Dissolved organic nitrogen (DON) is likely comprised of multiple compound classes with varying reactivities and turnover times, resulting in numerous roles in ocean biogeochemistry. Here, we present measurements of total DON and solid-phase extracted DON (SPE-DON) concentrations and delta 15N values from sampling sites across the global ocean. An optimized SPE protocol was developed to maximize total DON recovery, with a recovery of 41.0 +/- 9.4% for surface DON across the global ocean and 56.6 +/- 11.2% for deep water DON from the Sargasso Sea. SPE-DON concentrations were 1.8-1.9 mu M across most sampling sites, in contrast to greater variation in total DON concentration (4.0-5.9 mu M). However, in the equatorial upwelling zones, SPE-DON concentrations were slightly (0.3-0.4 mu M) higher than in other regions. The delta 15N of total DON in surface waters correlated well with the delta 15N of nitrate supplied to the euphotic zone from the subsurface. SPE-DON delta 15N was also correlated with nitrate delta 15N, but SPE-DON delta 15N values were confined to a narrower range compared to those of total DON. The combined concentration and delta 15N data indicate that while SPE-DON is biased toward long-lived DON, it still retains some reactive components introduced through regional inputs in the upper ocean, and even some of its longer-lived components may be labile on the timescales of deep ocean circulation. The longer average turnover time of SPE-DON suggests that greater molecular polarity and/or charge directly increase or are otherwise correlated with the biogeochemical lability of different DON pools.

While the ocean's photosynthetic production of organic matter rivals that on land, a combination of heterotrophy and sinking prevents significant accumulation of particulate organic matter (POM) in open ocean surface waters. The origins and fates of POM in ocean surface waters are unclear, in part due to the dominance of nonliving, altered material. From the natural nitrogen isotopic composition of chlorophyll and its degradation products, we estimate the fraction of particles from eukaryotic vs. prokaryotic phytoplankton. In subtropical gyres and along the eastern North Pacific margin, the eukaryotic-to-prokaryotic ratio in particles matches that of living phytoplankton. However, in the North Atlantic outside its subtropical gyre, particles have a lower eukaryotic-to-prokaryotic ratio than do the living phytoplankton. This discrepancy at least partly arises from preferential sinking of eukaryotic biomass, consistent with the canonical but disputed paradigm that cyanobacteria disproportionately fulfill the energetic demands of the upper ocean microbial community while eukaryotes drive export production. The prokaryotic bias in surface ocean particles may also result from slow decomposition of specific components of prokaryotic biomass, a possible bottleneck in the ocean's microbial loop. The different fates of organic matter produced by eukaryotic and prokaryotic phytoplankton affect the productivity of the surface ocean, carbon export to the interior, and the signals recorded in deep-sea sediments.

The Great Atlantic Sargassum Belt first appeared in 2011 and quickly became the largest interconnected floating biome on Earth. In recent years, Sargassum stranding events have caused substantial ecological and socio-economic impacts in coastal communities. Sargassum requires both phosphorus (P) and nitrogen (N) for growth, yet the primary sources of these nutrients fuelling the extensive Sargassum blooms remain unclear. Here we use coral-bound N isotopes to reconstruct N 2 fixation, the ultimate source of the ocean’s bioavailable N, across the Caribbean over the past 120 years. Our data indicate that changes in N 2 fixation were primarily controlled by multidecadal and interannual changes in equatorial Atlantic upwelling of ‘excess P’, that is, P in stoichiometric excess relative to fixed N. We show that the supply of excess P from equatorial upwelling and N from the N 2 fixation response can account for the majority of Sargassum variability since 2011. Sargassum dynamics are best explained by their symbiosis with N 2 -fixing epiphytes, which render the macroalgae highly competitive during strong equatorial upwelling of excess P. Thus, the future of Sargassum in the tropical Atlantic will depend on how global warming affects equatorial Atlantic upwelling and the climatic modes that control it.

The nitrogen (N) isotopic composition of coral tissue provides insight into N sources and cycling on reefs, and coral skeleton-bound organic matter (CS-δ15N) can extend these insights into the past. Across the Bermuda platform, we measured the δ15N of four coral species and their potential N sources, as well as an asymbiotic filter feeder as a comparative heterotroph and benthic macroalgae as a comparative autotroph. Organisms and organic N pools from the coral reefs exhibit a δ15N increase toward the Bermuda coast, likely due to anthropogenic N inputs. At all sites, the δ15N of bulk coral tissue is consistent with corals feeding dominantly on zooplankton-sized organic matter and some smaller suspended particulate N. The corals lack the trophic δ15N elevation that characterizes serpulids; this is consistent with internal recycling and retention of low-δ15N metabolic N by symbiont-bearing corals. The data are inconsistent with corals’ reliance on the dissolved inorganic N used by macroalgae at the same sites. Among coral species, two species with smaller polyps (1-2 mm) have ~1‰ lower bulk tissue δ15N than two counterparts with larger polyps (5-10 mm), perhaps due to differences in food source. Taxon-specific δ15N differences are also observed between coral tissue and skeleton-bound N, with larger differences in the two small-polyp species. In net, however, CS-δ15N mean values and spatial gradients were similar in the four species studied.

