Clumped isotopes in globally distributed Holocene coccoliths reveal their habitat depth

Abstract

Reliable temperature reconstructions are necessary to improve climate reconstructions and comparisons with paleoclimate model simulations. Most existing paleotemperature proxies are based on organic and inorganic remains of marine organisms. Despite the evidence that the habitat depth of coccolithophores and other phytoplankton depend on their ability to balance light, nutrients, and grazing pressure, calibrations of proxies based on photosynthesizers often assume they live in the surface ocean. Here we present the first globally distributed dataset of core top multi-species coccolith clumped isotopes (∆47), which show a clear latitudinal thermal gradient and demonstrate coccolith ∆47 sensitivity to temperature. The application of the most recent ∆47-temperature calibration for marine biogenic carbonates yield calcification temperatures implying deep habitats for tropical coccolithophores (from ~50 to up to ~150 m), which could photosynthesize with 1-10% of surface photosynthetic active radiation (PAR) levels. Given the low upper ocean temperature gradient of well-mixed high-latitude locations and the current uncertainties of ∆47 thermometry, coccolith ∆47 cannot be used to reliably constrain a specific habitat depth in these locations. Nevertheless, they are a good indicator of paleotemperatures of the mixed layer. We also use coccolith ∆47 to derive the first regression relating core top coccolith ∆47 and sea surface temperatures (SST). Although this formulation cannot be considered a proper coccolith-specific ∆47 calibration, since it ignores coccolithophore’s potential for calcification at depth, it facilitates comparison with temperature proxies like UK37', which are regressed to SST, rather than production temperature.