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The isotopic ratio 18O/16O of dissolved O2 in aquatic systems is affected by the preferential biological uptake of 16O (ε). Studies over the past six decades reveal that during incubation experiments, the isotopic effect of microorganism respiration (εorganism) varies in the range of −18‰ to −22‰. In contrast, natural variations in the deep‐ocean O2 concentration and δ18O levels show a considerably weaker effect (~ −10‰). The differences between these observations have been explained to result from either O2 uptake by sediments or organic particles that, due to diffusion‐limited respiration, are expected to weakly fractionate oxygen isotopes, by mixing processes or by weak fractionation at low temperatures. To gain better insight, we studied oxygen demand and δ18O in the deep, cold hypolimnion of Lake Stechlin between 2018 and 2021 as well as in various laboratory incubations. Our incubation results demonstrate an εorganism of about −24‰. Simple model calculations demonstrate a sediment O2 demand isotope effect (εSOD) of about −8.4‰, and a variated water‐column O2 demand isotope effect (εWOD) which is lower than εorganism, ranging from −13.9‰ in 2019 to −23‰ in 2021. Accompanying experiments indicate that the lower magnitude of εWOD may be related to respiration at organic particles lending to a weaker fractionation effect. Thus, variations in εWOD may reflect a changing partitioning of hypolimnion oxygen uptake between suspended particles and their containing microorganisms. Based on own incubation experiments with Daphnia carcasses, we discuss how possible changes of particle rigidity might influence εWOD. [ABSTRACT FROM AUTHOR]

The 18O content of atmospheric O2 is an important tracer for past changes in the biosphere. Its quantitative use depends on knowledge of the discrimination against 18O associated with the various O2 consumption processes. Here we evaluated, for the first time, the in situ 18O discrimination associated with soil respiration in natural ecosystems. The discrimination was estimated from the measured [O2] and δ18O of O2 in the soil-air. The discriminations that were found are 10.1 ± 1.5‰, 17.8 ± 1.0‰, and 22.5 ± 3.6‰, for tropical, temperate, and boreal forests, respectively, 17.9 ± 2.5‰ for Mediterranean woodland, and 15.4 ± 1.6‰ for tropical shrub land. Current understanding of the isotopic composition of atmospheric O2 is based on the assumption that the magnitude of the fractionation in soil respiration is identical to that of dark respiration through the cytochrome pathway alone (∼18‰). The discrimination we found in the tropical sites is significantly lower, and is explained by slow diffusion in soil aggregates and root tissues that limits the O2 concentration in the consumption sites. The high discrimination in the boreal sites may be the result of high engagement of the alternative oxidase pathway (AOX), which has high discrimination associated with it (∼27‰). The intermediate discrimination (∼18‰) in the temperate and Mediterranean sites can be explained by the opposing effects of AOX and diffusion limitation that cancel out. Since soil respiration is a major component of the global oxygen uptake, the contribution of large variations in the discrimination, observed here, to the global Dole Effect should be considered in global scale studies.

Anthropogenic‐induced variations of the Atlantic Meridional Overturning Circulation (AMOC) and the associated Deep‐Water Formation (DWF) are a major concern. Using measurements of triple oxygen isotopes in the deep North Atlantic, we present novel evidence for a dramatic decadal to centennial shift in ocean conditions at the source region of DWF. These measurements suggest a recent decrease in the percentage of photosynthetic O2 derived from the source regions of AMOC in the Nordic Seas compared to the Little Ice Age. 1‐D model simulations suggest that a reduction in photosynthetic O2 production can explain the observed decrease. Alternatively, it may indicate a substantial decrease in sea‐ice cover and thus increased air‐sea gas exchange, bringing the isotopic composition of O2 closer to equilibrium with the atmosphere. Our novel data can serve as a benchmark for climate models. Plain Language Summary: In this study, we use the isotopic signature of dissolved oxygen gas in the deep North Atlantic Ocean to demonstrate a recent dramatic change in the Nordic Seas. Water in the deep Atlantic at present was at the surface of the Nordic Seas several hundred years ago, during the Little Ice Age (LIA). The isotopic signature shows that the percentage of oxygen sourced from photosynthesis was greater during the LIA. This may indicate a greater rate of photosynthesis than at present. However, we know that when sea ice covers the ocean surface, it prevents the influx of atmospheric oxygen, while sunlight penetration through the ice keeps a high rate of photosynthesis and production of biological oxygen. Therefore, it is also possible that during the LIA, sea‐ice cover was much more extensive than today. Key Points: Oxygen isotopic evidence for a recent dramatic shift in the North Atlantic source region of deep water‐massesThe isotopic measurements suggest high extent of ice cover and/or higher primary production rates during the Little Ice AgeThese new observations can serve as a benchmark for climate models [ABSTRACT FROM AUTHOR]

