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Question 1. Which ion is considered conservative in seawater because its concentration varies only with dilution or evaporation? A) Chloride B) Silicate C) Ammonium D) Iron Answer: A Explanation: Chloride behaves conservatively; its ratio to other major ions remains constant, unlike nutrients that are biologically active. Question 2. The Practical Salinity Scale (PSS‑78) defines salinity based on which physical property? A) Density B) Conductivity C) Refractive index D) Freezing point Answer: B Explanation: PSS‑78 uses the electrical conductivity of seawater relative to a KCl standard to calculate salinity. Question 3. In the TEOS‑10 formulation, which variable is used to express seawater density? A) σ₀ B) ρ_sw C) γ_T D) α_S Answer: B Explanation: TEOS‑10 defines ρ_sw as the in‑situ density of seawater, incorporating temperature, salinity, and pressure. Question 4. The average residence time of sodium (Na⁺) in the ocean is on the order of: A) 10⁴ yr B) 10⁶ yr C) 10⁸ yr D) 10¹⁰ yr Answer: B Explanation: Sodium’s residence time (~10⁶ years) indicates it is essentially conservative on geological timescales. Question 5. Which of the following best describes chemical speciation of iron in oxygen‑rich surface seawater? A) Fe²⁺ free ion B) Fe³⁺ hydroxide colloids C) Fe‑organic complexes D) FeS precipitation Answer: C Explanation: In oxic surface water, Fe³⁺ rapidly complexes with organic ligands, keeping it dissolved despite low solubility of free Fe³⁺. Question 6. The equilibrium constant K₁ for the reaction CO₂ + H₂O ⇌ H⁺ + HCO₃⁻ increases with: A) Decreasing temperature B) Increasing temperature C) Decreasing salinity D) Increasing pressure Answer: A Explanation: Lower temperatures favor the formation of carbonic acid and thus increase K₁, enhancing acidity. Question 7. Total alkalinity (TA) of seawater is defined as the sum of: A) All cations minus anions B) All bases that can accept a proton C) All weak acids present D) All dissolved gases Answer: B
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Explanation: TA quantifies the water’s capacity to neutralize added acid, primarily from carbonate, borate, and hydroxide systems. Question 8. Which component contributes the most to seawater TA under typical open‑ocean conditions? A) Borate B) Carbonate C) Silicate D) Phosphate Answer: B Explanation: The carbonate system (HCO₃⁻ + 2CO₃²⁻) dominates TA, accounting for ~95 % of the total. Question 9. The Revelle factor is a measure of: A) Oceanic heat capacity B) Buffering capacity against CO₂ uptake C) Silicate dissolution rate D) Nutrient limitation D Answer: B Explanation: A high Revelle factor indicates that a given increase in atmospheric CO₂ leads to a relatively small increase in DIC, reflecting limited buffering. Question 10. Calcium‑carbonate saturation state (Ω) greater than 1 implies: A) Undersaturation, dissolution favored B) Supersaturation, precipitation favored C) No net reaction D) Complete dissolution Answer: B Explanation: Ω > 1 means the water is supersaturated with respect to CaCO₃ minerals, promoting precipitation. Question 11. The depth at which aragonite becomes undersaturated (Ω_arag < 1) in the Atlantic is called the: A) Lysocline B) Thermocline C) Halocline D) Pycnocline Answer: A Explanation: The lysocline marks the onset of rapid dissolution of aragonite shells. Question 12. Ocean acidification primarily reduces the concentration of which carbonate ion species? A) HCO₃⁻ B) CO₃²⁻ C) H₂CO₃ D) H₂O Answer: B Explanation: Lower pH shifts the equilibrium toward HCO₃⁻, decreasing CO₃²⁻, which is critical for calcifying organisms. Question 13. Which of the following organisms is most vulnerable to decreasing Ω_arag? A) Diatoms B) Coccolithophores C) Foraminifera (calcite) D) Radiolarians Answer: B Explanation: Coccolithophores produce calcite plates but are highly sensitive to aragonite undersaturation, affecting their growth.
