Reexamining the Relationship of Beryllium-7 Adsorption to Grain Size in the York River, Virginia

Transcription

1 Reexamining the Relationship of Beryllium-7 Adsorption to Grain Size in the York River, Virginia A thesis submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in Geology from the College of William and Mary by Rebecca Roper May 2003

2 Abstract Fractional grain size and net amount of 7 Be present was determined for 19 samples collected from the York River, Virginia, through the spring and summer. The data did not support the idea that there is a relationship between grain size and 7 Be in the bed sediments. This noticeable lack of a trend could be a result of other factors controlling the adsorption of 7 Be, such as salinity, ph and dynamics of the estuarine turbidity maximum. Additional studies that can further separate these controlling factors would be required to determine the extent to which grain size plays a role in the geochemistry of 7 Be in an marine environment.

3 Introduction Beryllium-7, a short-lived cosmogenic radionuclide (t 1/2 = 53.3 d) has been used extensively in recent years to try to understand rates of sedimentation and dynamics of particle transport in marine environments (Canuel et al., 1990; Dibb and Rice, 1988; Dibb, 1989, Olsen et al., 1986; Todd et al., 1989). It is produced in the atmosphere where it associates with aerosols and is therefore able to be harvested by rain to be deposited on the earth s surface. Such harvesting is most effective at delivering 7 Be to the surface during the spring and summer, when atmospheric conditions allow for mixing of the stratosphere where 7 Be is produced, and the troposphere where rain is able to remove the isotope (Canuel et al., 1989; Olsen et al., 1986). Once introduced into an estuary, 7 Be will readily associate with suspended inorganic particulate matter because of relatively shallow depths and high amounts of suspended particles. These conditions result in an abundance of 7 Be found in the particulate phase versus the dissolved phase (Dibb and Rice, 1988). Olsen et al. (1986) determined that there was no correlation between 7 Be sorption and particle composition, but did not suggest particle size as a controlling factor. Another study lumped silt and clay size particles together when determining 7 Be activity (Dibb and Rice, 1988). Given the popularity and usefulness of 7 Be as a tracer of sediment activity in marine environments, this investigation sought to 1) identify if variations in fractional grain size is a controlling factor on the sorption of 7 Be, 2) identify the extent to which silt to clay ratios control such sorption, and 3) to determine if silt and clay can be treated as a single entity during further investigations, or if such an assumption is inaccurate. Methods Cruises took place from May through October 2002 in the York River. Samples were collected using a Smith-MacIntyre bottom grab at intervals along the channel, leaving the sediment-water interface intact (Figure 1). Cores (inner diameter of 9 cm) taken from the grab were sectioned at 1 cm intervals up to 5 cm on board ship. The samples were transported back to the lab for analysis in a cooler.

4 Sediment samples were first processed for 7 Be analysis. If needed, cores from the same station and collecting date were combined to ensure sufficient mass for gamma counting. Homogenized samples were filled into a circular container approximately 6 cm in diameter and 2.2 cm in height. If needed, excess water was first centrifuged off. These were placed in an intrinsic planar germanium detector, which detected the gamma radiation produced from the decay of the 7 Be in the sample, for at least 25 hours (90000 sec). Disintegrations per minute (DPM) were calculated for each sample from the counts per minute, than corrected for any decay that occurred in the interval between collection and counting. Figure 1: Location of sampling stations in the York River. Inset shows study area. The procedure for fractional grain size analysis followed Stoke s laws of settling in principle g of sample was homogenized and deflocculated using 10 ml sodium metaphosphate solution. To ensure sufficient separation of clay particles, samples were allowed to sit over night or were placed on a magnetic stirrer for at least an hour. Sand and gravel were removed from the samples using a wet #230 sieve, and the silt and clay fraction were sieved into a 1000 ml graduated cylinder. 20 ml was pipetted off at specific