The isotopic composition of dissolved O2 (δ18O) in aquatic environments is strongly affected by the preferential uptake of the lighter isotopologue during biological consumption processes. Numerous studies have shown that during incubation experiments, the isotopic effect of microorganism respiration (εorganism) is on the order of −20‰. However, studies of the co‐variations of O2 and δ18O in natural environments show considerably weaker in situ fractionation (εapp). A possible explanation for this discrepancy is that a significant fraction of the O2 consumption is diffusion‐limited. Although this is a generally accepted mechanism in sediments, it cannot explain the weak fractionations observed in mid‐ocean sites. Here, we analyze a time series of O2, δ18O, and auxiliary data from the northern Gulf of Aqaba (Red Sea). Although an incubation experiment showed strong fractionation against the heavy isotopologue (εorganism = −24.5‰), the in situ εapp was only −14‰ in deep water isolated from the photic zone. We show that this result requires an additional O2 consumption mechanism with weak fractionation, rather than mixing, and suggest that this mechanism is diffusion‐limited respiration into aggregates of organic material. We estimate that this mechanism could be responsible for 30% of the O2 consumption in the Gulf and suggest that it may also constitute a major O2 consumption pathway in the world's oceans. [ABSTRACT FROM AUTHOR]

During the electrolysis of water in an acidified medium, ozone is produced, in association with oxygen, at the anode. This ozone is found to be depleted in heavy isotopes ([sup.18]O and [sup.17]O), with respect to the source water, following a strict mass-dependent rule. Our experiments also suggest that the isotopes are distributed at the apex and base positions of the bent ozone molecule in a random fashion, without obeying the zero-point energy constraint Endowed with these characteristics, the electrolytic ozone provides a source of reference that has a known internal heavy isotope distribution for spectroscopic studies. In addition, this ozone, when subjected to photolytic decomposition, can be used as a source of atomic oxygen with mass-dependent isotope ratios that can be varied by simply changing the water composition. Such an oxygen source is important for studying isotope effects in gas-phase recombination/exchange reactions such as COO + [O.sup.*] [right arrow] [[COOO.sup.*]] [right arrow] [COO.sup.*] + O.

A high-resolution record of carbon isotope changes during the late Holocene was obtained in core GA-112 from the easternmost Mediterranean. Over the past 3600 yr, a gradual [sup.13]C-depletion trend shown by both planktonic and benthic foraminifera corresponds to a global rise in atmospheric C[O.sub.2]. The global and local trends suggest that aridification and biomass destruction in the Nile basin played a major role in the global C[O.sub.2] rise in the late Holocene. Superimposed on the general [delta][sup.13]C trend are four mirror-image cycles in [delta][sup.13]C of planktonic and benthic foraminifera. Higher planktonic [delta][sup.13]C values reflect periods of high productivity off the coast of Israel. Simultaneously, decrease in the benthic [delta][sup.13]C values occurred because of degradation of higher organic-matter fluxes to the sediment. Therefore, large differences between the two records indicate periods of high productivity. The Eastern Mediterranean productivity fluctuations respond to a periodical replenishment of nutrients to the mixed layer via Nile runoff. Historical data of the Nile flood fluctuations for the past ~1400 yr show a remarkable coincidence between high and low Nile floods and high and low Eastern Mediterranean productivity levels, which are correlated with the periodic monsoonal activity over the Nile headwaters. Keywords: Eastern Mediterranean, foraminiferal [delta][sup.13]C, Nile-derived nutrients, productivity cycles, late Holocene.

The naturally occurring isotopes of oxygen are of mass numbers 16,17 and 18, and their ratio in atmospheric O(sub2) relies mainly on the isotopic composition of O(sub2) produced from terrestrial and aquatic plants. An experiment based on estimates of the size of the (super17)O/(super16)O anomaly in tropospheric O(sub2) was undertaken. It is argued that it mainly reflects the influences of biospheric cycling and stratospheric photochemical processes. It is proposed that the magnitude of the tropospheric (super17)O anomaly is useful as a tracer of global biosphere production.

Rationale Measurements of oxygen-17 excess (17Oexcess) in carbonates have become of great importance. However, to compare results obtained by different laboratories, it is necessary to normalize them to international standards. With this purpose in mind, we measured accurate and high precision δ17O and 17Oexcess values for NBS19 and NBS18, two international standards, for which δ18O values are already widely used as the references for carbonates. Methods The measurements are based on isotopic exchange at steady state between O2 and CO2 over hot platinum sponge at 750°C. Dual-inlet isotope ratio mass spectrometry (IRMS) measurements of the δ17O and δ18O values of this O2 allow δ17O values of CO2 to be obtained with very high precision (0.01 to 0.03 ‰) and, correspondingly, accurate 17Oexcess values with a precision of 5 per meg. Results We measured, for the first time, the δ17O values and 17Oexcess of CO2 liberated from NBS19 at 25°C (39.196 ± 0.026; 20.276 ± 0.015 ‰ and −227 ± 4 per meg, respectively) and NBS18 (17.591 ± 0.041 ‰; 9.253 ± 0.018 ‰ and 3 ± 5 per meg, respectively). The values are given versus VSMOW. Conclusions Accurate values of δ17O and 17Oexcess of the international standards NBS19-CO2 and NBS18-CO2 are now available. The new values should be used for normalization of measured oxygen isotope ratios of carbonates to allow meaningful comparison of results among different laboratories. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]