Download or read book Nitrogen Isotopes of Amino Acids in Marine Sediment a Burgeoning Tool to Assess Organic Matter Quality and Changes in Supplied Nitrate Delta 15N written by Wilson Sauthoff and published by . This book was released on 2016 with total page 98 pages. Available in PDF, EPUB and Kindle. Book excerpt: Compound-specific nitrogen (N) isotopic analysis (delta15N) of amino acids (AA), or CSI-AA, is a novel approach to understand N cycling. We expand upon initial observations at a productive, hypoxic margin to provide insight into source and transformation of sedimentary organic nitrogen from varied depositional conditions in a complex N cycling zone, the eastern tropical Pacific. delta15NAA patterns are generally well preserved, matching original mixed plankton inputs with some evidence for microbial degradation. There is appreciable delta15N offset between total N (bulk) and total AA N, where AA N is enriched. Source AA, previously understood to record changing baseline delta15N, are fundamentally different in marine sediments. We use modeling to rigorously test whether the total AA delta15N instead tracks changing baseline delta 15N. Lastly we employ this new understanding of delta15N AA in sediments to compare with published whole sediment (delta 15Nbulk) records, demonstrating the utility and wealth of information provided by CSI-AA.

Download or read book New Methods for Nitrogen Stable Isotope Measurements of Amino Acids written by Taylor A. B. Broek and published by . This book was released on 2014 with total page 74 pages. Available in PDF, EPUB and Kindle. Book excerpt: Compound specific isotope analysis of individual amino acids (CSI-AA) is a powerful tool for tracing nitrogen (N) source and transformation in biogeochemical cycles. Specifically, the delta15N value of phenylalanine (delta15N Phe) represents an increasingly used proxy for baseline delta15N values, with particular promise for paleoceanographic applications. Further, multiple studies have shown that delta15N values of Phe and glutamic acid (Glu) can be coupled to provide precise estimates of trophic position (TP), while simultaneously decoupling baseline delta15N values from the effects of trophic transfer. However, current derivatization/gas chromatography (GC) methods require expensive and relatively uncommon instrumentation, and have relatively low precision, posing significant issues for many potential applications. Here we present a new approach to CSI-AA based on a high-pressure liquid chromatography (HPLC) purification of underivatized amino acids, followed by offline elemental analysis-isotope ratio mass spectrometry (EA-IRMS). The utility of this approach is demonstrated in two different applications using natural materials: 1) the construction of a high precision paleoceanographic record from the delta15NPhe in proteinaceous coral tissue and 2) the TP estimation of five marine organisms using the coupled delta15N values of Glu and Phe. Both demonstrations were compared to values measured by an established GC based method. In all cases both methods produced equivalent values, however, values generated by the new HPLC/EA-IRMS approach had higher average precision in all cases. These results demonstrate that this approach represents a viable alternative to traditional GC based methods for delta15NAA measurement and may therefore allow significant expansion of CSI-AA applications, requiring only commonly available instrumentation to produce high precision delta15N AA values.

Download or read book Nitrogen in the Sea written by Etaro Wada and published by CRC Press. This book was released on 1990-12-07 with total page 226 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book provides essential information regarding the dynamics and rate processes of nitrogenous compounds in the sea. Topics discussed include characteristics and behavior of nitrogen at the atomic, molecular, and isotopic levels; elemental rate processes and physico-chemical and biological factors; the dynamics of nitrogen in several representative marine ecosystems; and current progress in isotope marine biogeochemistry. The book emphasizes the distribution and variation of nitrogen isotopes, which can provide a novel approach to understanding nitrogen metabolisms occurring in marine ecosystems. Nitrogen in the Sea: Forms, Abundances, and Rate Processes should be considered an indispensable reference tool for researchers and post-graduate students interested in the nitrogen cycle in aquatic ecosystems

