On the ability of the LODYC GCM to simulate the thermocline depth in the equatorial Atlantic

Using a multivariate model testing procedure that distinguishes between model inadequacies and data uncertainties, we investigate the ability of the LODYC GCM to simulate the evolution of the 20°C isotherm depth during the 1982–1984 FOCAL/SEQUAL experiment in the equatorial Atlantic. Two different versions of the model are considered: the Ri version which has a Richardson number dependent parameterization of vertical mixing and the new TKE version which uses a local estimation of the turbulent kinetic energy to parameterize vertical mixing. Some effects of the forcing uncertainties are considered by forcing the TKE version with three equally plausible wind stress fields whose differences are consistent with the measurement and sampling errors, and the drag coefficient indeterminacy. The resulting uncertainties in the model response are substantial and can be as large as the differences between simulations with the two GCM versions, which stresses the need to take the forcing uncertainties into account. Although only one Ri run is available, it is shown that the TKE parameterization significantly improves the representation of the equatorial upwelling and the simulation of the depth of the thermocline in the eastern Atlantic. However, there remain significant differences with the observations which cannot be explained by the forcing uncertainties that were considered. The two model versions perform better in the equatorial wave guide than in the 12°N-12°S domain, and they are better distinguished over large domains than along sections, which shows that a global multivariate view point must be used in model-reality comparisons. Finally, a comparison with a linear multimode model emphasizes the need for greater model complexity to properly simulate the equatorial upwelling and the thermocline variability in the tropical Atlantic.This paper was presented at the Second International Conference on Modelling of Global Climate Variability, held in Hamburg 7–11 September 1992 under the auspices of the Max-Planck-Institute for Meteorology. Guest Editor for these papers is L. Dümenil     

Mid-Holocene and Last Glacial Maximum climate simulations with the IPSL model—part I: comparing IPSL_CM5A to IPSL_CM4

The climates of the mid-Holocene (MH), 6,000 years ago, and of the Last Glacial Maximum (LGM), 21,000 years ago, have extensively been simulated, in particular in the framework of the Palaeoclimate Modelling Intercomparion Project. These periods are well documented by paleo-records, which can be used for evaluating model results for climates different from the present one. Here, we present new simulations of the MH and the LGM climates obtained with the IPSL_CM5A model and compare them to our previous results obtained with the IPSL_CM4 model. Compared to IPSL_CM4, IPSL_CM5A includes two new features: the interactive representation of the plant phenology and marine biogeochemistry. But one of the most important differences between these models is the latitudinal resolution and vertical domain of their atmospheric component, which have been improved in IPSL_CM5A and results in a better representation of the mid-latitude jet-streams. The Asian monsoon’s representation is also substantially improved. The global average mean annual temperature simulated for the pre-industrial (PI) period is colder in IPSL_CM5A than in IPSL_CM4 but their climate sensitivity to a CO₂ doubling is similar. Here we show that these differences in the simulated PI climate have an impact on the simulated MH and LGM climatic anomalies. The larger cooling response to LGM boundary conditions in IPSL_CM5A appears to be mainly due to differences between the PMIP3 and PMIP2 boundary conditions, as shown by a short wave radiative forcing/feedback analysis based on a simplified perturbation method. It is found that the sensitivity computed from the LGM climate is lower than that computed from 2 × CO₂ simulations, confirming previous studies based on different models. For the MH, the Asian monsoon, stronger in the IPSL_CM5A PI simulation, is also more sensitive to the insolation changes. The African monsoon is also further amplified in IPSL_CM5A due to the impact of the interactive phenology. Finally the changes in variability for both models and for MH and LGM are presented taking the example of the El-Niño Southern Oscillation (ENSO), which is very different in the PI simulations. ENSO variability is damped in both model versions at the MH, whereas inconsistent responses are found between the two versions for the LGM. Part 2 of this paper examines whether these differences between IPSL_CM4 and IPSL_CM5A can be distinguished when comparing those results to palaeo-climatic reconstructions and investigates new approaches for model-data comparisons made possible by the inclusion of new components in IPSL_CM5A.     

Sensitivity of simulated Asian and African summer monsoons to orbitally induced variations in insolation 126, 115 and 6 kBP

We have conducted four numerical experiments with an atmospheric general circulation model (AGCM) to investigate the sensitivity of Asian and African monsoons to small changes (–5 to +12%), with respect to present-day, in incoming solar radiation at the top of the atmosphere. We show that, during the mid-Holocene (6 kBP where kBP means thousands of years before present-day) and the last interglacial (126 kBP), the Northern Hemisphere seasonal contrast was increased, with warmer summers and colder winters. At the time of glacial inception (115 kBP) however, summers were cooler and winters milder. As a consequence, Asia and tropical North Africa experienced stronger (weaker) summer monsoons 6 and 126 kBP (115 kBP), in agreement with previous numerical studies. This present study shows that summer warming/cooling of Eurasia and North Africa induced a shift of the main low-level convergence cell along a northwest/southeast transect. When land was warmer (during the summer months 6 and 126 kBP), the monsoon winds converged further inland bringing more moisture into northern India, western China and the southern Sahara. The southern tips of India, Indochina and southeastern China, as well as equatorial North Africa became drier. When land was cooler (during the summer 115 kBP), the main convergence zone was located over the west Pacific and the wet (dry) areas were those that were dry (wet) 6 and 126 kBP. The location and intensity of the simulated precipitation maxima were therefore very sensitive to changes in insolation. However the total amount of monsoon rain in Asia as well as in Africa remained remarkably stable through the time periods studied. These simulated migrations of convective activities were accompanied by changes in the nature of precipitation events: increased monsoon rains in these experiments were always associated with more high precipitation events (> 5 mm day ^–1), and fewer light showers (1 mm day^–). Rainy days with rates between 1 and 5 mm day^–1 were almost unchanged.     

