The last deglaciation was a time of major reorganisation of marine carbon and nutrient pools in which the Southern Ocean was a major dynamical and biogeochemical player. As such, it provides an excellent case study for the testing of novel tracers of nutrient cycling in the past ocean. Here, we present species-specific silicon stable isotope (δ³⁰Si) data for two radiolarian species isolated from IODP Site U1537 in the Atlantic sector of the Southern Ocean (59°S, 41°W; 3713m water depth) across the last deglaciation, i.e. from 42ka to 7ka.

The species we focus on have fundamentally different habitats: Antarctissa denticulata/strelkovi live in the shallow subsurface to surface (< 200m), while Cycladophora davisiana appears to be a deep-dwelling (>200m) species that favours regions with extensive winter sea ice cover. Average δ³⁰Si values across our records are −0.96±0.45‰ and −2.60±0.95‰ (2SD) for A. denticulata/strelkovi and C. davisiana respectively (the large variance reflects a time-varying signal). This enables two first-order findings: firstly, our species-specific data confirm previous observations from mixed radiolarian samples that Southern Ocean radiolaria are isotopically lighter, by 2‰ or more, than those extracted from core-tops or deglacial sediment at lower latitudes. Secondly, there is a large difference in the δ³⁰Si value of the two species we investigate, and the isotopic offset between these two species is variable over our deglacial record (at 0.94 – 2.31‰). This may reflect the combination of a sensitivity to the concentration of ambient dissolved Si as well as its isotopic composition.