A 360.65-m-thick (hemi)pelagic sequence was recovered at Site 1237, spanning the interval from the early Oligocene to the Holocene. Clay-rich lithologies dominate the uppermost ~100 mcd, whereas nannofossil ooze is present in the lower ~260 mcd.
The sedimentary sequence at Site 1237 was divided into two major lithologic units (I and II), with each unit divided into two subunits (Table T8; Fig. F11). Unit I lithologies are rich in both terrigenous material and siliceous microfossils with various contributions from calcareous microfossils (Fig. F12). Subunit IA primarily consists of diatom- and/or nannofossil-bearing (silty) clay, and Subunit IB contains clayey nannofossil ooze. Unit II is composed of nannofossil ooze; Subunits IIA and IIB differ in micrite abundance. Differences among units and subunits are clearly apparent in color reflectance as well (Fig. F13). Ash layers are present between 0 and 167 mcd, frequently intercalated in Subunits IA through IIA (Fig. F14; Table T9).
For the upper ~148 mcd of the sedimentary sequence (Unit I through Subunit IIA), preliminary predictive relationships between reflectance and carbonate and total organic carbon (TOC), via a multiple linear regression, are strong for both carbonate and TOC (r2 = ~0.8 and ~0.9, respectively). Reflectance spectra show that goethite and hematite are present in Subunit IIB and confer a reddish color to the sediment.
The interval from early Oligocene to early Pliocene (Unit II) is marked by pelagic sedimentation. The transition toward a more hemipelagic environment since the early Pliocene (Unit I) is indicated by the increased siliciclastic component toward recent times and is consistent with the eastward motion of the Nazca plate, which moved Site 1237 to its modern position ~140 km off the coast of Peru (see "Introduction").
Minor siliciclastic components (goethite, hematite, and clay minerals) within the nannofossil ooze may indicate eolian transport of sediments to the site via the southeast trade winds from inferred arid areas of subtropical South America. The iron oxyhydroxides disappear in the upper Miocene (~8 Ma) section, probably as a result of the establishment of a reducing environment in response to an increase in the total organic carbon flux. Nevertheless, a simultaneous increase in the flux of total siliciclastic components suggests enhanced eolian supply since ~8 Ma.
Fifty-five ash layers at Site 1237 record volcanic eruptions from andesitic sources since ~9 Ma (Fig. F14). In the section older than 9 Ma, ash layers are essentially absent. The presence of ash layers since 9 Ma and maxima in ash layer frequency at ~7.5 Ma and between 3 and 1 Ma may reflect major phases of explosive volcanic activity, possibly associated with major phases of Andean uplift. However, variations in ash layer frequency are also influenced by a transport function that is linked to the paleoposition of Site 1237 or to climate-related variations in the strength and direction of atmospherical circulation. Andean uplift, in turn, may have exerted more influence on the regional wind and precipitation patterns, changing the climatic conditions in South America.
All lithologic units and subunits are defined by visual description, smear slide analyses, and color reflectance measurements and are further supported by trends in magnetic susceptibility and NGR (Fig. F11). However, lithologic changes between units are gradational. Therefore, we chose to define each of these transitional intervals of unit and subunit boundaries at the base of an appropriate ash layer identifiable in all holes (Fig. F15).
Technical problems with the gamma ray attenuation (GRA) bulk density sensor affected the data in the interval between ~170 and 240 mcd in Hole 1237B and in most intervals of Holes 1237C and 1237D. For the intervals in Hole 1237B where GRA bulk density measurements are reliable, they are highly correlated (r2 = 0.9) in a 1:1 relationship to the moisture and density (MAD) bulk density measurements. The smooth exponential downhole increase in both GRA and MAD bulk densities and the associated decrease in porosity are the result of compaction and dehydration (Fig. F11).
