Isotopic and stratigraphic evidence for a mass extinction
A 60 m sedimentary section in a basin contains a thin (5 cm) clay layer separating diverse fossil-rich limestone below from sparser, small-shelled fossils above. Below the clay, fossils include large brachiopods, corals and ammonites; above it, only thin-shelled bivalves and a few burrowing worms appear, and diversity recovers gradually over the next 12 m. The clay contains elevated iridium, shocked quartz grains and glass spherules. Carbonate carbon isotopes shift from +2‰ below the clay to −1‰ above it, and oxygen isotopes show a +1.5‰ shift in the same interval. A radiometric date from an ash bed 2 m below the clay gives 66.3 ± 0.2 Ma; another ash 8 m above gives 65.8 ± 0.2 Ma.
1. Interpret the iridium, shocked quartz and spherule evidence, and explain what it implies about the cause of the faunal change.
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Iridium anomaly: Iridium is rare in crustal rocks but enriched in some meteorites, so a sharp spike in a thin clay layer suggests an extraterrestrial source delivered rapidly.
Shocked quartz: Planar deformation features in quartz form under extreme, brief pressure - characteristic of impact, not ordinary volcanism or sedimentation.
Glass spherules: Quenched melt droplets ejected and deposited within the clay layer, again consistent with an impact ejecta layer.
Combined interpretation: The three lines of evidence converge on a large bolide impact at the time the clay was deposited.
Link to the fauna: The clay layer coincides with the collapse in diversity and the disappearance of large, specialised taxa. The impact is therefore a strong candidate cause: dust and aerosols would block sunlight, collapse photosynthesis, and disrupt food chains, while acid rain and wildfires would add stress. The survival of small, generalist, burrowing taxa fits this scenario. Note that the evidence is circumstantial for causation - correlation in time plus a plausible kill mechanism - so other stresses (e.g. volcanism, sea-level change) should also be considered.
2. Interpret the carbon and oxygen isotope shifts and assess whether they support or complicate the impact hypothesis.
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Carbon isotopes (δ¹³C from +2‰ to −1‰): A negative shift means the carbon entering the system was isotopically light. Possible sources include oxidation of organic matter, release of methane, or collapse of the biological pump as productivity fell. Any of these is consistent with a disrupted carbon cycle and ecosystem after the impact.
Oxygen isotopes (+1.5‰): In carbonate, a positive δ¹⁸O shift generally indicates cooling and/or a change in seawater composition. A post-impact cooling is consistent with sunlight-blocking aerosols.
Complication: The shifts are not unique to impact. Volcanic CO₂ release, sea-level fall and diagenesis can produce similar patterns. The isotope data therefore support the impact scenario but do not prove it on their own.
Judgement: Multiple independent lines of evidence (iridium, shocked quartz, spherules, isotopes, fossil turnover) together are far stronger than any single one, and the convergence is the key argument.
3. Use the radiometric dates to evaluate the timing and rate of the extinction, and state one limitation of the chronology.
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Timing: The clay lies between 66.3 ± 0.2 Ma (2 m below) and 65.8 ± 0.2 Ma (8 m above). The clay is therefore younger than 66.3 Ma and older than 65.8 Ma. If sedimentation were roughly constant over 10 m, the clay would be about 66.2 Ma, but this is an interpolation, not a measurement.
Rate: The faunal change is essentially instantaneous at the resolution of the section - the clay is only 5 cm thick - while recovery takes 12 m of section. So the extinction was abrupt and the recovery slow, which matches an impact-driven event.
Limitation: The uncertainty of ±0.2 Ma (200,000 years) is far larger than the duration of the extinction itself, so the dates cannot resolve the event's duration. Also, the ages assume constant sedimentation and no reworking; a hiatus or condensed section would distort the interpolation. Better dating (e.g. more ash beds, cyclostratigraphy) would be needed to test the timing precisely.