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doi:10.2204/iodp.proc.334.201.2014

Results and discussion

A total of 241 pore fluid samples were analyzed and the data are listed in Tables T1, T2, and T3.

Downhole distributions of Sr, B, Li, Mn, Ba, and H₄SiO₄ at Sites U1378 and U1379 are shown in Figures F2 and F3. The most striking signature within the slope sediment apron cored at these sites is a concomitant increase in Ba, Sr, and H₄SiO₄, which at Site U1379 occurs from ~300 to 450 mbsf, with maximum values of 7400 nM, 103 µM, and 435 µM, respectively. At Site U1378, there is a distinct maximum in Ba and H₄SiO₄ between 10 and 30 mbsf, with Ba ranging from 1067 to 1667 nM and H₄SiO₄ ranging from 560 to 700 µM; however, Sr decreases within these shallow sediments. In the sediment interval from ~200 to 400 mbsf, there are additional broad maxima in Ba (maximum = 7500 nM) and H₄SiO₄ (maximum = 740 µM) and a slight but distinct Sr maxima (maximum = 56 µM). Whereas this preliminary data set is not enough to fully constrain the reactions leading to these concentration changes, it is likely that the maxima in Ba and H₄SiO₄ are associated with alteration of reactive volcanic ash layers and dissolution of biogenic opal, and the Sr concentration reflects both ash alteration and carbonate diagenesis. At Site U1379 only two tephra layers are recognized in the uppermost 324 m of sediment, whereas 29 tephra layers were recognized between 309 and 500 mbsf, which correspond to the broad concentration maxima in Ba, Sr, and H₄SiO₄. Intense ash alteration within this interval is confirmed by a marked decrease in the ⁸⁷Sr/⁸⁶Sr ratios measured in pore fluids (Ross et al., submitted). Tephras at Site U1378 are more widely distributed (Fig. F2). In Unit I, shipboard sedimentologists identified 21 tephra layers, widely distributed within the background silty clay sediment. Alteration of these silicates releases H₄SiO₄, Ba, and Sr to the pore fluids. From the 49 tephra horizons described in Unit II, 38 occur between 205 and 333 mbsf, which is also consistent with the pore water profiles. A decrease in strontium isotope values occurs at Site U1378, but the isotopic distribution at Site U1378 is less congruent with Sr concentration data, possibly due to more carbonate diagenesis at this site (Ross et al., submitted).

Li and B profiles reported from the Costa Rica margin offshore Nicoya (Chan and Kastner, 2000; Kopf et al., 2000) and convergent margins elsewhere (Moriguti and Nakamura, 1998, Teichert et al., 2005) have shown these elements to be modified in response to moderate to high temperatures. Li is released from solid phases at high temperatures, whereas B concentration in fluids have been shown to decrease as alteration proceeds. Li concentrations measured in samples from Sites U1378 and U1379 are lower than seawater values (26 µM) and are relatively constant in the upper 200 mbsf. Deeper than 200 mbsf, there is an increase in concentration with depth, which at Site U1379 leads to a maximum of 74 µM at the bottom of the hole, suggesting diffusional interaction with fluid at a greater depth, where higher temperatures prevail. At Site U1378, Li concentrations increase rapidly to a maximum value of 59 µM at 490 mbsf, followed by a slight decrease to the base of the hole. The zone of elevated Li concentrations at Sites U1378 and U1379 correlates with the broad shear zone observed above the unconformity between the slope apron and underlying wedge material. At Site U1378, this zone extends from ~480 to 550 mbsf and corresponds to a horizon with depleted Cl and Ca concentrations, suggesting that the shear zone supports migration of fluids that originated from a source depth where temperatures are >80°C (see the “Expedition 334 summary” chapter [Expedition 334 Scientists, 2012a]; Torres et al., 2013). The decrease in Li concentrations below the shear zone at Site U1378 reflects the presence of a fluid at depth that has interacted with sediment and/or oceanic basement at a lower temperature. These inferences were confirmed by drilling deeper at this location during Expedition 344 (Harris, Sakaguchi, Petronotis, and the Expedition 344 Scientists, 2013; Torres et al., 2013).

Marine sediments constitute a major reservoir for boron (Ishikawa and Nakamura, 1993), an element that is a highly mobile and is involved in various diagenetic processes. An increase in concentration over the seawater value of 450 µM has been observed at various margin settings, and has been attributed to microbial alteration of organic matter, ammonium exchange reactions, and release from clays at higher temperatures (You et al., 1993, 1995). Although small, the shallow slope sediments cored at Site U1379 show slight enrichments in dissolved B (to 472 µM), and a more developed enrichment is apparent in shallow sediments from Site U1378, with values ranging from 493 to 594 µM in the upper 30 mbsf. Below these shallow maxima, B concentrations decrease with depth to values as low as 180 and 100 µM at Sites U1378 and U1379, respectively. Detrital clay minerals initially adsorb B onto the mineral surface, and with increasing burial during late diagenesis, this element is incorporated in tetrahedral sites of the clay structure, replacing Si (Williams et al., 2001). However, B is released during smectite to illite conversion, as documented experimentally and in field observations (Williams et al., 2001; Teichert et al., 2005; You et al., 1995). The lack of an increase in B concentrations with depth at these CRISP slope sites suggests that the B released at depth is overprinted by sorption and incorporation onto clays; a very slight concentration increase at the shear zones may be indicative of a component of clay dehydration reactions, evidenced in the low Cl values measured in pore fluids from the shear zone (see the “Expedition 334 summary” chapter [Expedition 334 Scientists, 2012a]). However, full evaluation of the processes driving B reactions in the CRISP region awaits results from boron isotopic analyses (Solomon laboratory, in progress).

Mn concentrations at Sites U1378 and U1379 are highly variable, and the discrete maxima in the reducing fluids at these sites possibly indicate alteration of Mn-bearing tephras.

Downhole distributions of Sr, B, Li, Mn, Ba, and H₄SiO₄ at Site U1381 are shown in Figure F4. Sr concentrations are close to a modern seawater value of 86 ?M from the seafloor to ~45 mbsf and increase slightly downhole from ~45 to ~95 mbsf to a maximum concentration of 104 µM. In the deepest ~10 m of the hole, Sr concentrations decrease slightly toward the sediment/basalt interface. The minor decrease in Sr concentrations observed at the base of the hole must be related to reactions in the oceanic basement, as confirmed by strontium isotope data (Ross et al., submitted).

In the shallowest sample analyzed at ~1.5 mbsf, H₄SiO₄ concentration (538 µM) is higher than bottom water concentration, which in this region is ~180 µM. Silica concentrations increase with depth to ~1080 µM, possibly in response to the dissolution of siliceous phyto- and zooplankton diatoms, radiolarians, and sponge spicules observed in larger abundances in lithostratigraphic Unit II (see the “Expedition 334 summary” chapter [Expedition 334 Scientists, 2012a]).

Ba concentrations throughout the sediment section are very low, between ~250 and 730 nM, and slightly above the bottom seawater concentration of ~240–470 nM. These concentrations are typical of pore fluid Ba values in the pelagic environment, where sulfate concentrations are too high for barite dissolution. No obvious trends are observed in the Ba concentration-depth profile, as expected for this environment. From ~25 mbsf to the bottom of the hole, Mn concentrations increase with depth, similar to the Li profile. This Mn increase has a diffusional shape, suggesting communication with deeper fluid in the oceanic basement that has a higher Mn concentration. As observed in the shallow sediments cored in the slope apron, B shows a slight increase over seawater values to 554 µM, with a general decrease toward seawater values at the bottom of the hole.

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