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doi:10.2204/iodp.proc.344.104.2013

Geochemistry

Inorganic geochemistry

We collected 39 APC whole-round samples (8–20 cm long), 17 XCB whole-round samples (14–30 cm), and 5 RCB whole-round samples (14–36 cm) at a frequency of 1–2 samples per core.

All samples were thoroughly cleaned for drill water contamination. The cleaned samples from all three holes were placed in Ti squeezers and squeezed at gauge forces of <30,000 lb. The inner diameter of the Ti squeezers is 9 cm; thus, the maximum squeezing pressure was 3043 psi (~21 MPa). The pore fluid was collected in syringes and passed through a 0.2 µm filter prior to analysis. The volume of pore fluid recovered varied with lithology and coring technique. Except for one ash layer in Section 344-U1414A-8H-4 from which we collected only 10 mL, the volume of fluid recovered ranges from 36 to 58 mL from the APC samples, 4 to 48 mL from the XCB samples, and 8 to 60 mL from the RCB samples. Because no sulfate–methane transition zone (SMTZ) was encountered at this site, the chemical compositions of the samples could not be corrected for drill fluid contamination. Specific aliquots of the pore fluids were used for shipboard analyses, and the remaining fluid from each sample was sampled for shore-based analyses (see “Geochemistry” in the “Methods” chapter [Harris et al., 2013b]).

Sediment samples from Core 344-U414A-40R to the bottom of the hole were too compacted or cemented to be processed for shipboard pore fluid geochemistry. Therefore, three whole-round sediment samples with lengths ranging from 20 to 30 cm were collected between ~345 and 380 mbsf for shore-based handling and analyses. These samples were cleaned in the same manner as the shipboard interstitial water samples and stored in double vacuum-sealed bags.

Three additional samples were collected for He isotope ratio analysis from Sections 344-U1414A-14H-5, 32X-2, and 39R-1. These samples were handled in the lower tween deck to avoid contamination with gas tank He present in the Chemistry Laboratory. They were cleaned in a N₂ glove bag at 4°C, squeezed as described above, and stored in Cu tubes that were crimped tightly. A quarter of each squeezed cake was frozen for shore-based microbiological studies.

Data for the major element concentrations are listed in Table T8, and Table T9 lists the minor element concentrations. The data are illustrated in Figures F29, F30, F31, and F32.

Salinity, chloride, and alkalis (sodium and potassium)

Downhole profiles of salinity, chloride, potassium, and sodium from Site U1414 are shown in Figure F29. Salinity values decrease slightly with depth to 32.5 between ~50 and 65 mbsf, just below the sulfate minimum at 38 mbsf (Fig. F30). As no lithologic change is observed at this depth (see “Lithostratigraphy and petrology”), the observed salinity decrease most probably reflects the minimum in sulfate and the steep decrease in alkalinity values below the sulfate minimum depth. Below ~65 and ~100 mbsf, salinity is nearly constant, between 33 and 34. Salinity slightly decreases to a minimum of 30 in the lowermost ~40 m of Hole U1414A.

Except for the very slight decrease in concentration from the seawater value to 553 mM in the uppermost ~14 m, Cl concentrations increase with depth to 573 mM at ~85 mbsf, probably because of volcanic tephra hydration to clays. Values remain rather constant to ~280 mbsf. Below this depth, Cl concentrations slightly decrease downhole (Fig. F29) to 555 mM (99% seawater value), likely because of silica phase transformation reactions at depth.

Sodium concentrations are almost constant and slightly below seawater value in the uppermost ~200 m, with values that fluctuate between 460 and 470 mM. Sodium concentrations slowly increase with depth, and at the bottom of the hole they approximately reach the seawater value of 480 mM (Fig. F29).

At and below the seawater/sediment interface, K concentrations are higher than the modern seawater value of 10.4 mM by 1–2 mM. These concentrations were observed at each of the other Expedition 344 drill sites and are most likely related to clay ion exchange reactions. K values fluctuate between 12.0 and 11.0 mM to ~280 mbsf. Below this depth, K concentrations decrease to 7.2 mM (~70% seawater value) at the bottom of the hole (Fig. F29). The significant K decrease below 280 mbsf is likely controlled by clinoptilolite formation, a K-Si–rich zeolite most common in high-silica marine sediments (see “X-ray diffraction analysis”).

