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 earlier 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 the 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 in marine sediments proceeds by a sequence of reactions typically observed with increasing depth below the seafloor (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. F27), Leg 202 sites can be divided into three categories:
Previous studies including those at 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 Ridge sites relative to the more rapidly accumulating Chile margin and Chile Basin sites (Fig. F27). 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, 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 follow 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) because of strong organic matter oxidation. 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).
A primary objective of Leg 202 is the study of variations in biological production in this eastern boundary current region and their role in modifying subsurface water masses characteristics such as oxygenation and nutrient inventories through time. TOC concentrations determined aboard ship were therefore used to give first insights about the richness of sediments in organic material and export production. TOC/total nitrogen (TN) ratios and Rock-Eval pyrolysis parameters were used to help identify the quality of the organic matter (e.g., origin and preservation).
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. Organic carbon concentration maxima do not exceed 4.5 wt%, even for sites at shallower water depths where biological production in the overlying waters is known to be high, such as in coastal upwelling areas. Average organic carbon concentrations are typically lowest for the three sites with limited degrees of sulfate reduction (Sites 1236, 1237, and 1241). Higher methane concentrations and partial to complete sulfate reduction are found at sites where sedimentary organic carbon concentrations are higher, with maximum values of at least 1.5 wt% (Sites 1233-1235, 1238-1240, and 1242). This indicates that the supply of labile organic matter controls the redox state of these sediments (Table T4). The availability of oxidants during diagenesis may also change both the quantity and composition of the organic matter in the upper several hundred meters of marine sediments.
TOC/TN ratios are commonly used to discriminate between marine and terrestrial organic matter. Marine organic materials have atomic TOC/TN ratios between 4 and 10, and terrestrial organic matter TOC/TN ratios are above 20. Whereas degradation during sinking and burial diagenesis can increase TOC/TN ratios, in Leg 202 sediments they are usually ~10 (Fig. F28), indicating that organic matter buried in sediments is mainly of marine origin along the southeast Pacific margin. One exception is Site 1241, where higher TOC/TN ratios are found. These high ratios may result from degradation, because the supply of land-derived organic matter is expected to be negligible at this site located on a ridge and away from the coast.
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 detrital and biogenic sediments based on site location make simple generalizations about site organic carbon concentrations difficult. For instance, the degradation process would be dominant in driving TOC changes if TOC decreases are associated with increases in TOC/TN ratios, whereas preservation would be the dominant process if an inverse correlation had been observed. The lack of a clear correspondence between these two parameters (Fig. F29) may indicate that both processes significantly influence the preservation of TOC. Results of Rock-Eval analyses of selected samples from several sites confirm this.
Dilution by siliciclastics appears to play an important role in controlling organic matter concentration in many of the sites, particularly at Sites 1233-1235 from the Chile margin, where sedimentation rates are extremely high (>50 cm/k.y. to >1 m/k.y). Although the high concentrations of volatile hydrocarbons at these three sites indicate that the burial flux of metabolizable organic matter is very important, TOC concentrations are low compared to other eastern boundary sites where coastal upwelling occurs, such as along the coastal branch of the Benguela Current. At the Chile margin sites, variations in TOC contents are mainly influenced by extremely high terrigenous sediment supply as a result of enhanced fluvial discharge in response to continental rainfall.
Results from two cores from the central Chile margin, close to Sites 1233-1235, document a decrease in TOC concentrations from the Holocene to the last glacial state (MIS 2) Hebbeln et al. (2002). The MARs of organic carbon, however, reveal a reversed pattern. Organic carbon MARs were higher during MIS 2 than during the Holocene as a consequence of higher glacial sedimentation rates. This pattern is very similar to that observed at Site 1233. Hence, the effect of dilution and varying sedimentation rates has to be considered for paleoproductivity reconstructions from Leg 202 sites, and a multiproxy approach should be preferred.
At Site 1237 on Nazca Ridge, the increase in TOC concentrations since 3.5 Ma can be attributed to a change in export production. A larger influence of degradation or preservation is unlikely according to the variations observed in TOC/TN ratios and Rock-Eval data. Dilution by calcium carbonate is minor. In addition, dilution by siliciclastics can be excluded because the supply of siliciclastics also increased over the last ~3.5 m.y. The increase in TOC contents could be the result of an intensification of coastal upwelling or the plate tectonic migration that moves Site 1237 closer to the coastal upwelling zone.
Similarly, export production and thus organic material delivery to the sediments seem to be the dominant factors in driving TOC variations in the sedimentary sequences of Sites 1238-1240 along the equatorial divergence. For instance, in these three sites, an organic-rich interval corresponding to Pliocene-Pleistocene times has been found. The generally good correlation between TOC variations and the presence of microfossils (diatoms in particular) in these intervals support this hypothesis.
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. F30A) and magnesium/calcium ratios are constant or decrease with increasing depth (Fig. F30C). 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. F30A, F30C). 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. F30B). 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. F30D). 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.
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. F31). 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. F31).
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. F32). The decline in interstitial sulfate with depth is accompanied by increases in alkalinity, phosphate, and ammonium (Fig. F32). 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 (Oyun et al., 1995) 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. F32).