Sedimentation Rates In The Hudson

Figure 6.3. Patterns of sediment accumulation in the Hudson determined from 137Cs distributions such as those shown in Fig. 6.2. Marginal areas of the estuary (especially the western margin off mid-Manhattan), coves and the inner harbor, are sites of rapid accumulation of sediment (from Olsen et al., 1981).

Figure 6.3. Patterns of sediment accumulation in the Hudson determined from 137Cs distributions such as those shown in Fig. 6.2. Marginal areas of the estuary (especially the western margin off mid-Manhattan), coves and the inner harbor, are sites of rapid accumulation of sediment (from Olsen et al., 1981).

invisible to the naked eye, that are created by the interaction of the bottom sediment with the water above. X-radiography of sediment cores, often us-ingportablemedicalX-ray equipment, is a sensitive technique for visualizing this sedimentary fabric. Like the human body, recently deposited sediment contains mostly water. The dense mineral grains that make up the relatively uncompacted sediment

Figure 6.4. Profiles of natural radionuclides in sediment cores collected off Manhattan (~km point 10). The activities and penetration depths of the short-lived radionuclides 234Th and7Be are greater in the western margin of the estuary (cores 2,3,5,6 and 7) than in a transition area leading to the main channel (cores 8 and 9), where little accumulation takes place (from Feng et al., 1998).

Figure 6.4. Profiles of natural radionuclides in sediment cores collected off Manhattan (~km point 10). The activities and penetration depths of the short-lived radionuclides 234Th and7Be are greater in the western margin of the estuary (cores 2,3,5,6 and 7) than in a transition area leading to the main channel (cores 8 and 9), where little accumulation takes place (from Feng et al., 1998).

absorb X rays strongly and are separated from one another by water, which blocks X rays much less effectively, providing a sharp contrast that can produce extremely sensitive images. Often a sediment core that appears featureless to the naked eye is revealed by X-radiography to contain striking features that can help in understanding the conditions under which it was deposited.

Sediment cores obtained by us from the western margin of the Hudson River, across from 79th

Figure 6.5

Accumulation rates determined from 7 Be profiles in cores collected off Manhattan The rates are determined by fitting a line to the log(activity) vs. depth profiles (see 1 in the text;from Feng et al., 1998).

Figure 6.5

Accumulation rates determined from 7 Be profiles in cores collected off Manhattan The rates are determined by fitting a line to the log(activity) vs. depth profiles (see 1 in the text;from Feng et al., 1998).

Street in Manhattan, illustrate this well (Hirschberg etal., 1996;Fengetal., 1998). On visual observation the cores appeared to be composed of a featureless, highly liquid mud that is easily stirred up into the overlying water. X-radiography of the cores (Fig. 6.6) revealed that this apparently featureless deposit is in fact highly structured, with numerous alternating bands of sediment of varying density in the upper part of the core. Such bands are called laminations, and are indicative of sediment deposited from rapidly flowing water. They result from the alignment of the individual min eral grains by the current flow. In the Hudson, most of the very muddy sediment is composed of clay minerals, which tend to have a flat or planar structure and become aligned in a parallel fashion by the current flow, much like bricks. The more closely aligned grains have less space, and hence less water, between them, and therefore absorb X rays more strongly, resulting in a lighter band on the X-rayfilm. The degree of alignment is partially controlled by the strength of the current flow, so laminations indicate that this sediment was deposited from water with strong and variable currents. That

Sediment-water interface Laminations \ Capitella burrows

1 cm

Core 79W6

Figure 6.6. X-radiograph of a sediment core collected on the western margin of the Hudson River estuary off Manhattan (~km point 10). Laminations evident in the X-rays constrain interpretations of sediment accumulation and the importance of physical or biological mixing on the sediment. Small relict burrows of the opportunistic polychaete Capitella are evident and are truncated by erosion events.

