Chapter 2. Sediment and chemical flux history in the Pocomoke Sound as defined by short lived isotopic analyses

Introduction

Pocomoke Sound, located in the southeastern part of Chesapeake Bay on the eastern shore of the central Delmarva Peninsula, is an ecologically threatened region in terms of water clarity, turbidity, dissolved oxygen, and the health of its submerged aquatic vegetation and the organisms that use this important habitat. Excess nutrient and sediment loads have resulted in excessive algal growth, oxygen depletion, and increased turbidity within the water column. For example, a six-year record (1986-1992) shows that dissolved oxygen in this mesohaline water body is depleted, especially during the summer months (www.baylink.org/atlas/). Increased turbidity is also a serious problem because it affects light reaching submerged aquatic vegetation living along the margins of Pocomoke Sound.

In order to determine processes that influence the sedimentary dynamics of the region, a study on short-lived isotopes and trace elements was conducted during the years 2001 and 2002. The short-lived isotopes, ⁷Be, ¹³⁷Cs, and ²¹⁰Pb are unique tracers of sediment in space and time and these isotopes were measured in surface samples and cores collected in the spring and fall of 2001. It was determined that from 1890 to about 1940, a significant portion of the sediment originated within the watershed. Between 1940 and 1950 there was a shift is sediment sources, with bank erosion becoming a prominent source. During this transition, the higher concentrations of redox sensitive elements (U, Mo, and Re) suggest that Pocomoke Sound was hypoxic. These changes can be directly linked to anthropogenic changes in the watershed. The results of the study demonstrated that the short lived isotopes are valuable in understanding changes in sediment regimes and with addition data may be useful in modeling the sedimentary dynamics of the area.

Short-lived radioisotopes, ⁷Be, ¹³⁷Cs, and ²¹⁰Pb, are unique spatial and temporal tracers of sediment processes. ²¹⁰Pb has been used extensively to detail the sedimentary record in Chesapeake Bay for over the last half century, but rarely in combination with ¹³⁷Cs or ⁷Be. Each isotope is introduced into the sediment via atmospheric fallout, thus the isotopes are deposited at nearly uniform activities across the region. Natural physical processes of erosion, transportation, and deposition deplete the sediment isotopic concentrations in some areas and concentrate them in others. Knowledge of the initial atmospheric flux of radioisotopes and their distribution in the depositional basin makes it possible to determine the dynamics of erosion, transport and deposition within the watershed. In this study, sedimentary processes in Pocomoke Sound and its surrounding watershed were investigated, (1) by defining the depositional centers with ⁷Be, (2) by using high-resolution profiles of ²¹⁰Pb and ¹³⁷Cs to define sediment accumulation rates, and (3) by defining environmental changes by chemical indices.

Study area -- Pocomoke Sound was formed by the erosion of the ancient Pocomoke River during lower sea level during Pleistocene glacial advances (Figure 2.1). Today, this river is the primary drainage for the south central part of the Delmarva Peninsula. The ~ 700 square mile watershed is 40 % agricultural lands, 38% forests, 18% wetlands, 1.25% urbanized, and less than >1.5% coastal marsh lands. These proportions have changed little since the middle of the 20^th century.

The terrestrial portion of the Delmarva Peninsula is topographically low. As land was cleared for habitation and farming, an extensive drainage system was constructed. This ditching, initiated in colonial times, has continued up to the present. Prior to the 1940's, the practice was limited. During and immediately after World War II, land reclamation increased, peaking in the early 1970's (Iivari, 1991). This was a time of massive row crop farming with a significant use of fertilizer (Zimmerman and Canuel, 2000). In early 1980's, many weirs were constructed to trap sediment (Iivari, 1991). Row-crop farming decreased at the same time, while poultry production increased (Figure 2.2).

