Rachael H. Rhodes

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.

Enhanced tropical methane production in response to iceberg discharge in the North Atlantic

The tropical impact of iceberg armadas The massive discharges of icebergs from the Greenland ice sheet during the Last Glacial Period are called Heinrich events. But did Heinrich events cause abrupt climate change, or were they a product of it? Methane levels represent a proxy for climate, because methane production increases mostly due to wetter conditions in the tropics. Rhodes et al. report a highly resolved record of atmospheric methane concentrations, derived from an ice core from Antarctica. Methane levels varied—i.e., the tropical climate changed—in response to cooling in the Northern Hemisphere caused by Heinrich events. Science, this issue p. 1016 Intense production of icebergs from the Greenland Ice Sheet fueled tropical methane production. The causal mechanisms responsible for the abrupt climate changes of the Last Glacial Period remain unclear. One major difficulty is dating ice-rafted debris deposits associated with Heinrich events: Extensive iceberg influxes into the North Atlantic Ocean linked to global impacts on climate and biogeochemistry. In a new ice core record of atmospheric methane with ultrahigh temporal resolution, we find abrupt methane increases within Heinrich stadials 1, 2, 4, and 5 that, uniquely, have no counterparts in Greenland temperature proxies. Using a heuristic model of tropical rainfall distribution, we propose that Hudson Strait Heinrich events caused rainfall intensification over Southern Hemisphere land areas, thereby producing excess methane in tropical wetlands. Our findings suggest that the climatic impacts of Heinrich events persisted for 740 to 1520 years.

Observing and modeling the influence of layering on bubble trapping in polar firn

Interpretation of ice core trace gas records depends on an accurate understanding of the processes that smooth the atmospheric signal in the firn. Much work has been done to understand the processes affecting air transport in the open pores of the firn, but a paucity of data from air trapped in bubbles in the firn-ice transition region has limited the ability to constrain the effect of bubble closure processes. Here we present high-resolution measurements of firn density, methane concentrations, nitrogen isotopes, and total air content that show layering in the firn-ice transition region at the West Antarctic Ice Sheet (WAIS) Divide ice core site. Using the notion that bubble trapping is a stochastic process, we derive a new parameterization for closed porosity that incorporates the effects of layering in a steady state firn modeling approach. We include the process of bubble trapping into an open-porosity firn air transport model and obtain a good fit to the firn core data. We find that layering broadens the depth range over which bubbles are trapped, widens the modeled gas age distribution of air in closed bubbles, reduces the mean gas age of air in closed bubbles, and introduces stratigraphic irregularities in the gas age scale that have a peak-to-peak variability of ~10 years at WAIS Divide. For a more complete understanding of gas occlusion and its impact on ice core records, we suggest that this experiment be repeated at sites climatically different from WAIS Divide, for example, on the East Antarctic plateau.

Isotopic and Elemental Changes in Winter Snow Accumulation on Glaciers in the Southern Alps of New Zealand

Abstract The authors present stable water isotope and trace element data for fresh winter snow from two temperate maritime glaciers located on opposite sides of the New Zealand Southern Alps. The isotopes δ18O and δD were more depleted at the eastern Tasman Glacier site because of prevailing westerly flow and preferential rainout of heavy isotopes as air masses crossed the Alps. The deuterium excess provided some indication of moisture provenance, with the Tasman Sea contributing ∼70% of the moisture received at Franz Josef and Tasman Glaciers. This source signal was also evident in trace elements, with a stronger marine signal (Na, Mg, and Sr) associated with snow from the Tasman Sea and larger concentrations of terrestrial species (Pb, V, and Zr) in air masses from the Southern and Pacific Oceans. Although postdepositional modification of signals was detected, the results indicate that there is exciting potential to learn more about climate trends and moisture source pathways and to learn from geochemic...

Synchronous volcanic eruptions and abrupt climate change ∼17.7 ka plausibly linked by stratospheric ozone depletion

Significance Cold and dry glacial-state climate conditions persisted in the Southern Hemisphere until approximately 17.7 ka, when paleoclimate records show a largely unexplained sharp, nearly synchronous acceleration in deglaciation. Detailed measurements in Antarctic ice cores document exactly at that time a unique, ∼192-y series of massive halogen-rich volcanic eruptions geochemically attributed to Mount Takahe in West Antarctica. Rather than a coincidence, we postulate that halogen-catalyzed stratospheric ozone depletion over Antarctica triggered large-scale atmospheric circulation and hydroclimate changes similar to the modern Antarctic ozone hole, explaining the synchronicity and abruptness of accelerated Southern Hemisphere deglaciation. Glacial-state greenhouse gas concentrations and Southern Hemisphere climate conditions persisted until ∼17.7 ka, when a nearly synchronous acceleration in deglaciation was recorded in paleoclimate proxies in large parts of the Southern Hemisphere, with many changes ascribed to a sudden poleward shift in the Southern Hemisphere westerlies and subsequent climate impacts. We used high-resolution chemical measurements in the West Antarctic Ice Sheet Divide, Byrd, and other ice cores to document a unique, ∼192-y series of halogen-rich volcanic eruptions exactly at the start of accelerated deglaciation, with tephra identifying the nearby Mount Takahe volcano as the source. Extensive fallout from these massive eruptions has been found >2,800 km from Mount Takahe. Sulfur isotope anomalies and marked decreases in ice core bromine consistent with increased surface UV radiation indicate that the eruptions led to stratospheric ozone depletion. Rather than a highly improbable coincidence, circulation and climate changes extending from the Antarctic Peninsula to the subtropics—similar to those associated with modern stratospheric ozone depletion over Antarctica—plausibly link the Mount Takahe eruptions to the onset of accelerated Southern Hemisphere deglaciation ∼17.7 ka.

Atmospheric methane variability: Centennial-scale signals in the Last Glacial Period

In order to understand atmospheric methane (CH4) biogeochemistry now and in the future, we must apprehend its natural variability, without anthropogenic influence. Samples of ancient air trapped within ice cores provide the means to do this. Here we analyze the ultrahigh-resolution CH4 record of the West Antarctic Ice Sheet Divide ice core 67.2–9.8 ka and find novel, atmospheric CH4 variability at centennial time scales throughout the record. This signal is characterized by recurrence intervals within a broad 80–500 year range, but we find that age-scale uncertainties complicate the possible isolation of any periodic frequency. Lower signal amplitudes in the Last Glacial relative to the Holocene may be related to incongruent effects of firn-based signal smoothing processes. Within interstadial and stadial periods, the peak-to-peak signal amplitudes vary in proportion to the underlying millennial-scale oscillations in CH4 concentration—the relative amplitude change is constant. We propose that the centennial CH4 signal is related to tropical climate variability that influences predominantly low-latitude wetland CH4 emissions.

Sea Ice Versus Storms: What Controls Sea Salt in Arctic Ice Cores?

The sea ice surface is thought to be a major source of sea salt aerosol, suggesting that sodium records of polar ice cores may trace past sea ice extent. Here we test this possibility for the Arctic, using a chemical transport model to simulate aerosol emission, transport, and deposition in the satellite era. Our simulations suggest that sodium records from inland Greenland ice cores are strongly influenced by the impact of meteorology on aerosol transport and deposition. In contrast, sodium in coastal Arctic cores is predominantly sourced from the sea ice surface and the strength of these aerosol emissions controls the ice core sodium variability. Such ice cores may therefore record decadal to centennial scale Holocene sea ice changes. However, any relationship between ice core sodium and sea ice change may depend on how sea ice seasonality impacts sea salt emissions. Field‐based observations are urgently required to constrain this.