Bulk δ15N Research Articles

Seasonal dynamics of zooplankton resource use revealed by carbon amino acid stable isotope values

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 531:143-154 (2015) - DOI: https://doi.org/10.3354/meps11319 Seasonal dynamics of zooplankton resource use revealed by carbon amino acid stable isotope values Jens M. Nielsen*, Monika Winder Department of Ecology, Environment and Plant Sciences, Stockholm University, 106 91 Stockholm, Sweden *Corresponding author: jens.nielsen@su.se ABSTRACT: Reliable tracer techniques are fundamental to retrieving accurate information on consumer dietary resource use in dynamic ecosystems. Analysis of δ13C values in essential amino acids has shown great promise in effectively capturing consumer food sources, since essential amino acids are not synthesized by heterotrophic organisms but instead routed directly from dietary sources to consumers, resulting in negligible 13C trophic discrimination. We assessed seasonal feeding patterns of a widespread key copepod species (Acartia spp.) in the northern Baltic proper using complementary approaches: bulk δ13C and δ15N values, δ13C values of essential amino acids, and quantitative phytoplankton taxonomic data. Our results showed distinct differences between Acartia and seston δ13C essential amino acid values measured at weekly to monthly sampling intervals, which indicated that Acartia preferentially utilized specific dietary resources that comprised only parts of the total phytoplankton composition (varying from 19.7% to 81.4% during the season). Results also indicated that care should be taken when inferring trophic position from bulk stable isotopes when consumers are highly selective, since isotope values of seston may not accurately reflect consumer specific diet resource uptake. Analysis of δ13C values in essential amino acids was shown to be a promising tool to accurately trace consumer resource use in complex natural systems. KEY WORDS: Stable isotopes · Amino acids · Acartia · Zooplankton · Phytoplankton · Food webs Full text in pdf format PreviousNextCite this article as: Nielsen JM, Winder M (2015) Seasonal dynamics of zooplankton resource use revealed by carbon amino acid stable isotope values. Mar Ecol Prog Ser 531:143-154. https://doi.org/10.3354/meps11319 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 531. Online publication date: July 02, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Testing the use of bulk organic δ13C, δ15N, and Corg:Ntot ratios to estimate subsidence during the 1964 great Alaska earthquake

During the Mw 9.2 1964 great Alaska earthquake, Turnagain Arm near Girdwood, Alaska subsided 1.7 ± 0.1 m based on pre- and postearthquake leveling. The coseismic subsidence in 1964 caused equivalent sudden relative sea-level (RSL) rise that is stratigraphically preserved as mud-over-peat contacts where intertidal silt buried peaty marsh surfaces. Changes in intertidal microfossil assemblages across these contacts have been used to estimate subsidence in 1964 by applying quantitative microfossil transfer functions to reconstruct corresponding RSL rise. Here, we review the use of organic stable C and N isotope values and Corg:Ntot ratios as alternative proxies for reconstructing coseismic RSL changes, and report independent estimates of subsidence in 1964 by using δ13C values from intertidal sediment to assess RSL change caused by the earthquake. We observe that surface sediment δ13C values systematically decrease by ∼4‰ over the ∼2.5 m increase in elevation along three 60- to 100-m-long transects extending from intertidal mud flat to upland environments. We use a straightforward linear regression to quantify the relationship between modern sediment δ13C values and elevation (n = 84, R2 = 0.56). The linear regression provides a slope–intercept equation used to reconstruct the paleoelevation of the site before and after the earthquake based on δ13C values in sandy silt above and herbaceous peat below the 1964 contact. The regression standard error (average = ±0.59‰) reflects the modern isotopic variability at sites of similar surface elevation, and is equivalent to an uncertainty of ±0.4 m elevation with respect to Mean Higher High Water. To reduce potential errors in paleoelevation and subsidence estimates, we analyzed multiple sediment δ13C values in nine cores on a shore-perpendicular transect at Bird Point. Our method estimates 1.3 ± 0.4 m of coseismic RSL rise across the 1964 contact by taking the arithmetic mean of the differences (n = 9) between reconstructed elevations for sediment above and below the 1964 earthquake subsidence contact. This estimate compares well with independent subsidence estimates derived from post-earthquake leveling in Turnagain Arm, and from microfossil transfer functions at Girdwood (1.50 ± 0.32 m). While our results support the use of bulk organic δ13C for reconstructing RSL change in southern Alaska, the variability of stable isotope values in modern and buried intertidal sediment required the analysis of multiple samples to reduce error.

