Site 1234 (proposed Site SEPAC-13B) is located ~65 km offshore (5 km off the continental shelf) and 60 km shoreward of the Peru-Chile Trench on a relatively flat bench in the middle of the continental slope at 1015 m water depth (Figs. F1, F39). Basement is likely continental crust. Precruise seismic survey data suggested mostly hemipelagic sedimentation at the site. The uppermost part of the seismic profile shows flat-lying reflectors with deformed but continuous layers farther below, possibly reflecting the original relief of the acoustic basement (Fig. F40).
This site was chosen to take advantage of the high sedimentation rates on the continental margin that will allow reconstructions of regional climate and oceanographic variability on millennial to centennial timescales for the late Quaternary. Specific goals were to
The vessel was on station by 0650 hr on 15 April, after a 288-nmi, 26-hr transit from the previous site. Hole 1234A was initiated with the recovery of a mudline core and advanced to 100.3 mbsf with 101% recovery, which was followed by XCB coring to the target depth of 205.2 mbsf with 84% recovery. The vessel was moved 10 m to the east, and Hole 1234B was initiated with an offset to target coring gaps in Hole 1234A. Another mudline was recovered, and piston coring advanced to the APC refusal depth at 93.8 mbsf with an average recovery of 104%. The hole was deepened to 182.4 m with the XCB. The vessel was offset 10 m east of Hole 1234B to target remaining coring gaps in Hole 1234C, which was advanced to a depth of 79.1 mbsf. Prior to coring Core 202-1234B-1H, a bottom-water temperature measurement was obtained using the APC temperature tool (APCT). Downhole APCT temperature measurements were attempted with Cores 202-1234A-4H, 7H, and 10H (33.8, 62.3, and 90.8 mbsf, respectively). The vessel left location at 2300 hr on 16 April in dynamic positioning mode because the next site (proposed Site SEPAC-14A) was only 7 nmi away.
We developed a composite depth section at Site 1234 (0.0240.4 mcd) and a splice representing a complete stratigraphic section from 0 to 94.4 mcd based primarily on hole-to-hole correlation of magnetic susceptibility data. Expansion of cores because of elastic rebound and degassing after recovery caused the composite depth section to be longer than the cored interval by ~18%.
The 239-mcd-thick sequence (203.8 mbsf) of Quaternary sediments (Fig. F41) is dominated by siliciclastic components, primarily of homogeneous dark olive-gray to dark gray silty clay and clay, with subtle and gradational color changes that mainly reflect small variations in sediment composition. Eight ash layers were recovered. The clastic/biogenic ratio increases and the quartz/feldspar ratio decreases downhole, indicating increased dilution of the biogenic material with detritus from the Andean provenance downcore.
Of the biogenic components, calcareous nannofossils are the most abundant overall, with decreasing concentration downhole. Diatoms are the next most abundant group and show maximum concentration in the middle part of the section. Foraminifer abundance peaks near the top and is low through to the base of the section. Fragments of mollusk shells are common, consistent with downslope transport of material from the continental shelf. Recognizable turbidites are rare; however, a few thin silt-rich layers were found, indicating that major turbidity flows are contained in the extensive channel system that characterizes this steep continental margin.
All fossil groups indicate that the entire sequence is of Quaternary age. The nannofossils suggest that the base of the cored interval is younger than 0.26 Ma. Calcareous nannofossils are generally moderately to well preserved in the samples examined, except for the core catcher samples from the lowermost six cores where recrystallization was significant. Planktonic foraminifers are present in all samples, but abundance and preservation vary markedly. Large fluctuations in the relative abundance of both planktonic foraminiferal and nannofossil taxa indicate large changes in surface water properties through time.
Site 1234 is within the highly productive coastal upwelling area near Concepción, and diatom floras are dominated by the upwelling-related taxa. Despite the dominance of the coastal upwelling forms, warm-water species are present in higher abundances below 62 mcd, whereas cold-water forms dominate the younger record. Benthic diatoms are present in most of the samples, indicating relatively persistent redeposition from the upper continental margin.
