Trace Element Variations in a Series of Sediment Cores 318
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TRACE ELEMENT VARIATIONS IN A SERIES OF SEDIMENT CORES 318. Figure 27. Variation of a range of elements with depth along a series of free fall and gravity cores. 319. F ee Fall Core No0 320. Free Fall Core No8 500 25... 321, Free Fall Core No 9 i i 1.1,1 Lt.! 5 w I 1100 laiam. Fe x01 Mn 322. Free Fall Core No14. 010 20 ‘0 140 290 590 topo 4,00 50p0 10,000 13.13.1111. Cu Ni Fex01 Mn - 21.5 a) a 10 1 20 25 323. Free Fall Core No 15 p.p.m. 3 2 4 . Free Fal I Core No 18 590 tgoo 2,goo___.5,900 Woo 13.13.m. Cup Ni Fe x0.1 Mn 20 25- 30- 325. Free Fall Core No 20 00 p.p.m. 326. Gravity Core , STN. 6274 5111 000 P.P.m. 30- 40- 50- 60- 70- 8 90- 327. Landergren (1969). In general, Mn shows an antipathetic relation- ship to Fe but this is not always well defined. The variation of the CaCO content within sediment cores (table 16) appears to be 3 too small to influence the trace element distribution within cores but the CaCO content of cores from different locations appear to 3 vary markedly indicating the influence of the sedimentary environ- ment on the CaCO content of the sediment. 3 The most significant feature of the data is the lack of significant enrichment of manganese in the upper layer of any of the free fall cores compared with the data for gravity core 6274 where manganese enrichment in the upper 10" of the core is more marked. Whilst this data w ,uld appear to indicate the absence of any significant migration of manganese in deep sea sediments in agreement with the findings of Chester and Hughes (1969), it may merely reflect the loss of the upper layers of sediment during free fall coring and for this reason no unambiguous interpretation of data can be given. In view of the high redox potential of the upper 2 ft of sediment (table 19) which in general exceeds +300 mV, and the absence of any well developed redox gradient in the sedi- ment, it is tentatively suggested that manganese remobilisation is not occurring on any significant scale in the upper 3 ft of sediment cores from area 4c. The significance of the erratic distribution of trace elements within the sediment cores is more difficult to define and can probably be interpreted only in terms of a complete study of the trace element partition between several phases within the sediment. Microscopic investigation of the acid leach residue of the sediment at intervals along the length of the core revealed small quantities of fragmented manganese oxides in the size range 50-100 µ which vary considerably in size and concentration down 328. the length of the core. Similar grains have been reported in ponded sediment from the North Atlantic (van Andel and Komer, 1969), in sediment cores from the Central Atlantic (Archer, pers. comm.) and in sediments from the Puerto Rico Trench (Conolly and Ewing, 1967). Van Andel (pers. comm.) reports that, although rounded micronodules have been observed in abundance in sediment cores from the Pacific, those from the Atlantic are "angular with broken edges and often rather spongy, botryoidal or con- sisting of numerous coagulating small spheres identical with the nattre of the crust on adjacent rocks". Attempts to separate these fragments for analysis by handpicking or using heavy liquids proved unsatisfactory because of the fine grain size of the parti- cles and their low concentration in the sediment. Selective methods of dissolution were not attempted because of the finding of Chester and Hughes (1967) that carbonates and adsorbed elements were also taken into solution under the conditions required for the dissolution of manganese oxides. For this reason,no quantitative estimate of the contribution of these particles to the variability of trace element distribution along the sediment core could be made. Particle counting techniques did, however, fail to reveal any correlation between the particle distribution in the sediment and the manganese content suggesting that other factors may influence the trace element variation in sediments. The origin of these granules in the sediment cores is not well understood but the fragmentary nature of the granules and the absence of concentric growth patterns within the granules suggests that these particles are not micronodules as suggested by Chester and Hughes (1969). This conclusion is supported by the fact that dissolution of the outer coating of the granules in 507: HC1 fails to reveal the presence of any nucleus on which the manganese coating could accrete. In addition, the distribution 329. of fragments within the sediment column shows no evidence of manganese migration since the fragments do not appear to be enriched in the upper layers of the cores and show no similarity with the sharp bands of micronodules and manganese spots described by Wangersky and Joensuu (1967) or the fine grained, opaque granules