Controls on the stratigraphic distribution and nitrogen isotopic composition of zinc, vanadyl and free base porphyrins through Oceanic Anoxic Event 2 at Demerara Rise

The Cenomanian–Turonian black shales of the Demerara Rise (DR) contain a wide range of tetrapyrroles and allow porphyrin analysis through Oceanic Anoxic Event 2 (OAE 2), making this site unique. The sediments provide an excellent opportunity to study the factors controlling the stratigraphic distribution and N isotopic composition of porphyrins. Three C33 bicycloalkanoporphryins (BiCAPs) are present as free bases (FBs) and as complexes with Zn and VO in quantities sufficient for compound-specific isotope analysis (CSIA) and were the focus of this study. The stratigraphic distribution of BiCAPs is controlled, foremost, by metal availability rather than sedimentary Eh/pH conditions or post depositional maturity. Titration of the local water column metal reservoir with sulfide and organic matter during OAE 2 resulted in a high concentration of FB BiCAPs and a very low concentration of metallo-BiCAPs. Conversely, high metal concentration occurs in sediments above and below the OAE, and Zn and VO porphyrin abundances mirror bulk metal concentration. The highest total concentration of porphyrins was where metal concentration was highest, suggesting that porphyrin preservation is enhanced by the increased stability of metal complexes. Unexpectedly, the total concentration of porphyrins was lowest during the heart of OAE 2, in an interval of higher total organic carbon (TOC) content, where enhanced preservation would be expected; we suggest this was the result of decreased preservation of extractable tetrapyrroles in the absence of available metals. The high reactivity of Zn2+ with sulfide limited the formation of Zn complexes to the non-sulfidic regions in the water column or at the sediment/water interface. The formation of VO porphyrins occurred exclusively within the sediments from available FBs or through transmetallation with other metallo-BiCAPS (Zn, Ni, Cu).The formation of the metal complexes had a N isotope effect. We observed significant differences in the δ15N values of the three most abundant BiCAPs. VO BiCAPs were systematically 15N-depleted by an average of 2.5±1.5‰ relative to FBs, and Zn BiCAPs were equivalent to FB BiCAPs within error (15N enriched by+0.1‰) but were variable (±1.5‰). The δ13C values of the Zn, VO and FB BiCAPs were equivalent, suggesting that the three compounds share a common chlorophyll source, in agreement with structural data. The significant difference in δ15N values between the VO and FB BiCAPs suggests that δ15N values of VO porphyrins are controlled by N isotopic effects during VO porphyrin formation. Under experimental conditions, the formation of Zn octaethylporphyrin had a fractionation (εproduct/reactant −3.9) consistent with the differences between FB and VO BiCAPs, supporting a diagenetic origin for the differences in δ15N. The δ15N values of the porphyrins in these ancient sediments have been used to reconstruct the δ15N value of the primary phototrophic biomass and the ecological balance of chlorophyll a producing organisms via the εpor proxy. If the δ15N values of porphyrins are indeed altered by metal–ligand reactions, the application for paleoenvironmental reconstruction may require a more complete analysis of the range of metalloporphyrins in a given structural class. The covariance of bulk, BiCAP, porphyrin class and kerogen δ15N through OAE 2 suggests that the variability in δ15N reflects primary N cycle dynamics and that the εpor proxy can be viable in ancient sediments despite these complexities.

Setting the stage for a global-scale trophic analysis of marine top predators: a multi-workshop review

Global-scale studies of marine food webs are rare, despite their necessity for examining and understanding ecosystem level effects of climate variability. Here we review the progress of an international collaboration that compiled regional diet datasets of multiple top predator fishes from the Indian, Pacific and Atlantic Oceans and developed new statistical methods that can be used to obtain a comprehensive ocean-scale understanding of food webs and climate impacts on marine top predators. We loosely define top predators not as species at the apex of the food web, but rather a guild of large predators near the top of the food web. Specifically, we present a framework for world-wide compilation and analysis of global stomach-contents and stable-isotope data of tunas and other large pelagic predatory fishes. To illustrate the utility of the statistical methods, we show an example using yellowfin tuna in a “test” area in the Pacific Ocean. Stomach-contents data were analyzed using a modified (bagged) classification tree approach, which is being prepared as an R statistical software package. Bulk δ15N values of yellowfin tuna muscle tissue were examined using a Generalized Additive Model, after adjusting for spatial differences in the δ15N values of the baseline primary producers predicted by a global coupled ocean circulation-biogeochemical-isotope model. Both techniques in tandem demonstrated the capacity of this approach to elucidate spatial patterns of variations in both forage species and predator trophic positions and have the potential to predict responses to climate change. We believe this methodology could be extended to all marine top predators. Our results emphasize the necessity for quantitative investigations of global-scale datasets when evaluating changes to the food webs underpinning top ocean predators under long-term climatic variability.