The benthic foraminiferal assemblage is dominated by high carbon flux indicators that are common in continental margin settings. Downhole variations may reflect shifts in bottom-water oxygenation related to either changes in deepwater circulation or changes in surface production and export of organic matter. For example, a distinct peak in the abundance of Bolivina spp. at ~175 to 223 mcd (>40% of total benthic foraminifers) points to an intense episode of seafloor dysoxia.
Gas, fluid, and sediment geochemical profiles are dominated by the influence of organic matter diagenesis, despite the relatively low organic carbon contents (average = 1.1 wt%; range = 0.23.0 wt% TOC). Sediments from Site 1234 were very gassy. Core liners were perforated after recovery to relieve the pressure and prevent excessive core expansion. High amounts of methane and low amounts of ethane indicate that the gas is biogenic in origin, resulting from in situ methanogenesis of the marine organic matter. Two intervals with high organic carbon concentrations are present at 54.590.2 mcd and at a depth of ~220 mcd with values up to 2.95 wt%. TOC/total nitrogen ratios range between 5 and 10, indicating a marine origin of the organic matter. This is consistent with pyrolysis results, which show that sedimentary organic matter is significantly degraded.
Calcium carbonate concentrations are low, ranging from 0.8 to 11.7 wt%, averaging 3.5 wt%, and increasing at depths >115 mcd. The good preservation of carbonate fossils in most samples indicates that the relatively low carbonate contents result mostly from dilution by siliciclastic material carried by intense river runoff rather than from dissolution and recrystallization associated with the degradation of organic matter. Sulfate reduction is complete by 9.7 mcd, with methane, apparently of biogenic origin, increasing rapidly to 9.7 mcd then remaining at high values throughout. High alkalinity (peak values > 63 mM), high ammonium concentrations (up to 11 mM), and high phosphate concentrations (up to >120 mM) result from organic matter degradation by sulfate reduction and methanogenesis, similar to observations at other continental margin sites. Dissolved calcium concentrations drop rapidly, consistent with the effects of authigenic mineralization reactions driven by the high alkalinity values. Magnesium/calcium ratios increasing to >30 from 17.9 to 40.4 mcd are consistent with authigenic precipitation of calcite in shallower sediments. Below that interval, magnesium/calcium ratios decrease with increasing depth to ~12 by 230 mcd, indicating that authigenic mineralization reactions may take up magnesium deeper in the sediments.
NRM intensities at Site 1234 were extremely high before and after AF demagnetization at peak fields up to 25 mT. AF demagnetized inclinations averaged approximately 30°, far from the expected inclination for an axial geocentric dipole (55°) at this site latitude (~36°S), indicating that the drill string overprint was substantial. The largest apparent overprints occurred when the APCT was used, perhaps due to the extra time the sediment sat in the steel core barrel while temperature measurements were made.
In spite of these difficulties, patterns of paleosecular variation are apparent that include excursional directions in both inclination and declination in three holes from 21 to 23 mcd, which may represent the Laschamp Excursion (~41 ka). If so, average sedimentation rates in the upper 23 mcd are ~50 cm/k.y. Extrapolation of either of these estimates suggests that Site 1234 records at least two, and perhaps up to four, of the late Pleistocene glacialinterglacial cycles at very high resolution.
In summary, the combination of continuous recovery in a composite section to ~94.4 mcd, a rich assemblage of biogenic fossils and terrigenous sediment components, promise of an improved paleomagnetic record after further removal of overprints, and sedimentation rates high enough to record millennial-scale climate variability over at least one full (and perhaps several) glacial cycles makes Site 1234 a perfect complement to Site 1233. Shipboard data provide evidence for substantial variations in biogenic production and suggest that the recovered sediment sequence will provide a detailed record of climate and biogeochemical changes of the late Quaternary.
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