Andrew J. Schauer

Onset of deglacial warming in West Antarctica driven by local orbital forcing

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate. Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago. An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently. Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere associated with an abrupt decrease in Atlantic meridional overturning circulation. However, significant warming in West Antarctica began at least 2,000 years earlier. Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics.

Hydroxylamine as an intermediate in ammonia oxidation by globally abundant marine archaea

The ammonia-oxidizing archaea have recently been recognized as a significant component of many microbial communities in the biosphere. Although the overall stoichiometry of archaeal chemoautotrophic growth via ammonia (NH3) oxidation to nitrite (NO2−) is superficially similar to the ammonia-oxidizing bacteria, genome sequence analyses point to a completely unique biochemistry. The only genomic signature linking the bacterial and archaeal biochemistries of NH3 oxidation is a highly divergent homolog of the ammonia monooxygenase (AMO). Although the presumptive product of the putative AMO is hydroxylamine (NH2OH), the absence of genes encoding a recognizable ammonia-oxidizing bacteria-like hydroxylamine oxidoreductase complex necessitates either a novel enzyme for the oxidation of NH2OH or an initial oxidation product other than NH2OH. We now show through combined physiological and stable isotope tracer analyses that NH2OH is both produced and consumed during the oxidation of NH3 to NO2− by Nitrosopumilus maritimus, that consumption is coupled to energy conversion, and that NH2OH is the most probable product of the archaeal AMO homolog. Thus, despite their deep phylogenetic divergence, initial oxidation of NH3 by bacteria and archaea appears mechanistically similar. They however diverge biochemically at the point of oxidation of NH2OH, the archaea possibly catalyzing NH2OH oxidation using a novel enzyme complex.

Holocene history of ENSO variance and asymmetry in the eastern tropical Pacific

El Niño shifted between the center and the East El Niño has changed quite a bit over the past 10,000 years. During some periods it was less variable than now, and during others it shifted from its current locale toward the central Pacific. Carré et al. analyzed the shells of mollusks from Peru to construct a record of the El Niño–Southern Oscillation (ENSO) in the eastern Pacific over the Holocene period. They compared this record with other records from the rest of the Pacific to reveal how much the strength and frequency of El Niños changed and how their positions varied. Science, this issue p. 1045 El Niño–Southern Oscillation warming of the sea surface was strongly skewed toward the central Pacific in the Mid-Holocene. Understanding the response of the El Niño–Southern Oscillation (ENSO) to global warming requires quantitative data on ENSO under different climate regimes. Here, we present a reconstruction of ENSO in the eastern tropical Pacific spanning the past 10,000 years derived from oxygen isotopes in fossil mollusk shells from Peru. We found that ENSO variance was close to the modern level in the early Holocene and severely damped ~4000 to 5000 years ago. In addition, ENSO variability was skewed toward cold events along coastal Peru 6700 to 7500 years ago owing to a shift of warm anomalies toward the Central Pacific. The modern ENSO regime was established ~3000 to 4500 years ago. We conclude that ENSO was sensitive to changes in climate boundary conditions during the Holocene, including but not limited to insolation.

The production of nitric oxide by marine ammonia-oxidizing archaea and inhibition of archaeal ammonia oxidation by a nitric oxide scavenger

Nitrification is a critical process for the balance of reduced and oxidized nitrogen pools in nature, linking mineralization to the nitrogen loss processes of denitrification and anammox. Recent studies indicate a significant contribution of ammonia-oxidizing archaea (AOA) to nitrification. However, quantification of the relative contributions of AOA and ammonia-oxidizing bacteria (AOB) to in situ ammonia oxidation remains challenging. We show here the production of nitric oxide (NO) by Nitrosopumilus maritimus SCM1. Activity of SCM1 was always associated with the release of NO with quasi-steady state concentrations between 0.05 and 0.08 μM. NO production and metabolic activity were inhibited by the nitrogen free radical scavenger 2-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (PTIO). Comparison of marine and terrestrial AOB strains with SCM1 and the recently isolated marine AOA strain HCA1 demonstrated a differential sensitivity of AOB and AOA to PTIO and allylthiourea (ATU). Similar to the investigated AOA strains, bulk water column nitrification at coastal and open ocean sites with sub-micromolar ammonia/ammonium concentrations was inhibited by PTIO and insensitive to ATU. These experiments support predictions from kinetic, molecular and biogeochemical studies, indicating that marine nitrification at low ammonia/ammonium concentrations is largely driven by archaea and suggest an important role of NO in the archaeal metabolism.