The subaqueous part of Pocomoke Sound can be conveniently divided into three geomorphic zones: (1) the lower Pocomoke River and associated tidal flats, (2) the Pocomoke River sub aqueous delta in the upper part of the sound, and (3) the non-filled incised relict river (Figure 2.1). The sinuous tidal river flows approximately 65 kilometers from Pocomoke City to the mouth of the Sound. This portion of the river is monotonously deep (~ 8 meters) and ~1 km wide, fringed with large tidal flats. The bottom of the river contains lag deposits. The tidal flats are comprised of very fine, highly water-saturated clays and marsh vegetation. Sediment deposition on the flats is the result of tidal lag deposition (van Straaten and Kuenen, 1958). In 1913, the Army Corps of Engineers began the dredging process to create a new passage through the mudflats at the mouth of the Pocomoke River, creating a straighter, more maneuverable navigation channel (Iivari, 1991).

In the upper part of the sound, between the Maryland boundary and Virginia's Freeschool marsh peninsula and Gulford Flats, is a large ~20 square kilometer delta. The water depth on the delta varies between 1 and 2 meters. The Freeschool marsh and Gulford Flats are founded on very coarse sands and gravels suggesting that they formed on a buried barrier. Southwestward from the delta between the ridge and the southern Maryland boundary is the thalwag of the incised submerged Pleistocene Pocomoke River. Bottom samples indicate that the sediment in the central portion of the sound is texturally mud (silty-clay) which grades into the fine sand near the shorelines.

Materials and Methods

This section describes material and methods used in analysis of surface sediment ⁷Be inventories (data given in Appendix 2.1), radionuclide measurements of ²¹⁰Pb, ⁷Be, ¹³⁷Cs, ⁴⁰K and ²²⁶Ra concentrations (data given in Appendix 2.2), and trace element distributions in sediment cores (data given in Appendices 2.3, 2.4). The radionuclide inventory of the sediment cores from Pocomoke Sound is summarized in Table 2.1. The detection limits of the "ultratrace" procedure used to measure trace metals in sediments from four cores are given in Table 2.2. Both tables are given at the end of this report.

Sampling Pocomoke Sound -- In April 2001, 18 surface samples were collected along the Pocomoke River at six locations from Pocomoke City down to the river mouth. At each location the uppermost 4 cm of sediment was collected from the thalwag and from each sidebank. In May 2001, 27 surface sediment samples were collected along 3 cross-axis transects in the upper Sound and at 3 deeper water sites in the lower Sound (Figure 2.1). These sites were reoccupied in September 2001. ⁷Be inventories were measured at each site to determine the spatial depositional pattern and identify areas of sediment storage.

For the retrospective analyses, four large volume push cores and one piston core were taken on along the axis of the sound. Three push cores were taken in the upper Sound on the subaqueous river delta. The fourth push core was collected on a tidal flat adjacent to the river to determine the history of sediment storage with the tidal flats. The piston core, taken in the deep channel in the lower Sound, integrated the sediment record from the Sound and the surrounding region.

The surface samples along the river were collected using an Ekman grab sampler. The surface samples in the Sound were collected by VanVeen and Ponar grab samplers. The Ekman and VanVeen samplers were large enough to allow subsamples to be obtained by inserting a 10.25 cm diameter plastic core liner into the sediment and carefully extracting a subcore. Each subcore was visually inspected prior to sampling to verify the integrity of the sediment/water interface. If any indication of sediment disturbance was visible, the subcore was discarded. The extracted subcores were sectioned into 1-cm intervals by inserting a piston in the bottom of the cylinder and carefully extruding the sediment in measured increments out the top of the core liner. Samples retrieved by the Ponar grab were a composite of the uppermost few centimeters of sediment.

The large volume cores were obtained by manually inserting a 10.25 cm diameter polycarbonate core liner into the sediment until refusal. The cores ranged in length from 45-70 cm. Care was taken at all times to minimize disturbance of the sediment/water interface. The cores were returned to the laboratory and x-radiographed prior to sampling. The x-rays were used to look for internal sedimentary structure and signs of post-depositional disturbance. The sediment was extruded in the same manner as the surface samples. The top 10 cm were sectioned at 1 cm intervals and at a 2 cm interval below 10 cm. Each section was weighed and the outermost sediment in contact with the core liner was removed to prevent contamination from smearing along the interior of the barrel. This method of sectioning resulted in depth intervals of known volume, allowing for direct calculation of bulk density, which is used in mass accumulation calculations.