Alkalinity, sulfate, ammonium, calcium, and magnesium

Sulfate, alkalinity, and ammonium show characteristic organic matter remineralization profiles in the uppermost ~80 m (Fig. F30).

Alkalinity increases from seawater value at the seafloor to a maximum of ~32 mM at ~33 mbsf. Below the maximum concentration, which occurs ~4 m above the sulfate minimum at 37 mbsf, alkalinity decreases steeply to ~80 mbsf, the depth of the lithostratigraphic Subunit IA/IB boundary (see “Lithostratigraphy and petrology”) and then more gradually to the bottom of Hole U1414A, where it reaches a minimum of 5.3 mM (Fig. F30). The decrease in alkalinity is mainly caused by carbonate diagenesis, as suggested by the Ca concentration-depth profile. Because of the large volume of pore fluid required for alkalinity analyses, only spot analyses were performed in the deeper sediment of this site.

Similar to other sites drilled in the incoming plate offshore Costa Rica (Site U1381, drilled during this expedition, and Sites 1039 and 1253, drilled during Ocean Drilling Program Legs 170 and 205, respectively, offshore Nicoya), sulfate does not reach zero concentration. No methane production was observed, and no SMTZ was recorded at this site. At Site U1414, sulfate concentrations decrease from seawater value to a minimum of 3.6 mM at ~37 mbsf, followed by an increase to ~15 mM at ~100 mbsf, and then remain remarkably constant between 15.0 and 16.0 mM to 260 mbsf. At this depth, sulfate concentrations steeply decrease with depth to a second minimum of 2.5 mM at ~330 mbsf and increase below that depth to 12.6 mM in the deepest sediment sampled at ~337 mbsf (Fig. F30).

Ammonium concentrations increase steeply from the seafloor, where concentrations are zero, to a maximum of 3.2 mM at ~14 mbsf. Ammonium concentrations then decrease to a minimum of 2.6 mM at 37 mbsf, the depth of the sulfate minimum concentration, and then show a slight increase with depth (Fig. F30). This is unlike typical ammonium concentration-depth profiles that increase with depth and reach a maximum value below the depth of the alkalinity maximum. Below the second ammonium maximum of 4.3 mM at ~150 mbsf, concentrations decrease to ~1.8 mM at the bottom of the hole. The observed coincidence in the depths of the sulfate and ammonium concentration minima suggests that at this site there may be favorable conditions for sulfate-reducing ammonium oxidation, according to the following net reaction described by Schrum et al. (2009):

The second sulfate concentration minimum at ~330 mbsf is atypical and likely reflects lateral flow of sulfate-depleted fluids that originated from oxidation of methane and/or other organic carbon sources landward of Site U1414 and migrated updip through the upper sediment of lithostratigraphic Unit III. Based on the diffusional profile between the minimum sulfate concentration at ~300 mbsf and the plateau of sulfate concentration of ~15 mM at ~270 mbsf, the bacterial sulfate reduction reaction is currently ongoing at Site U1414. This reaction will continue as long as the bacteria have a supply of carbon, as there is an abundance of sulfate in the pore fluids (Fig. F30). Isotopic and chemical analyses of the fluid recovered from depth at this site will help constrain the nature and timing of this process.

Ca concentrations decrease from seawater value near the seafloor to a minimum of 2.6 mM at ~37 mbsf, the depth of the sulfate concentration minimum (Fig. F30). At about this depth, alkalinity has a maximum that likely triggers diagenetic carbonate precipitation, consuming Ca and some of the other alkaline earth elements, particularly Mg and Sr. There is a marked increase in Ca concentration below this depth, to a maximum of ~13.5 mM (~28% higher than modern seawater value) at ~270 mbsf, most likely caused by silicate weathering reactions. Below this depth to the bottom of Hole U1414A, the Ca concentration profile mimics that of the sulfate profile. This profile is consistent with a second carbonate diagenetic reaction zone caused by ongoing sulfate reduction at this depth interval, whereas a fraction of the low calcium observed may be attributed to the chemical composition of the sulfate-depleted laterally migrating fluid discussed above.

