A boundary core with two signals
A marine drill core from the southern Indian Ocean spans the Cretaceous–Paleogene boundary at 80 m depth. Background iridium is 0.1 ppb; a 2 cm clay layer at the boundary contains 8.5 ppb Ir, 15 shocked-quartz grains per cm3, and nickel-rich spinels. Mercury rises from 20 ppb to 350 ppb in the same layer. Below the boundary, planktic foraminifera are diverse; above it, 90% of planktic species disappear, while only 30% of benthic species disappear. A fern-spore spike and a 2.5‰ negative δ13C excursion occur immediately above the boundary. An ash bed 40 cm below the boundary gives an 40Ar/39Ar age of 66.25 ± 0.05 Ma; the boundary gives 66.05 ± 0.05 Ma. Deccan Traps volcanism is dated at 66.3–65.5 Ma.
1. Evaluate the relative roles of the Chicxulub impact and Deccan volcanism in the extinction record. Which hypothesis better explains the abruptness, the Ir anomaly, and the Hg spike?
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The impact hypothesis better explains the abruptness and the diagnostic markers. The 2 cm boundary layer contains 8.5 ppb Ir (85 times background), shocked quartz and nickel-rich spinels; shocked quartz requires shock pressures found in impacts, not typical volcanism. The extinction is abrupt at the same horizon, followed by a fern-spore spike and a −2.5‰ δ13C excursion, consistent with a collapse in photosynthesis and a short-term carbon-cycle perturbation. Deccan volcanism was ongoing (66.3–65.5 Ma) and could have caused environmental stress, and the Hg spike shows a volatile input. But the ash bed 40 cm below the boundary (66.25 Ma) shows volcanism predates the extinction; the boundary (66.05 Ma) is the abrupt event. The best interpretation is impact as the trigger, with volcanism as a background stressor that may have reduced resilience.
2. Why did benthic foraminifera suffer less extinction than planktic foraminifera? Use the evidence in the core and your knowledge of marine ecosystems.
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Planktic foraminifera live in the surface ocean and depend on photosynthesis-driven food webs and carbonate chemistry. The impact would have blocked sunlight, cooled the surface and possibly acidified the ocean, hitting that habitat directly. Benthic foraminifera live on the seafloor and feed on sinking detritus; they experienced the same food-supply decline but had deeper refuges, lower metabolic demand and possibly more resistant life stages. The core shows 90% planktic versus 30% benthic extinction, which is consistent with a surface-ocean catastrophe rather than a purely seafloor one. Benthic taxa that relied on fresh phytodetritus were still vulnerable, which is why the extinction was not zero.
3. A critic argues that because the Deccan Traps were erupting at the same time, the Ir anomaly could be volcanic. Assess this argument and state what evidence would be needed to resolve it.
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Volcanic Ir is possible, but the Ir anomaly alone is not the strongest evidence. The critic must also explain the shocked quartz and nickel-rich spinels, which are impact-diagnostic. To resolve the source, measure platinum-group element ratios (for example Os/Ir and Ru/Ir) and Os isotopes: meteoritic values are distinct from mantle or volcanic values. Also date the boundary layer and the Deccan flows more precisely, and map the Hg and Ir spikes against the ash beds. If the Ir were volcanic, it should correlate with Hg and ash without shocked minerals; here it coincides with shocked quartz. Mercury and sulfur isotopes could further distinguish impact from volcanic sulfur. The argument is testable, not settled by the Hg spike alone.