LEG 202 SITE SUMMARIES

Site 1232 (proposed Site SEPAC-09A) 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 4072 m is ideal to monitor the influx of CPDW into the Chile Basin (Fig. F5) and variations in terrigenous sediment transported from southern Chile. Crustal ages are not well defined but are expected to be between 19 and 29 Ma based on 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. F42).

The primary paleoceanographic objectives at Site 1232 were to recover a nearly continuous sequence of Neogene hemipelagic sediments for paleoceanographic investigations to (1) reconstruct variations in the character of the Antarctic CPDW as it enters the Chile Basin from the south and (2) assess variations in the southernmost reaches of the northward-flowing Peru-Chile Current.

Three holes were drilled at Site 1232 (Table T2) during relatively high swell following a force 8 storm. Hole 1232A was cored with the APC from 0 to 112.5 mbsf and then was deepened with the XCB to 371.3 mbsf. Holes 1232B and 1232C included 10 APC cores to 90.1 mbsf and four APC cores to 33.2 mbsf depth, respectively. A composite section and splice documented nearly complete recovery for the upper 100 mcd.

The 390.6-mcd-thick (371.3 mbsf) sequence recovered at Site 1232 represents an expanded upper Pleistocene section (<1 Ma) of silty clay (inferred upper parts of turbidites and hemipelagic sediments) interbedded with >800 graded layers of silty sand beds with sharp, sometimes scoured, basal contacts (basal parts of distal turbidites) (Fig. F43). The silty sand layers are marked by high magnetic susceptibility and GRA bulk density, and low chroma (a* and b*) and reflectance (L*) values. Over the entire cored interval, the sandy silt turbidite layers range from a few millimeters to 118 cm in thickness (average = 3 cm) (Fig. F43) and are present throughout the section between silty clay layers that range from a few millimeters to 610 cm in thickness (average = 27 cm). The 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, consistent with a source area 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 Chile Rise, where the trench has relatively little bathymetric expression. 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 sediments are poor in organic carbon, typically <0.25 wt%. Organic carbon/organic nitrogen ratios indicate dominance of marine organic matter.

The abundance of calcareous microfossils at Site 1232 is generally low, and their preservation is generally poor to moderate. Diatom abundance and preservation fluctuate significantly and deteriorate noticeably at depths >202 mcd. Reworking of microfossils is apparent in all the fossil groups examined. In particular, freshwater diatoms such as Aulacoseira granulata and shallow-water benthic species such as Eunotia spp. are abundant in the intervals 43-69 and 193-241 mcd and indicate reworking from continental or nearshore sources. Calcareous nannofossils suggest an age of 0.26 Ma at 143 ± 5 mcd, 0.46 Ma at 259 ± 5 mcd, and <1.7 Ma for the deepest core at 382 mcd.

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 the removal of a strong drill string overprint. 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).

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 dominating the gas composition throughout.

The seismic reflection profile indicates an unconformity between acoustic basement (which may be lithified sediment) and the turbidite-dominated Pleistocene sequence. Extrapolation of the high late Pleistocene sedimentation rates (>450 m/m.y.) 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 Peru 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. The primary postcruise opportunity at Site 1232 will be to understand this record of turbidity flows and the record of terrigenous sediment components it provides. In addition, it may be possible to extract a paleoceanographic record from the pelagic sediments that are preserved between the turbidite layers, although such a record may be incomplete.

Site 1233 (proposed Site SEPAC-19A) 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). Predrilling survey data included high-resolution Parasound profiles and 3.5-kHz profiles that imaged the site to a depth of 110 mbsf (Figs. F44, F45) and 8-m sediment core (GeoB3313-3) that suggested continuous sedimentation at the site at rates of 100 cm/k.y. during the Holocene.

The intent of drilling at Site 1233 was to recover a very high resolution hemipelagic record of upper 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) assess variations in terrigenous sediment components off south-central Chile to reconstruct climate variability on land; (3) reconstruct sea-surface salinity anomalies that may record variations in river runoff or episodes of net glacier retreat in the fjord region of southern Chile; and (4) monitor changes in the signature of AAIW (proxies related to temperature, salinity, nutrients, and oxygen), which ventilates the South Pacific at intermediate depths.

Five APC holes were drilled at Site 1233 (Table T2). Hole 1233A was terminated after the first core was retrieved without a mudline. Hole 1233B provided a good mudline and was cored to 109.5 mbsf. Holes 1233C, 1233D, and 1233E were cored to depths of 116.3, 112.3, and 101.5 mbsf, respectively (Table T2). Seven downhole temperature measurements were taken with the APCT tool, yielding a temperature gradient of ~4.5°C/100 m.

A 136.1-mcd-thick (116.8 mbsf) upper Pleistocene hemipelagic sediment sequence was recovered at Site 1233 (Fig. F46). A composite section and splice based on high-resolution core logging data documented continuous recovery to the bottom of the sequence (Fig. F46). The sediment 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 will serve as regional stratigraphic markers. The relative paucity of turbidites in the basin suggests that sediment gravity flows were mostly channeled away from the basin in the extensive system of canyons that characterizes this portion of the Chile margin. Mineral assemblages are consistent with a siliciclastic provenance in both the Andes (relatively rich in feldspar) and the Coastal Range (relatively rich in quartz) in southern Chile. The quartz/feldspar ratio decreases downhole, indicating a change in provenance favoring Andean sources during older intervals.

