Site 1235 (proposed Site SEPAC-14A) is located on a gently sloping terrace of the upper continental slope, ~65 km shoreward of the Peru-Chile Trench and ~60 km offshore (Figs. F1, F39). This site was chosen to take advantage of the expected high sedimentation rates to decipher the late Quaternary history of continental climate and southeast Pacific oceanography on millennial to centennial timescales at a water depth appropriate to monitor the boundary between the subsurface Gunther Undercurrent and AAIW. Predrilling surveys, including gravity cores and seismic data, provided evidence for hemipelagic sedimentation at the site. The upper part of the seismic profile documents well-stratified, flat-lying reflectors (Fig. F42). At greater depths, deformed but continuous layers of moderate reflection appear to smooth the original relief of the acoustic basement, which is likely to be continental crust.
Site 1235 is located at a water depth of 489 m, ~526 m shallower than and 10 km to the northeast of Site 1234. We expect that these two sites have been influenced by similar surface water conditions, so major differences in physical, chemical, and paleontological properties can be attributed to depth-related effects.
The primary goals of Site 1235 are to
The vessel was offset 7.0 nmi from Site 1234 using the dynamic positioning system and with the drill string deployed to 375 m. The transit took 5.5 hr at an average speed of 1.3 kt, and the vessel was on location at 0412 hr on 17 April. Three APC holes were drilled at Site 1235 in an attempt to recover a complete stratigraphic section. Most cores contained numerous voids because of gas expansion during the retrieval process. Holes were drilled into the core liners using nonmagnetic brass drill bits to relieve some of the gas pressure before cores were processed on the catwalk. We were able to reach the maximum authorized penetration at Site 1235 using APC coring, so no XCB coring was attempted. Two bottom-water and six downhole temperature measurements were taken with the APCT in Holes 1235A and 1235B.
A nonmagnetic core barrel, cutting shoe, and flapper valve were deployed here for the first time during Leg 202 (Cores 202-1235C-3H, 5H, 7H, 9H, and 11H) after excessive magnetic overprint was discovered in cores recovered with standard tools. Cores retrieved with the nonmagnetic barrel had substantially less overprint than those retrieved in the regular steel barrels.
At Site 1235 we recovered a 215-m-thick hemipelagic sequence of rapidly accumulated upper Pleistocene to Holocene silty clay (Fig. F43). The planktonic foraminiferal assemblage is typical of the late Pleistocene Subzone PT1b, and the continued presence of the calcareous nannofossil species E. huxleyi to the bottom of Hole 1235A documents that the entire sequence is younger than 0.26 Ma (Zone NN21). Sedimentation rates are at least 80 cm/k.y. This allows us to provide materials for reconstruction of Pleistocene and Holocene climate and oceanography with millennial- to centennial-scale resolution and accomplishes one of our primary objectives at this site.
Three APC holes were drilled to ensure a complete recovery of the stratigraphic section. A composite section was constructed on the basis of high-resolution core logging data, and complete recovery was most likely achieved to ~171.45 mcd. Significant ship heave during drilling combined with gas expansion of the sediment after recovery made correlation between holes difficult. For example, precise splicing between holes was problematic in three intervals at ~45–49, 71–73, and 126–128 mcd, and as a result a formal splice was not constructed.
The hemipelagic sequence at Site 1235 is composed primarily of siliciclastic silty clay with little lithologic variability. The silt fraction is dominated by feldspars, with lesser amounts of quartz, pyroxenes, and volcanic glass. These mineral assemblages are consistent with a siliciclastic sediment provenance in both the Andes and the Coastal Range. The large supply of siliciclastic sediment onto the upper continental margin of central Chile mainly results from rivers that drain the mountainous regions of the Andes and the Coastal Range.
