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LEG 202 SITE SUMMARIES (continued)

Site 1233


Site 1233 (proposed Site SEPAC-19) is located 38 km offshore (20 km off the continental shelf) at 838 m water depth in a small forearc basin on the upper continental slope (Fig. F1). Here, shoreward of the Peru-Chile Trench, basement is likely continental crust. Predrilling survey data included high-resolution Parasound profiles that imaged the site to a depth of 80 mbsf (Fig. F35) and an 8-m sediment core (GeoB3313-3) that suggested continuous sedimentation at the site at rates of 100 cm/k.y. within the Holocene. The drilling strategy included multiple-APC coring to ensure complete stratigraphic coverage in a composite section.

The intent of drilling at Site 1233 was to recover a very high resolution hemipelagic record of late Quaternary sediments for paleoceanographic investigations to (1) reconstruct millennial- to century-scale changes of climate related to latitudinal shifts of the westerly wind belt, upwelling intensity, and productivity; (2) reconstruct sea-surface salinity anomalies indicative of variations in river runoff or episodes of net glacier retreat in the fjord region of southern Chile (continental climate); and (3) monitor changes in the signature of AAIW (nutrients, temperature, and salinity), which ventilates the Pacific to intermediate depths.


This was our second attempt to occupy proposed Site SEPAC-19A; our first attempt on 7 April was aborted prior to drilling because of unsuitable weather. Operations at Site 1233 began at 0945 hr on 12 April. On approach to the site, the JOIDES Resolution 3.5-kHz precision depth recorder documented seismic reflectors to a depth of 110 mbsf (Fig. F36). A request to drill to that depth was approved by ODP/Texas A&M University (TAMU) headquarters.

The first attempt at a mudline core (Core 202-1233A-1H) resulted in a full core barrel. The bit was raised 7 m, and APC coring in Hole 1233B was initiated with a shot from 844.0 meters below rig floor (mbrf), which obtained a good mudline and advanced to 109.5 mbsf. The vessel was offset 10 m southeast three times to core and drill Holes 1232C, 1232D, and 1232E to depths of 116.3, 112.3, and 101.5 mbsf, respectively. Optimization of coring offsets was guided by real-time correlation of magnetic susceptibility data obtained with the OSU Fast Track. A total of six downhole temperature measurements were taken at Site 1233. The transit to the next site began at 0400 hr on 14 April.

Scientific Results

Five APC holes were drilled at Site 1233 to ensure a complete stratigraphic overlap between cores from different holes (Fig. F37). Detailed comparisons between high-resolution core logging data, including records of magnetic susceptibility, GRA bulk density, natural gamma ray (NGR), and color reflectance demonstrated that the entire recovered sedimentary sequence to 116.4 mbsf (136 mcd) was recovered completely.

The magnetic susceptibility of the uppermost 9 mcd recovered at Site 1233 can be correlated directly to the 14C-dated sediment record of core GeoB 3313-1, implying Holocene sedimentation rates of ~1200 m/m.y. on the mcd scale (which is about 20% more than true depths due to gas expansion and elastic rebound of the sediments after recovery). Anomalous magnetic directions and low remanence intensities over a 2-m interval between 65 and 70 mcd in Holes 1233B, 1233C, and 1233D suggest that a geomagnetic field excursion is recorded, perhaps the Laschamp Excursion (~41 ka). This preliminary evidence suggests sedimentation rates that could be as high as ~1800 m/m.y. during the pre-Holocene, which would imply an age near 70–80 ka at the base of the recovered sediment sequence. With these sedimentation rates, a diverse record of biogenic and terrigenous sediment components promises an unprecedented Southern Hemisphere record of millennial- to century-scale climate variability. Moreover, a shipboard-derived magnetic paleointensity proxy suggests that variations in geomagnetic field strength are recorded that may form the basis for synchronization of the stratigraphy at Site 1233 with other high-resolution climate records from the Northern and Southern Hemispheres.

The recovered sedimentary sequence at Site 1233 consists primarily of clay and silty clay with varying amounts of calcareous nannofossils. Interbedded minor lithologies include thin silt-rich layers, which might represent distal turbidites, and five volcanic ash layers that offer the potential to provide regional stratigraphic markers. High magnetic susceptibility values are consistent with hemipelagic sedimentation with a predominant siliciclastic component. Preliminary predictive relationships between color reflectance and carbonate and TOC, developed via a multiple linear regression (R2 = 0.7 and 0.9, respectively), indicate substantial millennial-scale variability in these biogenic components.

High mean magnetic susceptibility values below ~10 mcd are consistent with high sedimentation rates and an increased terrigenous sediment supply during the last glacial interval. TOC contents and, to a lesser extent, calcium carbonate contents decrease from the Holocene to the glacial interval, consistent with either significant terrigenous dilution of the biogenic components or downhole diagenesis of organic matter.

