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Abstract:
The Cretaceous and Paleogene sediments recovered during Ocean Drilling Program Leg 207 can be divided into three broad modes of de-position: synrift clastics (lithologic Unit V), organic matter rich, laminated black shales (Unit IV), and open marine chalk and calcareousclaystones (Units III–I). The aim of this study is to provide a quantitative geochemical characterization of sediments representing these five lithologic units. For this work we used the residues (squeeze cakes) obtained from pore water sampling. Samples were analyzed for bulk parameters (total inorganic carbon, total organic carbon, and S) and by X-ray fluorescence for major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and selected minor (As, Ba, Co, Cr, Cu, Mo, Ni, Pb, Rb, Sr, U, V, Y, Zn, andZr) elements. Inductively coupled plasma mass spectrometry analyses for rare earth elements (REEs) were performed on acid digestions of the squeeze cake samples from Site 1258.The major element composition is governed by the mixture of a terrigenous detrital component of roughly average shale (AS) compositionwith biogenous carbonate and silica. The composition of the terrigenous detritus is close to AS in Units II–IV. For Unit I, a more weathered terrigenous source is suggested. Carbonate contents reach >60 wt% onaverage in chalks and calcareous claystones of Units II–IV. The SiO2 contribution in excess of the normal terrigenous detrital background indicates the presence of biogenous silica, with highest amounts in Units II and III. The contents of coarse grained material (quartz) are enhanced in Unit V, where Ti and Zr contents are also high. This indicates a high-energy depositional environment. REE patterns are generally similar to AS. A more pronounced negative Ce anomaly in Unit IV may indicate low oxygen conditions in the water column. The Cretaceous blackshales of Unit IV are clearly enriched in redox-sensitive and stable sulfide forming elements (Mo, V, Zn, and As). High phosphate content spoint toward enhanced nutrient supply and high bioproductivity. Ba/Al ratios are rather high throughout Unit IV despite the absence of sulfate in the pore water, indicating elevated primary production. Manganese contents are extremely low for most of the interval studied. Such an Mn depletion is only possible in an environment where Mn was mobilizedand transported into an expanded oxygen minimum zone (“open system”). The sulfur contents show a complete sulfidation of the reactiveiron of Unit IV and a significant excess of sulfur relative to that of iron, which indicates that part of the sulfur was incorporated into organicmatter. We suppose extreme paleoenvironmental conditions during black shale deposition: high bioproductivity like in recent coastal up-welling settings together with severe oxygen depletion if not presence of hydrogen sulfide in the water column.