Calcium carbonate concentrations range between 1 and 11 wt% (average = 5.4 wt%). Calcium carbonate and TOC contents (range = 0.4-2.5 wt%; average = 0.9 wt%) decrease downhole from the Holocene to the glacial interval, consistent with either significant terrigenous dilution of the biogenic components or downhole diagenesis. C/N ratios of TOC average 5.9, consistent with a marine origin of the organic matter. Preliminary predictive relationships between color reflectance spectra and carbonate and TOC, developed via a multiple linear regression, indicate substantial millennial- to centennial-scale variability in these biogenic components.

Calcareous nannofossils, foraminifers, and diatoms are abundant and well preserved at Site 1233. The presence of Emiliania huxleyi to the bottom of Hole 1233B indicates that the entire sequence is younger than 0.26 Ma. The planktonic foraminiferal assemblage, including Globigerina bulloides and Globorotalia inflata, is typical of eastern boundary settings in the transition from subtropical to the subpolar systems. The benthic foraminiferal assemblage represents 15% to 50% of the total foraminifers and indicates an upper to middle bathyal environment with high carbon fluxes to the seafloor. The diatom flora includes Chaetoceros spores, bristles, vegetative cells, and Thalassiosira species and reflects intermittent upwelling conditions at Site 1233. Freshwater diatoms, mainly Aulacoseira granulata, are in low abundance throughout the core. 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 (~41°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 paleomagnetic secular variation was recorded. Anomalous magnetic directions and low remanence intensities over a 2-m interval between 65 and 70 mcd in Holes 1233B, 1233C, and 1233D may represent the Laschamp geomagnetic field 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 correlation through paleointensity variations. Two intervals with exceptionally low remanence intensity (95-101 and 113-118 mcd) may reflect events of extreme sediment diagenesis.

A preliminary age model was constructed based on the correlation of the magnetic susceptibility of the uppermost 9 mcd to the ¹⁴C-dated sediment record of core GeoB 3313-1, recognition of the Laschamp geomagnetic excursion, and tentative constraints provided by correlation of relative paleointensity to similar data from Site 1089 that has been calibrated with oxygen isotope stratigraphy. MARs and LSRs estimated from this age model are 80 g/cm²/k.y. and 110 cm/k.y., respectively, for the Holocene and have maximum values of at least 150 g/cm²/k.y. and 160 cm/k.y., respectively, for much of the last glacial interval (MIS 2-3). Such high MARs and LSRs can be explained by extremely high fluvial sediment discharge in response to glaciation, heavy rainfall associated with the Southern Hemisphere westerly winds, and a dense drainage system in mountainous southern Chile. Prior to the Laschamp excursion, inferred sediment MARs and LSRs were 50 g/cm²/k.y. and 50 cm/k.y., respectively, suggesting that either transport pathways of terrigenous material and/or local syndepositional sediment focusing was different in the older interval. However, no major change in sediment composition accompanied this drastic shift in sedimentation rates.

Gas, fluid, and sediment geochemical profiles are dominated by the influence of organic matter diagenesis, despite the relatively low TOC contents at Site 1233. Sulfate reduction is complete by 5 mcd. Methane, apparently of biogenic origin, is found in high concentrations at depths >20 mcd. The zone of sulfate depletion is marked by a maximum in dissolved barium, possibly from barite dissolution. 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, indicating the possible presence of gas hydrates.

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 and properties of Earth's magnetic field on scales from millennia to centuries, perhaps even decades.

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 within the influence of the highly productive coastal upwelling center near Concepción, Chile (Figs. F1, F38). It is positioned on a relatively flat bench in the middle of the continental slope at 1015 m water depth, ~526 m deeper than and 10 km to the southwest of Site 1235 (Figs. F47, F48). 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. Precruise seismic survey data suggested mostly hemipelagic sedimentation at Site 1234. The uppermost part of the seismic profile shows flat-laying reflectors with undulating but continuous layers farther below, possibly reflecting the original relief of the acoustic basement (Fig. F48).

This site takes advantage of the high sedimentation rates on the continental margin that will allow reconstruction of regional climate and oceanographic variability on millennial to centennial timescales for the late Quaternary. Major objectives were to (1) assess late Quaternary history of productivity and coastal upwelling near Concepción, which is sensitive to regional winds, (2) assess variations in terrigenous sediment components off central Chile to reconstruct climate variability on land, (3) monitor changes in the boundary between oxygen-rich (nutrient poor) AAIW and oxygen-poor (nutrient rich) PCW, and (4) assess late Quaternary variations in paleomagnetic intensities and field directions in the southeast Pacific as a stratigraphic tool for comparison with similar data from the Northern Hemisphere.

Three holes were drilled at Site 1234 (Table T2). Hole 1234A was initiated with a mudline APC core, advanced to 100.3 mbsf, and deepened with XCB coring to the target depth of 205.2 mbsf. Hole 1234B was cored with the APC from the mudline to 93.8 mbsf and deepened with the XCB to 182.4 m. Hole 1234C was cored with the APC to a depth of 79.1 mbsf to target remaining coring gaps. Four downhole temperature measurements were taken with the APCT tool, yielding a temperature gradient of ~4.2°C/100 m. Most cores were affected by expanding gas during recovery. Holes were drilled in liners to relieve gas pressure to the extent possible, and major voids were closed prior to core logging operations.

A 239-mcd-thick (203.8 mbsf) upper Quaternary hemipelagic sediment sequence was recovered at Site 1234 (Fig. F49). A composite depth scale and splice based on high-resolution core logging data document complete recovery from 0 to 94.4 mcd and a nearly complete recovery to 240.4 mcd. Sediments consist primarily of homogeneous dark olive-gray to dark gray silty clay and clay. The dominant siliciclastic component provides high magnetic susceptibility and natural gamma radiation (NGR) values. The quartz/feldspar ratio decreases downhole, reflecting a change in the source region of siliciclastic sediment supply as basaltic to andesitic rocks from the Andes are relatively rich in feldspar, whereas the predominantly metamorphic rocks from the Coastal Range province are rich in quartz. Recognizable turbidites in the form of thin silt-rich layers are rare, indicating that major turbidity flows are contained in the extensive channel system that characterizes this steep continental margin. We recovered eight ash layers that may provide useful stratigraphic markers in the region.