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Explanation: Iron scarcity restricts growth despite abundant macronutrients, especially in the Southern Ocean and subarctic Pacific. Question 21. The solubility of gases in seawater generally decreases with: A) Increasing temperature B) Decreasing salinity C) Decreasing pressure D) Increasing pH Answer: A Explanation: Higher temperatures reduce gas solubility according to Henry’s law, leading to lower CO₂ uptake in warm waters. Question 22. Henry’s law constant for O₂ in seawater at 20 °C is approximately: A) 1.3 × 10⁻³ mol kg⁻¹ atm⁻¹ B) 2.5 × 10⁻³ mol kg⁻¹ atm⁻¹ C) 3.0 × 10⁻³ mol kg⁻¹ atm⁻¹ D) 4.0 × 10⁻³ mol kg⁻¹ atm⁻¹ Answer: B Explanation: The Henry’s law constant for O₂ at 20 °C is about 2.5 × 10⁻³ mol kg⁻¹ atm⁻¹, governing its equilibrium concentration. Question 23. Apparent Oxygen Utilization (AOU) is calculated as the difference between: A) Measured O₂ and its solubility at in‑situ temperature B) Surface O₂ and deep‑water O₂ C) Saturation O₂ and measured O₂ D) Atmospheric O₂ and seawater O₂ Answer: C Explanation: AOU = [O₂]ₛₐₜ − [O₂]ₘₑₐₐₛ, indicating the amount of O₂ consumed since water left the surface. Question 24. A water mass with high AOU values most likely experienced: A) Recent upwelling B) Strong photosynthetic production C) Extensive respiration D) Low temperature cooling Answer: C Explanation: High AOU reflects oxygen removal by biological respiration during the water’s transit. Question 25. The primary cause of an oxygen minimum zone (OMZ) in the eastern tropical Pacific is: A) High surface productivity B) Weak thermohaline circulation C) Cold water upwelling D) Deep‑sea hydrothermal activity Answer: A Explanation: Massive export of organic matter fuels respiration at depth, depleting O₂ and creating an OMZ. Question 26. Which chemical reaction dominates energy production at black‑smoker hydrothermal vents? A) Oxidation of H₂S to SO₄²⁻ B) Reduction of CO₂ to CH₄ C) Oxidation of
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Fe²⁺ to Fe³⁺ D) Nitrification of NH₄⁺ Answer: A Explanation: Sulfide oxidation provides electrons for chemoautotrophic microbes, forming the basis of vent ecosystems. Question 27. Cold seeps often release methane (CH₄) that can be oxidized anaerobically using which electron acceptor? A) O₂ B) NO₃⁻ C) SO₄²⁻ D) Fe(III) Answer: C Explanation: Anaerobic oxidation of methane (AOM) couples CH₄ oxidation to sulfate reduction, a key process at cold seeps. Question 28. The δ¹⁸O value recorded in foraminiferal calcite primarily reflects: A) Ambient seawater temperature B) Salinity C) Atmospheric CO₂ concentration D) Oceanic pH Answer: A Explanation: The isotopic fractionation of ¹⁸O/¹⁶O in calcite is temperature‑dependent, allowing paleotemperature reconstruction. Question 29. A higher δ¹³C in deep‑sea dissolved inorganic carbon (DIC) usually indicates: A) Recent photosynthetic uptake B) Input of respired CO₂ C) Upwelling of surface water D) Freshwater influx Answer: B Explanation: Respiration preferentially releases ¹²C, leaving the residual DIC enriched in ¹³C, raising δ¹³C values. Question 30. Radiocarbon (¹⁴C) ages of deep‑water masses are typically on the order of: A) 100 – 500 yr B) 1 kyr– 3 kyr C) 5 kyr– 10 kyr D) >20 kyr Answer: B Explanation: Deep‑water ¹⁴C ages reflect the time since water was last at the surface, usually 1– 3 kyr. Question 31. Thorium‑230 (²³⁰Th) is useful for estimating: A) Surface productivity B) Particle scavenging rates C) Ocean mixing time D) Hydrothermal fluxes Answer: B Explanation: ²³⁰Th is particle‑reactive; its removal from seawater onto sinking particles records scavenging efficiency. Question 32. Benthic flux of ammonium (NH₄⁺) from sediments is primarily driven by: A) Diffusive exchange B) Bioturbation C) Sulfate reduction D) Methanogenesis Answer: B Explanation: Macrofaunal bioturbation enhances pore‑water exchange, increasing NH₄⁺ release to the overlying water.
