Carbon Sources and Buffers
Give one natural and one human source of atmospheric CO2.
Natural: respiration, decomposition or volcanism. Human: fossil-fuel combustion or land clearing. Adapted skill: QCAA 2025 Paper 1 Q21.
In a supplied reaction, H+ combines with CO3 2-. What ion is formed?
Bicarbonate, HCO3-.
Why do thermal tolerance ranges differ among coral populations?
Species, acclimatisation, local thermal history and symbiont types can differ.
What is a proxy climate record?
An indirect record used to infer past climate, such as chemical signals in dated coral growth bands.
What do RCP numbers such as 2.6 and 8.5 describe?
Projected radiative forcing in watts per square metre by 2100 in those concentration pathways.
What carbon compound forms when dissolved CO2 reacts with water?
Carbonic acid, H2CO3. Adapted skill: QCAA 2025 Paper 2 Q11a.
For omega = ion product/Ksp, what does omega below 1 indicate?
Undersaturation: dissolution of exposed aragonite is thermodynamically favoured. Organisms can still regulate internal calcification conditions.
Why can two reefs with the same current temperature have different bleaching risk?
Their local thresholds and accumulated heat exposure can differ. A single temperature reading misses duration of stress.
Why are coral growth bands useful when interpreting a core?
They help date changes and estimate growth over time, allowing comparison with environmental records.
Which pathway generally implies stronger mitigation: RCP2.6 or RCP8.5?
RCP2.6. RCP8.5 represents much higher radiative forcing. These are scenarios, not probabilities.
Why can equal additions of CO2 cause a smaller pH change in buffered seawater than weakly buffered freshwater?
The carbonate system consumes or releases hydrogen ions, moderating the change. Freshwater buffering varies; the comparison assumes weaker buffering. Adapted skill: 2025 Paper 2 Q11b.
If ion product is 2 x 10^-7 and Ksp is 4 x 10^-7, calculate omega.
Omega = 0.5, indicating undersaturation.
Illustrative symbiont densities fall from 6 to 5 at Reef A and 6 to 1 at Reef B. Which shows the stronger bleaching signal?
Reef B loses more symbionts: five units rather than one. Adapted skill: QCAA 2025 Paper 2 Q4a-b.
In illustrative data, runoff rises in the same years as soil-derived humic material in coral bands. What proxy use is supported?
Coral chemistry may indicate historical runoff, because both records covary. Calibration and other evidence strengthen the inference. Adapted from 2025 Paper 1 Q13, viewed on Exam Insights.
Why can acidification affect corals through their food supply as well as their skeletons?
Changes in plankton abundance or composition can reduce prey availability for planktivorous corals. Adapted from 2025 Paper 1 Q3, viewed on Exam Insights.
How is carbonate cycling linked to geological processes?
Weathering supplies dissolved ions; carbonate formation, burial, dissolution and tectonic processes redistribute carbon over long periods.
If calcium and Ksp stay constant but carbonate concentration halves, how does omega change?
Omega halves because it is directly proportional to carbonate concentration under these assumptions.
Under a simplified heat-stress rule, two weeks each at 2 degrees above threshold accumulate how many degree heating weeks?
2 degrees x 2 weeks = 4 degree heating weeks. This simplified classroom calculation is not the full operational DHW algorithm.
A coral’s calcification rises in the lab but falls in the field as CO2 increases. What limitation of the lab result is indicated?
Laboratory conditions may exclude interacting field factors, so the lab response alone may not predict natural responses. Adapted skill: QCAA 2024 Paper 1 Q25.
Why might crown-of-thorns predation slow recovery after a heat event?
It removes surviving or recovering corals, adding mortality and limiting reef regrowth.
Why are deep carbonate sediments a long-term carbon reservoir?
Burial stores carbon away from rapid surface exchange; geological recycling and limited mixing make its return slow. Adapted from 2025 Paper 1 Q12, viewed on Exam Insights.
With other conditions constant, how do warming and increased salinity generally affect CO2 solubility?
Both generally reduce CO2 solubility. This isolated effect does not reverse acidification caused by continuing atmospheric CO2 increases.
Why might a higher proportion of heat-tolerant symbionts improve future heat tolerance without guaranteeing survival?
It can improve tolerance, but survival also depends on heat severity, coral physiology and other stressors. Adapted skill: 2025 Paper 2 Q4d.
A pH experiment uses one tank per treatment. Why are 20 corals per tank not 20 independent treatment replicates?
Corals share tank conditions. Independent replicated tanks are needed to separate treatment effects from tank effects; otherwise there is pseudoreplication.
Why are peer review, reproducibility and international collaboration valuable for reef projections?
They expose assumptions and errors, test whether findings hold, and combine evidence across regions and disciplines.