Lecture 3

• The rock forming elements make up the Earths crust and are the most abundant elements (carbon, oxygen, sodium, magnesium etc.)
• The principle source of most elements is the weathering of the ocean crust
• The rarest metals are found 12 orders of magnitude less than the rock forming elements.
• Despite not being an abundant rock forming element, Cl is an abundant element in seawater
• Na is a principle constituent of forming minerals
• Hydrogen and oxygen are the most abundant in seawater (given as water is h2o
• There is always lower concentrations of minerals in river water than in seawater (because it is more dilute) but the proportions in each are different.
• To measure the amount of salt in seawater you would evaporate a know volume of water and weigh the salt that is left over.
• You could also do a titration where you titrate halides against AgNO3 -> precipitate is silver halide. You would then weigh the precipitate and it will tell you the concentration of the silver chloride.
• To measure how much salt there is in seawater you can use conductivity measurements relative to KCl solution - used since 1978. It is measured as a ratio to the KCl and has no units.
• Salinity varies in the Atlantic and it depends on how the ocean circulates and ocean currents move around.
• The sub tropics have the highest salinity because precipitation is high but evaporation is higher which gives concentrations of salts at the sea surface.
• Salinity is lower in the tropics because of higher rainfall because this 'freshens' the sea surface
• Concentrations of major dissolved ions (those which have concentrations of > 1ppm by weight) can vary from place to place in the oceans, but relative proportions remain virtually constant.
• Major ions mainly show conservative behaviour (when they enter the oceans via seawater, it just hangs around and sits there).
• There is a small drop-off of Ca at the ocean surface because Ca is used during photosynthesis to create CaCO3 by the phytoplankton (eg cocoliffofors). It is a very tiny shift because there is so much calcium in seawater it will only make a tiny different.
• Na concentration stays the same wherever you are (vertically).
• Ratio of sodium ions to salinity is constant throughout oceans.
• Some major ions show small variations eg Calcium ions are utilised in surface waters and re-dissolved at depth.
• Mass balance states that whatever you do to an object, the total amount you end up with remains constant.
• Steady state says whatever goes in balances whatever goes out ➡️ no net change.
• Rivers are the principle source of many elements in seawater and is most important source of dissolved material and particulate material.
• Other inputs of material to the oceans include volcanic gases, dust, hydrothermal vent fluids, sediment pore waters, etc.
• The 1 box ocean model suggests rivers supply elements to the ocean and removal is proportional to concentration. This means the ocean is at a steady state. (the amount of anything coming into the ocean must be equal to whatever is leaving the ocean).
• The mean ocean residence time is how long an element is in the ocean before it is removed.
• F river x C river = V oceans x C ocean x k
• k = (F river x C river) / (V ocean x C ocean)
• k = rate constant for removal (yr-1)
• 1/k = time constant (yr) or residence time (t)
• Removal constant (k) is measure of geochemical reactivity. Removal occurs via a number of processes leading to formation or association with particles. Precipitation, scavenging, adsorption, biological uptake are all important.