Holocene climate inferred from glacier extent,lake sediment and tree rings at Goat Lake,Kenai Mountains,Alaska, USA

Lake sediment, glacier extent and tree rings were used to reconstruct Holocene climate changes from Goat Lake at 550 m asl in the Kenai Mountains, south‐central Alaska. Radiocarbon‐dated sediment cores taken at 55 m water depth show glacial‐lacustrine conditions until about 9500 cal. yr BP, followed by organic‐rich sedimentation with an overall increasing trend in organic matter and biogenic silica content leading up to the Little Ice Age (LIA). Through most of the Holocene, the northern outlet of the Harding Icefield remained below the drainage divide that currently separates it from Goat Lake. A sharp transition from gyttja to inorganic mud about AD 1660 signifies the reappearance of glacier meltwater into Goat Lake during the LIA, marking the maximum Holocene (postglacial) extent. Meltwater continued to discharge into the lake until about AD 1900. A 207 yr tree‐ring series from 25 mountain hemlocks growing in the Goat Lake watershed correlates with other regional tree‐ring series that indicate an average summer temperature reduction of about 1°C during the 19th century compared with the early–mid 20th century. Cirque glaciers around Goat Lake reached their maximum LIA extent in the late 19th century. Assuming that glacier equilibrium‐line altitudes (ELA) are controlled solely by summer temperature, then the cooling of 1°C combined with the local environmental lapse rate would indicate an ELA lowering of 170 m. In contrast, reconstructed ELAs of 12 cirque glaciers near Goat Lake average only 34 ± 18 m lower during the LIA. The restricted ELA lowering can be explained by a reduction in accumulation‐season precipitation caused by a weakening of the Aleutian low‐pressure system during the late LIA. Copyright © 2008 John Wiley & Sons, Ltd.     

Titan's hydrodynamically escaping atmosphere

The upper atmosphere of Titan is currently losing mass at a rate , by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating by CH₄ absorption. The hydrodynamic mass loss is essentially CH₄ and H₂ escape. Their combined escape rates are restricted by power limitations from attaining their limiting rates (and limiting fluxes). Hence they must exhibit gravitational diffusive separation in the upper atmosphere with increasing mixing ratios to eventually become major constituents in the exosphere. A theoretical model with solar EUV heating by N₂ absorption balanced by HCN rotational line cooling in the upper thermosphere yields densities and temperatures consistent with the Huygens Atmospheric Science Investigation (HASI) data [Fulchignoni, M., and 42 colleagues, 2005. Nature 438, 785-791], with a peak temperature of ∼185-190 K between 3500-3550 km. This model implies hydrodynamic escape rates of and , or some other combination with a higher H₂ escape flux, much closer to its limiting value, at the expense of a slightly lower CH₄ escape rate. Nonthermal escape processes are not required to account for the loss rates of CH₄ and H₂, inferred by the Cassini Ion Neutral Mass Spectrometer (INMS) measurements [Yelle, R.V., Borggren, N., de la Haye, V., Kasprzak, W.T., Niemann, H.B., Müller-Wodarg, I., Waite Jr., J.H., 2006. Icarus 182, 567-576].     

Phosphine photochemistry in the atmosphere of Saturn

A model is presented for the photochemistry of PH₃ in the upper troposphere and lower stratosphere of Saturn that includes the effects of coupling with NH₃ and hydrocarbon photochemistry, specifically the C₂H₂ catalyzed photodissociation of CH₄. PH₃ is rapidly depleted with altitude (scale height ～35 km) in the upper troposphere when K～10⁴cm²sec^?1; an upper limit for K at the tropopause is estimated at ～10⁵cm²sec^?1. If there is no gas phase P₂H₄ because of sublimation, P₂ and P₄ formation is unlikely unless the rate of the spin-forbidden recombination reaction PH + H₂ + M → PH₃ + M is exceedingly slow. An upper limit P₄ column density of ～2×10¹⁵cm^?2 is estimated in the limit of no recombination. If sublimation does not remove all gas phase P₂H₄, P₂ and P₄ may be produced in potentially larger quantities, although they would be restricted almost entirely to the lowest levels of our model, where T?100°K. Potentially observable amounts of the organophosphorus compounds CH₃P₂H₂ and HCP are predicted, with column densities of >10¹⁷ cm^?2 and production rates of ～2×10⁸cm^?2sec^?1. The possible importance of electronically excited states of PH_x and additional PH₃/hydrocarbon photochemical coupling paths are also considered.     