Comparison of the stable carbon and nitrogen isotopic values of gill and white muscle tissue of fish

The potential use of stable carbon and nitrogen isotope ratios (δ13C, δ15N) of fish gills for studies on fish feeding ecology was evaluated by comparing the δ13C and δ15N of gill tissue with the more commonly used white muscle tissue. To account for the effect of lipid content on the δ13C signatures, a study-specific lipid correction model based on C:N ratios was developed and applied to the bulk δ13C data. For the majority of species in the study, we found no significant difference in δ13C values between gill and muscle tissue after correction, but several species showed a small (0.3–1.4‰) depletion of 13C in white muscle compared to gill tissue. The average species difference in δ15N between muscle and gill tissue ranged from −0.2 to 1.6‰ for the different fish species with muscle tissue generally more enriched in 15N. The δ13C values of muscle and gill were strongly linearly correlated (R2=0.85) over a large isotopic range (13‰), suggesting that both tissues can be used to determine long-term feeding or migratory habits of fish. Muscle and gill tissue bulk δ15N values were also strongly positively correlated (R2=0.76) but with a small difference between muscle and gill tissue. This difference indicates that the bulk δ15N of the two tissue types may be influenced by different isotopic turnover rates or a different composition of amino acids.

Deep‐sea coral record of human impact on watershed quality in the Mississippi River Basin

AbstractOne of the greatest drivers of historical nutrient and sediment transport into the Gulf of Mexico is the unprecedented scale and intensity of land use change in the Mississippi River Basin. These landscape changes are linked to enhanced fluxes of carbon and nitrogen pollution from the Mississippi River, and persistent eutrophication and hypoxia in the northern Gulf of Mexico. Increased terrestrial runoff is one hypothesis for recent enrichment in bulk nitrogen isotope (δ15N) values, a tracer for nutrient source, observed in a Gulf of Mexico deep‐sea coral record. However, unambiguously linking anthropogenic land use change to whole scale shifts in downstream Gulf of Mexico biogeochemical cycles is difficult. Here we present a novel approach, coupling a new tracer of agro‐industrialization to a multiproxy record of nutrient loading in long‐lived deep‐sea corals collected in the Gulf of Mexico. We found that coral bulk δ15N values are enriched over the last 150–200 years relative to the last millennia, and compound‐specific amino acid δ15N data indicate a strong increase in baseline δ15N of nitrate as the primary cause. Coral rhenium (Re) values are also strongly elevated during this period, suggesting that 34% of Re is of anthropogenic origin, consistent with Re enrichment in major world rivers. However, there are no pre‐anthropogenic measurements of Re to confirm this observation. For the first time, an unprecedented record of natural and anthropogenic Re variability is documented through coral Re records. Taken together, these novel proxies link upstream changes in water quality to impacts on the deep‐sea coral ecosystem.

Constraining the internal variability of the stable isotopes of carbon and nitrogen within mantle diamonds

