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Changing sedimentation in tidal flat sediments of the southern North Sea from the Holocene to the present: a geochemical approach
Journal of Sea Research 44 (2000) 195±208
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Changing sedimentation in tidal ¯at sediments of the southern North Sea from the Holocene to the present: a geochemical approach O. Dellwig, J. Hinrichs, A. Hild, H.-J. Brumsack* Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany Received 13 October 1999; accepted 27 July 2000
Abstract This study presents geochemical evidence for a change in depositional energy conditions of tidal ¯at sediments (southern North Sea) from the Holocene, i.e. human unaffected, to present-day conditions. We investigated Holocene and present tidal ¯at sediments and suspended particulate matter (SPM) from the NW German coastal area (Spiekeroog Island back-barrier area and Jade Bay), as well as sediments from the Helgoland Island mud hole area. Samples were analysed for bulk parameters (TC, TIC), major (Al, Ca, Fe, Mg, K, P, Si, Ti), and trace elements (Ba, Pb, Rb, Sr, V, Zn, Zr). Enrichment factors versus average shale reveal four groups of elements for the investigated Holocene and present sediments. Fe, Mg, K, Ba, Rb, and V show a shale-like behaviour and enrichments of Ca and Sr re¯ect the occurrence of carbonate, whereas higher levels of P, Pb, and Zn in the present samples are due to pollution. The fourth group consists of Si, Ti, and Zr, which may be used as indicators of depositional energy because these elements are concentrated by particle sorting effects. The most pronounced geochemical difference between the Holocene and present tidal ¯at sediments is an enrichment of Zr in the present samples. As Zr is commonly associated with heavy minerals, this enrichment indicates a higher depositional energy environment in the present sediments, which can be traced to modern dike building. The same effect, i.e. increasing current velocities, is responsible for a general depletion of ®ne-grained, Al-rich, material in the present sediments. The examination of SPM shows that large amounts of this ®ne-grained material are present in the water column and may be transported from the intertidal system into the open North Sea. The comparison of a calculated Holocene clay accumulation rate with modern estimates of SPM deposition in the German Bight reveals about a two-fold higher deposition of ®ne material in the Holocene tidal ¯ats. As the sediments from the Helgoland mud hole show a geochemical composition similar to Holocene tidal ¯at sediments, we assume that the Helgoland mud hole may serve as a proximal depocentre in the southern North Sea for the SPM exported from the back-barrier systems. q 2000 Elsevier Science B.V. All rights reserved. Keywords: inorganic geochemistry; Holocene; present tidal ¯at sediments; SPM; Southern North Sea
1. Introduction
* Corresponding author. E-mail address: [email protected] (H.-J. Brumsack).
The southern North Sea coastline has changed drastically during the Holocene. A sedimentary wedge was deposited as a result of the bulldozing effect (Hagemann, 1969) caused by the climate-induced sea-level
1385-1101/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 1385-110 1(00)00051-4
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rise. This wedge consists of Pleistocene sands eroded from the ¯ooded shelf areas and of riverine detrital material delivered, e.g. by the Elbe and Weser. Between 9000 and 8000 BP tidal ¯at sedimentation became more widespread and after 7000 BP fully marine conditions prevailed in the southern North Sea (Eisma et al., 1981), which now has a maximum water depth of about 44 m. The sediments of the NW German coastal area with its barrier islands, wadden seas, and marshes are comparatively young. For instance, the East Frisian barrier islands (Fig. 1) have existed since 7500 BP (Streif, 1990). The morphology of the present coastline is largely due to human activity, viz. dike building and land reclamation beginning in the 11th century. As a consequence, the extent of periodically ¯ooded areas decreased, which led to a further increase in mean tidal range (Behre, 1993). At present the East Frisian Islands are dominated by mesotidal conditions (tidal range about 1.8±3.6 m), whereas macrotides (tidal range .3.6 m) occur in the upper estuaries of the Jade Bay and the river Weser (Davies, 1964; Hayes, 1975). According to sedimentological studies on sediments from the Spiekeroog Island back-barrier system (Fig. 1b) the human impact on coastal morphology led to a steeper than normal energy gradient from the barrier islands towards the coast (Flemming and Davis, 1994; Flemming and Nyandwi, 1994). This increase in depositional energy, i.e. higher tidal current velocities, caused a depletion of ®ne-grained material in the back-barrier system when compared with older sediments. Assuming an almost constant supply of detrital material to the tidal ¯at areas since their formation (approx. 7500 years BP) the presently observed lack of ®ne material should result from lower accumulation rather than enhanced erosion. Thus, larger amounts of ®ne-grained material remain suspended in the water column and may be exported into the open North Sea. The aim of our study is to identify higher energy conditions in the present tidal ¯at areas by geochemical means. For instance, increasing current velocities should be indicated by heavy mineral
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enrichments in the sediments, which can be traced by geochemical indicator elements, such as Ti or Zr. We have therefore compared the geochemistry of present and Holocene tidal ¯at sediments. While the present-day samples consist of surface sediments, the Holocene samples originate from the geological record and are unaffected by human activity. As the lacking ®ne-grained material is presumably suspended in the water column, suspended particulate matter (SPM) is supposed to be the decisive link between the present and Holocene conditions. Therefore, we investigated the geochemical composition of SPM from the Spiekeroog Island back-barrier area and further offshore in the southern North Sea (German Bight). In order to provide information on the fate of SPM, we examined samples from the Helgoland mud hole area, which is supposed to be an important local depocentre for SPM (e.g. Eisma and Kalf, 1987). 2. Geographical setting and sample material The locations of the sediment and suspended particulate matter (SPM) samples analysed are shown in Fig. 1a and b. The Holocene tidal ¯at sediments were sampled from selected intervals from two drill sites: (a) a core transect from the Wangerland area (5 cores, distance of about 3 km from SW to NE) representing sediments from a sheltered intertidal bay (Petzelberger, 1997) and (b) a drill core, which contains sediments from the ancient Jade Bay (location Schweiburg). The cores were drilled between 1996 and 1998 with the support of the Geological Survey of the Federal State of Lower Saxony, Hannover, using a technique described by Merkt and Streif (1970). Simpli®ed lithologies of the drill cores as well as of a core from the Jade Bay (location Arngast, see below) are presented in Fig. 2. The age determinations of several peat samples shown in Fig. 2 were supplied by M.A. Geyh (Geological Survey of the Federal State of Lower Saxony, Germany) and were calculated
Fig. 1. (a) Map of the study area showing the sampling/drilling locations for Holocene and present tidal ¯at sediments (asterisks), cores 99/43, 99/44, and 99/47 from the Helgoland mud hole area (open squares), and SPM (®lled circles). (b) Locations of sediment surface (asterisks) and SPM (®lled circles) sampling in the Spiekeroog Island back-barrier area. Water depth is given relative to nautical chart zero datum.
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Fig. 2. Lithologies of the investigated drill cores from the Wangerland transect (cores W1±W5) and from the locations Arngast and Schweiburg. The Holocene samples were taken from the tidal ¯at intervals. Average calibrated 14C ages (years BP) for the peat layers are denoted.
using the 14C age calibration program calib 3.0 (Stuiver and Reimer, 1993). The average ages (years BP) of the peat layers (Fig. 2) indicate that tidal ¯at sedimentation began about 5500 years BP. Fully marine conditions prevailed presumably until 2700±2200 years BP. The top of all cores consists of cultivated soil and/or salt marsh sediments, except for the Arngast core, which contains present tidal ¯at sediments (see below). The present tidal ¯at sediments consist of surface samples collected from the Swinnplate, which is located in the Spiekeroog Island back-barrier area (Fig. 1b). The samples were collected between winter 1994 and summer 1996 at 1±2 month intervals. The present-day sediments were supplemented by samples of one drill core (upper 2 m) from the Jade Bay (location Arngast,
Fig. 1a). Additionally we investigated three short cores (length 22±26 cm, cores 99/43, 44 and 47) from the Helgoland mud hole area (Fig. 1a), which were collected during a FK `Senckenberg' cruise (03/99). SPM samples (Fig. 1a and b) originate from the back-barrier tidal area and from the German Bight (cruises `Victor-Hensen' 8/97; FK `Senckenberg' 7/ 98, 3/99, 9/99, 1/00; FS `Heincke' 9/98, 8/99 and monthly sampling in the back-barrier tidal ¯at, from 5/94 to 8/95). SPM samples from the German Bight include two transects from Bremerhaven and Cuxhaven to Helgoland Island. An algal bloom during `Victor-Hensen' cruise 8/97 was responsible for high contents of biological material in SPM around Helgoland. A phytoplankton bloom occurred in June 1995 in the back-barrier area of Spiekeroog Island as well (Hild, 1997).
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Fig. 3. Ternary plot Al2O3´5-SiO2 ±CaO´2 (Brumsack, 1989) representing the major inorganic components of the investigated sediment samples. The light grey area indicates mean values of SPM (back-barrier and offshore). Average shale data from Wedepohl (1971).
