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doi:10.2204/iodp.proc.303306.111.2006

Geochemistry

Inorganic geochemistry

A total of eight interstitial water samples were extracted from 5 cm whole-round sediment sections from Hole U1312A with a resolution of one sample per core for the first six cores and thereafter one sample per core for 306-U1312A-9H and 12H covering a depth of 110.5 mbsf. Interstitial water samples were processed for routine shipboard geochemical analyses. For details of the interstitial water extraction procedure and analytical methods, see “Geochemistry” in the “Site U1312–U1315 methods” chapter. The concentrations of dissolved elements in Hole U1312A are given in Table T22, and their downhole profiles are illustrated in Figure F24. Cores 306-U1312A-1H to 10H (0–95 mbsf) exhibited frequent flow-in structures, probably caused by excessive heaving during coring, especially in the upper five cores (Table T18). Interstitial water data measured from these disturbed intervals are identified in Table T22 and indicated by open circles in Figure F24. Despite the apparent physical core disturbance in the upper cores, concentrations of some elements appear not to be affected, as shown in their downhole profiles (e.g., Mn). However, only the data points denoted by solid circles are utilized here for the downhole description. A noticeable change in most profiles shown in Figure F24 is observed below the boundary between the lithologic Units I and II.

Chlorinity, salinity, alkalinity, and pH

Chloride concentrations in Hole U1312A show a maximum value of ~570 mM at 34.5 mbsf, followed by a general downhole trend in decreasing values to 553 mM at 110.5 mbsf (Fig. F24A). The chlorinity maximum observed at 34.5 mbsf may correlate with those found by previous workers at 40–50 mbsf (see “Geochemistry” in the Site U1302/U1303, U1304, U1305, U1306, U1307, and U1308 chapters). Chlorinity maxima at this depth have been attributed to a remnant of higher salinity bottom water masses during the LGM preserved in the sediment pore spaces (e.g., McDuff, 1985; Adkins et al., 2002; Adkins and Schrag, 2003). The overall downhole trends of our shipboard interstitial water chlorinity data in the upper ~100 m of the sediment columns are similar to those reported during Expedition 303 see “Geochemistry” in the Site U1302/U1303, U1304, U1305, U1306, U1307, and U1308 chapters) and from other deep-sea sections. This implies that conservative chemical proxies preserved in the interstitial water samples collected from Hole U1312A may record properties of bottom water masses that prevailed during the LGM. The downhole salinity profile shows a similar trend to Cl^– decreasing from 35 to 33 g/kg between 34.5 and 110 mbsf (Fig. F24B).

Alkalinity increases with depth from 3.73 to 4.77 mM in the upper 82 mbsf, followed by a decrease to 3.96 mM at 110.5 mbsf (Fig. F24C). These values are comparatively lower than those reported from Hole U1308A (see “Geochemistry” in the “Site U1308” chapter). The pH profile in Hole U1312A does not exhibit any significant downhole trend, with values ranging from 7.01 to 7.41 (Fig. F24D).

Sodium, potassium, magnesium, and calcium

The interstitial water Na⁺, K⁺, and Mg²⁺ concentrations in Hole U1312A range from 432 to 454.3 mM, 8.4 to 9.4 mM, and 41.2 to 47.1 mM, respectively, and their downhole profiles exhibit trends that are roughly similar to that of Cl^– except for an increase in value between 82 and 110.5 mbsf (Fig. F24E).

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

Fe²⁺ concentrations in Hole U1312A exhibit downhole decreasing values from ~9.6 µM at 34.45 mbsf to 2.4 µM at 110.5 mbsf, with an interval of increased value (9.0 µM) at 82 mbsf (Fig. F24I). B concentrations, mostly as boric acid (H₃BO₃), in the interstitial water samples of Hole U1312A exhibit a downhole increasing trend (Fig. F24J).

