Stratigraphic signatures of mass extinctions: ecological and sedimentary determinants

Abstract

Stratigraphic patterns of last occurrences (LOs) of fossil taxa potentially fingerprint mass extinctions and delineate rates and geometries of those events. Although empirical studies of mass extinctions recognize that random sampling causes LOs to occur earlier than the time of extinction (Signor–Lipps effect), sequence stratigraphic controls on the position of LOs are rarely considered. By tracing stratigraphic ranges of extant mollusc species preserved in the Holocene succession of the Po coastal plain (Italy), we demonstrated that, if mass extinction took place today, complex but entirely false extinction patterns would be recorded regionally due to shifts in local community composition and non-random variation in the abundance of skeletal remains, both controlled by relative sea-level changes. Consequently, rather than following an apparent gradual pattern expected from the Signor–Lipps effect, LOs concentrated within intervals of stratigraphic condensation and strong facies shifts mimicking sudden extinction pulses. Methods assuming uniform recovery potential of fossils falsely supported stepwise extinction patterns among studied species and systematically underestimated their stratigraphic ranges. Such effects of stratigraphic architecture, co-produced by ecological, sedimentary and taphonomic processes, can easily confound interpretations of the timing, duration and selectivity of mass extinction events. Our results highlight the necessity of accounting for palaeoenvironmental and sequence stratigraphic context when inferring extinction dynamics from the fossil record.

Three‐fold nature of coastal progradation during the Holocene eustatic highstand, Po Plain, Italy – close correspondence of stratal character with distribution patterns

“…Phase 3 is marked by strong, mostly oblique progradation. Most coarse‐grained sediment was trapped nearshore, and distal prodelta areas were sediment starved (Amorosi et al ., ), testified by significant ecological and time condensation of macrobenthic remains (Scarponi et al ., , 2017a; Nawrot et al ., ). The delta system responded to the dominance of sediment supply over accommodation focusing sediment accumulation around the river mouth and along the nearshore.…”

Millennial‐Scale Age Offsets Within Fossil Assemblages: Result of Bioturbation Below the Taphonomic Active Zone and Out‐of‐Phase Production

“…Continental shelf sediments are valuable archives of oceanographic and biological history that allow inferences about the timing and causes of changes in climate, productivity, and seawater chemistry (Birks & Koç, ; Keigwin & Jones, ; Knies et al, Kobashi et al, ) and about evolutionary and ecological dynamics (e.g., Myhre et al, ; O'Dea et al, ; Pandolfi, ). However, as in pelagic sediments, bioturbational mixing by diffusion or advection increases time averaging (i.e., age offsets among co‐occurring individuals and species) of fossil assemblages and can alter the distribution of species in stratigraphic successions, thereby modifying community composition, co‐occurrence patterns, patterns of first and last appearances, and turnover rates (Alegret et al, ; Anderson et al, ; Lazarus et al, ; MacLeod & Huber, ; Nawrot et al, ; Tomašových & Kidwell, ). The time averaging of fossil assemblages is commonly approximated by dividing the thickness of the mixed layer by sedimentation rate (Anderson, ; Wheatcroft, ) and thus depends on the simultaneous estimation of (a) sedimentation rate and (b) depth of sediment mixing by bioturbation (Bentley et al, ; Bentley & Nittrouer, ; Wheatcroft & Drake, ; Wit et al, ).…”

Recognizing pulses of extinction from clusters of last occurrences

“…Elevated extinction rates during a period of major sea‐level change can produce multiple clusters of last occurrences that coincide with sequence‐stratigraphic surfaces (Holland 1995) due to the backscattering and concentration of last occurrences at lower stratigraphic surfaces (Holland & Patzkowsky 2015; Nawrot et al . 2018).…”