Worm cast mineralogy

Plotting difference maps for each clay mineral (worm cast value minus surface value) allows comparison of the concentrations between each location (Fig. 3.14). In all but two of the twelve samples smectite has a negative value with positive values predominating for both chlorite and inter-layer vermiculite. The worm casts appear to have less smectite than the estuary sediments, while inter-layer vermiculite and chlorite have higher concentrations in the worm cast samples. A comparison of the values for each location

Figure 3.13 - Average clay mineral relative percentages from worm cast, estuary, riverbank soil and glacial sediments. Error bars are one standard deviation. Average concentrations are calculated by averaging the concentrations (relative, carbonate-normalised, concentrations in estuary samples) in samples from a particular environment.

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Figure 3.14 - Difference maps of clay mineral concentrations in worm cast and surface sediments. (A) Difference in chlorite concentrations between worm cast and surface sample value. (B) Difference in inter-layer vermiculite concentration between worm cast and surface sample value. (C) Difference in smectite concentrations between worm cast and surface sample value. Negative smectite values and positive chlorite and inter-layer vermiculite values indicate that generally smectite and inter-layer vermiculite may be altering to inter-layer vermiculite and chlorite.

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Figure 3.15 - Difference graph for clay mineral concentrations in worm cast and surface sediments. Smectite concentrations appear generally lower in sample locations where bioturbation is more intense, conversely inter- layer vermiculite is also more concentrated in these areas.

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alongside worm cast density (Fig. 3.15) shows a more pronounced pattern with a relatively lower (or negative) value for smectite in the worm casts and there is a relatively higher (or positive) concentration of chlorite and inter-layer vermiculite.

3.6

Discussion

3.6.1 Sediment texture

Sediment in the Leirárvogur Estuary is texturally immature. Sorting is generally poor or very poor (fig. 3.4B) and this supports the interpretation that the transgression and filling of the estuary was initiated relatively recently (Ingólfsson, 1988; Le Breton et al., 2010). This may be compounded by the short transport length of river- supplied sediment, which would limit the degree of sorting. Modal sediment grain-size is well segregated and conforms to the coarse- fine-coarse tripartite facies model found in most modern estuaries (Dalrymple et al., 1992); with a fine to medium sand central portion, a coarse to very coarse marine end, and a pebble dominated bayhead delta. This division of grain-size confirms that sediment dispersal is controlled by both marine and fluvial processes.

Sediments at the marine end of the estuary have a coarser modal grain-size with lower fine fraction content and slightly better sorting (fig. 3.4); this reflects marine reworking. The bayhead delta, at the confluence of the Leirá and Laxá rivers, has the coarsest sediment (fig. 3.4A). The coarseness requires the majority of fluvially derived

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sediment present to have been carried as bedload, and this is most likely to occur during infrequent, high discharge events during storms or spring freshets (floods). Visual inspection of river waters during field sampling in low rainfall conditions during mid-summer showed very little suspended material in both the Leirá and Laxá Rivers. However, this observation is unlikely to be representative of river-borne concentrations during infrequent high discharge events. This contention is supported by published sample average discharge rates for rivers in the southwest of Iceland (Gíslason et al., 1996). Over a period of three years the larger of the two Leirárvogur Estuary rivers average discharge was found to be low (7.8 m3_{/s)compared to the}

total average for other rivers in southwest Iceland (83.9 m3_{/s). This may}

reflect the length and area of the Laxá River catchment compared to other rivers in the area, and because the river is not glacially-fed it may have a lower base stream flow and less glacially derived sediment. Similarly TDS content of the river is also low compared to local rivers and may reflect low denudation rates in a relatively old catchment (Gíslason et al., 1996).

The observation that the Leirárvogur Estuary Rivers may not, ordinarily, supply a large volume of suspended sediment to the estuary, and the indication that the estuary has only recently developed (Ingólfsson, 1988), may suggest that a proportion of the sediment in the estuary is entrained and reworked from local glacial sediments. Glacial

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sediment is actively being eroded on the northern and southern margins of the estuary, as well as in the vicinity of the bayhead delta and the frontal spit at the present day (fig. 3.1C & 3.2D). Glacial sediments found a few centimetres below surface sediments in portions of the present day inter-tidal flat (fig. 3.3F); also indicate extensive glacial sediment under parts of the estuary. Another potential source of glacially-derived sediment is from offshore, with sediment brought onshore through tides and storms. Similarly, active coastal erosion of glacial sediments are significant in the area (Ingólfsson, 1988), with eroded sediment suspended and possibly drawn into the estuary with flood tides.

A large, poorly-sorted, shingle washover beach on the grassy top of the bayhead delta (fig. 3.3B) is composed of smooth, flat, blade- shaped clasts and is characteristic of glacially-entrained sediment (Menzies, 2002). Potential sources for the sediment are from high discharge river events eroding glacial sediment within the catchment, or equally from the erosion and transport of glacial deposits surrounding and underlying the estuary. The presence of the washover indicates the importance of storm events and tides in controlling local grain-size in the upstream, eastern half of the estuary, while also underscoring the role of glacial sediment in supplying, either river-supplied or locally supplied sediment to the estuary.

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The finest modal grain-sizes and the largest fine fraction concentrations sampled were in sheltered areas close to some river inputs, and in the upper reaches at the eastern end of the estuary (fig. 3.4A&C). Fine grained sediment transported in suspension in rivers is likely to encounter a rapid decrease in flow velocity on encountering the estuarine water body. Flocculation of fine particles may also be induced due to the mixing of saline and fresh waters close to the turbidity maximum. Flocculation of sediment to form larger particles increases the settling velocity required for deposition of the particles and may result in deposition close to the river input points. This appears to be evidenced by the high fine fraction content in sediments proximal to river systems on the north-west and south-east sides of the estuary (fig. 3.4C).

The local variability in fine fraction weight percentage in areas of high sampling density, such as on the bayhead delta, and in the small creek behind the southern spit, suggests that there is the potential for a large degree of lateral and stratigraphic variation between proximal sample sites. Other areas, like the intertidal sandflat, display low fine fraction variability even where great distances separate sample sites (fig.3.4C).