Salinity, early ocean

Knauth LP (1998) Salinity History of the Earth s Early Ocean. Nature 395 554... 

If most of the degassing of primary volatile substances from the Earth s interior occurred early, the chloride released by reaction of HC1 with rock minerals would be found in the oceans and seas or in evaporite deposits, and the oceans would have a salinity and volume comparable to that of today. This conclusion is based on the assumption that there has been no drastic changes in the ratios of volatiles released through geologic time. The overall generalized reaction indicative of the chemistry leading to formation of the early oceans and sediments can be written in the form ... 

In their hypothesis of a glacial decrease in Southern Ocean CO2 leak, early workers considered two causes (i) an increase in biological export production and (ii) a decrease in the exposure rate of deep waters at the polar surface (Figure 7). One can imagine processes that would have reduced the evasion of CO2 from the Southern Ocean by either of the two mechanisms mentioned above. For instance, an increase in the input of dust and its associated trace metals to the Southern Ocean might have driven an increase in the rate of nutrient and carbon uptake by phytoplankton (Martin, 1990). Alternatively, an increase in the salinity-driven stratification of the Antarctic and/or a decrease in wind-driven upwelling could have lowered the rate of nutrient and carbon dioxide supply to the surface... 

The reconnaissance studies of fluid inclusions (Horita et al., 1991, 2002) suggest also that early Paleozoic seawater was —2.5 X depleted in SO4, compared to its present-day counterpart. From model considerations, based on the mineralogy and volume of evaporites, claims have been made also for changes in the potassium concentration of Phanerozoic oceans (Hardie, 1996), and for an increase in the total salinity, from the modem 35 ppt to —50 ppt in the Cambrian (Hay et al., 2001). The experimental confirmation for all these theoretical assertions is presently not available (see Chapter 6.21). 

The catalytically active, Wyoming-type montmorillonites are inferred to have developed their distinctive, high-Na smectite-rich compositions following deposition of granitic volcanic ash into a shallow, saline ocean during the Late Cretaceous. Oxygen isotopic data from Hadean zircons have been interpreted as indicating that oceans may have been present on the Earth by as early as 4,300-4,400 million years ago (62-63). While current evidence provides no information on the composition of the hypothesized Hadean seawater, the existence of oceans on the Hadean Earth is a necessary environment on the ancient Earth if catalytically active montmorillonites formed. 

The earliest application of electrodeless conductivity measurements appears to have been for measurement of salinity at various ocean depths (1, 11, 45, ). Other early uses have also included determination of the equivalent conductances of salts at high concentrations (3, 34, 53) and the monitoring of nitric... 

Defining a representative composition for the ocean end-member in the models is particularly challenging because nearly every aspect of the physical and chemical state of the coupled ocean/atmosphere system on the early Earth (temperature, pH, elemental composition, salinity, oxidation state, etc.) is poorly understood at present and continues to generate vigoroits debate. We adopt an ocean composition that appears consistent with the cirrrently available corrstraints. The composition of the model late Hadean seawater nsed in this cormnitrrication is given in Table 1. 

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