Site 1232 (proposed Site SEPAC-19A) is located in the Chile Basin between the Mocha and Valdivia Fracture Zones, ~50 km westward of the Peru-Chile Trench (Fig. F1). The water depth of 4069 m is ideal for monitoring the influx of CPDW into the Chile Basin and variations in terrigenous sediment transported from southern Chile. On the Nazca plate, the tectonic backtrack path of Site 1232 is roughly parallel to surface-ocean properties, such as sea-surface temperatures in the subpolar transition zone (Figs. F4, F6). Crustal ages are not well defined but are expected to be between 19 and 29 Ma based on the limited study of regional seafloor magnetic lineations. The total thickness of the sedimentary section was estimated at 470 m based on site-survey seismic profiles that revealed well-stratified, moderately reflective layers in a style characteristic of hemipelagic sediments (Fig. F33).
The primary objectives at Site 1232 were to recover a nearly continuous sequence of Neogene hemipelagic sediments for paleoceanographic investigations in order to
The drilling strategy included triple-APC coring until refusal and single-XCB coring to basement, with the expectation that sediments as old as the Miocene and perhaps the Oligocene would be recovered. However, Site 1232 revealed a dramatically different sequence than expected. The upper 382 mcd (362 mbsf) represents an expanded upper Pleistocene section (<1 Ma) of interbedded silty clay (inferred upper parts of turbidites and hemipelagic sediments) with thin beds of sandy silt (basal parts of distal turbidites). Given the apparent uniformity of the seismic section, we infer that all sediments above acoustic basement are similar to that of the cored interval. Given time constraints, development of an appropriate weather window to drill on the margin, and the unlikely prospect of finding pre-Quaternary sediments at Site 1232, we abandoned coring at 382 mcd (362 mbsf). However, we continued APC coring to 98.8 mcd (90.4 mbsf) in Hole 1232B and to 38.9 mcd (33.2 mbsf) in Hole 1232C to improve stratigraphic overlap between sediment sections so that the site objectives related to late Quaternary climate variations may be addressed.
Leg 202 began at 1200 hr on 29 March 2002 when the JOIDES Resolution was tied up in Valparaiso, Chile. We left port at first daylight on 4 April and headed to proposed Site SEPAC-19A on the Chile margin, the southernmost site we planned to drill. After a 515-nmi transit from Valparaiso, which included a brief 3.5-kHz survey of proposed Sites SEPAC-13B and SEPAC-14A, the JOIDES Resolution was driven off of Site SEPAC-19A by a force 8 storm and proceeded an additional 94 nm at low speed to occupy Site 1232 (proposed Site SEPAC-09A). Hole 1232A was initiated at 0500 hr on 8 April. Swell continued to be relatively high, and drilling proceeded with 12 APC cores and 27 XBC cores, with a total penetration of 371.3 mbsf. Hole 1232A was not logged. Holes 1232B and 1232C included ten APC cores to 90.1 mbsf and four APC cores to 33.2 mbsf depth, respectively. Operations at this site were concluded at 2200 hr on 11 April and were followed by a 94-nmi (~9 hr) transit back to Site SEPAC-19A under improved weather conditions.
The 382-m-thick sedimentary sequence recovered at Site 1232 is composed of gray silty clay and clay interbedded with >800 graded layers of dark gray silty sand beds with sharp, sometimes scoured, basal contacts (Fig. F34). Biogenic sediments are present in trace abundance, except in occasional thin layers within the dominant silty clay and clay. The silty sand layers, which we interpret as the basal parts of distal turbidites, have high magnetic susceptibility and GRA bulk density and low chroma (a* and b*) and reflectance (L*) values.
Over the full cored interval, the turbidite layers range from a few millimeters to 118 cm thick (average = 3 cm) and are present throughout the section between silty clay layers that range from a few millimeters to 610 cm thick (average = 27 cm). The sandy silt intervals account for ~9% of the total section. Thickness and frequency of the turbidites decrease downhole; recognized sandy silt intervals account for ~22% of the section from 0 to 20 mbsf, 15% of the section from 20 to 200 mbsf, and 3% of the section from 200 to 250 mbsf.
Within both interbedded lithologies, clay minerals and feldspar are common, whereas amphiboles, pyroxenes, mica, and quartz are present in minor amounts. Such a mineral assemblage is consistent with a source area in southern Chile that is dominated by andesitic volcanic rocks of the Andes Mountains. A plausible transport path for turbidity currents is via the southernmost extent of the Chile Trench north of the Chile rise, where the trench has relatively little bathymetric expression.
