Linking silicon isotopic signatures with diatom communities

The use of silicon isotope ratios (expressed as δ³⁰Si) as a paleolimnological proxy in lacustrine systems requires a better understanding of the role of lake processes in setting the δ³⁰Si values of dissolved Si (dSi) in water and in diatom biogenic silica (bSi). We determined the δ³⁰Si of modern dSi (δ³⁰Si_dSi) and bSi (δ³⁰Si_bSi) in three lakes in Lassen Volcanic National Park (LAVO), California (USA), and produced diatom assemblage compositional data from the modern system and from sediment core samples. In the modern systems, we observe the largest magnitude diatom Si isotope fractionations yet reported, at −3.4 and −3.9‰ for Fragilaria dominated samples. Using statistical approaches designed to condense multivariate ecological data, we can deconvolve assemblage-specific Si isotope fractionations from the combined diatom assemblage-δ³⁰Si data. For example, samples dominated by generally deeper water euplanktic species have low δ³⁰Si_bSi values (<−1.10‰). Conversely, samples dominated by shallow water planktic or benthic periphyton have higher δ³⁰Si_bSi values (>−0.14‰). These data suggest that δ³⁰Si records from LAVO lakes reflect species specific Si isotope fractionations and thus act as paleolimnological proxy for the aquatic-habitat of bSi production. Silicon isotope analysis should be coupled with diatom community composition data and other geochemical proxies for the most robust paleolimnological interpretations. We also construct a Si mass-balance for Manzanita Lake based on elemental fluxes. Despite a short residence time of ∼4 months, it is an efficient Si sink: about 30% of inflowing Si is retained in the lake sediments. An entirely independent Si isotope-based estimate agrees remarkably well. Burial fluxes of bSi derived from radiometrically dated sediment cores yield retention rates of about a factor of three higher, which might suggest groundwater is an important term in the lake Si budget.