Calcium carbonate concentrations are low, ranging from 1 to 12 wt% (average = 3.5 wt%). TOC concentrations are also low, ranging from 0.2 to 3.0 wt% (average = 1.1 wt%). TOC/TN ratios range between 5 and 10, indicating a mostly marine origin of the organic matter, although a positive correlation between C/N and TOC suggests that some contribution of terrigenous organic matter, or selective preservation effects, may contribute to the observed variability. Biogenic material decreases downhole, either indicating dilution of biogenic material with siliciclastic detritus, a decrease in biogenic productivity, or an increase in diagenetic dissolution of biogenic components.

Diatoms are abundant and are dominated by upwelling related taxa. Despite the dominance of the coastal upwelling forms, warm water species are present in higher abundance 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.

Calcareous nannofossils are abundant and moderately to well preserved, except for the core-catcher samples from the lowermost six cores where recrystallization was significant. The presence of Emiliania huxleyi suggests that the base of the cored interval has an age of <0.26 Ma. Planktonic foraminifers are present in all samples, but abundance and preservation vary markedly. The generally good preservation of carbonate fossils in most samples confirms that the relatively low carbonate contents result mostly from dilution by siliciclastic material rather than from dissolution. The benthic foraminiferal assemblage represents 20% to 98% of the total foraminifers and is dominated by species associated with high carbon fluxes in coastal upwelling regions. Downhole variations are interpreted to reflect shifts in bottom water oxygenation related to either changes in water masses at the seafloor or changes in surface production and the export of organic matter. For example, a distinct pronounced peak in the abundance of Bolivina spp. at ~175 to 223 mcd points to an intense episode of seafloor dysoxia.

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. Patterns of paleosecular variation are apparent and include directional excursions 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. The basal nannofossil age of <260 ka suggests higher sedimentation rates (average = >100 cm/k.y.) prior to 41 ka. Site 1234 likely records at least the last two late Pleistocene glacial-interglacial cycles at very high resolution.

Gas, fluid, and sediment geochemical profiles are dominated by the influence of organic matter diagenesis, despite the relatively low TOC contents. High amounts of methane and low amounts of ethane indicate that the gas is biogenic in origin. High alkalinity and high concentrations of ammonium and phosphate in interstitial waters also result from organic matter degradation by sulfate reduction and methanogenesis. Dissolved calcium concentrations drop rapidly below the sediment/water interface, consistent with the effects of authigenic mineralization reactions driven by the high alkalinity values. Magnesium/calcium ratios increase to >30 between 17.9 and 40.4 mcd, which is consistent with authigenic precipitation of calcite in shallower sediments. At greater depths, magnesium/calcium ratios decrease to ~12 by 230 mcd, indicating that authigenic mineralization reactions such as dolomite formation may take up magnesium deeper in the sediments.

With a nearly complete recovery to ~239 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, and evidence for substantial variations in biogenic production and bottom water properties all suggest that Site 1234 will provide an extraordinary record of Southern Hemisphere climate and biogeochemical changes of the late Quaternary.

Site 1235 (proposed Site SEPAC-14A) is located at a water depth of 489 m on a gently sloping terrace of the upper continental slope ~65 km shoreward of the Peru-Chile Trench and ~60 km offshore (Figs. F1, F38, F47). This site underlies a highly productive coastal upwelling zone off Chile, 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. Predrilling surveys including gravity cores and seismic data point to high hemipelagic sedimentation rates at the site. The upper part of the seismic profile documents well-stratified, flat-lying reflectors. 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 (Fig. F50).

This site was chosen to take advantage of expected high sedimentation rates to decipher the late Quaternary history of regional continental climate and oceanography on millennial to centennial timescales. Primary goals at Site 1235 were to (1) assess fluctuations in biological productivity and coastal upwelling that is sensitive to regional winds, (2) assess the late Quaternary history of terrigenous sediment components off central Chile to reconstruct climate variability on land, (3) assess variations in the boundary between oxygen-poor GUC water and oxygen-rich AAIW, and (4) monitor centennial- to millennial-scale variations in paleomagnetic intensities and field directions as a stratigraphic tool for comparison with similar data from the Northern Hemisphere.

Three APC holes were drilled at Site 1235 (Table T2). Holes 1235A, 1235B, and 1235C were cored to 181.3, 176.2, and 152.5 mbsf, respectively. A nonmagnetic core barrel, cutting shoe, and flapper valve were deployed here for the first time on Leg 202, resulting in sediment records with substantially less magnetic overprint than those retrieved with regular steel barrels (see Lund et al., this volume). Eight downhole temperature measurements were taken with the APCT tool, yielding a temperature gradient of ~3.8°C/100 m. Most cores were affected by expanding gas during recovery, and one core disintegrated entirely. Holes were drilled in liners to relieve gas pressure to the extent possible, and major voids were closed prior to core logging operations.

A 215.5-mcd-thick (181.3 mbsf) Pleistocene to Holocene hemipelagic sequence was recovered at Site 1235 (Fig. F51). A composite section and splice, constructed on the basis of high-resolution core logging data, indicated that complete recovery was most likely achieved to ~171.45 mcd, although some correlations between holes were problematic, especially at ~45-49, 71-73, and 126-128 mcd.

