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doi:10.2204/iodp.proc.340.207.2016

Results

Pore fluids

Some pore fluid results from the expedition have been presented previously (see the “Expedition 340 summary” chapter [Expedition 340 Scientists, 2013a]), and this report contributes additional data as discussed above. Both the major (see the “Expedition 340 summary” chapter [Expedition 340 Scientists, 2013a]) and minor element data (Table T1; Figs. F2, F3, F4, F5, F6, F7) show a variety of distributions that are likely influenced by a mixture of reactions that include organic carbon decomposition, carbonate dissolution and precipitation reactions, and volcanogenic sediment diagenesis. Ca and Mg are two of the more diagnostic elements for differentiating among these processes. For example, data from Site U1396 demonstrates Ca increases that mirror the Mg decreases (Fig. F4), which is quite likely driven by exchange reactions during diagenesis of volcanic glass (see the “Expedition 340 summary” chapter [Expedition 340 Scientists, 2013a]). By contrast, Site U1399 shows a decrease in Ca and Mg, which is likely a result, at least in part, of carbonate precipitation reactions at depth (Fig. F5). Sulfate varies from being quite close to the seawater value (~28 mM) throughout the profile (Site U1396; Fig. F4) to being depleted to ~10 mM at Site U1394 (Fig. F2). Manganese exhibits a variety of behaviors likely reflecting organic carbon oxidation reactions as well as variations in sedimentary reactive Mn content. The other elements, Si, B, Li, Sr, Rb, and Cs, all show a similar diversity in behaviors that are likely driven by these same or similar diagenetic reactions involving volcanic material, carbonate, biogenic silica, and organic matter (Figs. F2, F3, F4, F5, F6).

Whole-sediment digestion results

Whole-sediment digestion results are presented in Table T2. These results are for selected samples from Sites U1395, U1396, U1399, and U1400 and are not plotted separately, as there are too few data to construct meaningful interpretations. However, results do show considerable variability, which is consistent with the highly variable mixture of hemipelagic muds, turbidites, and volcaniclastic sediments.

Dithionite extraction results

Whole-core distributions

Dithionite-extractable iron (Fe_R) and manganese (Mn_R) as well as organic carbon (OC) generally have lower concentrations at the northern sites (Figs. F7, F8, F9) with highly variable CaCO₃ (0–81 wt%) as compared to the southern sites, which show more variation in their Fe_R, Mn_R, and OC concentrations with less variation in their CaCO₃ contents (Figs. F10, F11; Table T4).

At Site U1394, Fe_R, Mn_R, and OC have relatively low concentrations throughout the record. In the uppermost 25 m of sediment, these three components have average values of 0.39 ± 0.09 wt%, 0.03 ± 0.01 wt%, and 0.19 ± 0.1 wt%, respectively. Carbonate varies from 5 to 75 wt% within the uppermost 22 m of sediment. An ~87 m region produced no data, which reflects the fact that fine-grained material recovery within this interval was generally poor. From 111 to 197 meters below seafloor (mbsf), Fe_R and Mn_R are relatively constant, averaging 0.37 ± 0.03 wt% and 0.03 ± 0.01 wt%, respectively (Fig. F7A, F7B). Organic carbon averages 0.23 ± 0.1 wt% (Fig. F7C), and carbonate shows less variability in its concentration (55–60 wt%) compared to the upper 22 mbsf, with the exception of three low-concentration samples (Fig. F7D).

Some of the solid-phase concentrations at Site U1395 show a wider range in concentration compared to Site U1394. Fe_R, in particular, ranges from 0.18 to 0.75 wt% throughout the core with more variability below 60 mbsf (Fig. F8A). In contrast, Mn_R is particularly low, averaging 0.02 ± 0.02 wt%, with the exception of a single data point (0.09 wt%) at ~100 mbsf (Fig. F8B). Organic carbon averages 0.13 ± 0.08 wt% (Fig. F8C). Calcium carbonate varies between 0 and 76 wt%, and at ~169 mbsf, CaCO₃ decreases from 81 to 43 wt% (Fig. F8D).

Site U1396 solid phases, with the exception of CaCO₃, are relatively constant with depth (Fig. F9). Fe_R averages 0.35 ± 0.06 wt% with depth, with the highest sample at 0.48 wt% (Fig. F9A). Mn_R and OC are both uniformly low, averaging 0.02 ± 0.01 wt% and 0.13 ± 0.06 wt%, respectively (Fig. F9B, F9C). As is the case for Site U1395, calcium carbonate values show large variability ranging from 0 to 74 wt% throughout the core.

Site U1399 solid-phase data exhibit more variability in Fe_R, Mn_R, and OC with depth compared to Sites U1394–U1396 (Fig. F10). Fe_R has greater variability (0.46–1.4 wt%) in the uppermost 75 m of the core compared to the lowermost ~179 m of the core (0.42–1.06 wt%) (Fig. F10A). Mn_R is highly variable compared to the northern cores and has an average of 0.065 ± 0.046 wt% (Fig. F10B). Organic carbon ranges from 0.01 to 0.61 wt% with an average of 0.28 ± 0.15 wt% (Fig. F10C). CaCO₃ ranges from 2.5 to 36 wt% with depth, showing less variation compared to the northern sites (Fig. F10D).

Site U1400 solid-phase data are similar to those of Site U1399, with Fe_R, Mn_R, and, to a lesser extent, OC exhibiting more variation with depth and less variability in CaCO₃ compared to the northern sites (Fig. F11). Reactive Fe ranges from 0.4 to 1.4 wt% with an average of 0.8 ± 0.3 wt% (Fig. F11A). Mn_R values exhibit considerable scatter throughout the core, with an average value of 0.068 ± 0.037 wt% (Fig. F11B). Organic carbon shows a slight decrease to 200 mbsf and then increases with depth. However, OC is less abundant at Site U1400 compared to Site U1399, ranging from 0.1 to 0.4 wt% with an average of 0.23 ± 0.08 wt% (Fig. F11C). Calcium carbonate ranges from 2 to 35 wt% and demonstrates increases at approximately the same depths as Mn_R (Fig. F11D).

Discrete intervals

In addition to the results presented above, we analyzed samples from Sites U1399 and U1400 from focused or discrete sedimentary intervals (Table T5). These intervals were chosen based on abrupt lithology transitions. A total of nine different sections (5–6 samples in each section) were chosen, six from Site U1399 and three from Site U1400. The different sections include (in order of increasing depth) Sections 340-U1399B-3H-3, 7H-3, 11H-2, 16H-2, 19H-2, and 25H-2 and Sections 340-U1400B-16H-2, 26H-5, and 48X-1. Solid-phase data and visual lithology descriptions can be found in Table T5, and the results are plotted on Figures F12, F13, F14, F15, F16, F17, F18, F19, and F20. The general observation is that these changes in lithology often coincide with changes in reactive Fe and Mn as well as with OC and carbonate. Although organic carbon concentrations are generally low, they are typical for pelagic sedimentary environments (e.g., Ziebis et al., 2012). Concentrations of OC, Fe_R, and Mn_R are generally higher in the finer grain sediments as compared to the courser sedimentary horizons (Table T5).

Table T6 presents results for Fe, Mn, and Al from dithionite extractions on samples that were identified to be rich in volcanic ash/tephra. These sediments show that these layers can vary significantly in their Fe_R and Mn_R concentrations.

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