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 (Δ47), which show a clear latitudinal thermal gradient and demonstrate coccolith Δ47 sensitivity to temperature. The application of the most recent Δ47-temperature calibration for marine biogenic carbonates yields calcification temperatures implying deep habitats of ∼50 to ∼150 m for tropical coccolithophores, which could photosynthesize with 1-10% of surface photosynthetic active radiation (PAR) levels. Because of the uncertainties of Δ47 thermometry and of the low upper ocean temperature gradient, at well-mixed high-latitude locations, coccolith Δ47 cannot be used to reliably constrain a specific habitat depth. Nevertheless, Δ47 is a good indicator of paleotemperatures of the mixed layer. We also use coccolith Δ47 to derive the first regression relating core top coccolith Δ47 and sea surface temperatures (SST). Although this formulation cannot be considered a proper coccolith-specific Δ47 calibration, since it ignores coccolithophore's potential for calcification at depth, it facilitates comparison with temperature proxies like U37k′, which are regressed to SST, rather than production temperature.