The carbon and nitrogen isotope values of mantle xenoliths and xenocrysts are used to trace the cycling of volatiles in the deep Earth, for example, to place empirical constraints on the origin of diamond-forming carbon in the mantle. The global database for diamond shows that the δ13C value of peridotitic diamonds is very narrow, typically around −5‰, whereas eclogitic diamonds can show positive and very negative δ13C values resembling crustal carbonates and crustal organic carbon (<−40 to >+2‰); commonly interpreted to reflect a relationship between eclogitic diamond formation and subduction zone planet tectonics. Curiously, diamonds from both parageneses can show positive (crust-like) and negative (mantle-like) δ15N values (from <−40 to >+20‰). Most of these data are derived from single stage combustion gas sourced mass spectrometry, which produces simplistic datasets. By fragmenting single diamonds or using in-situ ion-beam techniques it is known that single diamonds can show large-scale heterogeneity for their carbon isotope values and nitrogen abundances, sometimes as large as entire populations of diamond across a few hundred micrometres. What is less well known is the scale of nitrogen isotope heterogeneity within single diamonds, and if the nitrogen isotope heterogeneity of single diamonds can provide an insight into why diamonds that show very restricted δ13C values show a much large range of δ15N values.To investigate the scale, and to determine the origin of the nitrogen isotope heterogeneity (source vs. fractionation during diamond-formation) shown for populations of mantle diamonds we have determined multiple δ13C–δ15N values and nitrogen abundances from 14 monocrystalline (MCDs) and 25 polycrystalline diamonds (PCDs) using step-wise oxidation gas sourced mass spectrometry. These data show that the heterogeneity shown for carbon and nitrogen isotope values from single diamond samples presented here is typically <5‰ and <8‰ respectively, both of which are comparable to the standard deviation for the mean mantle δ13C and δ15N values (±3 and ±4‰). However, there are samples that show much larger heterogeneities for δ13C and δ15N values (≤23‰ and ≤33‰ respectively), which cannot be generated by equilibrium stable isotope fractionation during, or prior to diamond-formation. These data suggest that isotopic heterogeneity may be present within the diamond-forming fluid on sub-mm scale, or that these diamonds formed during multiple diamond-formation events from isotopically distinct sources. From these 39 samples there are only 5 PCDs that show a larger range of carbon isotopes relative to nitrogen isotopes, but of these 5 samples only 2 show a range of δ13C values outside of analytical uncertainty. The remaining 34 samples show a greater isotopic heterogeneity for δ15N relative to δ13C values. The samples with the largest carbon and nitrogen isotopic heterogeneity are also the samples with low-bulk δ13C values (<−10‰), whilst there is no relationship between the ranges of nitrogen isotope values for a given sample and the corresponding bulk δ15N value or nitrogen content.These data show that δ15N values recorded in mantle diamonds are relatively heterogeneous, and can show both mantle-like (negative) and crustal (sedimentary)-like (positive) δ15N values within the same sample. We conclude that the large range of nitrogen isotope values seen within individual diamonds means that the observation of negative, mantle-like, nitrogen in mantle diamonds acquired by single-stage bulk combustion isotope ratio mass spectrometry cannot be used as a conclusive indicator for a mantle origin for the entirety of the diamond-forming carbon, and vice versa. Also, the behaviour of 15N/14N is not coupled with the behaviour of 14N/12C during diamond-formation. Instead, it appears that diamond-forming fluids can have positive and negative δ15N values, irrespective of their δ13C value(s). These data suggest that subduction induced nitrogen isotope heterogeneity may not be coupled with subduction induced carbon isotope heterogeneity in diamond-forming fluids.

Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the Ediacaran–Cambrian transition in South China

N and Corg isotope results are presented from six sections along a West–East transect in the South China Basin (SCB) covering both shallow and deeper domains, in order to investigate biogeochemical cycling, stratigraphic correlation and isotope systematics over the crucial Ediacaran–Cambrian transition. δ15N bulk values range between −3‰ and +7‰, while δ13Corg values range between −21‰ and −39‰. Similar isotopic trends have been identified for both these proxies across the basin, although hiatuses, differences in depositional setting and syn-depositional bacterial fermentation may have caused some inconsistencies. A trend towards negative N isotope values can be recognized above the Pc–C boundary in both shallow and deeper basin realms across the Yangtze platform. This negative δ15N excursion is probably a response to photic zone anoxia and intense nitrogen fixation/assimilation by diazotrophic cyanobacteria and green/purple sulfur bacteria (GSB and PSB). The Xiaotan section and a composite section from the Yangtze Gorges area show meaningful similarities in their nitrogen isotope trends, interpreted as chemocline fluctuations in the water column that testify to rapid mixing of water overlying the shallow platform. Using carbon isotope stratigraphy, we correlate boundary strata across the platform to test the wider significance of nitrogen isotopic variations. Increased bioturbation and food-chain complexity across the Ediacaran–Cambrian transition probably led to more frequent variations in nitrate isotope composition and related pool dimension during the early Cambrian, reflecting the beginning of a new more biologically controlled era.

Spatial segregation and similar trophic-level diet among eastern Canadian Arctic/north-west Atlantic killer whales inferred from bulk and compound specific isotopic analysis