Calcareous nannofossils, diatoms, and foraminifers vary from rare to abundant, and the species present are typical of an active upwelling zone on a continental margin. Microfossil preservation is good in the upper section and deteriorates downhole. Reworked benthic diatoms are found frequently. The ubiquitous presence of burrows and benthic faunal assemblages does not reveal any episodes of bottom-water anoxia. As at Site 1234, the benthic foraminiferal assemblage is dominated by species associated with high carbon fluxes in an oxygen minimum zone environment. Marked changes in the relative proportions of benthic foraminiferal species indicate variations in oxygenation or carbon fluxes at the seafloor that are probably related to temporal and spatial fluctuations of the upwelling system and/or shifts in subsurface water masses. A distinct peak in the abundance of Bolivina (>40% of total benthic foraminifers) between 78 and 118 mcd points to an intense episode of seafloor dysoxia. This interval comprises a narrower interval (~90–102 mcd) of distinctly decreased magnetic susceptibility and a* values (Fig. F44).
Sediment from Site 1235 contained abundant methane. Low amounts of ethane indicate that the gas is biogenic in origin, resulting from in situ methanogenesis of marine organic matter (TOC/total nitrogen ratios = 5–9). TOC contents are low throughout the core, averaging 0.6 wt% and ranging from 0.4 to 1.5 wt%. Calcium carbonate concentrations average 2.4 wt% and range from 0.3–15.5 wt%. The low organic carbon and carbonate contents relative to other continental margin settings indicate dilution with siliciclastic material. Organic matter degradation and carbonate dissolution also contribute to the observed variations of these components.
Authigenic pyrite and carbonate are present, especially in deeper intervals. Conspicuous carbonate concretions are intermittently present downhole. Intervals of abundant authigenic carbonate underlie zones of intense burrowing in at least three repetitive sequences. Color reflectance data document the presence of hematite and goethite (corresponding to positive a* values) near the top of these sequences, in each case overlying green sediments (negative a* values). Such systematic variations in mineralogy and color may indicate zones of strong redox contrast, perhaps related to major changes in sedimentation rate, productivity, and organic rain, or bottom-water oxygen content.
Sulfate reduction is complete at Site 1235 by 19.7 mcd. At this depth, the methane concentration increases rapidly to 55,648 ppmv and remains elevated at all greater depths. High alkalinity (peak values > 60 mM), ammonium (>8 mM), and phosphate (>200 mM) concentrations in interstitial waters result from organic matter degradation associated with sulfate reduction and methanogenesis. Dissolved calcium concentrations drop rapidly with depth to values <2 mM, consistent with the effects of authigenic carbonate mineralization driven by the high alkalinity. Magnesium/calcium ratios reach a high value of 41 at 53.6 mcd, consistent with precipitation of authigenic calcite in shallower sediments, but decrease to values as low as 2.4 at 210.3 mcd, indicating dolomite formation in deeper sediments. Calcium, strontium, and boron all increase significantly at depths >140 mcd, implying a source of these elements at depth.
Pore water chlorinity at Site 1235 decreases by >12%, from 553 mM at 1.5 mcd to 483 mM by 210.3 mcd. This change is accompanied by even larger decreases in salinity and sodium concentration. The decrease in chlorinity with depth at Site 1235, similar to that of Site 1233, is more extreme than those observed at the midslope sites on the Peru margin drilled during Leg 112 and indicates the presence of methane hydrates that dissociated either within subsurface sediments or after recovery.
Initial NRM intensities were extremely high, ranging from 0.5 to 2.1 A/m, and were characterized by steep positive inclinations (averaging demagnetization at peak fields up to 25 mT only partially reduced this overprint. To mitigate these analytical artifacts, a nonmagnetic core barrel, cutting shoe, and flapper valve were alternated with a standard steel core barrel for every other core of Hole 1235C. The nonmagnetic core barrel substantially reduced the viscous magnetic overprint and therefore may prove to be of significant benefit for future paleomagnetic studies in sediment of this type. Paleomagnetic secular variation features may be preserved at Site 1235 within the declination record, and this may provide a means of detailed correlation with Sites 1233 and 1234.
Site 1235 has clearly met our shipboard objectives of providing rapidly accumulating sediment for the study of century- to millennial-scale changes of climate, biogeochemistry, oceanography of surface and subsurface water masses, and paleomagnetic variations. Preliminary data on the fossil biota, geochemical composition of interstitial water and sediment, and terrigenous sediment components lead us to anticipate a rich paleoceanographic and paleoclimatic record, as long as stratigraphic challenges associated with recovery of gassy cores can be overcome.