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Uncovering the Magnitude of Arctic Climate Change

Periodic Reporting for period 1 - ARCTICO (Uncovering the Magnitude of Arctic Climate Change)

Reporting period: 2019-08-01 to 2020-08-31

A key challenge in climate change science is to provide informed constraints on the magnitude of future climate change. Uncertainties associated with such predictions remain large due to the shortness of our observational records (at best 150 years) and the absence of large climate shifts therein to serve as an analogue for future change. This is especially problematic when estimating Arctic climate change because the response in the Arctic is amplified relative to the global mean, making the Arctic the most sensitive and vulnerable environment with regards to global warming. Efforts to assess the magnitude of past (e.g. pre-industrial) climate changes using climate proxies are thus crucial to further our understanding of how the Arctic climate system will respond to continued global warming. The principal scientific objective of project ARCTICO was to constrain the magnitude of Arctic amplification in the past by refining our understanding of (1) how geochemical signals (Mg/Ca, B/Ca, δ11B, δ18O) are recorded in the Arctic planktonic foraminifera Neogloboquadrina pachyderma sinistral (NPS) and (2) how these signals relate to essential climate variables (e.g. temperature, salinity, carbonate chemistry). At the center of our work we test the hypothesis that in addition to temperature, sea water carbonate-ion concentration [CO3] exerts a strong secondary control on Mg/Ca – paleothermometry.
To address the objectives of this project, we selected a comprehensive calibration dataset from a rich archive of stratified plankton samples, which was paired with stable oxygen and carbon values of the water column taken at the time the plankton was removed from the water. The final dataset covered a large gradient in temperature, salinity, and carbonate chemistry to test the application of our geochemical proxies to a large range of hydrographic parameters. Next, we combined established and new analytical techniques in trace element and isotope geochemistry to derive and isolate carbonate system parameters from the temperature signal (e.g. Mg/Ca-palaeothermometry) recorded in NPS. This included three separate analytical techniques. Fist, we prepared samples for stable oxygen (δ18O) isotope analysis to determine the geochemical signature of the calcification environment (e.g. calcification depth) of NPS. This was followed by the preparation of sample material for boron isotope δ11B and finally trace element (El/Ca) analysis.

To estimate the influence of seawater carbonate ion concentration [CO3] on Mg/Ca values for our calibration dataset, we derived the relationship between Mg/Ca ratios (Mg/Ca[meas]/Mg/Ca[pred]) and water column [CO3]. As hypothesized our results describe a strong sensitivity of Mg/Ca to [CO3] at low concentrations that is best described by a power regression (r2=0.81). We note that the sensitivity described by our data is stronger than equations presented in Evans et al. [2016] and Morley et al. [2017] for other planktonic foraminifera and propose that the higher sensitivity can be explained by a non-linear response of [CO3] on Mg incorporation at various ambient temperatures. The steeper response supports the hypothesis that at lower temperatures the [CO3] effect is greater than at higher temperatures.

The δ11B data from NPS are lower than that of ambient borate B(OH)-4 ion and plot below the 1:1 line similar to G. bulloides (also non-symbiont baring foraminifera) and O. universa. The slope of the regression of 1.41 (±0.40) indicates that open ocean NPS display a pH sensitivity that is similar (within uncertainties) to that of aqueous borate ion with a slight negative offset from the 1:1 line. Similar to other non-symbiont baring species, the negative offset of NPS δ11B from the 1:1 line could be explained by changes in microenvironmental pH due to respiration since the effect of respiration could lower pH in the microenvironment and thus δ11Bborate [Henehan et al. 2016]. We note that the slope of the regression is slightly larger than 1 (within uncertainties), which could suggest that the effect of respiration on the microenvironment is greater at low pH, which increases the sensitivity of the δ11B-pH proxy for NPS. The regression supports the hypothesis that δ11B measured on NPS are sensitive to changes in pH and that δ11B can thus be used to reconstruct past changes in pH at high northern latitudes.

For the elemental incorporation of Boron over Calcium (B/Ca) into NPS we find that there is a departure of the predicted B/Ca to borate over bicarbonate relationship predicted by Hemming and Hanson [1992] with B/Ca values apparently unrelated to carbonate system parameters. We hypothesize that exceptionally low or constant calcification rates that are associated with NPS, prohibit a dependency of B/Ca values to carbonate system parameters as seen during inorganic calcite experiments [Uchikawa et al. 2017]. Practically, our findings thus prevent us from defining an empirical relationship between B/Ca and carbonate ion concentrations. For past reconstructions of sea surface temperatures an alternative proxy must therefore be used. A potential solution to this issue may be to define temperature, salinity, and carbonate chemistry sensitivities in culture experiments and to correct for carbonate chemistry sensitivities using δ11B similar to the approach developed by Evans and Gray (2019) for other planktonic foraminifera.
The scientific discoveries made during the project provide a proof of concept that Mg/Ca - Palaeothermometry is possible in Arctic environments when paired with boron isotope geochemistry. Given the uncertainties associated with available palaeoceanographic tools, our findings provides a major advancement in the field of paleoceanography. Considering the growing consensus in the scientific community that current trends in Arctic amplification will increase the frequency of extreme weather events over northern mid-latitudes, I anticipate that my findings will have immediate implications for future climate scenarios relevant to Europe and beyond.

The hands on training in isotope geochemistry and micropaleontology at MARUM, University of Bremen and the Alfred Wegner Institute for Polar Research provided the researcher with new technical skills and exposed her to the management, maintenance, and running costs associated with specialized equipment that she is eager to bring to her home institution in the long-term. This knowledge is invaluable when securing funding for infrastructure, in particularly when designing business models for the long-term maintenance, use, and feasibility of specialized equipment. In combination with the improved academic visibility and reputation, the MSCA fellowship will undoubtedly make the researcher more competitive for European funding calls upon return to her home institution where she is a tenured researcher.
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