Exploring Silicon Isotope Fractionation by Silicoflagellates: Results from a KOSMOS Experiment off Peru

Abstract. The Peruvian Upwelling is known for its exceptionally high surface water productivity and the presence of one of the world’s largest Oxygen Minimum Zones. The upwelling of silicate-rich subsurface waters typically supports diatom-dominated primary productivity in this region. However, warmer surface waters and subsequent changes in stratification and nutrient supply can cause a shift in plankton communities from diatoms to dinoflagellates and silicoflagellates, which affects the silicon (Si) and carbon (C) cycles.

In 2017, we investigated the Si cycle in a field experiment off the coast of Peru. Pelagic mesocosms (~55,000 L) were deployed for 50 days from February to April to simulate upwelling conditions, which coincided with a coastal El Niño. This unique setting allowed us to study the evolution of stable silicon isotopes in seawater (δ³⁰Si_dSi) and its direct comparison to the produced biogenic material (δ³⁰Si_bSi) without the influence of unaccountable water mass mixing. On day 12, approximately 40 % of the surface water of the mesocosms was replenished with nitrate-depleted deep water (low N:Si and N:P ratios), which strongly influenced the phytoplankton community. Prior to the addition of the deep water, the phytoplankton community was dominated by diatoms but shifted towards a pronounced dominance of flagellates, including silicoflagellates. At the beginning of the experiment, when diatoms dominated the phytoplankton community, the δ³⁰Si_dSi distribution in the surface water (+1.4 ‰ to +2.5 ‰) was within the same range as observed in previous seawater studies in the Peruvian upwelling. After deep water addition, low N:Si (0.02 to 0.2 mol/mol), strongly deviating from the preferred 1:1 ratios for diatoms, favored silicoflagellate (and dinoflagellate) growth and resulted in higher δ³⁰Si_dSi values (up to +4.1 ‰) in the surface waters. The strong increase in δ³⁰Si_dSi was associated with low δ³⁰Si_bSi values (-0.26 to +0.65 ‰) caused by high fractionation factors of stable silicon isotopes between seawater and silicoflagellates. For the first time, the field experiment allowed us to determine the Si isotope fractionation factor for silicoflagellates (ε³⁰_silico = -3.63 ‰), which is remarkably high compared to diatoms (-1.1 ‰) and offers a novel tool to study changes in the present and past marine silicon cycle.