High-Resolution Major and Minor Element Geochemistry of Sedimentary Sequences from the Lower Cretaceous (NW German Basin)

Hans-J. Brumsack
Institut für Chemie & Biologie des Meeres (ICBM), Postfach 2503, D-26111 Oldenburg, Germany
V. Rachold
Alfred-Wegener-Institut Forschungsstelle Potsdam, Telegrafenberg A 43, D-14473 Potsdam, Germany
E. Dehn
Institut für Chemie & Biologie des Meeres (ICBM), Postfach 2503, D-26111 Oldenburg, Germany
Variations in climate, weathering regime, sealevel, and paleoceanographic conditions are not only reflected e.g. by shifts in the isotopic composition of carbonate tests, but also by changes in sediment geochemistry. To be able to detect such changes, multidisciplinary approaches are required, providing paleontological and sedimentological information. Furthermore a precise analytical database has to be generated for large sample sets, including bulk parameters like organic carbon, several major elements and specific trace metals. Weathering effects due to the oxidation of reduced S (pyrite) and organic matter may significantly alter element patterns and can only be avoided by analyzing fresh material from drill sites.
The first example of such studies concerns Albian sequences from the NW German Basin. In this case two research drill sites were investigated, covering a total depth range of approx. 400 m and comprising more than 800 samples which were analyzed by XRF and coulometric methods for major and minor elements and bulk parameters. Even though the whole sequence shows little sedimentological variation, subtle changes become visible in major element ratios. Chemically the sediments may be regarded as a mixture of biogenous carbonate with terrigenous-detrital material. Biogenic silica is only present in the top of the sequence, paralleled by small increases in Ba. This increase may be interpreted as an intensification of bioproductivity. In contrast to this, the remaining intervals are lacking any clear indications of enhanced bioproductivity. Generally the sediments were deposited in an oxic environment. Concentrations of organic matter are generally low (on average around 0.3%) and the quality of this material suggests a dominating terrestrial source. This finding is supported by high Mn concentrations and low degrees of pyritization. Furthermore significant enrichments of redox-sensitive trace metals are lacking. One interval is characterized by cyclic variations of Ca/Al- and Si/Al-ratios which seem to be related to orbital parameters (Milancovitch-cycles). Whereas high-frequency changes in carbonate production are the most likely reason for Ca/Al-cyclicity, the lower frequency Si/Al changes must be related to variations in clay mineralogy, in particular the ratio of smectite to illite. Bioproductivity seems to react instantly on subtle changes in nutrient availability, possibly governed by 20, 40, and100 ka cycles. By contrast, lithological changes may be related to climate and associated changes in weathering conditions (400 ka cycles).
The second example concerns the Upper Barrêmian/Lower Albian sequence of paper shales and dark claystones, that culminates in the "fish shale" event, which may represent the boreal analogue to the "Selli-Level" in the tethyan realm. These sediments are characterized by elevated organic carbon concentrations reaching maximum levels of about 7 to 8%. The black and laminated paper shales are enriched in redox-sensitive and sulfide forming trace metals (As, Cd, Cu, Ni, Mo, Se, V, Zn) suggesting deposition under conditions of severe oxygen depletion even in the water column. Mn is present in comparably high levels in the carbonate phase which indicates the trapping potential of anoxic environments for this element, in analogy to the Recent Black Sea. The dark claystones by contrast are less enriched in trace metals and suffer from Mn depletion. This finding supports the hypothesis of suboxic or dysaerobic conditions in the water column and export of this element towards the Proto North Sea. The "fish shale" horizon represents the final black shale event in this Lower Cretaceous sequence and is followed by the "Hedbergellen" marls, which were deposited in a strictly oxic water column. In these marls slight metal enrichments are associated with Mn-oxyhydroxides, Se and As as typical examples for elements bound to reduced S are lacking any significant enrichments. Below the fish shale horizon major element ratios are documenting significant changes in clay mineralogy. Generally the concentrations of K and Al are increasing at the expense of Si. At the present stage of the investigation we are not able to exemplify if this change is related to a different source of the terrigenous-detrital material, or to variations in weathering intensity during this time interval.

responsible for this program: Hans Brumsack and Elisabeth Dehn

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