ALBERT

Description: We measured the respiratory isotope effect ϵresp for seven representative unicellular marine organisms. The bacterium Pseudomonas halodurans, the diatom Phaeodactylum tricornutum, the phytoflagellates Cryptomonas baltica and Dunaliella tertiolecta, the heterotrophic flagellates Paraphysomonas imperforata and Bodo sp., and the ciliate Uronema sp. exhibit ϵresp values in the range 14-26‰. We also measured ϵresp for three metazoans. The ϵresp for the copepod Acartia tonsa ranged from 17 to 25‰, while two larger organisms, the mollusk Mercenaria mercenaria and the salmon Salmo salmar, respire with a smaller ϵresp of 5-10‰. The average respiratory isotope effect of the dominant marine respirers (the bacteria, microalgae and zooplankton) is about 20 ± 3‰. An ϵresp of this magnitude supports the hypothesis that the photosynthesis-respiration cycle is responsible for the 23.5‰ enrichment in the δ18O ratio of atmospheric O2 relative to seawater (the Dole effect). The large value and high variability in the average ϵresp limits the usefulness of a proposed method using the δ18O of naturally fractionated dissolved O2 in seawater as a tracer of primary production in the oligotrophic ocean.

Description: We review the current understanding of the Dole effect (the observed difference between the δ18O of atmospheric O2 and that of seawater) and its causes, extend the record of variations in the Dole effect back to 130 kyr before present using data on the δ18O of O2 obtained from studying the Vostok ice core (Sowers et al., 1993), and discuss the significance of temporal variations. The Dole effect reflects oxygen isotope fractionation during photosynthesis, respiration, and hydrologic processes (evaporation, precipitation, and evapotranspiration). Our best prediction of the present-day Dole effect, +20.8‰, is considerably lower than the observed value, +23.5‰, and we discuss possible causes of this discrepancy. During the past 130 kyr, the Dole effect has been 0.05‰ lower than the present value, on average. The standard deviation of the Dole effect from the mean has been only ±0.2‰, and the Dole effect is nearly unchanged between glacial maxima and interglacial periods. The small variability in the Dole effect suggests that relative rates of primary production in the land and marine realms have been relatively constant. Most periodic variability in the Dole effect is in the precession band, suggesting that changes in this global biogeochemical term reflects variations in low-latitude land hydrology and productivity or possibly variability in low-latitude oceanic productivity.

Description: The isotopic composition of dissolved O2 in seawater, expressed as the δ18O of O2, is unique among the bioactive tracers of the aphotic zone in that it is not linearly related to oxygen utilization via the stoichiometry of organic matter decomposition. In fact, δ18O of O2 depends on the history of water mixing and O2 consumption in the sample studied (Craig and Kroopnick, 1970; Kroopnick and Craig, 1976). For this reason, the variation of δ18O of O2 with O2 concentration depends on regional circulation patterns and oxygen utilization rates. The δ18O of O2 can be used to chartacterize these processes by decoupling their effects. As an example of this assertion, we interpret the covariation between the concentration of O2 and its isotopic composition in the Pacific Ocean as reported by Kroopnick (1987), using four simple representations of seawater mixing and respiration. Kroopnick's data are in general accord with an elementary model of isopycnal mixing represented by diffusive exchange and oxygen utilization in the ocean's interior, coupled with atmospheric equilibrium at the point where the isopycnals outcrop at the sea surface. This specific result illustrates the general point that δ18O of O2 in seawater can serve as an important constraint on more extensive and sophisticated physical models used to estimate rates of oxygen utilization in the deep sea.

Description: A numerical algorithm based on Fermat's Principle was developed to simulate the propagation of Global Positioning System (GPS) radio signals in the refractivity field of a numerical weather model. The unique in the proposed algorithm is that the ray-trajectory automatically involves the location of the ground-based receiver and the satellite, i.e. the posed two-point boundary value problem is solved by an implicit finite difference scheme. This feature of the algorithm allows the fast and accurate computation of the signal travel-time delay, referred to as Slant Total Delay (STD), between a satellite and a ground-based receiver. We provide a technical description of the algorithm and estimate the uncertainty of STDs due to simplifying assumptions in the algorithm and due to the uncertainty of the refractivity field. In a first application, we compare STDs retrieved from GPS phase-observations at the German Research Centre for Geosciences Potsdam (GFZ STDs) with STDs derived from the European Center for Medium-Range Weather Forecasts analyses (ECMWF STDs). The statistical comparison for one month (August 2007) for a large and continuously operating network of ground-based receivers in Germany indicates good agreement between GFZ STDs and ECMWF STDs; the standard deviation is 0.5% and the mean deviation is 0.1%.

Description: Water vapor plays an important role in meteorological applications; GeoForschungsZentrum (GFZ) therefore developed a tomographic system to derive 3-D distributions of the tropospheric water vapor above Germany using GPS data from about 300 ground stations. Input data for the tomographic reconstructions are generated by the Earth Parameter and Orbit determination System (EPOS) software of the GFZ, which provides zenith total delay (ZTD), integrated water vapor (IWV) and slant total delay (STD) data operationally with a temporal resolution of 2.5 min (STD) and 15 min (ZTD, IWV). The water vapor distribution in the atmosphere is derived by tomographic reconstruction techniques. The quality of the solution is dependent on many factors such as the spatial coverage of the atmosphere with slant paths, the spatial distribution of their intersections and the accuracy of the input observations. Independent observations are required to validate the tomographic reconstructions and to get precise information on the accuracy of the derived 3-D water vapor fields. To determine the quality of the GPS tomography, more than 8000 vertical water vapor profiles at 13 German radiosonde stations were used for the comparison. The radiosondes were launched twice a day (at 00:00 UTC and 12:00 UTC) in 2007. In this paper, parameters of the entire profiles such as the wet refractivity, and the zenith wet delay have been compared. Before the validation the temporal and spatial distribution of the slant paths, serving as a basis for tomographic reconstruction, as well as their angular distribution were studied. The mean wet refractivity differences between tomography and radiosonde data for all points vary from −1.3 to 0.3, and the root mean square is within the range of 6.5–9. About 32% of 6803 profiles match well, 23% match badly and 45% are difficult to classify as they match only in parts.