Rhenium is among the most highly enriched dissolved species in seawater relative to its crustal abundance. Consequently, it shows authigenic enrichment more clearly than most other trace elements (Crusius and others, 1996). It behaves conservatively in the water column and is not generally affected by biological processes. Dissolved Re+7 (as ReO4-) is reduced to Re+6 (ReO3) and other more-reduced oxides and sulfides and deposited in sub-oxic sediments by processes demonstrated to act at the sediment-water interface and within the sub-oxic zone of shallow pore fluids. Colodner (1991) showed that Chesapeake Bay sediments with thicker sub-oxic zones (no O2 or H2S present) are more effective at sequestration of Re than more anoxic sediments with thin or absent sub-oxic zones. This implies that periods of transition from better oxygenated to more oxygen-depleted bottom waters and vice versa would show the greatest Re enrichments in Chesapeake sediments. Also, Re should be more concentrated in sediments deposited around the edges of the oxygen-depleted axial basins in the Chesapeake than in sediments in the deep centers of the basins. Leachable Re rather than total Re was measured in these sediments because it is more representative of the authigenic Re fraction in the sediment. Concentrations reported are parts per billion (ppb) of dry sediment weight.
Measurements of 15N and 13Corg in surface sediments in Chesapeake Bay by Hunt (1966) and Spiker (unpublished data) show significant differences along the length of the bay from north to south. Values of 13Corg increase from around -24 ppm in the northern bay to -20 ppm near the mouth. This is interpreted as the product of mixing of terrestrial and marine organic carbon sources, with the northern bay reflecting mostly river input and the mouth of the bay showing deposition of oceanic organic matter brought in by tidal circulation. Changes in the salinity at any given point in the bay produced by variations in streamflow would be expected to be reflected in the 13Corg signal (e.g., wetter climate = higher streamflow = lower average 13Corg).
A north-south transect of 15N of surface sediments (Spiker, unpublished data) shows a peak of 8.5 ppm 140 km south of the Susquehanna River (about 30 km south of the MD99-2209 site). Values are 1.5 to 2.0 ppm lower to the north and south. The mid-bay 15N peak is interpreted as resulting from greater denitrification in this portion of the bay caused by depletion of oxygen by decomposition of algal biomass. Increased anthropogenic or climatically-induced eutrophication of the bay would be expected to result in higher average 15N values due to more intense and areally extensive denitrification in the water column. Wetter or drier periods would also shift the 15N maximum south or north, respectively.
Several assumptions must be made when interpreting BSi data as a measure of diatom productivity: 1) the majority of the amorphous or opaline silica in the sediments is in the form of diatom frustules, 2) the proportion of diatom silica recycled in the water column remains approximately constant, and 3) post-depositional dissolution of silica is minor. In order to interpret biogenic silica as an index of trophic status, it must also be assumed that diatoms constitute most or a representative proportion of the total algal biomass. Dried Chesapeake Bay sediments typically contain 5 to 10 percent biogenic silica by weight. The balance of the sediment consists of organic matter (0.5 to 3 percent), calcareous shells (about 5 percent except in shell beds), and mineral grains (silt, clay, and sand; typically 80 to 95 percent). Under conditions of constant detrital sediment input, increases in percent biogenic silica indicate increased diatom blooms usually associated with greater nutrient supplies (especially nitrate), more spring runoff, and lower salinity (Cornwell and others, 1996; Baucom and others, 1999).
The 15N plot in Figure 10.1 shows that values fluctuate around 5.8 ppm between 1400 and 1750 A.D. About 1775, 15N begins to increase toward maxima of 10.1 ppm in 1952 and 9.8 ppm in 1972, with a slight decrease near the top of the core. The signal appears to have some cyclicity, with an amplitude of about +0.8 ppm and a period of about 60 to 80 years. The frequency of variability appears to decrease in the last 200 years of the record to about 20-year cycles, but this may be an artifact of the increase in sedimentation rate in this same period and the higher resolution of the data.
The 13Corg values are relatively constant from the base of the core to about 1775. The average value over this interval is -23.1 ppm +0.5. After 1775, the data are more variable (+0.5-1.0 ppm) but show no increasing or decreasing trend. The period of variability is fairly irregular over the full length of the core. As discussed above, 13Corg would be expected to vary with salinity due to mixing of light carbon from freshwater sources and heavy carbon from saltwater sources.
Data for leachable Re from the RD-98 core show positive excursions from two baseline levels rather than cyclical variation like N and C isotopes. The baseline in the lower part of the core (before about 1775 A.D.) is around 1.1 ppb with excursions of up to 1.0 ppb centered at 1540, 1670, and 1720. After 1775 the baseline drops to 0.9 ppb. This is followed by several small excursions (<0.5 ppb), a large excursion centered at 1936 of up to 1.9 ppb (highest data point off the scale of figure), and a smaller excursion (0.5 ppb) at 1970. The excursions are interpreted as being the result of transitions to lower oxygen conditions in Chesapeake bottom waters. Discussion of long-term trends and magnitude from these concentration data is difficult because they are affected by increased sedimentation rates over the last few centuries. This effect is suggested by the drop in Re baseline around 1775, a time of rapid increase in sedimentation rates in the bay. However, timing of excursions and the inferred transitions in oxygen concentrations derived from the concentration data are valid.