Download or read book Amino Acid Analysis of Marine Sediments Hauraki Gulf New Zealand written by and published by . This book was released on 2008 with total page 76 pages. Available in PDF, EPUB and Kindle. Book excerpt: Amino acid compositions, bulk organic carbon, total nitrogen, and total organic matter were measured for eight sediment samples from the Hauraki Gulf, New Zealand, in order to assess the source and fate of organic matter in a shallow marine environment. Samples were collected from near-shore and off-shore shore sites, as well as from a site receiving freshwater input from multiple rivers. A comparison of carbon, nitrogen, and amino acid compositions of samples between 1 cm depth (Uhle 2004) and 5 cm depth was used to assess changes in organic matter composition through time. Amino acid analyses in particular should be effective in deciphering organic decomposition through both the presence and abundance of amino acids. C:N values, which range from 6.5-9.3, are typical for marine sediments and indicate that the primary source for organic matter is marine despite potential terrigenous input from the surrounding environment. This interpretation is supported by carbon isotopes ( -20%0 and -22%0 for most sites) which is typical for marine-derived organic materials. C:N ratios do not appreciably vary from 1-5 cm depth, suggesting that there has not been significant depletion of organic matter in these young deposits. Furthermore, both D- and L-amino acids are present at 5 cm depth, supporting minimal organic matter decomposition. However, concentrations of amino acid decreases with depth, and the presence of D-alanine, D-aspartic acid, D-glutamic acid, glycine, and non-protein amino acids, at 5 cm depth, suggests at least a small contribution to the organic carbon source from bacteria. A decrease in [delta]15N with depth further suggests the possibility of bacterial reworking under anoxic conditions. Combined, these results support hypotheses of microbial reworking of organic matter through time, but are not clearly diagnostic of this process.

Download or read book Nitrogen Isotopes in Deep Time written by Colin Mettam and published by Cambridge University Press. This book was released on 2021-02-18 with total page 45 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nitrogen is an essential nutrient for life, and its sources and cycling have varied over earth history. Stable isotope ratios of nitrogen compounds (expressed as δ15N, in ‰) are preserved in the sedimentary record and track these changes, providing important insights into associated biogeochemical feedbacks. Here we review the use of nitrogen stable isotope geochemistry in unravelling the evolution of the global N cycle in deep time. We highlight difficulties with preservation, unambiguous interpretations, and local versus global effects. We end with several case studies illustrating how depositional and stratigraphic context is crucial in reliably interpreting δ15N records in ancient marine sediments, both in ancient anoxic (Archean) and more recent well oxygenated (Phanerozoic) environments.

Download or read book Development and Application of a New Method to Reconstruct Terrestrial Nitrogen Cycling from Isotopes of Plant Compounds in Soil written by Sara Katrin Enders and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Nitrogen availaiblity influences terrestrial productivity and global carbon cycling, with strong feedbacks to Earth's climate system. Natural abundance stable isotope ratios of nitrogen ([delta]15N) of plant leaves [delta]15N[subscript f] have provided information on large-scale patterns of N availbility in the modern environment. Few tools have been available, however, for quantitatively reconstructing past plant [delta]15N[subscript f]. We here explore the potential for chlorins, degradation products of the chlorophyll molecule with a history of use as paleo-proxies in subaqueous sediments, to be extracted from soils for use as a proxy for [delta]15N[subscript f]. In Chapter 1, we confirm that chlorins are present at depth in mineral soils. Pheophytin a (pheo a) is present in concentrations sufficient for isotopic analysis. We here test the hypothesis that [delta]15N[subscript f] is recorded in pheo a [delta]15N along the leaf-litter-soil continuum across an array of ecosystem conditions and plant functional types (C3, C4, legumes, woody plants). We report plant pheo a [delta]15N ([delta]15N [subscript phe]) and [delta]15N[subscript f] of six species across a climate gradient on Kohala Mountain of Hawaii, and find that [delta]15N[subscript phe] tracks [delta]15N[subscript f] over a wide range of environmental conditions. In Chapter 2, we examine the persistence and behavior of pheo a in soil depth profiles, through an isotope label tracer study that investigates recovery of pheo a enriched in 15N and 13C applied as isotopically-labelled leaf litter 15 years ago. We find that pheo a decays at rates comparable to bulk soil organic matter, and that pheo a has minimal downward mobility in the soil over time. In Chapter 3, we apply the [delta]15N[subscript phe] proxy in a 40 ky Siberian permafrost record to investigate changes in N cycling on that landscape through a cycle of glaciation and accompanying changes in climate, species, and atmospheric pCO2. We examine major drivers of N cycle dynamics, and compare the sensitivity of internal N cycling (recorded in [delta]15N[subscript phe]) with input-output fluxes (recorded in [delta]15N of bulk soil). The overarching picture revealed is one of bounded variability of the N cycle, with much greater sensitivity [delta]15N[subscript phe] relative to [delta]15N of bulk soil, which shows little variation over time. The rise in pCO2 accompanying deglaciation over the last ~20ky is not associated with isotopic evidence for progressive limitation by N; rather, the changing environmental conditions appear to stimulate new sources of N on this landscape, such as diazatrophic fixation or organic mineralization, and [delta]15N values indicate that N cycling stays within previous limits. These results depart from those of Free Air Carbon Dioxide (pCO2) Enrichment (FACE) experiments, which have seen a decline in [delta]15N[subscript f] and indications that that N may become more limiting to plant growth under elevated pCO2. This result suggests the overriding importance of other changing environmental variables for N cycling at high latitudes over past periods of pCO2 increase, or a compensating mechanism in the terrestrial N cycle beyond the window of experiments and within the window of global climate models.

