Pore Water Circulation Research Articles

DIAGENESIS AND MASS TRANSFER BETWEEN PERMO-TRIASSIC SANDSTONES AND INTERBEDDED MUDSTONES, ULSTER BASIN

Beds of siltstone, mudstone and shale are interbedded in the Permo-Triassic sandstones of the Ulster Basin, UK, at different stratigraphic levels. The paragenetic sequences of authigenic minerals both in the sandy and fine-grained sediments (mudstones and siltstones) indicate red bed diagenetic trend. Abundant authigenic quartz overgrowths and carbonate crystals observed near the sandstone-siltstone and sandstone-mudstone contacts, indicating that pore water and mass transfer from the fine-grained sediments to sandstone facies. The fine-grained sediments are the main source of ions for the authigenesis of quartz and feldspar overgrowths and carbonate precipitation. Mechanical compaction of fine-grained sediments, influx of gravity-driven acidic meteoric pore water, dehydration water, diffusion and convection are the most important means of mass transfer and pore water circulation. Pore water migration depends upon permeability distribution and pressure difference. (Received December 8, 2000; revised April 26, 2002) Bull. Chem. Soc. Ethiop. 2002, 16(1), 9-35.

Correction to “Anomalous temperatures in central Scotia Sea sediments—bottom water variation or pore water circulation in old ocean crust”

Correction to “Anomalous temperatures in central Scotia Sea sediments—bottom water variation or pore water circulation in old ocean crust”

Anomalous temperatures in central Scotia Sea sediments – Bottom water variation or pore water circulation in old ocean crust

We report low temperature gradients (from 60% to 12% of geothermal), and extrapolated temperatures offset from modern bottom water temperatures, in sediments from the central Scotia Sea. We examine possible causes, bearing in mind similar anomalous measurements 18 years previously, attributed at the time to instrumental error. Small temperature offsets (±0.1°C) may reflect short‐term bottom temperature variation within eddies of the Antarctic Circumpolar Current. Low sediment temperature gradients may be caused by horizontal advection of cold water within the upper oceanic basaltic layer (documented in younger ocean floor elsewhere), or by northward encroachment of colder bottom waters (from the Antarctic Peninsula shelf or Weddell Sea) for several years prior to measurement.

Organic matter processing in continental shelf sediments—the subtidal pump revisited

Abstract Pressure variations along an uneven permeable bottom, generated by the passage of gravity waves and by bottom currents, force water into the sediment below regions of high pressure, such as ripple troughs, and draw it out at regions of low pressure, such as ripple crests. The resulting pore water circulation brings organic matter and oxygen to the interior of the sediment, creates horizontal concentration gradients that can be as strong as the vertical gradients, and increases the flux of pore water constituents across the sediment-water interface. Here we review the present knowledge of pore water circulation and organic carbon mineralization in sandy continental shelf sediments. We argue that the circulation of water through the pores of the sediment enhances the mineralization rate of dissolved and particulate organic matter and that continental shelf sediments play a more important role in the oceanic carbon cycle than is generally thought.

The effects of wave‐induced pore water circulation on the transport of reactive solutes below a rippled sediment bed

The role of pore water flow in solute transport below a rippled sediment bed in a coastal environment is investigated. Based on the velocity field induced by the passage of gravity waves, a steady‐state transport equation is solved numerically over a range of field conditions. A consumption term governed by first‐order kinetics, a constant diffusivity and a constant boundary concentration slightly above the sediment‐water interface are assumed in the two‐dimensional continuum model. Significant spatial variations in concentration are found when there is an appreciable pore water flow. The concentration below a ripple trough can be many times higher than that at the same depth below a crest, and the concentration gradient in the horizontal can be of the same order of magnitude as that in the vertical. At high advective transport, a region of uniform concentration is found immediately below the ripple surface away from the crests. The rate of consumption over the sediment layer is compared with the corresponding rate in the absence of advective transport. Their ratio increases monotonously with the ripple slope and the ratio of the magnitudes of advective to diffusive transport and can reach up to 5 even for a relatively mild ripple slope of 0.1. As the magnitude of advective transport depends on the prevalent wave conditions, the rate of consumption and the distribution of solute concentration can vary appreciably with time. These results suggest that hydrodynamic conditions, sediment characteristics, and surface topography should be integral parts of field measurements of the distribution and flux of solutes in coastal sediments.

Secondary Porosity in the Brent Group (Middle Jurassic), Huldra Field, North Sea: Implication for Predicting Lateral Continuity of Sandstones?

ABSTRACT Middle Jurassic sandstones of the Brent Group from the Huldra Field, North Sea show extensive framework grain dissolution. Sandstones of the Etive Formation, which represent a laterally extensive sandstone facies, show more leaching than equally permeable sandstones of the Ness Formation, which are likely to be less continuous laterally. This variation in amount of secondary porosity is consistent with leaching and mass transfer associated with advection, and this may give information about internal flow properties within the reservoir. Kaolinite and relicts of feldspar are found in secondary pores, suggesting the formation of secondary porosity caused by feldspar dissolution associated with kaolinite precipitation. The least kaolinite is found in the sandstone facies of highest secondary porosity, suggesting that the mobility of aluminum is related to the rate of the pore-water flow. This is most likely explained by meteoric pore-water circulation due to the isotopic signature of the kaolinite, indicating formation at a temperature Secondary porosity is also formed at greater burial depths at temperatures higher than 60°C, but to a lesser extent. This leaching, which is selective for K-feldspar, is possibly enforced by the illitization of smectite in finer-grained associated sandstones and siltstones and by minor illitization of kaolinite. There is no net increase of porosity due to this process, and mass transfer is by diffusion.

The role of pore water circulation during the deformation of foreland fold and thrust belts

Volume‐loss strain accompanying pressure solution of calcite occurred within both the Umbria‐Marches Apennines of Italy and the Appalachian Mountains of western New York. Data from strain markers show that volume‐loss strain was greater within the shallow portions of the Apennines than within the Appalachians. Within the deeper portions of both fold and thrust belts, strain was nearly volume‐constant. Calcite solubility data suggest that downward circulation of meteoric water is necessary for the 35% volume‐loss strain of the limestones within the Apennines. Strain at a depth of about 1 km was volume‐constant and is interpretated as indicative of restricted pore fluid circulation. In the Appalachians, calcite comprises less than 1% of the clastic rocks, and a 10% volume‐loss of this calcite may occur during circulation of connate or dehydration water derived from dewatering of the shales but in an environment that restricts the circulation of meteoric water. Here, the volume of calcite removed (0.1% of the total rock) is so small that circulation of meteoric water is not necessary for strain by pressure solution.