Ocean Acidification & pH (Week 6 lecture 1 – ENVS 150)

Logarithms Refresher

  • Why Logs?
    • Used when variables span many orders of magnitude (e.g., 0.00001 to 1{,}000{,}000).
    • Compresses data range → easier visualization & calculation.
  • General Form
    • \log_{b}(x)=y \;\Longleftrightarrow\; b^{y}=x.
    • Two common bases:
    • b=10 ("common" log).
    • b=e (natural log, arises in many natural-science relationships).
  • Practice Examples (from assignment)
    • \log_{10}(100)=2 (because 10^{2}=100).
    • \log_{10}(1{,}000)=3.
    • \log_{10}(0.1)=-1.
    • Non-integer outputs:
    • \log_{10}(20)\approx1.30.
    • \log_{10}(50)\approx1.70 (should be > result for 20 & both between 1 and 2).

pH Fundamentals

  • Definition
    • \text{pH}=-\log_{10}([H^+]) where [H^+] = activity (≈ concentration) of hydrogen ions (mol/L).
  • Units
    • Scale is unitless (unlike °C, cm, etc.).
  • Logarithmic Nature
    • One-unit change = 10× change in [H^+].
    • From pH 7 → 6 = 10× more acidic.
    • From 7 → 5 = 100× more acidic (10 × 10).
  • Scale Reference Points (1 = most acidic, 14 = most basic)
    • 1: Gastric acid.
    • 2–3: Lemon juice, tomato juice.
    • 4–5: Coffee, soda.
    • 6: Milk.
    • 7: Pure water (neutral).
    • 8–9: Eggs, baking soda.
    • 10–12: Soaps, ammonia.
    • 13–14: Bleach, strong bases.
  • Ocean Baseline
    • Natural average seawater pH ≈ 8.25 (slightly basic).
    • "Ocean acidification" = trend toward lower (more acidic) pH; ocean still basic (>7) but becoming less basic.

Mole & Concentration Quick Note

  • Mole (Avogadro’s number) =6.02 \times 10^{23} particles.
  • Even large counts of H^+ usually <1 mol L⁻¹, giving negative powers of 10 (e.g., [H^+]=10^{-8} → pH 8).

Observational Evidence

Mauna Loa Observatory (Hawai‘i)

  • Atmospheric CO_2 monitored since 1958.
  • Data show:
    • Seasonal oscillation (higher in N-hemisphere winter → more respiration; lower in summer → more photosynthesis).
    • Overall upward trend in ppm over decades.
  • Ocean measurements since late 1980s:
    • Dissolved CO_2 increasing nearly in sync with atmosphere.
    • pH simultaneously decreasing.
  • Cultural context: Mauna Loa is sacred to Native Hawaiians (goddess Pele). Ongoing tension between research infrastructure & Indigenous stewardship.

Geological Context

  • 120 million-year record shows long-term increase in ocean pH (becoming more basic) — a natural trend.
  • Modern era (~ last 50–100 yrs) reverses this, producing a steep pH decline despite natural tendency.
  • Conclusion: Present acidification rate exceeds natural variability; human emissions are the driver.

Chemical Mechanism of Acidification

  1. CO_2 (gas) ↔ dissolves in seawater.
  2. CO2 + H2O \leftrightarrow H2CO3 (carbonic acid).
  3. H2CO3 \leftrightarrow HCO_3^- + H^+.
  4. HCO3^- \leftrightarrow CO3^{2-} + H^+.
    • Extra H^+ ions ↓ pH.
    • CO_3^{2-} availability decreases.
  5. Calcification impact:
    • Organisms need Ca^{2+}+CO3^{2-} \rightarrow CaCO3 (calcium carbonate) to build shells/skeletons.
    • Lower CO_3^{2-} = harder, costlier, or impossible shell formation.

Affected Organisms

  • Oysters & other bivalves.
  • Pteropods (sea butterflies).
  • Corals.
  • Foraminifera.
  • Diatoms (silica & carbonate forms).
  • Nautiluses, starfish, many planktonic species.
  • Consequences: Weaker shells, higher metabolic cost, reduced survival & reproduction, altered food webs.
  • ~30 % of anthropogenic CO_2 emissions absorbed by oceans.
    • Helps slow atmospheric warming.
    • But drives acidification.
  • Remaining fractions:
    • Land biosphere uptake.
    • Accumulation in atmosphere (greenhouse effect).

Future Projections

  • Historical record to ~2015 shown in gray; projections diverge:
    • High-emission pathway (red):
    • Rapid pH decline to ~7.75 by 2100 (current ~8.1).
    • Rapid-mitigation pathway (blue):
    • Continued slight decline until ~2050 → stabilization → gradual rebound.
    • Intermediate outcomes possible; extremes beyond modeled curves cannot be ruled out.

Key Takeaways & Significance

  • pH is logarithmic; small numeric changes = large chemical shifts.
  • Ocean acidification is measurable, accelerating, and tightly linked to human CO_2 emissions.
  • Chemical shifts disrupt marine calcifiers, cascading through ecosystems and economies (fisheries, reef tourism, etc.).
  • Mitigation (emission cuts) can slow/limit pH decline, offering biological systems better chances of adaptation.

Action Items (per lecture instructions)

  • Complete & submit assignment problems (log calculations + Mauna Loa graph questions).
  • Proceed to Week 6 — Lecture 2 when ready.