Unit I is characterized by calcareous and/or siliceous clays and clayey oozes of early Pliocene to Pleistocene age. The dominant lithologies are diatom nannofossil clay and clayey nannofossil ooze. Unit I is divided into two subunits primarily on the basis of visual core description, smear slide analyses, and magnetic susceptibility; trends in NGR and color reflectance are secondary to this classification.
The main components of Subunit IA are clay minerals, nannofossils, and diatoms. Changes in the relative percentages of components in smear slide samples are often subtle yet result in different lithologic classifications, such as diatom nannofossil clay, silty clayey diatom nannofossil ooze, diatom-bearing clay, and clayey diatom ooze, for almost identical lithologies within this subunit. Sediment color varies between shades of olive and olive gray. Mottling is common to abundant throughout Subunit IA. Trace fossils, often Zoophycos (Fig. F16), are common throughout this subunit. Infrequent black spots or smears on the cut surface contain up to 30% pyrite and minor amounts of micrite. Some pyritized burrows are observed. Trace amounts of reworked discoasters are also observed in the Pleistocene sediments.
Siliciclastic content ranges from ~90% to 40% (Fig. F12). Higher values of siliciclastics are present near the top of the hole and decrease below ~30 mcd. The combined sand and silt content of the siliciclastic fraction is ~10% with no downhole trend within Subunit IA. Away from discrete ash layers, the percentage of disseminated volcanic glass is generally ~10% and increases to ~60% near ash layers (e.g., at ~30 mcd). Siliceous microfossil percentages, including diatoms and sponge spicules, average ~15%, reaching maxima of >75% at ~5 and 40 mcd. Pyrite and micrite are nearly absent in the major lithologies.
Nineteen ash layers or significant ash patches are present within Subunit IA (Table T9). Twelve of them can be correlated between holes. These ash layers range between 2 and 14 cm thick with sharp basal contacts and diffuse upper contacts that grade into the dominant lithology. Ash colors range from light to dark greenish or brownish gray, dark gray, and black. Darker ash layers contain more pyrite.
Magnetic susceptibility values range between 25 and 50 instrument units in Subunit IA, with a broad minimum between 7 and 14 mcd (Fig. F11). After a sharp peak corresponding to a large ash layer just below ~20 mcd, the values decrease steadily to <25 units. NGR remains at 40-45 cps to a depth of ~30 mcd, decreasing slightly through the base of the subunit. NGR peaks mark ash layers. Bulk density, as determined from GRA and MAD measurements, increases steadily with increasing depth from ~1.2 to ~1.5 g/cm3 at the bottom of this subunit. Grain densities vary from 2.5 to 2.9 g/cm3 in Subunit IA and are, on average, lower than in the sequence below, reflecting a lower carbonate content (Fig. F11).
All sediment comprising Subunit IA is generally greenish (i.e., a* < 0) (Figs. F11, F13). Downhole trends in reflectance exhibit a very slight initial decrease in a* and b* values (from ~0 to just below 0 and ~15 to 10, respectively), whereas L* values steadily increase from ~40% to 50% (Fig. F11).
The sediment of Subunit IB is light greenish gray clayey nannofossil ooze that becomes increasingly white downcore. Mottling and other evidence of bioturbation are abundant, and Zoophycos burrows are common. Many of the circular burrow traces are very pale brown in color and are surrounded by gray halos (Fig. F17). Below 60 mcd, faint purple and green color bands become apparent in the sediment (Fig. F18) and are more obvious as the sediment becomes lighter downcore. These colored bands exhibit no distinct lithologic difference from the major lithology.
Siliciclastics make up 40% of the sediment for most of Subunit IB (Fig. 12) but decrease significantly at the base of the subunit. Minimal amounts of sand- and silt-sized siliciclastics are present. The percentage of siliceous microfossils is ~10%, whereas calcareous microfossils represent >50% of the sediment and increase to nearly 100% near the base of the subunit. Micrite is generally present in small amounts, except near 70 and 80 mcd, where percentages of up to ~30% are reached.
