Final Report Summary - PHOXY (Phosphorus dynamics in low-oxygen marine systems: quantifying the nutrient-climate connection in Earth’s past, present and future)
Phosphorus (P) is a key and often limiting nutrient for algae in the ocean. There is a strong positive feedback between P availability, algal growth and low oxygen in the ocean: when you have more algae, this can lead to less oxygen in deeper waters, which, in turn, can decrease the burial efficiency of P in sediments and increase P availability and algal production.
In the PHOXY project, we developed new methods to analyse P in present-day marine sediments to better understand what factors control oxygen-dependent changes in P burial. These methods included a range of micro-analysis techniques that allowed us to conclusively determine the elemental and mineral composition of P-bearing sediment particles. Such analyses are essential, because P is only a minor component of most sediments and the phases thus are difficult to identify. In our work, we specifically focused on sediments from the Black Sea, Baltic Sea, Arabian Sea and a Dutch marine lake (Grevelingen), in areas that suffer from either seasonal or permanent low oxygen conditions. Our results indicate that, besides oxygen concentrations, other factors such as the bottom water salinity and the input of Fe and Mn oxides and calcium-carbonate play a critical role in modulating the recycling of P from sediments. We show, for example, that in the Baltic Sea, lateral transfer of Fe and Mn oxides acts as a key driver of the formation of Fe(II)-P minerals (e.g. vivianite) while in the Black Sea, sinking calcium carbonate shells from surface waters play a critical role in the formation of the calcium-phosphorus mineral apatite. In addition to chemical controls, our work also demonstrates that the specific microbial community can be relevant: we find that in seasonally low oxygen settings, so-called cable bacteria, which carry out long-distance transport of electrons in sediments over scales of centimeters, can strongly modify Fe-P dynamics in the sediment. Finally, we demonstrate that care should be taken when interpreting major changes in P concentrations in sediments as an indicator for environmental change during deposition of the sediment. In some cases, such changes may be due to processes that occurred after deposition linked to, for example, a much later change in organic matter input to the sediment.
Our work reveals consistent evidence for a reduced burial efficiency of P relative to organic carbon in sediments of all low oxygen systems that we studied. In systems where P is limiting for algal growth, this has major environmental consequences. Sediment geochemical records for the Baltic Sea for the past 8000 years , for example, demonstrate that efficient recycling of P under low oxygen conditions always goes hand-in-hand with the occurrence of cyanobacteria, a major group of harmful algae. Efficient recycling of P linked to low oxygen conditions is also deduced from ancient sediments deposited during greenhouse periods in Earth’s past, such as the oceanic anoxic events in the Cretaceous and Jurassic oceans. Our modeling work for the ocean during these periods suggests that redox-dependent P recycling was a prerequisite for the formation of large deposits of organic matter on the seafloor during those times. Finally, our work demonstrates that enhanced recycling of P from sediments may contribute to the expansion of low oxygen conditions in the ocean on a time scale of 100.000 years upon continued global warming.
In the PHOXY project, we developed new methods to analyse P in present-day marine sediments to better understand what factors control oxygen-dependent changes in P burial. These methods included a range of micro-analysis techniques that allowed us to conclusively determine the elemental and mineral composition of P-bearing sediment particles. Such analyses are essential, because P is only a minor component of most sediments and the phases thus are difficult to identify. In our work, we specifically focused on sediments from the Black Sea, Baltic Sea, Arabian Sea and a Dutch marine lake (Grevelingen), in areas that suffer from either seasonal or permanent low oxygen conditions. Our results indicate that, besides oxygen concentrations, other factors such as the bottom water salinity and the input of Fe and Mn oxides and calcium-carbonate play a critical role in modulating the recycling of P from sediments. We show, for example, that in the Baltic Sea, lateral transfer of Fe and Mn oxides acts as a key driver of the formation of Fe(II)-P minerals (e.g. vivianite) while in the Black Sea, sinking calcium carbonate shells from surface waters play a critical role in the formation of the calcium-phosphorus mineral apatite. In addition to chemical controls, our work also demonstrates that the specific microbial community can be relevant: we find that in seasonally low oxygen settings, so-called cable bacteria, which carry out long-distance transport of electrons in sediments over scales of centimeters, can strongly modify Fe-P dynamics in the sediment. Finally, we demonstrate that care should be taken when interpreting major changes in P concentrations in sediments as an indicator for environmental change during deposition of the sediment. In some cases, such changes may be due to processes that occurred after deposition linked to, for example, a much later change in organic matter input to the sediment.
Our work reveals consistent evidence for a reduced burial efficiency of P relative to organic carbon in sediments of all low oxygen systems that we studied. In systems where P is limiting for algal growth, this has major environmental consequences. Sediment geochemical records for the Baltic Sea for the past 8000 years , for example, demonstrate that efficient recycling of P under low oxygen conditions always goes hand-in-hand with the occurrence of cyanobacteria, a major group of harmful algae. Efficient recycling of P linked to low oxygen conditions is also deduced from ancient sediments deposited during greenhouse periods in Earth’s past, such as the oceanic anoxic events in the Cretaceous and Jurassic oceans. Our modeling work for the ocean during these periods suggests that redox-dependent P recycling was a prerequisite for the formation of large deposits of organic matter on the seafloor during those times. Finally, our work demonstrates that enhanced recycling of P from sediments may contribute to the expansion of low oxygen conditions in the ocean on a time scale of 100.000 years upon continued global warming.