Chapter 6: The chemistry of seawater Salinity variation with depth II • Curves for high and low latitudes begin at different surface salinities • Halocline = layer of Salinity variations w/depth, latitude rapidly changing salinity The three-layered ocean • At depth, salinity is Seawater buffer system uniform Water’s Heat Capacity Surface salinity variation Surface salinity variation • High latitudes have low surface salinity • Pattern of surface – High precipitation and runoff salinity: – Low evaporation – Lowest in high • Tropics have high surface salinity latitudes – High evaporation – Highest in the – Low precipitation tropics – Dips at the • Equator has a dip in surface salinity Equator – High precipitation partially offsets high evaporation • Surface Figure 6-20 processes help explain pattern Chapter 6: The chemistry of seawater Global surface salinity II Salinity variations w/depth, latitude The three-layered ocean Seawater buffer system Figure 5-21 Water’s Heat Capacity 6-21 Pycnocline, thermocline and Ocean layering based on halocline density • Mixed surface layer (surface to 200 • Pycnocline = layer of rapidly changing meters) density – Low density; well mixed by waves, currents, • Thermocline = layer of rapidly changing tides temperature • Upper water (200 to 1000 meters) • Halocline = layer of rapidly changing – Intermediate density water containing salinity thermocline, pycnocline, and halocline (if • Barrier to vertical mixing of water present) • Deep water (below 1000 meters) – Cold, high density water involved in deep current movement Chapter 6: The chemistry of seawater Density and temperature variations with depth II Salinity variations w/depth, latitude The three-layered ocean Fig. 6-24 Seawater buffer system Water’s Heat Capacity Seawater buffer system pH Scale is a measure of Seawater buffer system the acidity or alkalinity of a solution. Water dissociates (break apart) into OH- and H+ ions. Note that reaction In pure water there are as goes in both many H+ as OH-: 10-7 Mol of directions! hydrogen ions = pH of 7 (neutral pH) 1 Mol= 6 x 1023 atoms, molecules, ions per liter, also called Avogadro’s number Fig. 6-17. The carbonic acid system in the ocean is responsible for the pH = -log[H+] relatively constant pH of seawater (pH of 7.8-8.2). The bicarbonate ions Negative logarithm of the hydrogen ion have the ability to bind with hydrogen ions and thus neutralize most acids. concentration More than 80% of Relative gas Solubility in Sea Water with carbon in seawater is in the form of Temperature - bicarbonate (HCO3 ), maintaining the average ocean pH at about 7.8-8.2. The abundance of bicarbonate makes seawater an ideal buffer system, i.e. changes in pH are compensated. Depth profiles of O and CO for Atlantic and Pacific 2 2 Depth profiles of pH for Atlantic and Pacific 6.15.