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doi:10.2204/iodp.proc.324.204.2013

Results and discussion

As XRF count rates for different elements are influenced by different factors, such as sample matrix, surface topography, or porosity, not accounted for in the postprocessing of the data derived from the XRF scanner, direct count rates of individual elements can only be compared with caution. This is especially the case when comparing the count rates of elements that are far apart in the periodic system of elements (e.g., light versus heavy elements). Based on these predefined conditions, the data interpretation in this contribution is entirely based on element ratios. As the cored sediments consist of limestone and volcaniclastic material, which basically represents a three-component mixture of (1) fresh volcanic glass, (2) altered volcanic glass to mainly smectite, and (3) biogenic and micritic carbonate, Si, Al, Fe, Mn, K, and Cl count rates have been normalized to Ca count rates. Because the volcaniclastic material furthermore represents a mixture of fresh and altered glass, Al, Fe, Mn, Ca, K, and Cl count rates have been normalized to the Si count rates. For better visualization of the geochemical patterns, the element ratios displayed in Figure F3 were smoothed by a two-point moving average. Averages of the different ratios for each individual core were calculated (Fig. F4), with the error bars representing the calculated variability (variance) characterizing each core.

Generally, two different systematic patterns in Si/Ca, Al/Ca, Fe/Ca, Mn/Ca, K/Ca, and Cl/Ca were observed in the investigated core material. One trend is a rhythmic pattern of increasing and decreasing element ratios with peak widths being variable throughout the entire core material (Fig. F3). The other one is a consistent downhole pattern of constantly decreasing/increasing element ratios superimposed on these rhythmic patterns (Figs. F3, F4). The latter is most prominent in the uppermost 110 m depth interval and is characterized by a steep increase in the Al/Ca, Fe/Ca, Mn/Ca, K/Ca, and Cl/Ca ratios (Figs. F3, F4). Below this depth, no clear element depth correlation can be observed and the average element ratios stay more or less constant throughout the remaining depth interval. The rhythmic patterns observed throughout the entire investigated core material are characterized by positive co-variations of Si/Ca, Al/Ca, Fe/Ca, Mn/Ca, K/Ca, and Cl/Ca with variable amplitudes and peak widths. Occasionally, the Cl/Ca ratio is anticorrelated to the Al/Ca, Fe/Ca, Mn/Ca, and K/Ca ratios.

The sediment investigated in this study is characterized by two main lithostratigraphic sequences, a sequence of marine limestone with intercalated chert and a roughly 110 m thick volcaniclastic sediment sequence with the limestone being deposited on top of the volcaniclastic material. The geochemical downhole trends of increasing Si/Ca, Al/Ca, Fe/Ca, Mn/Ca, K/Ca, and Cl/Ca clearly reflect this change in the principal lithology (Figs. F3, F4).

The cored volcaniclastic material is characterized, as already described above, by alternating layers of variable grain size, which are consistent with varying mixtures of predominantly altered glass, fresh glass, and biogenic as well as micritic carbonate and occasional intercalated thin layers containing higher abundances of radiolarians (Fig. F3). Geochemically, the mixture of carbonaceous material with the largely altered volcaniclastic material can be visualized as a dilution of the geochemical signal coming solely from the volcanic material. Because one key effect of submarine glass alteration to smectite is the nearly total loss of Ca (e.g., Stroncik and Schmincke, 2002), the observed Si/Ca, Al/Ca, Fe/Ca, Mn/Ca, K/Ca, and Cl/Ca records show the existing variations in the ratio of volcaniclastic material to carbonaceous material. The observed rhythmic patterns of Si/Ca, Al/Ca, Fe/Ca, Mn/Ca, K/Ca, and Cl/Ca correlate with the alternating layers of variable grain sizes observed in the core. An example is the medium grained, homogeneous-looking sediment characterizing the mixture of higher amounts of carbonate with volcanic material that corresponds to lows in the specific element ratios. On the other hand, the fine-grained material, being dominated by altered volcanic glass, corresponds to highs in the specific element ratios (Fig. F3). The higher amount of carbonaceous material, as well as the larger grain size in the medium-sized volcaniclastics, is most likely an indication for a shallower water hyaloclastite emplacement compared to the fine-grained material containing only minor amounts of carbonate.

In Figure F5, Al/Si, Fe/Si, Mn/Si, K/Si, and Cl/Si ratios are plotted versus Si. Even though the data show relatively large scatter, it is evident that Fe/Si, K/Si, and Cl/Si show negative trends with increasing Si content and that Al/Si shows positive trends with Si content in individual cores. This indicates that the geochemical signal introduced by the glass alteration process is superimposed on the “carbonate-volcaniclastic mixing signal” because alteration of basaltic glass to smectite results in a relative enrichment of Fe, K, and Cl and a relative depletion of Si and Al (e.g., Stroncik and Schmincke, 2002).

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