Isotopic Equilibrium Research Articles

The effects of incubation time, temperature and nitrogen concentration on the isotopic signature (δ15N) of the macroalga Chondrus crispus

Macroalgae can incorporate the isotopic signature (δ15N) of their external environment and be used as ‘bioindicators’ to map and identify between multiple sources of anthropogenic nitrogen. However, evidence suggests that the isotopic signature of their tissues can also be influenced by a range of other factors, which could confound their use as a monitoring tool. Hence, this study aimed to better understand the factors affecting macroalgae isotopic signatures, using Chondrus crispus as a model species. This was achieved by exposing C. crispus to 15N-enriched NO3− and/or NH4+ at varying concentrations, temperatures and incubation times under controlled laboratory conditions. Longer incubation times promoted macroalgae δ15N values closer to that of the external nitrogen source. However, even after 28 days incubation, macroalgae δ15N was still >30‰ lower than the source. This suggests that the δ15N of C. crispus is unlikely to reach isotopic equilibrium with its external environment within a time frame that is feasible for most field studies. However, C. crispus can be used to highlight relative isotopic differences between sources even at very low nitrogen concentrations (< 4 μM). Macroalgae incorporated the isotopic signature of external sources faster at higher nitrogen concentrations. This was fastest for macroalgae exposed to a combination of NH4+ and NO3−, followed by just NH4+, and slowest for NO3−. These results could have implications for bioindicator studies where proportions and concentrations of NO3− and NH4+ vary in time and space. Likewise, future field studies may have to account for temperature in their sampling design if temperatures vary spatially and/or seasonally, as the δ15N of C. crispus more closely resembled that of the external nitrogen source at 7 °C than 14 °C. Overall, this study found that the δ15N of C. crispus was not just a function of the source, as it was also influenced by incubation time, temperature and by the concentration and types of nitrogen present. As C. crispus can tolerate high concentrations of NH4+ and a wide range of temperatures, it could be a suitable bioindicator species for monitoring nitrogen rich effluents in many different environments providing the results are interpreted cautiously.

Variation in the diagenetic response of aragonite archives to hydrothermal alteration

Diagenesis of carbonate minerals is ubiquitous throughout the geologic record. Alteration is initiated immediately after deposition, or takes place in the endo- and exoskeletons as early as during the lifetime of a given carbonate-secreting biota, and can continue throughout the burial history of carbonate sediments and rocks. Variations in the diagenetic response of carbonate archives pose challenges for the reconstruction of past environmental conditions based on proxy data. This paper comparatively assesses alteration features of different aragonitic materials by experimentally-induced diagenesis. A multitude of factors lead to different reactivity and responses of a given aragonite archive, and provide insight to the interpretation of diagenetically altered material in the rock record. Chosen materials used in this study include relatively organic-rich samples such as coral skeletons and bivalve shells, and organic-lean abiotic carbonates such as speleothems and aragonite single crystals. Obtained datasets include distributions of elements, organics, carbon and oxygen isotope ratios, and crystallographic features. Observed variations in diagenetic responses include mineralogy of the diagenetic phases, rate and extent of mineral transformation, distribution of foreign ions in the crystal lattice (primarily Sr, Mg, and S), and the number of specific processes and products along diagenetic pathways. Alteration is shown to be primarily controlled by the initial diagenetic susceptibility of the sample (including porosimetry and structural characteristics, concentrations of organic material, and primary amounts of trace elements in the carbonate, such as Mg). Structural characteristics lead to initial internally “fluid-” or “rock-buffered” conditions, with low porosity and permeability resulting in a greater effect of internal fluids and organics. Differences in the amount of organic content and internal fluids affect transformation rates, secondary mineralogy, and isotope equilibria. Samples with relatively high porosity, high mineral transformation rates, and high primary Mg/Ca, may preferentially form secondary aragonite during fast equilibration with the diagenetic environment. Our results suggest that the degree and nature of diagenetic alteration of aragonite materials are strongly controlled by the micro- to nano-scale internal architecture governing the availability and transfer of aqueous fluids. Results of this study provide significant implications for the interpretation of diagenetic signals in carbonate archives, and have direct significance for the mechanistic understanding of carbonate diagenesis and (paleo)environmental conditions.

Geographical variations in the slope of the δ2H–δ18O meteoric water line over Europe: a record of increasing continentality

δ2H and δ18O values of precipitations follow an empirical linear relationship at the global scale that is called the Global Meteoric Water Line (GMWL) and characterized by a slope of 8. However, Local Meteoric Water Lines (LMWLs) may have different slopes S depending on their geographic situation. Monthly δ2H and δ18O of precipitation have been compiled from European International Atomic Energy Agency (IAEA) stations. Those data allowed the calculation of the slopes S of the δ2H–δ18O LMWL determined for each station. S increases with longitude ϕ from c. 5 (Portugal) to c. 9 (Russia) – they are positively correlated with relative humidity (RH), negatively with temperature and positively with the mean intra-annual amplitude of temperatures, which is a proxy of continentality. Slopes of 5–6, recorded in SW Europe, reflect mean RH (70–75%) and sea surface temperatures (c. 25°C) of the Central Atlantic Ocean where the main flux of moisture is formed before being transported by the westerlies. In addition, falling water droplets within an air column with a high RH (>80%) and low temperature are expected to escape sub-cloud evaporation. Therefore, slopes with values close to 9 are considered to reflect isotopic equilibrium conditions during the condensation of water vapour in clouds.