5 depths at very specific time intervals and dried to obtain the mass of particles still in suspension. 10 fractions of silt and clay were determined. Results 22 of the samples collected were analyzed for fractional grain size. All of the samples chosen had a very low fraction of sand, and any sand that was present was removed from consideration through sieving. Previous studies had validated that sand showed little tendency for 7 Be adsorption (Dibb and Rice, 1989). The overwhelming majority of samples were primarily clay. There was a handful that had higher percentages of silt, however they showed no noticeable trend. The highest average grain sizes were all within the 8-10 phi fraction, slightly more than 25%, and 4-5 phi the least represented (Table 1). Fractional grain size data showed no noticeable trends; there seemed to be no relationship with either the date or position of collection. Only 19 of the samples analyzed for grain size were also analyzed for 7 Be. Samples and were not run because of contamination due to overpenetration with the Smith-Macintyre grab during collection. Sample was also not analyzed in time for this study. Disintegrations per minute were calculated from the counts per minute given during gamma counting. A statistical error was given for each sample based on efficiency of the detector and counts per minute. This was used to give minimum DPM and maximum DPM, which were averaged to give the net DPM.

6 Percentage of Grain Size (phi) Sample (date km past mouth) Average Table 1: Fractional grain size data for 22 samples. Sand was removed from consideration for all samples shown.

7 Sample (date DPM (net)* DPM (-)* DPM (+)* km past mouth) Table 2: Calculated disintegrations per minute for 19 samples. *DPM calculated with statistical error as calculated by analyzing computer; each sample was given an error and calculated at minimum (-) and maximum (+) possible values. + samples showing a significant amount of 7 Be There was a large range of DPM values in the 19 samples. The most striking outlier is , which had 2 orders of magnitude more 7 Be detected than any other sample. This is a particularly interesting distinction when its grain size distribution is considered; there were no 9-10 phi grain sizes present. The majority of the samples had no significant 7 Be content; only 5 had detectable amounts considered significant (statistically, DPM less than 2 is considered insignificant). Given the relatively small amount of samples that yielded detectable amounts of 7 Be, it is difficult to distinguish significant trends in the data. As shown in Figure 2, the

8 samples containing 7 Be showed a distinctive increasing of percentages of smaller grain sizes. Although there are small variations in percentages, the ration of clay to silt is unmistakably large. Such evidence would seem to support a conclusion that a large clay to silt ratio increases 7 Be adsorption. However, samples containing negligible amounts of 7 Be show the same trend, a marked increase in the percentage of clay versus silt (Figure 3). There is a slightly higher percentage of the clay ratio represented in the samples containing significant amounts of 7 Be, however the difference is miniscule, and would most likely best be explained by other factors. Figure 4 is another graphical representation of the data. All samples contained a higher fraction of clay, but showed no trend that would support the conclusion that the amount of 7 Be detected in the samples is correlated with grain size. Figure 2: Showing the trend of higher percentages of the clay fraction in samples containing significant concentrations of 7 Be.

9 Figure 3: Showing the similar trend of higher percentages of the clay fraction in samples containing negligible amounts of 7 Be. Figure 4: Percentage of the smallest and largest grain sizes analyzed for and their relationship with net DPM/g of 7 Be. 15 samples are plotted. If there were a significant correlation between fraction grain size and the adsorption of 7 Be, such horizontal symmetry would not be expected.