The deeper part of the bay was sampled with an ~3-inch diameter piston corer. The total length of the core was 350 cm. In the laboratory, this core was split horizontally. Half the core was archived and the other half was sampled into 2 cm intervals. The bulk densities of these samples were calculated using the water content to calculate the porosity and assuming 2.76 g cm^-3 as the particulate density.

In the laboratory, all samples were dried in an oven at 60°C for at least 48 hours to determine water content. The dried samples were ground by hand to a fine powder using a ceramic mortar and pestle. Five grams of material was taken from each sample for loss on ignition (LOI). The material was heated in a muffle furnace for 6 hours at 450°C. The percent weight loss is an index of organic content. The material remaining after LOI was then used for ²¹⁰Pb analysis. A split (~1-2 grams wet) was taken for grain size analysis from each interval in the four push cores prior to drying. Grain size distributions were measured with a Beckman Coulter LS200 Particle Analyzer.

Radionuclide measurements -- ²¹⁰Pb activities were determined by measuring the activity of its granddaughter ²¹⁰Po, with the assumption that the two isotopes are in secular equilibrium. Because ²¹⁰Po has a relatively short half-life (138 days), secular equilibrium is quickly established with its parent isotopes. ²¹⁰Po is chemically leached from the sediment and autoplated onto a silver planchet for counting by alpha spectroscopy (Flynn, 1968). During the dissolution of the sample, a known amount of the tracer radioisotope ²⁰⁹Po is added. By comparison of the known activity of the tracer to that of the sample unknown, the ²¹⁰Pb activity in the sample is calculated. Triplicate analyses performed of a subset of samples defined the combined analytical and counting errors at 5%.

The ⁷Be, ¹³⁷Cs, ⁴⁰K and ²²⁶Ra concentrations were determined by gamma spectroscopy. The analyses were made with a Canberra low energy germanium detector coupled to a multi-channel analyzer. Detector efficiency was calibrated using a NIST-traceable standard. All activities are decay corrected to the date of sample collection. ²²⁶Ra was measured by averaging the activity its progeny ²¹⁴Pb and ²¹⁴Bi. The samples were stored in a sealed counting jar for a minimum of two weeks prior to analysis to insure radioactive equilibrium is established between ²²⁶Ra and its progeny.

Trace element analysis -- The trace element distribution in the sediment from core PC6B-3 was determined by a semi-quantitative method, whereas, the trace metals in sediments from the four cores were determined by what is referred to as the "ultratrace" procedure (Table 2.2). The latter procedure has a lower detection limit, which translates to higher degree of precision. Because the trace metals in the two data sets were measured by different methods, comparisons must be made with caution. However, where the measured of concentration is significantly higher than the lower limit of detection, such as in the case of zinc, comparisons between the two data sets may be appropriate.

To reduce the potential for operator error, the samples were randomized prior to submission. In addition, 20% of the samples were submitted in duplicate. The results indicate that for lithic elements, those elements that are part of mineral structure, the deviation ranged from 5 to 7%, whereas those elements which are adsorbed on the particles, and therefore more easily brought into solution, the deviation is less than 2%.

Results and Discussion

Seasonal Record -- ⁷Be, formed in the atmosphere by the collision of cosmic rays with oxygen and nitrogen, has a short half-life (< 54 days) and is a very particle reactive. The short half-life and geochemistry make ⁷Be useful for measuring the dynamics of active sedimentary processes. It also is useful in defining fluxes of other isotopes, such as ²¹⁰Pb and ¹³⁷Cs. The centers of active deposition are located by mapping the ⁷Be inventory (Appendix 2.1). The inventory is defined as the total amount of ⁷Be (dpm/cm²) at a sample site. The region with the highest inventory is a center of deposition. By making seasonal inventory maps, seasonal changes in depocenters are defined.

⁷Be inventory in Pocomoke Sound varied from 7.6 dpm cm^-2 at the mouth of the Pocomoke River to undetectable near the shorelines. Within the tidal portion of the river, ⁷Be varied between 0.6 to 2.2 dpm cm^-2; detectable and constant on the flats but absent, with the exception of one site, in the thalwag. The highest value (7.7 dpm cm^-2) occurs at the mouth of the river; ⁷Be is also high on both banks and on the upstream side of the river mouth deposit (Figure 2.3A). This value is similar to the 6.8 dpm g^-1 measured in suspended sediment, sampled in August 2002.