Killer whales in the Eastern Canadian Arctic (ECA) prey on narwhal, beluga, bowhead whales and seals, while further south in the north-west Atlantic (NWA), killer whales off the coast of Newfoundland and Labrador prey on both marine mammals and fish. Bulk and amino acid (AA) specific isotopic composition of dentinal collagen in teeth of 13 ECA/NWA killer whales were analysed to assess the degree, if any, of dietary specialization of killer whales across the region. Dentine was sampled from within annual growth layer groups (GLGs) to construct chronological profiles of stable nitrogen (δ15N) and carbon (δ13C) isotopic compositions for individual whales spanning 3–25 years. Interannual isotopic variation across GLGs was less than that among individuals, and median bulk δ15N values differed by up to 5‰ among individuals. Significant correlation between bulk δ15N values and baseline (source AA) δ15N values indicates much of the observed isotopic variation among individuals reflects foraging within isotopically distinct food webs, rather than diet differences. This interpretation is supported by consistent differences in bulk δ13C values between the two individuals with lowest source AA δ15N values and the remaining whales. After accounting for baseline isotopic variation, comparable δ15N values among individuals indicates similar trophic-level diet, although uncertainties in relative trophic15N enrichment of individual AAs currently limits trophic position estimates for top consumers. Further research is required to clarify seasonal movement patterns and possible diet shifts of ECA/NWA killer whales to better define their role in marine ecosystems across the region.

Quantifying the spatial variability of soil physical and chemical properties in relation to mitigation of diffuse water pollution

Understanding spatial variability of soil properties in response to land-use impacts is essential for evaluating the effectiveness of measures taken to address diffuse water pollution from agriculture. However, despite the growing emphasis on integrated catchment-scale implementation of land-use mitigation measures, the baseline landscape-scale evaluation of the spatial variability of key soil nutrients remains scarce. This study employs a high resolution geostatistical approach to characterise the spatial variability of parameters, including soil bulk density (BD), total soil carbon (TC), nitrogen (TN), phosphorus (TP), inorganic phosphorus (IP), organic phosphorus (OP), stable nitrogen isotope ratio (δ15N), C:N ratio, carbon storage and nitrogen storage in two study catchments with contrasting land uses (agricultural and semi-natural) that are subject to targeted management interventions to reduce flood risk and improve water quality. We found a stronger degree of spatial dependence of all soil properties in the agricultural than the semi-natural catchment, except for bulk density and δ15N. Furthermore, bulk density, TP, IP, OP, C:N ratio, δ15N and carbon storage showed a longer range or spatial auto-correlation in the agricultural catchment. The central tendency (median and mean) of all soil properties was also significantly different between the two catchments, with the exception of IP and δ15N. The spatial correlations between the soil properties pointed to the mechanisms that were responsible for the observed differences, whilst the krigged surfaces of soil variables identified most likely critical source areas for targeted land management interventions to improve water quality. Arable and intensive grasslands were identified as ‘high-impact’ land uses, associated with negative alteration of soil properties and increased diffuse water pollution, whilst moorland was a ‘low impact’ land use associated with improved water quality. A comparison with the national soil survey dataset shows that whilst it can be relied upon for the broad characterisation of carbon and TP stocks in the two study catchments, it underestimates the spatial variability of key soil properties in certain soil types and land uses. As the restoration of soil spatial heterogeneity may take several decades, a high resolution geostatistical approach should be included in the future design of catchment-scale monitoring schemes to inform catchment management strategies and elucidate the time frame over which landscape scale improvements in soil properties and corresponding ecosystem services can be achieved.

Evaluating nitrogen isotopes as proxies for depositional environmental conditions in shales: Comparing Caney and Woodford Shales in the Arkoma Basin, Oklahoma

In this study, we assessed the use of stable nitrogen isotopes (δ15N) to evaluate differences in water column conditions during the deposition of the Woodford Shale and Caney Shale in the Arkoma Basin in Oklahoma by comparing bulk δ15N values. The Woodford Shale was mainly deposited under anoxic bottom water column conditions, whereas the Caney Shale shows evidence of deposition under a suboxic/dysoxic deep water regime. Therefore, these two units can be used to test the utility of δ15N as a proxy for the differentiation of these intervals based on depositional environment and post-depositional processes, and provide insights into differences that may impact their assessments as reservoir and source rocks. We measured δ15Nbulk, organic carbon isotopes (δ13Corg), total organic carbon (TOC) and total nitrogen (TN) values from a core from the Arkoma Basin that contains sections of both Caney Shale and Woodford Shale. Bulk δ15N values are higher in the Caney Shale (average δ15Nbulk=9.5±0.8‰) than in the Woodford Shale (average δ15Nbulk=3.1±0.7‰); TOC values in the Woodford Shale are slightly higher than those in the Caney Shale (6±2% and 4±2%, respectively). No significant differences were identified in the δ13Corg and TN values between the two units. Given the depositional history of these units, we suggest that the difference in δ15N between the Caney Shale and the Woodford Shale primarily reflects differences in depositional environment, and that changes in δ15Nbulk values due to diagenesis or catagensis are subordinate.

