Cell geometry records the physiological responses of coccolithophores to Paleogene climate change

Abstract

The physiology and ecology of the calcifying marine phytoplankton group coccolithophores are expected to be susceptible to future changes in environmental conditions forced by increasing atmospheric carbon dioxide and temperature (e.g., Fabry et al., 2008). A key trait of the physiological state of a phytoplankton cell is cell size (e.g., Finkel et al., 2007), which we have captured from exquisitely preserved fossil coccospheres in biometric measurements of coccolith length, coccosphere size and number of coccoliths per coccosphere (Fig. 1). Our exquisitely preserved calcareous microfossils are obtained from sites where post-depositional alteration of the calcite is virtually absent. Typically these sites contain hemipelagic, clay-rich lithologies with high sedimentation rates that rapidly encapsulate calcareous microfossils, isolating them within an impermeable matrix (Bown et al., 2008). These sediments contain a remarkably high diversity of fossil coccolithophores and an unusually high abundance of intact coccospheres, where the original spherical-subspherical cell covering of coccoliths is completely preserved, instead of the typical disarticulation into loose coccoliths. The pristine preservation of fragile and taxonomically important central-area structures in coccoliths combined with the preservation of very small (<3μm)coccolithsenablesustostudycoccospheregeometry across a wide variety of Paleogene taxa at species level. We present material from Tanzania (Nicholas et al., 2006), New Jersey (Miller et al., 1998) and Californian (John et al., 2008) continental shelf environments, where the shallow water depth minimized water-column disaggregation and dissolution and the shelf setting promoted high sedimentation rates. The thick sequence of clay-rich oozes at deep-sea Labrador Sea site ODP 647A (Firth, 1989) has also yielded diverse coccospheres that illustrate a high-latitude North Atlantic Eocene assemblage. Intact fossil coccospheres can be interpreted on a cellular level through analogy with the cellular geometry of their modern counterparts (Gibbs et al., 2013). This facilitates an unprecedented insight into the cellular physiology of cells that are millions of years old and its link to population-level responses to climate change. Our biometric data (following Fig. 1; Gibbs et al., 2013 and Henderiks, 2008) reveals the cellular growth and calcification response of more than 30 coccolithophore species to long-term variability in sea-surface conditions during the Paleogene greenhouse interval, between 66 and 34 million years ago. Cell geometry from abundant coccospheres of the common Paleogene genera