Proxy development and innovation; Towards a mechanistic understanding of paleoproxies

Fig. 1: Planktonic foraminifer (juv. Orbulina universa) with its symbionts (Photo: Jelle Bijma/Howard Spero)

The marine carbonate system is an essential component of the global carbon cycle. Due to the CO2 exchange at the air-sea interface the oceans play an important role by regulating the amount of atmospheric carbon dioxide and are thus closely coupled to climate variability. Anthropogenic intervention alters the natural climate variability and raises the question of how environmental conditions might change in the future. To validate climate models, past climate reconstructions present the best opportunity. Useful information about the geological history of the marine carbonate system is preserved in ocean sediments. So-called "proxies" are used to reconstruct past ocean conditions. They are measurable descriptors that are closely correlated to desired but unobservable target parameters. For instance, the stable isotopic composition and (trace) element ratios within organic and inorganic compounds of microfossils, such as foraminifera (fig. 1), record environmental conditions during growth that can be measured by geochemical analyses.

Fig. 2: Sampling of diatoms (Photo: Alfred Wegener Institute/ Bärbel Hönisch)

Foraminifera (unicellular organisms secreting a carbonate shell) are among the most important recorders of past environmental conditions. Examples of proxies are δ18O (for temperature and the waxing and waning of ice sheets), Mg/Ca for temperature, δ13C can be used as a measure for the strength of the biological pump and δ11B is a proxy for pH. However, the precision and accuracy of the recorded target parameter is often biased by a dependency of the proxy on additional variables. Species specificity, various vital effects, controlled by abiotic factors like light intensity, nutrient availability, carbonate chemistry, etc., but also by the life processes of the proxy carrier, such as respiration, calcification and photosynthesis of possible symbionts complicate their interpretation. Hence, proxy relationships are not as robust as one would like them to be. (In situ pressure) culture experiments (figs 2-5) and field studies in concert with numerical (cell) modelling and a thorough understanding of the biomineralisation processes (fig. 6) enable us to identify the mechanisms that control the primary relationship. This knowledge is required to develop robust proxy relationships for high quality paleoclimatic reconstructions.

Fig. 3: Culturing of benthic foraminifera under controlled conditions (Photo: Alfred Wegener Institute/ Delphine Dissard)
Fig. 4: Nano-SIMS-picture of a benthic foraminifer (Photo: Alfred Wegener Institute/ Gernot Nehrke)