Sinking cities

When water supplies are abstracted from alluvial sands, interbedded clays compact and cause ground subsidence. Entire cities are being gently lowered, with serious consequences for those on low coastal sites.     

Lithological controls on biological activity and groundwater chemistry in Quaternary sediments

Depth profiles of solute chemistry and sulfate isotopic compositions are presented for groundwater and pore water in a sequence of Quaternary glacial outwash sediments. Sand units show evidence for hydraulic connection to the surface and thus modern sources of solutes. Finer‐grained sediments show a general pattern of increasing solute concentrations with depth, with sulfate derived from ancient rainwater and pyrite oxidation in the soil/drift. In these sediments sulfate has undergone bacterial sulfate reduction (BSR) to produce biogenic sulfide. In clay sediments, with d₁₀ ≤ 1·6 µm, high concentrations of sulfate and acetate now co‐exist, implying that BSR is inhibited. The correlation with smaller sediment grain size indicates that this is due to pore size exclusion of the sulfate reducing bacteria. Mechanical restriction of microbial function thus provides a fundamental limitation on microbial respiration in buried clay‐rich sediments, which acts as a control on the chemical evolution of their pore waters. Copyright © 2009 John Wiley & Sons, Ltd.     

The noble gas geochemistry of natural CO2 gas reservoirs from the Colorado Plateau and Rocky Mountain provinces, USA

Identification of the source of CO₂ in natural reservoirs and development of physical models to account for the migration and interaction of this CO₂ with the groundwater is essential for developing a quantitative understanding of the long term storage potential of CO₂ in the subsurface. We present the results of 57 noble gas determinations in CO₂ rich fields (>82%) from three natural reservoirs to the east of the Colorado Plateau uplift province, USA (Bravo Dome, NM., Sheep Mountain, CO. and McCallum Dome, CO.), and from two reservoirs from within the uplift area (St. John’s Dome, AZ., and McElmo Dome, CO.). We demonstrate that all fields have CO₂/³He ratios consistent with a dominantly magmatic source. The most recent volcanics in the province date from 8 to 10 ka and are associated with the Bravo Dome field. The oldest magmatic activity dates from 42 to 70 Ma and is associated with the McElmo Dome field, located in the tectonically stable centre of the Colorado Plateau: CO₂ can be stored within the subsurface on a millennia timescale.The manner and extent of contact of the CO₂ phase with the groundwater system is a critical parameter in using these systems as natural analogues for geological storage of anthropogenic CO₂. We show that coherent fractionation of groundwater ²⁰Ne/³⁶Ar with crustal radiogenic noble gases (⁴He, ²¹Ne, ⁴⁰Ar) is explained by a two stage re-dissolution model: Stage 1: Magmatic CO₂ injection into the groundwater system strips dissolved air-derived noble gases (ASW) and accumulated crustal/radiogenic noble gas by CO₂/water phase partitioning. The CO₂ containing the groundwater stripped gases provides the first reservoir fluid charge. Subsequent charges of CO₂ provide no more ASW or crustal noble gases, and serve only to dilute the original ASW and crustal noble gas rich CO₂. Reservoir scale preservation of concentration gradients in ASW-derived noble gases thus provide CO₂ filling direction. This is seen in the Bravo Dome and St. John’s Dome fields. Stage 2: The noble gases re-dissolve into any available gas stripped groundwater. This is modeled as a Rayleigh distillation process and enables us to quantify for each sample: (1) the volume of groundwater originally ‘stripped’ on reservoir filling; and (2) the volume of groundwater involved in subsequent interaction. The original water volume that is gas stripped varies from as low as 0.0005 cm³ groundwater/cm³ gas (STP) in one Bravo Dome sample, to 2.56 cm³ groundwater/cm³ gas (STP) in a St. John’s Dome sample. Subsequent gas/groundwater equilibration varies within all fields, each showing a similar range, from zero to ∼100 cm³ water/cm³ gas (at reservoir pressure and temperature).     

Quantifying the sequence stratigraphy and drowning mechanisms of atolls using a new 3-D forward stratigraphic modelling program (CARBONATE 3D)

This paper describes a new 3‐D forward numerical model (CARBONATE 3D) that simulates the stratigraphic and sedimentological development of carbonate platforms and mixed carbonate–siliciclastic shelves by simulating the following sedimentary processes: (1) Carbonate shallow, open‐marine production, dependent on water depth, restriction and sediment input; (2) Carbonate shallow, restricted‐marine production, dependent on water restriction; (3) Pelagic sediment production and deposition; (4) Coarse and fine siliciclastic input; (5) Erosion, transport and redeposition of sediment, dependent on currents, slope, depth and restriction as well as sediment grain‐size and composition; (6) Dissolution of subaerially exposed carbonate. In this paper the model is used to investigate the controlling mechanisms on the sequence stratigraphy of isolated carbonate platforms and atolls and to predict distinctive architectural signatures from different drowning mechanisms. Investigation of the mechanisms controlling atoll strata shows that although relative sea‐level is the major control, antecedent topography, environmental setting and early diagenesis have profound influence on what stratigraphic geometries and facies develop. Hence care must be taken if sea‐level curves are interpreted from real stratigraphies. Atoll drowning by fast sea‐level rise, by lowered production and by repeated exposure and fast subsequent sea‐level rises are investigated and different stratigraphic signatures for the respective mechanisms predicted. A fast relative sea‐level rise results in a bucket‐shaped morphology developed prior to drowning and a sharp transition from the platform margin facies to a pelagic cover. Drowning caused by lowered platform margin production is predicted to result in the development of a dome‐shaped, shallow‐water shoal over the whole platform top prior to drowning. Fourth order amplitudes of several tens of metres, typical of ‘icehouse’ settings, cause atoll drowning at subsidence rates where atolls subject to fourth order amplitude of only a few metres, typical of ‘greenhouse’ settings, can keep up with the rising sea‐level. In the resultant strata, vertical facies belts are less well developed but horizontally extensive facies bands are more prominent. High fourth order amplitudes (up to 80 m) without sufficient third order scale subsidence will not lead to drowning, however, as the platform can recover in each fourth order lowstand. These results suggest that atolls might be easier to drown in ‘icehouse’ rather than in ‘greenhouse’ conditions but only in situations with suitably high rates of longer‐term relative sea‐level rise or sufficient lag times.