Radiolarian size and silicification across the Paleocene-Eocene boundary and into the early Eocene

The Paleocene-Eocene Thermal Maximum (PETM) was a major environmental perturbation that drove range shifts, adaptation, and faunal turnover in marine and terrestrial clades. Open ocean planktonic responses—including body size trends—have been characterized for calcareous nannoplankton, dinocysts, and planktonic foraminifera. Relatively little, however, is known about the response of radiolarians, which share life modes and phylogenetic history with planktonic foraminifera but make their tests (i.e., ‘shells’) out of silica rather than calcium carbonate. Here, we compare radiolarian assemblage changes in the Southwest Pacific with assemblages from Atlantic and Indian Ocean sites. We also measured radiolarian test size before, during, and after the PETM in a pelagic sedimentary succession at Mead Stream, South Island, New Zealand. Our results show a substantial positive shift in mean radiolarian test size at Mead Stream at the assemblage level concurrent with the onset of the PETM, primarily due to an increase in the size and relative abundance of spumellarian radiolarians. We investigate oligotrophy, elevated seawater silica concentration, and redeposition as potential drivers for this size shift. Mead Stream radiolarians remain large for 1.3 Myr, long past the recovery of the carbon isotope excursion and a return to baseline in terrigenous sedimentation, bioturbation, and benthic foraminifera diversity, leaving the ultimate cause of the observed changes in test size uncertain. We find little evidence for a uniform morphological response to the event or its aftermath across sites, particularly as relates to the relative extent of silicification of different groups of radiolarians.