SUMMARY OF RESULTS (continued)
Geochemistry
The drilling of five sites in the region off Tasmania, in particular Sites 1168, 1170, 1171, and
1172, provided unique insights into both regional and temporal variation in organic and interstitial
water geochemistry. The results of the organic geochemical characterization of sediments cored
during Leg 189 have led to the following salient observations. First, a two-tiered carbonate
distribution is characteristic of Sites 1170, 1171, and 1172. In this distribution, Paleogene
sediments are generally carbonate poor, whereas Neogene sediments are carbonate rich. Second,
the transition from Paleogene carbonate-poor to Neogene carbonate-rich sediments appears to be
mostly quite abrupt, except at Site 1168 where it is gradual. Third, an antithetic relationship exists
between carbonate and TOC content. In this relationship, Paleogene sediments usually contain
<0.5 wt% TOC, whereas Neogene sediments are relatively free of organic matter. Fourth, organic
matter type determined primarily by Rock-Eval pyrolysis and paleosalinity characterizations using
C/S ratios provide a basis for intersite geochemical facies correlation from site to site. The facies
show distinct, and perhaps important, changes at the Paleocene/Eocene boundary, in the middle
Eocene, and near the Eocene/Oligocene boundary. Fifth, at Sites 1168, 1170, and 1171, methane
contents obtained from headspace gas measurements show an abrupt increase in the organic
carbon-containing Paleogene sediments.
Paleogene sediments with relatively high TOC and low CaCO3 contents exist at Sites 1168, 1170, 1171, and 1172 (Fig. 21). These sediments are bioturbated and have higher natural gamma values (Fig. 20), in particular Th/U ratios mostly >2. These characteristics indicate dysoxic to oxic conditions on the seafloor during deposition. Most of the relatively high TOC-low CaCO3 sediments may have had enhanced burial efficiency of organic matter associated with more rapid siliciclastic sedimentation. The generally poorer ventilation of the waters during this time also would have promoted high TOC-low CaCO3 sediments relative to the Neogene.
The high carbonate content of Neogene sediments reflects dominance of calcareous nannofossils; foraminifers are secondary in importance. The upward increase in carbonate content at all sites during Leg 189 is a consequence of a change from shallow-marine to pelagic open ocean conditions. The low organic carbon content of these carbonate-rich sediments suggests deposition through a well-mixed water column to a well-oxygenated seafloor. The transition from carbonate-poor to carbonate-rich sediments appears to be relatively abrupt at Sites 1170, 1171, and 1172, although higher resolution sampling at Site 1172 demonstrates that the change in carbonate content is gradual through ~10 m of section. This observation suggests that higher resolution sampling across this boundary at Sites 1170 and 1171 may better define the nature of this carbonate-rich/carbonate-poor boundary. Specifically, the presence of a condensed section can be inferred if the carbonate content increases gradationally across the boundary, whereas the presence of an unconformity can be considered where an abrupt change in carbonate content is observed. At Site 1168, the change from siliciclastic to carbonate sedimentation is gradational, perhaps because of its distance from processes affecting the other sites.
The similarity of geochemical facies between sites is significant because it suggests regional scale changes in seafloor and water-column conditions. Upper Paleocene-lower Eocene sediments encountered at Sites 1171 and 1172 contain elevated TOC content and variations in C/S and HI values indicative of fluctuations between terrestrial and marine inputs (Fig. 21). These characteristics suggest rapidly changing environmental conditions across the Paleocene/Eocene boundary in the Tasmanian region. A middle Eocene episode of elevated organic carbon burial was widespread in shallow-marine environments. During this time, a clear gradient in organic matter from terrestrially influenced to dominantly marine is observed from Site 1170 to Sites 1171 and 1172. These observations may reflect deposition at Site 1170 within the Australo-Antarctic Gulf, whereas Sites 1171 and 1172 were more strongly influenced by the Paleogene Pacific Ocean.
The highest organic carbon contents observed during Leg 189 are from upper Eocene sediments at Site 1168. Here, TOC content exceeds 5 wt% and is nearly wholly terrestrial in origin. Such characteristics were not encountered at any of the other sites and likely reflect the lower latitudes and proximity of Site 1168 to hinterland source regions, as well as the site location well within the restricted Australo-Antarctic Gulf. However, upper Eocene sediments at Sites 1168, 1170, and 1171 share a C/S record of brackish water conditions before the transition from carbonate-poor to carbonate-rich sedimentation. This signal is difficult to resolve within a setting of relative water deepening. Perhaps, the seaway conditions were deeper, but climatic fluctuations were sufficient to cause episodes of "freshening" of the water column and/or sediments. Alternatively, the brackish water signature in C/S values may actually represent unrecognized periods of decreased pyrite formation. In either case, more analyses will be needed to understand the significance of the geochemical record from this interval.
