Color banding is produced and maintained in response to the geochemical state of the system and, as a result, only corresponds to physical changes in the sediments when physical changes coincide with geochemical changes. In modern sediments this is frequently not the case. It is common that banding exists in cores of apparently homogenous grain size that contain no other apparent physical structure. In contrast, color bands also form horizontal laminae in sediments crossing complex physical structures that are the result of biological and hydrodynamic processes.
Color banding is only preserved at depth in sediments when the bands are buried below the zone of sulfate reduction; the lack of excess sulfur species prevents the AVS from being digenetically altered to pyrite. In organic rich sediments, when sulfate is depleted methanogenisis is the primary bacterially mediated decomposition reaction. The presence of methane gas maintains a highly reduced environment that prevents the diagenetic conversion of AVS to pyrite. The optimal conditions for preservation of banding is where methane gas is found at the sediment-water interface as well as throughout the rest of the sedimentary record.
There are two environmental conditions in Chesapeake Bay that produce this phenomenon. The first occurs in areas of low salinity where modern methanogenic processes produce gas. In the main stem of the Bay, the area north of the Bay Bridge, near latitude 39°00'; the presence of gas is indicated both in pore water data (Hill, 1988), and acoustic reflection records, which show signal saturation as a result of gas vesicles throughout the area. The other occurrence is within the most recent paleochannels underlying the current estuarine system (Hill and others, 1992). The proximity of gas to the sediment-water interface varies in these channels (Hagen and Vogt, 1999). Acoustic reflection shows the depth at which gas vesicles form an acoustically impenetrable layer. This impenetrable layer can be found at the sediment-water interface or at much greater depths; the processes controlling the depth of vesicle layer formation is not well understood, but it has been shown to vary seasonally in response to temperature fluctuations (Hagen and Vogt, 1999). How deep the gas can be found below the acoustically impenetrable layer is not known; the deepest core to date was 5 m and gas was found throughout the section. Core MD99-2207 was taken in a gas-charged region and penetrated >20 m into the sediment column and allows an opportunity to examine AVS and gas within the deepest continuous sediment cores yet available in Chesapeake Bay.
The system used to scan the cores consists of three parts. The first part is the base, which consists of a bed where the core rests, tracks which straddle the core bed, and the anchor for a stepping motor which drives the analytical head. The second part is the mobile analytical head, which contains an altered UV-Vis spectrophotometer coupled with a light source. A line of light is directed onto core surface, reflected light from the surface of the core is restricted through a slit into the monochrometer where it is detected by a photomultiplier tube. The analytical head is positioned along the core by means of a precision stepping motor. The third component of the system is the electronics box which includes the power supply, stepping motor controls, and data log.
Operationally, the system is manually optimized at a wavelength of 520nm to achieve the highest level of reflected light. From tests at MGS, 520nm appears to be the most sensitive wavelength to register changes in color in sediments. The linear scanning rate is approximately 1m/5 min. with a data sampling rate of 4X/sec. The system is capable of resolving bands ~3mm in width. The optical density (i.e. reflectance) is calibrated against the Kodak Grey Scale (KGS), used in black and white photography, which has 20 uniform steps. The response of the scanner system is linear over the entire range with typical r2 >0.96 for the calibration curve.
In the second regime, between ~720 cm - 985 cm (6,026 - 8,443 yr BP) the texture and color of the sediment changed. In this interval, the sediment appeared to have a lower water content, with a coarser grain size, and shell hash (finely divided shell material) was apparent throughout. Low frequency banding occurred in this interval with a period of ~30 cm (303 ±32 yrs). The banding does not appear to correspond to changes in grain size, though this determination will be made later as analyses continue. The bands were dominantly lighter in color than the baseline sediment color. Figure 12.2 shows the optical scan of Section 6 (750 -900 cm; ~6,350 - 7,850 yr BP).
