Benthic foraminiferal morphogroups track the recovery of the deep-marine ecosystem after the K/Pg boundary

Morphogroup analysis of benthic foraminifera is a useful tool that reflects the nature of the trophic continuum, and in particular the flux of organic matter to the sea floor and its source. In broad terms, agglutinated benthic foraminifera can be placed in four morphotypes based on their test morphology and feeding preference. Morphotype M1 constitutes tubular suspension feeders that trap food particles carried in suspension. Morphotypes M2 and M3 are the coiled multichambered forms, which are epifaunal detritivores, and M4 is the group of elongated and rectilinear infaunal deposit feeders. Their relative proportions tell us something about flux of particulate organic carbon (POM) to the sea floor and its mode of delivery, which is ultimately a function of water depth, currents, and the amount of surface-water phytoplankton production.

To assess the trophic structure of the benthic foraminiferal community across the Cretaceous/Paleogene boundary in the western Tethys, we examined >70 samples from the Scaglia Rossa Formation in Gubbio, Italy. Samples were mostly collected at 10 cm spacing. We also recalibrated the age model for the Scaglia Rossa Formation in Gubbio using the 2020 geologic time scale. In the top two meters of the Maastrichtian, a gradual increase in the proportion of M4 is observed (to ca. 20%) leading up to the K/Pg boundary. These values fall abruptly to 7% in the beds immediately above the boundary clay, with more variable values in the lower Paleocene. This pattern can be interpreted as reflecting a modest but short-lived reduction in the total sea-floor organic flux following the boundary event (but not a “Strangelove Ocean”). Morphotype M1 shows a major reduction above the boundary, and there is a concurrent increase in the M2 morphotype. This implies a reduction in the amount of POM arriving at the sea floor from suspension. The increase in M2 suggests that there was greater influence of organic matter from bacterial sources in the early Paleocene.

The recovery of the deep-marine ecosystem was prolonged, with M1 returning to Maastrichtian values approximately 3.4 m above the K/Pg boundary clay. Using our new age model, this is equivalent to 1.8 m.y. after the event. Our findings of a prolonged recovery are in line with the conclusions of nannofossil workers, who estimated that it took approximately 2 m.y. for the marine food web to fully reestablish itself after the K/Pg boundary event.