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Interstitial water

The salient characteristics of interstitial water from Hole M0027A are how fresh it is and how frequently and abruptly it alternates from fresh to salty downhole. Chloride concentrations vary from 41 to 530 mM, compared to 524 mM in the shallowest pore water sample (from 0.5 mbsf) that likely reflects ambient bottom water (Table T9; Fig. F41). The average chloride concentration over the 631 m sampled for pore water, weighted by depth interval, is 273 mM, and only 52% of that is in the shallowest sample; in other words, nearly half of the water in the drilled interval is fresh. We sampled at least five distinct layers of freshwater, some thin and some thick, at ~29, 79, 87, 183–341, and 419 mbsf. Backflow was observed in the drill pipe at 225 mbsf, within the thickest layer of freshwater, suggesting that an aquifer had been penetrated. These fresh layers alternate with salty layers at ~0–15, 47–71, 80–83, 96–167, and 351–410 mbsf that reach chloride concentrations of at least 434–529 mM chloride, 83%–101% of that in bottom seawater. Even with our average sampling interval of 7.3 m (87 pore water samples over 631 m sampled), it is clear that downhole gradients in the salinity of pore water are frequent and sharp. A central task in interpreting the pore water chemistry is to explain how these sharp chemical gradients are maintained in the face of chemical diffusion, which tends to soften such gradients and then, in time, to erase them altogether.

Beneath the lowest freshwater layer at 419 mbsf, the chloride concentration increases linearly with depth to 488 mM, 93% of that in bottom seawater, at 621 mbsf, near the bottom of the hole. Comparison of this profile with those from Holes M0028A and M0029A suggests that this increase continues with depth below the drilled interval and probably achieves concentrations well in excess of that in seawater. This linear increase below 419 mbsf is much more gradual than every other fresh to salty transition in Hole M0027A and suggests there are no more freshwater layers in this hole below that depth. This suggestion is reinforced by other aspects of the pore water chemistry discussed below. Three small peaks in this otherwise linear gradient, at 493, 507, and 549 mbsf, correlate with other chemical species and need to be investigated further.

Most of the major and minor ions in the seawater mimic the depth profile for chloride quite closely (Fig. F41), indicating that the interlayering of freshwater and seawater dominates the chemical profiles rather than reaction with sediment. These ions include Br, Na, K, Mg, Ca, and Sr. In discerning the effects of chemical reaction and differences in the chemistry of the various freshwater layers, therefore, ratios of these elements to chloride (Fig. F41) are more useful than concentrations. Sulfate (Fig. F42) mimics chloride except for the deepest interval, below 419 mbsf; over this interval, chloride increases linearly whereas sulfate stays at low concentrations, mostly <1 mM. Over the same deep interval, pH decreases and alkalinity increases tenfold, from the 2.3 mM typical of seawater (but in the freshwater layer) to a maximum of 26 mM at 549 mbsf (Fig. F42). Ammonium also increases greatly over this depth interval, mimicking alkalinity. These chemical changes are the characteristic signature of microbial oxidation of organic matter in anoxic sediments, utilizing sulfur in sulfate as an electron acceptor:

SO42– + 2CH2O = H2S + 2HCO3,

where CH2O represents generic organic matter. In this reaction, N in organic matter is liberated as ammonium and sulfate is reduced to H2S, which then typically precipitates as Fe sulfides, which eventually recrystallize to become pyrite. We detected an H2S odor in the cores only rarely during Expedition 313 (the only depth at which we detected it in Hole M0027A was at 236 mbsf), so this process is not likely to be very active at present over this depth interval. We did, however, observe frothing and bubbling of the cored mud at 565 and 577 mbsf, suggesting that microbes may be generating methane within this sulfate-depleted interval. Note also that the sulfur content of the sediments is relatively high below ~420 mbsf, reaching 2.7 wt% at 440 mbsf (Fig. F43).

Carbonate alkalinity generated by the above reaction causes the measured rise in the pore water but also drives precipitation of CaCO3 as follows:

Ca2+ + 2HCO3 = CaCO3 + H2O + CO2.

CaCO3 precipitation dominates near the alkalinity peak at 549 mbsf, as the ratio of Ca to chloride reaches a minimum over this same depth interval, at 538–555 mbsf. The highest concentration of carbonate C in the sediment was also measured near this interval, at 534 mbsf (although measurements over this depth range are sparse) (Fig. F43). Alkalinity then generally falls off with increasing depth because of less organic matter oxidation and/or continued precipitation of CaCO3 as Ca rises along with overall salinity. Although Ca concentration increases with depth over this interval, Ca/Cl does not, as a result of carbonate precipitation.

Over this same interval from 419 to 631 mbsf, relative to the steep increase in chloride, Na, K, and B decrease; Mg increases by 60%; Li and Sr nearly double; and Br, like Ca, shows no large net change, as illustrated by the ratios of each of these species to chloride (Fig. F41; Li/Cl, B/Cl, and Sr/Cl are not shown). Also increasing greatly over this interval is Ba (although Ba/Cl is more complicated), because of dissolution of barite from the sediment in response to microbial sulfate reduction and the resulting decrease in dissolved sulfate.

