«Nr.82 Kasten, S. EARLY DIAGENETIC METAL ENRICHMENTS IN MARINE SEDIMENTS AS DOCUMENTS OF NONSTEADY-STATE DEPOSITIONAL CONDITIONS Berichte, Fachbereich ...»
aus dem Fachbereich Geowissenschaften
der Universität Bremen
EARLY DIAGENETIC METAL ENRICHMENTS
IN MARINE SEDIMENTS AS DOCUMENTS OF
NONSTEADY-STATE DEPOSITIONAL CONDITIONS
Berichte, Fachbereich Geowissenschaften, Universität Bremen, Nr. 82
118 S., 30 Abb., 14 Tab., Bremen 1996
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Early diagenetic meta! enrichments in marine sediments as documents of nonsteady-state depositional conditions.
Berichte, Fachbereich Geowissenschaften, Universität Bremen, Nr. 82, 118 S., Bremen 1996.
ISSN 0931-0800 Early diagenetic metal enrichments in marine sediments as documents of nonsteady-state depositional conditions Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften im Fachbereich 5 der Universität Bremen vorgelegt von Sabine Kasten Bremen Tag des Kolloquiums: 27. Juni 1996
Prof. Dr. H.D. Schulz Prof. Dr. G. Wefer
Prof. Dr. V. Spieß Prof. Dr. H. Villinger Prcface This study was conducted as part of the Special Research Project (Sonderforschungsbereich, SFB) 261 "The South Atlantic in the Late Quaternary: Reconstruction of Material Budget and Current Systems" at the University of Bremen funded by the German Research Foundation (Deutsche Forschungsgemeinschaft). This work is submitted as a dissertation and has been supervised by Prof. Dr. Horst D. Schulz within the frame of the subproject TP A2 "Chemical processes and fluxes in sediment/porewater systems".
The thesis consists of foul' separate studies which have either been published or submitted for publication in international journals. Their thematic context as weIl as a general introduetion into the subject of the geoehemieal behaviour of metals in marine sediments is outlined in an introduetory chapter. A summary at the end of this workpresents the main results. For the purpose of a homogeneous appearance of the whole work also these two frame ehapters are written in English.
My own contribution to the publieation on "Solid-phase manganese in Southeast Atlantic sediments: Implieations for the paleoenvironment" (manuseript No. (Il)) eonsists of analytieal work (seleetive and bulk dissolution procedures and subsequent analysis), data proeessing and authorship. The remaining three manuseripts are based primarily on my own investigations whieh have been supplemented by data from other groups within the SFB 261. Manuseript No. (I) on "Rare earth elements in manganese nodules from the South Atlantic Oeean as indicators of oceanic bottom water flow" was realized in dose cooperation with Dr. Geoffrey P. Glasby. Mineralogical data were provided by Prof. Dr.
Günther Friedrieh; Prof. Dr. S.l. Andreev was added to the list of authors for helpful comments and diseussions and for providing aceess to Russian data on manganese nodules from the South Atlantie Oeean. The seleetive dissolution data presented in manuscript No.
(III) ("Simultaneous formation of iron-rich layers in sediments of the Amazon Deep-Sea Fan") are derived from an unpublished exam study of Tim FreudenthaI. The contributions of the other eo-authors to this paper eonsist of mineralogieal analyses (Dr. Franz X.
Gingeie) and data on geophysieal properties, description of the geophysieal methods used, and interpretation of the results (Dr. Tilo von Dobeneck). Manuseript No. (IV) ("Diserepaney between barium and biogenie constituents in sediments of the Ceani Rise and Sierra Leone Rise - effeet of nonsteady-state diagenesis?") eontains additional data provided by Ralf Haese and Dr. Matthias Zabel and uses published data by Dr. Carsten Rühlemann and Dr. Stefan Mulitza.
Table of contents
CHAPTERIIntroduction The early diagenetic processes occuning in sediments accumulating at a steady-state - i.e.
under depositional conditions constant with time - have been weIl studied (e.g. Berner, 1980). This simplif)ring assumption may not be true for real sedimentary systems in which periodically changing environmental conditions are common (e.g. Passier et al., 1996).
During the transition from one depositional environment to another or at the interface between the two sediment types nonsteady-state diagenetic processes are initiated which have a high potential for modif)ring the sedimentmy record. It is mainly during such transitions that pronounced solid phase signals which can be potentially preserved in the sedimentary record may form. Metal-rich layers are typical features of sediments deposited under nonsteady-state conditions which are often climatically induced. The present study deals with the formation of these kind of solid phase emichments in different diagenetic environments - focussing on the redistribution of manganese and iron. In addition the formation and preservation of trace metal signals and their use as paleoceanographic indicators is considered.
