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doi:10.2204/iodp.proc.341.106.2014

Geochemistry

Interstitial water chemistry

A total of 20 interstitial water (IW) samples were taken from Hole U1420A. Because of variable core recovery in the sandy to gravelly lithologies, the IW sample spacing was irregular. For example, just two samples were taken in the upper part of the hole (Cores 341-U1420A-2R and 5R), followed by a ~430 m CSF-A sampling gap. Deeper in the hole (Cores 341-U1420A-51R through 106R), sampling resolution continued to be irregular. Because of these unavoidable sampling gaps, the resulting downhole profiles of chemical parameters may be biased.

Whole rounds for IW analysis in Hole U1420A were 5 to 15 cm long. The applied squeezing pressures ranged from 8,000 to 32,000 psi, and the volumes of IW collected ranged from 8 mL (in sandy/gravelly material) to 30 mL (in muddy material). Splits of the IW samples were processed following methods outlined in “Geochemistry” in the “Methods” chapter (Jaeger et al., 2014). Some splits were analyzed at sea, and others were preserved for shore-based analysis of dissolved trace metals, oxygen/sulfur/calcium/strontium isotopes, dissolved inorganic carbon, and silica.

Alkalinity, pH, chloride, and salinity

The alkalinity and pH of the sea-surface water (sampled by pumping) at Site U1420 are 2.2 mM and 8.17, respectively. Alkalinity values in IW samples are consistently <12 mM (Fig. F15A). The highest alkalinity (11.6 mM) was observed at 478.0 m CSF-A. From this peak, concentrations gradually decrease downhole to a local minimum of 1.6 mM at 653.6 m CSF-A. A secondary maximum alkalinity of 5.5 mM occurs at 763.3 m CSF-A, followed by a gradual decrease to 1.2 mM at 1012.5 m CSF-A. The pH values range between 7.73 and 8.66; the upper range of these values (Fig. F15B) occurs deeper than 800 m CSF-A.

The sea-surface chloride concentration at Site U1420 is 509 mM. In IW samples, a chloride concentration of >365 mM was recorded in the shallowest sample (10.7 m CSF-A; Core 341-U1420A-2R), as well as between 478.0 and 653.7 m CSF-A (Cores 341-U1420A-51R through 69R) and deeper than 924.8 m CSF-A (Core 94R) (Fig. F15J). Chloride concentrations were 294–340 mM in Core 341-U1420A-5R (39.4 m CSF-A) and Cores 341-U1420A-80R through 94R (763.4–878.0 m CSF-A).

The low concentrations of chloride relative to surface seawater at Site U1420 were also reflected in salinity. The salinity of sea-surface water at Site U1420 is 31. In pore waters, salinity values ≥19 were recorded in the uppermost sample (10.7 m CSF-A; Core 341-U1420A-2R), between 478.0 and 653.7 m CSF-A (Cores 341-U1420A-51R through 69R), and deeper than 924.8 m CSF-A (Core 341-U1420A-94R) (Fig. F15I). The salinity is 16–18 in Core 341-U1420A-5R (39.4 m CSF-A) and Cores 80R through 94R (763.4–878.0 m CSF-A).

Dissolved ammonium, silica, and phosphate

Downcore variations in ammonium concentration often parallel those of alkalinity (Fig. F15D). The highest ammonium concentrations (2.5 mM) were recorded between 478.0 and 596.8 m CSF-A. At greater depths, ammonium decreased to a local minimum concentration of 1.4 mM at 653.6 m CSF-A. Another local maximum of 2.1 mM is at 763.3 m CSF-A, followed by a steep decrease to values near 1.2 mM deeper than 820.1 m CSF-A.

Silica trends are broadly similar to ammonium and alkalinity over most of the record. In the two uppermost samples (10.7 and 39.4 m CSF-A), silica decreases from 650 to 424 µM (Fig. F15H). A value of 650 µM was measured at 478.0 m CSF-A; concentrations decrease downcore to a local minimum of 206 µM (653.6 m CSF-A). Another local maximum of 782 µM is at 763.3 m CSF-A, followed by a downcore decrease to 146 µM at 1012.5 m CSF-A. Phosphate concentrations are variable but consistently <4 µM; lowest values are at depths deeper than 800 m CSF-A (Fig. F16J).

