4 results
Stratigraphic Evidence of Human Disturbance in an Estuary
- Grace S. Brush, Frank W. Davis
-
- Journal:
- Quaternary Research / Volume 22 / Issue 1 / July 1984
- Published online by Cambridge University Press:
- 20 January 2017, pp. 91-108
-
- Article
- Export citation
-
Prior to European settlement, oligohaline and mesohaline sections of Chesapeake Bay draining Piedmont saprolite supported diverse and abundant diatom and macrophyte populations. Compositional changes in diatoms and macrophytes in oligohaline sections correspond with 17th- and 19th-century deforestation and increased siltation, while effects on downstream populations were less notable. After deforestation, previously sparse diatom populations in a mesohaline estuary draining sandy Coastal Plain soils became more abundant. Fertilization of cultivated land was accompanied by increased production of both attached and free-floating diatoms. After the discharge of sewage, diatom populations increased enormously in the affected areas, followed by a dramatic decrease. The decrease suggests silica limitation after intense phosphorus enrichment. The loss of macrophytes and increase in planktonic diatoms in oligohaline areas in recent years resemble the historical sequences observed in lakes undergoing eutrophication. However, in the estuary, similar declines have also occurred in macrophyte populations in mesohaline areas where eutrophication is much less severe, but where chlorine and herbicide toxicity during the past 20 yr is similar to upstream areas.
Comparisons of 210Pb and Pollen Methods for Determining Rates of Estuarine Sediment Accumulation
- Grace S. Brush, E. Ann Martin, Ruth S. DeFries, Cynthia A. Rice
-
- Journal:
- Quaternary Research / Volume 18 / Issue 2 / September 1982
- Published online by Cambridge University Press:
- 20 January 2017, pp. 196-217
-
- Article
- Export citation
-
Comparisons of sedimentation rates obtained by 210Pb and pollen analyses of 1-m cores collected throughout the Potomac Estuary show good agreement in the majority of cores that can be analyzed by both methods. Most of the discrepancy between the methods can be explained by the analytical precision of the 210Pb method and by the exactness with which time horizons can be identified and dated for the pollen method. X-radiographs of the cores and the distinctness of the pollen horizons preclude significant displacement by reworking and/or mixing of sediments. Differences between the methods are greatest where uncertainties exist in assigning a rate by one or both methods (i.e., 210Pb trends and/or “possible” horizon assignments). Both methods show the same relative rates, with greater sediment accumulation more common in the upper and middle estuary and less toward the mouth. The results indicate that geochronologic studies of estuarine sediments should be preceded by careful observation of sedimentary structures, preferably by X-radiography, to evaluate the extent of mixing of the sediments. Time horizons, whether paleontologic or isotopic, are generally blurred where mixing has occurred, precluding precise identification. Whenever possible, two methods should be used for dating sediments because a rate, albeit erroneous, can be obtained isotopically in sediments that are mixed; accurate sedimentation rates are also difficult to determine where the time boundary is a zone rather than a horizon, where the historical record does not provide a precise date for the pollen horizon, or where scouring has removed some of the sediment above a dated pollen horizon.
3 - Transport and deposition of pollen in an estuary: signature of the landscape
- Edited by Alfred Traverse, Pennsylvania State University
-
- Book:
- Sedimentation of Organic Particles
- Published online:
- 06 January 2010
- Print publication:
- 30 June 1994, pp 33-46
-
- Chapter
- Export citation
-
Summary
Introduction
Pollen preserved in lake and peat sediments has been used to reconstruct changes in vegetation induced by climatic events (Davis, Spear, & Shane, 1980; Webb, Cushing, & Wright, 1983; Davis & Jacobson, 1985) and by anthropogenic alterations to the landscape (Brugam, 1978; Burden et al., 1986). Only recently has the record from estuarine sediments been similarly exploited (Brush, 1986; McGlone, 1988). In this chapter, we describe the general pathways of pollen in an estuary, and show from laboratory experiments and field observations how the transport and deposition of pollen in the estuary results in distributions in the sediments that record regional vegetation, land use and the effect of land use on rates and patterns of estuarine sedimentation. Our area of study is the Chesapeake Bay estuary extending between latitudes 37° and 39° 30' N in the mid-Atlantic region of the USA (Fig. 3.1).
Estuarine circulation
First, we examine the fluid motion, sediment motion, and salinity characteristics of Chesapeake Bay. Chesapeake Bay and many of its tributaries are partially mixed estuaries (Dyer, 1973), characterized by vertical, lateral and longitudinal gradients in salinity, but with no abrupt change from fresh to salt water as is found, for example, in the salt wedge environments of many fjords. On the other hand, Chesapeake Bay estuary is neither completely mixed nor homogeneous, as are estuaries characterized by large tidal flows and small river inflows. An example of an idealized, partially mixed estuary is shown in Figure 3.2A. When river water enters the estuary and meets ocean water, partial mixing occurs and causes lines of equal salinity to deform.
Paleontological methods in environmental science
- Grace S. Brush, Humaira Khan
-
- Journal:
- The Paleontological Society Special Publications / Volume 6 / 1992
- Published online by Cambridge University Press:
- 26 July 2017, p. 40
- Print publication:
- 1992
-
- Article
-
- You have access Access
- Export citation
-
Long term environmental changes, induced both by natural and anthropogenic causes, cannot be assessed by looking solely at historical records of temperature, rainfall, water quality, etc. Frequently, such records are nonexistent. Where they do exist, they are often too short to be of much use. However, sediments contain a stratigraphic record of environmental change that allows comparison of the historic period influenced largely by humans, with prehistoric time when climate was the major determinant of environmental conditions.
The stratigraphic record contains various fossilized organic materials which reflect environmental conditions at the time of the their deposition. Most abundant are pollen and spores of aquatic and terrestrial plants. Correspondence between pollen assemblages and vegetation has been found in all parts of the world. Therefore stratigraphic changes in pollen taxa reveal much information regarding the vegetational history of a region. Knowledge of the ecological requirements of various taxa can then be used to infer past climatic conditions. For example, dominance of Juglans (walnut) pollen indicates wet conditions while abundance of Pteridium (bracken fern) spores is an indicator of fire, suggesting drier conditions. Pollen can also be used to trace human disturbance to the environment. Sharp increases in the pollen of Ambrosia (ragweed) in sediment cores indicate large scale land clearance by man.
Seeds preserved in sediments provide another measure of temporal vegetational change. Generally, seeds are not dispersed far from the parent plant; hence they provide a more local record of vegetation than pollen. A decrease in seeds of aquatic plants and increase in seeds of higher ground taxa when accompanied by increased sedimentation rates is an indicator of infilling of an aquatic environment. If unaccompanied by increased sedimentation, the change more likely represents lowering of sea level. Disappearance of seeds of taxa sensitive to turbidity and eutrophication provide another long term record of human disturbance.
Distributions of diatoms are affected by temperature, salinity, oxygen, light availability and nutrient levels in the water. Therefore changes in diatom species preserved in the sediments can be used as indicators of climate, turbidity, anoxia and eutrophication in aquatic ecosystems.
Many inorganic substances preserved in sediments also provide a long term record of changes in the environment. Nitrogen and phosphorus can be measured in cores and used as a surrogate record of water quality in lakes and estuaries. Sharp increases in sedimentary accumulation of metals record wastewater discharge and fuel emissions, related to human activity.