Following the staging of (Walker 1978/79; Kasting et al. 1992), at Stage I
9 The Unique Invention of the Siliceous Sponges
255