Changes in North Atlantic nitrogen fixation controlled by ocean circulation

Abstract

In the ocean, the chemical forms of nitrogen that are readily available for biological use (known collectively as 'fixed' nitrogen) fuel the global phytoplankton productivity that exports carbon to the deep ocean. Accordingly, variation in the oceanic fixed nitrogen reservoir has been proposed as a cause of glacial-interglacial changes in atmospheric carbon dioxide concentration. Marine nitrogen fixation, which produces most of the ocean's fixed nitrogen, is thought to be affected by multiple factors, including ocean temperature and the availability of iron and phosphorus. Here we reconstruct changes in North Atlantic nitrogen fixation over the past 160,000 years from the shell-bound nitrogen isotope ratio ((15)N/(14)N) of planktonic foraminifera in Caribbean Sea sediments. The observed changes cannot be explained by reconstructed changes in temperature, the supply of (iron-bearing) dust or water column denitrification. We identify a strong, roughly 23,000-year cycle in nitrogen fixation and suggest that it is a response to orbitally driven changes in equatorial Atlantic upwelling, which imports 'excess' phosphorus (phosphorus in stoichiometric excess of fixed nitrogen) into the tropical North Atlantic surface. In addition, we find that nitrogen fixation was reduced during glacial stages 6 and 4, when North Atlantic Deep Water had shoaled to become glacial North Atlantic intermediate water, which isolated the Atlantic thermocline from excess phosphorus-rich mid-depth waters that today enter from the Southern Ocean. Although modern studies have yielded diverse views of the controls on nitrogen fixation, our palaeobiogeochemical data suggest that excess phosphorus is the master variable in the North Atlantic Ocean and indicate that the variations in its supply over the most recent glacial cycle were dominated by the response of regional ocean circulation to the orbital cycles.

Five decades of N2 fixation research in the North Atlantic Ocean

“…These authors also reported a poor relationship between N 2 fixation and dust deposition from a global database (Luo et al, 2012), suggesting that iron is not the ultimate controlling factor of marine diazotrophy on a global basis (Luo et al, 2014). This implies that while empirical evidence suggests dust explains short-term diazotrophic responses, it cannot explain N 2 fixation rates on a basin-scale or over geological time-scales in the NA (Straub et al, 2013).…”

Iron Fertilization of the Subantarctic Ocean During the Last Ice Age

“…Foraminifera-bound δ 15 N was measured at Princeton University following the protocol described in (23,32). In the intervals where foraminifera abundances were adequate, samples were measured in duplicate.…”

Deep-sea coral evidence for lower Southern Ocean surface nitrate concentrations during the last ice age

“…During nitrate uptake, phytoplankton preferentially consume N can be biased due to diagenetic alteration and contamination by foreign N input, with evidence for major artifacts from these processes in sediment records from both lowand high-latitude sites (11,12). The δ 15 N of organic matter bound within diatom and foraminifera microfossils avoids these issues and has been applied to reconstruct the nitrate consumption in the AZ and SAZ of the Southern Ocean, respectively (12,13).…”