Measurement of SLAP2 and GISP δ17O and proposed VSMOW‐SLAP normalization for δ17O and 17Oexcess

RATIONALE The absence of an agreed-upon δ(17)O value for the primary reference water SLAP leads to significant discrepancies in the reported values of δ(17)O and the parameter (17)O(excess). The accuracy of δ(17)O and (17)O(excess) values is significantly improved if the measurements are normalized using a two-point calibration, following the convention for δ(2)H and δ(18)O values. METHODS New measurements of the δ(17)O values of SLAP2 and GISP are presented and compared with published data. Water samples were fluorinated with CoF(3). Helium carried the O(2) product to a 5A (4.2 to 4.4 Å) molecular sieve trap submerged in liquid nitrogen. The O(2) sample was introduced into a dual-inlet ThermoFinnigan MAT 253 isotope ratio mass spectrometer for measurement of m/z 32, 33, and 34. The δ(18)O and δ(17) values were calculated after 90 comparisons with an O(2) reference gas. RESULTS We propose that the accepted δ(17)O value of SLAP be defined in terms of δ(18) O = -55.5 ‰ and (17)O(excess) = 0, yielding a δ(17)O value of approximately -29.6986 ‰ [corrected]. Using this definition for SLAP and the recommended normalization procedure, the δ(17)O value of GISP is -13.16 ± 0.05 ‰ and the (17)O(excess) value of GISP is 22 ± 11 per meg. Correcting previous published values of GISP δ(17)O to both VSMOW and SLAP improves the inter-laboratory precision by about 10 per meg. CONCLUSIONS The data generated here and compiled from previous studies provide a substantial volume of evidence to evaluate the various normalization techniques currently used for triple oxygen isotope measurements. We recommend that reported δ(17) O and (17)O(excess) values be normalized to the VSMOW-SLAP scale, using a definition of SLAP such that its (17)O(excess) is exactly zero.

Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change.

Significance The specific cause of the long-term decrease in stable nitrogen isotope ratio (15N/14N) of ice core nitrate beginning ∼1850 is a subject of debate, hindering the efforts to understand changes in the global nitrogen cycle. Our high-resolution record of ice core 15N/14N combined with model calculations suggests that the decrease is mainly caused by equilibrium shift in gas−particle partitioning of atmospheric nitrate due to increasing atmospheric acidity resulting from anthropogenic emissions of nitrogen and sulfur oxides. Our high-resolution record also reveals a leveling off of 15N/14N ∼1970, synchronous with changes in acidity and sulfate and nitrate concentrations. This leveling off suggests a measurable reduction in air pollution following the implementation of the US Clean Air Act of 1970. Nitrogen stable isotope ratio (δ15N) in Greenland snow nitrate and in North American remote lake sediments has decreased gradually beginning as early as ∼1850 Christian Era. This decrease was attributed to increasing atmospheric deposition of anthropogenic nitrate, reflecting an anthropogenic impact on the global nitrogen cycle, and the impact was thought to be amplified ∼1970. However, our subannually resolved ice core records of δ15N and major ions (e.g., , ) over the last ∼200 y show that the decrease in δ15N is not always associated with increasing concentrations, and the decreasing trend actually leveled off ∼1970. Correlation of δ15N with H+, , and HNO3 concentrations, combined with nitrogen isotope fractionation models, suggests that the δ15N decrease from ∼1850–1970 was mainly caused by an anthropogenic-driven increase in atmospheric acidity through alteration of the gas−particle partitioning of atmospheric nitrate. The concentrations of and also leveled off ∼1970, reflecting the effect of air pollution mitigation strategies in North America on anthropogenic NOx and SO2 emissions. The consequent atmospheric acidity change, as reflected in the ice core record of H+ concentrations, is likely responsible for the leveling off of δ15N ∼1970, which, together with the leveling off of concentrations, suggests a regional mitigation of anthropogenic impact on the nitrogen cycle. Our results highlight the importance of atmospheric processes in controlling δ15N of nitrate and should be considered when using δ15N as a source indicator to study atmospheric flux of nitrate to land surface/ecosystems.

