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Chlorophylls are produced by all photosynthetic organisms and are ideal targets for compound-specific isotopic studies of phytoplankton. In laboratory cultures, the difference between the nitrogen (N) isotope ratio (δ15N value) of chlorophyll and the δ15N value of biomass, known as epor, varies taxonomically, yielding potential applications for studying productivity in modern and ancient environments. Here we take advantage of the annual cyanobacterial bloom in Lake Erie, USA, to demonstrate epor patterns in a natural community. The resulting time series shows that environmental observations are similar to laboratory cultures: predicted epor endmember values range from 4.6‰ to 7.4‰ for eukaryotic algae, and −18‰ to −21‰ for cyanobacteria. Because the range and sensitivity of epor is similar between laboratory and natural settings, the data support the use of epor as a reliable tracer of the relative contributions of cyanobacteria and eukaryotic algae to nutrient utilization and primary production in lacustrine environments.

This study presents new high resolution sedimentary δN records from piston cores collected within and outside the present-day eastern south Pacific oxygen minimum zone along a latitudinal transect from 3.5°S to 15°S. Radiocarbon dating of foraminifera and organic matter show that the cores cover the Holocene and the last deglaciation with high sedimentation rate allowing interpretations at millennial to centennial timescale. High δN values, reaching 10‰ and large amplitude changes, with a magnitude of ~4‰ are observed in the southern part of the studied area during the last 18 ka BP. In contrast, the northern Peruvian cores located on the edge of the OMZ show low δN values varying from 4 to 6‰ with amplitude of only 1‰ during the same time period. δN values decrease in all the studied cores from the last deglaciation to the early Holocene (17 to 8.5 ka BP) and reach a minimum value during the mid-Holocene. The δN variations are attributed to microbial N-loss to N, e.g. denitrification and/or anammox, and the characteristic N-enriched signal that is recorded in the underlying sediments under suboxic to anoxic conditions where denitrifiers thrive. Surprisingly, δN values from cores located within the OMZ show similar values as the more northern cores located outside the OMZ between 5 and 8.5 ka BP. This minimum is not related to local changes in export production, reconstructed from sedimentary organic carbon, total nitrogen and bromine, but appears to be controlled by changes in the ventilation of the area. The low δN values recorded between 8.5 and 5 ka BP are well correlated with more arid conditions developed along the Peruvian margin and an increase of the sea surface temperature gradient along the Peruvian margin and between the West and East Pacific along the equator, implying an intensification of the Hadley circulation and climatic conditions similar to La Niña-like state. Consequently, these mid-Holocene conditions led to greater ventilation of subsurface waters that deepened the Peruvian oxycline then revealing similar conditions as observed today in the northern part of the study area., This work is a contribution of Sonderforschungsbereich 754 “Climate-Biogeochemistry interactions in the tropical ocean” (www.sfb754.de), which is supported by the Deutsche Forschungsgemeinschaft. Financial support was partly provided by French research program LEFE-EVE (INSU, CNRS) to P.M. We thank the anonymous reviewers for comments and suggestions on earlier version of this manuscript.

Bioavailable nitrogen is removed from the oceans in oxygen-deficient benthic and pelagic environments by denitrification. Future warming is predicted to reduce ocean oxygenation and to cause hypoxic regions to expand, potentially accelerating denitrification. A compilation of high-resolution sedimentary nitrogen isotope (delta N-15) records from the eastern tropical Pacific, North Pacific, and the Arabian Sea, and a global multi-site survey are presented as evidence for weak pelagic denitrification at the end of the Pliocene warm period. Mean delta N-15 values increased in the major oxygen minimum zones (OMZs) between 2.1 and 1.5 Ma. Pelagic denitrification strengthened during a period of long term global cooling, despite solubility driven increases in initial oxygen contents of Antarctic intermediate and Subantarctic mode waters ventilating the OMZs. This trend is opposite to the predicted mean trend for a cooling ocean as well as to the observed glacial-interglacial variation. Several alternatives to explain the shift are proposed, including a rise in net respiration, a progressive increase in the ventilation age of the deep ocean associated with million year scale, secular cooling, and a shoaling of the remotely ventilated thermocline to shallow depths corresponding to the zone of peak subsurface respiration. Given no evidence for a net increase in production, we assert that large-scale, climate-driven changes in ocean circulation regulate long timescale variations in the extent of pelagic denitrification. Additional data and modeling are required to fully explain the observations. (C) 2013 Elsevier B.V. All rights reserved.

