Ecological changes in marine macrobenthic assemblages across the Cretaceous/Palaeogene boundary in Patagonia: evidence for a productivity crisis and uncoordinated recovery

ABERHAN, M; WEIDEMEYER, S.; KIESSLING, W.; SCASSO, R.A.; MEDINA, F.A.

Uppsala

Congreso; 51st Palaeontological Association Annual Meeting; 2007

Paleontological Society

The mass extinction at the Cretaceous/Paleogene (K/Pg) boundary is generally explained by a severe, global crisis in primary productivity, following a catastrophic bolide impact. To test this hypothesis, we studied the taxonomic and ecological composition of the marine macrobenthos across the K/Pg boundary from so far poorly known Southern Hemisphere mid palaeolatitudes in the Neuquén Basin of Argentina. Consistent with the productivity scenario, the early Paleogene (Danian) mollusk-dominated benthic shelf ecosystems are characterized by (1) a stratigraphically limited low in macrofossil abundances in the earliest Danian, reminiscent of the „dead zone“ found in two Northern Hemisphere localities; (2) an increase in starvation-resistant, nonplanktotrophic deposit-feeders (nuculoid bivalves) and chemosymbionts (lucinid bivalves); (3) a reduction in the average body size of individuals as compared to those from Late Maastrictian assemblages; and (4) individuals with low metabolic rates and inactive lifestyles being more common than in the late Maastrichtian. Return to pre-extinction conditions of the various synecological attributes occurred over unequal time spans, indicating that recovery was uncoordinated with respect to ecological traits. Thus, pre-extinction abundance levels were reached at a short distance above the K/Pg boundary, whereas late Maastrichtian mobility levels were re-established somewhat later. Trophic composition once again resembles that of Maastrichtian samples only near the end of the studied interval, and mean body size within assemblages did not reach values typical of late Maastrichtian times in the study interval. Comparison of ecological patterns with those from other K/Pg boundary sites suggests that reduced food supply was a controlling factor in both Northern and Southern Hemispheres. It also affected macrobenthic marine faunas at various distances from the Chicxulub impact site. Finally, low nutrient availability was presumably more effective in siliciclastic environments as compared to carbonate settings. This later finding may be explained by the presence of a more starvation-resistant, low nutrient-adapted benthic fauna in oligotrophic carbonate shelf settings, as represented for example by the very small-sized bivalves and brachiopods of the Danish chalk.