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halocline separating water masses D) The depth of maximum chlorophyll concentration Answer: B Explanation: The lysocline marks the onset of rapid carbonate dissolution, often coinciding with a drop in Ω. Question 40. The term “Carbonate Compensation Depth (CCD)” refers to the depth at which: A) Primary production peaks B) Calcium carbonate dissolution balances precipitation C) Nutrient concentrations are highest D) Thermocline begins Answer: B Explanation: At the CCD, the rate of CaCO₃ dissolution equals its supply, so no net accumulation of carbonate sediments occurs. Question 41. Which of the following processes directly adds CO₂ to the surface ocean? A) Upwelling of deep water B) Atmospheric diffusion C) Biological photosynthesis D) Oceanic mixing Answer: B Explanation: Atmospheric diffusion (air‑sea gas exchange) transfers CO₂ from the atmosphere into seawater. Question 42. The primary chemical buffer that mitigates pH changes in the open ocean is: A) Borate system B) Phosphate system C) Carbonate system D) Silicate system Answer: C Explanation: The carbonate system (CO₂/HCO₃⁻/CO₃²⁻) provides the dominant buffering capacity in seawater. Question 43. A decrease in the Revelle factor would imply: A) Faster oceanic uptake of CO₂ B) Slower heat transport C) Increased silicate dissolution D) Reduced nitrate regeneration Answer: A Explanation: A lower Revelle factor indicates that a given rise in atmospheric CO₂ leads to a larger increase in DIC, meaning the ocean can absorb CO₂ more readily. Question 44. The term “non‑conservative ion” in oceanography most commonly refers to: A) Sodium B) Chloride C) Nitrate D) Magnesium Answer: C Explanation: Nitrate is removed by biological uptake and regenerated by remineralization, thus its concentration does not follow simple dilution. Question 45. Which of the following statements about seawater sulfate (SO₄²⁻) is correct? A) It is a limiting nutrient for phytoplankton B) It exhibits a residence time of ~10⁴ yr C) It is highly
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variable with depth D) It is primarily removed by denitrification Answer: B Explanation: Sulfate’s residence time (~10⁴ yr) makes it effectively conservative on short timescales. Question 46. The stoichiometric ratio of N:C in marine organic matter is approximately: A) 0. B) 0.10 C) 0.14 D) 0.18 Answer: C Explanation: Using the Redfield ratio (C:N = 106:16), N:C ≈ 16/106 ≈ 0.15, closest to 0.14. Question 47. In seawater, the dominant form of dissolved inorganic nitrogen is: A) NH₄⁺ B) NO₃⁻ C) N₂ O D) NO₂⁻ Answer: B Explanation: Nitrate (NO₃⁻) constitutes the largest fraction of DIN in most oceanic regions. Question 48. The “biological pump” primarily transports carbon from the surface ocean to the deep ocean via: A) Physical mixing B) Sinking of organic particles C) Diffusive gas exchange D) Thermohaline circulation Answer: B Explanation: Export of particulate organic carbon (POC) through sinking is the core mechanism of the biological pump. Question 49. Which process converts dissolved organic carbon (DOC) back to DIC in the deep ocean? A) Photosynthesis B) Chemosynthesis C) Respiration D) Nitrification Answer: C Explanation: Microbial respiration oxidizes DOC to CO₂, adding to the DIC pool. Question 50. The term “HNLC” stands for: A) High‑Nutrient, Low‑Chlorophyll B) High‑Nitrogen, Low‑Carbon C) High‑Nitrate, Low‑Calcium D) High‑Nutrient, Low‑Carbon Answer: A Explanation: HNLC regions have abundant macronutrients but low chlorophyll due to limitation by micronutrients such as iron. Question 51. Which of the following best describes the effect of increasing atmospheric CO₂ on seawater pH? A) pH increases (becomes more alkaline) B) pH decreases (becomes more acidic) C) pH remains unchanged D) pH fluctuates randomly Answer: B Explanation: Additional CO₂ dissolves to form carbonic acid, lowering pH.
