Estimating Annual CO2 Flux for Lutjewad Station Using Three Different Gap-Filling Techniques

Long-term measurements of CO2 flux can be obtained using the eddy covariance technique, but these datasets are affected by gaps which hinder the estimation of robust long-term means and annual ecosystem exchanges. We compare results obtained using three gap-fill techniques: multiple regression (MR), multiple imputation (MI), and artificial neural networks (ANNs), applied to a one-year dataset of hourly CO2 flux measurements collected in Lutjewad, over a flat agriculture area near the Wadden Sea dike in the north of the Netherlands. The dataset was separated in two subsets: a learning and a validation set. The performances of gap-filling techniques were analysed by calculating statistical criteria: coefficient of determination (R2), root mean square error (RMSE), mean absolute error (MAE), maximum absolute error (MaxAE), and mean square bias (MSB). The gap-fill accuracy is seasonally dependent, with better results in cold seasons. The highest accuracy is obtained using ANN technique which is also less sensitive to environmental/seasonal conditions. We argue that filling gaps directly on measured CO2 fluxes is more advantageous than the common method of filling gaps on calculated net ecosystem change, because ANN is an empirical method and smaller scatter is expected when gap filling is applied directly to measurements.

Late Pleistocene Interactions of East and West Antarctic Ice-Flow Regimes: Evidence from the McMurdo Ice Shelf

We present new interpretations of deglaciation in McMurdo Sound and the western Ross Sea, with observationally based reconstructions of interactions between East and West Antarctic ice at the last glacial maximum (LGM), 16 000, 12 000, 8000 and 4000 sp. At the LGM? East Antarctic ice from Mulock Glacier split, one branch turned westward south of Ross Island but the other branch rounded Ross Island before flowing southwest into McMurdo Sound. This flow regime, constrained by an ice saddle north of Ross Island, is consistent with the reconstruction of Stuiver and others (1981a). After the LGM, grounding-line retreat was most rapid in areas with greatest water depth, especially along the Victoria Land coast. By 12 000 sp, the ice-now regime in McMurdo Sound changed to through-flowing Mulock Glacier ice, with lesser contributions from Koettlitz, Blue and Ferrar Glaciers, because the former ice saddle north of Ross Island was replaced by a dome. The modern flew regime was established similar to 4000 BP. Ice derived from high elevations on the Polar Plateau but now stranded on the McMurdo Ice Shelf, and the pattern of the Transantarctic Mountains erratics support our reconstructions of Mulock Glacier ice rounding Minna Bluff but with all ice from Skelton Glacier ablating south of the bluff. They are inconsistent with Drewry's (1979) LGM reconstruction that includes Skelton Glacier ice in the McMurdo-Sound through-flow. Drewry's (1979) model closely approximates our results for 12 000-4000 BP. Ice-sheet modeling holds promise for determining whether deglaciation proceeded by grounding-line retreat of an ice sheet that was largely stagnant, because it never approached equilibrium flowline profiles after the Ross Ice Shelf, grounded, or of a dynamic ice sheet with flowline profiles kept low by active ice streams that extended northward from present-day outlet glaciers after the Ross Ice Shelf grounded.

A Coastal Transect of McMurdo Dry Valleys (Antarctica) Snow and Firn: Marine and Terrestrial Influences on Glaciochemistry

Samples of snow and firn from accumulation zones on Clark, Commonwealth, Blue and Victoria Upper Glaciers in the McMurdo Dry Valleys (similar to 77-78 degrees S, 161-164 degrees E), Antarctica, are evaluated chemically and isotopically to determine the relative importance of local (site-specific) factors vs regional-scale influences in defining glaciochemistry. Spatial variation in snow and firn chemistry confirms documented trends within individual valleys regarding major-ion deposition relative to elevation and to distance from the coast. Sodium and methylsulfonate (MS-), for example, follow a decreasing gradient with distance from the coast along the axis of Victoria Valley (350-119 mu gL(-1) for Na+; 33-14 mu gL(-1) for MS-); a similar pattern exists between Commonwealth and Newall Glaciers in the Asgaard Range. When comparing major-ion concentrations (e.g. Na-+,Na- MS-, Ca2+) or trace metals (e.g. Al, Fe) among different valleys, however, site-specific exposures to marine and local terrestrial chemical sources play a dominant role. Because chemical signals at all sites respond to particulates with varying mixtures of marine and terrestrial sources, each of these influences on site glaciochemistry must be considered when drawing temporal climate inferences on regional scales.