The regions near the Antarctic Peninsula in the Southern Ocean are highly productive, with notable phytoplanktonic blooms in the ice-free season. The primary productivity is sustained by the supply of nutrients from convective mixing with nutrient-rich subsurface waters, which promotes rapid phytoplankton growth as the sea ice melts in spring and summer. Surface waters are marked by the contrast between the warmer Drake Passage and the colder Weddell Sea, and seasonal duration of sea ice cover varies accordingly. Sea ice exerts multiple controls over primary production, by shading the light entering the ocean and stratifying the upper ocean with freshening by ice melt. However, the interaction between sea ice and productivity remains poorly characterized because satellites are unable to quantify biomass in partially ice-covered ocean, and direct measurements are too scarce to characterize the seasonally varying productivity. Here we evaluate productivity by assessing removal of nitrate from surface waters by biological nutrient utilization and study the associated change in δ15N of nitrate. We use a combination of bottle samples and in situ nitrate measurements from published databases, completed by two transects with isotopic measurements. The timing of sea ice melt date conditions the initiation of nitrate drawdown, but the annual minimum of nitrate only weakly correlates with sea ice concentration. As previously reported, we observe that δ15N of nitrate increases with nitrate depletion. Interestingly, the lowest nitrate depletion and δ15N values are found in the central region of N–S transects, where intermediate temperature and sea ice conditions prevail. Deeper mixing in waters that passed through the northern Bransfield Strait may explain higher nitrate concentration due to both a greater nitrate concentration at the beginning of the growth season and reduced productivity under light limitation in deeply mixed waters, confirmed by nitrogen isotope modeling. This highlights the importance of oceanographic controls on productivity patterns in sea ice regions in the Southern Ocean.

Foraminifera often form symbiotic relationships with photosynthetic algae, providing a host environment and inorganic nutrients in exchange for photosynthetic organic matter from the algal symbiont. To date, the history of this relationship has been studied in paleoceanographic records using the oxygen and carbon stable isotopes of foraminiferal calcite. More recently, photosymbiotic activity has been observed to impact the nitrogen isotope ratio (δ15N) of foraminiferal tissue and the organic matter incorporated into foraminiferal tests. Dinoflagellate symbiont-bearing species appear to be lower in δ15N than symbiont-barren species and more similar to their feeding sources, likely due to their retention of low-δ15N metabolic ammonium and thus a weaker amplitude for the “trophic enrichment factor”, the δ15N increase per trophic level that is widely observed in food webs. We report new glacial–interglacial foraminifera-bound δ15N (FB-δ15N) data from Deep Sea Drilling Program Site 516, located in the subtropical South Atlantic gyre, which contains multiple foraminifera species at adequately high abundance for interspecies comparison of foraminiferal nitrogen, carbon, and oxygen isotopes over a full glacial cycle. Our data show a conserved δ15N difference of 3 ‰–5 ‰ between dinoflagellate-bearing species and the other species, qualitatively consistent with, but greater in amplitude than, the δ15N difference observed in previous modern ocean and core-top studies. We propose that this greater amplitude at Site 516 is the result of the lateral transport of symbiont-barren species into the South Atlantic subtropical gyre, which appears to represent a small region of low thermocline nitrate δ15N surrounded by regions with higher thermocline nitrate δ15N. We demonstrate that FB-δ15N provides a clear signal of dinoflagellate endosymbiosis and that it may be able to identify other, weaker endosymbioses (e.g., with chrysophytes or pelagophytes). However, the data also suggest caution in regions with strong gradients, where species from contrasting environments may occur in a single sediment sample.

