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doi:10.2204/iodp.proc.313.104.2010

Geochemistry

Interstitial water

As in Hole M0027A, the pore water chemistry of Hole M0028A is dominated by the alternation of relatively fresh and salty layers (Table T9; Fig. F30). Although we have no samples from above 225 mbsf, we presume that multiple freshwater layers are present there as well, as our shallowest sample has a chloride concentration of 72 mM, only 14% of that found in present-day bottom seawater in the hole (524 mM). This is also the freshest water we sampled from Hole M0028A, and it is not nearly as fresh as its counterpart from Hole M0027A at 331 mbsf, which had only 41 mM Cl. The average pore water we sampled from Hole M0028A is also not as fresh: over the sampled interval of 439 m, from 225 to 664 mbsf, the average chloride concentration, weighted by depth interval, is 353 mM, 67% of that in bottom seawater, implying that about a third of the water is fresh, compared with half in Hole M0027A. The lower value for Hole M0028A is biased by the absence of samples from 0 to 225 mbsf; also, if the brine at the bottom of the hole is excluded, the freshwater fraction rises to 41%. Although the water from Hole M0028A may not be as fresh overall, we found significantly freshened water at a greater depth than in Hole M0027A: 214 mM Cl (41% of that in seawater) at 536 mbsf, 117 m deeper than the deepest fresh(er) water layer in Hole M0027A. We sampled three major freshwater layers (and several minor ones) in Hole M0028A, at <225–252, 270–324, and 517–536 mbsf. These were separated by relatively salty layers at 261–266, 335–407, and 551–664 mbsf (to the bottom of the hole). Although none of these layers could be readily correlated with those in Hole M0027A, either by depth, age, or lithologic facies, some similarities in the shape of the pore water profiles, especially for sulfate, suggest that some of the fresh and salty layers are laterally continuous between Holes M0027A and M0028A, 10 km apart, albeit with large changes in relative thickness. The minimum chloride content of each of the fresh layers increases with depth. The maximum chloride concentration measured, from 407 mbsf within the second of the three salty zones, which is complex and multilayered, is 607 mM, significantly higher than seawater and nearly identical to the 608 mM measured in the deepest sample. This value and the steady increase in chloride over 50 m to the bottom of the hole indicate that we encountered brines here for the first time during IODP Expedition 313 and that there is probably no more freshwater in this hole below that found at 536 mbsf. Brines with up to twice the salinity of seawater are a major feature of pore waters sampled at Ocean Drilling Program (ODP) Sites 902–906 on the outer shelf and upper to mid-slope off New Jersey during Leg 150 (Mountain, Miller, Blum, et al., 1994).

As in Hole M0027A, most major and minor ions in the seawater mimic the depth profile for chloride quite closely (Fig. F30), indicating that the interlayering of freshwater and seawater dominates the chemical profiles, rather than reaction with sediment. These ions again include Br, Na, Mg, Ca, and Sr. K is similar except for the lower 80 m, from 584 to 664 mbsf, over which K decreases and chloride increases. This trend is emphasized in the plot of K/Cl (Fig. F30) and is similar to that in Hole M0027A. Na also decreases relative to chloride over this deep interval as the waters become briny near the bottom of the hole, whereas Ca, Sr, Br, and Ba increase. In contrast to Hole M0027A, Mg, Li, and B are relatively trendless.

The sulfate profile resembles that for chloride except that microbial sulfate reduction has gone nearly to completion (<1 mM sulfate) within all layers of relatively freshwater below ~250 mbsf (i.e., within the entire interval that was cored), as well as within the saltiest water near the bottom of the hole. The same is true for Hole M0027A below 220 mbsf. Within the salty layers, in sharp contrast (except for the deepest, briny layer), sulfate and sulfate/chloride remain at seawater values; microbes have apparently been much more successful in oxidizing organic matter by reducing sulfate within the freshwater layers than within the shallower salty layers in both holes. Microbial activity appears to be especially intense within the deepest freshwater layer at 529 mbsf, which has the highest alkalinity and ammonium in the hole and still has some remaining sulfate (3.5 mM). At the base of this fresh layer 7 m deeper, at 536 mbsf, where sulfate is even lower at 0.4 mM, H₂S was detected for the only time in cores from Hole M0028A. As expected, alkalinity and ammonium are high wherever sulfate is low, as carbonate alkalinity and ammonium are the major products of organic matter oxidation. The exception is the deep briny layer near the bottom of the hole where, as in Hole M0027A, carbonate alkalinity is consumed by precipitation of CaCO₃ in response to increasing Ca, Ca/Cl, and salinity with depth as the water becomes briny. As expected for pore waters saturated with barite, Ba behaves inversely to sulfate in both fresh and salty layers. In Hole M0028A, pH varies inversely with alkalinity, which was not the case in Hole M0027A.

