Chemical oceanography Study Guide

📖 Core Concepts Marine chemistry = study of chemical composition & processes of oceans (seawater, gases, sediments, organisms). Biogeochemical cycles (C, N, P) link ocean chemistry to climate, biology, and geology. pH & carbonate system – CO₂ dissolves → $\mathrm{H2CO3}$ → $\mathrm{HCO3^-}$ + $\mathrm{H^+}$; more $\mathrm{H^+}$ → lower pH → lower carbonate saturation. Mg/Ca ratio of seawater controls which calcium‑carbonate mineral (calcite vs. aragonite) precipitates. DOM vs. POM – dissolved organic matter stays in solution (microbial loop); particulate organic matter sinks (biological pump). Chemoautotrophy – vent microbes harvest energy from redox reactions of H₂, H₂S, Fe²⁺, CH₄ rather than sunlight. Key instruments – CTD (conductivity‑temperature‑depth), mass spectrometer, chromatography, autonomous sensors, satellites. 📌 Must Remember Ocean surface pH fell from 8.15 (1950) → 8.05 (2020); a 0.1‑unit drop = 26 % increase in $[\mathrm{H^+}]$. DOM = 90 % of marine organic carbon; CDOM = 20–70 % of that carbon, peaks near river mouths. Mg/Ca ratio: low → “calcite seas”; high (slow spreading) → “aragonite seas”. Seafloor spreading releases Fe, S, Mn, Si via hydrothermal vents (global ion‑exchange). Ocean deoxygenation: global O₂ loss 1–2 % since mid‑20th C; projected 7 % loss next century. Human‑derived waste: 80 % of marine pollution originates on land. CTD measures conductivity → salinity, temperature, pressure; essential for profiling water columns. 🔄 Key Processes Carbonate Chemistry (acidification) CO₂ (air) → dissolves in seawater. $\mathrm{CO2 + H2O \rightleftharpoons H2CO3}$ $\mathrm{H2CO3 \rightleftharpoons HCO3^- + H^+}$ $\mathrm{HCO3^- \rightleftharpoons CO3^{2-} + H^+}$ More $ \mathrm{H^+}$ → lower pH, reduced $\mathrm{CO3^{2-}}$ → weaker carbonate saturation. Biological Pump (POM sinking) Production → POM formation → sinking → bacterial decomposition (releases nutrients & CO₂) → refractory fraction burial. Chemoautotrophic Energy Capture at Vents Oxidize reduced compounds (e.g., $\mathrm{H2S}$ + $½\mathrm{O2} \rightarrow \mathrm{SO4^{2-}} + \mathrm{H^+}$) → generate ATP → fix CO₂ via Calvin or rTCA cycles → support higher trophic levels. Mg/Ca Ratio Influence on Biomineralization High Mg/Ca → aragonite/high‑Mg calcite precipitation. Low Mg/Ca → low‑Mg calcite precipitation. Sampling Workflow (Shipboard) Deploy CTD → record profiles → trigger Nansen bottle rosette → collect water at selected depths → preserve for lab analysis (trace metals, nutrients, isotopes). 🔍 Key Comparisons DOM vs. POM DOM: dissolved, stays in water column, fuels microbial loop, 90 % of oceanic organic C. POM: particulate, sinks, drives vertical carbon flux (biological pump). Calcite Sea vs. Aragonite Sea Calcite Sea: low Mg/Ca, favors low‑Mg calcite skeletons. Aragonite Sea: high Mg/Ca, favors aragonite/high‑Mg calcite skeletons. Chemoautotrophy vs. Photosynthesis Chemoautotrophy: energy from redox of inorganic chemicals (vent fluids). Photosynthesis: energy from sunlight; dominant in surface photic zone. ⚠️ Common Misunderstandings “Acidification only means lower pH” – it also reduces carbonate ion concentration, directly threatening calcifiers. “All marine organic carbon is particulate” – 90 % is dissolved (DOM). “Higher CO₂ always means more carbon stored in ocean” – while total dissolved inorganic carbon rises, the speciation shifts toward bicarbonate, not carbonate, limiting sequestration in shells. “All hydrothermal vent organisms are chemosynthetic” – many are heterotrophs that rely on chemosynthetic primary producers. 🧠 Mental Models / Intuition “Carbonate buffering tank” – think of seawater as a large tank where added CO₂ fills the bicarbonate “bucket” first; only after the bucket overflows does carbonate drop, weakening shells. “Sinking conveyor belt” – POM behaves like a conveyor belt moving carbon from surface to depth; the slower the belt (e.g., deep water column), the more time bacteria have to remineralize carbon. “Mg/Ca thermostat” – the Mg/Ca ratio acts like a thermostat that sets the “mineral setting” for organisms: turn the knob low → calcite; turn it high → aragonite. 🚩 Exceptions & Edge Cases Local alkalinity spikes (e.g., upwelling of deep, CO₂‑rich water) can temporarily raise $[\mathrm{H^+}]$ beyond the global trend. Refractory DOM can persist for centuries, acting as a long‑term carbon sink despite overall high turnover of labile DOM. Cold seeps (different from hydrothermal vents) release methane and sulfide but have distinct microbial communities. 📍 When to Use Which Assessing carbonate saturation → use Mg/Ca ratio + pH to decide if organisms will favor calcite or aragonite. Tracing water mass origins → apply isotopic techniques (e.g., radiocarbon) rather than bulk chemistry. Measuring trace metals → prefer inductively coupled plasma mass spectrometry (ICP‑MS) over colorimetric methods for sensitivity. Monitoring large‑scale pH trends → deploy autonomous buoys/satellites; for high‑resolution depth profiles, use shipboard CTD‑mounted sensors. 👀 Patterns to Recognize Drop in pH + rise in $[\mathrm{HCO3^-}]$ → classic signature of ocean acidification. Elevated Fe, Mn, Si in deep‑water samples → indicator of hydrothermal vent influence. High CDOM absorbance + low chlorophyll near river mouths → terrestrial organic input dominates. Oxygen minimum zones coinciding with high nutrient concentrations → sign of eutrophication‑driven deoxygenation. 🗂️ Exam Traps “Ocean pH is 7” – the ocean is mildly alkaline (8); the 0.1‑unit drop is still above neutral. “All calcifying organisms will thrive with more CO₂” – extra CO₂ lowers carbonate saturation, harming calcifiers. “Higher Mg/Ca always means more calcite – it actually favors aragonite/high‑Mg calcite, not low‑Mg calcite. “DOM is only a short‑term carbon source – refractory DOM can last centuries, acting as a long‑term reservoir. “CTD measures only salinity – it measures conductivity, which is converted to salinity plus temperature and pressure. --- Use this guide for rapid recall; focus on the bolded terms and the cause‑effect arrows in the processes.