Sampling strategy

Our primary data are derived from extensive macrofossil collecting in three sedimentary section lines spanning the López de Bertodano Formation and K–Pg interval. All sub-sections that comprise the composite section were measured perpendicular to strike using a Jacob’s staff and tape measure. Fieldwork was undertaken during three field seasons to Seymour Island and encompassed the main outcrop of the López de Bertodano Formation in the southern part of the island24,27,44,51. The island is ice-free and exposure excellent. Sections DJ.959, DJ.957, DJ.952 and DJ.953 were made during the 1999 field season, and are located close to the central portion of the outcrop, commencing in the mid-levels of the López de Bertodano Formation representing the informal mapping units Klb7–9 and Ktplb10 of Macellari26 (Fig. 1). Section D5.251 (comprising sub-sections D5.212, D5.215, D5.218, D5.219, D5.220, D5.222 and D5.229) was measured and sampled during the 2006 field season and runs perpendicular to strike and approximately parallel to the southern coast of the island, beginning within the uppermost levels of the Snow Hill Island Formation. Sections D9.205, D9.206 and D9.207 were located at the northern end of the outcrop during the 2010 field season, and begin immediately below the K–Pg boundary. All three composite sections extend through the K–Pg boundary and the informal mapping unit Ktplb10 to the unconformable contact with the overlying Sobral Formation (Supplementary Figs 1–4).

Macrofossil collections were made systematically at varying scales during the different field seasons, with sample bins ranging on average from 1 m to intervals 10–15 m thick (see Supplementary Figs 2–4 and Supplementary Data 1 for illustration of sampling intervals and bin length in each individual section line). Changes in the size of sample bins within and between individual section lines were necessary during field collecting due to the nature of the ‘scarp and dip-slope’ topography that predominates on southern Seymour Island, whereby fossils are invariably more common on dip-slopes than scarps. Collections were made at each station until a representative collection of all the obvious macrofossil types had been obtained; just as it was not possible to standardize sample bin size, so it was not possible to use a standard collecting time either. For these reasons we chose to focus only on range-through data and standing species richness to estimate changes in taxonomic diversity across the K–Pg boundary. In Supplementary Fig. 7, we have plotted variations in sample species richness through all three of our sections, including data from Zinsmeister16 (Supplementary Fig. 6). These all show fairly regular variation around a sample mean, but no major trends that could be linked to any obvious form of either local or global environmental variation. Such small-scale fluctuations in species richness are an inevitable consequence of specimen collection in a scarp and dip-slope terrain, and are unlikely to represent any true response to environmental change.

Correlation between section lines and the construction of a composite section was achieved using several stratigraphic tie-points, notably the glauconite-rich beds that mark the K–Pg boundary and a further prominent glauconite-rich horizon present in all section lines ∼174 m below the K–Pg. To enable a full analysis of extinction patterns at the K–Pg boundary, field data derived from British Antarctic Survey sampling was also supplemented with additional data from Zinsmeister16 (Supplementary Fig. 6). Zinsmeister’s16 macrofossil collections were taken from a series of short (20 m) sections measured and sampled during a detailed along-strike mapping study of the K–Pg boundary across ∼5.5 km of southern Seymour Island. When plotting these additional fossil occurrences the base of the ‘Lower Glauconite’ horizon of Zinsmeister16 is taken as a reference plane, and assumed to be equivalent to the base of the glauconite-rich beds and K–Pg boundary in our composite measured section at a stratigraphic height of 1,007.5 m (refs 31, 51). For all these stratigraphic correlations, we assume planar bedding along strike. The unconformable contact at the base of the Sobral Formation is also useful as a tie-point, although it can be demonstrated that on a regional scale the degree of erosion of the upper levels of the López de Bertodano Formation changes subtly along strike across the island26,67.

The occurrence of glauconite-rich horizons, such as those that mark the K–Pg interval, suggest periods of slower, condensed sedimentation. The base of these units appears gradational in the field16,24,27,30 and high-resolution palynological studies18,23,24 show they are not associated with significant sedimentary hiatuses in the studied sections, but probably represent conformable facies boundaries24.