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sea‑ice formation, Mg²⁺ is preferentially excluded (brine rejection), altering its concentration in surrounding waters. Question 59. The “oxygen isotope effect” in foraminiferal calcite is primarily used to infer: A) Salinity changes B) Past sea‑surface temperature C) Ocean pH D) Nutrient concentrations Answer: B Explanation: δ¹⁸O in calcite varies with temperature, allowing reconstruction of paleo‑SST. Question 60. Which of the following reactions is the rate‑limiting step for the oxidation of H₂S in vent fluids? A) H₂S + O₂ → SO₄²⁻ + H⁺ B) H₂S + Fe²⁺ → FeS + H⁺ C) H₂S + NO₃⁻ → SO₄²⁻ + N₂ D) H₂S + CH₄ → C₂H₆ + S Answer: A Explanation: The direct oxidation of H₂S by O₂ is kinetically slow and controls sulfide removal at vents. Question 61. The “alkalinity anomaly” observed after a volcanic eruption in the ocean is primarily due to: A) Release of SO₄²⁻ B) Addition of H⁺ C) Increase in borate D) Input of Ca²⁺ and carbonate minerals Answer: D Explanation: Eruptions introduce carbonate ash, raising alkalinity by adding CaCO₃. Question 62. In the context of ocean carbon chemistry, the term “K₀” refers to: A) The first dissociation constant of carbonic acid B) The solubility constant for CO₂ in seawater C) The equilibrium constant for borate buffering D) The dissociation constant for phosphate Answer: B Explanation: K₀ is the Henry’s law constant for CO₂ solubility in seawater. Question 63. The primary control on the vertical distribution of nitrate in the thermocline is: A) Temperature B) Salinity C) Biological uptake D) Atmospheric deposition Answer: C Explanation: Phytoplankton consume nitrate in the euphotic zone, creating a subsurface maximum where remineralization supplies it. Question 64. The “silicate compensation depth” is analogous to the CCD but for silica; it marks the depth where: A) Silicic acid is saturated B) Si dissolution equals burial C) Silica dissolution
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exceeds supply D) No further silica precipitation occurs Answer: C Explanation: Below this depth, dissolution of biogenic silica exceeds its supply, preventing further accumulation. Question 65. Which of the following isotopic systems is most useful for dating sediments younger than ~10 kyr? A) ¹⁴C B) ²³⁰Th C) ²⁴⁰Pu D) ⁸⁷Sr Answer: A Explanation: Radiocarbon dating (¹⁴C) is effective for ages up to ~50 kyr, with high precision for <10 kyr. Question 66. The “oxygen minimum” observed in the Eastern Pacific is deepest at approximately what depth? A) 100 m B) 500 m C) 1000 m D) 3000 m Answer: C Explanation: The Eastern Pacific OMZ typically centers around 1000 m depth. Question 67. Which process is responsible for the conversion of nitrate to nitrite in oxygen‑deficient waters? A) Anammox B) Nitrification C) Denitrification D) Dissimilatory nitrate reduction to nitrite (DNRA) Answer: D Explanation: DNRA reduces NO₃⁻ to NO₂⁻ (and then to NH₄⁺) under low‑oxygen conditions. Question 68. The “alkalinity balance” equation for seawater includes contributions from all EXCEPT: A) Bicarbonate B) Borate C) Silicic acid D) Hydrogen sulfide Answer: D Explanation: Sulfide is generally negligible in the alkalinity balance compared to the other major contributors. Question 69. The “equation of state” for seawater links which three variables? A) Temperature, salinity, oxygen B) Pressure, density, sound speed C) Temperature, salinity, density D) Salinity, pH, alkalinity Answer: C Explanation: The equation of state expresses density as a function of temperature, salinity, and pressure (often approximated by depth). Question 70. A seawater sample shows a measured pH of 8.1 and an Ω_arag of 0.9. This water is: A) Supersaturated, favorable for aragonite growth B) At saturation, neutral C) Undersaturated, aragonite will dissolve D) Neutral with respect to calcite Answer: C Explanation: Ω_arag < 1 indicates undersaturation, leading to aragonite dissolution.