Download or read book An Examination of the Marine Nitrogen Cycle written by Fabian C. Batista and published by . This book was released on 2016 with total page 233 pages. Available in PDF, EPUB and Kindle. Book excerpt: Paleoceanographic studies typically employ stable nitrogen isotopic measurements of total combustible N (delta15NTN) from marine sediments to infer temporal changes in surface ocean marine N dynamics. However, delta 15NTN is a highly non-specific measurement, susceptible to confounding issues including non-marine N sources, and offers little, if any, information regarding degradation and/or alteration of primary delta 15N signals. Recent development of novel biomarker and microfossil based proxies provide new analytical capabilities to circumvent issues with delta 15NTN.

Download or read book Nitrogen in the Marine Environment written by Douglas G. Capone and published by Academic Press. This book was released on 2008-08-01 with total page 2079 pages. Available in PDF, EPUB and Kindle. Book excerpt: Since the first edition of Nitrogen in the Marine Environment was published in 1983, it has been recognized as the standard in the field. In the time since the book first appeared, there has been tremendous growth in the field with unprecedented discoveries over the past decade that have fundamentally changed the view of the marine nitrogen cycle. As a result, this Second Edition contains twice the amount of information that the first edition contained. This updated edition is now available online, offering searchability and instant, multi-user access to this important information.*The classic text, fully updated to reflect the rapid pace of discovery*Provides researchers and students in oceanography, chemistry, and marine ecology an understanding of the marine nitrogen cycle*Available online with easy access and search - the information you need, when you need it

Download or read book Organic Matter and Rice written by and published by Int. Rice Res. Inst.. This book was released on 1984 with total page 642 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Refining the Interpretation of Nitrogen Isotopes in Deep Time Systems written by Magali Ader and published by Frontiers Media SA. This book was released on 2022-08-01 with total page 173 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Nitrogen Cycling in Oxygen Deficient Zones written by Carolyn Buchwald and published by . This book was released on 2013 with total page 118 pages. Available in PDF, EPUB and Kindle. Book excerpt: The stable isotopes, [delta]15N and [delta]18O, of nitrite and nitrate can be powerful tools used to interpret nitrogen cycling in the ocean. They are particularly useful in regions of the ocean where there are multiple sources and sinks of nitrogenous nutrients, which concentration profiles alone cannot distinguish. Examples of such regions are "oxygen deficient zones" (ODZ). They are of particular interest because they are also important hot spots of fixed N loss and production of N2O, a potent greenhouse gas. In order to interpret these isotope profiles, the isotope systematics of each process involved must be known so that we can distinguish the isotopic signature of each process. One of the important processes to consider here is nitrification, the process by which ammonium is oxidized nitrite and then to nitrate. This thesis describes numerous experiments using both cultures of nitrifying organisms as well as natural seawater samples to determine the oxygen isotope systematics of nitrification. These experimental incubations show that the accumulation of nitrite has a large effect on the resulting [delta]18ONO3. In experiments where nitrite does not accumulate, [delta]18ONO3 produced from nitrification is between -1 to l%o. These values will be applicable for the majority of the ocean, but the nitrite isotopic exchange will be important in the regions of the ocean where nitrite accumulates, such as the base of the euphotic zone and oxygen deficient zones. [delta]18ONO2 was developed as a unique tracer in this thesis because it undergoes abiotic equilibration with water [delta]18O at a predictable rate based on pH, temperature and salinity. This rate, its dependencies, and how the [delta]18ONO2 values can be used as not only biological source indicators but also indicators of age are described. This method was applied to samples from the primary nitrite maximum in the Arabian Sea, revealing that the dominant source and sinks of nitrite are ammonia oxidation and nitrite oxidation with an average age of 37 days. Finally, using the isotope systematics of nitrification as well as the properties of nitrite oxygen isotope exchange described in this thesis, the final chapter interprets multiisotope nitrate and nitrite profiles in the Costa Rica Upwelling Dome using a simple ID model. The nitrite isotopes showed that there were multiple sources of nitrite in the primary nitrite maximum including (1) decoupling of ammonia oxidation and nitrite oxidation, (2) nitrate reduction during assimilation and leakage of nitrite by phytoplankton. In the oxygen deficient zone and secondary nitrite maximum, there were equal contributions of nitrite removal from nitrite oxidation and nitrite reduction. This recycling of nitrite to nitrate through oxidation indicates that the percentage of reduced nitrate fully consumed to N2 gas is actually smaller than previous estimates. Overall, this thesis describes new nitrogen and oxygen isotopic tracers and uses them to elucidate the complicated nitrogen biogeochemistry in oxygen deficient zones.

