Horizontal and Vertical Distributions of Elements in the Ocean
Ocean chemists are concerned with the spatial and temporal distributions of chemicals in the ocean and the interaction of these distributions with physical and biological processes that may change chemical concentrations. In turn, these chemical variations can have a profound impact on rates of biological processes. The distributions of dissolved elements in the ocean are generally controlled by the interplay of three processes. The uptake and release of dissolved elements on sinking particles (primarily sinking plankton) transports chemicals vertically within the ocean. The major flow of ocean currents, called the conveyor, transports ocean waters horizontally from the major sources of deep water in the Atlantic Ocean to the deep waters of the Pacific Ocean. As a result of this general circulation pattern, deep waters in the Atlantic are much younger (relative to the time since they left the surface) than deep waters in the Indian or Pacific Oceans. Finally, the distributions of some elements may reflect external sources that act as a source of elements to the ocean or a sink that removes elements.
Elemental distributions fall into five major categories. These are:
Conservative elements: These elements have nearly the same concentration vertically and horizontally in the ocean. The only process that create detectable changes in their concentrations are the addition or removal of water by precipitation or evaporation. Elements attain a conservative distribution when their residence time (the length of time it takes various input processes to add an amount of an element to the ocean that is equal to the amount of the element in the ocean) is much longer than the mixing time of the ocean (~1000 yrs). Conservative elements typically have residence times greater than several hundred thousand years. Sources or sinks of these elements are so small, relative to the amount in the ocean, that they do not produce appreciable impacts on the concentration. As an example, look at the distribution of sodium.
Nutrient-like elements: These elements are depleted at the ocean surface due to uptake from the water by plankton and incorporation into the biomass of the plankton. This process is dominated by plants, which are restricted to the sunlit zone of the ocean where there is enough light for photosynthesis (the euphotic zone). The euphotic zone may range from a few meters deep in particle rich coastal waters to 150 m in the clearest ocean waters. Much of the biomass produced by these plants sinks from the euphotic zone when they are eaten and excreted by zooplankton or if the plant population exhausts the chemical nutrients in the water. These particles are then eaten by bacteria and animals in the deep-sea and the chemicals are released back into the deep ocean waters (remineralized) below the euphotic zone. Remineralization enriches the deep waters with elements that are cycled by this process. The concentrations of nutrient-like elements will be much greater in deep waters of the Indian and Pacific Oceans because the conveyor circulation creates a greater age, which allows more chemical to be regenerated. The residence times of nutrient-like elements typically range from 1000 years (1 ocean mixing cycle) to several hundred thousand years. They have shorter residence times than conservative elements because some of the biogenic particles, with their elemental loads, inevitably reach the sediments and are removed from the ocean. As an example, see the distribution of phosphorous, an essential nutrient required by all living organisms. Nearly all of the phosphorous that reaches the surface ocean is removed by plants each year and phosphorous concentrations are almost zero. On the other hand, a small amount of strontium is used in the surface waters by Acantharia, a rare class of plankton that makes shells of SrSO4 (celestite). Concentrations of Sr are only slightly depleted near the sea surface. These slightly depleted elements are sometimes called bio-intermediate, while the depleted elements may be called bio-limiting (because their near zero concentrations can limit growth of some plankton).
Scavenged elements: These elements are rapidly sorbed onto sinking particles and removed to the sediments. The concentrations of scavenged elements generally decrease with time, therefore. This means that concentrations are lower in the mid-depths of the oceans than at the surface. The Indo-Pacific deep waters are older and more time has passed for sorption of the chemicals onto sinking particles. Concentrations are, therefore, lower in the deep waters of the Indian and Pacific Oceans than in the deep Atlantic. The residence time of scavenged elements is short, typically less than 1000 years. External processes will markedly change the concentration of these elements because inputs or outputs are large relative to rates of mixing. As an example, see the distribution of aluminum. The major input of aluminum is dust settling on the ocean surface and there is much more dust, and Al, in the Atlantic which receives large inputs from arid lands in northwest Africa (the Sahara and the Sahel). Concentrations of Al may also increase near the ocean bottom due to release from sediments.
Stable gases: The stable gases pass from the atmosphere into the surface waters of the ocean, where they reach saturation (the pressure of the gas in seawater equals the pressure of the gas in the atmosphere). The concentration at saturation of a gas (the solubility) depends on temperature. Cold water holds more gas than does warm water. Stable gas concentrations are higher in cold surface waters and when these cold waters sink into the ocean depths, they carry these gases along with them. These gases may be completely unreactive, such as the noble gases, or only very slightly reactive, such as nitrogen. The deep waters of the ocean are cold and have higher concentrations than the warm surface waters. There is little difference between the Atlantic and Pacific. As an example, see argon.
Radioactive elements: Seawater has many natural radioactive elements within it. These range from 14C (half-life = 5700 years), which is produced by cosmic rays colliding with nitrogen in the atmosphere, to the uranium series isotopes, which are decay products of uranium that was present when the Earth formed about 4.5 billion years ago. This uranium was produced when lighter elements captured neutrons during the collapse and explosion of stars (a supernova). The radioactive elements, particularly the Actinides, have complicated distributions in the ocean due to production from decay of their parent isotopes and scavenging removal by particles. Some are conservative, such as uranium, which forms a very stable complex with carbonate ion that has low chemical reactivity. The decay rate of uranium is very slow (238U half-life = 4.47×109 years), so radioactive decay removes very little. On the other hand, the isotopes of thorium are very particle reactive and the element is rapidly removed to the sediments. The isotope 234Th has a very short half-life (24 days) and it usually decays away before scavenging removes it, except in the euphotic zone areas of high particle production by plants. It’s activity (concentration x decay rate, where decay rate = ln(2)/half-life) in the deep-sea is very close to the decay rate of it’s parent isotope 238U, which is uniform in seawater. However, another thorium isotope, 230Th, has a much longer half-life (75,200 years) and chemical scavenging removes it from seawater much sooner than does radioactive decay. As a result, 230Th activity is much lower than the activity of its parent (234U – produced by decay of 238U to 234Th to 234U). Another thorium isotope, 232Th, has a very long half-life (1.4×1010 years) and it was all produced before the Earth formed.