Sediments consist primarily of homogeneous dark olive-gray to dark gray silty clay and clay with a dominant siliciclastic component and high magnetic susceptibility and NGR values. The silt fraction is dominated by feldspars, with lesser amounts of quartz, pyroxenes, and volcanic glass, consistent with a sediment provenance in both the Andes and the Coastal Range. The large supply of siliciclastic sediment onto the upper continental margin of Central Chile is considered to result mainly from rivers that drain the mountainous regions of the Andes and the Coastal Range.

Calcium carbonate concentrations range from 0.3 to 15.5 wt% (average = 2.4 wt%). TOC contents range from 0.4 to 1.5 wt% (average = 0.6 wt%) and are dominated by marine organic matter (TOC/TN ratios of 5 to 9). The relatively 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. 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 near the top of these sequences, in each case overlying green sediments (negative a* values).

Calcareous nannofossils, diatoms, and foraminifers vary from rare to abundant and are typical of a productive coastal upwelling zone. Microfossil preservation is moderate to good in the upper section but deteriorates downhole, especially for the calcareous nannofossils. Reworked benthic diatoms and pre-Pleistocene calcareous nannofossil are frequently present. The planktonic foraminiferal assemblage is typical of the upper Pleistocene Subzone Pt1b, and the continued presence of the calcareous nannofossil species Emiliania huxleyi to the bottom of the sequence documents an age younger than 0.26 Ma. Sedimentation rates are at least 80 cm/k.y. 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.

Initial NRM intensities were extremely high, ranging from 0.5 to 2.1 A/m, and were characterized by steep positive inclinations (averaging +79°) associated with a drill string magnetic overprint. AF demagnetization at peak fields up to 25 mT only partially reduced this overprint but revealed discrete intervals of low NRM intensity that are likely associated with substantial reduction diagenesis. The application of the nonmagnetic core barrel substantially reduced the viscous magnetic overprint. Paleomagnetic declinations are much less affected by overprints. Coherent patterns of paleomagnetic secular variation in all holes may provide a high-resolution regional stratigraphy at Sites 1234 and 1235.

Gas, fluid, and sediment geochemical profiles are dominated by the influence of marine organic matter degradation associated with sulfate reduction and methanogenesis. Sediments contained abundant methane and low amounts of ethane. A decrease in chlorinity with depth at Site 1235, similar to that of Site 1233, indicates the presence of methane hydrates that dissociated either within subsurface sediments or after recovery. Authigenic pyrite and carbonate are present, especially in deeper intervals. Conspicuous carbonate concretions are intermittent downhole. Dissolved calcium concentrations drop rapidly with depth to values <2 mM, consistent with the effects of authigenic carbonate mineralization driven by high alkalinity.

Site 1235 will provide a long sequence of extremely rapidly accumulating sediment for the study of century- to millennial-scale changes in climate, biogeochemistry, and oceanography of subsurface water masses, as well as variations in paleomagnetic declination. Preliminary data on the fossil biota, geochemical composition of interstitial water and sediment, and variations in the terrigenous sediment components lead us to anticipate a rich record of paleoceanographic and paleoclimatic change and chemical diagenesis, as long as the stratigraphic challenges associated with gassy and somewhat disturbed cores can be overcome.

Site 1236 (proposed Site NAZCA-10A) is located at a water depth of 1323 m atop Nazca Ridge, a fossil hotspot track with its modern expression at Easter Island (Fig. F1). The total thickness of sediment at Site 1236 is ~200 m. Seismic data reveal a well-stratified pelagic sequence with strong to moderately reflective layers from the sediment surface to ~120 mbsf. At depths from ~120 to ~200 mbsf, deformed but almost continuous layers of moderate reflection appear to smooth the relief of the acoustic basement (Fig. F52). Based upon a fixed hotspot model and on magnetic anomalies of the surrounding oceanic crust, basement ages are expected to be 30-35 Ma (Fig. F6). The ~20-km-wide plateau occupied by Site 1236 is punctuated by small volcanic spires a few hundred meters high that represent eroded remnants of an archipelago of volcanic islands. About 20 km to the southwest of Site 1236, a flat-topped guyot, which is presumably younger than the plateau surface, rises to within 350 m of present sea level. The existence of this relatively well preserved shallow bathymetric feature adjacent to the deeper, more eroded plateau surface suggests multiple episodes of volcanism in the region or the presence of a carbonate reef system that grew atop the volcanic edifice. A tectonic backtrack path on the Nazca plate moves Site 1236 ~20° westward, toward the center of the subtropical gyre, by 30 Ma (Fig. F6). Thermal subsidence would predict that the site was at shallower water depths during the early Miocene and, perhaps, above sea level very early in its history.

Today, Site 1236 is situated near the western edge of the northward-flowing Peru-Chile Current in an oligotrophic region of the subtropical gyre and at depths suitable for monitoring intermediate-water chemistry in the open ocean (Figs. F5).

The primary objectives at Site 1236 were to provide a continuous sequence of Neogene and Quaternary sediments to (1) improve on South Pacific Neogene timescales by combining magnetostratigraphy, biostratigraphy, and isotopic stratigraphy, (2) assess climate changes of the subtropical South Pacific focusing on major intervals of changing climate (e.g., the early Miocene intensification of Antarctic glaciation [~24 Ma], the middle Miocene climatic optimum [14-15 Ma], the expansion of the East Antarctic Ice Sheet [~13 Ma], and the mid-Pliocene intensification of the Northern Hemisphere glaciation [3.1-2.6 Ma], (3) examine the late Miocene to early Pliocene reorganization in South Pacific surface and intermediate-water circulation in response to the closure of the Isthmus of Panama (10-4 Ma), and (4) reconstruct changes in the boundary between Pacific Deep Water and AAIW on orbital and tectonic timescales.

