28 | Structure and dynamics of the North Sea benthos

4.2 Sediment particle size

H. Hillewaert

This section provides a descriptive summary of the available sediment particle size data from samples collected for NSBP 2000 and compares it with earlier information, including a statistical evaluation of similarities between matching stations from the 1986 and 2000 North Sea benthos surveys. Examples of more detailed sources of sediment data derived from geological surveys of parts of the North Sea are given in Section 5.1.

4.2.1 Methods

All sediment datasets were collated into a uniform database containing percentages for mud content (grain size <63 μm), sand (grain size between 63 μm and 2000 μm), gravel (grain size >2000 μm), median grain size, and sediment sorting coefficient, except the data for the Dutch continental shelf, for which only median grain size and mud content were available.

The datasets for which fractional data were available were reprocessed to yield uniformly calculated means, sorting coefficients, and descriptive assessments (see Figure 4.2.1).

The program GRADISTAT, Version 4.0 (Blott and Pye, 2001) was used for analyses.

Gravel

SandMud

Sandy gravelMuddy gravel

Gravelly sandGravelly mud

Muddy sandSandy mud

Gravel %

5%

30%

80%

Trace

Sand:Mud Ratio1:9 9:11:1

Figure 4.2.1. Standard descriptors of sediment type, according to % particle size contributions (after Folk, 1954).

4.2.2 Results and discussion

All the sediments sampled in the 2000 survey were relatively coarse grained, with 90% containing <10% silt/clay (Figure 4.2.2), and with most samples being predominantly sandy in nature. The proportion of silt/clay generally increased to the north. Gravelly sands and sandy gravels predominated in the south and east, and these are reflected in larger median grain sizes (Figure 4.2.3) and more poorly sorted sediments (Figure 4.2.4) in these areas.

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Figure 4.2.2. Percentage of mud content for NSBP 2000 stations. (Colour intensity increases with increasing % mud).

Figure 4.2.3. Median grain size for NSBP 2000 stations. (Colour intensity increases with increasing median size).

30 | Structure and dynamics of the North Sea benthos

Figure 4.2.4. Particle sorting for NSBP 2000 stations. (Increasing colour intensity indicates increasingly poorly sorted sediments).

These patterns are consistent with earlier descriptions of the distribution of sediment types in the region (Figure 4.2.5) and correspond approximately to variations in bathymetry and tidal current velocities. The sources of sediment are varied, and the present distribution is the result of a complex interaction of modern processes (tides, waves, and surges) with the effects of glaciations, changes in relative sea level, active sediment erosion, particularly of older Quaternary deposits, and relict features (Goldberg, 1973; Nio et al., 1981; Pantin, 1991; Basford et al., 1993; Irion and Zöllmer, 1999).

Comparing 1986 and 2000

A reduced dataset consisting of 1986 and 2000 samples was constructed according to the paired-station algorithm (see Section 3), which identified those that lay relatively close together. In all, 146 station pairs were thus retained.

The median grain size (µm) was the only parameter that could be reliably compared (Table 4.2.1). A two-tailed t-test conclusively demonstrated no statistically significant difference between the two datasets (P = 0.75 >0.05). The degree of correlation between the station pairs is illustrated in Figure 4.2.6.

Table 4.2.1. Mean and variance of median grain sizes in 1986 and 2000.

MEDIAN µm 1986 MEDIAN µm 2000

Mean 189.19 192.28 Variance 13179.3 17245.9 Observations 146 146

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Figure 4.2.5. North Sea sediment types (after Eisma, 1981).

0

200

400

600

800

1000

1200

0 200 400 600 800

Median (µ) 2000

Med

ian

(µ) 1

986

Median µ

Predicted Median µ

a.

-400

0

400

800

0 200 400 600 800Median (μ ) 2000

Res

idua

ls

b.

Figure 4.2.6. Top panel (a): relation between median grain sizes for 146 station pairs (1986 and 2000 data). Pearson correlation = 0.567, P <0.001. Bottom panel (b): plot of residuals against median μm in 2000.

32 | Structure and dynamics of the North Sea benthos

References

Basford, D. J., Eleftheriou, A., Davies, I. M., Irion, G., and Soltwedel, T. 1993. The ICES North Sea benthos survey: the sedimentary environment. ICES Journal of Marine Science, 50: 71–80.

Blott, S. J., and Pye, K. 2001. GRADISTAT: A grain size distribution and statistics package for the analysis of unconsolidated sediments. Earth Surfaces Processes and Landforms, 26: 1237–1248.

Eisma, D. 1981. Supply and deposition of suspended matter in the North Sea. Special Publication of the International Association of Sedimentologists, 5: 415–428.

Folk, R. L. 1954. The distinction between grain size and mineral composition in sedimentary rock nomenclature. Journal of Geology, 62(4): 344–359.

Goldberg, E. D. (Ed). 1973. North Sea Science. MIT Press, Cambridge, MA, USA.

Irion, G., and Zöllmer, V. 1999. Clay mineral associations in fine-grained surface sediments of the North Sea. Journal of Sea Research, 41: 119–128.

Nio, S. D., Shuttenhelm, R. T. E., and van Weering, Tj. C. E. (Eds). 1981. Holocene Marine Sedimentation in the North Sea Basin. Blackwell, Oxford. 515 pp.

Pantin, H. M. 1991. The sea-bed sediments around the United Kingdom: their bathymetric and physical environment, grain size, mineral composition and associated bed-forms. British Geological Survey Research Report SB/90/1. 47 pp.