From the shallowest pore water analyzed to approximately the depth of the sulfate reduction zone, we observe a slight decrease in Mg concentrations from ~50 to ~46 mM (Fig. F30), which is most likely caused by the carbonate diagenetic reactions that are also evidenced by the Ca concentration profile. Mg concentrations slightly increase to ~50 mM in the interval from ~70 to 132 mbsf, the depth range of the ammonium maximum, possibly caused by ion exchange reactions in clays. Below this depth, Mg concentrations decrease to the bottom of the hole.

Conspicuous in both the major element profiles discussed above and in the minor element profiles discussed below (Figs. F30, F31) are the pronounced concentration discontinuities at ~195 mbsf, a depth that corresponds to the lithostratigraphic Subunit IIA/IIB boundary (see “Lithostratigraphy and petrology”). These strong discontinuities coincide with the change from APC to XCB cores triggered by APC refusal, as well as a missing 4 m section in the sediment recovered.

Strontium, lithium, manganese, boron, barium, and silica

Downhole distributions of Sr, Li, Mn, B, Si, and Ba are shown in Figure F31. From ~0.56 to 27.9 mbsf, Sr concentrations slightly decrease from 84.5 µM to a low value of 68.8 µM, coincident with the first zone of active sulfate reduction that extends from the seafloor to 37.4 mbsf (Fig. F30). This decrease in Sr concentrations is concomitant with a decrease in calcium and is the result of authigenic carbonate precipitation. Below 37.4 mbsf to the base of Subunit IB at 145 mbsf (see “Lithostratigraphy and petrology”), Sr concentrations gradually increase to 115 µM. Within Subunits IIA and IIB, however, Sr concentrations increase more sharply, reaching 218 µM (~2.5 times seawater value) at the base of Subunit IIB. Lithostratigraphic Subunits IIA and IIB are dominated by nannofossil-rich calcareous ooze, and the large increase in Sr concentrations within this depth range is the result of carbonate recrystallization, as has been observed in other pelagic carbonate-rich sediments (e.g., Site 1039 off the Nicoya Peninsula of Costa Rica; Shipboard Scientific Party, 1997). Across the lithostratigraphic Subunit II/III boundary, there is a sharp increase in Sr concentrations that seems to coincide with a seismic reflector (see “Lithostratigraphy and petrology”), with a maximum value of 908 µM at 328 mbsf (~10 times seawater value). The composition of this fluid is different from that above the boundary, possibly due to more intense diagenetic reactions and a higher degree of carbonate diagenesis.

Li concentrations decrease from 27 µM at 0.56 mbsf to a minimum of 16 µM at 8.9 mbsf, suggesting that Li is controlled by secondary mineral precipitation, probably related to volcanic tephra alteration, and clay ion exchange reactions in the upper sediment column. Below this depth, Li concentrations gradually increase to ~31 µM at the base of lithostratigraphic Subunit IA at ~78 mbsf (Fig. F30). Below this depth, Li concentrations increase more sharply to 170 µM at the base of lithostratigraphic Subunit IIA. The profile through this depth interval reflects diffusion from lithostratigraphic Subunit IIB, where Li concentrations are relatively constant and enriched (170–182 µM; 7 times seawater value). The reason for the elevated Li concentrations is not immediately apparent, considering this unit is dominated by alternating nannofossil-rich calcareous ooze and sponge spicule–rich calcareous ooze (see “Lithostratigraphy and petrology”). Pore fluid samples were preserved for shore-based Li isotopic analyses, which will be critical for interpreting the cause of the Li enrichment within this unit. Below Subunit IIB, Li concentrations decrease to 103 µM at 336.7 mbsf.

Mn concentrations decrease abruptly from 53.1 µM at 0.56 mbsf to 4.8 µM at 3.3 mbsf, reflecting early organic matter diagenesis. Mn concentrations are low and variable from this depth to 52.4 mbsf, ranging from 0.6 to 3.2 µM. This depth interval encompasses the first zone of active sulfate reduction at this site (Fig. F30). Below this zone, from 60 to 170 mbsf, Mn concentrations increase and are variable, ranging from 1 to 8.5 µM. Deeper in the hole, Mn concentrations are low, not exceeding 1.6 µM, and relatively constant with depth (Fig. F31).