Constraining speleothem oxygen isotope disequilibrium driven by rapid CO2 degassing and calcite precipitation: Insights from monitoring and modeling

Oxygen isotopes are the most commonly applied speleothem proxy for reconstructing Quaternary changes in precipitation and/or temperature. These interpretations are either limited to qualitative wetting and drying trends or rely on theoretical, experimental and/or empirical equilibrium isotope fractionation factors for more quantitative constraints. These various fractionation factors have similar temperature sensitivities, but their absolute values differ, and cave calcite does not appear to generally precipitate in isotopic equilibrium with its drip water. Rapid CO2 degassing paired with calcite precipitation, both occurring under disequilibrium conditions, are a set of mechanisms commonly invoked to explain offsets between observed and equilibrium isotopic fractionation between cave calcites and drip waters. However, the relevance of these disequilibrium mechanisms to speleothem records remains unresolved. Here, we compare measured δ18O values of modern speleothem calcite from a tropical cave in Guam to calcite δ18O values predicted by a modified version of the ISOLUTION proxy system model. This extends the global comparison of cave drip water and modern calcite δ18O values to higher temperatures. We initialize the model using contemporaneous measurements of drip water (δ18O values, [Ca+], and pH), and cave air (CO2, and T) from four drip sites over 3.5 years of monitoring in the cave. Through this comparison, we show that for a slow drip-rate site, ventilation-driven CO2 degassing can explain seasonal variations in calcite oxygen isotope composition. At faster-dripping sites in this cave, the seasonal effect is limited. At these sites, the DIC reservoir is replenished by new drips faster than its isotopic composition can be modified by degassing CO2 and calcite precipitation, whether occurring each is occurring as an equilibrium or kinetic process. For the slow drip rate site, however, this is the first observation of cave air CO2 variations exerting a control on cave calcite oxygen isotope values. The confirmation of ventilation-driven processes controlling oxygen isotope values at a slow-drip site advances the process-based understanding of stalagmite formation that is required to move beyond the wetter-or-drier paradigm and make quantitative interpretations of speleothem oxygen isotope records.

Quantifying microbial growth and carbon use efficiency in dry soil environments via 18 O water vapor equilibration.

Soil microbial physiology controls large fluxes of C to the atmosphere, thus, improving our ability to accurately quantify microbial physiology in soil is essential. However, current methods to determine microbial C metabolism require liquid water addition, which makes it practically impossible to measure microbial physiology in dry soil samples without stimulating microbial growth and respiration (namely, the “Birch effect”). We developed a new method based on in vivo 18O‐water vapor equilibration to minimize soil rewetting effects. This method allows the isotopic labeling of soil water without direct liquid water addition. This was compared to the main current method (direct 18O‐liquid water addition) in moist and air‐dry soils. We determined the time kinetics and calculated the average 18O enrichment of soil water over incubation time, which is necessary to calculate microbial growth from 18O incorporation in genomic DNA. We tested isotopic equilibration patterns in three natural and six artificially constructed soils covering a wide range of soil texture and soil organic matter content. We then measured microbial growth, respiration and carbon use efficiency (CUE) in three natural soils (either air‐dry or moist). The proposed 18O‐vapor equilibration method provided similar results as the current method of liquid 18O‐water addition when used for moist soils. However, when applied to air‐dry soils the liquid 18O‐water addition method overestimated growth by up to 250%, respiration by up to 500%, and underestimated CUE by up to 40%. We finally describe the new insights into biogeochemical cycling of C that the new method can help uncover, and we consider a range of questions regarding microbial physiology and its response to global change that can now be addressed.

Controls on formation and alteration of early diagenetic dolomite: A multi-proxy δ44/40Ca, δ26Mg, δ18O and δ13C approach