The inferred sedimentation rates of 1200–1800 m/m.y. at Site 1233 (Fig. F38) are high, even for most upper continental slope settings (which commonly have accumulation rates in the range of hundreds of m/m.y.). The southern coast of Chile is marked by high rainfall and a dense drainage system of rivers in a mountainous region, and this combination of forcing factors on sediment supply may plausibly account for high sedimentation rates in this local basin, although local focusing of the regional supply may also occur. The paucity of turbidites in the basin suggests that downslope turbidity flows are mostly channeled away from the basin in the extensive system of canyons that characterizes this portion of the Chile margin.

Mineral assemblages at Site 1233 are consistent with a siliciclastic sediment provenance in both the Andes and the Coastal Ranges. Quartz/feldspar ratios decrease downhole, and this might indicate an increase during the last glacial interval in the supply of basaltic to andesitic Andean source rocks that are rich in feldspar relative to the predominantly metamorphic source rocks of the Coastal Range that are rich in quartz.

Calcareous nannofossils, foraminifers, and diatoms are abundant and well preserved at Site 1233. The continued presence of Emiliania huxleyi to the bottom of Hole 1233B documents that the entire sequence is younger than 0.26 Ma. However, the E. huxleyi acme zone (base at 0.08 Ma) was not observed because of the generally low abundance of the species.

The planktonic foraminiferal assemblage indicates an age younger than 0.65 Ma for the entire sequence. The observed species are typical of eastern boundary settings in the transition from the subtropical to the subpolar systems. Benthic foraminiferal assemblages represent 15% to 50% of the total foraminifers and indicate an upper to middle bathyal environment with high carbon fluxes to the seafloor. The downhole benthic foraminiferal succession likely reflects changes in bottom-water oxygenation that probably relate either to fluctuations in surface water productivity and/or subsurface circulation, including AAIW.

The diatom flora is dominated by Chaetoceros spores, bristles and vegetative cells, and Thalassiosira species and reflects intermittent upwelling conditions at Site 1233. Freshwater diatoms, mainly Aulacoseira granulata, are present in low abundance throughout the core but appear more consistently between 58 and 97 mcd. This interval is also marked by marine benthic diatoms that suggest sediment redeposition. Silicoflagellates, radiolarians, sponge spicules, and phytoliths are observed in most samples.

NRM intensities at Site 1233 were extremely high before and after AF demagnetization at peak fields up to 25 mT. AF demagnetized inclinations averaged approximately –52°, close to the expected inclination for an axial geocentric dipole (–59°) at this site latitude (~40°S), indicating that the drill string overprint was largely removed. The negative inclinations are consistent with normal polarity, and the entire sequence is interpreted to lie within the Brunhes Chron (0–0.78 Ma). Fine-scale variations in magnetic inclination and declination are consistent from hole to hole, suggesting that a high-fidelity record of geomagnetic secular variation is recorded. Anomalous magnetic directions and low remanence intensities over a 2-m interval between 65 and 70 mcd in Holes 1233B, 1233C, and 1233D suggest that a geomagnetic field excursion is recorded. Based on estimated sedimentation rates, this could be the Laschamp Excursion (~41 ka). Normalization of the demagnetized NRM intensity by MST-derived magnetic susceptibility suggests that these sediments may provide high-resolution records of relative geomagnetic paleointensity. These extraordinary records should provide opportunities for high-resolution regional correlation of marine and terrestrial records using paleomagnetic secular variation and perhaps global correlations through paleointensity variations. Two intervals with exceptionally low remanence intensity (95–101 and 113–118 mcd) may reflect events of extreme sediment diagenesis, suggesting that the paleomagnetic data from these two intervals should be considered with caution.

Gas, fluid, and sediment geochemical profiles are dominated by the influence of organic matter diagenesis, despite the relatively low organic carbon contents (average = 0.9 wt%; range = 0.4–2.5 wt% TOC). C/N averages 5.9, consistent with a marine origin of the organic matter. Sulfate reduction is complete by 5 mcd, with methane, apparently of biogenic origin, increasing by 20 mcd and remaining at high levels throughout. These findings are consistent with the downhole decrease in organic carbon content from ~2 wt% in the uppermost sediments to <1 wt% by 20 mcd. The zone of sulfate depletion is marked by a maximum in dissolved barium, possibly from barite dissolution. High alkalinity (peak values > 60 mM), high ammonium concentrations (up to 5 mM), and high phosphate concentrations (up to >200 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. Interstitial water chloride concentrations decrease by >10% over ~120 mcd, requiring some source of relatively fresh water at depth. Calcium carbonate content of the sediments averages 5.4 wt%, with a range of 1.24–10.75 wt%.

The combination of continuous recovery in a composite section to ~136 mcd, exceptionally high sedimentation rates apparently driven by input of terrigenous silt and clay in a hemipelagic setting, an extraordinary record of paleomagnetic field variations, clear geochemical signatures of organic matter diagenesis and authigenic mineralization, and a rich array of well-preserved biogenic and mineralogic tracers of paleoclimatic utility suggest that Site 1233 will provide unprecedented opportunities for understanding Southern Hemisphere and global climate changes on scales from millennia to centuries, perhaps even decades, and processes of sediment diagenesis.

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