Abstract Nitrous oxide (N2O) is an important greenhouse gas which destroys the ozone in the stratosphere. Primary sources of atmospheric N2O are nitrification and denitrification in terrestrial soils and the ocean, and the main sink is photolysis in the stratosphere. Studies have mostly focused on the climate‐related response of N2O during glacial‐interglacial periods. However, its mechanism of variation during the Holocene remains unclear. We present a high‐resolution N2O record from the South Pole Ice (SPICE) core covering the Holocene epoch. The millennial‐scale N2O trend agrees with existing records. We constructed a Holocene composite consisting of the new N2O measurements in SPICE and existing records from other ice core sites. The N2O composite reveals four distinct periods of N2O variation during 11.5–10.0 ka, 10.0–6.2 ka, 6.2–2.2 ka, and 2.2–1.4 ka, including two maxima in 11.0–10.0 ka and 3.0–2.2 ka and minima in 8.8–6.2 ka and approximately 1.4 ka. Apart from these, our new high‐resolution record from SPICE shows a short‐term N2O decrease around 2.8 ka which is not observed in other records possibly due to lower sample resolution and/or higher age smoothing. Comparison of our new Holocene N2O composite with the paleo‐proxy records suggests the plausible linkage of major monsoon (Asian, North African, South and North American, and Australian‐Indonesian monsoon) and upwelling (Arabian Sea and Eastern Tropical South Pacific) regions in regulating the atmospheric N2O during the Holocene.

The nitrogen isotopes of the organic matter preserved in fossil fish otoliths (ear stones) are a promising tool for reconstructing past environmental changes. We analyzed the 15 N/ 14 N ratio (δ 15 N) of fossil otolith-bound organic matter in Late Cretaceous fish otoliths (of Eutawichthys maastrichtiensis , Eutawichthys zideki and Pterothrissus sp.) from three deposits along the US east coast, with two of Campanian (83.6 to 77.9 Ma) and one Maastrichtian (72.1 to 66 Ma) age. δ 15 N and N content were insensitive to cleaning protocol and the preservation state of otolith morphological features, and N content differences among taxa were consistent across deposits, pointing to a fossil-native origin for the organic matter. All three species showed an increase in otolith-bound organic matter δ 15 N of ~4‰ from Campanian to Maastrichtian. As to its cause, the similar change in distinct genera argues against changing trophic level, and modern field data argue against the different locations of the sedimentary deposits. Rather, the lower δ 15 N in the Campanian is best interpreted as an environmental signal at the regional scale or greater, and it may be a consequence of the warmer global climate. A similar decrease has been observed in foraminifera-bound δ 15 N during warm periods of the Cenozoic, reflecting decreased water column denitrification and thus contraction of the ocean’s oxygen deficient zones (ODZs) under warm conditions. The same δ 15 N-climate correlation in Cretaceous otoliths raises the prospect of an ODZ-to-climate relationship that has been consistent over the last ~80 My, applying before and after the end-Cretaceous mass extinction and spanning changes in continental configuration.

The proportionality of oxygen-to-nitrogen isotope effects ( 18 ε/ 15 ε) is used as a key isotopic signature of nitrogen cycling processes in the environment. Dissimilatory nitrate reduction is observed to have an 18 ε/ 15 ε proportionality of ~0.9 in marine and ~0.6 in freshwater/terrestrial ecosystems. The origins of this difference are uncertain, with both geochemical and biological factors conceivably at play. One potential factor is variation in the isotope effect of nitrate reduction among different forms of the nitrate reductase enzyme. NarG nitrate reductases are observed to typically have an 18 ε/ 15 ε of ~0.9. However, a recent study uncovered an exception, with Bacillus NarG enzymes having an 18 ε/ 15 ε proportionality of ~0.6. This provides an opportunity to investigate genetic controls on 18 ε/ 15 ε. Furthermore, this atypical NarG signature also raises the question of whether intrinsic isotope signatures can evolve as the enzymes that produce them accumulate mutations through time. Here, we present data from site-directed mutagenesis experiments of key NarG residues, which suggest that the distinct Bacillus 18 ε/ 15 ε cannot be caused by single mutations alone and is potentially uncommon in nature. Variation in the intrinsic isotope effects of an enzyme through time may thus require more extensive evolutionary changes.

