Journal Article

Increasing subtropical North Pacific Ocean nitrogen fixation since the Little Ice Age

Sherwood, Owen A., Batista, Fabian C., Guilderson, Thomas P., McCarthy, Matthew D., Schiff, John T.

2014

Overview

Despite a reduction in nutrient supply to the North Pacific subtropical gyre, it has undergone a recent increase in nitrogen fixation, and here records of nitrogen isotopes preserved in Hawaiian corals show that this is a trend that could be linked to climate change since the end of the Little Ice Age. Corals record a North Pacific nitrogen boost The deep-sea Hawaiian gold coral Kulamanamana haumeaae is a remarkably long-lived species, sometimes attaining ages of thousands of years. Over the millennia these corals provide a unique geochemical time-series as they convert sinking phytoplankton and other tiny particles into a proteinaceous coral skeleton. Primary productivity in the North Pacific subtropical gyre has increased in recent decades despite a decline in nutrient supply. An ecosystem shift towards nitrogen-fixing plankton communities has been put forward as a possible explanation, but the cause for this shift remains unclear. Owen Sherwood and colleagues use nitrogen isotopic (δ 15 N) records from K. haumeaae corals to establish that the increase in nitrogen fixation had already began around 150 years ago, and that it may have been linked to Northern Hemisphere climate change since the end of the Little Ice Age. The North Pacific subtropical gyre (NPSG) plays a major part in the export of carbon and other nutrients to the deep ocean 1 . Primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters 2 , 3 . It is thought that this apparent paradox can be explained by a shift in plankton community structure from mostly eukaryotes to mostly nitrogen-fixing prokaryotes 2 , 3 , 4 . It remains uncertain, however, whether the plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend 5 . Here we analyse records of bulk and amino-acid-specific 15 N/ 14 N isotopic ratios (δ 15 N) preserved in the skeletons of long-lived deep-sea proteinaceous corals collected from the Hawaiian archipelago; these isotopic records serve as a proxy for the source of nitrogen-supported export production through time. We find that the recent increase in nitrogen fixation is the continuation of a much larger, centennial-scale trend. After a millennium of relatively minor fluctuation, δ 15 N decreases between 1850 and the present. The total shift in δ 15 N of −2 per mil over this period is comparable to the total change in global mean sedimentary δ 15 N across the Pleistocene–Holocene transition, but it is happening an order of magnitude faster 6 . We use a steady-state model and find that the isotopic mass balance between nitrate and nitrogen fixation implies a 17 to 27 per cent increase in nitrogen fixation over this time period. A comparison with independent records 7 , 8 suggests that the increase in nitrogen fixation might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 704/106/829/827 / 82/16 / Amino Acids - chemistry / Animals / Anthozoa - chemistry / Anthozoa - metabolism / Aquatic Organisms - metabolism / Archipelagoes / Climate Change / Climate variability / Community structure / Deep sea / Ecosystem / Environment / Environmental impact analysis / Fixation / Global warming / Hawaii / History, 15th Century / History, 16th Century / History, 17th Century / History, 19th Century / History, 20th Century / History, 21st Century / History, Ancient / History, Medieval / Holocene / Humanities and Social Sciences / Ice ages / Ice Cover / letter / Methods / multidisciplinary / Nitrates - analysis / Nitrogen / Nitrogen Fixation / Nitrogen Isotopes - analysis / Nutrients / Pacific Ocean / Plankton / Plankton - metabolism / Pleistocene / Primary production / Radiometric Dating / Science / Seawater - chemistry / Surface water / Time Factors / Tropical Climate