The full potential of the dolomite Ca isotope proxy only unfolds when combined with data of the other main elements (C, O, Mg) in the crystal lattice of Mg-carbonates. Data presented here reveal the level of complexity inherent to dolomite precipitation and alteration environments and add new constraints to the understanding of early diagenetic dolomite formation. Well-constrained Precambrian to Pleistocene dolomites were investigated, representing three characteristic formation and alteration environments: (i) sabkha, (ii) altered marine and (iii) lacustrine/palustrine dolomites. Primary sabkha dolomites with typically low cation ordering degree (COD), high δ13C and low δ44/40Ca values contrast with recrystallized sabkha dolomites with relatively high COD, low δ13C and high δ44/40Ca values. Both δ13C and δ44/40Ca values of sabkha dolomite bear witness to the relative effects of kinetic and equilibrium Ca isotope fractionation conditions. Primary sabkha dolomites display Δ44/40Cadolomite-fluid ranging from −0.4 to −1.3‰, whereas recrystallized dolomite is approaching isotopic equilibrium (Δ44/40Cadolomite-fluid ∼ 0‰). Using the fractionation factor deduced from the sabkha dolomite data set, recrystallized Precambrian dolomite points to a δ44/40Caseawater/pore fluid of about 1.2‰ (SRM 915a), a value that is distinctively lower compared to previously suggested ones and modern seawater. Altered marine dolomites display evidence for meteoric overprint, indicated by δ18O values as low as −4.62‰. Both Mg and Ca isotope signatures correlate with δ18O values of altered marine dolomites, whereas δ13C values lack correlation with the other isotope systems. We propose that freshwater circulated through silicate aquifers prior to reaching the dolostone units and Ca, Mg and O isotopes of altered marine dolomites reflect variable degrees of this meteoric overprint. Lacustrine/palustrine dolomites display a correlation between the isotope values of C, Mg and Ca. These dolomites are formed during pulses of marine ingression in swamp, playa and lake environments and are thus characterized by water-logged conditions (anaerobic) and saline, sulfate-rich fluids. Bacterial-sulfate reduction induces dolomite formation and leads to lower δ13C and δ44/40Ca as well as to higher δ26Mg values. The Ca isotope proxy acts as a benchmark against which other proxy data can be calibrated or processes tested. Taking COD and dolomite stoichiometry into consideration, the here documented multi-proxy isotope approach is promising and provides benchmarks against which proxy signals can be calibrated and tested. Interestingly, even the limited data sets shown here point to patterns that can be interpreted in a meaningful manner that is of relevance for dolomite research.

Mineralogical and Isotopic Characteristics of Sodic-Calcic Alteration in the Highland Valley Copper District, British Columbia, Canada: Implications for Fluid Sources in Porphyry Cu Systems

AbstractThe Highland Valley Copper porphyry Cu (±Mo) district is hosted in the Late Triassic Guichon Creek batholith in the Canadian Cordillera. Fracture-controlled sodic-calcic alteration is important because it forms a large footprint (34 km2) outside of the porphyry Cu centers. This alteration consists of epidote ± actinolite ± tourmaline veins with halos of K-feldspar–destructive albite (1–20 XAn) ± fine-grained white mica ± epidote. The distribution of sodic-calcic alteration is strongly influenced by near-orthogonal NE- and SE-trending fracture sets and by proximity to granodiorite stocks and porphyry dikes. Multiple stages of sodic-calcic alteration occurred in the district, which both pre- and postdate Cu mineralization at the porphyry centers.The mineral assemblages and chemical composition of alteration minerals suggest that the fluid that caused sodic-calcic alteration in the Guichon Creek batholith was Cl bearing, at near-neutral pH, and oxidized, and had high activities of Na, Ca, and Mg relative to propylitic and fresh-rock assemblages. The metasomatic exchange of K for Na, localized removal of Fe and Cu, and a paucity of secondary quartz suggest that the fluid was thermally prograding in response to magmatic heating. Calculated δ18Ofluid and δDfluid values of mineral pairs in isotopic equilibrium from the sodic-calcic veins and alteration range from 4 to 8‰ and −20 to −9‰, respectively, which contrasts with the whole-rock values for least altered magmatic host rocks (δ18O = 6.4–9.4‰ and δD = −99 to −75‰). The whole-rock values are suggested to reflect residual magma values after D loss by magma degassing, while the range of hydrothermal minerals requires a mixed-fluid origin with a contribution of magmatic water and an external water source. The O-H isotope results favor seawater as the source but could also reflect the ingress of Late Triassic meteoric water. The 87Sr/86Srinital values of strongly Na-Ca–altered rocks range from 0.703416 to 0.703508, which is only slightly higher than the values of fresh and potassic-altered rocks. Modeling of those data suggests the Sr is derived predominantly from a magmatic source, but the system may contain up to 3% seawater Sr. Supporting evidence for a seawater-derived fluid entrained in the porphyry Cu systems comes from boron isotope data. The calculated tourmaline δ11Bfluid values from the sodic-calcic domains reach 18.3‰, which is consistent with a seawater-derived fluid source. Lower tourmaline δ11Bfluid values from the other alteration facies (4–10‰) suggest mixing between magmatic and seawater-derived fluids in and around the porphyry centers. These results imply that seawater-derived fluids can infiltrate batholiths and porphyry systems at deep levels (4–5 km) in the crust. Sodic ± calcic alteration may be more common in rocks peripheral to porphyry Cu systems hosted in island-arc terranes and submarine rocks than currently recognized.

