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doi:10.2204/iodp.proc.344.201.2015

Results and discussion

A total of 187 pore fluid samples were analyzed, and the data are listed in Table T1. Figures F2, F3, and F4 show downhole Sr concentration and ⁸⁷Sr/⁸⁶Sr ratio depth profiles, in the context of the corresponding lithology and supporting pore fluid data.

Overriding plate

The upper sediment sections cored at all sites drilled on the upper plate show a significant decrease in the Sr isotopic ratios consistent with in situ alteration of tephra.

Site U1379

At Site U1379, ash alteration is more pronounced within the sediment of lithostratigraphic Subunit IIC and corresponds to an increase in strontium concentrations (Fig. F2). Release of nonradiogenic Sr during alteration of ash is well documented, and this reaction explains the decrease in Sr isotopic values at this site from a near-seawater ratio of 0.70908 to a minimum value of 0.70757 at 369 mbsf. The isotopic ratios within the remaining fluids of Unit II range from 0.70757 to 0.70829. Unit III is dominated by coarser grained sediment as well as several fault zones below ~600 mbsf that host freshened fluids, as exhibited by the broad interval of low Cl, which decreases sharply from ~560 mbsf and reaches its lowest concentration at ~680 mbsf (Expedition 334 Scientists, 2012c; Torres et al., 2013). Concomitant with the Cl decrease, dissolved Sr increases from ~41 µM at 560 mbsf to a broad maximum of ~70 µM, in spite of the overall dilution trend evidenced in the Cl profile. In this zone, Sr isotopes have a less radiogenic component.

Site U1378/U1380

At Site U1378, drilled in the middle slope, Cl shows a more pronounced decrease relative to Site U1379. Site U1380 is a continuation of the sequence drilled at nearby Site U1378, and pore fluid data obtained here show discrete anomalies that mark the unconformity separating the slope apron from the underlying wedge material (Torres et al., 2013) (Fig. F2). The Sr isotopic data in fluid from Site U1378 show a steady decrease from seawater value, which points to alteration of tephra throughout Unit II. This process is more pronounced within the sediment interval from 360 to 425 mbsf, where ⁸⁷Sr/⁸⁶Sr ratios show a broad minimum ranging from 0.70802 to 0.70791. Immediately below the zone displaying this broad minimum, ⁸⁷Sr/⁸⁶Sr ratios increase rapidly in samples collected just above the framework sediment, from 450 to 500 mbsf. The Sr isotopic composition in this zone ranges from 0.70834 to 0.70844, values that are similar to those measured within the unconformity drilled at Site U1379. The Sr concentration within the unconformity at Site U1380 is lower than that measured at Site U1379, reflecting the larger degree of dilution by fluid freshening in the middle slope region. Below the unconformity, Sr isotopic values show a steady increase with depth to a value of 0.70868 (at 552 mbsf), accompanied by a general increase in dissolved Sr concentration.

Site U1413

At Site U1413, Ca and Mg concentrations decrease from seawater values at the seafloor to a minimum of 1.1 and 39.8 mM, respectively, at the Unit I/II boundary (see the “Upper slope Site U1413” chapter [Harris et al., 2013e]), possibly reflecting precipitation of authigenic carbonates, an observation that is consistent with the sharp decrease in dissolved Sr concentration in these shallow sequences (Fig. F3). Strontium, however, is not fractionated during carbonate precipitation, and therefore the observed steady decrease in ⁸⁷Sr/⁸⁶Sr ratios from a near-seawater value of 0.70916 in the shallowest sample analyzed (4.4 mbsf) to 0.70899 at 24.4 mbsf indicates a component of ash alteration. Tephra were commonly observed in Unit I to a depth of ~35 mbsf. In Unit II, both the Sr concentration and its isotopic composition remain relatively constant in the depth interval from 24.4 to 135 mbsf, below which there is a marked decrease in ⁸⁷Sr/⁸⁶Sr ratio, which sharply declines from 0.70890 to 0.70824 at 178 mbsf. There is only a single, thin tephra layer reported in sediment from 40 to 140 mbsf, but increased occurrence was noted from 135 to 180 mbsf. These observations suggest that the alteration of reactive ash is controlling the Sr isotopic composition. Although not apparent in lithology, the Cl and Sr profiles display a different behavior below 200 mbsf. Cl shows a steady decrease, whereas Sr remains relatively constant. ⁸⁷Sr/⁸⁶Sr ratios also remain fairly constant to the bottom of the site at 578 mbsf.