Dissolved sulfate, calcium, magnesium, potassium, sodium, and bromide

Sulfate concentrations at Site U1420 range from 7.6 to 2.2 mM (Fig. F15C). The maximum concentration was recorded in the shallowest sample measured, at 10.7 m CSF-A. Sulfate concentrations increase slightly from 477.9 m CSF-A to the bottom of the hole.

Calcium concentrations at Site U1420 are consistently <20 mM (Fig. F16A). Between 548.0 and 653.6 m CSF-A, calcium concentrations increase sharply from 5 to 12 mM. From 763.3 to 965.1 m CSF-A, a second downcore increase occurs from 5 to 16 mM.

Magnesium concentrations vary between 14.9 and 28.1 mM at Site U1420 (Fig. F16C). Concentrations >25 mM were found at 10.7 m CSF-A and near 600 m CSF-A. Deeper than 600 m CSF-A, magnesium concentrations decrease until reaching the minimum value (14.9 mM) at 788.0 m CSF-A. At greater depths, magnesium increases to ~20 mM by the bottom of the hole.

Potassium concentrations at Site U1420 vary between 5.4 and 2.1 mM; the highest measured value is at 10.7 m CSF-A (Fig. F16B). Values decrease from 3.9 mM (548.0 CSF-A) to 2.2 mM (788.0 m CSF-A). At greater depths, values remained <2.5 mM.

Sodium concentrations at Site U1420 are highest (~300 mM) at 10.7 m CSF-A (Fig. F15K), between 478.0 and 653.7 m CSF-A (Cores 341-U1420A-51R through 69R), and deeper than 924.8 m CSF-A (Core 94R). Minima (as low as ~245 mM) were recorded at 39.35 m CSF-A and between 788.0 and 829.7 m CSF-A.

Bromide concentrations at Site U1420 are <0.6 mM in the two uppermost samples (10.7 and 39.4 m CSF-A) and between 788.0 and 877.9 m CSF-A. The highest concentrations were recorded between 478 and 653.7 m CSF-A (0.71 and 0.66 mM, respectively) and deeper than 924.8 m CSF-A (Fig. F15E).

Dissolved manganese, iron, barium, strontium, boron, and lithium

Dissolved manganese at Site U1420 is 7.3 µM in the uppermost sample (10.7 m CSF-A). Values decrease to 3.2 µM by 39.4 m CSF-A (Fig. F16I). In the deepest part of the hole (763.3–1012.5 m CSF-A), manganese increases slightly with depth from 3.0 to 6.0 µM.

Dissolved iron at Site U1420 ranges between 0 and 0.5 µM without any discernible depth trend (Fig. F16H).

Dissolved barium is below detection limit in the uppermost sample at Site U1420 (10.7 m CSF-A) (Fig. F16F). At greater depth, barium concentrations are variable but increase with sediment depth overall, from 6.3 µM at 39.4 m CSF-A to 38.7 µM at 938.0 m CSF-A.

Strontium concentrations increase sharply from 96.6 to 150.5 µM between 548.0 and 653.6 m CSF-A (Fig. F16G). From 763.3 to 965.1 m CSF-A, there is a second sharp downcore increase in concentrations from 100.4 to 212.1 µM.

Boron concentrations are 160 and 176 µM in the two uppermost samples (Fig. F16E). A maximum concentration of 288 µM was measured at 478.0 m CSF-A. At greater depths, boron concentrations decrease to a local minimum of 101 µM (653.6 m CSF-A). Another local maximum of 191 µM is at 763.3 m CSF-A, followed by a gradual concentration decrease to 83 µM at 1012.5 m CSF-A.

Lithium concentrations at Site U1420 are highest (18.0 µM) in the uppermost sample (10.7 m CSF-A) (Fig. F16D). At greater depths, lithium varies between 15.5 and 5.7 µM without any clear depth trend.

