Biomineral Geochemistry: Windows into Past Climates and Calcification

The ability of organisms to construct intricately shaped biominerals has fascinated researchers for centuries. It is now recognised that diverse organisms share the strategy of using amorphous intermediate phases during the mineralisation process. This article focuses on amorphous calcium carbonate (ACC) to explore how and why organisms use amorphous phases for biomineralisation and discusses the geochemical implications for palaeoenvironmental applications. We review ongoing efforts to mechanistically understand the effects of geochemistry and the transformation pathways of ACC on the corresponding proxy signals. We further consider how to quantify contributions to the offsets that are observed between the expected mineral compositions and the biological influences—a phenomenon known as ‘vital effects’, highlighting the importance of amorphous intermediates in geochemical (bio) mineralisation models.

The stable isotopic composition of marine biogenic carbonates is one of the main archives for paleoclimate reconstructions. Reading these archives accurately requires understanding of how different organisms make carbonate minerals, and how various biomineralization processes influence stable isotope fractionation. New developments in stable isotope measurements, laboratory experiments, and biomineralization modeling have progressively enabled us to disentangle the environmental and biological controls on the stable isotope proxies, and offer promise for a deeper understanding of how calcifying organisms record and respond to changes in Earth’s climate and carbon cycle through geologic time.

The concentrations of trace elements in carbonate biominerals can provide critical proxy records of past chemical and physical environmental conditions. However, the concentrations of these elements within biominerals are influenced by the diverse biological processes that govern mineralisation. This allows us to use the trace element content of biominerals grown under known conditions to infer the biological, physical, and dynamical processes that are involved in biomineralisation mechanisms. Here we introduce how key biomineralisation mechanisms can influence trace element incorporation, and we offer a high-level overview of how trace elements are used to infer the relative importance of these mechanisms in major groups of marine calcifiers.

The atmospheric partial pressure of CO2 (pCO2) is the key driver of climate variability. Boron isotopic compositions (δ11B) of marine calcium carbonates reveal pCO2 of the geologic past because boron isotope incorporation is sensitive to seawater pH, which closely reflects atmospheric pCO2. Biocarbonate δ11B values record environmental pH through a metabolic prism (so called “vital effects”), sometimes complicating interpretations. However, biocarbonate boron isotopes, coupled with boron concentrations (B/Ca), can also reveal the processes of calcification. Here, we review the link between seawater pH and the effective pH recorded by marine organisms via biomineralisation and summarise pCO2 reconstructions from boron isotopes for the Cenozoic (≈70 Ma to modern times), arguably the most significant contribution of this proxy system to date.

Calcium carbonate (CaCO3) forms various mineral polymorphs, including calcite, aragonite, and vaterite, each with distinct physicochemical properties. To benefit from these properties, marine organisms have evolved (some) control on the polymorphs from which their biomineral structures are built. This is achieved by modulating the conditions at their calcification sites and the nature of functional organic macromolecules that can serve as templates for carbonate crystallization. Environmental factors, such as seawater chemistry and ocean acidification, also affect polymorph selection, impacting organisms’ calcification pathways. Across geologic time, mass extinction events have influenced evolutionary-scale skeletal mineralogy trends. The organismal controls on CaCO3 polymorphism have significant implications for ecological and industrial applications, offering insights into the development of environmentally friendly materials with tailored properties.

The isotopic and elemental composition of calcium carbonate formed by marine organisms underpins a substantial portion of our knowledge of past climates. These geochemical ‘proxy’ systems have revolutionised our understanding of palaeoenvironmental change, but remain largely rooted in empiricism because of poorly understood biological ‘vital effects’. Here, we outline how this is both a problem and an opportunity—while some proxies have their basis in biological processes, this is the root cause of uncertainty in others. Moreover, integrating geochemistry into biomineralisation models provides additional constraint on cellular mechanisms; geochemical data have untapped potential in the field of biomineralisation and could be used to simultaneously understand the proxies in question and to determine why biomineralising organisms are sensitive to environmental change.