Site U1412

At Site U1412, drilled on the prism toe, borehole instability precluded sampling the décollement and recovering the underthrust sediment and igneous basement. The geochemistry of pore fluids largely indicates in situ diagenetic reactions. Because of the poor sample coverage, we grouped the Sr isotope data into three regions, constrained by sample recovery. In the shallowest section, the Sr isotopic values steadily decrease with depth, from the near-seawater value of 0.70918 at 1.4 mbsf to 0.70888 at 111 mbsf, likely reflecting volcanic ash alteration. Only three samples were analyzed between 176.4 and 196 mbsf, with values ranging from 0.70843 to 0.70856, and correspond to the Unit I/II boundary. Only five samples were analyzed from the deepest sections, 328.8–369.02 mbsf, and these display the lowest measured values at this site, from 0.70887 to 0.70837.

Incoming plate

Site U1381

Site U1381 was cored during both Expeditions 334 and 344. The data collected during both expeditions show highly consistent values (Fig. F4). The shallowest sample (1.45 mbsf) analyzed at this site has an ⁸⁷Sr/⁸⁶Sr ratio of 0.70918, which is within the uncertainty of the seawater value of 0.70917. The geochemical profiles obtained shipboard at Site U1381 indicate that the deep sediment at this site are influenced by diffusional communication with a fluid in the igneous basement (Expedition 334 Scientists, 2012d; see also the “Input Site U1381” chapter [Harris et al., 2013c). Most characteristic of this effect are sulfate profiles that show a reversal to seawater-like values at the bottom of the hole as observed here as well as in sites drilled offshore Nicoya during Ocean Drilling Program Legs 170 and 205 (Silver et al., 2000; Morris, Villinger, Klaus, et al., 2003). At Site U1381, Sr concentration increases with depth, concomitant with a decrease in the Sr isotopic values, corroborating the inference that the pore fluid within the upper basement has been modified by alteration of the basaltic crust.

Site U1414

The pore fluid composition in the uppermost 80 m of Site U1414 shows trends in alkalinity, sulfate, and ammonium characteristic of organic matter remineralization (see the “Input Site U1414” chapter [Harris et al., 2013b]). In addition, the alteration of ash layers commonly observed throughout the sediment sequence lead to the observed decrease in ⁸⁷Sr/⁸⁶Sr ratios, from 0.7091 measured in the shallowest sample analyzed (0.56 mbsf) to 0.70874 at ~150 mbsf, at the lithostratigraphic Unit I/II boundary. The dissolved Sr concentration remains relatively constant within this interval, suggesting that the main reaction involving strontium in Unit I is ash alteration, and there is very little carbonate precipitation. The large increase in Sr concentrations within Unit II, which is dominated by nannofossil-rich calcareous ooze, is the result of carbonate recrystallization, as has been observed in other pelagic carbonate-rich sediment (e.g., Site 1039 off the Nicoya Peninsula of Costa Rica; Shipboard Scientific Party, 1997). At the Unit II/III boundary (~300 mbsf) there is a sharp increase in Sr concentrations, with a maximum value of 941 µM at 327 mbsf (~10 times seawater value), suggesting more intense diagenetic reactions. This concentration change coincides with a seismic reflector and the onset of carbonate cementation (see the “Input Site U1414” chapter [Harris et al., 2013b]). The lowermost 66 m of the sediment column at Site U1414 (Unit III) is characterized by a sequence of lithified, calcareous, and siliceous cemented silt- and sandstone that probably lost its biogenic components as a result of diagenetic recrystallization. Fluids in this deepest sediment display the largest changes in Sr concentration as well as in its isotopic composition, which decreases from 0.70884 at 308 mbsf to 0.70861 measured in the deepest sample analyzed at ~337 mbsf. Basement was contacted at ~375 mbsf, but the deepest sediment was too cemented to obtain pore fluid samples. The ⁸⁷Sr/⁸⁶Sr ratio in the deepest sample analyzed is less radiogenic than the one measured at Site U1381 (0.70860). Intense carbonate recrystallization of these deepest sediments results in Sr concentrations that are almost an order of magnitude higher than those measured at the base of Site U1381.

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