T. Sowers - Academia.edu (original) (raw)

Papers by T. Sowers

Decoupling atmospheric methane isotope records during MIS 5-4 transition

This archived Paleoclimatology Study is available from the NOAA National Centers for Environmenta... more This archived Paleoclimatology Study is available from the NOAA National Centers for Environmental Information (NCEI), under the World Data Service (WDS) for Paleoclimatology. The associated NCEI study type is Ice Core. The data include parameters of ice cores with a geographic location of Antarctica. The time period coverage is from 52586 to -65 in calendar years before present (BP). See metadata information for parameter and study location details. Please cite this study when using the data.

materials on Science Online

widespread event 8,200 years ago. Geology 26, 6.

Atmospheric Measurement Techniques Discussions, 2017

We report results from intercomparison exercises between laboratories that conduct measurements o... more We report results from intercomparison exercises between laboratories that conduct measurements of stable carbon and hydrogen isotope ratios of atmospheric CH4 (δ13C-CH4 and…

Science, 2003

Paleoatmospheric records of trace-gas concentrations recovered from ice cores provide important s... more Paleoatmospheric records of trace-gas concentrations recovered from ice cores provide important sources of information on many biogeochemical cycles involving carbon, nitrogen, and oxygen. Here, we present a 106,000-year record of atmospheric nitrous oxide (N 2 O) along with corresponding isotopic records spanning the last 30,000 years, which together suggest minimal changes in the ratio of marine to terrestrial N 2 O production. During the last glacial termination, both marine and oceanic N 2 O emissions increased by 40 ± 8%. We speculate that our records do not support those hypotheses that invoke enhanced export production to explain low carbon dioxide values during glacial periods.

Science, 2006

One explanation for the abrupt increases in atmospheric CH 4 , that occurred repeatedly during th... more One explanation for the abrupt increases in atmospheric CH 4 , that occurred repeatedly during the last glacial cycle involves clathrate destabalization events. Because marine clathrates have a distinct deuterium/hydrogen (D/H) isotope ratio, any such destabilization event should cause the D/H ratio of atmospheric CH 4 (δD CH4 ) to increase. Analyses of air trapped in the ice from the second Greenland ice sheet project show stable and/or decreasing δD CH4 values during the end of the Younger and Older Dryas periods and one stadial period, suggesting that marine clathrates were stable during these abrupt warming episodes. Elevated glacial δD CH4 values may be the result of a lower ratio of net to gross wetland CH 4 emissions and an increase in petroleum-based emissions.

Nature, 2013

A new Greenland ice core, the NEEM core, reaches back to the last interglacial, the Eemian, 130-1... more A new Greenland ice core, the NEEM core, reaches back to the last interglacial, the Eemian, 130-115 ka (thousand years before year 1950), a period warmer than our current interglacial, and documents the Greenland climate and ice sheet response to Arctic warming. Here, the stratigraphy of the deep NEEM ice, spanning the major part of the Eemian, is reconstructed from folded ice using globally-homogenous parameters known from dated Greenland and Antarctic ice core records. The warmest Greenland surface temperatures found at NEEM are 8±4 o C warmer than the mean of the last millennium) are estimated from water stable isotopes at the onset of the Eemian (126 ka), followed by a gradual cooling trend very likely driven by summer insolation. During the Eemian the thickness of the NW Greenland ice sheet decreased by 400±250 m reaching surface elevations of 130±300 m lower than the present at 122 ka. The findings show a modest response of the Greenland ice sheet to the significant warming in the early Eemian. In addition, the ice core data at NEEM reveal significant surface melt during the Eemian. During the exceptional heat over Greenland in July 2012 melt layers formed at NEEM. With additional warming surface melt might become more common in the future.

Geology, 1997

Holocene climatic instability: A prominent, widespread event 8200 yr ago Email alerting services ... more Holocene climatic instability: A prominent, widespread event 8200 yr ago Email alerting services cite this article to receive free e-mail alerts when new articles www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geology www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click viewpoint. Opinions presented in this publication do not reflect official positions of the Society. positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political article's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to the science. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to

Atmospheric Chemistry and Physics, 2012

Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ense... more Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1σ Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age-gas age difference (age) at the firn-ice transition is calculated to be 182 +3 −9 yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major Published by Copernicus Publications on behalf of the European Geosciences Union. 4260 C. Buizert et al.: Gas transport in firn: multiple-tracer characterisation for NEEM differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records.

