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.
