Diagenetic processes in Holocene carbonate sediments: Florida Bay mudbanks and islands

The sedimentology, mineralogy and pore fluid chemistry of seven cores from the Holocene sediments of Florida Bay were studied to determine the physical processes and diagenetic reactions affecting the sediments. The cores were taken in a transect from a shallow mudbank onto a small adjacent island, Jimmy Key. Steady state models of pore fluid chemistry are used to estimate the rates of various reactions. In the mudbank sediments, little carbonate mineral diagenesis is taking place. No change in sediment mineralogy is detectable and pore water profiles of Ca²⁺, Mg²⁺ and Sr²⁺ show only minor variation. Chloride concentrations indicate substantial biological mixing of seawater from the bay into the sediments in one of the cores. Pore water analyses of sulphate and alkalinity show only a low degree of sulphate depletion and a decreasing extent of sulphate reduction downcore. Models of sulphate reduction in the mudbank show that there is substantial chemical exchange between the sediment pore fluids and water from the bay probably as a result of bio-irrigation. The sulphate and alkalinity data also suggest that the underlying Pleistocene rocks contain water of near normal seawater composition. Stratigraphic analysis and δ¹³C analyses of the organic carbon in the sediments of the island cores show that the sediments were primarily deposited in a subtidal mudbank setting; only the upper 20–30 cm is supratidal in origin. Nevertheless, island formation had a significant effect on pore fluid chemistry and the types of diagenetic reactions throughout the sediment column. Chloride in the sediment pore fluids is more than twice the normal seawater concentrations over most of the depth of the cores. The constant, elevated chloride concentrations indicate that hypersaline fluids which formed in ponds on the island are advected downward through the sediments. Models of the chloride profiles yield an estimate of 2·5 cm yr^?1 as a minimum advective velocity. Changes in pore water chemistry with depth are interpreted as indicating the following sequence of reactions: (1) minor high-Mg calcite dissolution and low-Mg calcite precipitation, from 0 to 35 cm; (2) Ca- or Mg-sulphate dissolution and low-Mg calcite precipitation, from 5 to 35 cm; (3) dolomite or magnesite precipitation together with sulphate reduction, from 35 to 55 cm; and (4) little reaction below 55 cm. In addition, one or more as yet unidentified reactions must be taking place from 5 to 55 cm depth as an imbalance in possible sources and sinks of alkalinity is observed. The imbalance could be explained if chloride is not completely conservative. Despite the pore fluid chemical evidence for diagenetic reactions involving carbonate minerals, no changes in sediment mineralogy were detected in X-ray diffraction analyses, probably because of the comparatively young age of the island.     

Fission track dating,thermal histories and tectonics of igneous intrusions in East Greenland

Fission track ages have been measured for 12 sphenes, 18 zircons and 25 apatites separated largely from Lower Tertiary magmatic rocks of East Greenland, with a few examples from Caledonian rocks. The sphene and zircon ages of Caledonian rocks agree with other radiometric ages but apatite is strongly discordant indicating that these rocks cooled very slowly over a 200 m.y. period. It was not until the Permian/Lower Jurassic that they finally cooled below 100 ° C, possibly as a consequence of uplift and erosion at this time in connection with extensive rifting. No evidence of a Tertiary imprint has been found in these rocks.Layered gabbros, such as Skaergaard, were emplaced at about the same time (ca. 54 m.y.) as the latest plateau basalts. Some evidence of syenitic activity from this period occurs in the Angmagssalik area ca. 400 km to the south but the syenites of Kangerdlugssuaq cluster around 50 m.y. The Gardiner ultramafic alkaline complex and some of the offshore gabbros apparently also were emplaced at about 50 m.y. Late dykes in the Kangerdlugssuaq area were emplaced over a considerable time span (43-34 m.y.) in keeping with their variable petrographic character, and the Kialineq centre was formed at 36.2±0.4 m.y.Intrusions of the Masters Vig area differ in age. Kap Simpson and Kap Parry to the northeast were emplaced around 40 m.y. whereas the Werner Bjerge complex is the youngest igneous activity so far identified in Greenland with an age of 30.3±1.3 m.y.Many apatites give strongly discordant ages of about 36 m.y. and these are concentrated in the area of a major domal uplift centred on Kangerdlugssuaq. The uplift is older than these ages but on field evidence post-dates the basalts. It probably formed in conjunction with alkaline magmatism at ca. 50 m.y. Cooling below ca. 200 ° was slow for these intrusions and was probably controlled by a number of factors including erosion of the dome, high heat flow caused by continuing dyke injection and regional plateau uplift. The last is believed to have taken place about 35 m.y. ago at the time of emplacement of the Kialineq plutons and last dykes. Renewed rapid erosion and declining heat flow at this time led to rapid cooling of the rocks now at the surface to below 100 °.     

Amphibole zonation in metabasites as a guide to the evolution of metamorphic conditions

In low grade metabasites the amphibole components tremolite, glaucophane, edenite and tschermakite have their activities controlled by interactions with the excess components albite, clinozoisite, chlorite, quartz and H₂O vapor. Three types of reaction are involved, (i) Those in which only components of condensed phases take part: isopleths of equilibrium constant are straight lines in the P-T plane. (ii) Dehydration reactions in which entropy change due to change in Al coordination is of the same sign as that due to dehydration: isopleths of constant K are positive at low pressure and negative at high pressure. (iii) Dehydration reactions in which entropy change due to Al coordination change is opposite in sign to that of dehydration: isopleths of constant K loop in the P-T plane with positive slopes at low and at high pressure. Zonation in naturally occurring amphiboles records the evolution of metamorphic conditions in particular rocks. In an example from the eastern Alps (Austria) early conditions calculated as 15 kb, 200 ° C evolve upgrade to 6 kb, 525 ° C implying concurrent heating and erosion. The record of evolving conditions may span some 30 Ma of geological history.     