The highest activity (2.0 ± 0.2 dpm g^-1) in the sound occurs in the deeper part of the system. There are slight differences between the spring (May) and fall (September). The sites on the riverside of the bay-mouth delta showed the most significant change. In the spring, ⁷Be was detected only on the seaward side of the deposit. In the fall, ⁷Be was detected both on the riverside and seaward side of the delta (Figure 2.4). This suggests that during the spring runoff much of the sediment by passes the bay-mouth deposit. The "weaker" transporting energy in the summer and fall permits some sediment to be trapped on the delta.

⁷Be is also valuable in estimating (A_o), a parameter essential in calculating ²¹⁰Pb chronologies. The short half-life of ⁷Be makes this isotope ideal for this purpose, because the presence of ⁷Be is evidence that the sediment was being transported within the last few months. By using ⁷Be, it was determined that the surface ²¹⁰Pb activity was ~ 4.0 dpm/g.

Lead-210 -- ²¹⁰Pb is a decay product within the uranium (²³⁸U) series. Radioactive equilibrium occurs when the activity of each successive isotope in a radioactive series has the same activity. Disequilibrium occurs when the series is broken. In nature, the ²³⁸U series is broken by the diffusion of ²²²Rn from minerals exposed at the earth's surface. ²²²Rn escapes into the atmosphere at a rate of ~ 42 atoms per minute per square centimeter of land surface (Faure, 1986). With a half-life of 3.8 days, ²²²Rn decays rapidly leading to the formation of ²¹⁰Pb through a series of very short half-life isotopes. This process produces excess ²¹⁰Pb in the atmosphere, from which it is purged by wet and dry deposition. Like ⁷Be, this isotope is particle reactive and once in the surface environment it is rapidly adsorbed on sediment.

There have been many ²¹⁰Pb age models developed. Robbins (1978), Oldfield and Appleby (1984), and Carroll and others (1995), Robbins and others (2001) Nie and others, (2001) have review the various methods, applications and approaches to ²¹⁰Pb age modeling. The chronologic model used in this study, developed by Robbins and others (2001), is a simple first order model. In this model the atmospheric ²¹⁰Pb flux and accumulation rate is assumed to be constant and any variability with the exception of ²¹⁰Pb decay is averaged by sedimentological processes. Results are given in Appendix 2.2. The surface activity is assumed to be constant and equal to the flux of ²¹⁰Pb divided by the sediment accumulation rate in grams per square centimeter. This parameter was determined from the surface activity by the ⁷Be. Excess ²¹⁰Pb activity (A _ex) for a given interval is defined as the difference between the total ²¹⁰Pb and ²²⁶Ra, and is calculated as follows:

where g is accumulative weight of sediment (g cm^-2), F is flux (dpm cm² yr^-1), R_s is accumulation rate(g/cm²/yr), λ is the decay constant of ²¹⁰Pb (ln2/t ^1/2 = 0.03114). This equation takes in to account sediment compaction and implicitly assumes no post- depositional mobility, sediment mixing or changes in sediment supply.

Numerous investigators have addressed post depositional ²¹⁰Pb mobility and in most environments this is not a problem (Urban and others, 1990). The degree of mixing is assessed by examination of x-radiographs, and indirectly by the nature of the trace metals profiles. The Pocomoke sound core x-radiographs show significant layering. The layers range from 2 to ~ 10 cm. in thickness. Therefore, if mixing is present, it occurs only within these layers.

A "best fit" curve of the measured data was calculated by least-squares optimization using the Marquardt-Levenberg (M-L) methods. Using the age formula (1), the ages of each level in the cores was calculated. These ages were used to compare with other data. While the age assignments are calculated from the accumulative weight of the sediment, the results are displayed in linear depth units. This procedure yields a first order age model. Comparing the best-fit curves (the "ideal curves") to raw data shows the degree to which the data conform to the ideal.