369-OR: [1,2-13C2]-L-Glutamine Mass Isotopomers Map Hepatic Mitochondrial Metabolism without Tracer Interference

Background: Tracers should be maximally descriptive with minimal interference to the metabolic system of interest. Divergent concepts of in vivo hepatic metabolism (e.g., anaplerosis) may have emerged, in part, from invalid assumptions and/or perturbations associated with common tracers. Aim: We evaluated three common substrate tracers for intrahepatic metabolism vis-à-vis a novel independent [1,2-13C2]-L-glutamine strategy. Methods: Overnight fasted Sprague-Dawley rats underwent primed, continuous infusions of reported tracer rates for lactate, propionate, acetate or glutamine. Plasma/freeze-clamped livers were obtained at 0, 5, 15, 30, 60, and 120 min. Plasma/tissue isotopomers were subject to steady state and kinetic Mass Isotopomer MultiOrdinate Spectral Analysis (MIMOSA) and reported relative to citrate synthase (CS). Results: In contrast to glutamine infusion, plasma insulin ([i]) glucose concentration ([g]), and/or plasma glucose turnover (Ra) were increased by acetate ([i] = 1.5 fold, p = 0.015; [g] = 129 mg/dl, p=0.021), lactate ([i]= 2.03 fold, p = 0.0001; [g] = 156mg/dl, p = 0.0001, Ra = 7.2 mg/kg/min, p=0.006), and propionate ([i]= 1.7 fold, p = 0.0013; [g] = 165 mg/dl, p = 0.0001, Ra = 7.8 mg/kg/min, p = 0.0017). [1,2-13C2]-L-glutamine quickly reached isotopic equilibrium without impacting mitochondrial metabolite concentration or glutamate anaplerosis. Transaminase exchange (Vx) was &amp;gt;&amp;gt;CS. PEPCK was 1.6 x CS with pyruvate only accounting for 76% of anaplerosis (remainder from propionate and glutamate anaplerosis). PK was less than CS while ATP citrate lyase (ACLy) was 18% of CS. Conclusion: Direct in vivo measurements of pyruvate cycling rates were low relative to CS while ACLy, IDH exchange, and non-pyruvate anaplerosis contributed substantially to hepatic metabolism. [1,2-13C2]-L-glutamine MIMOSA delivered an unencumbered, direct assessment of hepatic mitochondrial metabolism. Disclosure S. Siebel: None. R.L. Cardone: None. A. Abulizi: None. R. Raaisa: None. R.M. Williams: None. R. Sehgal: None. G. Butrico: None. G. Cline: None. D.L. Rothman: None. G.F. Mason: None. R. Kibbey: None. Funding National Institutes of Health (2K12DK094714-06, R01DK108283)

Effects of Prolonged Energy Restriction and Dietary Protein on Muscle Protein Synthesis and Proteome Dynamics in Obese Zucker Rats

Abstract Objectives High protein (HP) diets during short-term energy restriction (ER) attenuate energy-mediated reductions in muscle protein synthesis (MPS). MPS-adaptive responses to HP diets during prolonged ER are not well described. This study examined the effects of prolonged ER and HP on MPS and the synthesis rates of numerous individual muscle proteins. Methods Female 6-wk-old obese Zucker (leprfa+/fa+, n = 48) rats were randomized to one of four diet groups for 10 weeks: ad libitum-standard protein (AL-SP; 14% protein), AL-HP (35% protein), ER-SP, and ER-HP (both fed 60% of intake of AL-SP). At the start of week 10, D2O was administered by intraperitoneal injection and isotopic equilibrium was maintained daily by providing D2O in drinking water. Rats were euthanized after 1 week of labeling, and mixed-MPS (gastrocnemius), absolute mixed-MPS (mixed-MPS x muscle protein content), proteome dynamics, and protein half-lives [rate/d (k) = –ln(1-f)/d, where f is mixed-MPS and t is time in days; t1/2 (days) = ln(2)/k] were quantified. Results Mixed-MPS was not altered by energy status and protein intake. Gastrocnemius mass was lower (P &amp;lt; 0.001) in ER-fed rats than AL-fed rats and higher (P = 0.034) for AL-HP than AL-SP. As a result, absolute mixed-MPS was lower (P &amp;lt; 0.005) in ER than AL, regardless of dietary protein. Absolute synthesis in 24 of 26 myofibrillar, 32 of 61 mitochondrial, and 55 of 60 cytoplasmic measured proteins were lower in ER than AL (P &amp;lt; 0.05), regardless of dietary protein. The difference in absolute synthesis of myofibrillar, mitochondrial, and cytoplasmic proteins due to ER compared to AL was 28%, 16%, and 27%, respectively. Comparison of HP and SP within each energy state revealed lower turnover rates and prolonged half-lives for a majority of measured muscle proteins in HP than in SP in both ER and AL conditions (P &amp;lt; 0.001). Conclusions Prolonged ER in obese Zucker rats exerted a strong suppressive effect on myofibrillar, mitochondrial, and cytoplasmic MPS, suggesting reduced protein accretion contributed to lower gastrocnemius mass in ER-fed rats. Lower turnover rates of most muscle proteins in HP-fed rats without reductions in protein pool size (i.e., tissue mass) suggests prolonged HP intake, independent of energy, may prolong muscle protein lifespan of in obese Zucker rats. Funding Sources Supported by USAMRDC; authors’ views not official U.S. Army or DoD policy.