$Research paper thumbnail of Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice$

Nature

Rapid temperature change fractionates gas isotopes in unconsolidated snow, producing a signal tha... more Rapid temperature change fractionates gas isotopes in unconsolidated snow, producing a signal that is preserved in trapped air bubbles as the snow forms ice. The fractionation of nitrogen and argon isotopes at the end of the Younger Dryas cold interval, recorded in Greenland ice, demonstrates that warming at this time was abrupt. This warming coincides with the onset of a prominent rise in atmospheric methane concentration, indicating that the climate change was synchronous (within a few decades) over a region of at least hemispheric extent, and providing constraints on previously proposed mechanisms of climate change at this time. The depth of the nitrogen-isotope signal relative to the depth of the climate change recorded in the ice matrix indicates that, during the Younger Dryas, the summit of Greenland was 15 Ϯ 3 ЊC colder than today.

Measurements of trace gas species in ice cores are the primary means for reconstructing the compo... more Measurements of trace gas species in ice cores are the primary means for reconstructing the composition of the atmosphere. The longest such record comes from the Vostok core taken from the central portion of the East Antarctic ice sheet [Petit et al., 1999]. In general, the trace gas records from Vostok are utilized as the reference signal when correlating trace gas measurements from other ice cores. The underlying assumption implicit in such endeavors is that the bubbles recovered from the ice cores record the composition of the atmosphere at the time the bubbles were formed. Another implicit assumption is that the composition of the bubbles has not been compromised by the extremely long storage periods within the ice sheet. While there is ample evidence that certain trace gas records (e.g. CO2 and CH4) have probably not been compromised, anomalous nitrous oxide (N2O) measurements from the penultimate glacial termination at Vostok are consistent with in-situ (N2O) production [Sower...

ABSTRACT Atmospheric methane is a potent greenhouse gas that is responsible for ~20% of the total... more ABSTRACT Atmospheric methane is a potent greenhouse gas that is responsible for ~20% of the total increase in radiative forcing since the industrial revolution. Despite methane's importance, the spatial and temporal variability of sources and sinks is poorly understood. Measurements of methane from the West Antarctic Ice Sheet (WAIS) Divide 05A ice core (WDC05A, ~1.0-0.2 ka [Mitchell et al., 2011]), the WAIS Divide deep ice core (WDC06A, ~4.7-0.2 ka) and the Greenland ice core (GISP2D, ~2.8-0.2 ka) have been completed. These records have decadal scale resolution, analytical precision of <3 ppb, and are highly correlated with the only previous high resolution ice core methane record from Law Dome, Antarctica. The high degree of correlation between multiple ice cores demonstrates that the observed multidecadal variability is real and presents an opportunity to investigate the cause of this variability. In addition, these variations can be used to provide chronologic tie points between high resolution records because methane is a globally distributed greenhouse gas. Methane records from Antarctica and Greenland can be used to reconstruct the methane Inter-Polar Gradient (IPG) which is controlled by the latitudinal distribution of sources and sinks as well as the interhemispheric mixing time. The IPG provides a constraint on the global methane budget which can be used with other parameters such as the isotopologues of methane to constrain past scenarios of the global methane budget. Our IPG reconstruction reveals that over the past 2.8 ka the IPG was ~43 ppb with a standard deviation of 7 ppb. Over this time interval the IPG has a slightly decreasing trend punctuated by a short increase at ~1 ka. This observed trend in the IPG rules out the possibility of the late Holocene increase in methane concentrations coming from anthropogenic activities which occurred primarily in the extratropical Northern Hemisphere. An Eight Box Atmospheric Methane Model (EBAMM) is ideally suited to investigate variations in the latitudinal distribution of methane sources as well as the isotopologues of methane. Progress on our efforts to use this model to understand the late Holocene methane budget will be presented.