RADIOLARIANS EXHIBITED A THRESHOLD RESPONSE TO TEMPERATURE CHANGE DURING THE LATE NEOGENE (Invited Presentation)

Radiolarians have played a critical role in ocean food webs and geochemical cycles since the Cambrian. Despite this, radiolarians’ Neogene and Quaternary biodiversity, biogeography, and sensitivity to climate remain poorly understood. Therefore, the goal of this study was to compare radiolarian diversity dynamics at low versus high latitudes, to assess the ecological impacts of differential regional climate change during the last 10 million years. A new biodiversity curve was generated from the eastern equatorial Pacific (EEP), and examined alongside a previously-published radiolarian census from the Neogene Southern Ocean. Both were compared to regional temperature proxy records to better understand radiolarian climate sensitivity. From the EEP, approximately 5000 specimens were counted from each of 14 samples spanning 0-10 Ma, to ensure adequate documentation of rare species. These data were used to extrapolate total species richness, and to characterize paleocommunity structural dynamics through time. Our results indicate that tropical radiolarian diversity and community structure were remarkably high and stable throughout the Late Neogene (average species richness = 485 ± 36; average evenness (Pielou equitability index) = 0.84 ± 0.02). This stability was observed despite a gradual ~3°C decrease in sea surface temperature (SST) over the last 10 Ma, indicating that this change was below tropical radiolarians’ sensitivity threshold. By contrast, a profound loss of ~1/3 of Southern Ocean species, and establishment of a single dominant genus (Antarctissa), correlates with ~7°C regional SST drop from 5-0 Ma at high latitudes. The majority of species lost from the Southern Ocean were not observed in younger low latitude samples, suggesting that these species went globally extinct, rather than migrate to track their preferred temperature conditions. Our findings show that the relatively greater magnitude of high latitude temperature change elicited a significant extinction response and re-organization of radiolarian communities. With projected climate change similar in scale to that of the Late Neogene, we anticipate elevated extinction risk for high latitude radiolarians in the imminent future.