The Aeolian Flux of Calcium, Chloride and Nitrate to the McMurdo Dry Valleys Landscape: Evidence from Snow Pit Analysis

We have determined the flux of calcium, chloride and nitrate to the McMurdo Dry Valleys region by analysing snow pits for their chemical composition and their snow accumulation using multiple records spanning up to 48 years. The fluxes demonstrate patterns related to elevation and proximity to the ocean. In general, there is a strong relationship between the nitrate flux and snow accumulation, indicating that precipitation rates may have a great influence over the nitrogen concentrations in the soils of the valleys. Aeolian dust transport plays an important role in the deposition of some elements (e.g. C(2+)) into the McMurdo Dry Valleys' soils. Because of the antiquity of some of the soil surfaces in the McMurdo Dry Valleys regions, the accumulated atmospheric flux of salts to the soils has important ecological consequences. Although precipitation may be an important mechanism of salt deposition to the McMurdo Dry Valley surfaces, it is poorly understood because of difficulties in measurement and high losses from sublimation.

Monsoonal Circulation of the McMurdo Dry Valleys, Ross Sea Region, Antarctica: Signal from the Snow Chemistry

McMurdo Dry Valleys (MDV, Ross Sea region, Antarctica) precipitation exhibits extreme seasonality in ion concentration, 3-5 orders of magnitude between summer and winter precipitation. To identify aerosol sources and investigate causes for the observed amplitude in concentration variability, four snow pits were sampled along a coast-Polar Plateau transect across the MDV. The elevation of the sites ranges from 50 to 2400 m and the distance from the coast from 8 to 93 km. Average chemistry gradients along the transect indicate that most species have either a predominant marine or terrestrial source in the MDV. Empirical orthogonal function analysis on the snow-chemistry time series shows that at least 57% of aerosol deposition occurs concurrently. A conceptual climate model, based on meteorological observations, is used to explain the strong seasonality in the MDV. Our results suggest that radiative forcing of the ice-free valleys creates a surface low-pressure cell during summer which promotes air-mass flow from the Ross Sea. The associated precipitating air mass is relatively warm, humid and contains a high concentration of aerosols. During winter, the MDV are dominated by air masses draining off the East Antarctic ice sheet, that are characterized by cold, dry and low concentrations of aerosols. The strong differences between these two air-mass sources create in the MDV a polar version of the monsoonal flow, with humid, warm summers and dry, cold winters.

Solar Forcing Recorded by Aerosol Concentrations in Coastal Antarctic Glacier Ice, McMurdo Dry Valleys

Ice-core chemistry data from Victoria Lower Glacier, Antarctica, suggest, at least for the last 50 years, a direct influence of solar activity variations on the McMurdo Dry Valleys (MDV) climate system via controls on air-mass input from two competing environments: the East Antarctic ice sheet and the Ross Sea. During periods of increased solar activity, when total solar irradiance is relatively high, the MDV climate system appears to be dominated by air masses originating from the Ross Sea, leading to higher aerosol deposition. During reduced solar activity, the Antarctic interior seems to be the dominant air-mass source, leading to lower aerosol concentration in the ice-core record. We propose that the sensitivity of the MDV to variations in solar irradiance is caused by strong albedo differences between the ice-free MDV and the ice sheet.

The Effects of Joint ENSO-Antarctic Oscillation Forcing on the McMurdo Dry Valleys, Antarctica

Stable oxygen analyses and snow accumulation rates from snow pits sampled in the McMurdo Dry Valleys have been used to reconstruct variations in summer temperature and moisture availability over the last four decades. The temperature data show a common interannual variability, with strong regional warmings occurring especially in 1984/85, 1995/96 and 1990/91 and profound coolings during 1977/78, 1983/84, 1988/89, 1993/94, and 1996/97. Annual snow accumulation shows a larger variance between sites, but the early 1970s, 1984, 1997, and to a lesser degree 1990/91 are characterized overall by wetter conditions, while the early and late 1980s show low snow accumulation values. Comparison of the reconstructed and measured summer temperatures with the Southern Oscillation Index (SOI) and the Antarctic Oscillation (AAO) yield statistically significant correlations, which improve when phaserelationships are considered. A distinct change in the phase relationship of the correlation is observed, with the SOI-AAO leading over the temperature records by one year before, and lagging by one year after 1988. These results suggest that over the last two decades summer temperatures are influenced by opposing El Niho Southern Oscillation and AAO forcings and support previous studies that identified a change in the Tropical-Antarctic teleconnection between the 1980s and 1990s.

Status of the Zimmerwald SLR station

The Zimmerwald SLR station is operated in a monostatic mode with 532nm laser pulses emitted at adjustable frequencies of 90-110Hz with energies slightly less than 10mJ. A rotating shutter protects the CSPAD receiver from the backscatter of the transmit beam. These systems are located below the telescope in an operator room housed within the observatory building with the laser system located in a separated, air-conditioned part of the room. All hardware components may be automatically accessed by the control software and from remote if required. Thanks to the fully automatic and remotely controllable SLR operations, the Zimmerwald station is one of the most productive stations in the ILRS network. Key characteristics of the hardware are shown. Specialities like the tracking of the full GLONASS constellation, one-way ranging to the Lunar Reconnaissance Orbiter, and photon reception from bi-static experiments with the Graz SLR station are highlighted as well.