Headspace gases at all Leg 189 sites are best characterized as biogenic, although thermogenic inputs were observed at the base of most holes. Rock-Eval pyrograms from Paleogene sediments often display a double S2 peak character, suggesting the presence of bitumen. Tmax values obtained by Rock-Eval pyrolysis indicate that organic matter is immature generally, although values characteristic of the "oil window" occur at depth in most holes. Methane gas content is closely tied to pore-water sulfate concentrations at Sites 1168, 1170, 1171, and 1172 (Fig. 22), which in turn is closely tied to the lithostratigraphic separation between carbonate-rich and carbonate-poor sediments. The onset of methanogenesis immediately beneath the zone of sulfate reduction exhibits the characteristics of microbially driven diagenetic depth zonation. In this model, carbonate-poor Paleogene sediments contain sufficient organic matter for complete sulfate reduction, whereas the sulfate reducers in carbonate-rich Neogene sediments inhibit methanogens. At Site 1172, however, the presence of dissolved sulfate is likely inhibiting methanogenesis in the organic carbon-bearing Paleogene sediments. This observation is unusual because the TOC content of these low-gas sediments appears to be sufficient to have already driven sulfate concentrations to zero.
One of the surprising discoveries of the Leg 189 interstitial water geochemistry program was the presence of regionally extensive low-chloride (Cl-) pore fluids in the older sediments throughout the region, including on the west Tasmania margin (Site 1168), the STR (Sites 1170 and 1171), and the ETP (Site 1172) (Fig. 22). In general, the fresher pore waters are located in the older part of the cored interval but are in different age sediments and are not restricted to specific lithologies. These fresher fluids are manifested in the Cl- profiles by multiple distinct maxima and minima rather than smoothly decreasing values. Minimum Cl- values at the four sites range from 486 to 440 mM, a 13%-21% decrease relative to the mean seawater value (559 mM). At three of the sites, pore-water freshening coincides with the onset of methanogenesis; however, low Cl- fluids were encountered at Site 1172, where only traces quantities of methane are present (Fig. 22).
At present, the origin of the low-Cl- fluids is enigmatic. Low Cl- values in marine pore waters have been observed in environments ranging from accretionary prisms to passive continental margins. One possible external source of low-Cl- fluids in passive continental margins is the advection of meteoric waters from the continent (e.g., Austin, Christie-Blick, Malone, et al., 1998). However, the geographic separation of the sites (Fig. 3) and the general geologic setting make it improbable that there are links to continental recharge areas.
Possible internal sources that may provide low-Cl- fluids include (1) gas hydrate dissociation, (2) dehydration reactions of hydrous minerals, such as clays and biogenic opal, (3) clay membrane ion filtration (e.g., Kastner et al., 1991; Hesse and Harrison, 1981; Paull, Matsumoto, Wallace, et al., 1996), and perhaps (4) connate fluids. Gas hydrate dissociation is a common cause of such profiles in continental margin settings. However, crude estimates of the base of the gas hydrate stability zone (GHSZ) at Sites 1168, 1170, and 1171, assuming a pure methane and seawater system, indicate the low-Cl- fluids are below the stability zone. The depth of the hydrate stability zone will be sensitive to the chemistry of the pore fluids and incorporation of other gases into the hydrate structure (Dickens and Quinby-Hunt, 1997; Sloan, 1998). Therefore, a more rigorous calculation of gas hydrate stability may extend the depth of the stability zone. However, the presence of low-Cl- fluids at Site 1172, with only traces of methane present, appears to eliminate gas hydrates as a possible mechanism, at least at Site 1172.
At present, we cannot eliminate connate fluids as the possible source of fresher fluids. However, it is hard to imagine that the distinct maxima and minima evident in all Cl- profiles would not have been smoothed by diffusional processes that are clearly at work with other interstitial water constituents (Fig. 22). In fact, the jagged nature of the Cl- profiles suggests that the emplacement of fresher fluids has occurred relatively recently. Clay mineral reactions are a well-known source of fresh fluids, and clay content does increase in the older part of the sedimentary succession. Although reaction kinetics are also important, previous research, both experimental and natural observations, indicates that dehydration reactions (e.g., alteration of smectite) occur at elevated temperatures of 80°C or higher (e.g., Perry and Hower, 1972; Velde, 1983). Temperature measurements made during Leg 189 indicate geothermal gradients of ~50°-60°C/km, and all sites were drilled <1 km deep. Thus, the origin of these regionally extensive fresher pore fluids remains unresolved, and postcruise isotopic analysis of the interstitial waters will be required to better understand these diagenetically and/or paleoenvironmentally important fluids.
In addition to the low-Cl- fluids, pore-water profiles are characterized by reactions involving alteration of silicate minerals, carbonate recrystallization, and organic matter degradation (Fig. 22). Alteration of silicate minerals, within and below the cored section, lead to covarying decreases in Mg2+ and K+ and increases in Li+ within the pore fluids. The smooth, diffusional Mg2+ and K+ profiles observed at all sites are likely the result of uptake in reactions below the cored interval. Carbonate recrystallization is most pronounced in the Oligocene and Neogene pelagic carbonates, resulting in increases in interstitial Sr2+. The thinnest carbonate sequence (Site 1171) has the lowest Sr2+ concentrations with the thinnest interval of elevated Sr2+ (cf. Sr2+ to carbonate profiles in Fig. 21). As described above, organic matter reactions are most active in the thick, organic-rich Eocene sediments in the lower part of the cored intervals, which may result in differing sulfate gradients (i.e., thicknesses of the sulfate reduction zone) as sulfate is rapidly depleted in the organic-rich Eocene sections.