Download or read book Development and Application of Biogenic Mineral Bound Nitrogen Isotope Measurements to the Million Year Timescale written by Emma Ruth Kast and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: As an essential nutrient required by all organisms, biologically available nitrogen (?fixed N?) plays an integral role in biogeochemical cycling at Earth?s surface. The nitrogen isotope ratio (15N/14N, ?15N) of fixed N is sensitive to biogeochemical transformations that constitute the nitrogen cycle. Additionally, the ?15N of organisms increases with trophic level. ?15N can be a powerful tool for reconstructing the N cycle and the trophic level of organisms, yet poor preservation of N in the geologic record have limited our ability to apply ?15N on million-year timescales. This dissertation focuses on million-year time scale nitrogen cycling and trophic ecology using biogenic mineral-bound organic matter ?15N.A foraminifera shell-bound organic matter ?15N (FB-?15N) record is presented for the early Cenozoic (66 million years ago until present). There is a large decrease (by ~15 ? in the Pacific and by ~ 6 ? in the Atlantic) between 57 and 50 million years ago, before Eocene global cooling. This indicates expanded ocean suboxia in the Paleocene, followed by its tectonically driven demise.Mineral-bound ?15N methods are adapted for use with tooth enamel and enameloid, abundant fossils that are widely used in geochemical and paleontological studies of the past. Enamel(oid)-bound organic matter ?15N (?15NEB) has a long-term method precision of 0.7 ? (1?) and there is substantial evidence for preservation on million-year time scales. Modern shark tooth ?15NEB shows that despite significant intra- and inter-individual differences (up to 3 ?), ?15NEB captures ocean inter- and intra-basin differences in fixed N ?15N. Modern bovid (cow and bison) ?15NEB shows a close correspondence to collagen ?15N, and reflects the dietary plant ?15N on the terrestrial landscape.Applying ?15NEB to the fossil record, the high ?15NEB of the ancient mega-tooth shark Otodus megalodon provides evidence for a remarkably high trophic position. A 100-million-year fossil shark ?15NEB record from the North Atlantic largely corroborates fixed N ?15N changes observed in FB-?15N.This dissertation expands our understanding of marine nitrogen cycling and the trophic position of ancient sharks. This work establishes new mineral-bound ?15N tools and initiates mineral-bound N isotope investigation of the million-year geologic record.

