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doi:10.2204/iodp.proc.342.103.2014 GeochemistryThe geochemistry program carried out routine shipboard analyses for Site U1402, including
Sediment gas sampling and analysisOne headspace sample was taken from Hole U1402B as part of the routine environmental protection and safety monitoring program (Table T5). The concentration of methane (CH4) in Section 342-U1402B-1H-4 was 4.5 µL/L (ppmv). Other hydrocarbon gases were not detected. Interstitial water chemistryOne interstitial water sample was collected from Hole U1402B. The interstitial water was squeezed from a 10 cm whole-round sample from the bottom of Section 342-U1402B-1H-3, which was cut on the catwalk after Core 1H was sectioned. The top depth of this whole-round section is 3 mbsf. Interstitial water components are tabulated in Table T6. Depositional environment control on interstitial water chemistrySite U1402 is in a very different depositional environment compared to the other Expedition 342 sites. As a result, the interstitial fluid geochemistry data, although limited to only one sample, look very different than those from samples described in subsequent Expedition 342 site chapters. A notable example is in the high alkalinity and ammonium (NH4+) concentrations and low sulfate concentrations. Alkalinity in the interstitial water sample in Hole U1402B is 13.77 mM, considerably higher than the alkalinity of average seawater (2.33 mM based on the International Association of Physical Sciences of the Ocean [IAPSO]). Replicate alkalinity titrations of IAPSO standard seawater during operations at Site U1402 are consistent with the accepted alkalinity value (within 1%) for this standard. The pH of the interstitial water sample is 7.7, which is within the pH range of average seawater (7.7–8.1). The concentration of ammonium in Core 342-U1402B-1H is 1590 µM. Sulfate (SO42–) concentration is 9.1 mM. In the absence of additional interstitial water measurements, it is difficult to place these data into a geochemical framework. However, as compared to other interstitial water data collected during Expedition 342, this sample has higher alkalinity and ammonium concentrations and lower concentrations of sulfate, the hallmark signatures of organic matter consumption. Given the much shallower depth of Site U1402 relative to the other Expedition 342 sites, it is likely that the interstitial fluids at Site U1402 have been influenced by relatively high organic carbon contents (see “Sediment geochemistry”). High alkalinity and ammonium are most likely produced through the liberation of CO2 and nitrate, respectively, from decaying organic matter. Dissolved inorganic carbon (DIC) concentrations increase as carbonic acid forms and dissociates predominantly into bicarbonate, thus increasing alkalinity. As oxygen and less energetically favorable electron acceptors are depleted, the interstitial fluids develop ammonia at the expense of nitrate and concentrations of sulfate diminish. It is also possible that mobilization of Ba is responsible for low sulfate, though in the absence of Ba data from Hole U1402B, it is not possible to further examine this hypothesis. Sediment geochemistryConcentrations of total carbon, inorganic carbon, total organic carbon (TOC), and total nitrogen were measured from eight sediment samples (~5 cm3 plugs) taken from working halves of Cores 342-U1402B-1H and 2H (Fig. F8; Table T7). Inorganic carbon concentrations range from 0.2 to 2 wt%, which are equivalent to 1.6–17 wt% calcium carbonate assuming all of the inorganic carbon is present as calcite and/or aragonite. TOC concentrations are ~2 wt%, which is typical of mid-slope depositional environments. Total nitrogen concentrations range from 0.1 to 0.2 wt%. |
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