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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 (2-4). 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 [sup.15]N/[sup.14]N isotopic ratios ([δ.sup.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.sup.15]N decreases between 1850 and the present. The total shift in [δ.sup.15]N of -2 per mil over this period is comparable to the total change in global mean sedimentary [δ.sup.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. Gly 0.99 0.31 1.00 0.13 Glx 0.92 0.43 0.88 0.23 -0.01 0.03 Gly 0.36 0.005# 0.78 Glx 0.20 0.049# 0.46 0.001# 0.00 0.880, Recent satellite observations have shown that globally, the permanently oligotrophic subtropical gyres are expanding at a rate of 1% to 4% per year, generally commensurate with global decreases in net [...]

The nitrogen (N) isotopic composition (δ 15 N) of bulk sedimentary N (δ 15 N bulk ) is a common tool for studying past biogeochemical cycling in the paleoceanographic record. Empirical evidence suggests that natural fluctuations in the δ 15 N of surface nutrient N are reflected in the δ 15 N of exported planktonic biomass and in sedimentary δ 15 N bulk . However, δ 15 N bulk is an analysis of total combustible sedimentary N, and therefore also includes mixtures of N sources and/or selective removal or preservation of N-containing compounds. Compound-specific nitrogen isotope analyses of individual amino acids (δ 15 N AA ) are novel measurements with the potential to decouple δ 15 N changes in nutrient N from trophic effects, two main processes that can influence δ 15 N bulk records. As a proof of concept study to examine how δ 15 N AA can be applied in marine sedimentary systems, we compare the δ 15 N AA signatures of surface and sinking POM sources with shallow surface sediments from the Santa Barbara Basin, a sub-oxic depositional environmental that exhibits excellent preservation of sedimentary organic matter. Our results demonstrate that δ 15 N AA signatures of both planktonic biomass and sinking POM are well preserved in such surface sediments. However, we also observed an unexpected inverse correlation between δ 15 N value of phenylalanine (δ 15 N Phe ; the best AA proxy for N isotopic value at the base of the food web) and calculated trophic position. We used a simple N isotope mass balance model to confirm that over long time scales, δ 15 N Phe values should in fact be directly dependent on shifts in ecosystem trophic position. While this result may appear incongruent with current applications of δ 15 N AA in food webs, it is consistent with expectations that paleoarchives will integrate N dynamics over much longer timescales. We therefore propose that for paleoceanographic applications, key δ 15 N AA parameters are ecosystem trophic position, which determines relative partitioning of 15 N into source AA versus trophic AA pools, and the integrated δ 15 N AA of all common protein AA (δ 15 N THAA ), which serves as a proxy for the δ 15 N of nutrient N. Together, we suggest that these can provide a coupled picture of regime shifts in planktonic ecosystem structure, δ 15 N at the base of food webs, and possibly additional information about nutrient dynamics. [ABSTRACT FROM AUTHOR]