Eighteen ash layers and significant ash concentrations are present in Subunit IB. Nine of them can be correlated between holes (Table T9). In general, the ash layers are ~4-5 cm thick, often display subtle changes in color, and have sharp basal contacts and gradational upper contacts. The ashes are light to dark greenish or brownish gray, dark gray, and black in color. Darker ash layers contain more pyrite. Ash is often dispersed by bioturbation, even in cases where the basal contacts are sharp.
In Subunit IB, magnetic susceptibility values are lower than in Subunit IA and decrease from <10 instrument units to almost 0 by ~65 mcd (Fig. F11). NGR exhibits a similar trend (~40 to 20 cps). NGR peaks mark the presence of ash layers. GRA and MAD bulk density values continue to increase gradually from ~1.5 to 1.65 g/cm3.
All sediment within Subunit IB is generally greenish (i.e., a* < 0) (Figs. F11, F13). Increased variance in a* and b* compared to Subunit IA results from the green and purple color banding observed in this sediment. L* increases from 40% to 75% down to ~65 mcd and then remains steady to the base of the subunit.
Unit II contains early Oligocene to early Pliocene nannofossil ooze that changes abruptly from white to pale brown at ~162 mcd. Micrite abundance increases downcore. The ash layer frequency is greatest in the upper section of Unit II. Unit II is divided into two subunits based primarily on visual core description, smear slide analyses, color reflectance, and magnetic susceptibility.
Subunit IIA is composed of white bioturbated and mottled nannofossil ooze with abundant green and purple color banding (Fig. F19). Evidence for bioturbation exists in the form of very pale brown and gray mottles, ash patches above and below distinct ash layers, and the abundance of dispersed ash within the dominant lithology. Sulfides are present in the form of small black spots and smears on the exposed core surface (Fig. F19).
The siliciclastic content is only 0%-5% in Subunit IIA, and no sand or silt is noted within this interval (Fig. F12). Siliceous microfossils are absent to rare, whereas calcareous microfossils amount to nearly 100%. The percentage of calcareous microfossils decreases around 135 mcd as the amount of micrite increases. Pyrite is present between 120 and 140 mcd.
Fifteen ash layers and significant ash patches are present within Subunit IIA. Eleven of them can be correlated between holes (Table T9). The thickness of these ash layers varies between <1 and 20 cm. In general, the ash layers have sharp basal contacts. Some ash layers are diffuse, exhibiting signs of bioturbation and disturbance. Ash color is light to dark greenish or brownish gray, dark gray, and black, and grades to lighter colors toward the top. The upper contact grades into the major lithology. Individual ash layers are occasionally topped with a green band. X-ray diffraction (XRD) analysis shows that this greenish band is characterized by a mineral composition similar to the brownish gray basal part of the ash layer, with major amounts of clear volcanic glass, except the green band contains lesser amounts of biotite and plagioclase (Fig. F20).
Magnetic susceptibility values are ~0 for all of Subunit IIA (Fig. F11). NGR values of this subunit are characterized by a baseline around 12 cps (which is not distinguishable from background) with some variation to 30 cps, mostly from the presence of ash layers. GRA and MAD bulk density values continue to slightly increase downhole from 1.65 to 1.75 g/cm3, a trend continued from Unit I.
Reflectance measurements, a*, b*, and L*, remain generally constant at a greenish color (i.e. a* < 0) throughout this subunit (Figs. F11, F12, F13). Higher-frequency variations in a* and b* are the result of thin color bands.
Subunit IIB is composed of nannofossil ooze with micrite. The boundary between Subunits IIA and IIB is characterized by a sharp increase in micrite content and a corresponding decrease in calcareous microfossils (Fig. F12). The pale purple and green banding observed in Subunit IIA is no longer visible. The sediment color changes abruptly from grayish white to a fairly homogeneous pale brown at ~162 mcd (Fig. F21). White mottles, often surrounded by gray halos, and occasional distinct burrows are present within the pale brown sediment, decreasing in abundance downcore. Indurated sediments, partially lithified and composed of micrite and nannofossil ooze, are present in increasing abundance downcore and are especially prevalent at the base of Holes 1237B and 1237C (Fig. F22). Shallower than 250 mcd, these indurated sediment layers are usually associated with a white color. Below this depth, they match the color of the major lithology.