The global nitrogen cycle has been severely altered, with human interference resulting in more than double the amount of bioavailable nitrogen being released into ecosystems [1] . Introducing excess nitrogen into ecosystems has many negative environmental impacts including widespread eutrophication, ozone depletion, and acidification [1] . To predict ecosystem impacts and develop mitigation strategies to nitrogen pollution, a detailed understanding of the nitrogen cycle is required. Denitrification, or the conversion of bioavailable nitrate into inert N 2 gas, is a key release lever in the nitrogen cycle. The first step of this process, nitrate reduction to nitrite, produces an isotopic signature that researchers use to identify and quantify nitrogen removal rates in ecosystems. However, significant variation in the magnitude of the isotope fractionation for nitrate reduction ( 15 ε NR ) has been observed in both natural and laboratory settings [ 2 ] . It has been previously shown that microbial growth rates can impact 15 ε NR , with increasing growth rates correlating to increased magnitude of 15 ε NR [ 2 ] . However, this research has only been conducted in bacteria that use the membrane bound NarG enzyme to reduce nitrate. Recent work has indicated the potential importance of NapA (located in the periplasm of the cell) mediated nitrate reduction in ecosystems [ 3 ] , and so there is a need to characterize its isotopic fractionation under different growth rates as well. Therefore, we grew Shewanella loihica , a denitrifier that uses only NapA, at different growth rates in chemostat culture. We collected isotopic and proteomic samples to uncover what changes to microbial physiology may cause changes to 15 ε NR . Our preliminary data suggests NapA mediated nitrate reduction has the opposite trend of what has been previously

Nitrous oxide (N2O) is an important greenhouse gas with sources that respond to the biogeochemical processes on land and ocean. The major sources of atmospheric N2O are nitrification and denitrification occurring in terrestrial soils and the ocean. Here we present a new high-resolution atmospheric N2O record obtained from South Pole Ice (SPICE) core site covering the Mid- to Late Holocene (since ~5.5 ka). The N2O analysis was performed in a specialised wet extraction facility installed at Seoul National University that used small ice samples (<20 g) to yield a high precision (average standard deviation of ~1ppb) record. The new N2O data agree well with existing records on the millennial scale and reveal new details on the multi-centennial scale. Our results show a progressive increase in atmospheric N2O during 5.5 to 3.2 ka which correlates well with the increase of marine denitrification around the Arabian Sea (AS) and Peru-Chile Margin (PCM) as well as Indian monsoon precipitation around the same period. A local minimum in N2O is observed around 2.8 ka, possibly related to a sudden decrease in Western Tropical South (WTS) Pacific sea surface temperature and increased La-Nina like conditions which may have supressed denitrification along PCM. These conditions may have further influenced the monsoons and reduced denitrification in land soils. Our record also shows a local N2O maximum around 2.2 ka which may correspond to relaxed La-Nina like conditions around WTS Pacific. Subsequently, the N2O further dropped to attain a pronounced minimum around 1.4 ka. Similar N2O minima are also observed in Styx (Antarctica) and NEEM (Greenland) ice core records, demonstrating the robustness of the signals.

Improved knowledge of greenhouse gas-climate feedbacks is required to understand past and future climate changes. Atmospheric nitrous oxide (N2O) is of concern for its potential role in global warming and future stratospheric ozone destruction. Existing ice core N2O records for the Holocene have not been sufficiently consistent to allow an examination of small changes on sub-millennial time scales. Here, we present new high-resolution and high-precision N2O records obtained from the Greenland NEEM (North Greenland Eemian Ice Drilling) and Antarctic Styx Glacier ice cores. Our reconstruction shows, for the first time, a centennial-scale variability of ~10 ppb during the last 2000 years. Comparisons with proxy records suggest that centennial- to millennial-scale variations in N2O are driven, to a large extent, by changes in tropical and subtropical land hydrology and marine productivity.

Greenhouse gases (GHGs) trapped in ice wedges may provide useful information on biogeochemical environments in ground ice. Previous studies have reported highly elevated CO 2 and CH 4 mixing ratios in ice wedges. However, N 2 O mixing ratios in ice wedges remain unknown. Here, we present CO 2 , CH 4 and N 2 O mixing ratios in bubbles and plausible mechanisms of GHG formation for two lakeside ice wedges at Cyuie village near Yakutsk. The CO 2 gas age corresponds to the Last Glacial Maximum (18–19 ka). The δ(N 2 /Ar) values and bubble shapes indicate that the ice wedges formed by dry snow compaction rather than snowmelt water refreezing, while the δ 18 O and δD values of the ice indicate changes in the source area location and/or the climate during the Last Glacial Maximum. Using a dry extraction method, we obtained gas mixing ratios of 7–13% CO 2 , 5–130 ppm CH 4 and 100–5000 ppb N 2 O. The δ(O 2 /Ar) values imply that most of the O 2 was consumed by biological respiration. The CH 4 is negatively correlated with N 2 O and CO 2 . The N 2 O might have inhibited CH 4 production.

