Overview: We seek to quantify the nitrogen (N) and silicon (Si) diatom isotope proxy signatures produced in the Southern Ocean during the period of maximum diatom production and opal flux to the seafloor and to elucidate external vs. internal controls on these signatures to improve paleoceanographic reconstructions of surface-ocean nutrient cycling. Records of Southern Ocean productivity revealed by the isotopic signature of diatom-bound nitrogen (δ¹⁵N_DB) and by the silicon isotopic composition of diatom frustules (δ³⁰Si_BSi) have documented changes in overturning circulation and iron-stimulated biological production that likely worked together to contribute to carbon exchange between the ocean and the atmosphere. Interpretations for both proxies are often based solely based on expected trends produced by biological fractionation during the uptake of nitrate for δ¹⁵N_DB or of silicic acid for δ 30Si_Bsi. However, new data suggest that these simple fractionation models do not explain the spatial trends completely for either proxy and that the underlying mechanisms driving deviations vary across seasons. Understanding why the sedimentary data deviate from expectations will further the use of the diatom nutrient proxies.

Intellectual Merit: The proposed work will examine 1) the relationship between δ¹⁵N_DB and δ³⁰Si_BSi and surface nutrient uptake during a Southern Ocean spring bloom, 2) potential primary controls on the spatial variability observed for the relationships between diatom-bound nutrient isotope proxies, macronutrients, and biomass, and 3) possible alteration to the diatom proxy signals by processes occurring during their transit to the sediment. The effort builds off a comprehensive, field-based ground-truthing effort examining the relationship amongst the N and Si isotopic composition of surface nutrients, particles, and surface sediments during the late summer across the Southern Ocean. The meridional variability in suspended particle δ¹⁵N_DB values disagreed with the expected trend based on meridional patterns in surface ocean nitrate and with surface sediment δ¹⁵N_DB values potentially driven by the use of recycled nitrogen versus nitrate in late summer. Additionally, the silicon isotope results showed large, unexpected latitudinal gradients in the relationship between the dissolved and the biogenic phases that appeared driven by circulation rather than by changes in biological fractionation. Modeling suggested that, similar to the discrepancies for N isotopes, deviations for Si isotopes were related to the late season sampled driven by strong silicic acid depletion. Additional factors will be explored for the first time. Combining isotope and novel fluorescent labeling techniques the effect revealed in laboratory studies that some heavily silicified diatoms have stronger fractionation for both N and Si isotopes will be field-tested during blooms when lightly silicified forms proliferate over heavier silicified species. Isotope effects due to the dissolution of frustules in the water column also remain unresolved. The influence of the loss of lightly silicified taxa and of fractionation during the erosion of frustules on the fidelity of the sedimentary record will be evaluated. These potential drivers of isotopic variations will be addressed through field sampling during the primary growing season coupled with both shipboard and shore-based laboratory experiments.

Broader Impacts: Diatoms are heralded as passive surface-ocean recorders of the outsized role of the Southern Ocean in regulating atmospheric pCO₂. The proposed work will address gaps in understanding of how N and Si isotope proxies record surface nutrient conditions, including an examination of the effects of physiological adjustment of frustule silicification and alteration during sedimentation as potential influences on the isotope signals. The results will improve reconstructions of nutrient drawdown in the past, highlight optimal depositional conditions for robust reconstructions, and improve our understanding of observed spatial and temporal variability in existing diatom N and Si isotope records. This work will ultimately improve our understanding of global-scale climate change in the past. It will foster the careers of two early career scientists, impact education through training of undergraduate and graduate students, and improve instruction through the incorporation of data and results into Robinson’s & Brzezinski’s graduate oceanography courses and a Rhode Island School of Design research collaboration. The products of our collaboration with RISD will also be publicly displayed online and at events. We will broaden participation in STEM at URI through our participation in GSO’s REU, the SURFO program, as well as prioritizing underrepresented groups in recruitment for the graduate student position and at UCSB through the REEF program that focuses on underserved K-12 students.

This project involves field work in Antarctica.