Preservation effects on the isotopic and elemental composition of skeletal structures in the deep-sea bamboo coral Lepidisis spp. (Isididae)

Trace elements and stable isotopes (δ13C and δ15N) in deep-sea coral have been used as proxies to reconstruct past climate, and to investigate food web structure. However, there is a paucity of information regarding the effect of preservation on the chemical integrity of archived coral. In this study a live-caught colony of Bamboo coral (southern Australia), genus (Lepidisis), was sectioned into three pieces and stored for approximately one year to investigate the influence of preservation in ethanol and preservation in seawater, that mimics the early stages of fossilization, against a dry preserved control. Storage and preservation have no significant effect on the isotopic signature of Δ14C, bulk δ15N, δ15N and δ13C of individual amino acids, or C:N in the fibrillar protein matrix, with the only offset (~0.2‰) being observed in part of the δ13C record. In the high-magnesium calcite lattice Ba/Ca appears to be significantly different after storage in ethanol, whereas Δ14C, B/Ca, Mg/Ca, Sr/Ca, and U/Ca remain largely unaltered. Possible mechanisms responsible for these observed differences center around the decomposition, or contamination, by organics and we recommend further investigation, and caution when comparing samples with differing preservation histories.

A test of the nitrogen-limitation hypothesis for retarded eukaryote radiation: Nitrogen isotopes across a Mesoproterozoic basinal profile

Nitrogen limitation caused by trace metal scarcity under euxinic ocean conditions has been proposed as an explanation for the delayed radiation of eukaryotes until at least the late Mesoproterozoic. However, evidence for how the nitrogen cycle was operating during the middle Precambrian is, so far, rare. More specifically, it is unknown which steps in the biogeochemical nitrogen cycle, e.g. nitrogen fixation, nitrification or ammonification, were rate-limiting and thus controlling microbial community structures. The Mesoproterozoic Belt Supergroup in western Montana hosts a variety of facies ranging from shallow to deep water and thus offers the opportunity to address this issue. Bulk δ15N values show a clear trend from −1‰ in the deepest part of the basin to +5‰ along basin margins, which suggests that coastal areas were sufficiently oxygenated for aerobic nitrogen cycling. The total fractionation of carbon isotopes between carbonate and organic carbon (Δ13C) increases from 20‰ to 32‰ in the same direction, possibly indicating an ecological response to redox stratification and nitrogen speciation. Evidence from the Belt Supergroup is thus consistent with the idea that nutrient availability may have restricted early eukaryotic organisms to a narrow range of habitats, which thus prevented a global rise to ecological dominance until concentrations of fixed nitrogen increased in the global open ocean.

Range Expansion of the Jumbo Squid in the NE Pacific: δ15N Decrypts Multiple Origins, Migration and Habitat Use

Coincident with climate shifts and anthropogenic perturbations, the highly voracious jumbo squid Dosidicus gigas reached unprecedented northern latitudes along the NE Pacific margin post 1997–98. The physical or biological drivers of this expansion, as well as its ecological consequences remain unknown. Here, novel analysis from both bulk tissues and individual amino acids (Phenylalanine; Phe and Glutamic acid; Glu) in both gladii and muscle of D. gigas captured in the Northern California Current System (NCCS) documents for the first time multiple geographic origins and migration. Phe δ15N values, a proxy for habitat baseline δ15N values, confirm at least three different geographic origins that were initially detected by highly variable bulk δ15N values in gladii for squid at small sizes (<30 cm gladii length). In contrast, bulk δ15N values from gladii of large squid (>60 cm) converged, indicating feeding in a common ecosystem. The strong latitudinal gradient in Phe δ15N values from composite muscle samples further confirmed residency at a point in time for large squid in the NCCS. These results contrast with previous ideas, and indicate that small squid are highly migratory, move into the NCCS from two or more distinct geographic origins, and use this ecosystem mainly for feeding. These results represent the first direct information on the origins, immigration and habitat use of this key “invasive” predator in the NCCS, with wide implications for understanding both the mechanisms of periodic D. gigas population range expansions, and effects on ecosystem trophic structure.