Three holes were drilled at Site 1236 (Table T2). Hole 1236A was APC-cored to a depth of 173.1 mbsf and advanced with the XCB to a total depth of 207.7 mbsf, where basement was reached. Hole 1236B was piston cored and abandoned at 122.8 mbsf because of mechanical problems. In Hole 1236C, 13 APC cores were recovered to a refusal depth of 167.3 mbsf. The nonmagnetic core barrel assembly was deployed on nearly every second APC core at this and all subsequent sites. Six downhole temperature measurements were taken with the APCT tool, yielding a temperature gradient of ~3.9°C/100 m.

A 223.2-mcd-thick (207.7 mbsf) sequence of Neogene and upper Oligocene (~28 Ma) mostly calcareous sediments was recovered at Site 1236 (Fig. F53). A composite section and splice, based on high-resolution core logging data, documented complete stratigraphic recovery to ~129.7 mcd (~17-18 Ma). Pelagic sedimentation is prevalent at Site 1236, but gravity flow deposits during the early-middle Miocene and the impact of volcanic evolution of Nazca Ridge during the late Oligocene are also clearly represented. Calcium carbonate concentrations are >90 wt% throughout the section except for the lowermost (Oligocene) interval. TOC content was not measurable with shipboard techniques (<0.2 wt%).

Four major lithologic units are defined at Site 1236. Unit I (0-119.5 mcd) corresponds to the last 17 m.y. and contains primarily pelagic nannofossil ooze. Two isolated layers at depths of ~65 and ~98 mcd consist mostly of neritic grains and represent waning supply of gravity flows from a nearby carbonate platform during the late Miocene. A minor siliciclastic fraction in the uppermost ~40 mcd, corresponding to increased magnetic susceptibility values and containing goethite and hematite according to reflectance spectrometry results, is considered to represent eolian material transported from South America during the Pleistocene and Pliocene. Unit II (119.5-181.0 mcd; ~17 to ~23 Ma) consists mainly of nannofossil ooze (Subunit IIA) and unlithified calcareous mudstone (Subunit IIB), consistent with the presence of a pelagic sedimentary environment at Site 1236 since the late Oligocene. Frequent intercalations of well-sorted nonpelagic calcareous sediment (neritic grains and micrite), which sometimes fine upward, were supplied by gravity flows from nearby carbonate platforms. Micrite, which becomes dominant in Subunit IIB, reflects in situ recrystallization of nannofossils.

Allochthonous coarse-grained calcareous sediments in Unit III (181.0-207.7 mcd), including recrystallized larger benthic foraminifers, bryozoan fragments, and some remains and encrustations of red algae, indicate proximity of Site 1236 to a carbonate platform from ~25 to 23 Ma. The presence of glauconite within Unit III sediments is consistent with a shallow-water environment below the storm wave base. Volcaniclastic constituents with relatively high magnetic susceptibility are present through the upper Oligocene, suggesting proximity to volcanic islands at that time. Unit IV (207.7-222.3 mcd; >25 Ma) is a fine-grained chalk with abundant nannofossils and foraminifers overlying basalt (inferred basement), with no evidence for input of nonpelagic material. We infer, based on the presence of authigenic glauconite, a relatively shallow open-ocean environment on the order of hundreds of meters below sea level.

Calcareous nannofossils are generally abundant and moderately to well preserved throughout the interval. Planktonic foraminifers are common to abundant down to ~150 mcd (~20 Ma). Below this depth, foraminiferal abundance decreases and preservation deteriorates markedly. Calcareous nannofossil and planktonic foraminifer biostratigraphies indicate that the upper Pleistocene to upper Oligocene succession is essentially complete. Benthic foraminiferal assemblages document an oligotrophic, pelagic environment from Pliocene to middle Miocene time. Older intervals from the early Miocene and late Oligocene contain higher numbers of buliminids and nodosariids, reflecting shallower water depth and proximity to the oxygen minimum zone. The presence of poorly preserved larger benthic foraminifers, which have modern equivalents that host algal endosymbionts, indicates redeposition from a nearby carbonate platform during the Oligocene and early Miocene. Diatoms are only found close to the two ash layers and indicate either increased diatom production near the time of ash deposition or anomalous preservation as a result of the presence of ash. The presence of benthic and neritic diatoms at 82.3 and 206.7 mcd indicates redeposition from shallow-water environments.

NRM intensities are similar to the magnetic susceptibility profiles and reflect the major lithologic changes at Site 1236. A preliminary magnetostratigraphy was established for the upper 89 mcd (0-14 Ma) based on correlation to the geomagnetic polarity timescale, and this significantly augmented the biostratigraphy and provided a well-constrained chronology for the younger interval.

A long history of oligotrophic conditions with little input of nonbiogenic sediments at Site 1236 resulted in generally low MARs and linear sedimentation rates (LSRs) of <1.6 g/cm²/k.y. and <11 m/m.y., respectively. Winnowing processes may have contributed to the overall low MARs on this bathymetric rise. MARs are overwhelmingly controlled by carbonate MAR throughout the record. Higher rates of 0.6-1.6 g/cm²/k.y. (6-11 m/m.y.) in the lower to middle Miocene (>12 Ma) reflect the influence of nearby carbonate platforms and/or islands supplying significant amounts of nonpelagic calcareous material and siliciclastic and volcanic constituents. The lowest rates of ~0.4 g/cm²/k.y. (~4 m/m.y.) are typical for the lower upper Miocene (12-8 Ma) and in the Pleistocene (2-0 Ma). The uppermost Miocene to Pliocene (8-2 Ma, but especially 6-5 Ma) has a broad peak in MARs (0.5-0.9 g/cm²/k.y.) and LSRs (5-9 m/m.y.). Given no detectable changes in fossil preservation, these high MARs likely reflect an interval of increased productivity. Similar events are recorded at other Leg 202 sites (1237, 1238, 1239, and 1241) and at equatorial Pacific sites (846-853) during approximately the same time interval.

