Lecture 2 - Major Ions in Sea Water Why do we care about the major ions? What is the composition of seawater? What defines Major Ions? What are their concentrations? What are their properties?
Density: distributions and controls (salinity and temperature)
Density of Seawater σ = (ρ - 1) 1000 if ρ = 1.0250 gm/cm3 then σ= 25.0 σ as S σ as T Q. Why? σ What is salinity? What are and σ? What are their units?
Q. How is salinity measured? 1. gravimetric 2. analyze all the ions and sum 3. relative to halogens (Cl- + Br- + I-) using Knudsen equation from 1911 (n = 9 samples) S ‰ = 0.030 + 1.8059 Cl‰ (or gms per kg) 4. Conductivity UNESCO, 1981 Defined the Practical Salinity Scale (PSS) See Millero 1993 S = 35.000 (don’t use units like PSU)
Surface density, isopycnal outcrops Waters will move mostly along surfaces of constant density.
Sea Surface Salinity Q. Why does surface salinity vary? DS = 30 to 37 What are broad patterns and what controls salinity?
Evaporation and Precipitation Effects on Surface Salinity All original salinity signatures acquired at the sea surface Modified in the ocean interior by mixing. Becomes tracers for water masses.
How are the major ions of seawater defined? What are the major ions? Elements versus species? moles versus grams – conversions (See E&H Table 1.2)
How are the major ions of seawater defined? ans: major ions contribute to salinity (e.g. 35.000‰) salinity can be determined to 0.001 ppt = 1 ppm = 1 mg kg-1 Elements versus species? e.g., Na is an element Na+ is a species (cation) S is an element SO42- is a species (anion) What are the major ions? n = 11 ans: cations = Na+ > Mg2+ > Ca2+ ~ K+ > Sr2+ anions = Cl- >> SO42- > HCO3- > Br- > F- neutral = B(OH)3° written as main species moles versus grams – conversions (See E&H Table 1.2) 1 mol = 6.02 x 1023 atoms mol kg-1 = g(solute)/kg (water) g(solute)/mol. wt. 1 molNaCl = 1 mol Na+ + 1 molCl-
Concentrations molar (M) mol / ltr H2O molal mol / kg H2O SW mol / kg SW (H2O + salt) Q Why?? mol mmol 10-3 mmol 10-6 nmol 10-9 pmol 10-12 (Q How many atoms?)
Example: We want to make a solution with Na+ = 468.96 mmol/kg from NaCl(table salt)
From Pilson cations Na+ > Mg2+ > Ca2+ > K+> Sr2+ anions Cl- >>SO42- >HCO3-> Br->F- B(OH)3 Q. mol balance Q. charge balance Units Si and gases Liverpool and NIO DIC
Some major ions are conservative. These are Na+, K+, Cl-, SO42-, Br-, B(OH)3and F-. What does this mean? conservative. Q. How do you demonstrate this? What are the consequences? Do conservative major ions have a constant concentration in the ocean? Q Law of Constant Proportions(major ion/S‰ = constant) Knudsen equation ( S = 0.030 + 1.8050 Cl‰) More recently (S‰ = 1.8065 Cl‰) The Law breaks down in estuaries, evaporite basins, hydrothermal vents. Q
Some Major Ions are non-conservative Examples: Ca2+, Mg2+, Sr2+, Dissolved Inorganic Carbon (HCO3-) Non-conservative behavior due to: biological production hydrothermal ridge crest solutions river water (as in estuaries)
Nutrient Like Profiles Superposition of vertical biological flux on horizontal circulation Results in low surface water and high deep water concentrations. Results in higher concentrations in the older deep Pacific than the younger deep Atlantic
Example: Comparison of vertical profiles of nutrients from the Atlantic and Pacific PO4 Si Shallow remineralization Soft parts Deep remineralization Hard parts
Non-Conservative Major Elements Calcium (Ca) Ca = 0.1 / 10.2 = +1.0 % with depth Why?? CaCO3 (s) = Ca2+ + CO32- (from de Villiers, 1999)
Sr – also increases with depth (~2%) and N. Atl to N. Pac Distributions similar to PO4 (excellent correlation)
But why? The mineral phase Celestite (SrSO4) produced by Acantharia protozoa is proposed as the transport phase. Acantharia shell and cyst Acantharia are marine planktonic protozoans Examples from sediment traps at Bermuda
Inverse Mg – Ca Relationship from EPR at 17S; 113W (from de Villiers, 1999) Note significant variability in Mg (normalized to S = 35)! In this case ~1% variability. Hydrothermal Origin??
Black Smoker Fluids, East Pacific Rise , from Von Damm et al., (1985) Mg Ca Alk
River water ≠ seawater HCO3- > Cl- Ca2+ > Na+
Example of using seawater ratios: From Christner et al (2014) Nature, 512, 310 “A microbial ecosystem beneath the West Antarctic ice sheet” Crustal and seawater components to Subglacial Lake Whillans(SLW) waters The weathering products probably came from sulfide oxidation, carbonation reactions, and carbonate dissolution.