The study of metal behaviour m deep-sea sediments is as old as the discovery of manganese nodules during the H.M.S. Challenger Expedition from 1872 to 1876 (Murray and Renard, 1891). Research on manganese nodules between 1960 and 1980 primarily focussed on their economic potential as a source ofNi, Cu and Co. Currently, the exploit of manganese nodules is not considered to be economic. Today research related to manganese nodules concentrates more on the assessment of possible environmental implications of deep-sea mining, e.g. the international DISCOL project (Thiel, 1991) and the German TUSCH Research group (TUSCH Research Group, 1991). A second important aspect is the use of fenomanganese concretions in pa1eoceanography. In contrast to Mn and Fe phases, which are buried in suboxic and anoxic sediments and are subject to mobilization by reduction, the Mn and Fe oxyhydroxides, which ferromanganese nodules and crusts consist of, are assumed to be weIl preserved under oxic conditions at the sediment/water interface. Thus they represent valuable archives of oceanic conditions since their time of formation. Due to their slow growth rates of a few millimeters per million years (Dymond et al., 1984; Mangini, 1988), fenomanganese nodules and crusts can be considered as condensed stratigraphic sections that recorded oceanographic and geological conditions of the surrounding environment during accretion. The particular Fe and Mn mineral phases as weIl as the associated trace metals provide information on the changes in seawater composition and bottom water conditions with time (e.g. Segl et al., 1989; Mangini et al., 1990; Hein et al., 1992; McMurtry et al., 1994; see manuscript (I), chapter 2).
The distributions of trace metals in sediments have been studied for various purposes.
These are either related to using meta1concentrations in sediments as a primary signal - i.e.
to trace the input flux of a particular metal into the sediment - 01' to use them as proxy indieators to reconstruct the likely environment of deposition of a sediment section on account of the redox sensitivity of specific metals.
Examples for the use of metals in relation to their primary input into the sediments are (l) evaluation of hydrothermal activity from elements like U and Mo (e.g. Turekian and Bertine, 1971), (2) reconstruetion of glacial/interglacial ditferences in wind intensities and the position of wind systems from iron minerals which have been supplied to the sediment via eolian transport (e.g. Balsam et a1., 1995),01' (3) estimates of paleoproductivity from sedimentary Ba contents (e.g. Dymond et a1., 1992; Gingeie and Dahmke, 1994; Nürnberg, 1995; see manuscript (IV), chapter 5).
Due to the characteristics of many metals to respond to changes in redox conditions, the investigation of their distribution in the marine environment has gained much attention in relation to the formation of organic-rich black shales, as they are generally enriched by a number of trace metals. There is a substantial debate on the environment of formation of these deposits. One explanation is that bottom water anoxia led to enhanced preservation of deposited organic matter (e.g. Demaison and Moore, 1980). An alternative explanation could consist in an increased primary plankton production in the surface waters of the ocean which resulted in an increased settling and burial flux of organic matter (e.g. Calvert and Pedersen, 1993). The elucidation of the mechanisms on how transition metals are trapped in such deposits would not only help to explain their formation, but could also provide a valuable tool to assess whether sediments were deposited under oxic or anoxie bottom waters. Thus, the distribution and concentration of some trace metals in sediments could be used as a proxy indicator to examine the areal extent of anoxie environments - i.e.
bottom water anoxia, reducing sediments - in the geological past (Jacobs et al., 1987;
Francois, 1988; Calvert and Pedersen, 1993). Arecent attempt to reconstruct changes in the areal extent of reducing sediments has been undertaken by Hastings (1994) using the concentration of vanadium in foraminiferal calcite. Rosenthai et al. (1995) studied the possible influence of an expanded area of reducing sediments in glacial times in changing the oceanic Cd inventory, thereby decoupling Cd from P0 4, from the concentrations of authigenic Cd and U in glacial sediments.
All of the above described attempts that use metal concentrations in sediments as indicators to trace the primary input flux or to reconstruct the redox conditions during deposition of sediments, require a clear understanding of the early diagenesis of metals in different depositional environments. There is a vast amount of literature on the perturbation of the primary metal record during early diagenesis of organic matter. Recent studies have shown that besides the destruction of primary sedimentary signals by early diagenesis, distinct enrichments of numerous metals can be produced by post-depositional processes.
I~speciallynonsteady-state diagenesis has a high potential for producing and preserving metal accumulations in sediments (e.g. Oe Lange et al., 1994; Passier et al., 1996; see manuscripts (I1), (III) and (IV) in chapters 3 to 5). There is promising evidence that - if the processes which are active during nonsteady-state phases are fully understood - the metal enrichments themselves could be used as valuable 1I1dicators for reconstructing the depositional history. Therefore in order to model nonsteady-state accumulation of metals, their cyc1ing under these conditions has to be weIl understood.
The following parts of this introductory chapter give an overview on the supply of metals to the oceans, their transport to the sea-floor and the formation mechanisms of metal-rich layers in sediments.
Marine geochemistry of the metals examined in this study Like other elements, metals are supplied to the oceans from three major sources. These comprise (1) river input in both dissolved and particulate forms; (2) atmospheric input mainly occurring via rain that washes substances out of the atmosphere (e.g. eolian dust);
(3) hydrothermal activity that introduces material by the interaction of sea-water with newly forrned oceanic crustal basalt at ridge-crest spreading centres. This may occur via both high temperature hydrothermal activity and low temperature interaction with newly formed crust (Bruland, 1983).
The transport of trace metals to the sediments is strongly controlled by the abundance, production, sinking and decomposition of patticulate matter. Besides terrigenous material originating from river and atmospheric input, partic1es are produced primarily by phytoplankton in the surface water of the oceans. Together with the nutrients, metals are taken up actively or passively by these primary producers and are incorporated into their cells.