Gyttja

Dissimilatory Reduction in Surficial Sediments. Porewater profiles from a number of sites throughout Little Rock Lake show that sulfate is always depleted below the sediment-water interface (Figure 3). Sulfate depletion in porewaters occurs not onlyin the soft gyttja but also in sandy, littoral sites with organic contents < 10%. The observed depletion of sulfate and the occurrence of H2S indicate that the sediments are anoxic immediately below the sediment-water interface and that sulfate reduction occurs in surficial sediments. 

Figure 3. Porewater sulfate profiles at 5-meter sites in the north (acidified) and south (control) basins of Little Rock Lake during September, 1986, and January, 1987. Profiles were determined by porewater equilibrators placed in the lake for three weeks. Both sites are gyttja with > 90% water content.

Little Rock Lake sediments have relatively low iron content (-10 mg/g). As a result, even in short (1-3 hr) incubations of pelagic sediments with 35S042-, 20-30% of the reduced S-35 cannot be recovered by acid distillation. After a 24-hour incubation with north basin gyttja, 32% of reduced S-35 was recovered as AVS, 7% as CRS, and 58% as organic-S. In the same experiment, addition of Fe2+ (as FeCy or lowering the pH of the sediments (with HQ) resulted in a dramatic increase in the fraction of reduced S bound by Fe (Table III). 

Table III. Endproducts of Sulfate Reduction in Little Rock Lake Gyttja in Short-Term Lab Incubations With 35SC>42 Under Various Conditions...

The archaeological remains are found primarily in two layers, approximately 0.80-1.25 m below the surface (Figure 4). The uppermost finds are lying in peat, whereas the deeper lying finds are in a carbonate-rich gyttja. 

Figure 4 The find layers typically found in Nydam. Finds are primarily found in two layers -the Upper find layer (to the left of A) and the Lower find layer (to the left of B). The finds in the upper layers are surrounded by peat whereas those in the lower layers tend to be surrounded by gyttja...

Figure 7 Corrosion depth of a group of lance and spearheads excavated in 1994. (a) Plan view, where solid lines indicate modem excavations and dashed lines show excavations during the 19th century, (b) Vertical profile (seen from SE) where dashed line indicates interface between peat and gyttja. Solid line at each point represent a projection of the lance or spear head to the vertical view plane, so long steep lines indicate artefacts deposited in a steep angle. Numbers to the left are metres above sea level...

Weight loss has been measured for more than 40 modem iron coupons each exposed for approximately 2 years in the waterlogged peat and gyttja layers in Nydam. The results have demonstrated a close correlation between archaeological excavations in the area and measured corrosion rates (Figure 9). 

Organic matter (OM) Complex substances consisting mainly of C, H and O, with subordinate quantities of N, S, and other elements, produced by the growth of organisms. OM is present in trace amounts in virtually all sediments, is particularly abundant in some muds and silts, and is the dominant component of gyttja, peat and coal. 

---