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Question 77. Which of the following best describes the effect of increasing silicate concentrations on diatom productivity? A) No effect B) Decrease due to toxicity C) Increase up to a saturation point D) Immediate bloom regardless of other nutrients Answer: C Explanation: Diatom growth improves with more silicate until other factors become limiting. Question 78. The primary chemical reaction that buffers pH during the uptake of CO₂ by seawater is: A) CO₂ + H₂O ↔ H₂CO₃ B) HCO₃⁻ + H⁺ ↔ CO₂ + H₂O C) HCO₃⁻ ↔ CO₃²⁻ + H⁺ D) H₂O ↔ H⁺ + OH⁻ Answer: A Explanation: Dissolution of CO₂ forms carbonic acid, which then dissociates, providing the buffering capacity of the carbonate system. Question 79. In the context of oceanic carbon, “fCO₂” denotes: A) The fraction of total carbon that is inorganic B) The fugacity of CO₂, a corrected pressure term C) The fixed concentration of CO₂ D) The flux of CO₂ across the air‑sea interface Answer: B Explanation: Fugacity (fCO₂) accounts for non‑ideal behavior of CO₂ in seawater and is used in gas‑exchange calculations. Question 80. Which of the following processes directly produces nitrous oxide (N₂O) in the ocean? A) Nitrification B) Denitrification C) Anammox D) All of the above Answer: D Explanation: N₂O is a by‑product of both nitrification (especially hydroxylamine oxidation) and denitrification; it can also be released during anammox under certain conditions. Question 81. The “marine silica cycle” is largely controlled by the balance between: A) Silicate weathering and hydrothermal input B) Diatom uptake and dissolution of biogenic silica C) Volcanic ash deposition and precipitation D) Coral reef calcification and dissolution Answer: B Explanation: Diatom growth removes Si(OH)₄, while dissolution of their frustules returns silica to the dissolved pool. Question 82. The “thermodynamic equilibrium constant” for the reaction CO₂ + H₂O ↔ H⁺ + HCO₃⁻ (K₁) is temperature dependent. As temperature rises from 0 °C to 30 °C, K₁: A) Increases B) Decreases C) Remains constant D) Oscillates Answer: B Explanation: Higher temperatures shift the equilibrium toward CO₂, decreasing K₁.
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Question 83. Which of the following is a characteristic of “conservative mixing” of seawater masses? A) Changes in nutrient concentrations B) Linear variation of temperature and salinity C) Non‑linear changes in dissolved gases D) Rapid biological uptake of nutrients Answer: B Explanation: When two water masses mix conservatively, properties like temperature and salinity vary linearly between the endmembers. Question 84. The “alkalinity budget” of the open ocean is dominated by inputs from: A) Riverine runoff B) Atmospheric deposition of ammonia C) Weathering of continental rocks D) Hydrothermal vent fluids Answer: C Explanation: Continental weathering supplies bicarbonate and carbonate ions, the primary contributors to global alkalinity. Question 85. In seawater, the predominant form of dissolved iron under oxic conditions is: A) Fe²⁺ free ion B) Fe³⁺ hydroxide precipitate C) Fe‑organic complexes D) FeS mineral Answer: C Explanation: Fe³⁺ rapidly complexes with organic ligands (e.g., humic substances), keeping it in solution despite low solubility of the free ion. Question 86. The “biological pump” efficiency is greatest in regions with: A) Low primary productivity B) High export flux of POC C) Strong vertical mixing D) High surface temperature Answer: B Explanation: High export of particulate organic carbon enhances the transfer of carbon to depth, increasing pump efficiency. Question 87. Which of the following is NOT a typical component of marine total alkalinity? A) Bicarbonate B) Borate C) Phosphate D) Sulfate Answer: D Explanation: Sulfate does not contribute to alkalinity because it does not accept protons in the relevant pH range. Question 88. The term “pCO₂ anomaly” in a coastal upwelling zone indicates: A) Elevated CO₂ due to biological respiration B) Depleted CO₂ because of photosynthesis C) No change from atmospheric equilibrium D) Increased carbonate precipitation Answer: A Explanation: Upwelling brings CO₂‑rich deep water to the surface, raising pCO₂ relative to the atmosphere.
