A high-resolution geochemical investigation of the OAE 3 (Demerara Rise); new insights into the behaviour of trace metals and phosphorus

Author(s): Maerz, C.; Beckmann, B.; Kasten, S.; Kuester, K.; Wagner, T.; de Lange, G. J.
Author Affiliation(s): Primary:
Bremen University, Department of Geosciences, Bremen, Federal Republic of Germany
Other:
Alfred Wegener Institute for Polar and Marine Research, Federal Republic of Germany
University of Newcastle, United Kingdom
Utrecht University, Netherlands
Volume Title: AGU 2006 fall meeting
Source: Eos, Transactions, American Geophysical Union, 87( Fall Meeting Suppl.); American Geophysical Union 2006 fall meeting, San Francisco, CA, Dec. 11-15, 2006. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0096-3941 CODEN: EOSTAJ
Note: In English
Summary: Mid-Cretaceous Oceanic Anoxic Events (OAEs) are amongst the most prominent examples of wide-spread anoxia in Earth history. Investigating their causes and consequences was one of the major aims of ODP Leg 207, drilling Sites 1257-1261 on the Demerara Rise off the coast of Suriname. Here we present first results of geochemical high-resolution studies of OAE 3 that were performed on a 120 cm long black shale interval from Site 1261. The analytical spectrum included TOC, IC and Stot determination (Leco CS 200) as well as total acid digestion of sample splits and subsequent ICP-AES measurement of major and minor elements. A pronounced cyclicity pattern is documented in most parameters, consisting of 4 cycles in the investigated interval. Mean elemental enrichment factors are consistent with those reported from other anoxic sediments, especially OAEs, indicating a similar mode of formation. Redox-sensitive and/or sulphide-forming elements (e.g. Ni, V, Zn) and elements linked to organic matter deposition (e.g. Ba, Cu, P) are (highly) enriched. However, a closer look at the concentration-depth profiles of some elements (or element/Al ratios) reveals pronounced deviations from other, comparable black shale successions. Most obvious are the distributions of V/Al and Zn/Al over depth: In most studies of anoxic sediments, they show a good correlation with TOC or Stot, which is not the case in the investigated interval of OAE 3. We rather observe an anti-correlation, as highest V/Al and Zn/Al values are paralleled by lowest TOC contents. Calculation on a carbonate-free basis (CFB) does not alter this fact. We therefore conclude that the potential primary trace metal-organic matter relationship was somehow altered after sediment deposition. A similar, even more extreme pattern is observed for phosphorus. In more or less regular intervals, phosphorus contents rise rapidly around TOC maxima to peak values of several wt%, then drop again to very low background values. When calculated on a CFB, however, phosphorus peaks fall together with TOC minima. We applied a sequential phosphate extraction procedure to identify in which phase phosphorus is present in the sediments.
Year of Publication: 2006
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Anaerobic environment; Black shale; Chemical composition; Clastic rocks; Cretaceous; Demerara Rise; Depositional environment; High-resolution methods; Leg 207; Lithofacies; Mesozoic; Middle Cretaceous; ODP Site 1257; ODP Site 1261; Ocean Drilling Program; Organic compounds; Phosphorus; Sedimentary rocks; South America; Surinam; Total organic carbon; Trace metals
Coordinates: N092700 N092700 W0542100 W0542100
N090300 N090300 W0541900 W0541900
Record ID: 2009021085
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