Abstract
Abstract. The composition of planktonic foraminiferal (PF) calcite is routinely used to reconstruct climate variability. However, PF ecology leaves a large imprint on the proxy signal: seasonal and vertical habitats of PF species vary spatially, causing variable offsets from annual mean surface conditions recorded by sedimentary assemblages. PF seasonality changes with temperature in a way that minimises the environmental change that individual species experience and it is not unlikely that changes in depth habitat also result from such habitat tracking. While this behaviour could lead to an underestimation of spatial or temporal trends as well as of variability in proxy records, most palaeoceanographic studies are (implicitly) based on the assumption of a constant habitat. Up to now, the effect of habitat tracking on foraminifera proxy records has not yet been formally quantified on a global scale. Here we attempt to characterise this effect on the amplitude of environmental change recorded in sedimentary PF using core top δ18O data from six species. We find that the offset from mean annual near-surface δ18O values varies with temperature, with PF δ18O indicating warmer than mean conditions in colder waters (on average by −0.1 ‰ (equivalent to 0.4 °C) per °C), thus providing a first-order quantification of the degree of underestimation due to habitat tracking. We use an empirical model to estimate the contribution of seasonality to the observed difference between PF and annual mean δ18O and use the residual Δδ18O to assess trends in calcification depth. Our analysis indicates that given an observation-based model parametrisation calcification depth increases with temperature in all species and sensitivity analysis suggests that a temperature-related seasonal habitat adjustment is essential to explain the observed isotope signal. Habitat tracking can thus lead to a significant reduction in the amplitude of recorded environmental change. However, we show that this behaviour is predictable. This allows accounting for habitat tracking, enabling more meaningful reconstructions and improved data–model comparison.
Highlights
The chemical composition of planktonic foraminifera shells reflects the environmental conditions in which they precipitate and fossil shells serve as the prime source of information about the past state of the oceans
Through comparison of observed and predicted δ18O data of six common planktonic foraminifera we have demonstrated that the average geochemical signal preserved in a population of fossil shells shows a temperature-dependent offset from mean annual sea surface conditions
Our analysis indicates that spatial and temporal gradients in temperature may be underestimated by as much as 40 %, highlighting the need to account for climate-dependent habitat variability in the interpretation of palaeoceanographic records based on planktonic foraminifera
Summary
The chemical composition of planktonic foraminifera shells reflects the environmental conditions in which they precipitate and fossil shells serve as the prime source of information about the past state of the oceans. Planktonic foraminifera species are non-uniformly distributed across the world ocean, indicating they inhabit distinct ecological niches (Bé and Tolderlund, 1971). Habitat preferences are routinely used for palaeoenvironmental reconstruction based on fossil foraminifera assemblages. These habitat preferences affect reconstructions based on single species. While species-specific seasonality and calcification depth are often taken into account, it is often implicitly assumed that both remain constant in time and space. This assumption is at odds with observations from the presentday ocean, which likely has important implications for re-
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