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precipitation of silica Answer: A Explanation: Ω_Si > 1 means dissolved silica exceeds the equilibrium concentration for biogenic silica, favoring dissolution of siliceous particles. Question 96. The “oxygen isotope composition” of seawater (δ¹⁸O_sw) is primarily controlled by: A) Atmospheric CO₂ levels B) Global ice volume C) Biological productivity D) Riverine input of carbon Answer: B Explanation: During glacial periods, ¹⁶O is locked in ice, enriching seawater in ¹⁸O and raising δ¹⁸O_sw. Question 97. The “C:N:P” Redfield ratio can deviate toward higher C:N in oligotrophic gyres because: A) Increased nitrogen fixation B) Enhanced phosphorus recycling C) Preferential loss of nitrogen via denitrification D) Higher silica limitation Answer: C Explanation: Denitrification removes nitrogen, raising the C:N ratio of the residual organic matter. Question 98. Which process is the main source of nitrate in the deep ocean? A) Direct atmospheric deposition B) Upwelling of surface water C) Remineralization of sinking organic matter D) Hydrothermal vent discharge Answer: C Explanation: Decomposition of sinking organic material releases nitrate at depth. Question 99. The “acid-neutralizing capacity” of seawater is most directly measured by: A) Total alkalinity B) pH C) Dissolved inorganic carbon D) Conductivity Answer: A Explanation: Total alkalinity quantifies the amount of acid that can be neutralized without changing pH significantly. Question 100. The “hydrothermal plume” from mid‑ocean ridges is enriched in which trace metal? A) Iron B) Zinc C) Copper D) Manganese Answer: A Explanation: Hydrothermal fluids deliver high concentrations of Fe to the surrounding water column. Question 101. The primary factor determining the depth of the halocline in the Arctic Ocean is: A) Temperature gradient B) Salinity gradient C) Pressure gradient D) Nutrient gradient Answer: B Explanation: The halocline is a sharp salinity change separating fresher surface water from saltier deeper water.
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Question 102. In the carbonate system, the “buffer factor” (β) is defined as: A) dTA/dpH B) dDIC/dpH C) dTA/dDIC D) dCO₂/dpH Answer: B Explanation: β = ∂DIC/∂pH, indicating how DIC changes with pH and reflecting buffering strength. Question 103. The “solubility of calcium carbonate” increases with: A) Increasing temperature B) Decreasing pressure C) Decreasing CO₂ concentration D) Increasing Mg²⁺ concentration Answer: B Explanation: Higher pressure (greater depth) enhances CaCO₃ solubility, contributing to the CCD. Question 104. The “Mackenzie River” contributes significantly to the oceanic budget of which element? A) Iron B) Silicate C) Phosphate D) Nitrate Answer: C Explanation: The Mackenzie delivers large amounts of dissolved phosphate to the Arctic Ocean. Question 105. Which of the following gases has the longest atmospheric residence time? A) CO₂ B) O₂ C) N₂ O D) CH₄ Answer: A Explanation: CO₂ remains in the atmosphere for centuries to millennia, longer than the other listed gases. Question 106. The “pCO₂–pH” relationship in seawater is: A) Linear B) Exponential C) Logarithmic D) Quadratic Answer: C Explanation: Because pH is the negative logarithm of hydrogen ion concentration, the relationship between pCO₂ and pH is logarithmic. Question 107. The “biogeochemical pump” that transfers nutrients from deep to surface waters is primarily driven by: A) Upwelling B) Thermohaline circulation C) Wind‑driven Ekman transport D) All of the above Answer: D Explanation: Multiple physical processes bring deep nutrients to the euphotic zone. Question 108. In the context of ocean chemistry, “conservative temperature” (θ) is: A) Temperature corrected for pressure effects B) Temperature measured at the surface C) Temperature after accounting for salinity D) Temperature adjusted for pH Answer: A