Freshwater and saltwater layers above 419 mbsf have distinct chemistries. These differ not only between fresh and salty but also between individual fresh or salty layers. The three freshwater layers within the upper 100 mbsf, for example, have Na/Cl ratios that are lower and Mg/Cl ratios that are higher than those in the adjacent salty layers and seawater, whereas in the 158 m thick layer of freshwater at 183–341 mbsf and the deepest layer at 419 mbsf, these ratios vary in the opposite sense. These differences presumably reflect the different origins and histories of the various freshwater layers. By contrast, the five salty layers above 419 mbsf, as well as the water with seawater chloride concentration from near the bottom of the hole, all have Na/Cl ratios near that of seawater (with the exception of a single sample from 394 mbsf, which is discussed in detail below). The five freshwater layers generally have higher K/Cl, Ca/Cl, and Br/Cl ratios than in the adjacent salty layers and in seawater and also higher alkalinity, especially in the thick layer of freshwater at 183–341 mbsf. This thick fresh layer also has exceptionally high Si and B/Cl and low sulfate, ≤1 mM, which is much lower than would be expected from its chloride content, suggesting that microbial sulfate reduction has occurred within this interval as well, consistent with its higher alkalinity. Ba is high, too, in response to the low sulfate and resulting barite dissolution from the sediment.

Underlying the thick freshwater layer at 183–341 mbsf is a thick salty layer at 351–410 mbsf that is bounded on its upper side by a thin layer of highly cemented sand and on its lower by the deepest freshwater layer at 419 mbsf. This salty layer contains seawater that is more altered than any shallower in the hole but not as altered as that found below 419 mbsf. Its chloride concentration reaches 491 mM, 94% of the seawater value, its Na/Cl and sulfate are similar to values in seawater, but it has lost some K and Mg and gained Ca, Sr, and P. These are typical changes for a low-temperature reaction with igneous material, from which Ca is leached in exchange for Mg and K is taken up into secondary clay minerals and/or zeolites. The highest peak in ammonium in Hole M0027A occurs at the lower boundary of this salty layer, with a second large peak at its upper boundary. Between these two peaks, ammonium falls to a low concentration. This pattern is likely microbial in origin. One microbial community appears to be producing ammonium by exploiting the chemical gradients between fresh and salty water at the upper and lower boundaries of the salty layer, whereas a second process that may also be microbial is consuming ammonium in between. It may not be coincidental that dissolved phosphorus is high at these intervening depths. Note, however, that sediment within this interval at 351–410 mbsf contains almost no organic carbon, carbonate, or sulfur (Fig. F43).

Mn and Fe are both sensitive to redox conditions within the sediment, as both elements form extremely insoluble oxide and hydroxide minerals. Fe, in fact, is often lost from pore water prior to its recovery by squeezing; it is also easy to contaminate with at low concentrations. Mn data from pore water are generally much more reliable. The multiple peaks in these elements (Fig. F42) suggest discrete zones of reduction. Five of these zones show peaks in both Mn and Fe and are the most likely to reflect real processes in the sediment. The upper four, at 15–18, 75, 96, and 175 mbsf, all occur within the narrow and sharp transitions from fresh to salty water. Their presence within these transitions suggests they may be microbial in origin, as microbes are masters at exploiting chemical gradients. Further investigation of these zones is certainly warranted. Note that organic carbon is generally quite low in the sediment above ~170 mbsf except near the three layers of freshwater (Fig. F43).

The fifth and deepest peak, which lies at 394 mbsf within the salty layer discussed above, represents the single largest and most problematic chemical anomaly in the pore waters from Hole M0027A. It is defined by a single sample from the core catcher from Core 313-M0027A-139R. This sample has exceptionally high Mn, Fe, Li, B, K, Si, Ca, and Sr and low Na/Cl. For Si, Ca, and Sr, these are the highest concentrations and for Na/Cl the lowest value measured in Hole M0027A, and they are nearly so for Fe, B, and K. Whether this anomaly represents contamination of some sort we are still debating, but an identical anomaly occurs in Holes M0028A and M0029A from multiple samples that are less obviously disturbed. Its cause does not appear to be simple contamination by drilling mud. Drilling fluid is enriched relative to seawater in Si, Ca, and Sr (Table T9) but to nowhere near the concentrations measured in the pore water sample from 394 mbsf. Drilling fluid is also enriched in ammonium, Ba, Al, P, and, for the fluids used in Holes M0028A and M0029A (but not Hole M0027A), in Mn and Fe. It does not appear to be enriched in Li, B, and K, nor does it have low Na/Cl. The composition of drilling fluid is therefore a poor match for that of the pore water from 394 mbsf and for the similarly anomalous samples from Holes M0028A and M0029A.


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).

