Periodic Reporting for period 3 - SCrIPT (Stable Chromium Isotopes as a Productivity Tracer)
Reporting period: 2022-02-01 to 2023-07-31
The ocean plays an important role in the global carbon cycle. Indeed, the global ocean is taking up about 25 % of anthropogenic CO2 emissions, both associated to physical and biological processes. Yet, the efficacy of these processes in the near future remains uncertain, as the Earth's climate continues to warm. The SCrIPT project aims to quantify the strength of the marine biological carbon pump, a process, which governs the fixation of atmospheric CO2 by phytoplankton and its eventual export and sequestration in the ocean interior using a novel geochemical approach based on the geochemistry of Cr. A second aspect of SCrIPT relates to documenting the sensitivity of the biological carbon pump to climate, through the lens of paleoceanographic reconstructions using marine sediments. Indeed, the Earth's climate has undergone drastic climate fluctuations in the past and geological archives allow documenting the behaviour of the marine carbon cycle under different background conditions to better understand how the system will behave in the future in the face of anthropogenic warming.
The work performed thus far in the context of SCrIPT has aspired to increase the spatial resolution of Cr concentration/stable isotope composition measurement in seawater. Measurements now cover all major oceanic basins. Investigations have also targeted marine sediments (including pore waters) to unravel the geochemical behavior of Cr, as a prerequisite for pale oceanographic reconstructions. These preliminary results provided the necessary constraints that allowed for the first time to implement the geochemistry of Cr in an Earth system model of intermediate complexity (EMIC) in order to gain an improved understanding on the mechanisms that modulate the spatial distribution of Cr in the ocean. Data and model corroborate that the marine geochemistry of Cr is driven, in part, by the biological carbon pump, validating some of the hypotheses that have been outlined in the research proposal.
The stable isotope composition of Cr in seawater has been recognized as one of the most promising tools to quantify marine export production (i.e. the net export and subsequent sequestration of organic carbon to the ocean interior). Given the preliminary nature of our observations, the recognition of our ongoing work can certainly be considered as a significant achievement. If further corroborated, our observations based both on water-column samples (dissolved and particulate fractions) and marine sediments may provide a breakthrough in our ability to better characterize the marine biological carbon pump today but also under different climate regimes of the past.