Calcareous nannoplankton community composition across multiple early Eocene hyperthermal events at International Ocean Discovery Program (IODP) Site U1553 (Campbell Plateau, SW Pacific)

Carbon dioxide (CO₂) emissions are rapidly rising leading to warmer oceans, surface ocean acidification, and complex changes in marine biogeochemical cycling. Calcareous nannoplankton: single-celled marine haptophytes, are likely particularly susceptible to such environmental changes, because they form microscopic plates made out of calcium carbonate (calcite). As these organisms lie at the base of the marine food web, it is critical that we understand how they respond to climate change over longer (millennial) timescales so that we can better predict the long-term effects of current and future environmental change on marine communities.

The high CO₂ world of the early Eocene (~56 to 48 Ma) is characterized by multiple transient warming events (‘hyperthermals’), and is generally considered to be one of the best geologic analogues for future climate change. Here, we present preliminary, low-resolution calcareous nannoplankton assemblage data from the early Eocene of recently-drilled IODP Site U1553 (Campbell Plateau) in the South Pacific Ocean. Sediment cores recovered from Holes C and D at Site U1553 provide arguably one of the most complete and expanded early Eocene records yet from this relatively understudied region, including many of the previously recognized hyperthermals. This coupled with the high calcium carbonate content of the sediments, makes it an ideal case study for exploring millennial-scale changes in calcareous nannoplankton community composition and morphometry during transient warming events.

Within this presentation, we predominantly focus on the Paleocene-Eocene Thermal Maximum (PETM): the largest and best-studied of the early Eocene hyperthermals. Our results suggest that the turnover in nannoplankton species during this warming event was very similar to that observed at other southern high latitude sites such as Maud Rise. More minor ecological ‘jostling’ appears to have occurred prior to the onset of the PETM and following the event; however, the significance of these smaller changes in community composition have yet to be statistically analyzed at the time of writing. It is our aim to combine our assemblage counts with morphometric data to determine whether calcareous nannoplankton acted as a source or sink of carbon dioxide during the early Eocene hyperthermal events. We will also extend our dataset to include several of the smaller hyperthermals that succeeded the PETM, to elucidate whether calcareous nannoplankton exhibit a scaled or threshold response to warming.