Nitrogen isotope records of chloropigments can be used to reconstruct photic zone processes. However, the 15N offset between chlorophyll and biomass (epor = δ15Nbiomass − δ15Nchlorophyll) of cyanobacteria and eukaryotes grown in laboratory batch cultures differs significantly and the cause of the difference remains unknown. Here, in four experiments with Synechocystis sp. PCC 6803, values of epor were statistically invariant when cultures were maintained at pH 6.5–9.5 under constant growth conditions in a chemostat. The results suggest that the negative values of epor observed for freshwater cyanobacteria are not artifacts of batch culturing.

Natural variations in the ratios of nitrogen isotopes in biomass reflect variations in nutrient sources utilized for growth. In order to use δ 15 N values of chloropigments of photosynthetic organisms to determine the corresponding δ 15 N values of biomass – and by extension, surface waters – the isotopic offset between chlorophyll and biomass must be constrained. Here we examine this offset in various geologically-relevant taxa, grown using nutrient sources that may approximate ocean conditions at different times in Earth’s history. Phytoplankton in this study include cyanobacteria (diazotrophic and non-diazotrophic), eukaryotic algae (red and green), and anoxygenic photosynthetic bacteria (Proteobacteria), as well as environmental samples from sulfidic lake water. Cultures were grown using N 2 , NO 3 − , and NH 4 + as nitrogen sources, and were examined under different light regimes and growth conditions. We find surprisingly high variability in the isotopic difference (δ 15 N biomass − δ 15 N chloropigment ) for prokaryotes, with average values for species ranging from −12.2‰ to +11.7‰. We define this difference as e por , a term that encompasses diagenetic porphyrins and chlorins, as well as chlorophyll. Negative values of e por reflect chloropigments that are 15 N-enriched relative to biomass. Notably, this enrichment appears to occur only in cyanobacteria. The average value of e por for freshwater cyanobacterial species is −9.8 ± 1.8‰, while for marine cyanobacteria it is −0.9 ± 1.3‰. These isotopic effects group environmentally but not phylogenetically, e.g., e por values for freshwater Chroococcales resemble those of freshwater Nostocales but differ from those of marine Chroococcales. Our measured values of e por for eukaryotic algae (range = 4.7–8.7‰) are similar to previous reports for pure cultures. For all taxa studied, values of e por do not depend on the type of nitrogen substrate used for growth. The observed environmental control of e por suggests that values of e por could be useful for determining the fractional burial of eukaryotic vs. cyanobacterial organic matter in the sedimentary record.

Nitrogen isotopes of chlorins, degradation products of chlorophyll, reflect the isotopic composition of nutrient N utilized by marine phytoplankton communities. Here we show that in sediments of the eastern Mediterranean Pleistocene and Holocene, values of δ15N for chlorins and total nitrogen vary in concert, with a consistent offset of ∼ 5‰ reflecting the fractionation imparted during chlorophyll biosynthesis. Samples from the Integrated Ocean Drilling Program Sites 964 and 969 were analyzed at a sampling resolution of ∼ 4–10 cm, clustered around sapropel events 2, 3, 4 and 5 (∼ 100–170 ka). In low organic content sediments, chlorin values of ∼ 0‰ coincident with total nitrogen values of ∼+ 5‰ indicate that the latter reflects the original biomass and is not a consequence of diagenetic isotope enrichment. In sapropel horizons, the chlorin and total nitrogen values are 5‰ more negative (∼−5‰ and ∼ 0‰, respectively), resembling previously-reported, modern-day water-column particulates (∼ 0‰). We suggest that nutrient conditions in the Eastern Mediterranean correspond to three scenarios and that the similarity between sapropel and modern-day bulk δ15N is coincidental. Organic-poor marl sediments formed under oligotrophic conditions where surface productivity resulted from upwelling of Atlantic-sourced nitrate. Sapropels were characterized by enhanced diazotrophy that was likely fueled by increased riverine P fluxes to surface waters. Present-day conditions are dominated by anthropogenic N sources. These scenarios agree with a model of sapropel formation in which stratification caused by increased fresh-water inputs led to N fixation due to P:N nutrient imbalance. Enhanced production combined with stratification promoted and maintained anoxic deep waters, consequently increasing organic matter preservation. Such a model may be relevant to interpreting other episodes of intense organic matter deposition in past oceans.

