The Impact of Small-Scale Heterogeneity on Proxies in Biomineral Archives

Abstract:

From the pace of the ice ages to how the carbon cycle has changed through time, much of what we know about earth history and climate dynamics is based on chemical signatures locked within minerals. Recorded as trace element anomalies or as isotopic shifts, these chemical signatures reflect how mass and energy move across the planet, as well as the response of biological systems to these changes. When viewed at the sub-micron to nano-scale however, chemical composition rarely follows a simple relationship with environmental conditions. This is especially true for the preserved CaCO₃ skeletons of marine organisms, which often exhibit systematic patterns of high magnitude chemical variability at the sub-micron scale. While this biologically-driven variability can complicate the interpretation of climate records, it also represents a rich and largely untapped signal. Major advances in our understanding of both biomineralization and paleoproxies hinge on new techniques that can isolate small signals from this complexity.

To probe the mechanisms controlling biomineralization and sub-micron compositional variability, we use a suite of high spatial resolution tools: NanoSIMS, ToF-SIMS, and Atom Probe Tomography (APT), together with stable isotope labels and biomineral culture. In planktonic foraminifera we conducted modified pulse chase experiments using isotope tracers to measure ion transport rates during biomineralization. By varying elemental concentrations in the surrounding seawater during these pulse chase experiments, we induced systematic shifts in the ion transport rate. The magnitude of these shifts indirectly measure the elemental composition of the calcifying microenvironment, a key and previously unmeasured parameter affecting skeletal chemistry and paleoproxy systematics. Complementary isotope tracer experiments in coral and related experiments applying APT to the organic-mineral interface in foraminifera uncovered the response of calcification to ocean acidification and aspects of the organic templating process during skeletal nucleation, respectively. Collectively these experiments can help explain small-scale proxy heterogeneity, upscale this variability to bulk composition, and more accurately resolve specific environmental signals from the geochemical record.