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doi:10.2204/iodp.proc.313.105.2010

Geochemistry

Interstitial water

Like Holes M0027A and M0028A, Hole M0029A displays alternating layers of fresher and saltier water within the upper part of the hole (Table T8; Fig. F25). Unlike earlier holes, however, Hole M0029A contains an increasingly saline brine from 345 mbsf to the deepest sample at 748 mbsf, in which chloride concentration reaches 995 mM, nearly twice the concentration in seawater. Brine was also just barely penetrated near the bottom of Hole M0028A from 627 to 664 mbsf. From 0 to 330 mbsf in Hole M0029A, there are seven relatively fresh layers, with minimum chloride concentrations from 19 to 313 mM, alternating with saltier layers with maximum concentrations from 373 to 509 mM. We were able to define these many layers in spite of the fact that the upper 200 mbsf was the only spot cored in this hole. Excluding brine in the lower half of the hole, the average chloride concentration in these alternating layers, weighted by depth interval, is 314 mM, nearly identical to the 309 mM calculated for Hole M0028A, indicating that, overall, 40% of the pore water in the upper half of the hole is fresh. The deepest layer of fresher water, at 315–324 mbsf, is shallower in Hole M0029A than in the other two holes. Brine in the bottom half of Hole M0029A approaches an asymptotic composition of ~1000 mM chloride, indistinguishable from similar asymptotic brine concentrations measured during Ocean Drilling Program (ODP) Leg 150 at Sites 902, 903, 904, and 906 farther offshore, from the upper slope at a water depth of 445 mbsl to the mid-slope at 1123 mbsl (Mountain, Miller, Blum, et al., 1994). Such convex-upward, asymptotic depth profiles for a nonreactive species like chloride are typical of one-dimensional advective-diffusive profiles that suggest upwelling of pore water, which in this case is a brine.

As in Holes M0027A and M0028A, most of the major and minor ions in the seawater closely mimic the depth profile for chloride (Figs. F25, F26). Ratios of these ions to chloride can therefore reveal the composition of waters from different sources within the upper half of the hole and the composition of the brine within the lower half. As in earlier holes, fresher and saltier layers each have distinctive, perhaps unique, compositions. Of the seven layers of fresher water in the upper half of the hole, for example, five have lower Na/Cl than seawater and two have higher Na/Cl. These same five layers have higher K/Cl than seawater, but the other two do not. The upper four layers have higher Br/Cl than seawater and the lower three do not. Some layers have higher Mg/Cl than seawater and some lower, whereas still others have similar ratios to seawater. All the fresher layers have higher Ca/Cl than seawater, but some are much higher than others. All layers of fresher water have a different pH from the adjacent salty layers and, although most are higher, at least one, at 277–286 mbsf, is much lower. It remains to be seen whether these compositions can be correlated laterally with compositionally similar layers in the other two holes. As brine becomes more concentrated with depth in Hole M0029A, Na, K, and Mg decrease relative to chloride and Br, Li, B, Ca, and Sr increase. Many of these changes in elemental ratio to chloride are approximately linear with depth. The directions of change are generally consistent with those seen in Holes M0027A and M0028A but are much more definitive because the changes are so much larger in the saltier brine in Hole M0029A.

Based on the ratio of sulfate to chloride, sulfate has been selectively reduced within freshwater layers, as was the case in Holes M0027A and M0028A. Alternatively, fresher waters have a source of sulfate independent of chloride (i.e., not seawater). Saltier layers all have sulfate to chloride ratios near that of seawater. Unlike the other holes, there is not a large increase in either alkalinity or ammonium within any of these layers, fresh or salty, nor was H₂S detected at any depth. Below the depth of the deepest freshened layer, sulfate drops to low concentrations, mostly <1 mM, presumably because of microbial sulfate reduction during oxidation of organic matter. Total sulfur (TS) in the sediment ranges as high as 4.2 wt% over this interval and pyrite as high as 5%–6% (Fig. F27). As expected, alkalinity, ammonium, phosphorus, and Ba increase greatly over the interval in which sulfate decreases. The distribution of apatite in the sediment is also greatly affected. Alkalinity then begins a slow decrease toward the bottom of the hole, as Ca increases and CaCO₃ precipitates in response. Increases in ammonium and Ba slow over this interval until they, too, decrease again near the bottom of the hole. Phosphorus undergoes this increase and then decreases over a much shorter depth interval, presumably because it is released by oxidation of organic matter and then is quickly reprecipitated as apatite (Fig. F27).

Like Holes M0027A and M0028A, Hole M0029A shows a large chemical anomaly in a single interval, comprising large peaks in Mn, Fe, Si, B, Li, and K. This time, the usual peaks in Ca and Sr are quite muted, if present at all, and there is no prominent low in Na/Cl. There is, however, an extreme low in pH, as there was in Hole M0028A. The anomalous interval in Hole M0029A is at 277–286 mbsf, shallower than the similar anomalies at 394 and 407 msbf in, respectively, Holes M0027A and M0028A. It also occurs in a layer that is not particularly salty, unlike the earlier cases. The cause of this single anomaly in each of the three holes remains unknown.

Sediment chemistry and mineralogy

Sediment samples were analyzed for total carbon (TC), total organic carbon (TOC), and TS concentrations. Total inorganic carbon (TIC) was calculated as the difference between TC and TOC (Table T9). Bulk sample X-ray diffraction (XRD) data were produced for the same samples (Table T10). The upper part of Hole M0029A was spot-cored, and sampling was sparse for these analyses; only three samples were measured above the deepest freshwater layer at 330 mbsf, at 7, 16, and 50 mbsf. We are thus unable to infer the effects of fresh versus salty layers as was done for Holes M0027A and M0028A. Instead, the samples document the effect of increasing salinity with depth from 330 mbsf to the bottom of the hole.

The uppermost sample at 7 mbsf is relatively rich in TIC and carbonate minerals and poor in TOC and TS (Figs. F27 and F28). The next deepest sample at 17 mbsf has the highest quartz concentration measured in Hole M0029A, 70%, and lies at the top of the first salty layer. The sample at 552 mbsf has the highest concentrations of TOC, TS, and pyrite measured in any sample recovered during Expedition 313. As in Holes M0027 and M0028A, TS correlates very well with pyrite as measured by XRD (Fig. F27). As noted above, apatite varies systematically with dissolved phorphorus, and siderite correlates with apatite, as it did in Holes M0027A and M0028A (Fig. F28). As dissolved Ca and Ca/Cl increase with depth in brine, alkalinity decreases steadily below 379 mbsf (Fig. F26), indicating precipitation of CaCO₃, mainly as calcite (Fig. F28). TIC is relatively high and variable over this depth interval, reaching a maximum concentration of 1.6 wt% at 696 mbsf, nearly the same depth at which alkalinity peaks within the general decline, at 690 mbsf.

As in Holes M0027A and M0028A, quartz dominates the mineralogy of the sediment in Hole M0029A, ranging in concentration from 37% to 70% in analyzed samples (mean = 59%) (Table T10; Fig. F29). Other minerals that exceed 6% are, in order, plagioclase, kaolinite, other clay minerals, and calcite.

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