A 282.9-m sedimentary sequence was recovered from four holes at Site 1240. Basement was reached in Hole 1240A, where basalt was recovered. The sediments at Site 1240 are mainly nannofossil ooze and diatom nannofossil ooze. Siliciclastic contents are very low and primarily consist of clay minerals. Eight ash layers are present and can be correlated between holes. The presence of brown glass in some of the layers may indicate a different source of ash layers between Site 1240 and previous sites.
A single lithologic unit with three subunits is defined at Site 1240 (Table T8). Subunit boundaries are defined by physical properties and minor lithologic differences (Figs. F12, F13). Subunit IA (0-142.2 mcd; ~0-1.7 Ma) is composed of nannofossil ooze with relatively high percentages of diatoms. Sediments are highly bioturbated, showing very heterogenous color mottling. Subunit IB (142.2-206.4 mcd; ~1.7-2.1 Ma) is enriched in diatoms and siliciclastic components relative to the other two subunits. Color banding (centimeter scale) is frequently present throughout this subunit. Darker bands are associated with the occurrence of pennate diatoms. The overall color is more reddish yellow than in Subunit IA, represented by high values of a* (Fig. F12). Subunit IB is also characterized by relatively low values in grain density, probably as a result of a high biogenic silica content (Fig. F14) and high values of total organic carbon (TOC) (see "Geochemistry") and chlorins (Fig. F15). All physical properties show a substantial change in Subunit IB relative to the adjacent subunits, with sharp transitions over intervals of apparently <1 m. Ash layers are absent within Subunit IB. Sediments and physical properties in Subunit IC (206.4-282.9 mcd; ~2.1-2.6 Ma) are very similar to those in Subunit IA.
Compositional differences observed between subunits may reflect the history of primary productivity at Site 1240. Characteristics of Subunit IB indicate an enhancement of equatorial upwelling and thus increased primary productivity between ~1.7 and 2.1 Ma, probably as a result of intensified atmospheric and oceanic circulation. Increased siliciclastic content in that interval also suggests more effective eolian transport toward Site 1240, supporting a potential intensification of atmospheric circulation.
A single lithologic unit (Unit I) is defined and divided into three subunits (Table T8) on the basis of visual core description, smear slide analysis, magnetic susceptibility, color reflectance, P-wave velocity, NGR, moisture and density (MAD), and GRA bulk density measurements. Depth boundaries of the subunits are based primarily on changes in physical properties (Fig. F12). All three subunits contain nannofossil ooze; however, in Subunit IB, nannofossil-bearing diatom ooze becomes more frequent. Subunit IB is distinguished from Subunits IA and IC by relatively low values of magnetic susceptibility and GRA bulk density, high P-wave velocity (VP) and NGR, low lightness (L*), and high a*.
At Site 1240, GRA bulk density and MAD bulk density are well correlated (r2 = 0.94) (Fig. F14). Porosity mirrors changes in bulk density. Grain density is largely controlled by carbonate content, which is typical in sediments composed mostly of carbonate and opal microfossils (r2 = 0.63) (Fig. F14). Predictive relationships between reflectance and either carbonate or TOC via a multiple linear regression are relatively good for both components (i.e., r2 = ~0.8). Organic pigments are detected in absorbance features at 410, 510, 560, and 650 nm in reflectance spectra (Fig. F15). The strongest absorption feature at 650 nm, which persists throughout the sediment column, is probably a result of chlorins (i.e., chlorophyll-related pigments). The chlorin optical index is well correlated to the TOC (r2 = ~0.7) (Fig. F15).
Subunit IA is dominated by diatom-bearing nannofossil ooze and nannofossil ooze. Clay- (diatom) bearing nannofossil ooze is present in the top ~34 mcd and some foraminifer- (diatom) bearing nannofossil ooze is sporadically present throughout the subunit. Diatom nannofossil ooze and nannofossil diatom ooze are present below ~100 mcd. The sediment color alternates between pale olive and light olive gray. These colors are not homogeneously distributed but form amorphous mottles throughout the entire subunit, indicating intense bioturbation. Vertical burrows (5-20 cm in length) and Zoophycos traces are also common (Fig. F16).
The sediments are dominated by biogenic components (Fig. F17), which consist mainly of nannofossils (~40% to ~80%) and diatoms (~10% to ~40%), with minor amounts of foraminifers (~0% to ~15%), spicules, silicoflagellates, and radiolarians (the last three combined = ~3%). Below ~100 mcd, an increase in diatom abundance from 20% to 40% accompanies a decline in nannofossil abundance from 80% to 40%. Siliciclastics, mostly clay minerals, represent a minor component (~5%) of Subunit IA. Micrite is usually present, with values increasing to 5% in the interval of 100-125 mcd. Thin intervals (~40 cm) of color banding coincide with increases in diatom abundance (Fig. F17).
