Effects of photosymbiosis and related processes on planktic foraminifera-bound nitrogen isotopes in South Atlantic sediments

Abstract. Foraminifera often form symbiotic relationships with photosynthetic algae, providing a host environment and inorganic nutrients in exchange for photosynthetic organic matter from the algal symbiont. To date, the history of this relationship has been studied in paleoceanographic records with the oxygen and carbon stable isotopes of foraminiferal calcite. More recently, photosymbiotic activity has been observed to impact the nitrogen isotope ratio (δ¹⁵N) of foraminiferal tissue and the organic matter incorporated into foraminiferal tests. Dinoflagellate symbiont-bearing species appear to be lower in δ¹⁵N than symbiont-barren species and more similar to their feeding sources, likely due to their retention of low-δ¹⁵N metabolic ammonium and thus a weaker amplitude for the “trophic enrichment factor,” the δ¹⁵N increase per trophic level that is widely observed in food webs. We report new glacial/interglacial foraminifera-bound δ¹⁵N (FB-δ¹⁵N) data from Deep Sea Drilling Program Site 516, located in the subtropical South Atlantic gyre, which contains multiple foraminifera species at adequately high abundance for inter-species comparison of foraminiferal nitrogen, carbon, and oxygen isotopes over a full glacial cycle. Our data show a conserved δ¹⁵N difference of 3–5 ‰ between dinoflagellate-bearing species and the other species, qualitatively consistent with, but greater in amplitude than, the δ¹⁵N difference observed in previous modern ocean and core-top studies. We propose that this greater amplitude is the result of lateral transport of symbiont-barren species into the South Atlantic subtropical gyre, which appears to represent a small region of low thermocline nitrate δ¹⁵N surrounded by regions with higher thermocline nitrate δ¹⁵N. The data point to FB-δ¹⁵N as the best available proxy for dinoflagellate symbiosis. However, they also suggest caution in regions with strong gradients, where species from contrasting environments occur in a single sediment sample.