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Our recent steady‐state mass‐balance modeling suggests that most global carbonic‐acid weathering of silicate rocks occurs in the vadose zone of aquifer systems not on the surface by atmospheric CO2. That is, the weathering solute flux is nearly equal to the total global continental riverine carbon flux, signifying little atmospheric weathering by carbonic acid. This finding challenges previous carbon models that utilize silicate weathering as a control of atmospheric CO2 levels. A robust analysis utilizing global estimates of groundwater carbon concentration generated by a geospatial machine learning algorithm was coupled with recharge flux in a geographic information system environment to yield a total global groundwater carbon flux of between 0.87 and 0.96 Pg C/year to the surface environment. On discharging to the surface, the carbon is speciated between 0.01 and 0.11 Pg C/year as CaCO3; 0.35 and 0.38 Pg C/year as CO2 gas; and 0.49 and 0.51 Pg C/year as dissolved HCO3−. This total weathering carbon flux was calculated for direct ocean discharge (0.030 Pg C/year); endorheic basins (0.046 Pg C/year); cold‐wet exorheic basins (0.058 Pg C/year); warm‐dry exorheic basins (0.072 Pg C/year); cold‐dry exorheic basins (0.115 Pg C/year); and warm‐wet exorheic basins (0.448 Pg C/year). [ABSTRACT FROM AUTHOR]

The estimated current global mean nitrogen concentration (geogenic + anthropogenic) in the active continental freshwater aquifer element pool is 1.1 mg/L as N, or between four and five times greater than the assumed geogenic mean. This concentration, combined with groundwater flux, generates a continental mass flux of 17 Tg N/y (teragrams of nitrogen, as N, per year) as a result of direct ocean discharge (0.67 Tg N/y), endorheic basins (1.2 Tg N/y), and cold‐wet (0.82 Tg N/y); cold‐dry (1.4 Tg N/y); warm‐dry (1.6 Tg N/y); and warm‐wet (11 Tg N/y) exorheic basins. These values are derived from a geospatial machine learning algorithm and combined groundwater‐modeled recharge in an ArcGIS environment. This active continental freshwater aquifer mass flux is between 35% and 40% of the continental integrated riverine system discharge, thus a significant component of the Earth's active continental freshwater nitrogen budget. We estimate the active continental freshwater aquifer volume to be between 1.4 and 2.8 million km3 suggesting a legacy of between 1.5 and 3.1 Pg as N (petagrams nitrogen as N) with mean residences of 90–180 years. Plain Language Summary: Globally, the current average nitrogen concentration in groundwater is 1.1 mg/L, or more than 4 times the estimated natural uncontaminated value. This nitrogen contributes between 35% and 40% of the total nitrogen carried in the rivers on Earth. A freshwater aquifer volume of between 1.4 and 2.8 million km3 combined with a concentration of 1.1 mg/L results in a mass legacy of between 1.5 and 3.1 Pg as N with a mean residence time of between 90 and 180 years.. Key Points: The current nitrogen concentration in the active global aquifer element pool is 1.1 mg/L as N or 4 to 5 times the estimated geogenic meanA mass flux of 17 Tg N/y (Tetragrams of nitrogen, as N, per year) and is between 35% and 40% of the total nitrogen riverine fluxThe current global mass legacy is between 1.5 and 3.1 billion tons with a residence time of 90–180 years. [ABSTRACT FROM AUTHOR]