Three ash layers are present in Subunit IA (Table T9). These ashes are present both as patches and layers and, in all cases, show evidence of bioturbation (Fig. F18). The ash color varies between brownish, gray, and dark gray. The ash layers contain clear platy and vesicular glass. Accessory minerals include biotite, quartz, and feldspars, with rare occurrences of clinopyroxene and amphiboles. The uppermost ash layer (25.69-25.71 mcd) contains trace amounts of brown volcanic glass.
Overall, magnetic susceptibility is low (~1-4 instrument units) through this subunit, but the values are higher and more variable than those in Subunit IB (Fig. F12). NGR remains between ~5 and 19 cps in Subunit IA. GRA and MAD bulk densities increase between 0 and 45 mcd as a result of dewatering and compaction and remain stable at ~1.4 g/cm3 for the rest of the subunit. Lightness increases slightly from 0 to ~40 mcd and then remains stable through the base of the subunit.
The lithology in Subunit IB is variable and consists of nannofossil-bearing diatom ooze, diatom nannofossil ooze, and diatom-bearing nannofossil ooze. Clay-bearing diatom and/or nannofossil ooze also occur frequently. Color oscillates primarily between dark olive, olive, and olive gray. Intervals containing centimeter-thick color bands are frequent within Subunit IB (Fig. F17). The darker-colored bands contain higher abundances of pennate diatoms, such as Thalassiothrix spp., the major component of diatom mats (see "Biostratigraphy"). Bioturbation, mottling, and burrow traces including Zoophycos are less abundant here than in Subunit IA. The intervals with color banding exhibit little evidence of bioturbation.
Nannofossil contents are slightly lower (~40% to ~60%) and diatoms generally more abundant (~30% to ~50%) in Subunit IB than in Subunit IA (Fig. F13). Other biogenic components (foraminifers, spicules, and silicoflagellates) are also present but in smaller percentages (<5%) than in Subunit IA. Siliciclastics, mainly clay minerals, are variable but, on average, are more abundant in this subunit (~10%). Micrite abundance in Subunit IB is similar to that in other subunits (0%-10%).
The physical properties data show abrupt shifts at 142 and 206 mcd, defining the boundaries of Subunit IB (Fig. F12). In Subunit IB, magnetic susceptibility is relatively low (~0-2 instrument units), whereas NGR is relatively high. NGR is more variable than in Subunit IA.
Bulk density shifts to lower values as a result of high porosity and then remains very stable (~1.3 g/cm3) throughout the subunit. Grain density is low in this subunit, consistent with greater concentrations of siliceous components (Fig. F14). P-wave velocities show consistently high values in Subunit IB (Fig. F12). High concentrations of biogenic silica are expected to result in a high P-wave velocity (e.g., Hamilton and Bachman, 1982; Dunbar, 2001) because angular grains in contact form a rigid framework conducive to the transmission of sound waves. A combination of low density and high compressional wave velocity, similar to the situation in Subunit IB, has been attributed elsewhere to the presence of abundant biogenic silica (e.g., Weber et al., 1997; Weber, 1998). Lightness values are distinctly lower than those in Subunits IA and IC. All color measurements at Site 1240 plot in the "yellow" domain of the a*-b* space (Fig. F19), but sediment in Subunit IB comprises a distinct color population characterized by a redder hue (i.e., a* > 0).
Subunit IB is characterized by relatively high TOC concentration (~3.4 wt%) (Fig. F12), which coincides with high chlorin abundance (Fig. F15). Carbonate concentrations are relatively low (~40 wt%) in this subunit (see "Geochemistry") in parallel with low grain density values (Fig. F13). Sedimentation rates appear to increase in Subunit IB from ~8 to ~13 cm/k.y. However, the age model for Site 1240 is tentative at this time and allows for alternative interpretation (see "Age Model and Mass Accumulation Rates").
The lithology in Subunit IC is mainly diatom-bearing nannofossil ooze and diatom nannofossil ooze. Sediment color is olive, pale olive, and light olive gray. Color mottling is abundant throughout the subunit, indicating intense bioturbation. Burrow traces, including Zoophycos (Fig. F16), are commonly present, as observed in Subunit IA. The sediments near the basaltic basement contain vivid green mottles, indicative of enrichment in the smectite group of minerals, such as nontronite (Fig. F20), resulting from alteration of the basalt.