Core 79W6

Figure 6.6. X-radiograph of a sediment core collected on the western margin of the Hudson River estuary off Manhattan (~km point 10). Laminations evident in the X-rays constrain interpretations of sediment accumulation and the importance of physical or biological mixing on the sediment. Small relict burrows of the opportunistic polychaete Capitella are evident and are truncated by erosion events.

much (at least) is obvious, since we know that the Hudson River here is strongly tidal, with the current reversing itself on ebb and flood twice daily. It is the scale of the laminations, their number, and the distance between them that may provide us with some new information about sediment deposition processes at this site.

As noted above, radioactive dating of these laminated sediments using the short half-life natural radionuclides 7Be and 234Th indicates that the laminations have been deposited over a period of less than a few months. The number of laminations is therefore too few to have been produced by individual ebb and flood currents, which occur twice daily. Instead the laminations are more consistent with another, longer term variation in the strength of the tidal currents, the neap and spring tide cycle in tide height caused by the phases of the moon. During spring tides, the times of the month with the strongest tides, the tidal currents become significantly greater and the laminations may reflect this monthly cycle. The observed pattern of laminations is not that simple, however, as closer inspectionoftheX-radiographs shows. Some of the laminations show faint, dark (less dense) streaks originating at the bottom of the individual laminations and extending upward into the next lamination above. These are burrows made by a small marine worm (Capitella sp.) that lives near the surface of muddy sediment and feeds by ingesting deposited sediment. The burrow interior is filled with watery sediment and is less dense than the surrounding mud. Some of the Capitella burrows are abruptly truncated, especially at the interface between laminations (Fig. 6.6). This could indicate that in addition to sediment deposition, erosion or removal of sediment is also taking place.

Natural changes in the estuarine 'equilibrium' are complemented by those caused by human activities. In particular, changes in bottom morphology caused by dredging or sand mining shift the equilibrium strongly in favor of sediment accumulation. A clear example of this is seen in the lower harbor where excavation of coarse-grained sediments in the 1960s caused a series of'borrow' pits to be created. The natural bottom in this area is sand and little mud is deposited, yet following the creation of the borrow pits, they began to accumulate fine-grained sediment. Measurement of 7Be and 210Pb profiles in one such pit (Fig. 6.7; Sneed, 1985) shows that on the short term (<1 y) indicated by 7Be, sediments are accumulating at rates of ~7 cm y-1. On the time scale of the halflife of 210Pb, however, rates are ~4 cm y-1. This is consistent with the long-term average rate of accumulation in the pit, based on the thickness of mud accumulated since the pit was created. X-radiography of the cores indicates that they are not disturbed by mixing by a benthic fauna. Thus, differences in accumulation rates on different time scales are likely related to aperiodic high-energy events such as storms that can erode sediment from the pits. These studies suggest that the estuarine equilibrium surface is truly dynamic on annual to multiannual time scales in the Hudson River/New

1 2 3 456789 10 1 2 345678 910

1 2 3 456789 10 1 2 345678 910

Figure 6.7. Profiles of 7Be and excess 210Pb in two sediment cores collected from a 'borrow' pit in New York Harbor. Such pits were created by sand mining in the 1960s and are now sites of fine-grained sediment accumulation. The decreases in the natural radionuclides with depth give indicators of the sediment accumulation rate (equation 1 in the text). No biological mixing is occurring in these deposits and the 7Be results from cores collected in June 1983 (Fig. 6.7b) and February 1984 (Fig. 6.7a) show that short-term rates are ~7 cm y-1. Accumulation on the longer term, indicated by excess 210Pb, is slower, presumably due to erosion of material from the pits by storms (data from Sneed 1985).

Figure 6.7. Profiles of 7Be and excess 210Pb in two sediment cores collected from a 'borrow' pit in New York Harbor. Such pits were created by sand mining in the 1960s and are now sites of fine-grained sediment accumulation. The decreases in the natural radionuclides with depth give indicators of the sediment accumulation rate (equation 1 in the text). No biological mixing is occurring in these deposits and the 7Be results from cores collected in June 1983 (Fig. 6.7b) and February 1984 (Fig. 6.7a) show that short-term rates are ~7 cm y-1. Accumulation on the longer term, indicated by excess 210Pb, is slower, presumably due to erosion of material from the pits by storms (data from Sneed 1985).

York Harbor system, causing large variations in patterns of sediment accumulation.

0 0

Post a comment