Time Transfer Experiments Using GLONASS P-code Measurements from RINEX Files

We have used GLONASS P-code measurements from different geodetic GPS/GLONASS receivers involved in the IGEX campaign to perform frequency/time transfer between remote clocks. GLONASS time transfer is commonly based on the clock differences between GLONASS system time and the local clock computed by a time transfer receiver. We choose to analyze the raw P-code data available in the RINEX files. This also allows working with the data from geodetic receivers involved in the IGEX campaign. As a first point, we show that the handling of the external frequency in some of the IGEX receivers is not suited for time transfer applications. We also point out that the GLONASS broadcast ephemerides give rise to a considerable number of outliers in the time transfer, compared to the precise IGEX ephemerides. Due to receiver clock resets at day boundaries, which is a characteristic of the R100 receivers from 3S-Navigation, continuous data sets exceeding one day are not available. Invthis context, it is therefore impossible to perform RINEX-based precise frequency transfer with GLONASS P-codes on a time scale longer than one day. Because the frequencies used by GLONASS satellites are different, the time transfer results must be corrected for the different receiver hardware delays. After this correction, the final precision of our time transfer results corresponds to a root-mean-square (rms) of 1.8 nanoseconds (ns) (maximum difference of 11.8 ns) compared to a rms of about 4.4 ns (maximum difference of 31.9 ns) for time transfer based on GPS C/A code observations. ? 2001 John Wiley & Sons, Inc.     

Boron isotope ratios of surface waters in Guadeloupe,Lesser Antilles

Large variations are reported in the B concentrations and isotopic ratios of river and thermal spring waters in Guadeloupe, Lesser Antilles. Rivers have δ¹¹B values around 40‰ and B concentrations lower than 30 μg/L, while thermal springs have δ¹¹B of 8–15‰ and B concentrations of 250–1000 μg/L. River samples strongly impacted by hydrothermal inputs have intermediate δ¹¹B and B contents. None of these surface water samples have δ¹¹B comparable to the local unweathered volcanic rocks (around 0‰), implying that a huge isotopic fractionation of 40‰ takes place during rock weathering, which could be explained by preferential incorporation of ¹⁰B during secondary mineral formation and adsorption on clays, during rock weathering or in the soils. The soil-vegetation B cycle could also be a cause for such a fractionation. Atmospheric B with δ¹¹B of 45‰ represents 25–95% of the river B content. The variety of the thermal spring chemical composition renders the understanding of B behavior in Guadeloupe hydrothermal system quite difficult. Complementary geochemical tracers would be helpful.     

Using Mg Isotopes to Trace Cyanobacterially Mediated Magnesium Carbonate Precipitation in Alkaline Lakes

This study assesses the potential use of Mg isotopes to trace Mg carbonate precipitation in natural waters. Salda Lake (SW Turkey) was chosen for this study because it is one of the few modern environments where hydrous Mg carbonates are the dominant precipitating minerals. Stromatolites, consisting mainly of hydromagnesite, are abundant in this lake. The Mg isotope composition of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments were measured. Because Salda Lake is located in a closed basin, mass balance requires that the Mg isotopic offset between Lake Salda water and precipitated hydromagnesite be comparable to the corresponding offset between Salda Lake and its water inputs. This is consistent with observations; a ??²⁶Mg offset of 0.8?C1.4??? is observed between Salda Lake water and it is the incoming streams and groundwaters, and precipitated hydromagnesite has a ??²⁶Mg 0.9?C1.1??? more negative than its corresponding fluid phase. This isotopic offset also matches closely that measured in the laboratory during both biotic and abiotic hydrous Mg carbonate precipitation by cyanobacteria (Mavromatis, V., Pearce, C., Shirokova, L. S., Bundeleva, I. A., Pokrovsky, O. S., Benezeth, P. and Oelkers, E.H.: Magnesium isotope fractionation during inorganic and cyanobacteria-induced hydrous magnesium carbonate precipitation, Geochim. Cosmochim. Acta, 2012a. 76, 161?C174). Batch reactor experiments performed in the presence of Salda Lake cyanobacteria and stromatolites resulted in the precipitation of dypingite (Mg₅(CO₃)₄(OH)₂·5(H₂O)) and hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O) with morphological features similar to those of natural samples. Concurrent abiotic control experiments did not exhibit carbonate precipitation demonstrating the critical role of cyanobacteria in the precipitation process.     