of manganese minerals formed by the precipitation of remobilised manganese and described by Lynn and Bonatti (1965). By'contrast, the fragments display irregular surfaces similar to ground frag- ments of granular manganese crusts suggesting a fragmentary origin for these particles and indicating that the independent growth of these granules below the sediment-water interface does not occur. To illustrate possible differences in the origin of these fragments, a comparison of the texture of these fragments from area 4c and from core CUSP 15P from the North Pacific is presented in fig. 28. Two possibilities may be invoked to account for the occur- rence of these granules. The first is that the crustal fragments are dispersed down the sediment core on impact of the corer with overlying manganese crustal material. This hypothesis is rejected in view of the low probability of contact of the corer with a manganese crust at the sediment surface. The second is that the granules result from the fragmentation of granular crusts on the sca floor during normal erosional processes and the particles become distributed on the sediment surface by bottom currents activity. Whilst this hypothesis is unsatisfactory in many respects, recent work by Riedel and Funnel (1964), Laughton (1967) and Ericson et al (1961) has shown the importance of erosional processes on the deep sea floor and it is not inconceivable the sediment slumping, bottom current activity or even the influence of benthonic organisms could lead to this type of fragmentation. Wiseman (pers. comm.) has shown the author nodules from the Pacific Ocean in which fragmentation has occurred along certain cleavage planes ILLUSTRATION OF MANGANESE GRANULES 330. Figure 28a. Photograph of manganese granules from area 4c sediment core. Magnification 40 x. Figure 28b. Photograph of manganese micronodules from core CUSP 15P from the North Pacific. Magnification 15 x. Specimen donated' by Dr. R. Chester. 3310. 0 332. and manganese has subsequently been deposited on the cleaved surface, suggesting that fragmentation of manganese nodules can occur spontaneously within the marine environment. Whilst discussion of the transport of fine grained particles has been mainly restricted to tidal environments (Inman 1949, Menard 1950, Bagnold 1963, 1968, Hunt 1967), the observation of bottom current velocities of the order of 9-18 cms./sec. by Swallow and Worthington (1961) and the suggestion of Heezen et al (1966) that geostrophic bottom currents are the principle agents controlling the shape of the continental rise indicates that the lateral transport of fine grained material may occur in a pelagic environment, possibly by turbulent suspension. A recent discussion of this problem has been presented by van Andel and Komar (1969). The antipathetic relation of Mn and Fe in the sediment core and the marked enrichment of Fe relative to Mn in the sediment compared with manganese nodules indicates that iron must be present in the sediment predominantly in some phase other than manganiferous granules, possibly in the clay mineral fraction. This distribution of iron between several phases is in agreement with the findings of Chester and Hughes (1967) that only 1.5% of the total iron is present in the form of micronodules and 95% in clay minerals. The correlation of Ni and Mn suggests that these elements may be associated together in the manganiferous phase, although s•me partitioning between phases may occur. This agrees with the conclusion of Chester and Hughes (1967) that Ni is partitioned equally between manganiferous micronodules and clay minerals in a North Pacific deep sea clay core and there appears to be no evidence that Ni is incorporated exclusively into the Mn02 phase of deep sea sediments as suggested by Carvajal and Landergren (1969), The poor correlation of Cu with Mn and the enrichment of Cu relative to Mn in the sediment suggest that Cu is also partitioned between several 333. phases. The problem is, however, complicated since the manganese fragments may well be thermodynamically metastable with respect to dissolution below the sediment-water interface and compositional changes of these fragments may therefore be occuring within the sediment due to preferential dissolution of the adsorbed species on the surface of the granules. A strict comparison of the composition of these fragments with that of nodules may therefore be invalid. It is significant in this context, however, that Cronan and Tooms (1969) have shown that sub-surface nodules have a composition similar to that of surface nodules from the same region. From the preceeding discussion it is suggested that manganese crustal fragments play an important role in influencing the trace element distribution in pelagic sediments from area 4c and it is tentatively concluded that these fragments are not forming as micronodules below the sediment-water interface.