3. Material and methods Sampling was performed on the Holocene drill cores as well as on the Arngast core at 5±10 cm intervals depending on lithology and at 1 cm intervals on the three short cores from the Helgoland mud hole. The samples were stored in PE-bags, sealed, and immediately frozen. At our laboratory the samples were freeze-dried and homogenised in an agate mortar. The ground powder was used for all subsequent geochemical analyses. The present surface samples from the Spiekeroog back-barrier area were handled in the same way as the drill core samples. A total number of 615 sediment samples (Holocene 349, present 194, Helgoland mud hole 72) were analysed for major elements (Al, Ca, Fe, Mg, K, P, Si, Ti) and trace metals (Ba, Pb, Rb, Sr, V, Zn, Zr) by XRF (Philips PW 2400, equipped with a Rh-tube) using fused lithiumtetraborate glass discs. Heavy minerals (density .2.89 g cm 23) of ®ve present tidal ¯at samples were separated from the ,200 mm fraction using CHBr3. The heavy mineral separates were analysed by XRF. Total carbon (TC) was determined by combustion using an IR-analyser Leco SC-444. Inorganic carbon (TIC) was determined with a UIC Coulometrics Inc. CM 5012 CO2 coulometer coupled to a CM 5130
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acidi®cation module (Huffman, 1977; Engleman et al., 1985). The total organic carbon (TOC) content was calculated as the difference between TC and TIC. SPM samples were collected in the back-barrier tidal ¯at area (378 samples) and the German Bight (79 samples). Onboard ship, 0.2±4 dm 3 of seawater were ®ltered through preweighed Millipore ®lters (0.45 mm, for multi-element analyses) and Whatman quartz ®ber ®lters (0.7 mm, for TC and TIC analyses). The ®lters were rinsed with 18 MV water, dried at 608C and re-weighed for the determination of total suspended material retained on the ®lters. For multielement analysis, the Millipore ®lters were digested in closed PTFE autoclaves (Heinrichs et al., 1986) at 1808C in a mixture of HNO3, HClO4 (puri®ed by sub-boiling distillation) and HF (suprapure). The major elements Al, Ca, Fe, Mg, P, and Ti as well as the minor elements Ba, Pb, Sr, V, Zn, and Zr were analysed by ICP-OES (Perkin±Elmer Optima 3000XL). Contamination effects can be excluded after measurements of ®lter and onboard procedural blanks. For an estimate of mean SiO2 content of SPM, we analysed 56 back-barrier samples collected with a ¯ow-through centrifuge (FK `Senckenberg' 7/98, 9/ 99) by XRF (glass discs) and 26 offshore ®lter samples (FS `Victor Hensen' 8/97) by thin-®lm XRF (Wehausen, 1995). Precision and accuracy of the latter method are 8.2% (2s ) and 15%, respectively. Inorganic carbon was determined coulometrically from the quartz ®bre ®lters with the UIC instrument, TC was determined from a second quartz ®lter by high temperature combustion and coulometric detection of CO2 on a StroÈhlein Coulomat 702. From stations where only one quartz ®lter was available, splits were used for TIC and TC. TOC was obtained by the difference of TC and TIC. Precision and accuracy of all measurements were checked by parallel analysis of international reference materials (GSD-3, GSD-5, GSD-6, GSS1, GSS-6, LKSD-1), as well as in-house standards (see Appendix A).
4. Results and discussion 4.1. Geochemistry of tidal ¯at sediments and SPM Fig. 3 shows a ternary plot (Brumsack, 1989) of the
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Fig. 4. Ternary plot Al±Ca±TOC of SPM samples from the Spiekeroog island back-barrier area. Mean Holocene and present tidal ¯at sediments as well as mean Helgoland mud hole deposits are plotted for comparison. Average shale data from Wedepohl (1971).
distribution of three main inorganic components of the investigated samples. The three poles represent the end-members quartz (SiO2), clay (Al2O3), and carbonate (CaO). The quartz-rich sand ¯at sediments plot close to the SiO2 pole. The facies change with increasing clay and carbonate content to mixed and mud ¯at sediments. In comparison to the anthropogenically unaffected Holocene samples, the present sediments show a less well developed mud ¯at character and lower clay contents. This difference is unlikely to be a result of sample selection as was shown by sedimentological investigations (Flemming and Ziegler, 1995), which con®rmed that most parts of the Spiekeroog Island back-barrier area consist of mixed and sand ¯at sediments while mud ¯ats are relatively rare. SPM samples contain highest clay contents and plot close to mud ¯at sediments. As SiO2 determinations of back-barrier and offshore SPM bear large analytical uncertainties (see Section 3) only ranges are presented. As SPM shows only little variation in the above parameters, the ternary plot in Fig. 4 considers the organic matter content (TOC) instead of quartz. For comparison, the values of average shale and mean tidal ¯at sediments (present and Holocene samples) as well as mean Helgoland mud hole sediments are also shown. The geochemistry of the mud hole samples will be discussed in Section 4.4.
The SPM is characterised by essentially constant Ca/Al ratios and highly variable TOC contents, which increase offshore owing to algal blooms. The Holocene tidal ¯at sediments are enriched in TOC when compared with the present sediments. This ®nding supports the muddy character of the Holocene sediments (compare Fig. 3) as TOC shows a positive relation with increasing clay content (TOC versus Al2O3: r 2 0:9: Regarding the similarity of the Ca/Al ratios of SPM and tidal ¯at sediments we suggest a detrital rather than biogenic carbonate source for both back-barrier and offshore SPM. This assumption is in accordance with results of Salomons (1975), who demonstrated that the carbonate in the ®ne fraction of tidal ¯at sediments (southern North Sea) consists of material from the English Channel (approx. 80%) and from riverine input (approx. 20%). In order to provide an overview about elemental composition we calculated enrichment factors versus average shale (EFS) for different tidal ¯at facies (Holocene and present), Helgoland mud hole sediments, and SPM (Fig. 5). The EFS values are calculated as follows: EFS elementsample =Alsample = elementav: shale =Alav: shale
The normalisation to Al eliminates dilution effects caused by quartz, carbonate, and organic matter, and allows us to compare sediments of different genesis. The element Al was chosen because its abundance is directly linked to clay minerals and its content in sediments is not signi®cantly affected by biogenic cycles or pollution. The differentiation between sand, mixed, and mud ¯at is based on lithological core descriptions and on SiO2 contents (SiO2 , 65% mud ¯at, SiO2 65±80% mixed ¯at, SiO2 . 80% sand ¯at). EFS values of SPM and mud ¯at sediments as well as Helgoland mud hole samples and mixed ¯at sediments are plotted together due to their geochemical similarity as seen from the ternary plot (Fig. 3). A compilation of the average values for sediments and SPM is presented in Table 1. Fig. 5a±c shows that the present and Holocene sediments are characterised as a ®rst approximation by comparable EFS values. The detrital elements Ba, Fe, K, Mg, Rb, and V plot close to the average shale composition. Elements enriched in comparison to
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Fig. 5. Enrichment factors versus average shale (EFS) for selected major and minor elements of different Holocene and present tidal ¯at facies, SPM (back-barrier and offshore), and Helgoland mud hole deposits. Average shale data from Wedepohl (1971).
average shale can be divided into three groups: (1) Ca, Sr; (2) P, Pb, Zn; and (3) Si, Ti, Zr. The enrichment in Ca and Sr re¯ects the presence of carbonates while higher amounts of the second group of elements are
due to anthropogenic activity. P shows elevated concentrations especially in the present mud and mixed ¯at sediments as well as in SPM re¯ecting an enhanced nutrient input (Radach et al., 1990). Pb is
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Table 1 Average values of bulk parameters (%), major elements (%) and selected trace metals (mg kg 21) for Holocene and present tidal ¯at sediments, Helgoland mud hole deposits, and SPM from the study area Element
TOC TIC SiO2 TiO2 Al2O3 Fe2O3 MgO CaO K2O P2O5 Ba Pb Rb Sr V Zn Zr SPM (mg kg 21) a
Holocene
Present
Mud hole
Mud ¯at
Mixed ¯at
Sand ¯at
Mud ¯at
Mixed ¯at
Sand ¯at
1.8 1.4 61.4 0.60 9.5 4.6 1.6 6.6 2.1 0.16 258 17 95 202 93 61 305
1.1 1.1 71.3 0.53 7.4 3.0 1.2 5.1 1.9 0.11 270 13 72 158 59 44 409
0.3 0.7 81.7 0.33 5.0 1.5 0.6 3.3 1.5 0.06 258 7 48 112 26 20 327
1.8 1.2 57.8 0.46 7.4 3.2 1.6 6.4 1.8 0.24 256 40 70 232 73 95 326
1.3 0.9 75.8 0.38 5.5 1.8 0.8 4.0 1.6 0.11 294 16 40 140 29 22 414
0.3 0.3 87.9 0.29 3.5 0.8 0.4 2.0 1.2 0.05 242 9 22 82 12 10 401
1.2 1.5 68.3 0.46 6.9 2.6 1.2 6.6 1.8 0.09 268 24 68 193 52 64 395
SPM Backbarrier
Offshore
4.5 1.5 49.9 0.52 10.5 4.6 1.9 8.2 2.0 0.37 270 63 n.d. a 296 104 184 140 55
6.6 1.4 30 0.45 8.8 4.2 1.8 7.1 1.6 0.51 296 122 n.d. a 252 90 363 96 7
n.d. not determined.
