Charles B. Officer

Chesapeake Bay Anoxia: Origin, Development, and Significance

Anoxia occurs annually in deeper waters of the central portion of the Chesapeake Bay and presently extends from Baltimore to the mouth of the Potomac estuary. This condition, which encompasses some 5 billion cubic meters of water and lasts from May to September, is the result of increased stratification of the water column in early spring, with consequent curtailment of reoxygenation of the bottom waters across the halocline, and benthic decay of organic detritus accumulated from plankton blooms of the previous summer and fall. The Chesapeake Bay anoxia appears to have had significant ecological effects on many marine species, including several of economic importance.

Terminal cretaceous environmental events.

The geologic record of terminal Cretaceous environmental events indicates that iridium and other associated elements were not deposited instantaneously but during a time interval spanning some 10,000 to 100,000 years. The available geologic evidence favors a mantle rather than meteoritic origin for these elements. These results are in accord with the scenario of a series of intense eruptive volcanic events occurring during a relatively short geologic time interval and not with the scenario of a single large asteroid impact event.

Discussion of the behaviour of nonconservative dissolved constituents in estuaries

Abstract A simplified relation for the loss of a dissolved constituent in an estuary is derived. The relation is applicable to steady state, one dimensional, tidal averaged conditions. Caution should be exercised in its use as well as in the measurement procedures and the interpretation of measurements of dissolved constituents, for such simplifications may not be appropriate requiring more complex estuarine hydrodynamic effects to be considered.

The Cretaceous-Tertiary Transition

The fossil sequences from cores across the Cretaceous-Tertiary boundary show a, range of transition times and transition time intervals depending on the fossil indicators and the location of the site. These variations, together with the pattern of iridium distribution with depth at some sites, differences in total amounts of iridium, variations in noble metal abundances normalized to extraterrestrial concentrations, the depositional effects that might be expected in a reducing environment, and the clay mineralogy of the boundary layer clays, put into question the interpretation that an extraterrestrial event was the cause of the faunal changes and the iridium anomaly in the vicinity of the Cretaceous-Tertiary transition. It seems more likely that an explanation for the changes during the transition will come from continued examination of the great variety of terrestrial events that took place at that time, including extensive volcanism, major regression of the sea from the land, geochemical changes, and paleoclimatic and paleoceanographic changes.

Box Models Revisited

A methodology in terms of box models has been reexamined for the investigation of conservative and nonconservative quantities in estuaries. Both one and two dimensional models and both tidal exchange and circulation effects are included. Various types of loss relations, sources and sinks, and vertical exchanges are considered. The box model results are tested against analytic solutions of the same problems where available and against two more refined, hydro-dynamic numerical model results for a nonconservative loss problem and for a suspended sediment distribution.

Dynamics of mixing in estuaries

The one-dimensional, advective-dispersive equation for a non-conservative substance is reformulated such that salinity replaces longitudinal distance as one of the independent variables. This general formulation is used to discuss the concept of concentrate-salinity mixing curves. The importance of temporal variations and loss terms in distorting these curves is illustrated by solving the transformed equation for simple cases. Procedures for measurement, interpretation and prediction of mixing curves are discussed.

Rates of late Cenozoic tectonism in the Vallecito–Fish Creek basin, western Imperial Valley, California

The kinetics of continental rifting are recorded in the late Neogene sediments of the western Imperial Valley, California. The Gulf of California opened in its present form about 4 m.y. ago, capturing the Colorado River. Some 5 km of deltaic and fluvial sediments accumulated in the Vallecito–Fish Creek area during the interval 4.3−0.9 m.y. ago. Initial sedimentation rates were 5.5 mm yr −1 , which diminished in an exponential fashion to 0.5 mm yr −1 . When isostatic adjustments are accounted for, this implies tectonic subsidence rates of 1.5 mm yr −1 , decreasing systematically to 0.1 mm yr −1 . After 0.9 m.y. ago, right-lateral shearing along the Elsinore fault zone cut across the basin, causing it to be tilted and uplifted at a mean rate of 5.9 mm yr −1 . The basin was concurrently rotated as a unit 35° clockwise. The dissected pediments and badlands that now characterize the area were formed and eroded in the past 0.9 m.y.

Deposition of channel deposits near the Cretaceous-Tertiary boundary in northeastern Mexico: Catastrophic or normal sedimentary deposits?

Investigation of Cretaceous-Tertiary (K-T) boundary channel deposits in three sections in northeastern Mexico reveal three lithologically and mineralogically distinct units separated by three or more disconformities. The units include a spherule-rich layer (unit 1), laminated sandstone (unit 2), and rippled sand and shale layers (unit 3). On the basis of paleontological, mineralogical, and geochemical evidence, the K-T boundary is just above the top of unit 3. Although the deposits have previously been interpreted as a tsunami deposit related to a presumed K-T impact at Chicxulub on Yucatan, we have found no evidence of a nearby impact. We suggest that deposition occurred in an incised valley setting and over an extended period of time.

Dynamic deformation of volcanic ejecta from the Toba caldera: Possible relevance to Cretaceous/Tertiary boundary phenomena

Plagioclase and biotite phenocrysts in ignimbrites erupted from the Toba caldera, Sumatra, show microstructures and textures indicative of shock stress levels higher than 10 GPa. Strong dynamic deformation has resulted in intense kinking in biotite and, with increasing shock intensity, the development of plagioclase of planar features, shock mosaicism, incipient recrystallization, and possible partial melting. Microstructures in quartz indicative of strong shock deformation are rare, however, and many shock lamellae, if formed, may have healed during post-shock residence in the hot ignimbrite; they might be preserved in ash falls. Peak shock stresses from explosive silicic volcanism and other endogenous processes may be high and if so would obviate the need for extraterrestrial impacts to produce all dynamically deformed structures, possibly including shock features observed near the Cretaceous/Tertiary boundary. 38 references, 3 figures.

Mixing, sedimentation rates and age dating for sediment cores

Abstract Analytic solutions to the advection-diffusion equation useful as a guide for the interpretation of mixing, sedimentation rate and age dating effects in sediment cores have been derived. Various simplified mixing models appropriate to 14 C and 210 Pb measurements have been considered. The emphasis has been on a discussion of the various interrelations and dependencies among the mixing coefficient, sedimentation rate and radioactive decay constant parameters in determining the 14 C and 210 Pb concentrations as a function of depth. Porosity variations within the sediment column are also considered and the transformed advection-diffusion equation including porosity effects is derived.