Mercury (Hg) as indicator of large-scale volcanism in marine sediments provides new insights into relative timing between biological and environmental changes, mass extinctions and delayed recovery. We analyzed Hg concentrations linked to Deccan volcanism, TOC, δ¹³C, δ¹⁸O and faunal changes for C29r that spans 250 ky below the Cretaceous-Paleogene boundary (KPB) at Elles, Tunisia. The sediment accumulation rate is 8.6 cm/ky making this the most complete and expanded sequence known worldwide. Sample analyses at 10 cm intervals yield a data point every 1160 years. Relatively high Hg/TOC ratios are first recorded 1.6 m above the C30n/C29r boundary (base zone CF2). Sustained high pulsed Hg/TOC ratios lasted from 200 to 135 ky pre-KPB (calculated from sedimentation rates), correlative with global warming and decreased δ¹³C values. Variable Hg/TOC ratios and cooling between 133 and 90 ky was followed by low ratios, warming and decreased δ¹³C between 90-56 ky. Fluctuating Hg/TOC, cooling and increased δ¹³C prevailed between 55 to 18 ky. Maximum Hg/TOC ratios and rapid warming during the last 18 ky culminate in the KPB mass extinction. Lower frequency Hg/TOC ratios continue through the early Danian C29r and implicate Deccan volcanism also in the delayed recovery after the mass extinction.

Faunal turnovers mirror Hg/TOC ratios, δ¹⁸O and δ¹³C trends. Analyses of three size fractions (38-63 mm, >63 mm, >150 mm) reveal strong dwarfing and high-stress environmental conditions. Dwarfed (38-63 mm) disaster opportunist Guembelitria cretacea are common throughout C29r but reached maximum stress (85%) during the last 8 ky prior to the mass extinction. Small, low oxygen tolerant biserial species (38-63 mm, >63 mm) dominated C29r at times of increased biotic stress and climate warming (C29r up to 90 ky) suggesting disoxic conditions. Larger, more complex species are generally rare. The mass extinction occurred during the last 8-10 ky, a time of maximum Hg/TOC ratios linked to Deccan volcanism, rapid climate warming and ocean acidification.

What role, if any, the Chicxulub impact played in this mass extinction is still controversial. The common assumption that this impact occurred precisely at the KPB is contradicted by evidence of primary impact spherule deposition ~100 ky prior to the mass extinction in Mexico.