Oceanography (2+1)

Chemical History of the Ocean

          As conditions on the earth changes during the course of geologic time, there must have been corresponding changes in the chemical composition of the ocean. These chemical adjustments would have stemmed from the regulatory processes,much indirect evidence has been obtained by studying ancient sedimentary rock strata on the continents. These continental rocks have weathered under the attack of water containing dissolved atmospheric gases. As a result, most of the major cations have found their way to the ocean. Calcium reached saturation quite early, judging from ancient calcium carbonate and calcium sulfate deposits existing on land today. Potassium accumulated in appreciable quantities as early as 1.2 billion years ago because sediments of that age contain  glauconite, a potassium-rich claylike mineral. An abundance of magnesium caused the formation of strata of dolomite, a calcium magnesium carbonate mineral. The earliest water-laid sediments appear to be about three billion years old.

          The first form of life must have been anaerobic,but eventually an organism capable of photosynthesis evolved. Photosynthesising organisms produced an excess of oxygen that gradually escaped in to the atmosphere. The amount of carbon dioxide in the atmosphere and dissolved in the sea decreased by being incorporated into organic matter. The hydrogen ion concentration (acidity) of seawater was established and held constant by the buffering action of the dissolved silica-clay mineral system and the dissolved carbon dioxide-solid carbonate system. As oxygen accumulated the ocean and terrestrial protions of the earth changed from the primordial reducing condition to an oxidizing one. Only in locations removed from contact with air or oxygenated water do reducing conditions persist today. One such location is in the sediments on the ocean floor. Not only are the bulk of bottom sediments isolated from contact with oxygenated seawater, but organic matter incorportated in these sediments decomposes and consumes whatever oxygen may have  become trapped in interstitial water when the sediment settled to the bottom. The oxidizing state at the interface of sediment and water has led to the formation of ferromanganese precipitates. These and organic detritus must have begun adsorbing trace metals soon after an excess of oxygen accumulated in the ocean and atmosphere.  

          Considerable geological evidence leads us to conclude that the volume of seawater in the ocean has been increasing throughout the earth’s history. The chemical composition of the world ocean, however became very nearly what it is at present shortly after the advent of photosynthesizing organisms that is, about 1.5 billion carbonate-secreting orgranisms and with temperature, for instance, would have caused a decreased solubility of gases and a concomitant decrease in biological activity. Under such conditions, nutrient concentrations would have increased and carbonates would have precipitated. Nevertheless, except for the possible appearance of some new substance capable of ion exchange or chemisorptions, the effects on regulatory processes in the world ocean today are limited to changes in the rates of inorganic and biochemical reactions.