Deglacial–Holocene environmental changes at the Pacific entrance of the Strait of Magellan

We show environmental and nitrogen cycle changes between 13 and 2.5kyr BP (from the latest deglaciation to the Holocene) based on biogeochemical records in a sediment core from the Pacific entrance of the Strait of Magellan. Organic carbon, total nitrogen, and C37 alkenone contents were low during 13–9kyr BP but increased rapidly at 8kyr BP. The relative contribution of tetra-unsaturated C37 alkenone, used as a salinity proxy, suggests low salinity during 13–9kyr BP changing to high salinity at 8kyr BP. Planktic foraminifer data showed that Globigerina bulloides was more abundant during the deglacial–early Holocene than in the mid–late Holocene, whereas abundances of Neogloboquadrina pachyderma showed the opposite pattern. This evidence plus biogenic opal data suggest that sea level rose, surface waters were relatively stratified, and in situ productivity was low except for organisms with opal tests from 13 to 9kyr BP. Bulk δ15N values were high (9–11‰) relative to present-day values during the deglacial–early Holocene, with consistently lower values (~7‰) after ~8kyr BP. These relatively high δ15N values most likely reflect increased nutrient utilization in the Southern Ocean, which resulted in high-δ15N water being advected to the Patagonian fjords, although local denitrification caused by the delivery of copious terrestrial materials and organic matter to the ocean cannot be ruled out. By ~8kyr BP continental influences and surface stratification were reduced and characteristic Holocene conditions were established, with increased influence of Southern Ocean water in the western Strait of Magellan. Sea surface temperatures showed millennial-scale changes during the deglaciation and early Holocene, with relatively warm temperatures (11–12°C) corresponding to the Antarctic counterpart of the Younger Dryas cooling event of the Northern Hemisphere. The longer term pattern (~10kyr time scale) in alkenone-derived sea surface temperatures generally follows orbital changes in insolation during austral spring, suggesting a controlling influence of austral spring insolation on southern Patagonian climate.

Deglacial nitrogen isotope changes in the Gulf of Mexico: Evidence from bulk sedimentary and foraminifera‐bound nitrogen in Orca Basin sediments

Constraining variations in marine N2‐fixation over glacial‐interglacial timescales is crucial for determining the role of the marine nitrogen cycle in modifying ocean productivity and climate, yet paleo‐records from N2‐fixation regions are sparse. Here we present new nitrogen isotope (δ15N) records of bulk sediment and foraminifera test‐bound (FB) nitrogen extending back to the last ice age from the oligotrophic Gulf of Mexico (GOM). Previous studies indicate a substantial terrestrial input during the last ice age and early deglacial, for which we attempt to correct the bulk sediment δ15N using its observed relationship with the C/N ratio. Both corrected bulk and FB‐δ15N reveal a substantial glacial‐to‐Holocene decrease of δ15N toward Holocene values of around 2.5 ‰, similar to observations from the Caribbean. This δ15N change is most likely due to a glacial‐to‐Holocene increase in regional N2‐fixation. A deglacial peak in the FB‐δ15N of thermocline dwelling foraminifera Orbulina universa probably reflects a whole ocean increase in the δ15N of nitrate during deglaciation. The δ15N of the surface dwelling foraminifera Globigerinoides ruber and the corrected bulk δ15N show little sign of this deglacial peak, both decreasing from last glacial values much earlier than does the δ15N of O. universa; this may indicate that G. ruber and bulk N reflect the euphotic zone signal of an early local increase in N2‐fixation. Our results add to the evidence that, during the last ice age, the larger iron input from dust did not lead to enhanced N2‐fixation in this region. Rather, the glacial‐to‐Holocene decrease in δ15N is best explained by a response of N2‐fixation within the Atlantic to the deglacial increase in global ocean denitrification.

Tracking aquatic change using chlorin‐specific carbon and nitrogen isotopes: The last glacial‐interglacial transition at Lake Suigetsu, Japan

Joint carbon and nitrogen isotope measurements were made from chlorins (chlorophyll a, phaeophytin a and pyrophaeophytin a) extracted from the last glacial‐interglacial transition sediments of Lake Suigetsu, central Japan. These data highlight both the potential and difficulty of using chlorin‐specific isotopes to track aquatic change from lake sediments. δ13C and δ15N of the three chlorins show coherent patterns with time, supporting the theory that phaeophytin a and pyrophaeophytin a are early diagenetic products of chlorophyll a and that despite this transition, their isotopic signatures remain intact. However, our data suggest that the isotopic composition of phaeophytin a and pyrophaeophytin a can be imprecise proxies for the isotope composition of chlorophyll a, possibly owing to the complex array of factors which affect the synthesis, transformation and sedimentation of these phaeopigments in nature. The total accumulation of organic matter in Lake Suigetsu appears to be controlled by the balance of allocthonous and authocthonous material as reflected by the C/N ratio. However, both bulk organic and chlorin‐specific δ13C show similar changes, suggesting that the first order variability in bulk organic δ13C reflects aquatic change. By contrast, there is no similarity between chlorin and bulk δ15N, suggesting that interpretation of bulk δ15N in this setting is compromised by diagenetic alteration. The isotopic composition of chlorins are interpreted to reflect the response of aquatic primary productivity to post‐glacial environmental change. However, further research into the synthesis and transformation of chlorins in the modern environment is required in order to facilitate a more rigorous approach to interpreting isotope ratios in chlorins extracted from sediments.

