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The oceanographic and geographic settings of Leg 202 sites influence their biogeochemical environments, as reflected in the volatile hydrocarbon, interstitial water, and sediment geochemistry. The latitudinal range for Leg 202 (from ~41°S to ~7.5°N) is significantly larger than those ranges for ODP legs that focused on the productive regimes of major eastern boundary currents: Leg 167 sites from the California Current region (~30–40°N) and Leg 175 sites from the Benguela Current region (~4°–32°S). Water depths at Leg 202 sites range from 489 to 4072 m, affecting the delivery of organic matter to the sediment/water interface. Linear sedimentation rates range over at least two orders of magnitude, from <1 to >100 cm/k.y. The oxidation of organic matter is the major influence on interstitial water geochemistry, with resulting effects on volatile hydrocarbon geochemistry and on authigenic carbonate mineralization reactions. In addition to the prevailing influence of organic matter degradation, two sites are affected by methane hydrates (Sites 1233 and 1235), and a third site shows the signature of fluid flow in the underlying basement (Site 1240).

Organic Matter Oxidation: Sulfate Reduction, Methanogenesis, and Nutrient Regeneration

Organic matter oxidation in marine sediments proceeds by a sequence of reactions typically observed with increasing depth (aerobic respiration, denitrification, manganese reduction, iron reduction, sulfate reduction, and methane fermentation). The geochemical sampling and analytical strategies employed during Leg 202 are best suited to observing the effects of organic matter oxidation through the depletion of sulfate, an oxidant, and the production of methane, a product of methane fermentation. On the basis of sulfate profiles (Fig. F20), sites can be divided into three categories:

  1. Those with no to limited sulfate reduction (Nazca Ridge sites [1236 and 1237], and the deeper Cocos Rise site [1241]);
  2. Those with an intermediate degree of sulfate reduction (the deeper Carnegie Ridge site [1238]); and
  3. Those with complete sulfate reduction (Chile Basin site [1232], Chile margin sites [1233–1235], and the shallower Carnegie Ridge and Cocos Rise sites [1239 and 1241]).

Previous studies including some Peru margin sites have found a relationship between interstitial sulfate gradients with depth and bulk sedimentation rates, with steeper sulfate decreases (i.e., shallower depth of sulfate disappearance) related to faster sedimentation rates. This relationship appears to be generally true for Leg 202 sites, with deeper sulfate "zero" depths for the shallower of the Carnegie Ridge and Cocos Rise sites relative to the more rapidly accumulating Chile margin and Chile Basin sites (Fig. F20). However, further evaluation of this requires definition of sedimentation rates for the Chile margin and Chile Basin sites, all with relatively shallow sulfate disappearance depths of <30 mcd (Table T4).

The categorization of sites by sulfate profiles is clearly linked to the production of methane at those sites, as the presence of interstitial sulfate is known to inhibit methanogenesis in marine sediments (Claypool and Kvenvolden, 1983). The sites with limited sulfate reduction and thus high sulfate concentrations had very low methane concentrations, whereas the deeper Carnegie Ridge site (1238) with intermediate sulfate concentrations had only low to moderate levels of measured methane (Table T4). In contrast, all the sites with complete sulfate depletion had high measured methane values, with high methane concentrations being reached at depths coincident with or just deeper than the depth of sulfate disappearance (Table T4). Note that the limited depth resolution of both interstitial water and volatile hydrocarbon sampling makes a more detailed assessment impossible.

Because the oxidation of organic matter is important in the production of alkalinity in interstitial waters, the values of the alkalinity maxima for these sites also follows the division on the basis of the sulfate profiles. Sites with limited sulfate reduction have relatively low peak alkalinity values (2 to >5 mM), the site with intermediate sulfate reduction has a moderately high alkalinity maximum (>17 mM), and the sites with complete sulfate reduction have the largest alkalinity maxima (>25 mM to as high as 60 mM) (Table T4). The regeneration of phosphorus (as dissolved phosphate) and nitrogen (as dissolved ammonium) are also closely linked to organic matter oxidation, and the peak values at the sites follow the same division by redox intensity (Table T4).