Siliciclastics are minimal to absent in Subunit IIB. Minor amounts of silt to very fine sand-sized grains and volcanic glass are observed at 170-180 and ~250 mcd (Fig. F22). Calcareous microfossils and micrite each represent ~50% of the lithology. A notable increase in micrite content to nearly 100% and a proportional decrease in calcareous microfossils occurs at ~170 mcd, followed by a decrease in micrite abundance to almost 0 (and increase in calcareous microfossils). A second interval of decreased micrite content is present between ~235 and 275 mcd.
Three ash layers are present in Subunit IIB. They are light to dark brownish gray and range from 2 to 8 cm in thickness. All of them can be correlated between holes. The deepest ash layer is present at ~167 mcd (Table T9).
Magnetic susceptibility is generally low, increasing slightly between ~150 and 180 mcd, 210 and 250 mcd, and 280 and 360 mcd, apparently unrelated to changes in major lithology (Fig. F11). NGR is constant at a very low level. GRA and MAD bulk density have relatively constant values near ~1.8 g/cm3.
Subunit IIB is characterized by a reddish hue (i.e., a* > 0), increased chromaticity (i.e., a* and b* higher than in the sediments above), and variable luminance (L*) (Fig. F11). Within the reddish sediment, the less chromatic parts (i.e., low a* and low b*) are also the brightest (i.e., highest L*). Three separate populations of reddish sediment are distinguishable in the L*-a*-b* color space (Fig. F13), and a different population characterizes each of the upper, middle, and lower parts of this subunit (Fig. F11).
A total of 55 ash layers and significant patches are present between 6 and 167 mcd. Thirty-five of them can be correlated among holes (Table T9). These ash layers primarily consist of clear silt- to sand-sized volcanic glass associated with minor amounts of plagioclase (oligoclase to labradorite), biotite, quartz, amphibole, and pyroxene (based on preliminary smear slide observations).
Volcanic ash deposition substantially began in the early late Miocene (~9 Ma), although minor amounts of volcanic glass are present at ~180 mcd (~11 Ma) and ~250 mcd (~17 Ma) (Fig. F12). Volcanic events, as represented by the presence of ash layers, were most frequent at ~7.5 Ma and 1-3 Ma (Fig. F14). The composition of ash layers suggests an andesitic source. Pouclet et al. (1990) reported from the nearshore sedimentary sequences (90°-14°S) collected off the coast of Peru during ODP Leg 112 that intense volcanic activity occurred between 17 and 16 Ma, 13 and 12 Ma, 10 and 8 Ma, and 5-0 Ma. In addition, ignimbrite eruption intensified in the central volcanic zone of the Andes (15° and 28°S) between 12 and 5 Ma (Gregory-Wodzicki, 2000).
Andesitic ash is nearly absent at Site 1236 (~21°S); thus, two possibilities exist for ash layer deposition beginning at ~9 Ma at Site 1237. One possibility is that Site 1237 was closer to the volcanic source than Site 1236. This is supported by the paleoposition of Site 1237 at 81°W at ~9 Ma, the same as the present position of Site 1236 (see "Introduction"). Alternatively, zonal easterly winds able to transport volcanic materials were mostly restricted to north of Site 1236 at 21°S.
The increasing frequency of ash layer deposition at ~7.5 Ma indicates either a change in the position or intensity of zonal wind favorable for ash transport to Site 1237 or a change in the volcanic activity of the central volcanic zone of the Andes. If the southeast trades were enhanced, they would also promote intense coastal upwelling. Coincidence of maxima of siliceous fossils and organic carbon (see "Geochemistry") with volcanic ash layer frequency may reflect this situation (Fig. F23). Increases in siliceous microfossils and organic carbon since ~7.5 Ma could also be explained by eastward motion of the Nazca plate, bringing Site 1237 closer to the upwelling zone off Peru (Fig. F23).
