Radiolarians exhibit a threshold response to climate change during the late Neogene

Radiolarians have played a major role in ocean food webs and geochemical cycles since the Cambrian. Despite this, radiolarians’ Neogene and Quaternary biodiversity, biogeography, and sensitivity to climate are very poorly understood. We 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 first ever complete tropical radiolarian 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 each of 13 EEP samples, approximately 5000 specimens were counted to ensure adequate documentation of rare species. These data were used to construct a range-through diversity curve, and to characterize paleocommunity structural dynamics through time. Our preliminary results indicate that tropical radiolarian diversity and community structure were remarkably high and stable throughout the Late Neogene (average diversity = 490 taxa, sd = 22; average evenness (Pielou equitability index) = 0.84, sd = 0.02). This stability was observed in spite of a gradual ~3°C decrease in tropical SST over the last 10 Ma, indicating that this change was below tropical radiolarians’ diversity sensitivity threshold. By contrast, a major extinction of ~1/3 of Southern Ocean taxa, and dramatic change in community structure, correlates with a ~7°C polar SST drop from 5-0 Ma. These findings suggest that the relatively greater magnitude of high latitude climate change elicited a significant extinction event and re-organization of local radiolarian communities. With projected polar warming similar in scale to cooling in the Late Neogene, many high latitude radiolarian species may be at elevated extinction risk in the imminent future.