Atmospheric vapour and precipitation are not in isotopic equilibrium in a continental mountain environment

AbstractIsotopic exchange with atmospheric vapour can strongly influence the isotopic values of evaporating surface water bodies (e.g., lakes), influencing our understanding of hydrological processes across aquatic and terrestrial environments. Rather than measure the isotopic values of the atmosphere directly, it is much more common to estimate values by assuming equilibrium with local precipitation. This assumption may introduce large errors, thereby biasing hydrological inferences and understanding. The pattern and magnitude of this error has been quantified only in a few circumstances. We compared observations of vapour and precipitation isotope values over a four‐year record collected in a montane environment in the central Rocky Mountains of North America. We further investigated factors and conditions promoting disequilibrium. Scenario comparisons assessed the impact of theoretical and methodological elements on the accuracy of the equilibrium assumption. We found that the equilibrium assumption was not well supported by direct and continuous observations of vapour isotopes using tower‐based laser isotope spectroscopy, particularly during the summer months. Across all scenarios, errors associated with the equilibrium assumption were high, credibly ranging from 14 to 154 ‰ for δ2H and 1.5 to 16.3 ‰ for δ18O. Environmental covariates (e.g., vapour pressure deficit, air pressure) helped explain some of the apparent disequilibrium. Although the equilibrium assumption for estimating atmospheric vapour isotope values may not be applicable in a continental montane environment, our findings highlight opportunities for using direct vapour isotope measurements to better understand vapour sources, air mass mixing, and phase changes over complex mountainous terrain, which in turn may better constrain regional‐ to global‐scale hydrological process estimates, such as evapotranspiration rates and the water budgets of mountain lakes.

Biosignatures Associated with Freshwater Microbialites.

Freshwater microbialites (i.e., lithifying microbial mats) are quite rare in northern latitudes of the North American continent, with two lakes (Pavilion and Kelly Lakes) of southeastern BC containing a morphological variety of such structures. We investigated Kelly Lake microbialites using carbon isotope systematics, phospholipid fatty acids (PLFAs) and quantitative PCR to obtain biosignatures associated with microbial metabolism. δ13CDIC values (mean δ13CDIC −4.9 ± 1.1‰, n = 8) were not in isotopic equilibrium with the atmosphere; however, they do indicate 13C-depleted inorganic carbon into Kelly Lake. The values of carbonates on microbialite surfaces (δ13C) fell within the range predicted for equilibrium precipitation from ambient lake water δ13CDIC (−2.2 to −5.3‰). Deep microbialites (26 m) had an enriched δ13Ccarb value of −0.3 ± 0.5‰, which is a signature of photoautotrophy. The deeper microbialites (>20 m) had higher biomass estimates (via PLFAs), and a greater relative abundance of cyanobacteria (measured by 16S copies via qPCR). The majority of PLFAs constituted monounsaturated and saturated PLFAs, which is consistent with gram-negative bacteria, including cyanobacteria. The central PLFA δ13C values were highly depleted (−9.3 to −15.7‰) relative to δ13C values of bulk organic matter, suggesting a predominance of photoautotrophy. A heterotrophic signature was also detected via the depleted iso- and anteiso-15:0 lipids (−3.2 to −5.2‰). Based on our carbonate isotopic biosignatures, PLFA, and qPCR measurements, photoautotrophy is enriched in the microbialites of Kelly Lake. This photoautotrophy enrichment is consistent with the microbialites of neighboring Pavilion Lake. This indication of photoautotrophy within Kelly Lake at its deepest depths raises new insights into the limits of measurable carbonate isotopic biosignatures under light and nutrient limitations.

Are the phosphate oxygen isotopes of Saharan dust a robust tracer of atmospheric P source?

Abstract Aeolian dust is a significant source of phosphorus (P) in various ecosystems. The Saharan desert is the major dust source on earth and it contributes atmospheric P to terrestrial ecosystems which often depend on these inputs. Since estimations on the contribution of dust to the atmospheric P cycle vary between studies, accurate estimations on the aerosol-P inputs to ecosystems can be enhanced by an identification method for dust from the other atmospheric P sources. In the current study, we explored the use of the oxygen stable isotopes in phosphate, δ18OP, as a marker for desert dust P. To characterize the δ18OP in Saharan dust, we analyzed atmospheric particles sampled in Puerto Rico, which has good exposure in summer to African dust with little exposure to wildfires, local, or anthropogenic influence on atmospheric inputs. Additionally, to evaluate possible isotopic fractionations in desert dust sources, which could affect the soil δ18OP before transport, we analyzed the δ18OP in Israeli desert soils that served as an analog. Saharan dust storms sampled at Puerto Rico were found to have δ18OP values of 22.5 ± 0.3‰ in the resin fraction (which represents labile inorganic P), similar to previously reported values of Saharan dust sampled in Israel. The data from Israel desert soils demonstrated that biological activity in desert crusts can have a small effect (with a maximum difference of 3.3‰ between the topsoil and the subsoil) on the δ18OP of desert soils, mainly when developed soil crust is present, as a result of intracellular isotopic equilibration between oxygen in phosphate and soil water. However, biological crusts are not considered a major dust source. Additionally, Saharan dust sampled in Israel and in Puerto Rico had a δ18OP similar to apatite minerals derived from sedimentary phosphorites. Thus, we suggest the dust source had little or no biological activity. Furthermore, the δ18OP in Saharan dust was distinctive from other atmospheric P sources, such as pollen and wildfire ash, and as a result, the δ18OP can be used to distinguish between these atmospheric P main sources.