Volatile hydrocarbons

Headspace gas samples in Hole U1420A were collected as warranted by sediment recovery; the sampling resolution is relatively low and irregular. Methane was the dominant hydrocarbon gas present, but at very low concentrations (<11 ppmv) in Cores 341-U1420A-2R, 4R, and 5R (Fig. F15F). Deeper than 410 m CSF-A, methane concentrations range between 1,700 and 33,000 ppmv throughout Hole U1420A. Ethane was detected in headspace gas samples deeper than 244.9 m CSF-A, but concentrations are consistently low (<5 ppmv) (Fig. F15G). The C₁/C₂ ratio is correspondingly high (1,200–35,000), indicating no threat to drilling operations.

Bulk sediment geochemistry

Discrete core samples were analyzed from Site U1420 for total carbon, total nitrogen (TN), and total inorganic carbon. From these analyses, total organic carbon (TOC) and calcium carbonate (CaCO₃) were calculated as described in “Geochemistry” in the “Methods” chapter (Jaeger et al., 2014) In total, 41 samples were analyzed from Hole U1420A (Cores 341-U1420A-2R through 106R). Discrete samples were selected in collaboration with the Lithostratigraphy group to ensure that the common lithologies were analyzed.

The TOC content at Site U1420 is usually between 0.4 and 0.9 wt% with no apparent downhole trend (Fig. F17A). The maximum TOC content was found at 866.1 m CSF-A (1.21 wt%; Section 341-U1420A-91R-1) but was not associated with any obviously biogenic rich lithology (see “Lithostratigraphy”).

The TN content at Site U1420 generally is <0.1 wt% (Fig. F17B). Elevated TN contents were observed in Cores 341-U1420A-83R and 84R at 788.3, 789.1, and 798.1 m CSF-A (0.06, 0.07, and 0.10 wt%, respectively).

Organic carbon to TN (C/N) ratios range between 11 and 77 (Fig. F17C). Relatively high values of this ratio occur near 700 m CSF-A, but otherwise there is no consistent trend with depth.

Measured CaCO₃ contents range between 1.4 and 4.2 wt% at Site U1420 (Fig. F17D). Relatively high concentrations at the site (4.2 and 3.8 wt%) were associated with mud-rich samples at 558.8 and 788.0 m CSF-A (Cores 341-U1420A-59R and 83R; see “Lithostratigraphy”). Between 600 and 690 m CSF-A, CaCO₃ contents decrease downcore and then increase around 800 m CSF-A. At greater depths, there is considerable scatter in the data.

Interpretation

The IW and sediment composition at Site U1420 are consistent with moderate rates of organic matter remineralization. The TOC contents at Site U1420 are low, but respective accumulation rates are substantially higher because of high sedimentation rates (see “Stratigraphic correlation”). Relatively high organic carbon to TN (C/N) ratios reflect the dominance of terrigenous organic matter (Walinsky et al., 2009). Determination of inorganic N is required to fully assess marine and terrigenous inputs to the organic matter record at Site U1420.

Higher C/N ratios (600–800 m CSF-A) are associated with low CaCO₃ and higher TOC contents and lower porosity (see “Physical properties”), consistent with variations in terrigenous input as a source of variability. Deeper than 875 m CSF-A, scatter in CaCO₃ contents and a decrease in TOC are associated with the presence of shallow-water benthic foraminiferal taxa and overall reduced foraminifer abundances (see “Paleontology and biostratigraphy”). Despite the low abundance of biosiliceous microfossils and paucity of volcanic ash (see “Lithostratigraphy”), dissolved silica concentrations are substantially higher than seawater, implying that opal dissolution is occurring within these sediments. The broad similarities between the silica and ammonium profiles suggest coupled dissolution of biosilica and the degradation of biosilica-associated organic matter. This hypothesis is supported by overall lower C/N ratios (Fig. F17C) wherever dissolved ammonium and silica are high, suggesting that a higher proportion of reactive marine organic matter is available within these intervals (Fig. F15).