Atmospheric nitrous oxide (N2O) is a greenhouse gas and ozone-depleting substance whose emissions are substantially perturbed by current human activities. Although air trapped in polar ice cores can provide direct information about N2O evolution, analytical precision was not previously sufficient for high temporal resolution studies. In this work, we present a highly improved analytical technique with which to study N2O concentrations in ancient-air-trapped ice cores. We adopt a melt-refreezing method to extract air and use a gas chromatography-electron capture detector (GC-ECD) to determine N2O concentrations. The GC conditions are optimized to improve the sensitivity for detecting N2O. Retrapped N2O in ice during the extraction procedure is precisely analyzed and corrected. We confirmed our results using data from the Styx Glacier ice core in Antarctica by comparing them with the results of a dry-extraction method. The precision estimated from the pooled standard deviation of replicated measurements of the Styx ice core was 1.5 ppb for ∼20 g of ice, a smaller sample of ice than was used in previous studies, showing a significant improvement in precision. Our preliminary results from the Styx Glacier ice core samples have the potential to define small N2O variations (a few parts per billion) at centennial time scales.

Reconstruction of the long-term surface temperature history in Antarctica is important for a better understanding of human-induced climate changes, especially since the Industrial Revolution. We present here a surface temperature history spanning the last century at Styx Glacier, located on the eastern coast of northern Victoria Land, which is reconstructed using borehole logging data. Our results indicate that surface temperatures in the 20th century were 1.7 +/- 0.4 degrees C higher than the long term averages over 1600-1900 Common Era, indicating regional warming over the eastern coast of northern Victoria Land. However, we found no evidence for significant warming across the northern Victoria Land since the mid-20th century. A global reanalysis as well as the reconstruction of proxy records demonstrate that the climate in this region was more affected by changes in the Southern Hemisphere Annular Mode than in the Amundsen-Bellingshausen Sea Low. Plain Language Summary The western coast of the Ross Sea, northern Victoria Land, is one of several regions around the world where the temperature history is highly uncertain. Here we provide a temperature reconstruction using borehole logging data from Styx Glacier. The reconstructed temperature history indicates that the surface temperature at Styx Glacier in the 20th century is higher than in previous centuries, although there is no significant trend since similar to 1950s. The lack of recent warming trend off the western Ross Sea is in contrast to the warming in the Antarctic Peninsula and West Antarctica.

Understanding processes controlling the atmospheric methane (CH4) mixing ratio is crucial to predict and mitigate future climate changes in this gas. Despite recent detailed studies of the last ∼ 1000 to 2000 years, the mechanisms that control atmospheric CH4 still remain unclear, partly because the late Holocene CH4 budget may be comprised of both natural and anthropogenic emissions. In contrast, the early Holocene was a period when human influence was substantially smaller, allowing us to elucidate more clearly the natural controls under interglacial conditions more clearly. Here we present new high-resolution CH4 records from Siple Dome, Antarctica, covering from 11.6 to 7.7 thousands of years before 1950 AD (ka). We observe four local CH4 minima on a roughly 1000-year spacing, which correspond to cool periods in Greenland. We hypothesize that the cooling in Greenland forced the Intertropical Convergence Zone (ITCZ) to migrate southward, reducing rainfall in northern tropical wetlands. The inter-polar difference (IPD) of CH4 shows a gradual increase from the onset of the Holocene to ∼ 9.5 ka, which implies growth of boreal source strength following the climate warming in the northern extratropics during that period.

Because blue ice forms where ice flows up to the surface and ablates by sublimation, we may get old ice on the surface. We can also obtain large amount of ice samples for same ages on the surface because the same age ice horizontally extends on the surface and vertical age change is small. This paper reports the exploration on the blue ice at Victoria Land, East Antarctica in austral summers of 2014-2017, and preliminary results of geochemical analysis for the ice. The surface blue ice samples collected from the Elephant Moraine in 2014-2015 show scattered CO2 and CH4 concentrations within neighboring ice, and very low total air content of 0.03-0.04 ml/g ice, indicating alteration along the crack surfaces. The mean deuterium excess was low at -3.4%, very likely due to isotopic alteration. On the other hand, the 10 m core collected from the Tarn Flat in 2015-2016 shows relatively constant greenhouse gas concentration levels of pre-industrial interglacials and the water isotope ratios were observed to gradually increase upward. To obtain unaltered samples at the Elephant Moraine, 15 blue ice cores including 10 m cores were drilled in 2016-2017. Furthermore, ground penetration radar (GPR) survey for stratigraphic analysis and measurement of 3-dimensional extensity of a tephra layer were conducted in the field. GPR results show the bed rock laid down to 450-800 m from the surface and we found tephra layer tilted with an angle of 20-59 degrees in the surface. For the future study, we urgently need noble gas isotope analysis for accurate ages as well as shallow or intermediate-depth drilling to obtain unaltered samples with continuous ages.