A molecular and isotopic study of the macromolecular organic matter of the ungrouped C2 WIS 91600 and its relationship to Tagish Lake and PCA 91008

Abstract– Insight into the chemical history of an ungrouped type 2 carbonaceous chondrite meteorite, Wisconsin Range (WIS) 91600, is gained through molecular analyses of insoluble organic matter (IOM) using solid‐state 13C nuclear magnetic resonance (NMR) spectroscopy, X‐ray absorption near edge structure spectroscopy (XANES), and pyrolysis‐gas chromatography coupled with mass spectrometry (pyr‐GC/MS), and our previous bulk elemental and isotopic data. The IOM from WIS 91600 exhibits similarities in its abundance and bulk δ15N value with IOM from another ungrouped carbonaceous chondrite Tagish Lake, while it exhibits H/C, δ13C, and δD values that are more similar to IOM from the heated CM, Pecora Escarpment (PCA) 91008. The 13C NMR spectra of IOM of WIS 91600 and Tagish Lake are similar, except for a greater abundance of CHxO species in the latter and sharper carbonyl absorption in the former. Unusual cross‐polarization (CP) dynamics is observed for WIS 91600 that indicate the presence of two physically distinct organic domains, in which the degrees of aromatic condensation are distinctly different. The presence of two different organic domains in WIS 91600 is consistent with its brecciated nature. The formation of more condensed aromatics is the likely result of short duration thermal excursions during impacts. The fact that both WIS 91600 and PCA 91008 were subjected to short duration heating that is distinct from the thermal history of type 3 chondrites is confirmed by Carbon‐XANES. Finally, after being briefly heated (400 °C for 10 s), the pyrolysis behavior of Tagish Lake IOM is similar to that of WIS 91600 and PCA 91008. We conclude that WIS 91600 experienced very moderate, short duration heating at low temperatures (&lt;500 °C) after an episode of aqueous alteration under conditions that were similar to those experienced by Tagish Lake.

Stable N isotope composition of nitrate reflects N transformations during the passage of water through a montane rain forest in Ecuador

Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO3 −–N, NH4 +–N and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15–0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural 15N abundance in NO3 − during the passage of rain water through the ecosystem and bulk δ15N values in soil to detect N transformations. Depletion of 15N in NO3 − and increased NO3 −–N fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO3 − concentrations progressively decreased and were enriched in 15N but did not reach the δ15N values of solid phase organic matter (δ15N = 5.6–6.7‰). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the δ15N value of NO3 − in litter leachate was smaller (δ15N = −1.58‰) than in the other quarters (δ15N = −9.38 ± SE 0.46‰) probably because of reduced mineralization and associated fractionation against 15N. Nitrogen isotope fractionation of NO3 − between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO3 −–N from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO3 − gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream.

Source of Nitrous Oxide Emissions during the Cow Manure Composting Process as Revealed by Isotopomer Analysis of and amoA Abundance in Betaproteobacterial Ammonia-Oxidizing Bacteria

A molecular analysis of betaproteobacterial ammonia oxidizers and a N(2)O isotopomer analysis were conducted to study the sources of N(2)O emissions during the cow manure composting process. Much NO(2)(-)-N and NO(3)(-)-N and the Nitrosomonas europaea-like amoA gene were detected at the surface, especially at the top of the composting pile, suggesting that these ammonia-oxidizing bacteria (AOB) significantly contribute to the nitrification which occurs at the surface layer of compost piles. However, the (15)N site preference within the asymmetric N(2)O molecule (SP = delta(15)N(alpha) - delta(15)N(beta), where (15)N(alpha) and (15)N(beta) represent the (15)N/(14)N ratios at the center and end sites of the nitrogen atoms, respectively) indicated that the source of N(2)O emissions just after the compost was turned originated mainly from the denitrification process. Based on these results, the reduction of accumulated NO(2)(-)-N or NO(3)(-)-N after turning was identified as the main source of N(2)O emissions. The site preference and bulk delta(15)N results also indicate that the rate of N(2)O reduction was relatively low, and an increased value for the site preference indicates that the nitrification which occurred mainly in the surface layer of the pile partially contributed to N(2)O emissions between the turnings.