Sedimentary Organic Matter: Competing Influences and the Importance of Dilution

In contrast to the wide ranges of sulfate reduction, alkalinity generation, and methane production, the sedimentary organic carbon concentrations have a much narrower range for Leg 202 sites. The average organic carbon concentrations are typically lowest for the three sites with limited degrees of sulfate reduction (Sites 1236, 1237, and 1241), indicating that the supply of labile organic matter to serve as reductant is the limiting factor in setting the redox state of these sediments (Table T4). Many sites show an initial decline of organic carbon concentrations with increasing depth, consistent with organic matter oxidation. However, the competing influences of organic carbon supply from primary productivity, the effects of water depth at a site on organic carbon delivery to sediments, and the importance of dilution by detritial and biogenic sediments based on site location make simple generalizations about site organic carbon concentrations difficult.

Authigenic Mineralization: Calcium and Magnesium Profiles in Interstitial Waters

Alkalinity produced by organic matter oxidation can drive carbonate mineralization reactions, including precipitation and replacement reactions producing calcite and/or dolomite. If precipitation reactions are sufficiently intense, they can result in substantial decreases in interstitial calcium, in contrast to the more typical conservative increases of calcium with depth driven by reactions in basement seen in many marine sediments. Decreases in calcium can drive large increases in magnesium/calcium ratios, and high values of this ratio appear to be necessary to promote authigenic dolomite formation.

At sites with little to no sulfate reduction, interstitial calcium typically is constant or increases with increasing depth (Fig. F21A) and magnesium/calcium ratios are constant or decrease with increasing depth (Fig. F21C). In contrast, the deeper Carnegie Ridge site (1238) with an intermediate degree of sulfate reduction (Table T4) has a calcium minimum ~40% lower than seawater values, and magnesium/calcium ratios increase to nearly 8 just shallower than the sulfate minimum (Fig. F21A, F21C). The depletion of calcium is more pronounced at sites with complete sulfate reduction and with minimum calcium concentrations at least 80% lower than seawater concentrations (Fig. F21B). The depth of the calcium minimum is typically shallower than the alkalinity maximum (Table T4). The large decreases in calcium drive substantial increases in magnesium/calcium ratios to values as high as 40 (Fig. F21D). Higher magnesium/calcium ratios more effectively promote dolomite formation, which acts as a sink for magnesium. Sites with more intense sulfate reduction via organic matter oxidation are likely to be influenced more heavily by authigenic carbonate mineralization reactions, and this must be considered when evaluating their utility for ocean history reconstructions.

Chloride Profiles: Gas Hydrates in Two Chile Margin Sites

The two shallower Chile margin sites (1233 and 1235) show clear evidence of the influence of methane hydrates in interstitial chloride profiles based on the large decreases in chloride with increasing depth (Fig. F22). These gradients are significantly larger than those observed at the deeper water, midslope Peru margin basin sites drilled during Leg 112 (Sites 682, 683, 685, and 688; site water depths range from 3072 to 5071 m). Decomposition of gas hydrates could explain the observed chloride decreases as a result of dilution, either through in situ decomposition at depth in the sediment column below the hydrate stability zone or as an artifact during sediment recovery. In contrast to Sites 1233 and 1235, the deeper-water Chile margin site (1234) has a more conservative chloride gradient with depth (Fig. F22).

Fluid Flow in Basement: Panama Basin Site 1240

A Panama Basin site (1240) has a relatively thin total sediment cover, and drilling reached basement at 289.2 mcd. Organic matter diagenesis influences the interstitial water chemistry, as indicated by the sulfate profile (Fig. F23). The decline in interstitial sulfate with depth is accompanied by increases in alkalinity, phosphate, and ammonium (Fig. F23). However, sulfate reduction is not complete at this site. The return of sulfate and alkalinity values toward seawater values near the sediment/basalt interface, along with those of other elements, indicates flow of relatively unaltered seawater in the underlying basement. Large-scale horizontal advection of such waters through oceanic crust in the central equatorial Pacific has been inferred from interstitial water geochemistry and is thought to be responsible for the low conductive heat flow (i.e., low thermal gradients) observed in that region. This influence of advective flow at Site 1240 means that any estimates of the influence of processes like authigenic mineralization based on interstitial water geochemistry may tend to underestimate their effects at this site. The incomplete sulfate reduction is not because of lack of oxidizable organic matter, as indeed the middepth sediments at this site are relatively organic carbon rich, but is the result of the effectiveness of the resupply of sulfate from above and below the sediment column, resulting in distinctive interstitial profiles (Fig. F23).

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