Etiologies, observations and reporting of estuarine finfish lesions.

Lesions in estuarine finfish are associated with a variety of organisms including parasites and bacterial, viral, and fungal infectious agents. In addition, trauma, suboptimal water quality, and other abiotic stress factors may result in the loss of homeostasis. We have observed solitary ulcerative lesions on menhaden sampled from the Chesapeake Bay, Maryland, the Pimlico River, North Carolina, and the St. Johns River, Florida. Histologically, the lesions demonstrated a marked chronic inflammatory infiltrate and granulomas in response to fungal hyphae throughout large areas of exposed necrotic muscle. Gram-negative rod-shaped bacteria were also observed in the lesions, a common finding in ulcers of aquatic organisms. Similar observations in menhaden and other species have been described previously in the literature as ulcerative mycosis, mycotic granulomatosis, red spot disease, and epizootic ulcerative syndrome. Despite the many different known causes of fish lesions, the popular press and the scientific literature have recently emphasized Pfiesteria piscicida and other Pfiesteria-like dinoflagellates (and their bioactive compounds) as the primary causative agent for finfish lesions, particularly mycotic granulomatous ulcers in Atlantic menhaden. While some laboratory data suggest that Pfiesteria may play a role in field-observed lesions, much more cause-and-effect evidence is needed to determine the importance of other risk factors, both alone or and in combination with Pfiesteria. In order to better understand the etiology of lesion initiation and progression in estuarine finfish, accurate assessments of environmental conditions collected on appropriate temporal and spatial scales, and fish morphological indicators consistent with gross and histological pathologic terminology, should be used for reporting fish lesion observations and kills. Further, this outlook will help to avoid bias and may foster a broader perspective for examining the health of estuarine systems in general.     

Determination of post-glacial land uplift from the three precise levellings in Finland

We have calculated vertical velocities and their change in time from three precise levellings in Finland, with central epochs 1902, 1946, and 1986. Models with and without simultaneous determination of heights give the same results for velocities. Evidence for velocity change in time appears at first sight strong but on closer examination turns out to be inconclusive. We offer an alternative explanation in terms of levelling errors and estimate them. A new uplift map closely reproduces main features of earlier maps from partial data, but does not support some details in them. Received: 27 January 1997/ Accepted: 20 April 1998     

Spatial analysis of anthropogenic impact on karst geomorphology (Slovenia)

The objective of this contribution is analyzing, visualizing, interpreting and discussing the impacts of anthropogenic influences on karst geomorphology through various geomorphologically based environmental indicators. The primary data sources are digital terrain models (DTMs) as continuous surface data, supported by aerial photographs, satellite images, topographic maps and databases of natural and anthropogenic features. The sources are supplemented by written information about surface changes and in situ inspections. Spatial analyses as quantitative methods in combination with enhanced visualizations as qualitative methods performed in a geographical information system (GIS) on different data sets are introduced as an important methodology. This enables explaining many anthropogenic influences on the terrain surface (landform), which were not perceived before by classical surveying techniques and verifications.     

Statistical Analysis of Anisotropic Damage of the Bukit Timah Granite

Summary This paper presents a method to analyze probabilistic anisotropic initial damage of a rock mass. A three-dimensional geometric model of cracks in a rock mass is established in terms of several parameters, such as orientations, spacing and normalized sizes of cracks in the rock mass. The dip, dip direction, spacing, and normalized size of cracks of the Bukit Timah granite in Singapore were obtained from visual inspection of rock cores, color TV imaging, and impression packer tests in the boreholes at a site of geological investigation. Using the measured data, probabilistic distribution laws of geometric parameters of cracks on the surface of the boreholes are derived. Based on these statistical distributions and using Monte-Carlo simulation method, the distribution of anisotropic initial damage of the Bukit Timah granite is derived and found to have a beta distribution. This anisotropic initial damage distribution of a rock mass can be used in probabilistic analysis of rock mass responses to both static and dynamic loads.     

The dependence of the climate sensitivity on convective parametrization: statistical evaluation

Two sensitivity experiments, in which CO₂ is instantaneously doubled, have been performed with a general circulation model to determine the influence of the convective parametrization on simulated climate change. We have examined the spatial structure of changes in the annual mean and annual cycle for surface temperature and precipitation for both experiments; similarly we have examined changes in the variance for these two fields. We have also computed a range of test statistics in order to obtain reliable measures of the signal-to-noise ratio in the climate change signal from each experiment. We have computed test statistics for the entire globe and for five different region and we contrast the global response with the response in the Australian region taken as a representative sample.We find that the highest signal-to-noise ratios in the change from 1 * CO₂ to 2 * CO₂ are for the change in surface temperature for both experiments with little difference in the global averages between the experiments. Globally averaged precipitation shows a greater noise level but perhaps the greatest contrast between experiments. There are generally significant increases in the temporal and spatial variability of precipitation in the change from the 1 * CO₂ to 2 * CO₂ and with some differences apparent between the two experiments. The temporal variability of surface temperature does not change significantly in any of the 2 * CO₂ cases, and there is little difference between the experiments. There is a significant decrease in the spatial variability of surface temperature in all 2 * CO₂ experiments in all cases and with significant differences in the seasonal variations between different experiments. The spatial variability of precipitation increases in all 2 * CO₂ cases and also with substantial differences in the seasonal variations between the experiments. There are accompanying significantly different spatial pattern correlations for both surface temperature and precipitation. In general we find that the global changes are fairly robust with the differences associated with convective parametrization schemes being very small. However, at the regional level, there are marked differences between experiments with changes both in the means and in the spatial and temporal variances but often with low levels of significance.