As in Hole M0027A, the major fresh and saltwater layers appear to have distinct chemistries. The uppermost freshwater layer at <225–252 mbsf, for example, has exceptionally high Na/Cl and B values, whereas the lowermost layer at 517–536 mbsf has very high K/Cl and the middle layer at 270–324 mbsf actually has lower K/Cl than in seawater. These distinct chemistries, along with the shape of the sulfate profiles, suggest that it may be possible to correlate some of these layers laterally by their composition. A preliminary test of this hypothesis works well for the thickest freshwater layer from Hole M0027A, at 183–341 mbsf, which appears to correlate with the middle layer in Hole M0028A, at 270–324 mbsf. Freshwater from these layers has similarly high ratios to chloride for Br, Li, Na, B, Ca, and Sr. The two waters share high alkalinity, ammonium, and P and low pH, Mg/Cl, and sulfate. They are not identical, but they certainly are similar. Whether they are sufficiently dissimilar from the other freshwater layers above and below to correlate them uniquely requires additional analysis. If this interpretation is correct, then the salty layer immediately above in Hole M0028A, from 261 to 266 mbsf, would correlate with the much thicker layer in Hole M0027A at 96–167 mbsf. Both of these layers have a small salty "shoulder" on their lower edge in both the chloride and sulfate profiles (Figs. F30, F31). The salty layer immediately below in Hole M0028A, at 335–407 mbsf, would correlate with a similar layer at 351–410 mbsf in Hole M0027A. Each of these salty layers displays multiple peaks for many elements and contains within it the largest chemical anomaly in each hole, a sharp peak defined by greatly elevated Mn, Fe, Si, B, Li, K, Ca, and Sr, at 394 mbsf in Hole M0027A and 407 mbsf in Hole M0028A. Below these salty layers lie the deepest freshwater layer in each hole, at 517–536 mbsf in Hole M0028A and 419 mbsf in Hole M0027A. Deeper still, Hole M0028A has a thick layer at 543–610 mbsf within which sulfate reaches the seawater concentration, which would correlate with the several lesser peaks in sulfate in Hole M0027A between 493 and 593 mbsf. If these correlations are accurate, the various layers would cross several sequence boundaries identified in seismic reflection profiles, indicating that hydraulic connectivity would be independent of these boundaries to some extent.

The multipeaked salty layer at 335–407 mbsf in Hole M0028A with the large chemical anomaly does not display the simple pattern of elevated ammonium and phosphorus seen at 351–410 mbsf in Hole M0027A that we attributed to microbial activity, but it does contain elevated concentrations of both of these chemical species. The large chemical anomaly at 407 mbsf in Hole M0028A is nearly identical to that located at 394 mbsf in Hole M0027A, except that here it is defined by three samples from 407 to 419 mbsf rather than by a single sample. It is even more extreme: it has the highest concentrations in the hole for Mn, Fe, Si, K, Li, and Ca and nearly so for B and Sr. As in Hole M0027A, it has the lowest Na/Cl ratio measured. Unlike Hole M0027A, it also has the highest P and the lowest pH measured. Its origin is presently unknown. Profiles for Mn and Fe are simpler in Hole M0028A than in Hole M0027A, with far fewer peaks, largely because we did not core the upper 220 mbsf. The only other depth in Hole M0028A at which both Mn and Fe are elevated is 600 mbsf.

Sediment chemistry and mineralogy

Sediment samples were analyzed for total carbon (TC), total organic carbon (TOC), and total sulfur (TS) concentrations. Total inorganic carbon (TIC) was calculated as the difference between TC and TOC (Table T10). Bulk sample X-ray diffraction (XRD) data were produced for the same samples (Table T11).

The patterns seen in Hole M0027A of high concentrations of TOC and TS in the quartz-poor freshwater layers and low concentrations in the quartz-rich salty layers are present in Hole M0028A (Figs. F32, F33, F34) but are not as obvious, in large part because sampling was sparser, but the clear distinction between fresh and salty water is not as obvious in Hole M0028A, either. The three salty layers sampled are generally quartz rich and clearly have lower TOC and TS, but the pattern for the freshwater layers is more complicated. The uppermost freshwater layer is clay rich and has high TOC and TS, but mainly along its upper and lower boundaries, suggesting a role for chemical reaction. Concentrations of TOC and TS are lower overall in Hole M0028A than they are in Hole M0027A, with maximum concentrations of 2.9 and 3.1 wt%, respectively.

As in Hole M0027, TS correlates very well with pyrite as measured by XRD (Fig. F32). The near absence of both TS and pyrite below ~620 mbsf, in the deeper part of Hole M0028A that is occupied by sulfate-free brine, indicates that this brine lost its sulfate somewhere else. It may well have ascended from greater depth, as suggested for the similar brine in Hole M0029A. As expected from the depth profiles for alkalinity and dissolved Ca, TIC is relatively high (0.7 wt%) within the brine because of precipitation of CaCO₃.

Quartz and kaolinite dominate the mineralogy of Hole M0028A even more than they did in Hole M0027A. Quartz accounts for 24% to 81% (mean 64%) of the total XRD signal for a given sample, and kaolinite for 0%–54% (mean 8.9%) (Table T11). The only other minerals with a maximum signal above 6% are chlorite and goethite.

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