The best-fit data for the six cores shows that each have a very good and significant "goodness of fit" with the R² values ranging from a low of 0.88 in the core data from the tidal flat to 0.98 for the deepest core on the Pocomoke delta. The r² values for the other cores varied between 0.97 and 0.96 (Figure 2.5). Core 1 has the highest r² value.

The very high r² values indicate that the ²¹⁰Pb chronologies are a good first order approximation. Using the estimated chronologies, the average accumulation rates can be calculated. Cores 2 and 3 have an average accumulation rate of 0.25 and 0.29 cm yr^-1 respectively. On the seaward side of the bay mouth deposit, the average accumulation rate (Core 1) was calculated to be 0.47 cm yr^-1. The highest rate was calculated to be 1.8 cm/yr in the thalwag of the incised river valley. On the tidal flat, the sediment was calculated to be accumulating at an average of 0.62 cm yr^-1.

Examination of the raw data against the "ideal" curve for each core is very revealing. With the exception of Core 1 whose excess ²¹⁰Pb almost perfectly reflects the ideal curve, the top portions of Cores 2, 3, 4, and 6 fall above the ideal curves. Close examination of this portion of these curve shows that the slopes of these data is much shallower than the data deeper in the core. This shallow slope is suggestive of more rapid accumulation than the calculated average rates. The calculated age at the point of cross over varied between 1953 +/- 4yrs in core 1 to 1959+/-4 yrs on the tidal flat. There is a suggestion that in the top portion of the cores, the ²¹⁰Pb has a shallower slope.

Cesium-137 -- In July 1945, nuclear weapon tests began releasing ¹³⁷Cs and other radioactive nuclides into the environment for the first time. Over the 50 years since those tests, much research has been done to understand the movement and fate of the radionuclides in the environment. Beginning in the late 1950's many atmospheric radiological monitoring stations were established around the country. The closest to Chesapeake Bay region is in Sterling, Va. The ¹³⁷Cs record was derived from the ⁹⁰Sr depositional record. Using the ⁹⁰Sr/¹³⁷Cs ratio measured by the atmospheric sampling program for the 1959-1963 period, ¹³⁷Cs was found to be 1.32 times the ⁹⁰Sr activity (www.eml.doe.gov/databases/). The ¹³⁷Cs fallout records from the central and northeastern portion of the country are all similar in terms of total inventory, thus it is assumed to be universal through the watershed and sound. Radiological fallout varied with the number and magnitude of nuclear test. The record shows the largest fluxes occurred during the early 1962-1963 and declined after the ratification of the Nuclear Test Ban Treaty (Figure 2.6).

Overlying the ¹³⁷Cs distribution on the ²¹⁰Pb time scale shows that the ¹³⁷Cs peak in cores 1, 2, 3 coincide with 1962-1963 maximum production (Figure 2.7). This confirms the ²¹⁰Pb chronology. ¹³⁷Cs distribution in core 4 is different. In this core, the ¹³⁷Cs flux increases from the late 1950's to a maximum after 1980. In core 6, ¹³⁷Cs activity ranges from less than 0.1 to a maximum of 0.86 dpm g^-1. These values translate to a range of fluxes form a low of 1 to a maximum of 5.4 dpm cm^-2 yr^-1. The maximum concentration does not match the peak production but peaks between 1966 and 1967. This shift or delay in maximum ¹³⁷Cs has been recognized by others (Robbins et al. 2001 and Nie et al. 2001). This time delay is believed to be a function of varying rates of erosion and transportation of sediment from the supplying watershed. In addition, there is a secondary concentration peak that apparently correlates with an event which occurred in 1972. This is the year of Hurricane Agnes, which was a major sedimentological event in Chesapeake Bay region (Nie et al., 2001; Helz et al., 1985).