Interstitial water chemistry at Site 1236 is consistent with low input of organic matter in an oligotrophic setting. Diagenesis of opal and calcite is minimal, as reflected in silicate, calcium, and magnesium measurements of interstitial waters.

Site 1236 provides an essentially complete Neogene to Quaternary sequence relatively unmodified by burial diagenesis and recovered with the APC to 188 mcd (~23 Ma). The tight framework of bio- and magnetostratigraphic age controls together with good preservation of calcareous microfossils extending to the late Oligocene will provide a solid base for detailed studies that aim to reconstruct the long-term history of sea-surface and intermediate-water characteristics and regional climate in the southeast Pacific.

Site 1237 (proposed Site NAZCA-17A) is located at a water depth of 3212 m on a relatively flat bench on the easternmost flank of Nazca Ridge, ~140 km off the coast of Peru (Fig. F1). Nazca Ridge, a fossil hotspot track with its modern expression at Easter Island, terminates just outboard of the Peru-Chile Trench, where it is deformed and subducted beneath Peru. About 19 km to the west of Site 1237, an abrupt scarp (probably and active normal fault) rises 600 m to the summit of the ridge. Eastern Nazca Ridge is covered by a thick drape of pelagic sediment to its shallowest reaches. The seismic record reveals well-stratified reflective layers, which clearly drape the underlying bathymetry from the sediment surface to acoustic basement (Fig. F54). The basement age is expected to be between 40 and 45 Ma, based upon a hotspot model and magnetic anomalies of the surrounding oceanic crust. Considering the thermal subsidence over this time span, the site may have been within several hundred meters of sea level in its early history.

The modern water depth of Site 1237 reflects the transition zone between the relatively oxygen-rich (nutrient poor) remnants of CPDW, which enter the Peru Basin as bottom water through the Peru-Chile Trench, and the relatively oxygen-poor (nutrient rich) PCW (Fig. F5). During its early history, the site may have occupied depths of the modern PCW or its paleoequivalent. Today, Site 1237 is situated near the eastern edge of the northward-flowing Peru-Chile Current, close to the productive upwelling system off Peru, and underlies the path of eolian dust that originates in the Atacama Desert of Chile (Fig. F33). Given the tectonic backtrack ~20° westward toward the center of the oligotrophic gyre (Fig. F6), and assuming there were no compensating changes in the environment, we expect that accumulation rates of both biogenic and eolian sediments at Site 1237 would have been lower in the past.

The primary objectives at Site 1237 were to provide a continuous and complete sedimentary sequence from the late Cenozoic to (1) improve on regional chronological frameworks by combining magnetostratigraphy, biostratigraphy, isotopic stratigraphy, and orbital tuning methods; (2) assess climate and biogeochemical changes of the subtropical South Pacific, focusing on major intervals of changing global climate, such as the early Miocene intensification of Antarctic glaciation (~24 Ma), the middle Miocene climatic optimum (14-15 Ma), the expansion of the East Antarctic Ice Sheet (~13 Ma), the mid-Pliocene intensification of the Northern Hemisphere glaciation (3.1-2.6 Ma), and the transition from early Pleistocene climate cycles dominated by ~41-k.y. rhythms to those of the late Pleistocene that are dominated by ~100-k.y. rhythms (~0.9 Ma); (3) examine the impact of tectonic events on regional climate, productivity, biogeochemical cycles, and ocean circulation, including the opening of Drake Passage (~25 Ma), the major uplift of the Andes (probably within the past 15 m.y.), and the closure of the Isthmus of Panama (10-4 Ma); and (4) reconstruct changes in the boundary between PCW and Circumpolar Deep Water on orbital and tectonic timescales.

Four APC holes were drilled at Site 1237 (Table T2). Hole 1237A was terminated after retrieving the first core without a mudline. Hole 1237B recovered the mudline, and piston coring advanced until refusal at 317.4 mbsf. This represented the deepest penetration with the APC since Leg 162 and one of the five deepest APC penetrations in ODP history. Hole 1237C was cored to refusal at 315.3 mbsf. Hole 1237D was spot-cored to cover coring gaps left in the previous holes in four intervals to a total depth of 281.1 mbsf. The nonmagnetic core barrel was used on approximately every second APC core. Ten downhole temperature measurements were taken with the APCT tool, yielding a temperature gradient of ~3.1°C/100 m.