Total carbon, total organic carbon, and total sulfur

TOC and TS are relatively abundant in all five layers occupied by freshwater in Hole M0027A and are nearly absent in the salty layers, except for the deepest salty layer below 420 mbsf to the bottom of the hole (Fig. F43). This is because the five salty layers above 420 mbsf are all extremely quartz rich, based on XRD analyses, whereas the freshwater layers all reside in quartz-poor sediment that is rich in detrital and authigenic minerals, including pyroxene, epidote, chlorite, magnetite, goethite, and the clay minerals kaolinite, glauconite, and mica + illite, along with mixed-layered clays (Figs. F44, F45). Pyrite is also abundant in three of the five freshwater layers; its relative abundance as determined by XRD correlates extremely well with TS (Fig. F43). These minerals are all present at low abundance or not at all in the salty layers, based on XRD. All but the uppermost salty layer at 0–15 mbsf also lack calcite, plagioclase, and K-feldspar. TOC and TS are especially abundant in the two deepest freshwater layers; within the thickest of these, at 183–341 mbsf, concentrations reach 7.4 wt% TOC and 4.0 wt% S.

These observations are consistent with the lack of evidence for sulfate reduction within the salty layers, except below 420 mbsf; as noted above, pore water sulfate profiles mimic chloride above 420 mbsf, and the sulfate/chloride ratio is constant at the seawater value within these salty layers (Fig. F43). Within the freshwater layers, by contrast, the presence of organic carbon allows for microbial sulfate reduction, so at least some of the sulfur, present as pyrite in the sediment, is likely authigenic in origin. Within the salty layer below 420 mbsf, where pore water sulfate has clearly been reduced, some or all of the pyrite is certainly authigenic.

TIC, or carbonate carbon, is highest at 0–22 mbsf within the uppermost salty layer and its transition to freshwater, where it is present mainly as calcite and high-Mg calcite, based on XRD analyses (Fig. F44); near the transition from freshwater to saltwater at ~90–110 mbsf, where it is present mainly as high-Mg calcite and dolomite; within the thick freshwater layer from 183 to 341 mbsf, where it is present as both calcite and dolomite; and within the transition to saltwater as the bottom of the hole is approached, at ~420–600 mbsf, where it is present as calcite, high-Mg calcite, and dolomite. Large intervals in Hole M0027A, particularly within layers occupied by saltwater, are essentially carbonate free. As noted in the discussion of pore water chemistry above, the highest concentration of TIC measured in Hole M0027A was 3.2 wt% at 534 mbsf, within the deep salty layer where sulfate reduction has occurred, producing carbonate alkalinity and inducing CaCO3 precipitation. This highest TIC concentration correlates with a similarly large XRD peak in high-Mg calcite.

As noted in the above discussion of pore water, the thick salty layer at 351–410 mbsf has exceptionally high concentrations of ammonium at its upper and lower boundaries, where it adjoins freshwater. Within the salty layer, ammonium concentrations are much lower, whereas dissolved phorphorus is much higher. Apatite content as determined by XRD is likewise low within this salty layer but higher along its boundaries; in fact, apatite concentrations are higher along several of the freshwater/saltwater boundaries in Hole M0027A than elsewhere, suggesting that it may be authigenic (Fig. F43).

Mineralogy by X-ray diffraction

The semiquantitative abundance of a number of minerals has been determined by XRD from the ratio of the intensity of a peak for a single mineral divided by the sum of the integrated intensities for all peaks in a scan (Table T11). The mineralogy of Hole M0027A is heavily dominated by quartz, which accounts for 6%–86% of the signal for a given sample (mean = 62%). Other minerals with maximum signals above 6% are, in order from highest concentration to lowest, kaolinite, plagioclase, chlorite, K-feldspar, Mg calcite, calcite, clays, siderite, micas and illite, mixed-layer clays, pyroxene, pyrite, and glauconite. As noted above, the depth profile for pyrite abundance correlates extremely well with that for TS measured chemically on the same sample (Fig. F43). The abundance of carbonate minerals also correlates well with the chemical measurement of carbonate carbon (TIC), but only when all carbonate minerals are considered; XRD data can thus be used to estimate the relative proportions of calcite, high-Mg calcite, dolomite + ankerite, and possibly rhodochrosite (Fig. F44) and siderite (Fig. F45).

Many of these minerals show similar abundance patterns with depth in Hole M0027A and thus correlate with one another. The strongest correlations are found between dolomite + ankerite and rhodochrosite; pyroxene, epidote, and amphibole; siderite and apatite (Fig. F44); kaolinite, chlorite, magnetite, goethite, glauconite, and, to a lesser extent, pyrite and gibbsite, all of which are anticorrelated with quartz; and clays, mica + illite, mixed-layered clays, and, to a lesser extent, smectite (Fig. F45).

The distribution of these minerals with depth in Hole M0027A correlates well with the distribution of freshwater and saltwater layers, which thus appears to be determined largely by lithology. The five salty layers above 420 mbsf contain abundant quartz and not much else, whereas the five freshwater layers contain abundant detrital and authigenic minerals, including pyroxene, epidote, chlorite, magnetite, goethite, and the clay minerals kaolinite, glauconite, and mica/illite, along with mixed-layered clays (Figs. F44, F45). Pyrite is also abundant in three of the five freshwater layers, whereas it is present at very low concentration or not at all in the salty layers. Plagioclase and K-feldspar are especially abundant within the uppermost 22 mbsf.