Ba²⁺ concentrations are very low (<1.0 µM) and it was challenging to obtain sensible values using inductively coupled plasma–atomic emission spectroscopy. In order to make the slope more sensitive to pore water Ba²⁺ concentrations, the intensity of the standard of the Ba²⁺ values needed to be tuned to the lower end of the calibration curve. Without performing this regression, obtained Ba²⁺ values would be too unrealistic to be valid (Table T22). However, shifting the intercept to zero made the calibration curve more sensitive to the lower values and allowed us to obtain more realistic Ba²⁺ values, listed as Ba²⁺* in Table T22. Downcore Ba²⁺* values show an increasing trend from 0.26 to 0.69 µM between 34.5 and 82 mbsf (Fig. F24K). Values then decrease to 0.39 µM at 110.5 mbsf. The highest Ba²⁺* value was measured at 82 mbsf, near the boundary between lithologic Units I and II.

The Li⁺ and Sr²⁺ profiles show opposite trends to each other. Li⁺ concentrations in Hole U1312A are highest in the shallowest samples and decrease with depth, while the reverse is observed with Sr²⁺ values (Fig. F24L, F24N). Sr²⁺ is usually expelled in the pore water from the carbonate constituents in the sediments during dissolution and reprecipitation. This is one hypothesis for the downhole increase of Sr²⁺ (Baker et al., 1982; De Carlo, 1992). Mn²⁺ concentrations are also higher in the shallowest samples but decrease rapidly with depth (Fig. F24M).

Dissolved silica and sulfate

Dissolved silica (H₄SiO₄) concentrations in Hole U1312A range from 425.3 to 640.9 µM, and its downhole profile exhibits an initial trend in increasing values to a depth of 53.45 mbsf followed by decreasing values thereafter to the deepest sample measured (Fig. F24O). The lowest dissolved silica concentration (425 µM) was measured at 110.45 mbsf, coinciding with an interval composed of white nannofossil ooze sediments (see “Lithostratigraphy” and “Biostratigraphy”). The elevated dissolved silica contents between 34.5 and 53.5 mbsf likely reflect the initial presence of biogenic silica in the sediments and its subsequent dissolution.

Sulfate (SO₄^2–) concentrations show a moderate decrease with depth and its downhole profile appears to parallel that of pH (Fig. F24P, F24D).

Organic geochemistry

Volatile hydrocarbons

Headspace gas analysis was conducted as part of the standard protocol required for shipboard safety and pollution prevention monitoring. A total of 25 headspace samples from Hole U1312A with a sample resolution of one sample per core were analyzed (Table T23). Methane was the only hydrocarbon gas detected at the site. The concentrations of methane in Hole U1312A are relatively constant and at a natural background level (1.7–3.8 ppmv). Slight fluctuations in methane concentrations of ~3 ppmv are observed in the upper ~130 mbsf. However, the concentration of methane below 140 mbsf remains uniform at ~2 ppmv (Fig. F25).

Sedimentary bulk geochemistry

Sediment samples for the analysis of solid-phase bulk inorganic C, total C, and total N (TN) were collected from the working halves from Hole U1312A at a resolution of two samples per core. In addition, splits of sediments from interstitial water squeeze cake (see “Geochemistry” in the “Site U1312–U1315 methods” chapter) samples were also used for investigations of solvent-extractable organic matter (see below) as well as for bulk geochemical analyses. Data from the bulk geochemical analysis performed on 58 samples are shown in Table T24. See “Geochemistry” in the “Site U1312–U1315 methods” chapter for analytical methods and the derivation of total organic carbon (TOC) values. Descriptions and discussion of data and subsets follows with reference to the identified lithologic units (see “Lithostratigraphy”). Note that the geochemical data may have suffered from flow-in and distortion during coring (see “Stratigraphic correlation;” Table T18) with respect to sample depth assignments and absolute values.

Downhole variations of calcium carbonate contents for Hole U1312A are shown in Figure F26. For comparison, CaCO₃ data from Site 608 drilled at the same location were also plotted. CaCO₃ for Hole U1312A is very high, averaging 90.4 wt% with a range from 58.9 to 98.3 wt%. Notable fluctuations in carbonate content between 59 and 95 wt% are recognized in the upper ~40 mbsf (lithologic Subunit IA), and the lowest carbonate contents for Hole U1312A are found within this unit. On average, the CaCO₃ concentration for Subunit IA is 84 wt%. In Subunit IB, carbonate contents generally increase downcore from a minimum value of ~65 wt% at 40 mbsf toward values of ~95 wt% at 70 mbsf. The boundary between lithologic Units I and II in Hole U1312A is confined as a local minimum in CaCO₃ at ~80 mbsf and is in accordance with the earlier findings from Site 608 (Ruddiman, Kidd, Thomas, et al., 1987). Being dominantly composed of a white nannofossil ooze with only a minor amount of clay, Unit II of Hole U1312A displays relatively constant (90–98 wt%) and high (average = 95 wt%) carbonate contents, except two low values (~82 wt%) at 81.95 and 110.45 mbsf.