Evaluation of geochemical records as a paleoenvironmental proxy in the hypercalcified demosponge Astrosclera willeyana

The geochemistry of the calcium carbonates of marine organisms is an excellent proxy for reconstruction of the paleoceanographic history. However, previous studies of hypercalcified demosponges (sclerosponges) are considerably fewer than those of corals, foraminifers, and bivalves. Here, we investigated stable oxygen (δ18O) and carbon (δ13C) isotopes and minor and trace element (Mg, Sr, Ba, Pb, and U) to Ca ratios of 36 living sclerosponges (Astrosclera willeyana) collected from Kume Island in the Ryukyu Islands, southwestern Japan, to evaluate the utility of geochemistry as a paleoenvironmental proxy. The δ18O, δ13C, and Sr/Ca deviations of the coevally precipitated skeleton were extremely small and almost constant at all skeletal portions, strongly suggesting that within-skeletal variations in the chemical components are negligibly small for non-symbiotic sclerosponges. Mean δ18O, δ13C, and Sr/Ca values (N = 36), falling within a quite narrow range, showed no significant evidence for intraspecific (inter-specimen) variations in the sclerosponges. The sclerosponges δ18O and δ13C were consistent with those of the aragonites precipitated in isotopic equilibrium with seawater at the growth site. The sclerosponge Sr/Ca was close to that of inorganically precipitated aragonite, and the estimated partition coefficient of 1.1 (almost unity) is identical to previously reported values for different species (Ceratoporella nicholsoni). Consequently, these results suggest that A. willeyana sclerosponge, having little vital effects on the geochemistry, is a robust indicator of paleocean environments (seawater δ18O, temperature, and dissolved inorganic carbon δ13C). Further, our evaluation study documents that sclerosponges living in deeper ocean environments can support the reconstruction of spatial and vertical paleoceanographic changes in conjunction with coral proxy records. The sclerosponge U/Ca showed little within-skeletal and intraspecific variations, but the heterogeneity and individual difference of the Mg/Ca, Ba/Ca, and Pb/Ca were relatively large, the reasons of which still remain unresolved.

Novel reverse radioisotope labelling experiment reveals carbon assimilation of marine calcifiers under ocean acidification conditions

Abstract Ocean acidification by anthropogenic carbon dioxide emissions is projected to depress metabolic and physiological activity in marine calcifiers. To evaluate the sensitivity of marine organisms against ocean acidification, the assimilation of nutrients into carbonate shells and soft tissues must be examined. We designed a novel experimental protocol, reverse radioisotope labelling, to trace partitioning of nutrients within a single bivalve species under ocean acidification conditions. Injecting CO2 gas, free from radiocarbon, can provide a large contrast between carbon dissolved in the water and the one assimilated from atmosphere. By culturing modern aquifer organisms in acidified seawater, we were able to determine differences in the relative contributions of the end members, dissolved inorganic carbon (DIC) in seawater and metabolic CO2, to shell carbonate and soft tissues. Under all pCO2 conditions (463, 653, 872, 1,137 and 1,337 μatm), radiocarbon (Δ14C) values of the bivalve Scapharca broughtonii shell were significantly correlated with seawater DIC values; therefore, shell carbonate was derived principally from seawater DIC. The Δ14C results together with stable carbon isotope (δ13C) data suggest that in S. broughtonii shell δ13C may reflect the kinetics of isotopic equilibration as well as end‐member contributions; thus, care must be taken when analysing end‐member contributions by a previous method using δ13C. The insensitivity of S. broughtonii to perturbations in pCO2 up to at least 1,337 µatm indicates that this species can withstand ocean acidification. Usage of radioisotope to dope for tracer experiments requires strict rules to conduct any operations. Yet, reverse radioisotope labelling proposing in this study has a large advantage and is a powerful tool to understanding physiology of aquifer organisms that can be applicable to various organisms and culture experiments, such as temperature, salinity and acidification experiments, to improve understanding of the proportions of nutrients taken in by marine organisms under changing environments.