most enriched in offshore SPM due to adsorption onto particle surfaces by physical and biological processes (Kersten et al., 1992; Tappin et al., 1995). Back-barrier SPM and present sediments show almost the same Pb enrichments indicating frequent redistribution processes inside the intertidal system. Considering the shale-like EFS values of the Holocene sediments it is obvious that the observed Pb enrichment is to a large extent caused by pollution. Zn shows a similar distribution but is not enriched in the sand ¯at sediments. This is possibly due to the different carrier phases of Pb and Zn. Pb is more strongly in¯uenced by atmospheric input (50%) than Zn (30%), while the riverine contribution of Zn exceeds Pb by a factor of two (Schwedhelm and Irion, 1985). Owing to the comparatively low riverine input into the back-barrier area, atmospheric input seems to be the most important factor. Additionally, Pb is more particle reactive than Zn (Tappin et al., 1995) and therefore more ef®ciently concentrated in SPM. The third group consists of elements, which can be used as indicators of depositional energy conditions. Enrichments in Si re¯ect higher amounts of coarse-
grained quartz, whereas high Ti and Zr values point towards elevated heavy mineral contents (e.g. ilmenite, rutile, zircon). Only SPM is characterised by a shale-like composition while the sediments are enriched in these elements owing to energy-driven sorting effects. 4.2. Geochemical indications of changes in depositional energy The most signi®cant geochemical difference of detrital elements between the investigated present and Holocene tidal ¯at sediments is seen for Zr (Fig. 5), an indicator element for heavy minerals (see above). Especially the present sand ¯at sediments exhibit a pronounced Zr enrichment in comparison to the Holocene sediments. Heavy mineral enrichments are often associated with particle sorting effects due to wave action and high current velocities. Thus, the observed high Zr levels provide a ®rst indication of a rise in energetic conditions from the Holocene to the present system. According to Ludwig and Figge (1979) heavy minerals are continuously washed out from Pleistocene sands in the Southern North Sea
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Fig. 6. Scatter plot of Zr vs TiO2 of sediments (Holocene and present tidal ¯at, Helgoland mud hole) and SPM. Bold lines indicate heavy mineral separates and average shale (Wedepohl, 1971).
during storm events and are deposited at the seaward edge of the barrier islands. Highest heavy mineral concentrations of more than 3% in bulk sediments are found in water less than 10 m deep. If tidal currents are strong enough, heavy minerals may also enter the back-barrier area through the inlets between the barrier islands (e.g. Otzumer Balje, Fig. 1b). Fig. 6 shows a scatter plot of Zr and TiO2 for sediments and SPM. The broad correlation of TiO2 and Zr in present sediments, which tends towards the ratio determined for heavy mineral separates from the Spiekeroog Island back-barrier area, indicates that both elements are associated with the heavy mineral fraction. However, Zr seems to be a more sensitive indicator of heavy minerals than Ti, as seen from the less steep regression line of the present sediments (Zr/ Ti 0.25) in comparison to the heavy mineral separates (Zr/Ti 0.48). The different behaviour of Ti and Zr results from the 30-fold higher abundance of Ti in average shale when compared with Zr. Thus, small amounts of heavy minerals added to a clay-rich system will have a larger effect on Zr than on Ti contents. For that reason, we assume that Ti is to a larger extent bound to clay minerals, in addition to the heavy mineral fraction. Offshore SPM samples reveal a geochemical composition similar to average shale, while some back-barrier SPM samples are slightly
enriched in Zr due to resuspension processes (see below). The Holocene samples plot between these two end-members (heavy minerals and shale). Hence, we postulate that the Holocene sediments represent a mixture of material similar to the present heavy mineral-rich sediments and shale-like SPM. Elevated depositional energy conditions are considered to be the decisive factor responsible for the physical separation of coarser sediments and SPM seen from Fig. 6. In contrast, during the Holocene this separation was less effective due to lower energetic conditions. Therefore, it appears likely that during the Holocene higher amounts of ®ne-grained material were deposited in the back-barrier sediments. The scatter plot of TiO2 versus Al2O3 (Fig. 7) shows similar differences between the Holocene and present tidal ¯at sediments and SPM as seen from Fig. 6, i.e. the Holocene samples again plot between present sediments and SPM. While a correlation between TiO2 and Al2O3 is seen for the Holocene sediments, no such correlation is observed for the present sediments. This again indicates that in the Holocene sediments most of the Ti is bound in the clay mineral lattice. However, the fact that the regression line shows a signi®cant positive intercept re¯ects the presence of small amounts of heavy minerals. The present tidal ¯at sediments show no such correlation
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Fig. 7. Scatter plot of TiO2 vs Al2O3 of sediment (Holocene and present tidal ¯at, Helgoland mud hole) and SPM. The bold line indicates the average shale ratio (Wedepohl, 1971).
as a result of the higher and more variable contribution of heavy minerals. Higher Al2O3 contents of the Holocene tidal ¯at sediments are consistent with elevated mud contents whereas the present deposits show a signi®cant lack of this ®ne fraction. A depletion of the mud fraction has also been observed by Flemming and Nyandwi (1994) and Flemming and Davis (1994) on the basis of sedimentological investigations in the back-barrier area of Spiekeroog Island. They related this phenomenon to
modern dike building because the mainland dike seems to act as an energy barrier for the deposition of the ®ne fraction. It was concluded that the dike increases the normal cross-shore energy gradient and therefore, inhibits the deposition of ®ne material with settling velocities ,0.5 cm s 21, which predominantly remains suspended in the water column. On the other hand, before the onset of dike building the coastal area was characterised by a different morphology. The Holocene coast contained extensive
Fig. 8. Scatter plots of (a) Ti and (b) Zr vs Al of SPM from the German Bight (offshore) and Spiekeroog Island back-barrier area. Element concentrations are given per volume of seawater. Bold lines indicate average shale ratio (Wedepohl, 1971).
O. Dellwig et al. / Journal of Sea Research 44 (2000) 195±208 Table 2 Comparison of present and Holocene estimates of clay accumulation in the German Wadden Sea Time period
Author
Clay accumulation in the Wadden Sea (Mt yr 21)
Present Present Holocene
Kirby (1987) Puls et al. (1997) This study
1.24 1 ^ 1a 2.3
a Value was calculated for the entire German Wadden Sea, i.e. the coasts of Lower Saxony and Schleswig±Holstein.
salt marshes and lagoons as well as seaward decreasing stands of reed, causing ¯at energy gradients (Van der Woude, 1981; Streif, 1990; Dellwig et al., 2000). Recent examples of such systems may be the mesotidal coastlands of South Carolina and Georgia, USA, as well as the brackish-marine Tidal Lands in Florida, USA (Hayes, 1975; Reineck, 1984 and references therein). In Fig. 8a and b the concentrations of Ti and Zr in Spiekeroog back-barrier and offshore SPM samples are plotted versus Al. It should be noted that the concentrations are given per volume of seawater rather than on a dry weight basis. Ti is well correlated to Al in both compartments and plots close to the average shale ratio (Fig. 8a). Ti enrichments similar to those observed in the sediments (Fig. 7) can hardly be found in SPM whereas the more sensitive energy indicator element Zr shows a large scatter in backbarrier SPM (Fig. 8b). This pattern is probably caused by heavy minerals resuspended by wave action and tidal currents. In contrast, offshore samples from the much less turbid German Bight waters are characterised by a composition similar to average shale with respect to both, Ti and Zr. At a station located 1 km off the Otzumer Balje outlet (Fig. 1a and b), Ti/ Al (av. 0.056; av. shale 0.053) and Zr/Al (av. 18; av. shale 18) ratios were at the shale level. This implies that the heavy minerals remain within the back-barrier area and only shale-like material is exported (back) into the open North Sea. Assuming that the Holocene SPM composition did not differ signi®cantly from present SPM, and particulate matter input has remained approximately constant over time, the observed depletion of ®ne-grained sediments in the back-barrier area should be the result of SPM export
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out of the intertidal ¯ats, while coarse-grained sands and heavy minerals are still accumulating. Consequently, this ®nding leads to the assumption that the total amount of SPM in the water column is signi®cantly higher under present high-energy conditions than in the Holocene. 4.3. Comparison of Holocene and present-day clay accumulation From the results discussed so far, we assume a higher clay accumulation in the Holocene than in the present tidal ¯ats. Large amounts of clay from the present tidal ¯at sediments are suspended in the water column due to an anthropogenically induced high-energy environment. During the Holocene this ®ne fraction was deposited to a larger extent. In order to estimate the clay accumulation in the study area, we compare present SPM budgets and the Holocene clay accumulation in Table 2. For calculation of the average Holocene mass ¯ux, we assumed a time period of 7500 years for the deposition of the coastal sedimentary wedge of Lower Saxony (sediment volume 25 £ 10 9 m 3), consisting predominantly of tidal ¯at and brackish water sediments (Hoselmann and Streif, 1997). Based on a mean dry density of 1.25 t m 23 for the Holocene sediments (mixed ¯at sediments, Puls et al., 1997), the total deposited mass amounts to 4.2 Mt yr 21. For comparison with the present SPM accumulation, we calculated the mean clay content of the Holocene sediments including tidal ¯at and brackish water sediments (Dellwig, 1999) from their mean Al2O3 content (9.2%) and the Al2O3 content of average shale (16.8%). This procedure assumes a dilution of clay material with quartz, which holds for the sediments used in this study (compare EFS values). From the mean Al2O3 content we inferred a mean clay content of 55% for Holocene sediments, which is equivalent to a clay deposition rate of 2.3 Mt yr 21. This value is about two times higher than present estimates (Table 2). It should be noted that the deposition rate given by Puls et al. (1997) integrates the entire German coastal area, including the coastlands of Lower Saxony and Schleswig± Holstein. Therefore, their value has to be divided by a factor of approximately 2, the area of the Lower Saxonian coastlands amounting to about 43% of the
