Ocean Archaean

Kusky, T. M. Kidd, W. S. F. 1992. Remnants of an Archaean oceanic plateau, Belingwe Greenstone Belt, Zimbabwe. Geology, 20, 43-46. 

Fig. 7. Model section of the shelf, slope and deeper waters of an Archaean ocean. 

It is likely that the progressive oxidation of the atmosphere also influenced the oceans. Certainly the chemistry of the Archaean oceans was different from that of modern oceans and was richer in Ca, Mg, K, Fe, Ba, Ce, and Si and lower in S, P, Mo, Re, U, and Os. These differences, in part, reflect the different redox conditions of the Archaean Earth... 

There are two reasons why it is likely that the Archaean sulfur cycle might have been very different from that which is observed in the modern. First, as will be discussed later in this chapter, the Archaean atmosphere had very low levels of oxygen, so that there was no oxidative weathering of sulfide in crustal rocks at this time. This means that no sulfate was delivered to the Archaean oceans through weathering and consequently the Archaean ocean was probably very low in sulfate. Second, the removal of sulfate from the ocean is bacterially mediated. The operation of this... 

If the Zahnle and Sleep (2002) model for COa-drawdown is correct, and C02 was principally stored in the oceanic crust during the Archaean, this does not necessarily negate the calculations of Kramers (2002), but it does shift the time of C02-drawdown back to perhaps the mid-Archaean. However, a problem with the Zahnle and Sleep (2002) model is that, unlike the Urey cycle, the Archaean oceanic weathering cycle has no inbuilt temperature feedback and needs a "thermostat" to maintain equable surface temperatures on the early Earth. This problem could be solved (just) if there were very high levels of COa in the atmosphere, but the better solution is to include methane as a significant component of the Archaean atmosphere. 

Thus in the Archaean, when initially there was no life - or life of a different kind, when the atmosphere was more C02-rich and less oxygenic, and when landmasses were smaller, but weathering processes more aggressive, it is to be expected that sea water had a very different composition from the present day. In fact, in an early study Walker (1983) suggested that in the Archaean oceans had a lower pH and a lower carbonate and sulfate content, but higher concentrations of Ca, Fe2+, Ba, Si, Na, Cl and bicarbonate. 

Quantifying the hydrothermal input into the Archaean oceans... 

TABLE 5.5 Estimates of the pH, element ratios and composition of Archaean sea water compared to modern sea water. Modern sea water data from Holland (2003) and Bruland and Lohan (2003). Elemental con-centrations that were higher in the Archaean oceans are indicated in italics. ... 

Present-day oceans fmmol.l"1) Archaean oceans (mmoLl-1) Reference... 

The regular precipitation of iron oxides from the Archaean oceans is also thought to have had a strong control on phosphate levels. Phosphate is important because it is a major nutrient which controls oxygen productivity. Bjerrum and Canfield (2002) calculated the dissolved phosphate content of the Archaean... 

Desrochers, J.-P, Hubert, C., Ludden, J.N., and Pilote, P., 1993. Accretion of Archaean oceanic plateau fragments in the Abitibi greenstone belt, Canada. Geology, 21, 451-4. 

Puchtel, I.S., Hofmann, A.W., Jochum, K.P., Mezger, K., Shchipansky, A.A., and Samsonov, A.V., 1997. The Kostomuksha greenstone belt, NW Baltic Shield remnant of a late Archaean oceanic plateau Terra Nova, 9, 87-90. 

Van Kranendonk, M.J., Webb, G.E., and Kamber, B.S., 2003. Geological and trace element evidence for a marine sedimentary environment of deposition and biogenicity of 3.45 Ga stromatolitic carbonates in the Pilbara Craton and support for a reducing Archaean ocean. Geobiology, 1, 91-108. 

Walker, J.G.C., 1983. Possible limits on the composition of the Archaean ocean. Nature, 302, 518-20. 

Ohmoto H, Felder RP (1987) Bacterial activity in the warmer, sulphate-bearing, Archaean oceans. Nature 328 244-246... 

A study of phosphorus in rocks 3.5 to 3.2 billion years old... R. E. Blake et al. Phosphate oxygen isotopic evidence for a temjjerate and biologically active Archaean ocean. 2010. Nature 464(7291), p. 1029. DOI 10.1038/nature08952. 

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