Chemical, isotopic and physical evidence indicate that some of the groundwater in the Chesapeake Bay crater is remnant Early Cretaceous North Atlantic sea water, probably 100-145 million years old, with an average salinity of about 700/00, which is twice that of modern sea water. Intact water from a Cretaceous ocean Geologists and oceanographers generally have to rely on indirect evidence from the analysis of solid materials in deep sediment cores when estimating temperature and salinity levels of the ancient oceans. But now a team from the US Geological Survey has identified a body of underground water that is actually a remnant of a past ocean water, a body of Early Cretaceous North Atlantic sea water. Deep drilling at the site of the Chesapeake Bay impact crater has yielded groundwater with an isotope and chemical composition signature that together with model analysis suggests that it was trapped in the sediments before the impact occurred around 35 million years ago. The water may have lain undisturbed for much longer: it has an average salinity of about 70 parts per thousand, twice that of modern sea water, and is probably 100-145 million years old. High-salinity groundwater more than 1,000 metres deep in the Atlantic coastal plain of the USA has been documented in several locations.sup.1,2, most recently within the 35-million-year-old Chesapeake Bay impact crater.sup.3,4,5. Suggestions for the origin of increased salinity in the crater have included evaporite dissolution.sup.6, osmosis.sup.6 and evaporation from heating.sup.7 associated with the bolide impact. Here we present chemical, isotopic and physical evidence that together indicate that groundwater in the Chesapeake crater is remnant Early Cretaceous North Atlantic (ECNA) sea water. We find that the sea water is probably 100-145 million years old and that it has an average salinity of about 70 per mil, which is twice that of modern sea water and consistent with the nearly closed ECNA basin.sup.8. Previous evidence for temperature and salinity levels of ancient oceans have been estimated indirectly from geochemical, isotopic and palaeontological analyses of solid materials in deep sediment cores. In contrast, our study identifies ancient sea water in situ and provides a direct estimate of its age and salinity. Moreover, we suggest that it is likely that remnants of ECNA sea water persist in deep sediments at many locations along the Atlantic margin., Author(s): Ward E. Sanford [sup.1] , Michael W. Doughten [sup.1] , Tyler B. Coplen [sup.1] , Andrew G. Hunt [sup.2] , Thomas D. Bullen [sup.3] Author Affiliations: (1) US Geological [...]

Kohout convection is the name given to the circulation of saline groundwater deep within carbonate platforms, first proposed by F.A. Kohout in the 1960s for south Florida. It is now seen as an Mg pump for dolomitization by seawater. As proposed by Kohout, cold seawater is drawn into the Florida platform from the deep Straits of Florida as part of a geothermally driven circulation in which the seawater then rises in the interior of the platform to mix and exit with the discharging meteoric water of the Floridan aquifer system. Simulation of the asymmetrically emergent Florida platform with the new three-dimensional (3-D), finite-element groundwater flow and transport model SUTRA-MS, which couples salinity- and temperature-dependent density variations, allows analysis of how much of the cyclic flow is due to geothermal heating (free convection) as opposed to mixing with meteoric water discharging to the shoreline (forced convection). Simulation of the system with and without geothermal heating reveals that the inflow of seawater from the Straits of Florida would be similar without the heat flow, but the distribution would differ significantly. The addition of heat flow reduces the asymmetry of the circulation: it decreases seawater inflows on the Atlantic side by 8% and on the Gulf of Mexico side by half. The study illustrates the complex interplay of freshwater-saltwater mixing, geothermal heat flow, and projected dolomitization in complicated 3-D settings with asymmetric boundary conditions and realistic horizontal and vertical variations in hydraulic properties. Keywords: Kohout convection, Floridan aquifer system, Boulder Zone, geothermal, heat flow, dolomitization, groundwater.

Active reflux may occur during periods of platform-top brine generation, but the role and fate of these brines after reflux events are uncertain. We have used a numerical flow model to investigate and quantify the response of reflux brines to changes in platform-top salinity. Simulations suggest that reflux brines, originally concentrated to gypsum saturation (150 [per thousand]), have a relatively long platform residence time, on the order of 100 times the duration of the reflux event. When brine-generating conditions cease, brines will continue to sink through the platform, entraining seawater, a variant of reflux circulation we term latent reflux. Mesosaline brines intercepted by drilling of carbonate margins by the Ocean Drilling Program may have originated from Pleistocene reflux event(s) on the adjacent platform top and be currently moving by latent reflux. Latent-reflux circulation could deliver a significant quantity of dissolved reactants to platform carbonates, including Mg for dolomitization. Keywords: reflux, brines, carbonate platform, latent reflux, dolomitization.