In order to place Figure 12.2 in context, refer to Figure 12.1 which shows the entire calibration range of the KGS scale and the reflectance range found in Section 6. Note that lighter shades of color are lower KGS values and, as the color darkens, the KGS value increases. The range of data in Section 6 spans a 4.5 KGS unit interval from 7 to 11.5m, which is a limited range in comparison to banded sediments found in other parts of the Bay; thus although visible, the contrast is not striking. Increasing the contrast helps to visualize the banding, to this end, an enhanced scale gray scale was used to generate a representation of the cores color variation.
Figure 12.2 also shows the measured KGS as a function of depth; included is enhanced representation of the color variability in the core (the banded bar above the graph). This section of the core indicates a transition in environmental conditions based on the presence of a distinct banding pattern, absent in the younger sediments. Note that there is a distinct baseline, uniform in color, with light colored bands occur at relatively equal intervals with a spacing of ~30cm between bands.
The third regime starts at a depth of approximately 985cm (8,843 yr BP). At this time there is a sharp transition, and all of the older sediments, deeper in the sedimentary record, are highly banded. Figure 12.3 (Section 11, 1500 -1650cm; ~11,150 - 11,400 yr BP) is representative of the banding found in the deeper portion of the sedimentary record. Within these sediments the range of color variation is low but significantly elevated above baseline. The frequency of banding is high with major units occurring at roughly 10 cm (16 ±3 yrs), and minor bands occurring inside the major units. An interesting feature of these bands is that the color bands coincide with physical changes in sediment characteristics; the bands were clearly demarcated by thin layers of sand. This is an unusual occurrence in relation to more modern sediments in the Bay. In addition, some parting of the core occurred at these layers similar to what has been found in shallower gas-charged sediments as a result of the release of pressure which allows the core to expand.
In general, it appears that the sediments in core MD 99-2207 represent a transgressional sequence. The older sediments (>8,450 yr BP) appear to have been subjected to a higher energy environment, strongly influenced by periodic high energy events. The preservation of the color banding and the parting of the sediment during sampling, suggests that the water column was fresh enough to produce and maintain a methanic environment. As water depths increased the sediments became increasingly isolated and only more severe events were preserved in the record. These events were probably major storm events which mixed the water column and produced more aerobic conditions. This could be one mechanism that produces the lighter colored bands; future analyses of the redox metals will help to establish if this is the case. The youngest sediments (<~6,000 yr BP) show a relatively insulated environment capable of sustaining relatively uniform depositional conditions.
Brush, G., Hill, J.M., and Unger, M., 1998, Pollution history of the Chesapeake Bay: NOAA Technical Memorandum NOS ORCA 121.
Chmelik, F.B., 1967, Electro-osmotic core cutting: Marine Geology, v. 5, p. 321-325.
Hagen, R.A., and Vogt, P.R., 1999, Seasonal variability of shallow biogenic gas in the Chesapeake Bay: Marine Geology, v. 158, p. 75 - 88.
Hill, J.M., 1988, Physiographic distribution of interstitial waters in Chesapeake Bay: Maryland Geological Survey Report of Investigations No. 49.
Hill, J.M., Halka, J.P., Conkwrigth, R.D., Koczot, and Coleman, S., 1992, Distribution and effects of shallow gas on bulk estuarine sediment properties: Continental Shelf Research, v. 12, no. 10, p. 1219 -1230.
Hill, J.M., Brush, G., and Park, J.P., 1996, Geochemical history of Chesapeake Bay - natural and anthropogenic influences [abs.]: SETAC meeting, Washington D.C.
Hill, J.M., and Brush, G., 1999, Historical environmental conditions of the Chesapeake Bay based on sediment geochemistry [abs.]: Estuarine Research Federation Annual Meeting, New Orleans, LA.
Hill, J.M., and Brush, G., in prep, Recent changes in sedimentary environment in northern Chesapeake Bay.
U.S. Department of Interior, U.S. Geological Survey
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