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total German Wadden Sea area (LozaÂn et al., 1994). The distinctly higher Holocene accumulation rate of ®ne-grained material con®rms the postulated change in depositional energy due to dike building as was shown by our geochemical data set. 4.4. SPM, where does it go? The comparison of Holocene and present clay accumulation rates has shown that large amounts of ®negrained material are currently not deposited within the tidal ¯at system and are presumably transported further offshore. As most of the North Sea consists of sandy sediments, the main area of deposition for ®ne-grained material is the western Skagerrak/ Norwegian Channel, where presently about 50±70% of the total mass accumulation of the North Sea occurs (Eisma and Kalf, 1987; Eisma and Irion, 1988). An additional more proximal depocentre is supposed to be the area of the Helgoland mud hole in the southern North Sea. Considering the ternary plots (Figs. 3 and 4) as well as the EFS values shown in Fig. 5b, sediments from the present Helgoland mud hole reveal a geochemical composition similar to mixed ¯at sediments. Despite partly higher carbonate amounts, owing to mussel shell fragments, the Helgoland mud hole deposits geochemically seem to be more related to the Holocene than to the present mixed ¯at sediments. This is also supported by the scatter plots of Al, Ti, and Zr (Figs. 6 and 7) as most samples show a close similarity to the Holocene tidal ¯at sediments. Only a few samples tend towards present tidal ¯at sediments due to elevated Zr contents in the top 5 cm of the investigated cores. Due to the shallow water depth west of the Helgoland mud hole (10±20 m) we presume that during extreme storm events heavy mineral-rich North Sea sediments have been transported to the mud hole area. Additionally ®ne-grained material may have been eroded from the top sediment layer by wave action. Nevertheless our geochemical data indicate that under normal conditions the Helgoland mud hole sediments consist of two endmembers (1) shale-like and (2) quartz-rich material (Fig. 3). While the quartz end-member is most likely represented by North Sea sediments, the shale-like material seems to be SPM which partly originates from the back-barrier areas. Therefore, an increasing
SPM export from the tidal ¯at areas, induced by man-made high-energy conditions, should enhance sediment accumulation especially in the nearby Helgoland mud hole. This assumption is con®rmed by a comparison of Holocene and present-day sedimentation rates. Von Haugwitz et al. (1988) proposed an average sedimentation rate of about 3 mm yr 21 for Helgoland mud hole sediments deposited before coastal morphologies were altered by human impact (8000±1500 years BP). In contrast, present estimates are signi®cantly higher with a range between 5 and 18 mm yr 21 (e.g. FoÈrstner and Reineck, 1974; Dominik et al., 1978; Eisma et al., 1984; Baumann, 1991). According to Von Haugwitz et al. (1988) this difference in sedimentation rates may be explained by dredged harbour mud originally dumped into the mouth of the river Elbe. However, on the basis of our results the increasing SPM export from the back-barrier intertidal systems also has to be considered because this process contributes large amounts of ®ne material to the southern North Sea. 5. Conclusions In this study inorganic geochemical analyses of Holocene and present-day tidal ¯at sediments as well as SPM are used to evidence changes in depositional energy at the NW German coast. The geochemistry of the investigated tidal ¯at sediments shows an increase in depositional energy from Holocene to present conditions, i.e. the change from a natural to a human-affected environment. While the present sediments contain higher amounts of heavy minerals as shown by enrichments in Zr, the Holocene sediments are characterised by elevated clay contents. According to sedimentological investigations by Flemming and Nyandwi (1994) on back-barrier sediments of the study area this difference is caused by human in¯uence on the coastal morphology, e.g. dikebuilding. The examination of SPM demonstrates that in the present situation the ®ne-grained material is predominantly suspended in the water column of the tidal ¯at area and therefore may be exported into the southern North Sea while heavy mineral-rich sediments remain in the back-barrier systems. An estimate of the Holocene clay accumulation indicates a significantly higher deposition rate of ®ne-grained material
O. Dellwig et al. / Journal of Sea Research 44 (2000) 195±208
than at present. Besides the western Skagerrak/ Norwegian Channel we assume that the Helgoland mud hole may serve as a proximal depocentre in the southern North Sea for the SPM exported from the back-barrier systems. Acknowledgements The authors wish to thank H. Streif and J. Barckhausen (Geological Survey of the Federal State of Lower Saxony, Germany) for the supply of Holocene sediment material and for the lithological core descriptions. Further thanks to the crews of FK `Senckenberg', FS `Heincke', and FS `Victor Hensen' as well as to R. Reuter and B. Warning. We appreciate the support of K. Bente (Institute of Crystallography/Mineralogy and Material Science Leipzig, Germany) and B. Hansen (IGDL GoÈttingen, Germany), who enabled the investigation of heavy mineral separates. Two anonymous reviewers are thanked for their constructive comments. This study was funded by the German Science Foundation (DFG) through grants No. Scho 561/3-1, 4-1 and forms part of the interdisciplinary special research program ªBio-geochemical changes over the last 15 000 years Ð continental sediments as an expression of changing environmental conditionsº and the German Federal Environment Of®ce (VBA/ BMBF, grant no. 03F0112A/B) as part of the project ªEcosystem Research Lower Saxonian Wadden Sea È SF)º. (O Appendix A The precision and accuracy of the analysed elements is presented in the following table: Element
Method
TC TC TIC Al Ca Fe K
Coulometry StroÈhlein IR-analyser Coulometry UIC XRF
Precision SD 2s (%) 1.2 4.0 1.1 1.3 0.9 1.1 1.5
Accuracy (%) 1.5 1.3 0.2 1.1 4.8 0.9 1.6
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(continued) Element
Mg P Si Ti Ba Pb Rb S V Zn Zr Al Ca Fe K Mg P Ti Ba Pb Sr V Zn Zr
Method
ICP-OES
Precision SD 2s (%)
Accuracy (%)
3.2 3.5 0.8 1.5 2.8 5.6 3.3 1.3 3.0 3.4 2.3 4.9 5.4 4.9 8.8 5.7 8.1 6.0 6.1 12.4 5.3 7.6 4.7 5.6
3.1 6.7 0.8 2.3 3.9 2.7 2.4 1.5 3.1 4.9 1.5 4.0 2.1 2.6 15.2 1.9 4.3 2.6 3.5 5.1 3.7 5.5 7.7 11.8
References Baumann, M., 1991. Die Ablagerung von Tschernobyl±RadiocaÈsium in der Norwegischen See und in der Nordsee, Berichte, Fachbereich Geowissenschaften, UniversitaÈt Bremen 14. Behre, K.-E., 1993. Die nacheiszeitlichen Meeresspiegelbewegungen und ihre Auswirkungen auf die KuÈstenlandschaft und deren Besiedlung. In: Schellnhuber, H.-J., Sterr, H. (Eds.). KlimaaÈnderung und KuÈste. Springer, Berlin, pp. 57±76. Brumsack, H.-J., 1989. Geochemisty of recent TOC-rich sediments from the Gulf of California and the Black Sea. Geol. Rundsch. 78, 851±882. Davies, J.L., 1964. A morphogenetic approach to world shorelines. Z. Geomorph. 8, 127±142. Dellwig, O., 1999. Geochemistry of Holocene coastal deposits (NW Germany): Palaeoenvironmental reconstruction. PhD thesis, University of Oldenburg. Dellwig, O., Watermann, F., Brumsack, H.-J., Gerdes, G., Krumbein, W.E., 2000. Sulphur and iron geochemistry of Holocene coastal peats (NW Germany): a tool for palaeoenvironmental reconstruction. Palaeogr. Palaeoclimatol. Palaeoecol. (in press). Dominik, J., FoÈrstner, U., Mangini, A., Reineck, H.-E., 1978. 210Pb
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and Cs chronology of heavy metal pollution in a sediment core from the German Bight (North Sea). Senckenb. Marit. 10, 213±227. Eisma, D., Kalf, J., 1987. Dispersal, concentration and deposition of suspended matter in the North Sea. J. Geol. Soc. Lond. 144, 161±178. Eisma, D., Irion, G., 1988. Suspended matter and sediment transport. In: Salomons, W. (Ed.). Pollution of the North Sea Ð An Assessment. Springer, Berlin, pp. 20±35. Eisma, D., Mook, W.G., Laban, C., 1981. An early Holocene tidal ¯at in the Southern Bight. Spec. Publ. Int. Assoc. Sedimentol. 5, 229±237. Eisma, D., Jacobs, E., Berger, G., Stolk, A., 1984. Mud-accumulation in the German Bight, ®rst results. Proc. of a Congress of the Working Group on Marine Sediments in Relation to Pollution, ICES Papers Rostock Meeting. Engleman, E.E., Jackson, L.L., Norton, D.R., 1985. Determination of carbonate carbon in geological materials by coulometric titration. Chem. Geol. 53, 125±128. Flemming, B.W., Davis, R.A., 1994. Holocene evolution, morphodynamics and sedimentology of the Spiekeroog barrier island system (Southern North Sea). Senckenb. Marit. 24, 117±155. Flemming, B.W., Nyandwi, N., 1994. Land reclamation as a cause of ®ne grained sediment depletion in backbarrier tidal ¯ats (Southern North Sea). Neth. J. Aquat. Ecol. 28, 299±307. Flemming, B.W., Ziegler, K., 1995. High-resolution grain size distribution patterns and textural trends in the backbarrier environment of Spiekeroog island (Southern North Sea). Senckenb. Marit. 26, 1±24. FoÈrstner, U., Reineck, H.-E., 1974. Die Anreicherung von Spurenelementen in den rezenten Sedimenten eines Pro®lkernes aus der Deutschen Bucht. Senckenb. Marit. 6, 175±184. Hagemann, B.P., 1969. Development of the western part of the Netherlands during the Holocene. Geol. Mijnb. 48, 373±388. Hayes, M.O., 1975. Morphology of sand accumulation in estuaries: an introduction to the symposium. In: Cronin, L.E. (Ed.). Estuarine Research. Academic Press, New York, pp. 3±22. Heinrichs, H., Brumsack, H.-J., Loft®eld, N., KoÈnig, N., 1986. Verbessertes Druckaufschlub system fuÈr biologische und anorganische Materialien. Z. P¯anzenernaÈhr. Bodenkd. 149, 350±353. Hild, A., 1997. Geochemie der Sedimente und Schwebstoffe im RuÈckseitenwatt von Spiekeroog und ihre Beein¯ussung durch biologische AktivitaÈt. Forschungsz. Terramare Ber. 5, 1±33. Hoselmann, C., Streif, H., 1997. Bilanzierung der holozaÈnen Sedimentakkumulation im niedersaÈchsischen KuÈstenraum. Z. Dt. Geol. Ges. 148, 431±445. Huffman Jr, E.W.D., 1977. Performance of a new automatic carbon dioxide coulometer. Microchem. J. 22, 567±573. Kersten, M., FoÈrstner, U., Krause, P., Kriews, M., Dannecker, W., Garbe-SchoÈnberg, C.-D., HoÈck, M., Terzenbach, U., Graûl, H., 1992. Pollution source reconnaissance using stable lead isotope
