Seawater Formation and Origin of Life

Changes in the ocean area, depth, and volume of the oceans were accompanied by significant qualitative transformation of seawater composition.

During the Early Archaean (c. 4-3 x 109yr BP), the dissolved volcanic products with accompanying gases (HCl, HF) predominated in seawater, besides boric acid, CO2, CH4, and other hydrocarbons, and SiO2. Seawater was acidic, with pH c. 1-2.

Gradually the composition of seawater underwent changes as a result of neutralization of acids by carbonates (K, Na, Ca, Mg), which formed on land during weathering of volcanic rocks and were washed down to the oceans. Little by little the seawater became a chloride solution with Al, Fe, Mn, and a small amount of sulfates. As a result of further chemical weathering on the land surface seawater acquired chloride-carbonate composition and sedimentation of CaCO3, MgCO3, FeCO3, and MnCO3 began.

At the end of the Archaean (c. 4-3 x 109yr BP), the first green plants appeared, whose life-supporting processes were based on photosynthesis, with resulting production of oxygen. It was a turning point in the

Table 1 Changes of the basic parameters of surface waters and the oceans

Time

(109 years)

Total mass of water in hydrosphere (1024 g)

Total volume of hydrosphere (106 km3)

Total volume of oceans (km2)

Total area of oceans (106 km2)

Average depth of the ocean (km)

Sea level in relation to the average elevation of the Earth crust H (km)

Sea level in relation to its actual level H2 (km)

-4.0

0.02

0.02

0.02

509

0.04

0.01

-2.49

-3.5

0.09

0.09

0.09

508

0.18

0.10

-2.40

-3.0

0.22

0.22

0.22

506

0.44

0.25

-2.25

-2.5

0.43

0.42

0.42

504

0.83

0.53

-1.97

-2.0

0.66

0.64

0.63

499

1.26

1.00

-1.50

-1.5

0.90

0.88

0.86

488

1.76

1.50

-1.00

-1.0

1.10

1.07

1.04

462

2.25

1.88

-0.62

-0.5

1.27

1.24

1.20

418

2.87

2.18

-0.32

0.0

1.42

1.39

1.34

361

3.71

2.50

0.00

T x 109

Figure 2 Changes in total ocean area and gradual rise of sea level.

T x 109

Figure 2 Changes in total ocean area and gradual rise of sea level.

evolution of the atmosphere leading to its transformation into the oxygen-type one.

In the Early and Middle Proterozoic (c. 2 x 109 yr BP), rather numerous traces of plant organic life became evident. The case in point is calcareous algae, or stromatolites, known from the carbonate formations of Huronian series of the Lower Proterozoic in Canada. Free oxygen being available, the oxidation of sulfur and hydrogen sulfide began, seawater received a sulfate ion; therefore, its composition gradually became chloride-carbon-sulfate.

The graphic evidence of transition from reduction to oxidation in the atmosphere and the ocean in the Early Proterozoic was mass accumulation of thick strata of banded ferruginous quartzite (jaspilites), many of which aged from 1.9 to 2.2 x 109yr BP.

It is thought that the amount of free oxygen in the atmosphere equaling one per mille of its present-day concentration (the Jury point) was reached c. 1.2 x 109yr BP. Since then the formation of thick acid-leached weathering crusts enriched with iron hydroxides and thus red-and brown-colored soil began on land.

Due to the presence of even a small amount of free oxygen in the atmosphere and hydrosphere, first oxygen-consuming living organisms appeared in the oceans. The most ancient fossils (worm tubes) were found in the Middle Proterozoic sedimentary rocks. The Vendian sedimentary formations embed the fossils of at least 20 genera of sea animals, mainly Coelenterata (jellyfish) and Arthropoda.

At the turn of the Phanerozoic, an important milestone had been achieved. Around 600 x 106yr BP, practically between the Vendian and the Paleozoic, the amount of free oxygen in the atmosphere exceeded 1% of the present-day concentration (the so-called Paster point). It was this circumstance that caused the evolutionary explosion in the beginning of the Phanerozoic, when practically all types of marine animals except chordates were widely distributed. About 400 x 106 yr BP, the concentration of free oxygen was already 10% of the present-day values, providing for the formation ofthe atmospheric ozone layer and the penetration of life on the land. Further on, during just several dozens of million years, the terrestrial vegetation evolved rapidly and the present-day concentration of oxygen in the atmosphere was achieved due to the process of photosynthesis.

The composition of hydrochloric sediments and buried brines of sea origin gives evidence that already in the Cambrian the composition of seawater was the same as nowadays. In other words, final stabilization of the present-day salt composition of the World Ocean took place between 1.5 and 0.5 x 105yr BP.

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