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.

The Nile Delta formed by progression of a complex system of deltaic fans throughout the Pleistocene, with the Modern delta being formed from sediments supplied by at least ten distinct distributaries throughout the Holocene. With an average elevation of approximately 1 m above sea level within 30 km of the coast and a predicted rise in sea level of 1.8—5.9 mm/yr the subsidence of the northern delta has become a topic of major concern to the Egyptian population and government. The Nile Delta is home to more than 50 million people and the major agricultural production area for Egypt. We evaluated the modern rates of subsidence of sections of the northeastern Nile Delta (a total length of 110 km, up to 50 km from the coastline) using persistent scatterer radar interferometry techniques applied to 14 ERS-1 and ERS-2 scenes. The area covered includes the present active depocenter of the Damietta promontory, and the nearby Mendesian depocenter that was active up to recent times (to 2500 years ago). The highest subsidence rates (~8 mm/yr; twice average Holocene rates) do not correlate with the distribution of the thickest Holocene sediments, but rather with the distribution of the youngest depositional centers (major deposition occurred between ~3500 yr BP and present) at the terminus of the Damietta branch. The adjacent, slightly older (8000—2500 yr BP) Mendesian branch depositional center is subsiding at slower rates of 2—6 mm/yr. Results are interpreted to indicate that: (1) modern subsidence in the Delta is heavily influenced by compaction of the most recent sediments, and (2) highly threatened areas are at the terminus of the Damietta and possibly the Rosetta branches, where active deposition is occurring.

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‐CH₄ m⁻² yr⁻¹) from the marsh was high and compatible with its net annual CO₂ uptake (F_CO2: −21.0 g C‐CO₂ m⁻² yr⁻¹). In contrast, F_CH4 was small (2.3 g C‐CH₄ m⁻² yr⁻¹) and accounted for a minor portion of the atmospheric carbon budget (F_CO2: −151.8 g C‐CO₂ m⁻² yr⁻¹) at the cropland. At the seasonal scale, soil temperature associated with methane (CH₄) production provided the dominant regulator of F_CH4 at the marsh (R² = 0.86). At the diurnal scale, plant‐modulated gas flow was the major pathway for CH₄ 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 CH₄. 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.