On the other hand, ash layer deposition may be related to Andean uplift (Kono et al., 1986). From the late Oligocene to middle Miocene, the central Andes rose but probably did not yet impede subtropical wind circulation (Gregory-Wodzicki, 2000). The lack of an Andean barrier to subtropical wind circulation could potentially promote eolian transport into the subtropical southeast Pacific via the southeast trades. The progressive uplift is thought to have changed the direction in trade winds from a more zonal flow prior to the uplift toward a more meridional flow in response to the uplift as indicated by results of modeling studies (e.g., Hay and Brock, 1992). Enhancement of the meridional flow would promote coastal upwelling and higher productivity at Site 1237, which is possibly indicated by maxima in mass accumulation rates in both carbonate and organic carbon from ~8 to 5 Ma (see "Age Model and Mass Accumulation Rates" and "Geochemistry").
Frequent ash layers between 1 and 3 Ma may suggest enhanced tectonic events. Frequent volcanic activities during this period are also reported from Sites 502 and 503 near Central America (Ledbetter, 1982), suggesting that volcanic activity could reflect regional-scale tectonic events in the eastern equatorial to south Pacific margin, possibly associated with the major uplift phase of the northern Andes during the Pliocene.
Color reflectance measurements plot in two main regions in the a*-b* color plane: the generally greenish sediments of Unit I and Subunit IIA group in the second quadrant, whereas the reddish sediments of the Subunit IIB cluster more tightly in the first quadrant (Fig. F13), suggesting that multiple chromophores are responsible for the general sediment color.
Preliminary predictive relationships between reflectance, carbonate, and TOC developed via a multiple linear regression reveal high correlation coefficients for carbonate (r2 = ~0.8) and for TOC (r2 = ~0.9) in the upper ~148 mcd of the pelagic sequence drilled at Site 1237 (Unit I and Subunit IIA) (Fig. F24). No prediction for these components was attempted in Subunit IIB, where carbonate values are close to 100 wt% and TOC contents are below shipboard detection limits. The predicted TOC exhibits strong ~8-m cycles in the first 160 mcd (spanning the last ~8 m.y. based on the shipboard age model), suggesting a ~400-k.y. cyclicity.
Reflectance spectra of Subunit IIB were examined to identify the chromophore(s) responsible for the pervasive red color of the sediment (Fig. F25). A sequential plot of raw spectra shows that Subunit IIB differs from those of the overlying units by exhibiting repetitive trends toward the red part of the visual domain (i.e., 600-700 nm) (Fig. F25A). First derivatives of the reflectance (Fig. F25B) show that peaks typical of goethite (i.e., ~535 nm for the principal peak and ~435 nm for the secondary peak) and hematite (i.e., ~465 nm) are present in Subunit IIB components (Fig. F25C). Q-mode factor analysis of the first derivatives confirmed that all spectra collected below 149.2 mcd are similar; therefore, the sediment contains both hematite and goethite. However, VARIMAX-rotated R-mode factor analysis failed to extract individual factors for hematite and goethite, suggesting they covary in the depth-time domain.
Extraction of a quantitative goethite estimate was precluded by the anomalously large amplitude of the secondary goethite peak, sometimes stronger than the principal peak, as well as the proximity of this secondary peak to the peak characteristic of hematite. Nevertheless, preliminary uncalibrated estimations of hematite variation using the peak height at 565 nm show stepwise increases at 163.7 and 168.3 mcd, an abrupt decrease between 169 and 171 mcd, a gradual increase between 273 and 343 mcd, and, again, a slight decrease between 343 and 360 mcd (Fig. F25D).