ratios ( 206/207Pb). In: Vernet, J.-P. (Ed.). Impact of Heavy Metals on the Environment. Elsevier, Amsterdam, pp. 311±325. Kirby, R., 1987. Sediment exchanges across the coastal margins of NW Europe. J. Geol. Soc. Lond. 144, 121±126. LozaÂn, J.L., Rachor, E., Reise, K., Von Westernhagen, H., Lenz, W., 1994. Warnsignale aus dem Wattenmeer. Blackwell, Berlin. Ludwig, G., Figge, K., 1979. Schwermineralvorkommen und Sandverteilung in der Deutschen Bucht. Geol. Jb. D 32, 23±68. Merkt, J., Streif, H., 1970. Stechrohr-BohrgeraÈte fuÈr limnische und marine Lockersedimente. Geol. Jb. 88, 137±148. Petzelberger, B.E.M., 1997. Geologische Untersuchungen zur Landschaftsgeschichte des juÈngeren HolozaÈns in der ehemaligen Crildumer Bucht im Wangerland, Ldkr. Friesland. Probl. KuÈstenforsch. suÈdl. Nordseegebiet 24, 301±309. Puls, W., Heinrichs, H., Mayer, B., 1997. Suspended particulate matter budget for the German Bight. Mar. Poll. Bull. 34, 398± 409. Radach, G., SchoÈnfeld, W., Lenhart, H., 1990. NaÈhrstoffe in der Nordsee Ð Eutrophierung, Hypertrophierung und deren Auswirkungen. In: LozaÂn, J.L. (Ed.). Warnsignale aus der Nordsee. Verlag Paul Parey, Berlin, pp. 48±65. Reineck, H.E., 1984. Aktu-Geologie klastischer Sedimente. Senckenberg-Buch 61. Verlag Waldemar Kramer, Frankfurt. Salomons, W., 1975. Chemical and isotopic composition of carbonates in recent sediments and soils from western Europe. J. Sediment. Petrol. 45, 440±449. Schwedhelm, E., Irion, G., 1985. Schwermetalle und NaÈhrelemente in den Sedimenten der deutschen Nordseewatten. Cour. Forsch.Inst. Senckenberg 73, 1±119. Streif, H., 1990. Das ostfriesische KuÈstengebiet. GebruÈder BorntraÈger, Berlin. Stuiver, M., Reimer, P.J., 1993. Extended 14C database and revised calib 3.0 14C age calibration program. Radiocarbon 35, 215± 230. Tappin, A.D., Millward, G.E., Statham, P.J., Burton, J.D., Morris, A.W., 1995. Trace metals in the Central and Southern North Sea. Estuar. Coast. Shelf Sci. 41, 275±323. Van der Woude, J.D., 1981. Holocene palaeoenvironmental evolution of a perimarine ¯uviatile area. Acad. Proefschr., Amsterdam. (cited after: Reineck, H.-E., 1984. Aktugeologie klastischer Sedimente. Senckenb.-Buch 61, Verlag Waldemar Kramer, Frankfurt a.M.). Von Haugwitz, W., Wong, H.K., Salge, U., 1988. The mud area southeast of Helgoland: a re¯ection seismic study. Mitt. Geol.PalaÈont. Inst. Univ. Hamburg 65, 409±422. Wedepohl, K.H., 1971. Environmental in¯uences on the chemical composition of shales and clays. In: Ahrens, L.H. (Ed.). Physics and Chemistry of the Earth. Pergamon, Oxford, pp. 305±333. Wehausen, R., 1995. RoÈntgen¯uoreszenzanalytische Untersuchungen an Schwebstoffen aus dem NiedersaÈchsischen Wattenmeer. Diploma Thesis, University of Oldenburg, Germany.
www.elsevier.nl/locate/seares
Changing sedimentation in tidal ¯at sediments of the southern North Sea from the Holocene to the present: a geochemical approach O. Dellwig, J. Hinrichs, A. Hild, H.-J. Brumsack* Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany Received 13 October 1999; accepted 27 July 2000
Abstract This study presents geochemical evidence for a change in depositional energy conditions of tidal ¯at sediments (southern North Sea) from the Holocene, i.e. human unaffected, to present-day conditions. We investigated Holocene and present tidal ¯at sediments and suspended particulate matter (SPM) from the NW German coastal area (Spiekeroog Island back-barrier area and Jade Bay), as well as sediments from the Helgoland Island mud hole area. Samples were analysed for bulk parameters (TC, TIC), major (Al, Ca, Fe, Mg, K, P, Si, Ti), and trace elements (Ba, Pb, Rb, Sr, V, Zn, Zr). Enrichment factors versus average shale reveal four groups of elements for the investigated Holocene and present sediments. Fe, Mg, K, Ba, Rb, and V show a shale-like behaviour and enrichments of Ca and Sr re¯ect the occurrence of carbonate, whereas higher levels of P, Pb, and Zn in the present samples are due to pollution. The fourth group consists of Si, Ti, and Zr, which may be used as indicators of depositional energy because these elements are concentrated by particle sorting effects. The most pronounced geochemical difference between the Holocene and present tidal ¯at sediments is an enrichment of Zr in the present samples. As Zr is commonly associated with heavy minerals, this enrichment indicates a higher depositional energy environment in the present sediments, which can be traced to modern dike building. The same effect, i.e. increasing current velocities, is responsible for a general depletion of ®ne-grained, Al-rich, material in the present sediments. The examination of SPM shows that large amounts of this ®ne-grained material are present in the water column and may be transported from the intertidal system into the open North Sea. The comparison of a calculated Holocene clay accumulation rate with modern estimates of SPM deposition in the German Bight reveals about a two-fold higher deposition of ®ne material in the Holocene tidal ¯ats. As the sediments from the Helgoland mud hole show a geochemical composition similar to Holocene tidal ¯at sediments, we assume that the Helgoland mud hole may serve as a proximal depocentre in the southern North Sea for the SPM exported from the back-barrier systems. q 2000 Elsevier Science B.V. All rights reserved. Keywords: inorganic geochemistry; Holocene; present tidal ¯at sediments; SPM; Southern North Sea
1. Introduction
* Corresponding author. E-mail address: [email protected] (H.-J. Brumsack).
The southern North Sea coastline has changed drastically during the Holocene. A sedimentary wedge was deposited as a result of the bulldozing effect (Hagemann, 1969) caused by the climate-induced sea-level
1385-1101/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 1385-110 1(00)00051-4
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rise. This wedge consists of Pleistocene sands eroded from the ¯ooded shelf areas and of riverine detrital material delivered, e.g. by the Elbe and Weser. Between 9000 and 8000 BP tidal ¯at sedimentation became more widespread and after 7000 BP fully marine conditions prevailed in the southern North Sea (Eisma et al., 1981), which now has a maximum water depth of about 44 m. The sediments of the NW German coastal area with its barrier islands, wadden seas, and marshes are comparatively young. For instance, the East Frisian barrier islands (Fig. 1) have existed since 7500 BP (Streif, 1990). The morphology of the present coastline is largely due to human activity, viz. dike building and land reclamation beginning in the 11th century. As a consequence, the extent of periodically ¯ooded areas decreased, which led to a further increase in mean tidal range (Behre, 1993). At present the East Frisian Islands are dominated by mesotidal conditions (tidal range about 1.8±3.6 m), whereas macrotides (tidal range .3.6 m) occur in the upper estuaries of the Jade Bay and the river Weser (Davies, 1964; Hayes, 1975). According to sedimentological studies on sediments from the Spiekeroog Island back-barrier system (Fig. 1b) the human impact on coastal morphology led to a steeper than normal energy gradient from the barrier islands towards the coast (Flemming and Davis, 1994; Flemming and Nyandwi, 1994). This increase in depositional energy, i.e. higher tidal current velocities, caused a depletion of ®ne-grained material in the back-barrier system when compared with older sediments. Assuming an almost constant supply of detrital material to the tidal ¯at areas since their formation (approx. 7500 years BP) the presently observed lack of ®ne material should result from lower accumulation rather than enhanced erosion. Thus, larger amounts of ®ne-grained material remain suspended in the water column and may be exported into the open North Sea. The aim of our study is to identify higher energy conditions in the present tidal ¯at areas by geochemical means. For instance, increasing current velocities should be indicated by heavy mineral
197
enrichments in the sediments, which can be traced by geochemical indicator elements, such as Ti or Zr. We have therefore compared the geochemistry of present and Holocene tidal ¯at sediments. While the present-day samples consist of surface sediments, the Holocene samples originate from the geological record and are unaffected by human activity. As the lacking ®ne-grained material is presumably suspended in the water column, suspended particulate matter (SPM) is supposed to be the decisive link between the present and Holocene conditions. Therefore, we investigated the geochemical composition of SPM from the Spiekeroog Island back-barrier area and further offshore in the southern North Sea (German Bight). In order to provide information on the fate of SPM, we examined samples from the Helgoland mud hole area, which is supposed to be an important local depocentre for SPM (e.g. Eisma and Kalf, 1987). 2. Geographical setting and sample material The locations of the sediment and suspended particulate matter (SPM) samples analysed are shown in Fig. 1a and b. The Holocene tidal ¯at sediments were sampled from selected intervals from two drill sites: (a) a core transect from the Wangerland area (5 cores, distance of about 3 km from SW to NE) representing sediments from a sheltered intertidal bay (Petzelberger, 1997) and (b) a drill core, which contains sediments from the ancient Jade Bay (location Schweiburg). The cores were drilled between 1996 and 1998 with the support of the Geological Survey of the Federal State of Lower Saxony, Hannover, using a technique described by Merkt and Streif (1970). Simpli®ed lithologies of the drill cores as well as of a core from the Jade Bay (location Arngast, see below) are presented in Fig. 2. The age determinations of several peat samples shown in Fig. 2 were supplied by M.A. Geyh (Geological Survey of the Federal State of Lower Saxony, Germany) and were calculated
Fig. 1. (a) Map of the study area showing the sampling/drilling locations for Holocene and present tidal ¯at sediments (asterisks), cores 99/43, 99/44, and 99/47 from the Helgoland mud hole area (open squares), and SPM (®lled circles). (b) Locations of sediment surface (asterisks) and SPM (®lled circles) sampling in the Spiekeroog Island back-barrier area. Water depth is given relative to nautical chart zero datum.