Extreme silicon isotope fractionation due to Si organic complexation: Implications for silica biomineralization

A combination of theoretical predictions and isotopic equilibration experiments using the three-isotope method have been performed to assess Si isotope fractionation among minerals, and aqueous species in the presence of dissolved catechol. Aqueous Si in abiotic ambient temperature aqueous solutions is dominated by the IV-coordinated H4SiO40 species, but the presence of aqueous catechol provokes the formation of a VI-fold Si-catechol complex. Results show an equilibrium Si fractionation factor of ∼19‰ between the VI-fold coordinated Si-catechol complex and the IV-fold coordinated aqueous silicic acid, an amplitude never previously observed for silicon. The fractionation between V-fold Si-organo complexes (with diolate, glyconate or methyllactate groups) and silicic acid has also been estimated through theoretical predictions to be about −10‰. These extreme fractionations can be used to improve our ability to interpret the Si isotope compositions of natural solids, and in particular those associated with marine silica biomineralization processes (e.g. sponge spicules).

In vitro application of carbonic anhydrase to accelerate the equilibration of 18O between H2O and CO2 for the rapid measurement of 18O/16O isotope ratios in aqueous samples

ABSTRACT A novel method of the accelerated equilibration of 18O between CO2 and H2O for the measurement of the 18O/16O isotope ratios in aqueous samples with natural isotope abundances is presented. This rapid equilibrium method is based on the in vitro application of the enzyme carbonic anhydrase (CA). The CA from bovine erythrocytes was adsorptively fixed to 3-mm glass beads with an etched surface. After the addition of this carrier-fixed CA catalyst to the water sample, the isotope equilibrium was already reached after 1 h. The previously used non-catalysed 18O isotope exchange in water samples needs about 24 h. Whole blood samples also showed fast 18O isotope equilibration, which definitely results from the native presence of CA in erythrocytes. By shortening the time for sample preparation, the CA catalysed technique can significantly increase the throughput of the samples to be measured, and also 18O and 2H measurement by means of isotope ratio mass spectrometry (IRMS) may be synchronized. The 2H and 18O sample preparation can be performed in the same reaction vessel because cross-effects at the simultaneous use of Pt and CA catalysts do not occur.

Oxygen isotope fractionation between gypsum and its formation waters: Implications for past chemistry of the Kawah Ijen volcanic lake, Indonesia

Abstract Gypsum (CaSO4·2H2O) provides an opportunity to obtain information from both the oxygen isotopic composition of the water and sulfate of its formation waters, where these components are commonly sourced from different reservoirs (e.g., meteoric vs. magmatic). Here, we present δ18O values for gypsum and parent spring waters fed by the Kawah Ijen crater lake in East Java, Indonesia, and from these natural samples derive gypsum-fluid oxygen isotope fractionation factors for water and sulfate group ions of 1.0027 ± 0.0003‰ and 0.999 ± 0.001‰, respectively. Applying these fractionation factors to a growth-zoned gypsum stalactite that records formation waters from 1980 to 2008 during a period of passive degassing, and gypsum cement extracted from the 1817 eruption tephra fall deposit, shows that these fluids were in water-sulfate oxygen isotopic equilibrium. However, the 1817 fluid was &amp;gt;5‰ lighter. This indicates that the 1817 pre-eruption lake was markedly different, and had either persisted for a much shorter duration or was more directly connected to the underlying magmatic-hydrothermal system. This exploratory study highlights the potential of gypsum to provide a historical record of both the δ18Owater and δ18Osulfate of its parental waters, and provides insights into the processes acting on volcanic crater lakes or any other environment that precipitates gypsum.

Oxygen Isotope Fractionation between Phenocrysts and Melt: Equilibrium Estimation for the Alkaline Lavas of Changbaishan Volcano (Northeast China)

The mechanisms of the occurrence of nonequilibrium oxygen isotope fractionation between olivine and plagioclase phenocrysts and the groundmass of rocks (from alkaline basalts to alkaline rhyolites) of Changbaishan volcano are considered. The basaltic trachyandesite–trachyte–comendite–pantellerite rock series shows a systematic increase of the δ18O values of feldspar (from 5.4 to 7.8‰) and the rock groundmass (from 5.6 to 7.1‰). In contrast, the δ18О values of olivine decrease from 5.35‰ typical of mantle peridotites to significantly lower values (3.88–4.14‰). This is accompanied by changes in olivine composition from the Mg-rich (Fo = 74–79) to almost pure fayalite (Fo = 1), thus indicating a positive correlation between the Mg–number and δ18О of olivine. It is shown that feldspar and the rock groundmass are close to the oxygen isotopic equilibrium, whereas olivine–rock groundmass and olivine–feldspar pairs did not approach the oxygen isotopic equilibrium. The main mechanisms leading to the oxygen isotopic disequilibrium in the mineral–melt system are considered. An approach to the estimation of oxygen isotope equilibrium between the phenocrysts and melts is proposed. It takes into account the degree of melt polymerization (NBO/T ratio). This approach makes it possible to explain the positive correlation between δ18О values and olivine Mg-numbers in the Changbaishan lavas, as well as the occurrence of low δ18O values (down to +3‰) in ferrous olivine from alkaline–salic rocks. The δ18О values observed in the olivine and the rock groundmass reflect changes in the degree of melt polymerization during crystal fractionation. The proposed model does not require an additional geological process, in particular, the contribution of a low–δ18О material in the formation of the Changbaishan differentiated rock series.