An investigation of Pluto’s troposphere using stellar occultation light curves and an atmospheric radiative-conductive-convective model

We use a radiative-conductive-convective model to assess the height of Pluto’s troposphere, as well as surface pressure and surface radius, from stellar occultation data from the years 1988, 2002, and 2006. The height of the troposphere, if it exists, is less than 1 km for all years analyzed. Pluto has at most a planetary boundary layer and not a troposphere. As in previous analyses of Pluto occultation light curves, we find that the surface pressure is increasing with time, assuming that latitude and longitude variations in Pluto’s atmosphere are negligible. The surface pressure is found to be slightly higher ( μbar in 1988,  μbar in 2002, and 18.5 ± 4.7 μbar in 2006) than in our previous analyses with the troposphere excluded. The surface radius is determined to be . Comparison of the minimum reduced chi-squared values between the best-fit radiative-conductive-convective (i.e., troposphere-included) model and best-fit radiative-conductive (i.e., troposphere-excluded) shows that the troposphere-included model is only a slightly better fit to the data for all 3 years. Uncertainties in the small-scale physical processes of Pluto’s lower atmosphere and consequently the functional form of the model troposphere lend more confidence to the troposphere-excluded results.     

Holocene climate and glacier variability at Hallet and Greyling Lakes,Chugach Mountains,south-central Alaska

Evidence from lake sediments and glacier forefields from two hydrologically isolated lake basins is used to reconstruct Holocene glacier and climate history at Hallet and Greyling Lakes in the central Chugach Mountains of south-central Alaska. Glacial landform mapping, lichenometry, and equilibrium-line altitude reconstructions, along with changes in sedimentary biogenic-silica content, bulk density, and grain-size distribution indicate a dynamic history of Holocene climate variability. The evidence suggests a warm early Holocene from 10 to 6 ka, followed by the onset of Neoglaciation in the two drainage basins, beginning between 4.5 and 4.0 ka. During the past 2 ka, the glacial landforms and lacustrine sediments from the two valleys record a remarkably similar history of glaciation, with two primary advances, one during the first millennium AD, from ~500 to 800 AD, and the second during the Little Ice Age (LIA) from ~1400 to 1900 AD. During the LIA, the reconstructed equilibrium-line altitude in the region was no more than 83 ± 44 m (n = 21) lower than the modern, which is based on the extent of glaciers during 1978. Differences between the summer temperature inferred from the biogenic-silica content and the evidence for glacial advances and retreats suggest a period of increased winter precipitation from 1300 to 1500 AD, and reduced winter precipitation from 1800 to 1900 AD, likely associated with variability in the strength of the Aleutian Low.

Late Pleistocene and Holocene glaciation of the Fish Lake valley,northeastern Alaska Range,Alaska

We reconstructed a chronology of glaciation spanning from the Late Pleistocene through the late Holocene for Fish Lake valley in the north‐eastern Alaska Range using ¹⁰Be surface exposure dating and lichenometry. After it attained its maximum late Wisconsin extent, the Fish Lake valley glacier began to retreat ca. 16.5 ka, and then experienced a readvance or standstill at 11.6 ± 0.3 ka. Evidence of the earliest Holocene glacial activity in the valley is a moraine immediately in front of Little Ice Age (LIA) moraines and is dated to 3.3–3.0 ka. A subsequent advance culminated at ca. AD 610–900 and several LIA moraine crests date to AD 1290, 1640, 1860 and 1910. Our results indicate that ¹⁰Be dating from high‐elevation sites can be used to help constrain late Holocene glacial histories in Alaska, even when other dating techniques are unavailable. Close agreement between ¹⁰Be and lichenometric ages reveal that ¹⁰Be ages on late Holocene moraines may be as accurate as other dating methods. Copyright © 2009 John Wiley & Sons, Ltd.     

