Michelle R. Koutnik

South Polar Layered Deposits of Mars: The cratering record

Data from the Mars Orbiter Laser Altimeter (MOLA) and Mars Orbiter Camera (MOC) instruments aboard the Mars Global Surveyor (MGS) were used in a detailed search of a selected part of the South Polar Layered Deposits (SPLD) for impact craters. Impact craters with diameters from 0.8 to 5 km were identified from a MOLA-derived shaded relief map and were primarily validated using individual MOLA tracks and, in select cases, MOC narrow angle images. The resultant crater population determined in this study is at least four times the density of the crater population previously recognized. From these new statistics, we estimate the mean apparent surface age of the SPLD to be 30–100 Ma, depending on the established production model isochrons used. All of these craters are considerably shallower than other Martian craters in the same diameter range. We attribute this shallowness to be the cause of the lower detection rates of previous studies. There is a correlation between crater depth and rim height, which suggests that both erosion and infilling have affected the crater forms. A similar study of the north polar layered deposits uncovered no craters in this diameter range. A limited population of craters smaller than 800 m was uncovered in higher-resolution MOC narrow angle images. These do not appear to have been degraded to the same degree. This separate population implies a surface exposure age of only 100,000 years and perhaps indicates an event that erased all small craters and degraded and infilled the larger ones.

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 31 ka 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 0–8 ka period but no correlation for the 8–15 ka 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.

Deglacial temperature history of West Antarctica

Significance The magnitude and timing of Antarctic temperature change through the last deglaciation reveal key aspects of Earth’s climate system. Prior attempts to reconstruct this history relied on isotopic indicators without absolute calibration. To overcome this limitation, we combined isotopic data with measurements of in situ temperatures along a 3.4-km-deep borehole. Deglacial warming in Antarctica was two to three times larger than the contemporaneous global temperature change, quantifying the extent to which feedback processes amplify global changes in polar regions, a key prediction of climate models. Warming progressed earlier in Antarctica than in the Northern Hemisphere but coincident with glacier recession in southern mountain ranges, a manifestation of changing oceanic heat transport, insolation, and atmospheric CO2 that can further test models. The most recent glacial to interglacial transition constitutes a remarkable natural experiment for learning how Earth’s climate responds to various forcings, including a rise in atmospheric CO2. This transition has left a direct thermal remnant in the polar ice sheets, where the exceptional purity and continual accumulation of ice permit analyses not possible in other settings. For Antarctica, the deglacial warming has previously been constrained only by the water isotopic composition in ice cores, without an absolute thermometric assessment of the isotopes’ sensitivity to temperature. To overcome this limitation, we measured temperatures in a deep borehole and analyzed them together with ice-core data to reconstruct the surface temperature history of West Antarctica. The deglacial warming was 11.3±1.8∘C, approximately two to three times the global average, in agreement with theoretical expectations for Antarctic amplification of planetary temperature changes. Consistent with evidence from glacier retreat in Southern Hemisphere mountain ranges, the Antarctic warming was mostly completed by 15 kyBP, several millennia earlier than in the Northern Hemisphere. These results constrain the role of variable oceanic heat transport between hemispheres during deglaciation and quantitatively bound the direct influence of global climate forcings on Antarctic temperature. Although climate models perform well on average in this context, some recent syntheses of deglacial climate history have underestimated Antarctic warming and the models with lowest sensitivity can be discounted.

Holocene accumulation and ice flow near the West Antarctic Ice Sheet Divide ice‐core site