The major lithologic components of Subunit IC are of biogenic origin (Fig. F13). Nannofossils are predominant (~40%-70%), with lesser amounts of diatoms (~20%-40%). Abundances of the other biogenic components (foraminifers, spicules, silicoflagellates, and radiolarians) are slightly higher than in Subunit IB. Siliciclastics, primarily clay, are present in relatively low percentages (~0%-10%). In contrast to the previous subunits, volcanic glass is present as a minor component of the major lithology throughout Subunit IC. Micrite is present in amounts similar to the previous subunits (~5%).
Five ash layers are present in Subunit IC. Three of them are correlative between Holes 1240A and 1240B (Table T9). Some of the layers are diffuse as a result of bioturbation, whereas others have a clear, sharply defined base (Fig. F18). The ash color varies between light gray, dark gray, and black. With the exception of the layer at 268.03-268.12 mcd, ash layers in Subunit IC are mainly composed of platy and vesicular clear volcanic glass, with feldspars, biotite, and rare amphiboles. The ash layer at 249.46-249.54 mcd contains very small amounts of brownish glass. The ash layer at 268.03-268.12 mcd is composed of brown glass shards.
Physical properties are similar to those recorded in Subunit IA (Figs. F12, F14). Magnetic susceptibility values oscillate between ~0 and 3 instrumental units. NGR values are stable with some oscillations near ~15 cps, although values increase sharply near the base of the subunit (below 272.5 mcd). Bulk density values are more variable than in Subunit IA (~1.4 g/cm3). Lightness increases in relation to Subunit IB.
The recovered sedimentary sequence at Site 1240 forms a continuous record of the last ~2.6 m.y. (late Pliocene-Pleistocene) with unusually high sedimentation rates for a pelagic site (~8 cm/k.y. for Subunit IA; ~13 cm/k.y. for Subunits IB and IC). Such high sedimentation rates, together with the dominantly biogenic composition of the sediments, indicate a relatively high productivity regime. This interpretation is consistent with the present geographic location of the site beneath the equatorial upwelling zone, suggesting that this highly productive system has been active throughout the last 2.6 m.y.
Primary production at Site 1240 was dominated by calcareous nannoplankton. Diatoms were also important particularly between ~1.7 and 2.1 Ma (Subunit IB). Characteristics of the sediment during this time period (i.e., presence of layers enriched in pennate diatoms, higher TOC weight percentages, high chlorins, and increased sedimentation rates) are consistent with an overall increase in productivity at ~2.1 Ma, likely resulting from an increase in the strength of the equatorial upwelling system, leading to higher nutrient availability. Enhanced equatorial upwelling in the Pacific has been attributed to intensified atmospheric and oceanic circulation caused by an increased meridional temperature gradient (Farrell et al., 1995).
Intensified atmospheric circulation during this interval is further suggested by increased siliciclastic content within Subunit IB, which is most likely of eolian origin. A synchronous strengthening of the equatorial upwelling system and enhancement of eolian supply is consistent with results from Ocean Drilling Program (ODP) Leg 138 (Hovan, 1995). The dominance of clay-sized siliciclastic particles may indicate that siliciclastic transport to Site 1240 occurred via wet deposition. This enhanced siliciclastic input could provide additional nutrients, further contributing to the increase of primary productivity.
Grain compositions of ash layers are mostly similar to those for Sites 1238 and 1239, suggesting that the majority of the ash at Site 1240 might also originate from northern South American volcanism. However, some ash layers with brown volcanic glass as an accessory component (one in Subunit IA and one in Subunit IC) indicate the presence of an additional volcanic source. The presence of ash layers with minor amounts of brown volcanic glass, also observed at Site 1241, suggests that Central America is a possible source region (Cadet et al., 1982). In addition, the presence of an ash layer fully composed of brown glass (268.03-268.12 mcd) may indicate a hotspot source, such as Galapagos.
Ash layers are absent in Subunit IB, although ash layers were deposited at the more southern Sites 1238 and 1239 during the same time interval (~1.7-2.1 Ma) (Fig. F21). The distribution of ash layers in the equatorial Pacific is mainly controlled by the extent and location of the volcanic eruption, the high-altitude winds that are predominantly westerly, low-altitude winds that are dominated by the southeast trade winds, and by the directions of the surface currents. Today, Sites 1238-1240 are located below the path of the southeast trade winds. A weakening of the trade winds system or a strengthening of upper-atmosphere westerlies may have restricted the distribution of ash to the west and prevented a long-range transport of wind-blown material to the more distal location of Site 1240. Alternatively, the eastward flow of the very strong EUC at Site 1240 may have offset the atmospheric fall-out pattern toward the east. However, the absence of ash in Subunit IB could also a result of an episode of relatively infrequent explosive volcanism in northern South America.