Atmospheric and deep marine carbon cycle reconstructions using boron

An important goal in modern isotope geochemical marine research is the identification of short- or long-term perturbations of the marine environment. Since the chemical and physical composition of the oceans in the past cannot be measured directly, indirect techniques are needed.

Marine calcifiers incorporate many major and trace elements very systematically depending on physical and chemical ambient seawater conditions. This includes elements such as boron that are highly diagnostic for reconstructing the state of the carbon cycle, and hence atmospheric CO2. However, the resulting elemental ratios and isotopic compositions observed in such marine biogenic calcite not only depend on the location of sampling but also on the biology of the respective organism. For example, foraminifera incorporate various major and minor elements differently compared with cold-water corals, yet the species-specific internal variability observed in these marine organisms is systematic.

One important aspect of the research carried out within the Marine Geosystems group is the assessment to which degree an elemental or isotopic composition within certain species differs from local seawater compositions and to detect the controlling factors in setting this offset. Once the biological factor has been quantified, many marine calcifiers can be used to create marine records ranging from short (decadal) to very long (multi-millennial).

In one major research area within the Marine Geosystems group, boron isotopes are used to quantify the dissolved inorganic carbon concentration in surface seawater, and in turn are one of very few tools available to reconstruct past atmospheric CO2 concentrations. In combination with other geochemical tracers such as surface seawater temperature reconstructions using foraminiferal Mg/Ca or oxygen isotopic compositions, direct estimates can be made on the resulting greenhouse gas forcings caused by variable atmospheric CO2 concentrations during key climatic transitions at present and in the geological past.

In our group, boron isotopes are also used in deep marine settings where cold-water coral boron isotopic compositions tell us about the enhanced glacial deep marine storage of dissolved inorganic carbon in regions such as the deep Southern Ocean. Atmospheric CO2 levels were about a third lower during the Last Glacial Maximum ~22 thousand years ago compared with modern pre-industrial levels. This deficit of glacial atmospheric CO2 was stored in the deep ocean due to vastly different deep ocean circulation patterns. With help of other additional parameters measured on the same coral (for example neodymium isotopes), we are able to identify these glacial circulation patterns and their consequences for the carbon cycle.