An ascending-brine model is proposed to address the observed isotope geochemistry, solute composition, and solute and water fluxes in the coastal sabkha of the Emirate of Abu Dhabi. Mass-balance measurements document that >95% of the solutes are derived from ascending continental brines; minor amounts are derived from rainfall and from groundwater entering from up-gradient areas. Nearly 100% of the annual water loss is from evaporation and not lateral discharge. Direct rainfall on the sabkha and subsequent recharge to the underlying aquifer account for ~90% of the annual water input to the system; the remaining 10% comes from both lateral and ascending groundwater flow. Thus, the water and solutes in this system are from different sources. Solute concentrations of conservative (i.e., nonreactive) elements in the coastal, sabkha-covered aquifer are consistent with the fluid pore volumes of ascending brine calculated from hydrologic properties. Calcium to sulfate ratios and sulfur isotopes are consistent with this source of solute from the underlying Tertiary formations. Recharging rainwater dissolves halite and other soluble minerals on the surface, causing the solution to become more dense and sink to the bottom of the aquifer where it vertically mixes with less dense ascending brines. Solutes are returned to the surface by capillary forces and recycled or lost from the system by eolian or fluvial processes. Thus, the system becomes vertically mixed, consistent with the presence of tritium throughout the aquifer; but there is essentially no horizontal mixing of seawater with groundwater. The observed seawater solutes in the supra-tidal zone come from interstitial seawater trapped by the rapid progradation of the sediments into the Arabian Gulf and are not refluxed or laterally mixed. The ascending-brine model contrasts significantly with both the seawater-flooding and evaporative-pumping models previously proposed as a source of solutes to the coastal sabkha of the Emirate of Abu Dhabi. Use of these earlier models leads to incorrect conclusions and raises serious questions about their applicability in the evaluation of sabkhat in the geologic record. Keywords: Arabian Gulf, Abu Dhabi, brine, sabkha environment, salt flat.

Reactive-transport simulations were used to determine the spatial patterns and rates of dolomitization on a 40-km long, 2-km thick carbonate platform subjected to geothermal convection at temperatures sufficient to supersede kinetic limitations. A link was found between dolomitization and precipitation of anhydrite.

Introduction It has been recognized that diffusion into the rock matrix of a fractured porous media is important in the transport of radionuclides. Neretnieks (1980, 1981) first pointed out the [...]

Introduction Recharge is frequently the most important factor in evaluating regional aquifer systems in add and semiarid environments, and unfortunately it often is also the most difficult to quantify. Additionally, [...]

The timing of changes in the stage and salinity of Double Lakes of Lynn County, Texas, was estimated using dissolved-chloride profiles across an underlying shale layer. Lake conditions over the past 30 to 50 ka can be inferred from the chloride profiles by using the advective velocity of the pore water through the shale and an appropriate coefficient of molecular diffusion. The profiles suggest that net-evaporative conditions existed over the southern High Plains for the past 50 ka; a period of increasing salinity in the lake began at [approximately] 20 ka and reached current levels at [approximately] 5 ka. In addition, deflationary conditions were present for at least 4 ka, and likely began or were accelerated during the most recent altithermal period at [approximately] 5 ka. This type of lake-brine record may also exist in many other saline lake environments throughout the Great Plains of North America.

A variable density groundwater flow model is used to examine the pattern, magnitude and interaction of density-driven seawater circulation and reflux circulation mechanisms, for mesosaline platform-top waters (50 per thousand) and brines concentrated up to saturation with respect to gypsum (150 per thousand) and halite (246 per thousand). The study reveals that the persistence of geothermal circulation is dependent on the rate of brine reflux, and also on the magnitude and distribution of permeability.

Geochemical mixing theory suggests that the mixing of seawater and calcite-saturated fresh ground wanter can result in a solution that is undersaturated with respect to calcite. Previous studies of the mixing of such waters in carbonate rocks along certain coastlines have indicated that this mixing effect may be responsible for significant amounts of calcite dissolution and porosity development. In this study, potential rates of porosity development by calcite dissolution are assessed by combining geochemical mixing theory with the hydrodynamics of fresh-water-salt-water mixing zones in a coupled reaction-transport model. Results from the reaction-path model PHREEQE are used with a variable-density ground-water flow and solute-transport model to simulate an idealized cross section of a coastal carbonate aquifer. Results of the simulations indicate that the dissolution process is sensitive to fresh-water chemistry, ground-water velocities, and sea-level movement. Dissolution potential was evaluated at three field sites, and evidence from those sites is in general agreement with the simulation results. Dissolution rates indicated by the model show that under the proper conditions this dissolution mechanism can produce significant increases in porosity over relatively short spans of geologic time (tens of thousands of years).

The magnitude and distribution of permeability within a carbonate platform have a significant influence on seawater circulation. Depending on the values of horizontal and vertical permeability, heat transfer within a carbonate platform takes place via conduction, free convection or forced convection. Seawater circulation may decline rapidly as a function of depth as indicated by depth-dependent relations for permeability and porosity in carbonate platforms.