A 360.6-mcd-thick (317.4 mbsf) Oligocene (~31 Ma) to Holocene pelagic sediment sequence was recovered at Site 1237 (Fig. F55). A composite depth section and splice, based on high-resolution core logging data, document complete recovery for the entire sequence. The sequence was divided into two major lithologic units, and each unit was divided into two subunits (Fig. F55). The upper ~100 mcd of the sequence (Unit I) reflects pelagic sedimentation with significant terrigenous (probably mostly eolian) and siliceous microfossil components. In Subunit IA (0-41 mcd), calcium carbonate concentrations are <50 wt% (and often just a few percent), and TOC contents are relatively high for an open-ocean setting (0.3-1.9 wt%). The sediment gradually changes downhole from nannofossil-bearing (silty) clay to nannofossil ooze in Subunit IB (41-100 mcd). Below ~100 mcd (Unit II), the sedimentary sequence is dominated by nannofossil ooze, calcium carbonate values are >90 wt%, and TOC values fall below detection limits (<0.2 wt%). Micrite contents increase at depths >148 mcd (Subunit IIB) and are likely associated with recrystallization of calcareous nannofossils. A marked color change from more greenish younger sediment to reddish older sediment containing goethite and hematite occurs at ~164 mcd (8-9 Ma). The abrupt color change appears to result from diagenetic reduction processes or analytical detection problems (i.e., matrix effects) rather than from a possible change in the primary signal of dust source areas, based on evidence from Site 1236 (l500 km southwest of Site 1237 and farther from the probably source region), where the reddish color signal persists into the Pleistocene interval. At approximately the depth where the color change occurs, discrete ash layers and dispersed ash begin to appear in the sediment. Fifty-five ash layers were deposited during the last ~9 m.y. The Pliocene to Pleistocene interval contains significant amounts of both terrigenous material and siliceous microfossils.

Calcareous nannofossils and foraminifers are generally abundant or common and well to moderately well preserved throughout, indicating that a complete late Pleistocene to late Oligocene succession was recovered. Calcareous nannofossils at the base of the hole indicate an age of <31.5 Ma. Marked changes in the relative proportions of benthic foraminiferal species within the Pleistocene to late Pliocene assemblage indicate variations in carbon fluxes at the seafloor that are probably related to temporal and spatial fluctuations of the coastal upwelling system. At greater ages, the benthic foraminiferal assemblage is consistent with an oligotrophic, pelagic environment. Diatoms are abundant and are usually well preserved from 0 to 60 mcd, but abundance decreases and preservation deteriorates at greater depths, and diatoms are essentially absent at depths >174 mcd. Diatoms provide additional biostratigraphic control down to ~136.7 mcd (7 Ma). The diatom assemblage from 0 to 60 mcd is typical of a coastal upwelling zone on a continental margin. At greater depths, upwelling forms are present only occasionally with some oceanic forms.

The paleomagnetic stratigraphy at Site 1237 is excellent with clear definitions of all chrons and subchrons over the last 5 m.y. and from 7 to 13 Ma. Even fine-scale features and short polarity subchrons, such as the "Cobb Mountain Event" within the Matuyama Chron, may be present. Based on shipboard data, the polarity assignments for the interval from 5 to 7 Ma and for the lowermost part of the sequence between 13 and 31 Ma are uncertain because the polarity sequence allows for several possible interpretations.

Total MARs and LSRs range between 0.6 and 2.5 g/cm²/k.y. and between 5 and 23 m/m.y., respectively. Before 8 Ma (>150 mcd), the rates are generally lower (<1.2 g/cm²/k.y. and <9 m/m.y., respectively) and driven by the carbonate MAR, as is typical for oligotrophic settings far from the continents. Between 8 and 4 Ma, MARs are higher and reach maximum values at 6-5 Ma, most likely associated with anomalously high production of carbonate-secreting plankton. The enhanced rain of organic carbon that follows from high productivity would also support the hypothesis of a diagenetic boundary that reduced the reddish oxides at <150 mcd. The Miocene to early Pliocene biogenic bloom was also found at Site 1236, as well as in the equatorial Pacific (6.7-4.5 Ma). At ages <5 Ma, carbonate MARs decreased gradually at Site 1237 and reached minimum values during the last 2 m.y.

Noncarbonate accumulation rates gradually increased from 12 Ma to the Holocene, constituting about half of the total MAR at ~3 Ma and dominating the total MAR during late Pliocene and Pleistocene. This trend represents the tectonic drift of the Nazca plate eastward toward both the eolian dust source in the Atacama Desert and the region of coastal upwelling. Ash layers become more frequent after ~9 Ma and may record the onset of intense volcanism and accompanying tectonic uplift of the Andes. If so, the topographic barrier of an early mountain range would have enhanced the steering of westerly and trade winds along the coast, resulting in stronger meridional flow, enhanced eolian transport, coastal upwelling, and biogenic productivity within the eastern boundary current.

Interstitial water chemistry reflects minor influence from organic matter diagenesis, a limited degree of biogenic opal dissolution, and a minor signature of biogenic calcite diagenesis. Dissolved silicate concentrations have highest values in the depth range where diatoms are reported to have the best preservation and abundance.

Site 1237 provides a complete upper Paleogene to Neogene sequence relatively unmodified by burial diagenesis and fully recovered with the APC. Good preservation of calcareous microfossils to the base of the Oligocene and the tight framework of bio- and magnetostratigraphic age controls will provide an excellent stratigraphic reference section for the South Pacific, with opportunities for further refinement based on an orbitally tuned timescale. Detailed studies will aim to reconstruct the long-term history of Andean uplift and continental climate as well as the evolution of upwelling, biota, biogeochemistry, and sea-surface and intermediate-water characteristics in the southeast Pacific.

Site 1238 (proposed Site CAR-2C) is located ~200 km off the coast of Ecuador on the southern flank of Carnegie Ridge at 2203 m water depth (Fig. F1). The region is draped with pelagic sediment ~400-500 m thick on a bench that slopes gently to the south (Fig. F56). Basement at Site 1238 likely consists of basalt formed at the Galapagos hotspot at ~11-13 Ma. A tectonic backtrack path on the Nazca plate moves Site 1238 ~600 km westward and slightly to the south relative to South America, probably to shallow water depths by 10-12 Ma (Fig. F6).