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Fig. 2. Lithologies of the investigated drill cores from the Wangerland transect (cores W1±W5) and from the locations Arngast and Schweiburg. The Holocene samples were taken from the tidal ¯at intervals. Average calibrated 14C ages (years BP) for the peat layers are denoted.
using the 14C age calibration program calib 3.0 (Stuiver and Reimer, 1993). The average ages (years BP) of the peat layers (Fig. 2) indicate that tidal ¯at sedimentation began about 5500 years BP. Fully marine conditions prevailed presumably until 2700±2200 years BP. The top of all cores consists of cultivated soil and/or salt marsh sediments, except for the Arngast core, which contains present tidal ¯at sediments (see below). The present tidal ¯at sediments consist of surface samples collected from the Swinnplate, which is located in the Spiekeroog Island back-barrier area (Fig. 1b). The samples were collected between winter 1994 and summer 1996 at 1±2 month intervals. The present-day sediments were supplemented by samples of one drill core (upper 2 m) from the Jade Bay (location Arngast,
Fig. 1a). Additionally we investigated three short cores (length 22±26 cm, cores 99/43, 44 and 47) from the Helgoland mud hole area (Fig. 1a), which were collected during a FK `Senckenberg' cruise (03/99). SPM samples (Fig. 1a and b) originate from the back-barrier tidal area and from the German Bight (cruises `Victor-Hensen' 8/97; FK `Senckenberg' 7/ 98, 3/99, 9/99, 1/00; FS `Heincke' 9/98, 8/99 and monthly sampling in the back-barrier tidal ¯at, from 5/94 to 8/95). SPM samples from the German Bight include two transects from Bremerhaven and Cuxhaven to Helgoland Island. An algal bloom during `Victor-Hensen' cruise 8/97 was responsible for high contents of biological material in SPM around Helgoland. A phytoplankton bloom occurred in June 1995 in the back-barrier area of Spiekeroog Island as well (Hild, 1997).
O. Dellwig et al. / Journal of Sea Research 44 (2000) 195±208
Fig. 3. Ternary plot Al2O3´5-SiO2 ±CaO´2 (Brumsack, 1989) representing the major inorganic components of the investigated sediment samples. The light grey area indicates mean values of SPM (back-barrier and offshore). Average shale data from Wedepohl (1971).
3. Material and methods Sampling was performed on the Holocene drill cores as well as on the Arngast core at 5±10 cm intervals depending on lithology and at 1 cm intervals on the three short cores from the Helgoland mud hole. The samples were stored in PE-bags, sealed, and immediately frozen. At our laboratory the samples were freeze-dried and homogenised in an agate mortar. The ground powder was used for all subsequent geochemical analyses. The present surface samples from the Spiekeroog back-barrier area were handled in the same way as the drill core samples. A total number of 615 sediment samples (Holocene 349, present 194, Helgoland mud hole 72) were analysed for major elements (Al, Ca, Fe, Mg, K, P, Si, Ti) and trace metals (Ba, Pb, Rb, Sr, V, Zn, Zr) by XRF (Philips PW 2400, equipped with a Rh-tube) using fused lithiumtetraborate glass discs. Heavy minerals (density .2.89 g cm 23) of ®ve present tidal ¯at samples were separated from the ,200 mm fraction using CHBr3. The heavy mineral separates were analysed by XRF. Total carbon (TC) was determined by combustion using an IR-analyser Leco SC-444. Inorganic carbon (TIC) was determined with a UIC Coulometrics Inc. CM 5012 CO2 coulometer coupled to a CM 5130
199
acidi®cation module (Huffman, 1977; Engleman et al., 1985). The total organic carbon (TOC) content was calculated as the difference between TC and TIC. SPM samples were collected in the back-barrier tidal ¯at area (378 samples) and the German Bight (79 samples). Onboard ship, 0.2±4 dm 3 of seawater were ®ltered through preweighed Millipore ®lters (0.45 mm, for multi-element analyses) and Whatman quartz ®ber ®lters (0.7 mm, for TC and TIC analyses). The ®lters were rinsed with 18 MV water, dried at 608C and re-weighed for the determination of total suspended material retained on the ®lters. For multielement analysis, the Millipore ®lters were digested in closed PTFE autoclaves (Heinrichs et al., 1986) at 1808C in a mixture of HNO3, HClO4 (puri®ed by sub-boiling distillation) and HF (suprapure). The major elements Al, Ca, Fe, Mg, P, and Ti as well as the minor elements Ba, Pb, Sr, V, Zn, and Zr were analysed by ICP-OES (Perkin±Elmer Optima 3000XL). Contamination effects can be excluded after measurements of ®lter and onboard procedural blanks. For an estimate of mean SiO2 content of SPM, we analysed 56 back-barrier samples collected with a ¯ow-through centrifuge (FK `Senckenberg' 7/98, 9/ 99) by XRF (glass discs) and 26 offshore ®lter samples (FS `Victor Hensen' 8/97) by thin-®lm XRF (Wehausen, 1995). Precision and accuracy of the latter method are 8.2% (2s ) and 15%, respectively. Inorganic carbon was determined coulometrically from the quartz ®bre ®lters with the UIC instrument, TC was determined from a second quartz ®lter by high temperature combustion and coulometric detection of CO2 on a StroÈhlein Coulomat 702. From stations where only one quartz ®lter was available, splits were used for TIC and TC. TOC was obtained by the difference of TC and TIC. Precision and accuracy of all measurements were checked by parallel analysis of international reference materials (GSD-3, GSD-5, GSD-6, GSS1, GSS-6, LKSD-1), as well as in-house standards (see Appendix A).
4. Results and discussion 4.1. Geochemistry of tidal ¯at sediments and SPM Fig. 3 shows a ternary plot (Brumsack, 1989) of the
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Fig. 4. Ternary plot Al±Ca±TOC of SPM samples from the Spiekeroog island back-barrier area. Mean Holocene and present tidal ¯at sediments as well as mean Helgoland mud hole deposits are plotted for comparison. Average shale data from Wedepohl (1971).
distribution of three main inorganic components of the investigated samples. The three poles represent the end-members quartz (SiO2), clay (Al2O3), and carbonate (CaO). The quartz-rich sand ¯at sediments plot close to the SiO2 pole. The facies change with increasing clay and carbonate content to mixed and mud ¯at sediments. In comparison to the anthropogenically unaffected Holocene samples, the present sediments show a less well developed mud ¯at character and lower clay contents. This difference is unlikely to be a result of sample selection as was shown by sedimentological investigations (Flemming and Ziegler, 1995), which con®rmed that most parts of the Spiekeroog Island back-barrier area consist of mixed and sand ¯at sediments while mud ¯ats are relatively rare. SPM samples contain highest clay contents and plot close to mud ¯at sediments. As SiO2 determinations of back-barrier and offshore SPM bear large analytical uncertainties (see Section 3) only ranges are presented. As SPM shows only little variation in the above parameters, the ternary plot in Fig. 4 considers the organic matter content (TOC) instead of quartz. For comparison, the values of average shale and mean tidal ¯at sediments (present and Holocene samples) as well as mean Helgoland mud hole sediments are also shown. The geochemistry of the mud hole samples will be discussed in Section 4.4.