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Question 115. The “biological removal” of dissolved inorganic carbon (DIC) from surface waters occurs mainly through: A) Calcification B) Photosynthetic uptake C) Gas exchange D) Hydrothermal vent activity Answer: B Explanation: Photosynthesis converts DIC into organic carbon, decreasing DIC concentration. Question 116. The “marine nitrogen budget” is most significantly affected by which process on a global scale? A) Nitrogen fixation B) Denitrification C) Nitrification D) Anammox Answer: B Explanation: Denitrification removes fixed nitrogen from the ocean, representing the largest net loss. Question 117. The “euphotic zone” is defined as the depth where: A) Light intensity is >1 % of surface value B) Temperature is constant C) Salinity is uniform D) Oxygen is at saturation Answer: A Explanation: The euphotic zone extends to the depth where photosynthetically active radiation falls below ~1 % of its surface value. Question 118. In the context of ocean acidification, “buffer capacity” is highest in waters that have: A) High total alkalinity B) Low temperature C) High CO₂ concentration D) Low salinity Answer: A Explanation: High alkalinity provides greater ability to neutralize added acids, reducing pH change. Question 119. The “marine silica cycle” is strongly linked to which other biogeochemical cycle? A) Carbon B) Nitrogen C) Phosphorus D) Iron Answer: A Explanation: Diatom growth couples silica uptake with carbon fixation, linking the silica and carbon cycles. Question 120. The “thermodynamic solubility product” (K_sp) for aragonite is: A) Higher than for calcite B) Lower than for calcite C) Equal to that of calcite D) Independent of temperature Answer: A Explanation: Aragonite is more soluble, thus its K_sp is larger than that of calcite. Question 121. The “deep‑sea oxygen minimum” observed in the North Atlantic is primarily caused by: A) High surface productivity B) Weak ventilation C) Elevated temperature D)
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Increased river runoff Answer: B Explanation: Limited deep‑water formation and circulation reduce oxygen replenishment, creating an OMZ. Question 122. The “magnesium/calcium ratio” in seawater is used as a proxy for: A) Past temperature B) Silicate weathering intensity C) Sea‑level change D) Ocean alkalinity Answer: A Explanation: Mg/Ca ratios in foraminiferal shells are temperature‑dependent, serving as paleotemperature indicators. Question 123. The “acid–base chemistry” of seawater is dominated by which buffer pair? A) H₂CO₃/HCO₃⁻ B) HSO₄⁻/SO₄²⁻ C) H₃PO₄/H₂PO₄⁻ D) NH₄⁺/NH₃ Answer: A Explanation: The carbonate system (H₂CO₃/HCO₃⁻) provides the main buffering capacity in seawater. Question 124. The “oxygen utilization” in a water mass increases most rapidly when: A) Temperature rises B) Biological respiration intensifies C) Salinity decreases D) Atmospheric CO₂ declines Answer: B Explanation: Respiration consumes oxygen, raising AOU and indicating water‑mass aging. Question 125. The “pH scale” commonly used for seawater is based on: A) Total hydrogen ion concentration B) Free hydrogen ion concentration C) Activity of hydrogen ions D) Concentration of carbonate ions Answer: C Explanation: Seawater pH is defined using the hydrogen ion activity, not the concentration. Question 126. The “first dissociation constant” (K₁) of carbonic acid decreases with increasing: A) Salinity B) Temperature C) Pressure D) pH Answer: B Explanation: Higher temperatures shift the equilibrium toward CO₂, lowering K₁. Question 127. The “hydrothermal alteration” of seawater chemistry typically adds which ion to the surrounding water? A) Na⁺ B) Mg²⁺ C) Fe²⁺ D) Cl⁻ Answer: C Explanation: Hydrothermal fluids are rich in Fe²⁺, which can be released into the ambient seawater.