Do organic ligands affect calcite dissolution rates?

Steady state Iceland-spar calcite dissolution rates were measured at 25 °C in aqueous solutions containing 0.1 M NaCl and up to 0.05 M dissolved bicarbonate at pH from 7.9 to 9.1 in the presence of 13 distinct dissolved organic ligands in mixed-flow reactors. The organic ligands considered in this study include those most likely to be present in either (1) aquifers at the conditions pertinent to CO₂ sequestration or (2) soil/early diagenetic environments: acetate, phthalate, citrate, EDTA⁴⁻, succinate, d-glucosaminate, l-glutamate, d-gluconate, 2,4-dihydroxybenzoate, 3,4-dihydroxybenzoate, fumarate, malonate, and gallate. Results show that the presence of <0.05 mol/kg of these organic anions changes calcite dissolution rates by less than a factor of 2.5 with the exception of citrate and EDTA⁴⁻. The presence of 0.05 mol/kg citrate and EDTA⁴⁻ increases calcite dissolution rates by as much as a factor of 35 and 500, respectively, compared to rates in organic anion-free solutions. Further calcite dissolution experiments were performed in the presence of organic polymers similar to bacterial exudates, cell exopolysaccharides, and analogs of microbial cell envelopes: alginate, lichen extract, humic acid, pectin, and gum xanthan. In no case did the presence of <100 ppm of these organics change calcite dissolution rates by more than a factor of 2.5. Results obtained in this study suggest that the presence of aqueous organic anions negligibly affects calcite forward dissolution rates in most natural environments. Some effect on calcite reactivity may be observed, however, by the presence of organic anions if they change substantially the chemical affinity of the fluid with respect to calcite.     

Rise and Fall of an Egyptian Oasis: Artesian Flow,Irrigation Soils,and Historical Agricultural Development in El‐Deir,Kharga Depression,Western Desert of Egypt

The present study examines the geoarchaeological history of an oasis in Kharga Depression in central Egypt. El‐Deir is renowned for its Ptolemaic temple and Roman fortress on the road from former Hibis (Kharga) to the Nile Valley. During the survey, spring mounds and irrigation soils belonging to an ancient agricultural zone were discovered, and further documented by ceramics found on the site. Our methodology combines the geomorphological interpretation of landforms (especially yardangs) with ceramics and ¹⁴C‐dated charcoal to distinguish and date former agricultural areas in El‐Deir. The results show that the oasis experienced several phases of soil accretion and destruction through time. Playa sediments were deposited in the humid early Holocene and severely eroded by deflation before the onset of irrigated agriculture between Pharaonic and Persian times. Very fast vertical soil accretion occurred in the Ptolemaic period, but irrigation soils were later destroyed during the Roman period by a combination of wind deflation and flash floods (second to fourth century A.D.), suggesting a period of climate instability. The case of El‐Deir invites reevaluation of constructive agencies for the development of irrigated land and destructive agencies as limiting factors for the sustainability of agricultural practices in late antiquity.     

Dawsonite synthesis and reevaluation of its thermodynamic properties from solubility measurements: Implications for mineral trapping of CO2

Over the last decade, a significant research effort has focused on determining the feasibility of sequestering large amounts of CO₂ in deep, permeable geologic formations to reduce carbon dioxide emissions to the atmosphere. Most models indicate that injection of CO₂ into deep sedimentary formations will lead to the formation of various carbonate minerals, including the common phases calcite (CaCO₃), dolomite (CaMg(CO₃)₂), magnesite (MgCO₃), siderite (FeCO₃), as well as the far less common mineral, dawsonite (NaAlCO₃(OH)₂). Nevertheless, the equilibrium and kinetics that control the precipitation of stable carbonate minerals are poorly understood and few experiments have been performed to validate computer codes that model CO₂ sequestration.In order to reduce this uncertainty we measured the solubility of synthetic dawsonite according to the equilibrium: , from under- and oversaturated solutions at 50-200 °C in basic media at 1.0 mol · kg⁻¹ NaCl. The solubility products (Q_s) obtained were extrapolated to infinite dilution to obtain the solubility constants (. Combining the fit of these values and fixing  at 25 °C, which was derived from the calorimetric data of Ferrante et al. [Ferrante, M.J., Stuve, J.M., and Richardson, D.W., 1976. Thermodynamic data for synthetic dawsonite. U.S. Bureau of Mines Report Investigation, 8129, Washington, D.C., 13p.], the following thermodynamic parameters for the dissolution of dawsonite were calculated at 25 °C: , and . Subsequently, we were able to derive values for the Gibbs energy of formation (, enthalpy of formation ( and entropy ( of dawsonite. These results are within the combined experimental uncertainties of the values reported by Ferrante et al. (1976). Predominance diagrams are presented for the dawsonite/boehmite and dawsonite/bayerite equilibria at 100 °C in the presence of a saline solution with and without silica-containing minerals.