Periodic Reporting for period 4 - SCrIPT (Stable Chromium Isotopes as a Productivity Tracer)
Berichtszeitraum: 2023-08-01 bis 2025-04-30
The SCriPT project investigates how chromium (Cr) moves through the modern ocean and what this can tell us about past ocean conditions. Chromium isotopes have often been used as a signal of oxygen levels in ancient oceans, but SCriPT shows that this interpretation may be too simple. A key result is that chromium in surface waters is strongly influenced by biological productivity, especially where phytoplankton grow actively. When phytoplankton take up carbon during photosynthesis and some of this organic matter sinks, chromium can be removed from surface waters at the same time. In this sense, the chromium cycle is partly linked to the oceanic carbon cycle, because chromium export can follow the export of organic carbon from the surface to deeper waters. However, once particles sink and degrade in the deep ocean, chromium release does not always follow the same pattern as carbon respiration or nutrient regeneration. This means that chromium is connected to carbon cycling, but not in a simple one-to-one way. The project also shows that sediments and seafloor processes can release chromium back into bottom waters, adding another layer of complexity. In some regions, such as low-productivity parts of the Pacific and Atlantic, water mass mixing and ocean circulation appear to control chromium distributions more than local biology. Overall, SCriPT suggests that chromium records in marine sediments may reflect a combination of biological productivity, carbon export, circulation, sediment processes and oxygen conditions, rather than oxygen alone.
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.