10 Discussion The lack of a noticeable correlation between adsorption of 7 Be and fractional grain size in this study supports the conclusion that the assumption that mud need not be fractionalized in future investigations involving 7 Be. However, it does suggest that factors controlling adsorption are varied and not easily separated. The potential for the introduction of error in this study was significant. Fractional grain size was determined manually, a method yielding less perfect results than an automated system. Such error might include inexact measurement of pipette depth in the water column, inexact pipette withdrawals, and even contamination of samples through sloshing and dripping of neighboring samples. Any such error alone might be negligible, however the net effect could be significant. Other lab procedures could have introduced error as well, including the possibility of poorly homogenized samples and a significant passage of time between collection and analysis. However, the lack of a trend is likely the result of other factors. All samples were collected in an estuarine turbidity maximum (ETM), an area characterized by a very high concentration of suspended sediments. Because 7 Be is found in bed sediments in areas of deposition, the unusually high amount of resuspension taking place in that area of the York may have affected the chemistry of 7 Be adsorption. Even though the majority of 7 Be present is probably in the particulate phase (Dibb and Rice, 1988), it is possible that suspended particles encompassed the majority of that particulate phase. Salinity also seems to play a huge role in the geochemistry of 7 Be (Dibb and Rice, 1988; Dibb and Rice, 1989). It is also a major factor in the position and dynamics of an ETM (Geyer et al, 2000; Geyer, 1993). The presence of the ETM may suggest a significant role of salinity in the dynamics of the area, which may then have had more of an effect on the presence of 7 Be than grain size. Further, the ETM in the York River estuary is found above muddy deposits (Friedrichs et. al, in press), and the specificity of grain sizes involved in the ETM may have been the controlling factor in the grain size distribution, which may be operating on a time scale which makes it possible that 7 Be adsorption was held at a minimum.

11 Other controlling factors on the sorption of 7 Be (after Dibb and Rice, 1989) are ph and atmospheric influx, neither of which were separated during this study, but could prove more of a controlling factor than grain size. Conclusions Given the complex geochemistry of 7 Be in dynamic estuaries, a more sophisticated study where controlling factors can be taken into consideration is needed to better elucidate an understanding of the role that grain size has on the adsorption of 7 Be. The data reported here shows that grain size does not have any noticeable effect on the presence of 7 Be in the York River, however, other controlling factors (salinity, ph, etc) may have overwhelmed the data, not allowing any trends that may be there to show. Separating such controlling variables may allow for further investigations to better understand any role that grain size may play, and a better understanding of the dynamics of 7 Be in an estuary will prove invaluable to better understanding sedimentation in such environments. Acknowledgements: The author thanks Heidi Romine for her invaluable help in developing procedures and understanding the complexity of 7 Be. Also Carl Friedrichs, who provided funding and support and the geology department at William and Mary for support and suggestions.

12 References Canuel, E.A., Martens, C.S. and Benninger, L.K, 1990, Seasonal variations in 7 Be activity in the sediments of Cape Lookout Bight, North Carolina, Geochimica et Cosmochimica Acta, 54, Dibb, J.E., 1989, Atmospheric deposition of beryllium-7 in the sediments of Chesapeake Bay. Journal of Geophysical Research, Dibb, J.E and D.L. Rice, 1988, The geochemistry of beryllium-7 in Chesapeake Bay, Estuarine, Coastal and Shelf Science, 28, Dibb, J.E. and D.L. Rice, 1989, Temporal and spatial distribution of beryllium-7 in the sediments of Chesapeake Bay. Estuarine, Coastal and Shelf Science, 28, Friedrichs, C.T., Lin, J., Scully, M.E., Battisto, G.M., Schaffner, and Kuo, A.Y., Formation of the turbidity maximum in the York River. In press, School of Marine Science, Virginia Institute of Marine Science Geyer, W.R., 1993, The importance of suppression of turbulence by stratification on the estuarine turbidity maximum, Estuaries, 16, Geyer, W.R., Throwbridge, J.H., and Bowen, M.M, 2000, The dynamics of a partially mixed estuary, Journal of physical oceanography, 30, Larsen, I.L. and N.H. Cutshall, Direct determination of 7 Be in sediments. 1981, Earth Planeary Science leters., 54,

13 Olsen, C.R., I.L. Larsen, P.D. Lowry, and N.H. Cutshall, 1986, Geochemistry and deposition of 7 Be in river-estuarine and coastal waters. J. Geophys. Res., 91, Todd, J.F., George T.F. Wong, C.R. Olsen, and I.L. Larsen, 1989, Atmospheric depositional characteristics of Beryllium-7 and Lead-210 along the southeastern Virginia coast, Journal of Geophysical Researc,