Figure 10.2 (A) shows a comparison of leachable Re concentrations between high-resolution samples from the upper 50 cm of cores RD-98 and MD99-2209. These data show strong correlation between the two cores. The RD-98 samples were collected at regular 3-cm intervals. Samples from MD99-2209, however, were collected from the centers of 10 pairs of alternating black and gray color bands in the sediment. The data show that Re is not strongly affected by the process controlling color banding (i.e., formation of iron monosulfides such as hydrotroilite).
Figure 10.2 (B) shows combined Re data for all of core RD-98 and the portion of core MD99-2209 below 300 cm. Like the duplicated interval shown in the upper plot of Figure 10.2, the region of overlap between data sets from the two cores (3.0 to 4.5m depth, 300 to 700 yr BP) shows their data to be highly correlated, with depth offsets of no more than 25 cm. Radiocarbon data indicate that a hiatus or unconformity is present in the MD99-2209 core at a depth of approximately 820 cm (see Fig. 6.1). The missing gap represents about 3500 years between 2300 and 5800 yr BP (see Fig. 10.2B). Sample resolution between the base of core RD-98 and the hiatus in MD99-2209 (700 to 2300 yr BP) is fairly low because of relatively low sedimentation rates. Re concentrations are similar to those in the bottom part of the RD-98 core (1.1 ppb +0.5), with lows around 750 and 1500 yr BP and a peak at 1200 yr BP. Immediately below the hiatus is an anomalously high Re peak followed by an extreme low. These are interpreted as having been produced by remobilization of Re orginally present in the sediments due to oxygen penetration during the period of probable non-deposition. The part of the core deposited between 6000 and 6800 yr BP shows a plateau around 1.1 ppb with brief (<100 yr) negative excursions of about 0.3 ppb. Between 6800 and 7600 BP there is a gradual decline in Re concentrations to 0.7 ppb except for an anomalous high value around 7500 yr BP. This is consistent with a transition to more restricted estuarine conditions that existed in the bay when sea level was lower.
Results for BSi concentration show a gradual decrease from near the base of the core (10.6% BSi) to about 4.4% in 1882; after this values start to increase again. Because BSi, like Re, is a concentration parameter, it must be corrected for total detrital sediment flux (Baucom and others, 1999) before diatom productivity changes over time can be interpreted. Work is in progress to determine the history of BSi flux; initial indications are that BSi flux has increased at nearly the same pace as sedimentation rate (see Chapter 6).
Core RD-98 data for 13Corg, 15N, and Re (A, B, and C, respectively) show systematic changes over the length of the core. Isotope measurements of 13Corg and 15N are generally inversely correlated, especially prior to about 1890 (W3b, W4, W6, W12, W13, and W16). This is consistent with a scenario where higher streamflow during wet periods brings more terrestrial carbon (isotopically more negative) into the bay, while also enriching the bay in nutrients. This in turn drives increased phytoplankton productivity and more denitrification in the water column, yielding sediments enriched in 15N (isotopically more positive). The two isotopes are more closely correlated after 1890 at the site indicating a change in the nitrogen source to one more closely tied to dry periods. One possibility is an increased contribution to the nitrogen budget from ammonia and nitrate in riverbed and submarine groundwater discharge.
Leachable Re data variations are loosely correlated with 13Corg and inversely correlated with 15N. This pattern is best seen at the wet periods W3a/b, W12, W13, and W17, but is not consistent across the transition period between 1850 and 1750. Maximum Re enrichments correlate with transitions from stable climate periods (wet, dry, or average) to more extreme periods (wet or dry) based on PHDI (e.g., peaks at approximately 1970, 1935, 1735, 1540, and 1420). Such transitions would be expected to create greater thicknesses of sub-oxic sediment by either partially oxygenating previously anoxic sediments (wet to dry transition), or removing oxygen from previously oxygenated sediments (dry to wet transition). The increase in sub-oxic sediment would remove Re from bottom waters. Changes in sedimentation rate could have similar effects. The correlation with 13Corg may indicate a connection between Re and salinity. Additional analysis of Re mass flux data, and comparison with other trace metal data and BSi data will provide more insight on interpretation of Re cycling in this system.
Prior to significant human impact on the bay, the general relationships between the three parameters discussed was as follows. Heavier 15N from increased denitrification would correlate with lighter 13Corg derived from increased riverine discharge, and variable Re based on conditions preceding the change and the duration of the shift. This conclusion is supported by correlation with the PHDI and paleo-salinity data in Figure 10.3 D and E. Agricultural development of the watershed and construction of flood-control dams in the 1930s changed these relationships, producing more variable 13Corg, greater oxygen depletion and denitrification indicated by heavy 15N, and lower average concentrations of leachable Re with extreme excursions.
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U.S. Department of Interior, U.S. Geological Survey
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