No abstract

Measurements of 81Kr/Kr in deep groundwater from the Nubian Aquifer (Egypt) were performed by a new laser‐based atom‐counting method. 81Kr ages range from ∼2 × 105 to ∼1 × 106 yr, correlate with 36Cl/Cl ratios, and are consistent with lateral flow of groundwater from a recharge area near the Uweinat Uplift in SW Egypt. Low δ2H values of the 81Kr‐dated groundwater reveal a recurrent Atlantic moisture source during Pleistocene pluvial periods. These results indicate that the 81Kr method for dating old groundwater is robust and such measurements can now be applied to a wide range of hydrologic problems.

To understand the carbon and energy exchange between the lake surface and the atmosphere, direct measurements of latent, sensible heat, and CO 2 fluxes were taken using the eddy covariance (EC) technique in Western Lake Erie during October 2011 to September 2013. We found that the latent heat flux (LE) had a marked one-peak seasonal change in both years that differed from the diurnal course and lacked a sinusoidal dynamic common in terrestrial ecosystems. Daily mean LE was 4.8 ± 0.1 and 4.3 ± 0.2 MJ m À2 d À1 in Year 1 and Year 2, respectively. The sensible heat flux (H) remained much lower than the LE, with a daily mean of 0.9 ± 0.1 and 1.1 ± 0.1 MJ m À2 d À1 in Year 1 and Year 2, respectively. As a result, the Bowen ratio was <1 during most of the 2 year period, with the lowest summer value at 0.14. The vapor pressure deficit explained 35% of the variation in half hourly LE, while the temperature difference between the water surface and air explained 65% of the variation in half hourly H. Western Lake Erie acted as a small carbon sink holding À19.0 ± 5.4 and À40.2 ± 13.3 g C m À2 in the first and second summers (May-September) but as an annual source of 77.7 ± 18.6 and 49.5 ± 17.9 g C m À2 yr À1 in Year 1 and Year 2, respectively. The CO 2 flux (F CO2 ). Similar to LE, F CO2 had noticeable diurnal changes during the months that had high chlorophyll a months but not during other months. A significantly negative correlation (P < 0.05) was found between F CO2 and chlorophyll a on monthly fluxes. Three gap-filling methods, including marginal distribution sampling, mean diurnal variation, and monthly mean, were quantitatively assessed, yielding an uncertainty of 4%, 6%, and 10% in LE, H, and F CO2 , respectively.

No abstract

Net ecosystem carbon dioxide (F CO2 ) and methane (F CH4 ) exchanges were measured by using the eddy covariance method to quantify the atmospheric carbon budget at a Typha-and Nymphaea-dominated freshwater marsh (March 2011 to March 2013) and a soybean cropland (May 2011 to May 2012) in northwestern Ohio, USA. Two year average annual F CH4 (49.7 g C-CH 4 m À2 yr À1) from the marsh was high and compatible with its net annual CO 2 uptake (F CO2 : À21.0 g C-CO 2 m À2 yr À1). In contrast, F CH4 was small (2.3 g C-CH 4 m À2 yr À1) and accounted for a minor portion of the atmospheric carbon budget (F CO2 : À151.8 g C-CO 2 m À2 yr À1) at the cropland. At the seasonal scale, soil temperature associated with methane (CH 4 ) production provided the dominant regulator of F CH4 at the marsh (R 2 = 0.86). At the diurnal scale, plant-modulated gas flow was the major pathway for CH 4 outgassing in the growing season at the marsh. Diffusion and ebullition became the major pathways in the nongrowing season and were regulated by friction velocity. Our findings highlight the importance of freshwater marshes for their efficiency in turning over and releasing newly fixed carbon as CH 4 . Despite marshes accounting for only~4% of area in the agriculture-dominated landscape, their high F CH4 should be carefully addressed in the regional carbon budget.