32 research outputs found
Terminus-driven retreat of a major southwest Greenland tidewater glacier during the early 19th century : insights from glacier reconstructions and numerical modelling
Peer reviewedPublisher PD
Five millennia of surface temperatures and ice core bubble characteristics from the WAIS Divide deep core, West Antarctica
Bubble number densities from the West Antarctic Ice Sheet (WAIS) Divide deep core in West Antarctica record relatively stable temperatures during the middle Holocene followed by late Holocene cooling. We measured bubble number density, shape, size, and arrangement on new samples of the main WAIS Divide deep core WDC06A from similar to 580m to similar to 1600 depth. The bubble size, shape, and arrangement data confirm that the samples satisfy the requirements for temperature reconstructions. A small correction for cracks formed after core recovery allows extension of earlier work through the brittle ice zone, and a site-specific calibration reduces uncertainties. Using an independently constructed accumulation rate history and a steady state bubble number density model, we determined a temperature reconstruction that agrees closely with other independent estimates, showing a stable middle Holocene, followed by a cooling of similar to 1.25 degrees C in the late Holocene. Over the last similar to 5 millennia, accumulation has been higher during warmer times by similar to 12%degrees C-1, somewhat stronger than for thermodynamic control alone, suggesting dynamic processes
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In situ cosmogenic radiocarbon production and 2-D ice flow line modeling for an Antarctic blue ice area
Radiocarbon measurements at ice margin sites and blue ice areas can potentially be used for ice dating, ablation rate estimates and paleoclimatic reconstructions. Part of the measured signal comes from in situ cosmogenic šâ´C production in ice, and this component must be well understood before useful information can be extracted from šâ´C data. We combine cosmic ray scaling and production estimates with a two-dimensional ice flow line model to study cosmogenic šâ´C production at Taylor Glacier, Antarctica. We find (1) that šâ´C production through thermal neutron capture by nitrogen in air bubbles is negligible; (2) that including ice flow patterns caused by basal topography can lead to a surface šâ´C activity that differs by up to 25% from the activity calculated using an ablation-only approximation, which is used in all prior work; and (3) that at high ablation margin sites, solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10%. As part of this effort we model two-dimensional ice flow along the central flow line of Taylor Glacier. We present two methods for parameterizing vertical strain rates, and assess which method is more reliable for Taylor Glacier. Finally, we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error. The results presented here can inform ongoing and future šâ´C and ice flow studies at ice margin sites, including important paleoclimatic applications such as the reconstruction of paleoatmospheric šâ´C content of methane
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Variable relationship between accumulation and temperature in West Antarctica for the past 31,000 years
The Antarctic contribution to sea level is a balance between ice loss along the margin and accumulation in the interior. Accumulation records for the past few decades are noisy and show inconsistent relationships with temperature. We investigate the relationship between accumulation and temperature for the past 31âka using high-resolution records from the West Antarctic Ice Sheet (WAIS) Divide ice core in West Antarctica. Although the glacial-interglacial increases result in high correlation and moderate sensitivity for the full record, the relationship shows considerable variability through time with high correlation and high sensitivity for the 0â8âka period but no correlation for the 8â15âka period. This contrasts with a general circulation model simulation which shows homogeneous sensitivities between temperature and accumulation across the entire time period. These results suggest that variations in atmospheric circulation are an important driver of Antarctic accumulation but they are not adequately captured in model simulations. Model-based projections of future Antarctic accumulation, and its impact on sea level, should be treated with caution
Seasonal temperatures in West Antarctica during the Holocene
The recovery of long-term climate proxy records with seasonal resolution is rare because of natural smoothing processes, discontinuities and limitations in measurement resolution. Yet insolation forcing, a primary driver of multimillennial-scale climate change, acts through seasonal variations with direct impacts on seasonal climate1. Whether the sensitivity of seasonal climate to insolation matches theoretical predictions has not been assessed over long timescales. Here, we analyse a continuous record of water-isotope ratios from the West Antarctic Ice Sheet Divide ice core to reveal summer and winter temperature changes through the last 11,000âyears. Summer temperatures in West Antarctica increased through the early-to-mid-Holocene, reached a peak 4,100âyears ago and then decreased to the present. Climate model simulations show that these variations primarily reflect changes in maximum summer insolation, confirming the general connection between seasonal insolation and warming and demonstrating the importance of insolation intensity rather than seasonally integrated insolation or season duration2,3. Winter temperatures varied less overall, consistent with predictions from insolation forcing, but also fluctuated in the early Holocene, probably owing to changes in meridional heat transport. The magnitudes of summer and winter temperature changes constrain the lowering of the West Antarctic Ice Sheet surface since the early Holocene to less than 162âm and probably less than 58âm, consistent with geological constraints elsewhere in West Antarctica4-7
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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[superscript 1,2]. 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[superscript 3,4]. An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently[superscript 2,5]. 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[superscript 6] associated with an abrupt decrease in Atlantic meridional overturning circulation[superscript 7]. 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.Keywords: Last glacial period, Carbon Dioxide, High resolution, Chronology, Ice core, Circulation, Abrupt climate change, Atmospheric Co2, Greenland, Polar ic
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Precise interpolar phasing of abrupt climate change during the last ice age
The last glacial period exhibited abrupt DansgaardâOeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archivesš. Ice cores show that Antarctica cooled during the warm phases of the Greenland DansgaardâOeschger cycle and vice versa[superscript 2,3], suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw[superscript 4â6]. 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[superscript 7â9]. 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[superscript 2,3,10]. 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 Âą 92 years (2Ď) for DansgaardâOeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 Âą 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 DansgaardâOeschger dynamics
Freshwater mussels in a California North Coast Range river: occurrence, distribution, and controls
Freshwater mussels in California's rivers are potentially very useful as indicators of watershed health and recorders of watershed changes. We report the occurrence and habitat of mussel populations within a continuous 8-km section of the South Fork Eel River in the Northern Coast Range of California. The primary goals of our study were 1) to compile information on species composition and population density, and 2) to examine whether spatial distribution and variability were related to geomorphology and hydrology. We found numerous individuals of 2 species (Margaritifera falcata and Anodonta californiensis), with the spatial distribution of both species characterized by high variability. Mussels in this system live almost exclusively in pools (with a few in runs), near the channel banks, and especially among sedge root-mat substrate. High discharges almost certainly provide more of a constraint on the distribution and persistence of mussels in the South Fork Eel than do low summer flows, so we used the Hydrologic Engineering Center's River Analysis System (HEC-RAS) hydraulic model to estimate physical conditions during high flows when in-channel investigations were not feasible. In all flow regimes (summer, winter, 5-y flood, and the largest floods on record), mussels were found in areas of lower boundary shear stresses and lower velocities. Our study suggests that, at various spatial scales, mussels appear to be distributed in a manner that protects them from the highest flow-induced stresses