The SPM is characterised by essentially constant Ca/Al ratios and highly variable TOC contents, which increase offshore owing to algal blooms. The Holocene tidal ¯at sediments are enriched in TOC when compared with the present sediments. This ®nding supports the muddy character of the Holocene sediments (compare Fig. 3) as TOC shows a positive relation with increasing clay content (TOC versus Al2O3: r 2 0:9: Regarding the similarity of the Ca/Al ratios of SPM and tidal ¯at sediments we suggest a detrital rather than biogenic carbonate source for both back-barrier and offshore SPM. This assumption is in accordance with results of Salomons (1975), who demonstrated that the carbonate in the ®ne fraction of tidal ¯at sediments (southern North Sea) consists of material from the English Channel (approx. 80%) and from riverine input (approx. 20%). In order to provide an overview about elemental composition we calculated enrichment factors versus average shale (EFS) for different tidal ¯at facies (Holocene and present), Helgoland mud hole sediments, and SPM (Fig. 5). The EFS values are calculated as follows: EFS elementsample =Alsample = elementav: shale =Alav: shale
The normalisation to Al eliminates dilution effects caused by quartz, carbonate, and organic matter, and allows us to compare sediments of different genesis. The element Al was chosen because its abundance is directly linked to clay minerals and its content in sediments is not signi®cantly affected by biogenic cycles or pollution. The differentiation between sand, mixed, and mud ¯at is based on lithological core descriptions and on SiO2 contents (SiO2 , 65% mud ¯at, SiO2 65±80% mixed ¯at, SiO2 . 80% sand ¯at). EFS values of SPM and mud ¯at sediments as well as Helgoland mud hole samples and mixed ¯at sediments are plotted together due to their geochemical similarity as seen from the ternary plot (Fig. 3). A compilation of the average values for sediments and SPM is presented in Table 1. Fig. 5a±c shows that the present and Holocene sediments are characterised as a ®rst approximation by comparable EFS values. The detrital elements Ba, Fe, K, Mg, Rb, and V plot close to the average shale composition. Elements enriched in comparison to
O. Dellwig et al. / Journal of Sea Research 44 (2000) 195±208
201
Fig. 5. Enrichment factors versus average shale (EFS) for selected major and minor elements of different Holocene and present tidal ¯at facies, SPM (back-barrier and offshore), and Helgoland mud hole deposits. Average shale data from Wedepohl (1971).
average shale can be divided into three groups: (1) Ca, Sr; (2) P, Pb, Zn; and (3) Si, Ti, Zr. The enrichment in Ca and Sr re¯ects the presence of carbonates while higher amounts of the second group of elements are
due to anthropogenic activity. P shows elevated concentrations especially in the present mud and mixed ¯at sediments as well as in SPM re¯ecting an enhanced nutrient input (Radach et al., 1990). Pb is
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Table 1 Average values of bulk parameters (%), major elements (%) and selected trace metals (mg kg 21) for Holocene and present tidal ¯at sediments, Helgoland mud hole deposits, and SPM from the study area Element
TOC TIC SiO2 TiO2 Al2O3 Fe2O3 MgO CaO K2O P2O5 Ba Pb Rb Sr V Zn Zr SPM (mg kg 21) a
Holocene
Present
Mud hole
Mud ¯at
Mixed ¯at
Sand ¯at
Mud ¯at
Mixed ¯at
Sand ¯at
1.8 1.4 61.4 0.60 9.5 4.6 1.6 6.6 2.1 0.16 258 17 95 202 93 61 305
1.1 1.1 71.3 0.53 7.4 3.0 1.2 5.1 1.9 0.11 270 13 72 158 59 44 409
0.3 0.7 81.7 0.33 5.0 1.5 0.6 3.3 1.5 0.06 258 7 48 112 26 20 327
1.8 1.2 57.8 0.46 7.4 3.2 1.6 6.4 1.8 0.24 256 40 70 232 73 95 326
1.3 0.9 75.8 0.38 5.5 1.8 0.8 4.0 1.6 0.11 294 16 40 140 29 22 414
0.3 0.3 87.9 0.29 3.5 0.8 0.4 2.0 1.2 0.05 242 9 22 82 12 10 401
1.2 1.5 68.3 0.46 6.9 2.6 1.2 6.6 1.8 0.09 268 24 68 193 52 64 395
SPM Backbarrier
Offshore
4.5 1.5 49.9 0.52 10.5 4.6 1.9 8.2 2.0 0.37 270 63 n.d. a 296 104 184 140 55
6.6 1.4 30 0.45 8.8 4.2 1.8 7.1 1.6 0.51 296 122 n.d. a 252 90 363 96 7
n.d. not determined.
most enriched in offshore SPM due to adsorption onto particle surfaces by physical and biological processes (Kersten et al., 1992; Tappin et al., 1995). Back-barrier SPM and present sediments show almost the same Pb enrichments indicating frequent redistribution processes inside the intertidal system. Considering the shale-like EFS values of the Holocene sediments it is obvious that the observed Pb enrichment is to a large extent caused by pollution. Zn shows a similar distribution but is not enriched in the sand ¯at sediments. This is possibly due to the different carrier phases of Pb and Zn. Pb is more strongly in¯uenced by atmospheric input (50%) than Zn (30%), while the riverine contribution of Zn exceeds Pb by a factor of two (Schwedhelm and Irion, 1985). Owing to the comparatively low riverine input into the back-barrier area, atmospheric input seems to be the most important factor. Additionally, Pb is more particle reactive than Zn (Tappin et al., 1995) and therefore more ef®ciently concentrated in SPM. The third group consists of elements, which can be used as indicators of depositional energy conditions. Enrichments in Si re¯ect higher amounts of coarse-
grained quartz, whereas high Ti and Zr values point towards elevated heavy mineral contents (e.g. ilmenite, rutile, zircon). Only SPM is characterised by a shale-like composition while the sediments are enriched in these elements owing to energy-driven sorting effects. 4.2. Geochemical indications of changes in depositional energy The most signi®cant geochemical difference of detrital elements between the investigated present and Holocene tidal ¯at sediments is seen for Zr (Fig. 5), an indicator element for heavy minerals (see above). Especially the present sand ¯at sediments exhibit a pronounced Zr enrichment in comparison to the Holocene sediments. Heavy mineral enrichments are often associated with particle sorting effects due to wave action and high current velocities. Thus, the observed high Zr levels provide a ®rst indication of a rise in energetic conditions from the Holocene to the present system. According to Ludwig and Figge (1979) heavy minerals are continuously washed out from Pleistocene sands in the Southern North Sea
O. Dellwig et al. / Journal of Sea Research 44 (2000) 195±208
203
Fig. 6. Scatter plot of Zr vs TiO2 of sediments (Holocene and present tidal ¯at, Helgoland mud hole) and SPM. Bold lines indicate heavy mineral separates and average shale (Wedepohl, 1971).
during storm events and are deposited at the seaward edge of the barrier islands. Highest heavy mineral concentrations of more than 3% in bulk sediments are found in water less than 10 m deep. If tidal currents are strong enough, heavy minerals may also enter the back-barrier area through the inlets between the barrier islands (e.g. Otzumer Balje, Fig. 1b). Fig. 6 shows a scatter plot of Zr and TiO2 for sediments and SPM. The broad correlation of TiO2 and Zr in present sediments, which tends towards the ratio determined for heavy mineral separates from the Spiekeroog Island back-barrier area, indicates that both elements are associated with the heavy mineral fraction. However, Zr seems to be a more sensitive indicator of heavy minerals than Ti, as seen from the less steep regression line of the present sediments (Zr/ Ti 0.25) in comparison to the heavy mineral separates (Zr/Ti 0.48). The different behaviour of Ti and Zr results from the 30-fold higher abundance of Ti in average shale when compared with Zr. Thus, small amounts of heavy minerals added to a clay-rich system will have a larger effect on Zr than on Ti contents. For that reason, we assume that Ti is to a larger extent bound to clay minerals, in addition to the heavy mineral fraction. Offshore SPM samples reveal a geochemical composition similar to average shale, while some back-barrier SPM samples are slightly
enriched in Zr due to resuspension processes (see below). The Holocene samples plot between these two end-members (heavy minerals and shale). Hence, we postulate that the Holocene sediments represent a mixture of material similar to the present heavy mineral-rich sediments and shale-like SPM. Elevated depositional energy conditions are considered to be the decisive factor responsible for the physical separation of coarser sediments and SPM seen from Fig. 6. In contrast, during the Holocene this separation was less effective due to lower energetic conditions. Therefore, it appears likely that during the Holocene higher amounts of ®ne-grained material were deposited in the back-barrier sediments. The scatter plot of TiO2 versus Al2O3 (Fig. 7) shows similar differences between the Holocene and present tidal ¯at sediments and SPM as seen from Fig. 6, i.e. the Holocene samples again plot between present sediments and SPM. While a correlation between TiO2 and Al2O3 is seen for the Holocene sediments, no such correlation is observed for the present sediments. This again indicates that in the Holocene sediments most of the Ti is bound in the clay mineral lattice. However, the fact that the regression line shows a signi®cant positive intercept re¯ects the presence of small amounts of heavy minerals. The present tidal ¯at sediments show no such correlation
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Fig. 7. Scatter plot of TiO2 vs Al2O3 of sediment (Holocene and present tidal ¯at, Helgoland mud hole) and SPM. The bold line indicates the average shale ratio (Wedepohl, 1971).
as a result of the higher and more variable contribution of heavy minerals. Higher Al2O3 contents of the Holocene tidal ¯at sediments are consistent with elevated mud contents whereas the present deposits show a signi®cant lack of this ®ne fraction. A depletion of the mud fraction has also been observed by Flemming and Nyandwi (1994) and Flemming and Davis (1994) on the basis of sedimentological investigations in the back-barrier area of Spiekeroog Island. They related this phenomenon to
modern dike building because the mainland dike seems to act as an energy barrier for the deposition of the ®ne fraction. It was concluded that the dike increases the normal cross-shore energy gradient and therefore, inhibits the deposition of ®ne material with settling velocities ,0.5 cm s 21, which predominantly remains suspended in the water column. On the other hand, before the onset of dike building the coastal area was characterised by a different morphology. The Holocene coast contained extensive
Fig. 8. Scatter plots of (a) Ti and (b) Zr vs Al of SPM from the German Bight (offshore) and Spiekeroog Island back-barrier area. Element concentrations are given per volume of seawater. Bold lines indicate average shale ratio (Wedepohl, 1971).