Tracers associated with the intensity of organic matter degradation are inconsistent. Although alkalinity and phosphate are low throughout the record, ammonium concentrations are relatively high (up to 2.5 mM). Organic matter degradation within the uppermost 40 m CSF-A and deeper than 478.0 m CSF-A is low to moderate, implying the presence of refractory terrigenous material. The discrepancy between phosphate/alkalinity and ammonium concentrations can be explained by the rapid consumption of alkalinity and phosphate by authigenic mineral precipitation, whereas the higher ammonium concentrations overall could indicate that few clay minerals are present to act as sorption sites (see “Lithostratigraphy”).

Organic matter degradation pathways at Site U1420 seem to be dominated by sulfate reduction and especially methanogenesis; dissimilatory iron and manganese reduction are negligible (Froelich et al., 1979). Total sulfate depletion is not reached at Site U1420, although sulfate reduction may be occurring within the uppermost ~40 m CSF-A, given the onset of methanogenesis shallower than 244 m CSF-A. However, IW sulfate concentrations are always significantly lower than respective seawater concentrations (by 60% at 11 m CSF-A and even lower downcore). This relationship indicates significant dilution through the entire sediment profile by sulfate-poor freshwater, which hinders the determination of the diagenetic component to the sulfate record. The dissolved barium profile precludes contamination with sea-surface water pumped downhole during drilling. The low sulfate concentrations in all but the uppermost sample may influence the dissolution of barite deeper than ~10 m CSF-A. However, barium concentrations are extremely low, perhaps because of the limited formation of biogenic barite in shallow-shelf water (von Breymann et al., 1992). Dissolved strontium covaries with calcium, indicating control by the dissolution and precipitation of carbonate minerals.

The chlorinity, salinity, and sodium profiles document significant IW freshening in all recovered samples with respect to the overlying seawater. The freshening is particularly extreme at 39.4 m CSF-A and in the interval between 763.4 and 896.2 m CSF-A. Dehydration of clay minerals can be excluded as a source of freshwater at Site U1420 because of the relatively shallow burial depth and corresponding low temperature (Saffer and McKiernan, 2009). Low IW salinities have been associated previously with methane hydrate dissociation (e.g., Hesse, 2003; Torres et al., 2004). However, given the low to moderate methane concentrations analyzed in headspace samples, methane hydrate dissociation is an unlikely explanation for IW freshening at Site U1420. Alternatively, studies on marine IW salinities adjacent to modern ice sheets of Greenland (Ocean Drilling Program Leg 152; Gieskes et al., 1998; DeFoor et al., 2011) and Antarctica (Lu et al., 2010) suggest that substantial IW freshening might be related to glacial meltwater discharge events. Site U1420 is located within a glacial shelf-crossing trough (Carlson and Bruns, 1997; Worthington et al., 2010), and the IW freshening may be related to burial of freshwater associated with glacial meltwater or associated with modern submarine groundwater discharge driven by onshore/nearshore hydrostatic gradients.

To estimate the impact of freshwater dilution on the IW profiles, concentrations of all chemical parameters were normalized to downcore chloride concentrations. Selected normalized profiles are shown in Figure F18. Although bromide and magnesium profiles deeper than 760 m CSF-A are affected by dilution from less saline waters, all other profiles maintain their original trends after normalization. The upper boundary of the low-salinity layer (600–650 m CSF-A) is also a sink for ammonium, alkalinity, and magnesium and a source of calcium and strontium, which could reflect the dissolution of biogenic calcite and precipitation of authigenic dolomite. In contrast, within the low-salinity layer (deeper than 763 m CSF-A), sinks in ammonium, alkalinity, calcium, and strontium co-occur. These initial IW composition patterns indicate that at least two distinct fluid bodies must be present at Site U1420, separated at ~650 m CSF-A. Physical properties data indicate a drop in porosity and an increase in bulk density at the top of the low-salinity layer, which could indicate that the less saline waters deeper than this depth are protected from diffusive equilibration with the overlying, more saline waters (see “Physical properties”).

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