Trace metals and sediment texture -- The trace metals concentrations in the Pocomoke Sound sediments the fall into four categories: fine sediment lithic, coarse sediment lithic, anthropogenic, and authigenic (Appendices 2.3, 2.4). The fine sediment lithic elements are those elements (Al, B, K, Cr, Ni, Ga, Rb, Tl, Th, and the rare earths) that are associated with the crystalline structure or adsorbed on the fine silts and clays. The coarse sediment lithic is those elements (Li, Mg, Ca, Ba, Mn, Fe, Co), which are associated with coarser silts and sands. The anthropogenic elements (Cu, Zn, As, Se, Cd, Sb, Bi) are derived from anthropogenic sources. As these elements are used as additives in a variety of manufactured products (such as tires, fertilizers etc.) a single source is difficult to name. The authigenic elements (U, Mo, and Re) are those that react to changing redox conditions in the system.

The authigenic redox sensitive elements (U, Mo, and sometimes V) are concentrated in sediments by changes in the physiochemical conditions of the water (Morford and Emerson, 1999). The stable form of uranium in oxygenated waters is +VI. In marine waters this species forms a complex with carbonate ions. In anoxic systems, uranium is reduced to the insoluble +IV and is adsorbed on particlate material or is deposited as a sulfide. Molybdenum (VI) is the stable form in oxic systems and exists as MoO₄^-2 Reduction of Mo (VI) to Mo (IV) and authigenic enrichment of Mo in the sediment occurs under reducing conditions via precipitation across the sediment interface (Crusius and Thompson, 1996). Vanadium on occasion has the same properties as molybdenum element, but is also an element that mirrors iron. Because iron is ubiquitous it is difficult to separate the vanadium which is associated with iron from that which is reacting to oxic conditions.

The metal profiles in all cores are similar. Zinc, one of the easiest metals to measure and compare among cores (Figure 2.8), increased in concentration greatly, beginning in the late 1940s and early 1950s. This increase reached its maximum around the mid 1980's and remained constant until the present. The distribution of the redox sensitive elements (uranium and molybdenum) exhibits a decrease through the late 1900's. The spike in the late 1940's corresponds to the rapid changes in land used in the watershed (Figure 2.9) and suggests a short-term nearly anoxic interval in the Sound beginning shortly after the increase use of fertilizer during and immediately after World War II.

The core with the longest and best-resolved record is core PC-6B (Figure 2.10). In this core, the anthropogenic elements zinc and lead have the same pattern, increasing greatly between 1950 and 1970 (Figure 2.10). Lithic element(s) (i.e. aluminum) co-varies with the organic index, loss on ignition, and are in opposition to sand percentages. The metal distribution also reflects changes in sediment intensity, with spikes occurring during Hurricane Agnes.

Size analysis of the sediment confirms that most of the sediment in the Sound is silty clay. There is an indication that the post 1950 sediment was slightly coarser. The organic content of the sediment, as measured by LOI, ranged from 4 to 8 %.

Conceptual Models -- There have been many efforts to use short-lived isotopes to model the history of sedimentary dynamics in aqueous systems. Ritchie and Mc Henry (1990) proposed a box model that related the atmospheric fallout of ¹³⁷Cs to its distribution in the depositional basin. Edgington and Robbins (2000) successfully modeled 100 years of sediment transport in Lake Michigan using ¹³⁷Cs. To develop a rigorous sediment transport model for any sedimentary system, analysis of many depositional sites is necessary. Although only five core sites were sampled in this study, they contain sufficient data on the Pocomoke Sound's sedimentary history to formulate preliminary sediment dynamics models. In an ideal equilibrium world (case 1), the distribution of the short-lived isotopes would follow physical laws, with isotopes introduced directly a basin of deposition in equilibrium with atmospheric flux. As a result, the ¹³⁷Cs record would mirror the atmospheric flux, and the concentration of ²¹⁰Pb, initially equal to the atmospheric flux, would decrease logarithmically with depth (Figure 2.11a). In this case the atmospheric to core inventory ratio for both isotopes would be one (Figure 2.12).