Download or read book Stable Isotopes of Nitrite as a Tool for Understanding Nitrogen Cycling and Loss in Oxygen Deficient Zones written by Taylor Martin and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Nitrogen (N), a key nutrient in the global ocean, has a complicated biogeochemical cycle. In the ocean, nitrogen is mainly found as nitrate (NO3-), but it also accumulates as nitrite (NO2-) within oxygen deficient zones (ODZs). Nitrite is a key intermediate in several processes that produce nitrous oxide (N2O), a potent greenhouse gas. Using natural abundance stable isotope measurements of NO3- and NO2- help to capture an integrated signal of the N cycle processes. My doctoral work aims to use stable isotopes of N and oxygen (O) in order to further our understanding of the pathways that produce and consume NO2-, which will help us better predict the global impacts of nitrogen cycling both at present and in the future. Many N cycle processes are microbially mediated and impart one or more unique isotope effects on its N substrates and products, and it is important to quantify these isotope effects in order to use numerical models to elucidate relative contributions of different N cycle processes. In Chapter 1, I use pure culture experiments to better constrain estimates of the N and O isotope effects of microbial nitrite reduction, a key step in the process of denitrification, which leads to loss of bioavailable N. I then use the values from this experiment, in conjunction with previous estimates of other N cycle process isotope effects, to model observed NO3- and NO2- data from the Arabian Sea (Chapter 2) and explore the necessity for oxygen-requiring processes within the ODZ. In Chapter 3, I expand upon the results from previous chapters by applying the results to a global 3D inverse model of the N cycle, which includes isotopic fractionation, in order to examine the NO2- production and consumption processes within marine ODZs. I also examine the sensitivity of NO3- and NO2- concentration and isotope profiles to controlling parameters of N cycle processes, such as oxygen thresholds and isotope effects, in order to examine the roles of individual processes in N loss, and their potential for future change.

Download or read book Compound Specific Isotope Analysis of Amino Acids in Freshwater Ecosystems written by Prarthana Ghosh and published by . This book was released on 2014 with total page 178 pages. Available in PDF, EPUB and Kindle. Book excerpt: Investigation of the delta^13 C and delta^15 N of individual compounds, commonly referred to as compound specific isotope analysis (CSIA), along with traditional techniques such as stomach content, dietary tracers, and bulk stable isotopic analysis (BSIA), provide a detailed description of trophic dynamics of organisms. CSIA analysis of amino acids (CSIA-AA) in particular, has been used successfully in marine, estuarine, and more recently, terrestrial systems to estimate trophic position (TP) among organisms. Despite the analytical potential of this technique, its application in freshwater systems has remained limited. The goal of this dissertation is to bridge this knowledge gap by assessing the validity and efficacy of the CSIA-AA technique, through studies ranging from laboratory microcosm experiments, to natural freshwater ecosystems in Alabama. In Chapter 2, patterns of trophic enrichment were investigated in a freshwater phytoplankton-grazer model through a controlled feeding experiment in microcosms. Trophic fractionation among amino acids was consistent with previously published results from marine studies. TP determined by CSIA-AA provided a better estimate of trophic status than BSIA, extending the potential of this technique in freshwater systems. Chapter 3 examined the isotopic underpinnings of the phenotypic plasticity in bluegills from the Sipsey River main channel and its backwater habitats, thereby extending CSIA-AA to fluvial and lacustrine environments. BSIA of delta^13 C indicated that the main channel bluegills were subsidized by allochthonous detrital carbon, compared to bluegills from the two floodplain lakes that relied on submerged macrophytic vegetation. Estimates of trophic length using CSIA placed bluegills from all three sites at comparable TP, between trophic levels 3 and 4, suggesting some degree of omnivory. In Chapter 4, the biotic factors affecting the significant difference in mercury concentrations between the largemouth bass from the Sipsey and Black Warrior Rivers were identified, using dual CSIA of triglyceride fatty acids (TGFA; delta^13 C) and amino acids (delta^15 N). Results indicate that increased presence of sulfate-reducing bacteria, coupled with detritus based food web, was the primary driver of the elevated mercury levels in the Sipsey bass. Data presented in this dissertation clearly demonstrate the potential of CSIA-AA and CSIA-TGFA to provide high-resolution insights into complex food web interactions.

