Abstract:
We present the first nitrogen isotope (?15N. measurements of living planktic foraminifera, paleoceanographically important zooplankton, from the Southern Ocean. This study builds on previous work in the subtropical ocean (near Bermuda. where, under conditions of complete nitrate consumption, foraminifera record the annual average ?15N of nitrate supplied to the euphotic zone. Time-series measurements from the Bermuda region demonstrate that foraminifera ?15N can likely also record seasonal changes in upper ocean ammonium recycling. In the polar ocean, we expect regional and seasonal variations in the degree of nitrate consumption to be an additional influence on foraminifera ?15N. Here, our focus is on investigating the regionally- and species-specific signals captured by foraminifera in Subantarctic waters. Nine foraminifera species were collected from upper ocean net tows in winter 2015 (between Cape Town and the Antarctic sea-ice edge at 56.4°S, 0.3°E. and late summer 2016 (between Cape Town and Marion Island at 46.9°S, 37.7°E.. Consistent with expectations from the progressive, northward drawdown of nitrate by phytoplankton (and their preferential removal of the lighter isotope, ?14N. across the Southern Ocean, the bulk tissue ?15N of most foraminifera species mirrors the northward rise in nitrate and suspended particulate organic N ?15N in late summer. In winter, however, tissue ?15N exhibits no clear north-south trend, and is significantly higher than in late summer (by 3-4‰.. Here, we explore the roles of nutrient dynamics (nitrate supply, the degree of nitrate consumption, and ammonium cycling. as well as food availability and species-specific characteristics (e.g., depth habitat, feeding preferences, and symbiotic activity. in explaining these observations. These enquiries are essential for the reliable interpretation of fossil foraminifera-bound ?15N from sedimentary archives, and for our understanding of the biological effect on global carbon and climate cycles. - Abstract as displayed in the - Abstract booklet. There was no presentation.