Ideal conditions rarely occur in a nature. Changes in climate, land use, or geophysical conditions, such as sea level rise, lead to changes in sedimentation on the time scales that can be measured by these isotopes. In a situation where much of the sediment in the basin is derived from the watershed (Case 2), the ¹³⁷Cs record in the basin would be a combination of the internal basin record and material added by erosion and transport from the watershed. The exact distribution would be a combination of natural and anthropogenic factors. In this case, the peak ¹³⁷Cs concentration would not necessarily match the fallout peak (1962-1963) but, depending on the timing and intensity of erosion and transport, would be younger. The over all ²¹⁰Pb activity, due to focusing, would initially be higher. If this system were in equilibrium, the ²¹⁰Pb profile with depth would have a logarithmic decrease (Figure 2.11b). In this case, the core inventory to atmospheric flux ratio would be greater than one for both ¹³⁷Cs and ²¹⁰Pb. Pulses of sediment would be reflected by abrupt increases in the general ¹³⁷Cs and ²¹⁰Pb curves.

In a case where most of the sediment reaching the basin was exhumed, "old" sediment not exposed to the atmosphere during the late 1950's and early 1960's, the sediment would be depleted in ¹³⁷Cs (Case 3). However, because the atmospheric supply of ²¹⁰Pb and ⁷Be is continuous and, in most large systems, a large percentage is adsorbed on the sediment during transport, the record in the sediment would approximate the ideal (Figure 2.11c). In this case, the atmospheric to core inventory for ¹³⁷Cs would be much less than one, whereas the ²¹⁰Pb ratio would be approximately one.

Multi-decadal Sedimentation -- The core transect in the sound lacks a three dimensional aspect, so that a sediment budget cannot be estimated. The data do, however, provide the means to estimate any variability in temporal fluxes. On a decadal scale, comparison of the total inventory of the short live isotopes at a site with the inventory of atmospheric fluxes gives a glimpse of temporal sediment variability. The total isotopic inventory is an easily calculated parameter. Inventory is defined as the total amount of an isotope at a site and is calculated by:

where I is inventory (dpm/cm²); A_dpm/g is activity in dpm per gram; Rho is dry bulk density (g/cm³); and Z is sample interval (cm). The nominal time that inventory is useful is a function of the isotopic half-life and/or the time of introduction. With a ~ 22 years half life, ²¹⁰Pb, has an effective nominal limit of 100 years; with a ~ 30 year half life and an introduction in 1950's, the effective time limit for ¹³⁷Cs is 50 years. Since ¹³⁷Cs has been in the environment for ~ 50 years, for comparative purposes, the ²¹⁰Pb inventory must be adjusted.

Because ²¹⁰Pb is constantly being replenished, its inventory has been constant for over a millennia, varying only with the nuances of atmospheric circulation. The ²¹⁰Pb inventory was determined to be ~ 32.dpm/cm² in New England (Cochran and others, 1998). Using the data from the Surface Air Sampling Program (SASP - www.eml.doe.gov/databases/), it was determined that the ²¹⁰Pb concentration at Sterling Virginia was 60% that of New England; thus it is estimated to be ~ 19 dpm cm^-2. Adjusting for the last fifty years, the inventory would be an estimated 16.0 dpm cm^-2. Because most of the ¹³⁷Cs was added to the environment during a short period, 1959-1963, the inventory has decayed over time. The maximum ¹³⁷Cs inventory 9.2 dpm cm^-2 occurred during 1963 in the Chesapeake Bay region. Today, through decay, the inventory of this isotope is approximately 5 dpm cm^-2.

The ²¹⁰Pb inventory in cores 1, 2, 3, and 4 are 10.0, 16.6, and 14.1 dpm cm^-2 respectively (Appendix 2.2). As these values are very close to the estimated atmospheric inventory and there is no evidence of loss or disruption in the records for this period (Figure 2.5), the sediment deposited on the Pocomoke delta for the last fifty years approximates equilibrium with the ²¹⁰Pb atmospheric flux. The inventory on the Pocomoke tidal flat of 29.3 dpm/cm², is almost twice the atmospheric inventory, where as core PC-6B has an estimated inventory of 150 dpm/cm², approximately ten times the expected ²¹⁰Pb estimated atmospheric inventory.

¹³⁷Cs in cores 1, 2, and 3 have an estimated inventory of 1.5, 2.2, and 2.2 (dpm cm^-2) respectively (Table 1). These values are about 20% of the expected inventory. However, Core 4, from the Pocomoke River tidal flat has an inventory of 11.4 (dpm cm^-2); about twice the estimated atmospheric inventory, and Core PC-6B has an inventory of 36.7 (dpm cm^-2); about seven times the estimated atmospheric inventory.

