Award: OCE-1732139

It is becoming increasingly clear that society needs to understand how the Earth will react to a changing climate. The oceans are key to the response both as a sink for heat, but also in how ocean biogeochemistry will change to mitigate or accelerate change. Insights on the ocean?s response to changing climate can come from an examination of variations in the relative proportions of stable isotopes of a variety of elements in the Earth?s ocean. Relevant to this project is the use of stable isotopes of silicon to reconstruct how key aspects of ocean productivity change through time. The variations are driven by the combined influence of biological fractionation and ocean circulation. Organisms like sponges and diatoms take up dissolved silicon from seawater and use it to produce structural elements like sponge spicules and diatom frustules. Diatoms dominate this process producing 15,000 metric tons of opaline silica skeletons each year across the global ocean. Organisms tend to select for the lighter isotopes of elements causing the silicon isotopic composition of diatoms to differ from that of the dissolved silicon in seawater. The process is called biological fractionation and can be used to diagnose the level of diatom growth that has occurred in an area of the ocean through an examination of the isotopic composition of the local diatoms or the dissolved silicon in the seawater. Moreover, as diatoms die or are eaten, they settle to the sea floor forming a stacked record of past diatom activity in the overlying water. Histories of diatom productivity reconstructed from the Si isotopic composition of diatoms collected from dated sediment cores can reveal how diatom growth has been affected by changing ocean conditions over millennia. Such reconstructions rely on knowledge of how the measured isotopic composition of diatoms comes about. Knowledge of how much dissolved silicon is supplied to diatoms and its isotopic composition are vital components needed to understand the surface conditions that give rise to isotope values measured in sediments. Silicon is suppled through ocean circulation that brings deep waters of the sea into the surface where diatoms grow. The isotopic composition of deep waters appears to be controlled by the interaction between the dissolution of diatoms sinking from the surface and the subduction of surface waters and their associated dissolved silicon. The relevant circulation occurs on grand sales spanning the entire globe and is known as the meridional overturning circulation. One complete circulation takes roughly 1000 years, but this slow process interacts with the rain of diatoms from the surface to set the global distribution of silicon and its isotopes in the sea. Understanding these relationships is key to applying silicon isotopes to climate reconstructions. This project focused on Si isotopes in dissolved silicon in the northeast Pacific along a line of latitude running from Tahiti to Alaska. This area is of particular interest as the north Pacific lies at the end of the meridional circulation and contains the oldest waters that have the highest dissolved silicon concentrations in the sea. One hypothesis as to the origin of the high dissolved silicon concentrations was that hydrothermal inputs from the sea floor create a buoyant plume of water with high dissolved silicon. We set out to test this concept and to evaluate the how the high concentrations affect the interaction between circulation and diatoms in this area. Our data refute the hydrothermal hypothesis. Hydrothermal waters have a uniquely low silicon isotope value which we did not observe. However, during our project a new hypothesis for the formation of the dissolved silicon maximum arose that proposed that the old waters at mid depth in the north Pacific constituted a shadow zone in the general circulation that would allow dissolved silicon from sinking diatoms to accumulate. That idea came from computer modeling, but our measurements fit the model providing the first empirical evidence in support of this new paradigm. In summary, this project developed a new model for how the silicon cycle of the north Pacific interacts with the overturning circulation. These concepts will aid in refining the interpretation of silicon isotope records from the region and provide empirical validation for the shadow-zone circulation concept. These results have been communicated to our fellow scientists at formal scientific meetings. We have also incorporated ideas on stable isotope variations as oceanographic tools for understanding ocean dynamics into outreach programs that bring UCSB marine science research to K-12 schools reaching roughly 20,000 students. Our approach is to make students aware of the diversity of ocean science disciplines in ways that will make them better stewards of the ocean and to excite some about oceanography as a possible career. Last Modified: 01/19/2023 Submitted by: Mark A Brzezinski