IODP Proceedings    Volume contents     Search
iodp logo



Inorganic geochemistry

Interstitial water chemistry

A total of seven interstitial water samples were extracted from 5 cm whole-round sediment sections from Hole U1314A with a resolution of one sample per core for the first five cores and thereafter one sample per core from Cores 306-U1314A-9H and 12H, covering a depth of 102.9 mbsf. Interstitial water samples were processed for routine shipboard geochemical analyses (see “Geochemistry” in the “Site U1312–U1315 methods” chapter). The concentrations of dissolved elements from Hole U1314A are given in Table T22, and their downhole profiles are illustrated in Figure F24. Three samples from Cores 306-U1314A-2H, 6H, and 12H appeared to be disturbed by flow-in during coring (see “Stratigraphic correlation;” Table T21) and are identified by open circles in Figure F24. Like in Hole U1312A, the visually apparent disturbance in these sediment sections appears not to have affected the overall downhole pattern of the pore water chemistry. However, data from the undisturbed sections are the only ones used for the description.

Chlorinity, salinity, alkalinity, and pH

Chloride (Cl) concentrations in Hole U1314A range from ~540 to ~586 mM between 17.4 and 74.4 mbsf (Fig. F24A). The highest Cl value was measured at 74.4 mbsf.

The downhole salinity value of 34 g/kg prevails to 26.9 mbsf then decreases to 33 g/kg at 36.4 mbsf. The lowest salinity of 32 g/kg was measured at 74.4 mbsf (Fig. F24B).

Alkalinity increases from 5.38 to 7.46 mM between 17.4 and 74.4 mbsf (Fig. F24C). These values are similar to those reported from Site U1313 but lower than those reported from Site U1304 (see “Geochemistry” in the “Site U1304” chapter).

pH values in Hole U1314A range from 7.41 to 7.56 between 17.4 and 74.4 mbsf (Fig. F24D). These values are more or less similar to those reported from Sites U1312 and U1313 as well as from Site U1304 (see “Geochemistry” in the “Site U1304” chapter).

Sodium, potassium, magnesium, and calcium

Calcium (Ca2+), potassium (K+), and magnesium (Mg2+) concentrations range from 8.4 to 5.2, 12.4 to 10.5, and 48.7 to 37.4 mM, respectively, and their downhole profiles show roughly similar decreasing trends with depth (Fig. F24F, F24G, F24H). Na+ concentrations range from ~494 to ~446 mM throughout the profile and show a similar pattern as Cl (Fig. F24E).

Iron, boron, barium, lithium, manganese, and strontium

Iron (Fe2+) concentrations in Hole U1314A increase from 25.5 to 31.8 µM between 17.4 and 26.9 mbsf and then decrease sharply to 6.5 µM at 36.4 mbsf (Fig. F24I). The lowest value of 6.5 µM is measured within a lithologic interval displaying darker and black streaks, suggesting the presence of diagenetic iron minerals. An Fe2+ concentration of 22.8 µM is measured at 74.4 mbsf.

Boron (B) concentrations, mostly as boric acid (H3BO3), increase between ~440 and 465 µM from 17.4 to 36.4 mbsf (Fig. F24J). A boron value of 457 µM is measured at 74.4 mbsf. Barium (Ba2+) concentrations range from 17 to 18.3 µM throughout the profile (Fig. F24K). Highest Ba2+ values are measured at this site in comparison to Sites U1312 and U1313.

The lithium (Li+) and strontium (Sr2+) concentrations in Hole U1314 increase slightly with depth (Fig. F24L, F24N). Sr2+ is usually expelled in the pore water from the carbonate constituents in the sediments during their dissolution and reprecipitation. This is one hypothesis for the downhole increase of Sr2+ (Baker et al., 1982; De Carlo, 1992). It may be mentioned here that the Sr2+ values are 2–4 times lower than those measured at Sites U1312 and U1313, and in general the overall carbonate values are lower as well (see below).

Manganese (Mn2+) concentrations sharply decrease from 46.8 to 15.9 µM between 17.4 and 36.4 mbsf. However, they remain uniform from 36.4 to 74.4 mbsf (Fig. F24M).

Dissolved silica, sulfate, and ammonium

Dissolved silica (H4SiO4) concentrations show a downhole increasing trend (Fig. F24O). The highest dissolved silica concentration of 570 µM is measured at 74.4 mbsf and likely reflects the initial presence of biogenic silica in the sediments and its subsequent dissolution.

Sulfate (SO42–) concentrations range from 20.5 to 1.1 mM between 17.4 and 74.4 mbsf (Fig. F24P). However, both high and low SO42– values of 20.5 and 1.1 mM are measured at 17.4 and 26.9 mbsf, respectively.

Ammonium (NH4+) concentrations increase from 0 to 14.7 µM between 17.4 and 74.4 mbsf (Fig. F24P). These concentrations are almost 10 times lower that those measured in the pore water from Hole U1313A.