Today, Site 1238 is situated under the eastern reaches of the equatorial cool tongue, in an open-ocean upwelling system near the equator. The nutrient-rich Equatorial Undercurrent (EUC) supplies waters that upwell here and along the coast of Peru and Ecuador, driving exceptionally high primary productivity. Sediments at the site are likely to record changes in upwelling and biological production, as well as long-term changes in upper-ocean temperature and pycnocline depth. The modern water depth is appropriate to monitor PCW south of Carnegie Ridge, roughly at the sill depth of Panama Basin (Fig. F5).

The primary objectives at Site 1238 were to provide a continuous sedimentary sequence of Neogene age to (1) assess the history of near-surface water masses, including the eastern reaches of the equatorial cold tongue, (2) assess changes in biogeochemistry and biota that are linked to variations in nutrients, productivity, and fluxes of organic matter and biogenic sediments to the seafloor, (3) monitor temporal and vertical fluctuations of PCW, (4) assess oceanographic changes that are related to the closure of the Isthmus of Panama, and (5) record changes in the occurrence and frequency of volcanic ashes, which might be associated with active tectonic phases of the northern Andes.

Three holes were drilled at Site 1238 (Table T2). Hole 1238A was cored with the APC to 204.5 mbsf and then deepened with the XCB to refusal at 430.6 mbsf. Holes 1238B and 1238C were APC cored to 201.0 and 167.5 mbsf, respectively, to cover coring gaps. The nonmagnetic core barrel was used on approximately every second APC core. Hole 1238A was logged using the triple combo tool string with the Lamont Multi-Sensor Spectral Gamma Ray Tool (MGT) on top, and then by the FMS-sonic tool string. Downhole logging operations experienced smooth borehole conditions and calm seas (peak heave of <2 m), and as a result the data were of excellent quality from the density, porosity, and FMS tools that require good borehole contact. Six downhole temperature measurements were taken with the APCT tool, yielding an anomalously high temperature gradient of ~12.7°C/100 m.

A 468.2-mcd-thick (430.6 mbsf) sequence of middle Miocene (~12 Ma) to Holocene pelagic sediment was recovered at Site 1238 (Fig. F57). Based on high-resolution core logging data, a composite section and splice documented complete recovery of the section cored with the APC to a depth of 225.4 mcd. XCB cores from Hole 1238A were appended onto the mcd scale, using a 10% growth rate of mcd relative to mbsf. The dominant lithologies at Site 1238 are diatom nannofossil ooze and nannofossil diatom ooze, reflecting primarily biogenic sedimentation in a moderate- to high-productivity pelagic setting.

Calcium carbonate contents range between 17 and 94 wt% and generally increase with depth, but with substantial variability. TOC contents range between 0.1 and 4.3 wt%, and vary inversely to calcium carbonate. Consistently low TOC/TN ratios (3.4-11.8) indicate the dominant marine origin of organic matter. Color and lithologic banding is pervasive on a scale of meters (10⁴ to 10⁵ yr, based on shipboard age models) and provides evidence for regional variability associated with Earth's orbital cycles. Similar to the core logging data, the downhole logs document cyclic variations in sediment properties on the scale of meters that reflect alternations between carbonate- and opal-rich sediments. The downhole logs reveal that uranium concentration predominantly drives variations in natural gamma measured in core logs, and both properties are well correlated with TOC. Uranium variations likely reflect changes in sedimentary redox conditions in response to fluxes of organic matter to the seafloor, meaning that the primary lithologic variations at Site 1238 are probably driven by changing productivity of surface waters.

One lithologic unit, divided into two subunits, was defined at Site 1238. Subunit IA (0-403 mcd) contains bioturbated nannofossil ooze, diatom nannofossil ooze, and nannofossil diatom ooze with varying abundance of clay and foraminifers. High productivity during this latest Miocene to Pliocene interval is indicated by high concentrations of diatoms, organic carbon, and organic pigments in the sediment. An anomalous event of low calcium carbonate and high TOC and U contents occurs from 85 to 104 mcd (~1.7-2.0 Ma). Similar coeval events are found at Sites 1239 and 1240, and at other equatorial sites (e.g., 846 and 847), they are associated with high accumulations of opal. Several ash layers occur within the upper ~300 m (<5 Ma), including the regionally correlative ash layer L (~230 ka). The frequency of ash layers increased during the past 3 m.y. and may represent a phase of intensified volcanism in the northern Andes.

Subunit IB (~403-467 mcd), the base of the recovered sequence, is characterized by partially lithified diatom and nannofossil oozes interbedded with chalk and occasional chert horizons. Increasing lithification is documented by increases in bulk density and decreases in porosity. The presence of chert and micrite indicates significant opal and carbonate diagenesis, respectively, perhaps driven by the relatively high heat flow at Site 1238.

Calcareous nannofossils are generally abundant, and planktonic and benthic foraminifers are common to abundant, with good to moderate preservation throughout the section, except in the lower Pleistocene and middle Miocene. Diatoms are common to abundant, except at depths >430 mcd (~9 Ma), where diatoms are essentially barren. This absence of diatoms at depth is probably due to opal diagenesis and chert formation. The various microfossil groups provide a well-constrained biostratigraphy. The lower Pliocene-upper Miocene section contains a minor component of reworked older microfossils. Calcareous nannofossils indicate a basal age of 10.4-11.6 Ma. However, considering possible reworking of that indicator, and considering a planktonic foraminifer datum (11.7 Ma) at 423-437 mcd, a hiatus may be present at ~420 mcd, spanning the interval of 12 Ma to ~8 Ma. Paleomagnetic measurements were compromised by drill string overprints, as well as diagenesis of magnetic minerals, and thus did not provide useful information for age control.