Changing sedimentation in tidal flat sediments of the southern North Sea from the Holocene to the present: a geochemical approach

This study presents geochemical evidence for a change in depositional energ y conditions of tidal flat sediments (southern North Sea) from the Holocene , i.e. human unaffected, to present-day conditions. We investigated Holocen e and present tidal flat sediments and suspended particulate matter (SPM) f rom the NW German coastal area (Spiekeroog Island back-barrier area and Jad e Bay), as well as sediments from the Helgoland Island mud hole area. Sampl es 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 s how a shale-like behaviour and enrichments of Ca and Sr reflect the occurre nce of carbonate, whereas higher levels of P, Pb, and Zn in the present sam ples are due to pollution. The fourth group consists of Si, Ti, and Zr, whi ch may be used as indicators of depositional energy because these elements are concentrated by particle sorting effects. The most pronounced geochemic al difference between the Holocene and present tidal Rat sediments is an en richment of Zr in the present samples. As Zr is commonly associated with he avy minerals, this enrichment indicates a higher depositional energy enviro nment 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 fine-grained, Al-rich. material in the present sedime nts. The examination of SPM shows that large amounts of this fine-grained materi al are present in the water column and may be transported from the intertid al system into the open North Sea. The comparison of a calculated Holocene clay accumulation rate with modern estimates of SPM deposition in the Germa n Eight reveals about a two-fold higher deposition of fine material in the Holocene tidal flats. As the sediments from the Helgoland mud hole show a g eochemical composition similar to Holocene tidal flat sediments, we assume that the Helgoland mud hole may serve as a proximal depocentre in the south ern North Sea for the SPM exported from the back-barrier systems. (C) 2000 Elsevier Science B.V. All rights reserved.