Organic geochemistry

Volatile hydrocarbons

A total of 27 headspace samples from Hole U1314A with a sample resolution of one sample per core were analyzed (Table T23). With the exception of one sample from 178.9 mbsf, methane was the only hydrocarbon gas detected in this hole. Concentrations of CH4 in Hole U1314A range from 1.3 to 4.8 ppmv (average = 2.2 ppmv), which is slightly higher than atmospheric background level. Although overall low, a subsurface increase toward ~5 ppmv between 36.4 and 45.9 mbsf and a subsequent decrease to average values below ~50 mbsf is notable (Fig. F25). CH4 concentrations slightly above the average also occur at 178.9 and ~250 mbsf. At 178.9 mbsf, ethane (C2 = 1.9 ppmv) and ethene (C2= = 1.95 ppmv) were also detected.

Sedimentary bulk geochemistry

Sediment samples for the analysis of solid-phase inorganic carbon, total organic carbon (TOC), and total nitrogen (TN) were collected from the working halves in Hole U1314A at a resolution of two samples per core. In addition, splits of squeeze cakes from interstitial water samples were also used for bulk measurements to investigate solvent-extractable organic matter (see below). Data for the bulk geochemical analysis were performed on a total number of 62 samples as shown in Table T24 (see “Geochemistry” in the “Site U1312–U1315 methods” chapter for analytical methods and the derivation of TOC values).

CaCO3 contents in Hole U1314A range from 3.7 to 70.5 wt% (average = 34.1 wt%). A ±20–30 wt% variability around the average persists throughout the profile. Overall, the average carbonate concentration increased from ~20 wt% at 250 mbsf to a mean of ~40 wt% CaCO3 at the seafloor (Fig. F26A). The comparison of CaCO3 and color reflectance data (L*) at expanded depth scale (Fig. F26B) shows a nearly perfect correlation between these two, implying that the carbonate content is mainly controlling the variability in the lightness profile.

TOC and TN contents range from 0 to 0.5 wt% and 0.09 to 0.22 wt%, respectively, in sediments of Hole U1314A (Fig. F27). Overall, TOC contents from Hole U1314A are slightly higher compared to Sites U1312 and U1313, even though the maximum TOC is lower at Site U1314. Fluctuations in TOC of 0.2 wt% are present throughout the downcore profile. It appears that these fluctuations might be superimposed on an overall variability at low frequency as shown by the shaded area in Figure F27 (left panel). Furthermore, a correlation between TOC and magnetic susceptibility data is obvious (Fig. F28). The relatively high susceptibility is explained by the presence of more magnetic minerals probably caused by increased terrigenous input (see “Physical properties”). Thus, the covariance of TOC and susceptibility can be explained by the dominance of terrigenous organic matter. Indeed, this explanation is in accordance with molecular analysis of extractable organic compounds (see below), at least as observed for the upper ~100 m of the hole.

Extractable organic matter sources

In Hole U1314A, eight samples were used for an initial investigation of the solvent-extractable matter (Table T25). As for Site U1313, the flame ionization detector (FID) installed on the gas chromatograph had to be used for compound detection instead of the mass spectrometer. However, the use of the FID allowed us to obtain a visual overview of the extractable compound inventory and to identify its major constituents (see “Geochemistry” in the “Site U1312–U1315 methods” chapter). Extractable organic matter (EOM) from Hole U1314A consists mainly of the same compounds as found at Site U1313, namely a series of n-alkanes in a carbon atom number range of ~C20–C33 and homologs of C37–C39 unsaturated methyl and ethyl ketones (alkenones). Differences in relative proportions of these compound classes, as illustrated in Figure F29, can be used for an initial organic matter characterization in terms of terrigenous versus marine sources. See “Geochemistry” in the “Site U1313” chapter for details on the origin of these compound classes and the derivation of marine and terrigenous EOM proportions, and “Geochemistry” in the “Site U1312–U1315 methods” chapter for methodology. From this initial characterization it appears that all Pleistocene samples from Hole U1314A show relatively high proportions (~65%–90%) of terrigenous EOM except a single sample from the late Pliocene that shows a relatively higher marine proportion (Fig. F30).

Alkenone-derived sea-surface temperatures

As for Site U1313, alkenone-derived sea-surface temperatures (SSTs) were calculated for Hole U1314A. The available preliminary SSTs range from 9.7° to 13.5°C (Table T25) and are comparable to modern ocean-atlas annual mean SSTs in the relevant area. However, these initial SST data show a lower temperature variability (3.8°C) than the ~6°C difference between modern and Last Glacial Maximum SST obtained by foraminiferal transfer functions in this area of the North Atlantic (Pflaumann et al., 2003). One possible explanation for this discrepancy could be that none of the limited (and randomly chosen) shipboard samples originates from a full glacial interval.