Chilean margin sedimentary N isotope records have been the focus of paleoceanographic studies examining the extent of water-column denitrification in the eastern South Pacific in the past. Here we use 15N/14N of nitrate and surface sedimentary N along the Chilean coast to investigate the relative contributions of water-column denitrification and surface nitrate assimilation by phytoplankton to the sedimentary N isotope record. Off northern and central Chile, subsurface enrichment of 15NO3- is associated with the lowest oxygen concentrations and maximum nitrate deficits, the product of water-column denitrification, locally at 21°S and through the transport of denitrified waters to the south. While elevated, the δ15N of pycnocline nitrate shows no distinct trend with distance from the OMZ and is nearly homogenous within the shallow subsurface layer, presumably due to lateral circulation along the margin. Moreover, an isotopically depleted and relatively uniform layer exists within the shallow subsurface, possibly as a result of the remineralization of newly fixed nitrogen that may work to further homogenize the δ15N of the upwelling nitrate. Whereas the high δ15N of sedimentary N in the region is clearly a product of denitrification and its isotopic imprint on nitrate along the margin, the northward increase in sedimentary δ15N from higher southern latitudes also reflects the degree of surface layer nitrate consumption by phytoplankton. The northward increase in sedimentary δ15N corresponds to a regional decrease in the surface nitrate concentration, and isotopic fractionation during nitrate assimilation is apparent in shallow nitrate δ15N. A comparison of the δ15N in shallow subsurface nitrate and sedimentary N suggests that, north of ∼30°S, nearly complete nitrate consumption causes the δ15N of sediments to converge on that of the nitrate supply, such that denitrification should be the dominant signal in downcore δ15N records from these latitudes. Moreover, the lateral homogeneity of the denitrification signal in the subsurface within this region suggests that such records will provide robust reconstructions of denitrification intensity.

Surface sediment diatom-bound δ 15 N along a latitudinal transect of 170°W shows a previously unobserved increase to the South of the Antarctic Polar Front. The southward δ 15 N increase is best explained by the combination of two changes toward the South, a decrease in the isotope effect of nitrate assimilation ( e ) and an increase in the degree of nitrate consumption, both associated with shoaling of the mixed layer into the seasonal ice zone (SIZ). New downcore records show high amplitude changes in diatom-bound δ 15 N during the last ice age, with intervals of higher δ 15 N, including the last glacial maximum, the transition between marine isotope stages 5 and 4, and marine isotope stage 6, while other intervals are similar in δ 15 N to interglacial sediments. Variation in the range of 0–3‰, as seen in previously published records, may be entirely due to changes in e . However, the observed magnitude of the change of 4–10‰ in the three new records and the locations of these records relative to the modern meridional gradient in mixed layer depth appear to require increased nitrate consumption to explain the high- δ 15 N intervals. The new sites are near the modern Southern Antarctic Circumpolar Current Front (SACCF), and one of the sites has been shown to be associated with sporadic summer sea ice during the LGM. As with other Antarctic sites, the available proxy data suggest that they were characterized by lower export production. Based on these and other observations, we propose that the weak southward nitrate decrease in the modern Antarctic surface was a fully developed “nutrient front” in the glacial Antarctic, associated with the SACCF. Both modern ocean and paleoceanographic work is needed to test this hypothesis, which would have major implications for atmospheric CO 2 .

Temporal changes in oceanic denitrification, the bacterial reduction of nitrate under suboxic conditions, highlight the potential importance of N inventory changes and the production of N 2 O on the climate system. At the same time, the cause of the globally observed variation in denitrification remains unclear. High-resolution benthic foraminiferal oxygen isotope and bulk sediment nitrogen isotope records from ODP Site 1234 on the Chile Margin record integrated denitrification changes within the Peru–Chile Upwelling system over the last ∼70 ka. Denitrification changes in the southeast Pacific are coherent with Antarctic climate changes recorded by the Byrd ice core δ 18 O record, and lead northern hemisphere climate events. The southern-hemisphere character of the Chile margin δ 15 N record suggests that episodes of reduced denitrification in the SE Pacific represent times when more oxygen was supplied as the result of changes in the ventilation and preformed nutrient content of Subantarctic Mode Water (SAMW), which forms in the Subantarctic zone of the Southern Ocean and feeds into the low-latitude thermocline.