Download or read book Nitrogen in Sedimentary Materials written by Anastasios Emmanuel Nychas and published by . This book was released on 1978 with total page 178 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Studies of Nitrogen and Organic Carbon Cycling in the Eastern Pacific Ocean Using Natural Abundance Isotopes written by Margot Elizabeth White and published by . This book was released on 2021 with total page 202 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis uses a variety of natural abundance isotope measurements to expand our understanding of nitrogen and organic carbon cycling within the ocean. Chapters 2 & 3 use stable isotopes of nitrate to investigate recent trends in nitrogen cycling in the southern California Current System (sCCS). In Chapter 2 we show that surface ocean warming during the 2015-2016 El Niño event led to reduced nitrate supply and increased reliance on recycled nitrate produced via euphotic zone nitrification. We also identified spatial and temporal variability in euphotic zone nitrate utilization associated with iron limitation of phytoplankton. Chapter 3 uses a time series of stable isotopes of nitrate along with other physical and chemical measurements to examine recent biogeochemical changes in a partially isolated marine basin off the coast of California, whose bottom waters become deoxygenated between annual flushing events. Our data confirm an unprecedented increase in water column denitrification in basin bottom waters in recent years that was likely caused by a long-term trend of decreasing oxygen concentrations outside the basin. Chapter 4 uses a variety of chemical and isotopic measurements to characterize organic matter across oxygen gradients in the eastern tropical North Pacific (ETNP). This work describes unique organic matter dynamics along the oxycline and shows that optical properties of dissolved organic matter effectively identify locations where intense oxic remineralization takes place, whereas most other bulk measurements of organic matter do not provide insight into unique processes within these environments. We also document the impact of a passing hurricane on water column properties. Chapter 5 uses thermal oxidation in conjunction with a chemical processing method to show that the current paradigm of marine DOC cycling needs revision. Our results show that even the most 14C-depleted DOC fraction exhibits a vertical radiocarbon gradient, suggesting the addition and removal of DOC from this reservoir on shorter timescales than current models predict. We propose a new hypothesis for marine DOC cycling that would revise the water column lifetime of marine DOC to only 2,300 years, approximately doubling the estimated flux of carbon through the deep ocean.

Download or read book Nitrogen Isotopes in a Global Ocean Biogeochemical Model written by Christopher J. Somes and published by . This book was released on 2009 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: We present a new nitrogen isotope model incorporated into the three-dimensional ocean component of a global Earth System Climate Model designed for millennial timescale simulations. The model includes prognostic tracers for the stable nitrogen isotopes, 14N and 15N, in the nitrate (NO3−), phytoplankton, zooplankton, and detritus variables of the marine ecosystem model. The isotope effects of algal NO3− assimilation, water column denitrification, and zooplankton excretion are considered as well as the input of newly fixed nitrogen by diazotrophs. A global database of [delta]15NO3− observations is compiled from previous studies and compared to the model results on a regional basis where sufficient observations exist. The model is able to qualitatively and quantitatively reproduce the observed patterns such as high subsurface values in denitrification zones, the meridional and vertical gradients in the Southern Ocean, and the meridional gradient in the Central Equatorial Pacific. The observed subsurface minimum in the Atlantic is underestimated presumably owing to too little nitrogen fixation there. Sensitivity experiments show that algal NO3− assimilation, nitrogen fixation and water column denitrification have strong effects on the simulated distribution of nitrogen isotopes, whereas the effect from zooplankton excretion is weaker. Both water column and sedimentary denitrification also have important indirect effects on the nitrogen isotopes distribution by reducing the fixed nitrogen inventory, which creates an ecological niche for diazotrophs and stimulates additional nitrogen fixation. Water column denitrification has a strong but rather localized effect on the nitrogen isotope distribution in model versions without iron limitation of diazotrophy, in which a tight coupling of nitrogen fixation exists. However, including iron limitation of diazotrophy inhibits a tight coupling between water column denitrification and nitrogen fixation in the Eastern Pacific and shifts the main location of nitrogen fixation from the Eastern Tropical Pacific to the Western Tropical Pacific, which results in a better agreement with N' = NO3−-16PO43− and [delta]15NO3− observations. Thus, our model results suggest that iron limitation of diazotrophy can modulate the feedback between denitrification and nitrogen fixation in the ocean. We speculate that a feedback response time on the centennial to millennial time scale may exist between denitrification and nitrogen fixation, producing imbalances in the global oceanic fixed nitrogen cycle, which may well have contributed to past changes of atmospheric CO2 via the biological pump.