TOC and TN contents range from 0 to 0.9 wt% and from 0.08 to 0.18 wt%, respectively (Fig. F27). On average, TOC and TN are 0.13 and 0.1 wt%, respectively, in Hole U1312A. Despite low mean TOC contents, the upper 80 mbsf (lithologic Unit I) shows higher variability in TOC, occasional peak values as high as 0.9 wt%, and an average of 0.3 wt%. In contrast, Unit II is characterized by extremely low TOC concentrations (<0.1 wt%).

TN concentrations are low overall and relatively uniform; downcore fluctuations are noiselike and might therefore result from measurements at the lower end of instrumental precision. However, a slight gradient in TN from ~0.15 wt% at the top toward the average 0.1 wt% value at the boundary between lithologic Units I and II is obvious. This is in line with the decreasing abundance of clay content over the same depth range (see “Lithostratigraphy;” Fig. F5). Therefore, sedimentary TN in Hole U1312A might result from the tendency of clay to absorb ammonium ions generated during the degradation of organic matter (Müller, 1977). Because of this and TOC contents <0.2 wt% for most of the samples from Hole U1312A, TOC/TN ratios, often used as an indicator for the nature of organic matter in sediments (i.e., marine versus terrestrial derived) (Emmerson and Hedges, 1988; Meyers, 1994), were not calculated.

Sedimentary organic geochemistry

In Hole U1312A, eight samples were collected for geochemical analyses of the solvent extractable organic matter. These were splits of the remaining sediments after interstitial water squeeze-out as described in “Geochemistry” in the “Site U1312–U1315 methods” chapter. The bulk data corresponding to these samples are shown in Table T24 and are underlined. Because of the dominant presence of CaCO₃ and the generally low TOC, only five out of the eight samples were processed. Because of the low organic carbon contents, the total organic solvent extracts of these samples were completely colorless. Therefore, the total extracts were submitted directly to gas chromatograph (GC)-mass spectrometer (MS) analysis without further silica gel separation. Sedimentary organic compounds were only detected in one sample at 24.95 mbsf (see below). In all other samples investigated, no n-alkanes or alkenones could be detected, or they were below the detection limit of the GC-MS system used. Sample 306-U1312A-3H-4, 145–150 cm, revealed the presence of a series of hopanes, as shown by the m/z 191 extracted ion chromatogram in Figure F28. The carbon atom number distribution of these compound types ranges from C₂₆ (T_S) to C₃₅ with the C₃₀ homolog most abundant and a regular decrease from C₃₁ to C₃₅ homohopanes. The isomerizations of these compounds at C₁₇ and C₂₂ (i.e., the observed 17(α) H-configuration and the dominance of 22S over 22R isomers for C₃₁–C₃₅ hopanes) clearly indicate that they do not occur in their biological configuration but rather in a thermodynamically more stable configuration. Such “mature” isomeric hopane mixtures are typically found in oils and source rocks (e.g., Peters and Moldowan, 1993; Killops and Killops, 2004). Their occurrence in Hole U1312A at 24.95 mbsf (>0.78 Ma, based on the occurrence of the Brunhes/Matuyama reversal at 18.40 mbsf; see “Paleomagnetism” and Table T17) thus can be interpreted as contributions of organic matter derived from ancient sedimentary source rocks, possibly eroded by glaciers and transported by icebergs to the site location. Recently, comparable hopane mixtures have been shown to occur specifically within Heinrich layers 2 and 3 from samples of the Labrador Sea and also in the western and northern North Atlantic (Rashid and Grosjean, 2004). The identification of this mature hopane mixture in Hole U1312A may thus support the occurrence of “Heinrich-type” sedimentation prior to ~0.8 Ma.

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