The West Antarctic Ice Sheet Divide Core (WDC) provided a high-resolution climate record from near the Ross-Amundsen Divide in Central West Antarctica. In addition, radar-detected internal layers in the vicinity of the WDC site have been dated directly from the ice core to provide spatial variations in the age structure of the region. Using these two data sets together, we first infer a high-resolution Holocene accumulation-rate history from 9.2 thousand years of the ice-core timescale and then confirm that this climate history is consistent with internal layers upstream of the core site. Even though the WDC was drilled only 24 kilometers from the modern ice divide, advection of ice from upstream must be taken into account. We evaluate histories of accumulation rate by using a flowband model to generate internal layers that we compare to observed layers. Results show that the centennially averaged accumulation rate was over 20 percent lower than modern at 9.2 thousand years before present (B.P.), increased by 40 percent from 9.2 to 2.3 thousand years B.P., and decreased by at least 10 percent over the past 2 thousand years B.P. to the modern values; these Holocene accumulation-rate changes in Central West Antarctica are larger than changes inferred from East Antarctic ice-core records. Despite significant changes in accumulation rate, throughout the Holocene the regional accumulation pattern has likely remained similar to today, and the ice-divide position has likely remained on average within 5 kilometers of its modern position. Continent-scale ice-sheet models used for reconstructions of West Antarctic ice volume should incorporate this accumulation history.

Millennially averaged accumulation rates for the Vostok Subglacial Lake region inferred from deep internal layers

Abstract Accumulation rates and their spatio-temporal variability are important boundary conditions for ice-flow models. The depths of radar-detected internal layers can be used to infer the spatial variability of accumulation rates. Here we infer accumulation rates from three radar layers (26, 35 and 41 ka old) in the Vostok Subglacial Lake region using two methods: (1) the local-layer approximation (LLA) and (2) a combination of steady-state flowband modeling and formal inverse methods. The LLA assumes that the strain-rate history of a particle traveling through the ice sheet can be approximated by the vertical strain-rate profile at the current position of the particle, which we further assume is uniform. The flowband model, however, can account for upstream strain-rate gradients. We use the LLA to map accumulation rates over a 150 km × 350 km area, and we apply the flowband model along four flowbands. The LLA accumulation-rate map shows higher values in the northwestern corner of our study area and lower values near the downstream shoreline of the lake. These features are also present but less distinct in the flowband accumulation-rate profiles. The LLA-inferred accumulation-rate patterns over the three time periods are all similar, suggesting that the regional pattern did not change significantly between the start of the Holocene and the last ~20 ka of the last Glacial Period. However, the accumulation-rate profiles inferred from the flowband model suggest changes during that period of up to 1 cma–1 or ~50% of the inferred values.

Formulating an inverse problem to infer the accumulation-rate pattern from deep internal layering in an ice sheet using a Monte Carlo approach

Using a Monte Carlo (MC) method, we determine the accumulation-rate profile along a flowband, the influx of ice into the upstream end of the flowband and the age of an internal layer. The data comprise the depth profile of the internal layer, a few velocity measurements at the surface and the average accumulation at one location. The data in our example were collected at Taylor Mouth, a flank site off Taylor Dome, Antarctica. We present three alternative formulations of this inverse problem. Depending on the formulation used, this particular inverse problem can have up to four solutions, each corresponding to a different spatial accumulation-rate pattern. This study demonstrates the ability of a MC method to find several solutions to this inverse problem, and how to use a Metropolis algorithm to determine the probability distribution of each of these different solutions. The only disadvantage of the MC method is that it is computationally more expensive than other inverse methods, such as the Gradient method.

Identifying Dynamically Induced Variability in Glacier Mass-Balance Records

AbstractGlacier mass balance provides a direct indicator of a glacier’s relationship with local climate, but internally generated variability in atmospheric circulation adds a significant degree of noise to mass-balance time series, making it difficult to correctly identify and interpret trends. This study applies “dynamical adjustment” to seasonal mass-balance records to identify and remove the component of variance in these time series that is associated with large-scale circulation fluctuations (dynamical adjustment refers here to a statistical method and not a glacier’s dynamical response to climate). Mass-balance records are investigated for three glaciers: Wolverine and Gulkana in Alaska and South Cascade in Washington. North Pacific sea level pressure and sea surface temperature fields perform comparably as predictors, each explaining 50%–60% of variance in winter balance and 25%–35% in summer balance for South Cascade and Wolverine Glaciers. Gulkana Glacier, located farther inland, is less closely...