The photochemistry of hydrocarbons in the atmosphere of Titan

Detailed photochemical models are constructed for two model atmospheres: (1) 100% CH₄ and (2) 50% H₂, 50% CH₄. Both models predict large column densities of C₂H₂ and C₂H₆ (～1 cm atm) for eddy mixing rates ～10⁵ cm² sec^?, which are comparable to rates appropriate for Jupiter. These column densities vary inversely with the eddy diffusion coefficient. The models confirm the interpretation by Danielson et al. (1973) of the 12μ feature in the spectra of Gillet et al. (1973) as emission by C₂H₆ in a thermal inversion region. The $\text{C}{}_2\text{H}{}_6\text{C}{}_2\text{H}{}_2$ mixing ratio is sensitive to the net escape rate of H atoms from the exobase.     

The ion mass loading rate at Io

The Io plasma torus, composed of mostly heavy ions of oxygen and sulfur, is sustained by an Iogenic mass loading rate of ∼10³⁰ amu s⁻¹ = 1.6 × 10²⁸ SO₂ s⁻¹ or approximately 10³ kg s⁻¹(A.L. Broadfoot et al., 1979, Science 204, 979-982). We argue on the basis of available power sources, reanalysis of F. Bagenal (1997, Geophys. Res. Lett. 24, 2111-2114), HST UV remote sensing, and detailed model calculations that at most 20% of this mass leaves Io in the form of ions, i.e., ≤3 × 10²⁷ × (n_e,0/3600 cm⁻³) ions s⁻¹, where n_e,0 is the average torus electron density. For the Galileo spacecraft Io pass in December 1995, the ion mass loading rate was ≤3 × 10²⁷ ions s⁻¹, whereas for the Voyager epoch with lower n_e,0 (=2000 cm⁻³), this rate would be ≤1.7 × 10²⁷ ions s⁻¹, consistent with the D.E. Shemansky (1980, Astrophys. J. 242, 1266-1277) mass loading limit of ≤1 × 10²⁷ ions s⁻¹. We investigate the processes that control Io’s large scale electrodynamic interaction and find that the elastic collision rate exceeds the ionization/pickup rate by at least a factor of 5 for all atmospheric column densities considered (10¹⁶-10²¹ m⁻²) and by a factor of ∼100 for the most realistic column density. Consequently, elastic collisions are mostly responsible for Io’s high conductances and thus generate Io’s large scale electrodynamic interaction such as the generation of Io’s electric current system and the slowing of the plasma flow. The electrodynamic part of Io’s interaction is thus best described as an ionosphere-like interaction rather than a comet-like interaction. An analytic expression for total electron impact rates is derived for Io’s atmosphere, which is independent of any particular model for the 3D interaction of torus electrons with its atmosphere.     

Preliminary Results from an Astrophysically Relevant Radiation Transfer Experiment

The results of a diffusive radiation transport experiment in a simple geometry are presented. The experiment depends primarily on two variables, the target density and the temperature drive, which are characterized well. The experiment is designed to verify and validate radiation transport in codes. The codes can then be used to model astrophysical systems. The results of the experiments are found to be in good agreement with simulation results.     

Introduction to the JoPL special issue, “Holocene paleoenvironmental records from Arctic lake sediment”

The 18 papers in this Special Issue of the Journal of Paleolimnology report new records of Holocene environmental and climate change from Arctic lake sediment. At least 15 distinct physical, chemical, and biological properties were analyzed at lakes located across the North American Arctic and subarctic, and northwestern Europe. The studies are notable for their multi-proxy approach (eight present data for at least five different proxies), and for the high quality of their geochronological control. Three of the studies analyzed sediment from more than one lake to test the influence of contrasting physiographic settings on the response of proxies to the same climate forcing. The sedimentary sequences analyzed in seven studies extend beyond 11.5?cal?ka, providing evidence for pronounced climate shifts that took place during the late-glacial period. Two-thirds extend beyond 8?cal?ka; many of these records were interpreted in terms of the shift in temperature and moisture that occurred during the transition from the warm early to middle Holocene to the cooler late Holocene. These records contribute to the growing network of sites that is needed to reconstruct the spatial pattern of this pronounced paleoclimate transition, and to address how ocean-atmospheric circulation changed with the mean?state of climate.