Analysis of atmospheric inputs of nitrate to a temperate forest ecosystem from Δ17O isotope ratio measurements

(dry deposited HNO3 and wet deposited NO3 )i n northern Michigan is derived from atmospheric deposition. To test this idea, soil, rainfall, and cloud water were sampled in a temperate forest in northern Lower Michigan. The fraction of the soil solution NO3 pool directly from atmospheric deposition was quantified using the natural isotopic tracer, D 17 O. Our results show that on average 9% of the soil solution NO3 is unprocessed (no microbial turnover) N derived directly from the atmosphere. This points to the potential importance of anthropogenic N deposition and contributes to the long‐standing need to improve our understanding of the impacts of atmospheric nitrogen processing and deposition on forest ecosystems and forest productivity. Citation: Costa, A. W., G. Michalski, A. J. Schauer, B. Alexander, E. J. Steig, and P. B. Shepson (2011), Analysis of atmospheric inputs of nitrate to a temperate forest ecosystem from D 17 O isotope ratio measurements, Geophys. Res. Lett., 38,

Triple water‐isotopologue record from WAIS Divide, Antarctica: Controls on glacial‐interglacial changes in 17Oexcess of precipitation

Measurements of the 17Oexcess of H2O were obtained from ice cores in West and East Antarctica. Combined with previously published results from East Antarctica, the new data provide the most complete spatial and temporal view of Antarctic 17Oexcess to date. There is a steep spatial gradient of 17Oexcess in present-day precipitation across Antarctica, with higher values in marine-influenced regions and lower values in the East Antarctic interior. There is also a spatial pattern to the change in 17Oexcess between the Last Glacial Maximum (LGM) and Holocene periods. At coastal locations, there is no significant change in 17Oexcess. At both the West Antarctic Ice Sheet Divide site and at Vostok, East Antarctica, the LGM to Early Holocene change in 17Oexcess is about 20 per meg. Atmospheric general circulation model (GCM) experiments show that both the observed spatial gradient of 17Oexcess in modern precipitation, and the spatial pattern of LGM to Early Holocene change, can be explained by kinetic isotope effects during snow formation under supersaturated conditions, requiring a high sensitivity of supersaturation to temperature. The results suggest that fractionation during snow formation is the primary control on 17Oexcess in Antarctic precipitation. Variations in moisture source relative humidity play a negligible role in determining the glacial-interglacial 17Oexcess changes observed in Antarctic ice cores. Additional GCM experiments show that sea ice expansion increases the area over which supersaturating conditions occur, amplifying the effect of colder temperatures. Temperature and sea ice changes alone are sufficient to explain the observed 17Oexcess glacial-interglacial changes across Antarctica.

An automated sampler for collection of atmospheric trace gas samples for stable isotope analyses

Research focused on the isotopic composition of CO2 exchange between terrestrial ecosystems and the atmosphere has been historically constrained by the need for personnel to be present at remote field sites for sample collection. In practice, this has limited sampling frequency and duration, and potentially even biases sampling events to fair weather periods. We have developed an automated sampling system that can be installed and used for unattended collection of 100-ml air samples in remote areas. The sampler was designed with the primary goal of collecting samples for analysis of CO2 concentration and its isotopic composition in ecosystem-atmosphere flux research, but several other potential applications are also discussed. Laboratory tests examined potential artifacts associated with sampler components. These tests included evaluation of potential isotopic exchange between atmospheric CO2 and sampler component materials and the effects of sample exposure to these materials for up to 5 days and under a wide range of temperatures (10–50 ◦ C). Some of the rejected component materials influenced either CO 2 mole fraction or CO2 isotopic content. Exposure of air at subambient CO2 concentrations to all sampler components in an intact system for 5 days resulted in a [CO2] value that was 0.9mol mol −1 higher than for an equivalent sample collected by the sampler but not stored. Associated exposure-induced errors in δ 13 Co f CO 2 were generally small, ranging between 0.03 and 0.17‰ for 0 day versus 5 days exposure, respectively. These error values were within the sampling precision associated with a PreCon continuous flow mass spectrometer analysis. A more substantial exposure-induced error was observed for δ 18 Oi n CO 2 (0.29 and 0.88‰, respectively). The potential for isotopic exchange between CO2 and sampler components increased under a combination of elevated temperature and multiple-day storage treatments. These errors were small and of similar magnitude between 10 and 40 ◦ C, but unacceptably large at 50 ◦ C. Finally, we compared Keeling plots created with samples collected by the sampler with those collected simultaneously by a manual method and found no detectable differences between the two approaches. Based on these results, we conclude that sampler induced isotopic exchange for air samples held up to 5 days between 10 and 40 ◦ C is largely within the overall precision limits of a PreCon continuous flow mass spectrometer measurement.