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 using 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 interspecies 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 at Site 516 is the result of the 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. We demonstrate that FB-δ15N provides a clear signal of dinoflagellate endosymbiosis and that it may be able to identify other, weaker endosymbioses (e.g., with chrysophytes or pelagophytes). However, the data also suggest caution in regions with strong gradients, where species from contrasting environments may occur in a single sediment sample.