Marine clastic sediments

The first modem publication of Li isotope data from sediments came from Chan et al. (1994a), who reported the isotopic compositions of unaltered turbidites from DSDP Hole 477 and All A, from the Gulf of California. These samples have been reanalyzed (LH Chan, written communication 2003), yielding lighter values. Similarly, Chan and Frey (2003) suggested a range for marine clastic sediments (5 Li = -1 to +5.6) based on new data and re-analysis of samples previously considered to be isotopicaUy heavy. 

Siliciclastics are composed of various silicate grains. Carbonates consist mainly of only the two minerals dolomite and calcite. Clastic sediments have been transported over long distances, whereas carbonates are formed on-site (mostly marine). Clastic sediments are relatively chemically stable they form an intergranular pore space. 

Lithium isotope studies of sediments and sedimentary rocks have thus far concentrated on marine clastic and carbonate material. No systematic description of the effects of diagenetic processes on sediments has been made. Clay rich sediments are important to Li budgets in near-surface systems, as they concentrate Li relative to marine carbonates, which are among... 

Figure 15. Diagram showing the major components of the global calcium cycle with b Ca values (denoted as 5). The modem residence time of Ca in the oceans is about 1 million years (Holland 1978 1984). Abbreviations used are SW = seawater, Sed = sedimentation, clastic = clastic sediments, carb = marine carbonate sediments, hydrol = mid-ocean ridge hydrothermal systems, lith = continental lithosphere.

Schwarcz and Bumie (1973) reviewed sulfur isotope abundances in strata-bound sulfide deposits in clastic sediments not associated with volcanic rocks and concluded that two patterns were evident. One was a broad distribution ranging from arotmd the 5 S of sea water to values 25%o lower while the other was a narrow distribution around a 5 8 of —50%o with respect to oceanic sulfate. The former pattern was identified with shallow marine or brackish-water environments while the latter occurred in deep, euxinic basins. They explained the first distribution on the basis of Fig. 6.2.4a and assumed a closed system with an average 32/ 34 ratio of about 1.025. The second distribution pattern applied to deep basins and Schwarcz and Bumie (1973) concluded that the systems were fully open, in which case the isotopic selectivity was much larger as is the case in a number pf modem euxinic basins (e.g. Black Sea see pp. 338, 412). 

The scatter diagram of TS vs. TOC illustrates the S/C ratios of our samples (Fig. 7). Although the study area is a normal marine environment Le. clastic sediments overlain by oxic waters of typical oceanic salinity), most of the S/C ratios in the sediments are considerably lower than the average ratio of 1/2.8 obtained for reduced sulfur and organic carbon in sediments beneath oxygenated seawater (Berner, 1982). By contrast, the S/C ratios in our samples are mostly lower than 1/10, typically occurring in fresh water environments. Similarly, low S/C ratios have been observed in Amazon inner shelf muds (Alter et al, 1986 Alter and Blair, 1996). The authors attribute the low ratios to the oxidation power of iron oxides and reworking of sediments. Unlike the Amazon case, our samples are mostly from the slope and submarine canyons rather than the shelf and, therefore, deserve further discussion. 

Pickering, K.T., Hiscott, R.N., and Hein, F.J. 1989. Deep Marine Environments Clastic Sedimentation and Tectonics. Unwin Hyman, London, 416pp. 

Silts are clastic sediments derived from pre-existing rocks, chiefly by mechanical breakdown processes. They are composed mainly of fine quartz material. Silts may occur in residual soils, but they are not important in such instances. However, silts are commonly found in alluvial, lacustrine, fluvio-glacial and marine deposits. These silts tend to interdigitate with deposits of sand and clay. Silts are also present with sands and clays in estuarine and deltaic sediments. Lacustrine silts are often banded. Marine silts may also be banded. Wind-blown silts are generally uniformly sorted. 

Clays are clastic sediments, i. e., they consist mainly of the remains of pre-existing rocks which have been broken down by weathering and/or erosion. The clay minerals are present in the form of very small particles (<0.002 mm) which have been deposited mainly in water — fresh, brackish or marine. Geologically the clays, along with shales, marls, etc., are classed as argillaceous rocks. The term "clay" is more especially reserved for material which has no pronounced bedding planes and which forms a plastic mass when wet. The principal constituents are the clay... 

The matrices and sources of the sediments listed in Table 4.2 are sometimes unclear. Those that are known are highly weighted toward clastic (quartz- and aluminosilicate-rich) marine sediments from coastal environments. Some of these reference materials, such as MESS-3 (NRC-Canada), MAG-1 (USGS) and the Arabian Sea and Pacific Ocean samples (IAEA 315, and 368), could provide excellent examples of clastic marine sediment representing the main repositories of organic matter in the ocean (Hedges and Keil, 1995). The listed materials fail to include both open-ocean opal and carbonate oozes, as well as pelagic red clays. 

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