Fossil data analysis

Over 6,000 fossils of benthic molluscs (bivalves and gastropods) were examined during this study, with 5,710 identified to at least generic level, these have been combined with >700 cephalopod macrofossils18 for an examination of overall diversity of the molluscan fauna. Following taxonomic identification and reassessment (Supplementary Note 1), first and last occurrence data from individual section lines were used to construct a composite range chart using the stratigraphic tie points outlined above. Changes in stratigraphic bin size were accounted for by taking the base of the stratigraphic bin in which a species first occurred as the first appearance, and the top of the stratigraphic bin in which a species last occurred as the last appearance. While this introduces a degree of error into the results (for example, where a sampling bin straddles the K–Pg boundary in a single stratigraphic section), it is negligible given the expanded nature of the succession. A presence–absence data set based on this range data (Supplementary Data 1) was used to calculate standing species richness variations throughout the section, supplemented with additional collections from the overlying Paleocene Sobral Formation24,44 and a literature review to identify range-through taxa from older, underlying formations (Supplementary Table 1). To assess changing rates of biotic turnover through the succession, the presence–absence data set was split into 10 m bins and both extinction (E r ) and origination (O r ) rates calculated for each 10 m bin using the boundary-crosser methodology outlined by Foote38:

where N bt =number of range-through taxa, N ft =number of taxa that originate within any given 10 m bin and cross the top boundary of that bin and N bl =number of taxa that cross the bottom boundary of the bin but have their last occurrence within the bin. These should be considered as ‘extinction’ and ‘origination’ rates only in the local context, and are not expressed relative to bin duration. Available evidence suggests overall sedimentation rates remained high throughout deposition of the López de Bertodano Formation at 0.1–0.2 mm per year (ref. 17) (Supplementary Fig. 5) indicating that any variation in bin duration is likely to have a negligible effect on the magnitude of biotic turnover rates.

To test the hypothesis of multiple extinction events and visually assess the pattern of taxonomic turnover more generally through the Maastrichtian, we also employed the stratigraphic abundance method of Meldahl40. Stratigraphic abundance (S, the percentage of sample intervals in which a given taxon occurs) was calculated using a recently modified method41 to enable us to include data from all three studied sedimentary sections in addition to data from Zinsmeister16:

where N occurrence 1 is the number of occurrences of a given species in section 1, and N sample 1 is the number of samples in section 1. A plot of S versus last occurrence provides a visual estimate of the likely position of an extinction horizon based on the disappearance of the most common taxa in an assemblage (Fig. 3c), and along with a plot of the frequency distribution of last occurrences in a stratigraphic section (Supplementary Fig. 7), can be compared with simulated models of sudden and gradual extinction40. We included all molluscan taxa in this analysis, including a reanalysis of the nekton to include additional data from field collections made in 2010 (data reanalysed from ref. 18). To investigate the extent to which the data is influenced by the ‘Signor-Lipps Effect’39 due to the sampling strategy, 50 and 95% confidence intervals were calculated for all benthic taxa with >5 occurrences during the Maastrichtian within two BAS section lines (sections A and B, Fig. 1c). We applied the ‘classical’ method as summarised by Marshall42, and illustrate these confidence intervals as range extensions (Supplementary Figs 3 and 4) using the following equation:

Average gap size between fossil occurrences also provides an unbiased point estimate of the true time of appearance or disappearance in any given stratigraphic section assuming random fossil recovery42, and was calculated for the same taxa using:

where r C,i is the length of the range extension, r unbiased is the average gap between fossil occurrences as a percentage of that taxon’s stratigraphic range, C is the desired confidence level (expressed as a decimal; 0.5 and 0.95), H is the number of observed fossil occurrences for a species in an individual section line and R is the observed stratigraphic range of the taxon in the same section line. Confidence intervals were only calculated for data from two sections directly sampled by the authors (sections A and B, Fig. 1c), both of which have extended records from the Maastrichtian into the Paleocene. Because of the constraints of the sampling strategy outlined above, confidence intervals were not applied to any composite data set that is derived only from range-through data (Fig. 2).

Pyrite petrography

Polished blocks were made from 21 bulk sediment samples collected throughout composite section D5.251 (Supplementary Table 2). These were examined using an FEI Quanta 650 scanning electron microscope (SEM) in back-scatter mode to identify microfacies and quantify the diameter of pyrite framboid populations. The size and distribution of pyrite framboids in both ancient and modern sediments are interpreted to result from local redox conditions45,46. In modern environments, syngenetic framboids form in a narrow iron reduction region developed at the redox boundary, but cease growing in the underlying fully anoxic sulphate reduction zone. Under fully euxinic conditions (where free H 2 S occurs in the water column), syngenetic framboids grow to a maximum diameter of 6–7 μm in the water column before gravity causes them to sink to the seabed45. Framboid populations formed under these conditions will exhibit both a small size range and a small s.d. In dysoxic settings, conditions on the seabed are often weakly oxygenated, leading to framboid development in the pore water of the underlying sediments. Here the size range is controlled primarily by the availability of reactants and therefore framboid populations typically grow to larger sizes (up to 20 μm) with a correspondingly higher s.d.46. Supplementary Figure 8 presents a ‘Box and Whisker’ plot showing the stratigraphic distribution of the sampled horizons and illustration of framboid populations.

Data availability

The authors declare that all data supporting the findings of this study are available within the article and its Supplementary Information Files.