O. Dellwig et al. / Journal of Sea Research 44 (2000) 195±208 Table 2 Comparison of present and Holocene estimates of clay accumulation in the German Wadden Sea Time period
Author
Clay accumulation in the Wadden Sea (Mt yr 21)
Present Present Holocene
Kirby (1987) Puls et al. (1997) This study
1.24 1 ^ 1a 2.3
a Value was calculated for the entire German Wadden Sea, i.e. the coasts of Lower Saxony and Schleswig±Holstein.
salt marshes and lagoons as well as seaward decreasing stands of reed, causing ¯at energy gradients (Van der Woude, 1981; Streif, 1990; Dellwig et al., 2000). Recent examples of such systems may be the mesotidal coastlands of South Carolina and Georgia, USA, as well as the brackish-marine Tidal Lands in Florida, USA (Hayes, 1975; Reineck, 1984 and references therein). In Fig. 8a and b the concentrations of Ti and Zr in Spiekeroog back-barrier and offshore SPM samples are plotted versus Al. It should be noted that the concentrations are given per volume of seawater rather than on a dry weight basis. Ti is well correlated to Al in both compartments and plots close to the average shale ratio (Fig. 8a). Ti enrichments similar to those observed in the sediments (Fig. 7) can hardly be found in SPM whereas the more sensitive energy indicator element Zr shows a large scatter in backbarrier SPM (Fig. 8b). This pattern is probably caused by heavy minerals resuspended by wave action and tidal currents. In contrast, offshore samples from the much less turbid German Bight waters are characterised by a composition similar to average shale with respect to both, Ti and Zr. At a station located 1 km off the Otzumer Balje outlet (Fig. 1a and b), Ti/ Al (av. 0.056; av. shale 0.053) and Zr/Al (av. 18; av. shale 18) ratios were at the shale level. This implies that the heavy minerals remain within the back-barrier area and only shale-like material is exported (back) into the open North Sea. Assuming that the Holocene SPM composition did not differ signi®cantly from present SPM, and particulate matter input has remained approximately constant over time, the observed depletion of ®ne-grained sediments in the back-barrier area should be the result of SPM export
205
out of the intertidal ¯ats, while coarse-grained sands and heavy minerals are still accumulating. Consequently, this ®nding leads to the assumption that the total amount of SPM in the water column is signi®cantly higher under present high-energy conditions than in the Holocene. 4.3. Comparison of Holocene and present-day clay accumulation From the results discussed so far, we assume a higher clay accumulation in the Holocene than in the present tidal ¯ats. Large amounts of clay from the present tidal ¯at sediments are suspended in the water column due to an anthropogenically induced high-energy environment. During the Holocene this ®ne fraction was deposited to a larger extent. In order to estimate the clay accumulation in the study area, we compare present SPM budgets and the Holocene clay accumulation in Table 2. For calculation of the average Holocene mass ¯ux, we assumed a time period of 7500 years for the deposition of the coastal sedimentary wedge of Lower Saxony (sediment volume 25 £ 10 9 m 3), consisting predominantly of tidal ¯at and brackish water sediments (Hoselmann and Streif, 1997). Based on a mean dry density of 1.25 t m 23 for the Holocene sediments (mixed ¯at sediments, Puls et al., 1997), the total deposited mass amounts to 4.2 Mt yr 21. For comparison with the present SPM accumulation, we calculated the mean clay content of the Holocene sediments including tidal ¯at and brackish water sediments (Dellwig, 1999) from their mean Al2O3 content (9.2%) and the Al2O3 content of average shale (16.8%). This procedure assumes a dilution of clay material with quartz, which holds for the sediments used in this study (compare EFS values). From the mean Al2O3 content we inferred a mean clay content of 55% for Holocene sediments, which is equivalent to a clay deposition rate of 2.3 Mt yr 21. This value is about two times higher than present estimates (Table 2). It should be noted that the deposition rate given by Puls et al. (1997) integrates the entire German coastal area, including the coastlands of Lower Saxony and Schleswig± Holstein. Therefore, their value has to be divided by a factor of approximately 2, the area of the Lower Saxonian coastlands amounting to about 43% of the
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total German Wadden Sea area (LozaÂn et al., 1994). The distinctly higher Holocene accumulation rate of ®ne-grained material con®rms the postulated change in depositional energy due to dike building as was shown by our geochemical data set. 4.4. SPM, where does it go? The comparison of Holocene and present clay accumulation rates has shown that large amounts of ®negrained material are currently not deposited within the tidal ¯at system and are presumably transported further offshore. As most of the North Sea consists of sandy sediments, the main area of deposition for ®ne-grained material is the western Skagerrak/ Norwegian Channel, where presently about 50±70% of the total mass accumulation of the North Sea occurs (Eisma and Kalf, 1987; Eisma and Irion, 1988). An additional more proximal depocentre is supposed to be the area of the Helgoland mud hole in the southern North Sea. Considering the ternary plots (Figs. 3 and 4) as well as the EFS values shown in Fig. 5b, sediments from the present Helgoland mud hole reveal a geochemical composition similar to mixed ¯at sediments. Despite partly higher carbonate amounts, owing to mussel shell fragments, the Helgoland mud hole deposits geochemically seem to be more related to the Holocene than to the present mixed ¯at sediments. This is also supported by the scatter plots of Al, Ti, and Zr (Figs. 6 and 7) as most samples show a close similarity to the Holocene tidal ¯at sediments. Only a few samples tend towards present tidal ¯at sediments due to elevated Zr contents in the top 5 cm of the investigated cores. Due to the shallow water depth west of the Helgoland mud hole (10±20 m) we presume that during extreme storm events heavy mineral-rich North Sea sediments have been transported to the mud hole area. Additionally ®ne-grained material may have been eroded from the top sediment layer by wave action. Nevertheless our geochemical data indicate that under normal conditions the Helgoland mud hole sediments consist of two endmembers (1) shale-like and (2) quartz-rich material (Fig. 3). While the quartz end-member is most likely represented by North Sea sediments, the shale-like material seems to be SPM which partly originates from the back-barrier areas. Therefore, an increasing
SPM export from the tidal ¯at areas, induced by man-made high-energy conditions, should enhance sediment accumulation especially in the nearby Helgoland mud hole. This assumption is con®rmed by a comparison of Holocene and present-day sedimentation rates. Von Haugwitz et al. (1988) proposed an average sedimentation rate of about 3 mm yr 21 for Helgoland mud hole sediments deposited before coastal morphologies were altered by human impact (8000±1500 years BP). In contrast, present estimates are signi®cantly higher with a range between 5 and 18 mm yr 21 (e.g. FoÈrstner and Reineck, 1974; Dominik et al., 1978; Eisma et al., 1984; Baumann, 1991). According to Von Haugwitz et al. (1988) this difference in sedimentation rates may be explained by dredged harbour mud originally dumped into the mouth of the river Elbe. However, on the basis of our results the increasing SPM export from the back-barrier intertidal systems also has to be considered because this process contributes large amounts of ®ne material to the southern North Sea. 5. Conclusions In this study inorganic geochemical analyses of Holocene and present-day tidal ¯at sediments as well as SPM are used to evidence changes in depositional energy at the NW German coast. The geochemistry of the investigated tidal ¯at sediments shows an increase in depositional energy from Holocene to present conditions, i.e. the change from a natural to a human-affected environment. While the present sediments contain higher amounts of heavy minerals as shown by enrichments in Zr, the Holocene sediments are characterised by elevated clay contents. According to sedimentological investigations by Flemming and Nyandwi (1994) on back-barrier sediments of the study area this difference is caused by human in¯uence on the coastal morphology, e.g. dikebuilding. The examination of SPM demonstrates that in the present situation the ®ne-grained material is predominantly suspended in the water column of the tidal ¯at area and therefore may be exported into the southern North Sea while heavy mineral-rich sediments remain in the back-barrier systems. An estimate of the Holocene clay accumulation indicates a significantly higher deposition rate of ®ne-grained material
O. Dellwig et al. / Journal of Sea Research 44 (2000) 195±208
than at present. Besides the western Skagerrak/ Norwegian Channel we assume that the Helgoland mud hole may serve as a proximal depocentre in the southern North Sea for the SPM exported from the back-barrier systems. Acknowledgements The authors wish to thank H. Streif and J. Barckhausen (Geological Survey of the Federal State of Lower Saxony, Germany) for the supply of Holocene sediment material and for the lithological core descriptions. Further thanks to the crews of FK `Senckenberg', FS `Heincke', and FS `Victor Hensen' as well as to R. Reuter and B. Warning. We appreciate the support of K. Bente (Institute of Crystallography/Mineralogy and Material Science Leipzig, Germany) and B. Hansen (IGDL GoÈttingen, Germany), who enabled the investigation of heavy mineral separates. Two anonymous reviewers are thanked for their constructive comments. This study was funded by the German Science Foundation (DFG) through grants No. Scho 561/3-1, 4-1 and forms part of the interdisciplinary special research program ªBio-geochemical changes over the last 15 000 years Ð continental sediments as an expression of changing environmental conditionsº and the German Federal Environment Of®ce (VBA/ BMBF, grant no. 03F0112A/B) as part of the project ªEcosystem Research Lower Saxonian Wadden Sea È SF)º. (O Appendix A The precision and accuracy of the analysed elements is presented in the following table: Element
Method
TC TC TIC Al Ca Fe K
Coulometry StroÈhlein IR-analyser Coulometry UIC XRF
Precision SD 2s (%) 1.2 4.0 1.1 1.3 0.9 1.1 1.5
Accuracy (%) 1.5 1.3 0.2 1.1 4.8 0.9 1.6
207
(continued) Element
Mg P Si Ti Ba Pb Rb S V Zn Zr Al Ca Fe K Mg P Ti Ba Pb Sr V Zn Zr
Method
ICP-OES
Precision SD 2s (%)
Accuracy (%)
3.2 3.5 0.8 1.5 2.8 5.6 3.3 1.3 3.0 3.4 2.3 4.9 5.4 4.9 8.8 5.7 8.1 6.0 6.1 12.4 5.3 7.6 4.7 5.6
3.1 6.7 0.8 2.3 3.9 2.7 2.4 1.5 3.1 4.9 1.5 4.0 2.1 2.6 15.2 1.9 4.3 2.6 3.5 5.1 3.7 5.5 7.7 11.8
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