B concentrations increase from near seawater value at the seafloor to a maximum of 642 µM at 27.9 mbsf (~1.4 times seawater value), likely related to clay ion exchange reactions. B concentrations then decrease steadily to 299 µM at ~145 mbsf at the base of Subunit IB. B concentrations increase slightly in Subunit IIA and then remain constant and near seawater value in Subunit IIB. B concentrations are variable and range between 350 and 450 µM in Subunit III (Fig. F31).

The dissolved Si profile at Site U1414 reflects lithology and silicate mineral diagenesis. Si concentrations monotonically increase from 573 µM at 0.56 mbsf to 1032 µM at 150 mbsf, reflecting a gradual increase in the amount of biogenic opal in the sediments from the silty clay/sand dominating Subunit IA through the nannofossil-rich clay in Subunit IB. A sharp decrease in Si concentrations in Subunit IIA reflects the change from the nannofossil-rich clay of Subunit IB to the nannofossil-rich calcareous ooze in Subunit IIA. The marked increase in Si concentrations (Fig. F31) across the lithostratigraphic Subunit IIA/IIB boundary (from 631 µM at 185 mbsf to 1465 µM at 208 mbsf) indicates a change in fluid rock chemistry in Subunit IIB, dominated by the opal-A/dissolved silica equilibrium at the in situ temperature of ~35°–45°C. The distinct decrease in Si concentrations to 612 µM near the base of Subunit IIB (269–288 mbsf), which coincides with a clear decrease in dissolved K concentrations (Fig. F29), suggests that Si within this zone may be controlled by clinoptilolite formation, a K-Si–rich zeolite. Clinoptilolite was not observed in the XRD data between ~150 and 250 mbsf, but it is present within the depth interval depleted in dissolved silica and potassium (see “X-ray diffraction analysis”).

Pore fluid Ba concentrations are elevated with respect to bottom water concentration (~0.4 µM), ranging between 0.5 and 2.57 µM from 0.56 to 64.5 mbsf. This elevated Ba may reflect release from barite dissolution at the sulfate minimum. Below ~65 mbsf, Ba concentrations are low and constant to the Subunit II/III boundary, ranging from 0.55 to 1.0 µM. Below ~65 mbsf, the Ba concentration profile is a mirror image of the sulfate profile. It reaches a maximum value of 17.6 µM at 319.5 mbsf and then decreases to 9.4 µM at 336.7 mbsf (Fig. F31). The Ba profile is likely dominated by the stability of the mineral barite (BaSO₄). The Ba increase may be the result of in situ barite dissolution or the migration of Ba in the low-sulfate fluid. Postcruise solid-phase Ba analyses will be important for determining the origin of the elevated Ba in Subunit III and the time history of the fluid flow.

Organic geochemistry

Organic geochemistry data for Site U1414 are listed in Table T10, and methane concentrations are plotted in Figure F32. At this site, we detected very low concentrations of methane (2–40 ppmv), consistent with our observation that sulfate does not reach depletion. C₂₊ was not detected.

Organic and inorganic carbon distributions are illustrated in Figure F33 and listed in Table T11. Total carbon, inorganic carbon, and CaCO₃ abruptly increase at the base of lithostratigraphic Unit I at 145 mbsf, where concentrations change from ~2 to 5 wt%, 0.5 to 10 wt%, and 2 to 88 wt%, respectively. Total organic carbon concentrations in lithostratigraphic Unit II decrease slightly from ~2.0 to <0.8 wt%. At the boundary between lithostratigraphic Units II and III, total organic carbon values increase to 2.2 wt%. Total nitrogen decreases from 0.20 to 0.01 wt% in lithostratigraphic Units I and II and increases to 0.16 wt% in Unit III. The calculated C/N ratio is ~9 in Unit I and >10 and variable in lithostratigraphic Units II and III.

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