The abundance of calcareous microfossils (nannofossils and foraminifers) at Site 1232 is generally low, and their preservation is generally poor to moderate. Foraminifer species suggest substantial changes in climate, varying between polar (Neogloboquadrina pachyderma) and eastern boundary (Globigerina bulloides and Globorotalia inflata) assemblages. Rare specimens of subtropical foraminifer species with warm-water affinities, such as Globigerinoides ruber, were found in a few samples. Diatom abundance and preservation fluctuate significantly and deteriorate noticeably deeper than 202 mcd (185 mbsf; Hole 1232A). Marine diatoms abundance is highest in the upper 100 m of the sediment record, and the assemblage is dominated by coastal upwelling and eastern boundary current forms, such as Chaetoceros spp.
Reworking of microfossils is apparent in all of the fossil groups examined. In particular, freshwater and benthic diatoms, such as Aulacoseira granulata, and shallow-water benthic species, such as Eunotia spp., are especially abundant in the intervals at 43–69 mcd (37–56 mbsf; Hole 1232A) and 193–241 mcd (176–224 mbsf; Hole 1232A) and suggest reworking from continental sources.
Ages of the sediment are inferred from calcareous nannofossils and planktonic foraminifers. The nannofossils suggest an age of 0.26 Ma at 143 (241
NRM intensities at Site 1232 were extremely high both before (mean = 1.41 A/m) and after alternating-field (AF) demagnetization (0.21 A/m) at peak fields up to 20 mT. The large drop in remanence intensity after demagnetization is due to a strong drill string positive magnetic overprint that is substantially but incompletely removed after shipboard demagnetization. Aside from drilling disturbance and the drill string overprint, the NRM after demagnetization is characterized by negative inclination (normal polarity) throughout the sequence and therefore is interpreted to represent the Brunhes Chron (0–0.78 Ma).
A complete composite section and splice were developed in the interval from 0 to 42.7 mcd (0–37.6; mbsf in Hole 1232B). Father downhole, correlation between cores in different holes form a floating splice to 100.4 mcd (83.94 mbsf; Hole 1232A). We primarily used magnetic susceptibility data acquired with the OSU Fast Track system for the correlation. This track analyzes individual 1.5-m core sections in about 2 min and allows stratigraphic correlation to keep pace with and guide coring operations.
Additional core logging data, including magnetic susceptibility, GRA density, natural gamma radiation, and P-wave velocity, were acquired with the ODP MST. Reflectance spectra and point-sensing magnetic susceptibility were measured on the archive-half MST. The Geotek line-scan digital imager was operated on its own track to record red-green-blue color images at a pixel resolution of ~100 µm. These three additional tracks operated at a slower rate than the Fast Track, typically at 5–20 min/s, in order to optimize data quality. These data will be used later to refine hole-to-hole correlation and to interpret lithologic composition in greater detail. Physical property measurements of moisture and density yielded excellent relationships to GRA density, which indicates minimal coring disturbance at this site.
Results from gas, interstitial water, and sedimentary composition measurements at Site 1232 reflect the influences of organic matter diagenesis and the low biogenic content of sediments at this site. The sediments were moderately gassy, with methane first appearing at 27.3 mcd (23.0 mbsf, Hole 1232A) and dominating the gas composition throughout. The disappearance of dissolved sulfate in interstitial water (<2 mM) at this depth, coupled with high alkalinity concentrations (>30 mM) and elevated phosphate and ammonium concentrations (>50 mM and >2 mM, respectively), is consistent with organic matter degradation being a primary influence on interstitial water and gas composition at this site. Interstitial waters have dissolved silicate concentrations <600 µM, below saturation with respect to biogenic opal. Dissolved calcium concentrations decrease from seawater values at the sediment water interface to <6 mM by 27.2 mcd (23.0 mbsf), with a pronounced minimum of <4 mM at 93.4 mcd (85.0 mbsf; Hole 1232A). The sediments are poor in organic carbon, typically <0.25 wt%. Organic carbon/organic nitrogen ratios indicate dominance of marine organic matter. Calcium carbonate contents are typically low, less than a few weight percent, although isolated peaks were found with values as high as 27 wt%.
The combination of the age constraints from biostratigraphy and magnetostratigraphy suggest siliciclastic-dominated sediment accumulation rates of >450 m/m.y., roughly a factor of 10 to 20 higher than regional pelagic sedimentation. The seismic reflection profile indicates an unconformity between the bottom reflector and the turbidite-dominated Pleistocene sequence. Extrapolation of late Pleistocene sedimentation rates to the unconformity suggests that much of the sediment accumulation here has occurred within the last million years. A similar situation has been described on the Peruvian continental margin, with a change from carbonate to terrigenous deposition in the late Pleistocene (0.93–0.44 Ma). Whether these dramatic secular changes in sedimentation off western South America can be attributed to changes in tectonics or climate remains a question. On the fine scale, inferred sedimentation rates imply that the turbidite layers observed at Site 1232, which have an average depth spacing of ~30 cm, have an average recurrence time on the order of hundreds of years. Great earthquakes associated with active tectonism of southern Chile, as well as century- to millennial-scale climate changes, may serve as possible triggers for large turbidity flows that transport continental sands and silts to Site 1232.