Nitrogen isotopic composition of planktonic foraminifera from the modern ocean and recent sediments

Haojia Ren, Daniel M. Sigman, Robert C. Thunell and Maria G. Prokopenko

Limnol. Oceanogr., 57(4), 2012, 1011-1024 | DOI: 10.4319/lo.2012.57.4.1011

ABSTRACT: We investigated the controls on the δ15N values of shell-bound organic matter of planktonic foraminifera (foraminifera-bound δ15N, or FB-δ15N). The bulk biomass δ15N of live foraminifera collected from plankton tows at Sta. S in the Sargasso Sea is within ∼1‰ of the FB-δ15N of the same species picked from surface sediments from the low-latitude North Atlantic. The FB-δ15N value in the surface sediments is strongly correlated with the δ15N of thermocline nitrate, the dominant source of new N to the euphotic zone. The three euphotic-zone-dwelling, symbiotic, spinose species, Globigerinoides ruber, Globigerinoides sacculifer, and Orbulina universa, have a FB-δ15N similar to or slightly higher than that of the nitrate supply to the euphotic zone, whereas the deeper-dwelling, non-spinose, and/or asymbiotic forms have higher δ15N. In the Cariaco Basin sediment trap samples, the FB-δ15N of O. universa varies substantially (1.2‰ between the lowest and highest value), in some cases in step with δ15N changes in the bulk sinking N, while the subeuphotic-zone-dwelling, asymbiotic, and/or non-spinose species are generally higher in FB-δ15N and less variable through the time series. The higher and less temporally variable FB-δ15N values of the deeper dwellers are consistent with their partial reliance on subsurface suspended particulate nitrogen, the δ15N of which is elevated and relatively stable over time. As an alternative, possibly additional explanation for the lower FB-δ15N of the euphotic-zone dwellers (despite their likely tendency to feed on high-δ15N zooplankton), the dinoflagellate symbionts may reduce the δ15N elevation of a foraminifera's biomass relative to its diet by reducing the efflux of low-δ15N ammonium.

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