Impact of iron release by volcanic ash alteration on carbon cycling in sediments of the northern Hikurangi margin

We present geochemical data collected from volcanic ash-bearing sediments on the upper slope of the northern Hikurangi margin during the RV SONNE SO247 expedition in 2016. Gravity coring and seafloor drilling with the MARUM-MeBo200 allowed for collection of sediments down to 105 meters below seafloor (mbsf). Release of dissolved Sr2+ with isotopic composition enriched in 86Sr (87Sr/86Sr minimum = 0.708461 at 83.5 mbsf) is indicative of ash alteration. This reaction releases other cations in the 30-70 mbsf depth interval as reflected by maxima in pore-water Ca2+ and Ba2+ concentrations. In addition, we posit that Fe(III) in volcanogenic glass serves as an electron acceptor for methane oxidation, a reaction that releases Fe2+ measured in the pore fluids to a maximum concentration of 184 μM. Several lines of evidence support our proposed coupling of ash alteration with Fe-mediated anaerobic oxidation of methane (Fe-AOM) beneath the sulfate-methane transition (SMT), which lies at ∼7 mbsf at this site. In the ∼30-70 mbsf interval, we observe a concurrent increase in Fe2+ and a depletion of CH4 with a well-defined decrease in δ13C-CH4 values indicative of microbial fractionation of carbon. The negative excursions in δ13C values of both DIC and CH4 are similar to that observed by sulfate-driven AOM at low SO42− concentrations, and can only be explained by the microbially-mediated carbon isotope equilibration between CH4 and DIC. Mass balance considerations reveal that the iron cycled through the coupled ash alteration and AOM reactions is consumed as authigenic Fe-bearing minerals. This iron sink term derived from the mass balance is consistent with the amount of iron present as carbonate minerals, as estimated from sequential extraction analyses. Using a numerical modeling approach we estimate the rate of Fe-AOM to be on the order of 0.4 μmol cm−2 yr−1, which accounts for ∼12% of total CH4 removal in the sediments. Although not without uncertainties, the results presented reveal that Fe-AOM in ash-bearing sediments is significantly lower than the sulfate-driven CH4 consumption, which at this site is 3.0 μmol cm−2 yr−1. We highlight that Fe(III) in ash can potentially serve as an electron acceptor for methane oxidation in sulfate-depleted settings. This is relevant to our understanding of C-Fe cycling in the methanic zone that typically underlies the SMT and could be important in supporting the deep biosphere.

A large epeiric methanogenic Bambuí sea in the core of Gondwana supercontinent?

Carbon isotope compositions of both sedimentary carbonate and organic matter can be used as key proxies of the global carbon cycle and of its evolution through time, as long as they are acquired from waters where the dissolved inorganic carbon (DIC) is in isotope equilibrium with the atmospheric CO2. However, in shallow water platforms and epeiric settings, the influence of local to regional parameters on carbon cycling may lead to DIC isotope variations unrelated to the global carbon cycle. This may be especially true for the terminal Neoproterozoic, when Gondwana assembly isolated waters masses from the global ocean, and extreme positive and negative carbon isotope excursions are recorded, potentially decoupled from global signals. To improve our understanding on the type of information recorded by these excursions, we investigate the paired δ13Ccarb and δ13Corg evolution for an increasingly restricted late Ediacaran-Cambrian foreland system in the West Gondwana interior: the basal Bambuí Group. This succession represents a 1st-order sedimentary sequence and records two major δ13Ccarb excursions in its two lowermost lower-rank sequences. The basal cap carbonate interval at the base of the first sequence, deposited when the basin was connected to the ocean, hosts antithetical negative and positive excursions for δ13Ccarb and δ13Corg, respectively, resulting in Δ13C values lower than 25‰. From the top of the basal sequence upwards, an extremely positive δ13Ccarb excursion is coupled to δ13Corg, reaching values of +14‰ and −14‰, respectively. This positive excursion represents a remarkable basin-wide carbon isotope feature of the Bambuí Group that occurs with only minor changes in Δ13C values, suggesting change in the DIC isotope composition. We argue that this regional isotopic excursion is related to a disconnection between the intrabasinal and the global carbon cycles. This extreme carbon isotope excursion may have been a product of a disequilibria between the basin DIC and atmospheric CO2 induced by an active methanogenesis, favored by the basin restriction. The drawdown of sulfate reservoir by microbial sulfate reduction in a poorly ventilated and dominantly anoxic basin would have triggered methanogenesis and ultimately methane escape to the atmosphere, resulting in a13C-enriched DIC influenced by methanogenic CO2. Isolated basins in the interior of the Gondwana supercontinent may have represented a significant source of methane inputs to the atmosphere, potentially affecting both the global carbon cycle and the climate.