The nearly homogeneous nannofossil ooze of Subunit IIB shows no significant lithologic variation during the late Oligocene to late Miocene. The siliciclastic fraction, mainly clay minerals, is low with little variability from 3% to 5%, whereas color variations result from changing concentrations of goethite and hematite (Fig. F25). Authigenic formation of goethite and other more disordered oxyhydroxides in marine sediments can occur in the oxic zone (e.g., Haese et al., 1998, and references therein), but dehydration of goethite to hematite is unlikely in subaqueous conditions. The iron oxyhydroxides are mobilized in the form of Fe2+ as the sediments experience reduction with increasing depth below the oxic zone. At Site 1237, goethite and hematite contents increase with depth, suggesting a terrigenous rather than authigenic origin (Fig. F25). Both goethite and hematite can be formed as weathering products in soils and can be transported by winds as flakes or coatings on other mineral particles. Goethite is favored over hematite in conditions of increased precipitation and/or decreasing temperature (Kampf and Schwertmann, 1983). The presence of iron oxyhydroxides implies that the sediment of Subunit IIB contains a far-field eolian component (Pye, 1987).
Today, Site 1237 lies in a zone that extends off the coasts of Peru, Ecuador, and Colombia, receiving dust transported by the southeast trade winds from the deserts of Peru and Chile (Prospero and Bonatti, 1969). Prior to the major uplift of the Andes, the trade winds probably had a more zonal distribution and the zone of eolian hematite and goethite deposition extended over both Sites 1236 and 1237. Coarse-grained material such as volcanic ash did not usually reach Site 1236 because of its southern marginal position within the trade winds belt. Iron hydroxides are detectable at Site 1237 between ~31 and ~8 Ma; however, during that time interval, siliciclastic mass accumulation rates were an order of magnitude lower than at present. These low rates, together with the fine grain size of the terrigenous component, reflect the distant position of the site relative to the continent (i.e., ~1800 to 100 km) (Fig. F23).
The persistence of iron oxyhydroxides in sediments younger than the early late Miocene at Site 1236 indicates that diagenesis is the likely cause of their abrupt disappearance at ~8 Ma at Site 1237. Dissolution of iron oxides may have occurred following an increase in the organic carbon flux around that time (see "Age Model and Mass Accumulation Rates" and "Geochemistry"). A period of reducing conditions might be indicated by the presence of green and purple-black banding (Lind et al., 1993; Giosan, 2001), which is ubiquitous in the sediments above Subunit IIB. Nevertheless, the siliciclastic mass accumulation rate also increased at ~8 Ma (see "Age Model and Mass Accumulation Rates") suggesting an increased supply in the eolian dust, even though the iron hydroxide component is gone. An increase in the amount of siliciclastic materials at ~3 Ma, some yielding small amounts of reworked discoasters, possibly supplied from the Tertiary outcrops in the Peru coastal region, suggests the presence of a hemipelagic component. However, eolian dust was also a likely contributor to the siliciclastic fraction as suggested by the increase in iron oxides at Site 1236 during the late Pliocene-Pleistocene.
The presence of eolian material at Site 1237 strengthens the hypothesis put forward at Site 1236 that arid and semiarid conditions existed in subtropical South America from the late Oligocene to early Miocene. Aridity in this region could have been the result of loss of humidity by the southeast trade winds along their continental path across South America and the presence of the adjacent cool Peru-Chile Current (Frakes, 1979). Variability in the dust deposition at Site 1237 may have resulted from changes in the wind strength and/or from changes in continental aridity in the source area in response to climatic fluctuations. Hematite maxima during the late Oligocene and early late Miocene seem to coincide with Antarctic ice sheet maxima (Zachos et al., 2001, and references therein) (Fig. F23), suggesting that the aridity in the source area might have been affected by these major climatic events. Peak micrite abundance also coincides with the hematite maxima, indicating a potential linkage between the marine environment, diagenesis, and climate.