Figure 2.12 is a plot of the atmospheric flux to core inventory ratios for all the cores. The ²¹⁰Pb in cores 2 and 3 are very close to one whereas ¹³⁷Cs is much less. This suggests that the sediment that has been deposited on the delta for the last fifty years has a large "exhumed" fraction. This pattern seems to be evident in core 1, but the smaller ²¹⁰Pb ratio is suggestive a higher content of older sediment. The inventory in cores 4 and 6 exceeds the expected inventory for both isotopes. The ¹³⁷Cs concentration is suggestive of a large contribution from the watershed and the large ²¹⁰Pb concentration is suggestive of a sediment focusing. Together, this suggests that the tidal flats are storage areas for sediment eroded from the watershed.

The accumulation rates in all the cores but core PC-6B are relatively slow, and as a result the temporal resolution is poor. However, the rate of accumulation in core PC-6B is extremely high. As a result, this core illustrates many sedimentological changes that have occurred within the sound over the last fifty years. Figure 2.13 is a plot of the ¹³⁷Cs distribution. The figure includes the flux inventory at Sterling and the data from core 6. The ¹³⁷Cs data is decay corrected and plotted as percent inventory (¹³⁷Cs core/¹³⁷Cs flux *100). The plot shows a slight increase at about 1963 which is assumed to be the result of maximum atmospheric production. The highest percentage occurred in 1968, with a flux of about 60 percent the atmospheric contribution. Between 1968 and 1971, the introduction of ¹³⁷Cs enriched sediment declined rapidly to about 20 percent. The only exception was the increase in 1972, which corresponds to the sediment derived from the flooding due to Hurricane Agnes. Nie and others (2001), and Helz and others (1985) also recognized this increase in the bay. After this event there is a significant drop off in enriched ¹³⁷Cs sediment between 1973 and 1976. This drop off in sediment delivery is believed to be the result of sediment flushing of material that had been stored in temporary storage sites, in the case of the Pocomoke system on the tidal flats. This phenomenon has been recognized other places within the Bay (Nie and others, 2001) and elsewhere in other places within the country (Horowitz, 2003). Since 1976, there appears to be a steady flux of ¹³⁷Cs tagged sediment. The data suggests that this is about 20 percent the amount still in the watershed.

Conclusions

The goal of this project was use radionuclides to measure the sedimentary dynamics of Pocomoke Sound. Using the shortest-lived isotopes ⁷B, the seasonal deposition centers were defined and the A_T(0) for ²¹⁰Pb was estimated. Using the latter value and the ²¹⁰Pb activity at depth in each core, sanitizing the data by best fitting procedures to a standard curve, a chronology for each core was calculated. This information was then used to examine, in detail, the ¹³⁷Cs distribution and the trace metal data. The results of this analysis showed that there has been a significant increase in anthropogenic elements since the late 1940's (Figure 2.14). This is the period when the Delmarva Peninsula became more accessible from the Baltimore-Washington metroplex. ¹³⁷Cs, was found to be a useful tool in determining changes in sedimentation within the system. This data showed that there were three major stages of sedimentation. The pre-1950's, the time in which the system was equilibrium with the agriculture activity in the watershed. Urbanization and agricultural activity changed during and immediately following World War II resulting increasing sediment flux. Around 1970, the increased environmental awareness coupled with changes in agriculture (row crops to chickens), the sediment flux was diminished. Correspondingly, there was an apparent change in sources of sediment to the deeper parts of the system. The chemistry and sediment accumulation rates suggest that this source was bank erosion.

Acknowledgements

We would like to acknowledge Owen Bricker who helped improve the original manuscript significantly. Also we acknowledge Deb Willard, Tom Cronin, and others who assisted in the field and lab work. Also we acknowledge the dedication of Heather Kamanski who prepared the samples for α - analysis.

References

U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubsdata.usgs.gov/pubs/of/2004/1350/chapters/chapter2.htm
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