7%). Micrite is nearly absent. The abundance of both siliciclastics and foraminifers decreases with depth, whereas nannofossils and diatoms become more significant.
A pelagic sediment sequence 447.06 mcd thick was recovered at Site 1241, spanning the interval from the late Miocene to the Holocene. The sediment at Site 1241 consists of various types of nannofossil ooze. A distinct interval of laminated diatom ooze is present near the base of the sequence between ~420 and 440 mcd. Biogenic constituents are predominantly nannofossils, but some intervals include significant amounts of diatoms, foraminifers, and/or siliciclastics, mainly clay. Interbedded minor lithologies include volcanic ash and lapilli layers. Physical properties reflect the mixture of a carbonate component, characterized by high bulk density, high reflectance, and low magnetic susceptibility and natural gamma radiation (NGR), with secondary constituents that dilute the carbonate signal. Organic pigment absorption features are detectable in reflectance spectra at 410 and 650 nm throughout the sediment.
Based on major lithologic components, textural characteristics, and physical properties, a single lithologic unit was defined and divided into three subunits. Subunit IA is composed primarily of clayey foraminifer-nannofossil ooze that exhibits cyclic variability in physical properties. Subunit IB contains nannofossil ooze with variable minor contributions from diatoms and foraminifers in a sequence whose overall homogeneity is clearly expressed in the physical properties. Subunit IC is composed primarily of clay diatom-bearing nannofossil ooze with laminated diatom ooze near the base. Partially lithified nannofossil ooze underlies the laminated diatom ooze and sits atop basaltic sands and vesicular basalt fragments that are indicative of basement.
High diatom and total organic carbon (TOC) fluxes during the early part of the late Miocene are consistent with the tectonic path followed by Site 1241 on the Cocos plate, suggesting a decrease in biogenic production as the site gradually exited the active equatorial upwelling zone. Coeval low fluxes of nannofossils could be attributed to ecological variability and/or increased dissolution during the Miocene "carbonate crash" (Lyle et al., 1995). Subsequent increases in carbonate accumulation rates between ~7 and 5 Ma might be coincident with the "biogenic bloom" described in other regions of the tropical Indo-Pacific (Peterson et al., 1992; Farrell et al., 1995).
Seventy-two volcaniclastic horizons record both hotspot volcanic activity as well as volcanic eruptions in Central America. The decrease in brown volcanic glass and scoria toward recent times, paralleled by an increase in clear glass and pumice, is consistent with the eastward plate motion, which moves the site away from the Galapagos hotspot and toward the Central American volcanic arc. Increased volcanic activity between 0 and 2.5 Ma and 8 and 9 Ma agrees with high ash accumulation intervals previously described in the Caribbean.
A single lithologic unit was defined and divided into three subunits on the basis of visual core description, smear slide analysis, color reflectance, magnetic susceptibility, NGR, moisture and density (MAD), and GRA bulk density measurements (Table T7; Fig. F14). Unit I is composed entirely of pelagic ooze dominated by nannofossils with variable enrichments of foraminifers, diatoms, and siliciclastics, primarily clay, whose relative distributions determine the presence of the three subunits (Fig. F15). Basaltic basement directly underlies the pelagic sequence.
Throughout Unit I at Site 1241, GRA bulk density and MAD bulk density are well correlated to each other (r2 = 0.89) and porosity mirrors changes in bulk density (r2 = 0.9) (Fig. F16). Grain density is controlled in part by carbonate content (r2 = 0.5). Opaline silica content also affects the grain and bulk densities, as low values occurring between 350 and 440 mcd coincide with a distinct increase in diatom content (Fig. F16). The lightness parameter L* is strongly controlled by carbonate content (r2 = 0.8). A multiple regression between reflectance and carbonate improves this correlation only slightly (r2 = 0.9). However, the relationship between reflectance and TOC is weak (i.e., r2 = ~0.4 in a multiple linear regression). Reflectance measurements plot in the "yellow" domain of the a*-b* color plane (Fig. F17), although most of the data fall in the "green" quadrant (i.e., a* < 0). Absorption features ascribed to chlorophyll-related pigments (i.e., chlorins) are detectable at 650 and 410 nm in reflectance spectra measured in sediment at Site 1241 (Fig. F18). Unlike at previous sites where chlorins have been detected (i.e., Sites 1238-1240), at Site 1241 the normalized depth of the absorption feature at 650 nm is not correlated to the TOC content. However, both TOC and chlorin contents are highest in the lowermost subunit of the sedimentary sequence recovered at Site 1241.
Subunit IA is primarily composed of clayey foraminifer-nannofossil ooze. The sediment is dominated by foraminifers (25%-45%) and nannofossils (30%-60%) with significant amounts of siliciclastics, mainly clays (10%-20%) (Fig. F15). Diatoms are a minor component (<12%) as are the other siliceous biogenic components (i.e., the sum of sponge spicules, radiolarians, and silicoflagellates is 7%). Micrite is nearly absent. The abundance of both siliciclastics and foraminifers decreases with depth, whereas nannofossils and diatoms become more significant.
Light-dark color cycles are present on decimeter scales throughout the subunit, with sediment color ranging from olive gray to light olive gray and pale olive (Fig. F19). Color mottling and discrete vertical burrows, including Zoophycos traces, are common to abundant throughout Subunit IA. Green and purple-gray color mottles and burrow halos are visible in the pale olive-colored intervals.
Fourteen of the nineteen volcaniclastic horizons observed in Subunit IA can be correlated between Holes 1241A and 1241B (Table T8). The volcaniclastic horizons include ash layers and lapilli layers or patches (Table T8; Fig. F20). The ashes contain primarily clear glass, with minor amounts of brown glass, plagioclase, biotite, pyroxenes, amphiboles, and quartz. A patch of ash at 27.66 mcd contains only brown glass with rare associated minerals. The lapilli layers and patches contain pumice fragments averaging 3-5 mm in diameter.
Magnetic susceptibility is relatively high in Subunit IA, where values range from 24 instrument units at the top of the subunit to ~3.5 instrument units at 48 mcd (Fig. F14). Natural gamma counts are also elevated (25-35 cps) and variable in this interval. Lightness (L*) is relatively low, below ~60%. The high-amplitude variability in physical properties, high magnetic susceptibility and NGR values, and low lightness levels help to distinguish Subunit IA from the sediment below. The base of Subunit IA is defined by the base of the ash layer at 51.6 mcd. This layer is recognized in Holes 1241A and 1241B and lies immediately below the shifts in physical properties and the obvious color cycles typical of Subunit IA.
Homogeneous nannofossil ooze dominates Subunit IB, with variable contributions from diatoms and foraminifers (foraminifer-bearing nannofossil ooze, diatom-bearing nannofossil ooze, and clay foraminifer-bearing nannofossil ooze). Overall, siliciclastics are minor, as are other siliceous biogenics and micrite. Sediment color varies slightly between light greenish gray, pale olive, and yellowish gray. Mottling and discrete burrows are less obvious in Subunit IB when compared to the sediment above. However, green and purple-gray color bands and burrow halos are pervasive.
Nannofossil abundance varies between 60% and 85%. Diatom abundance is lower, between 3% and 15%. Foraminifer abundance continues to steadily decline until ~200 mcd and then remains constant at ~3%. The siliceous microfossil abundance averages ~3% with little variability. Micrite presence, although minor, is more common than in Subunit IA, with average values of ~5% and a distinct maximum of ~15% between 160 and 185 mcd.
Subunit IB contains 23 volcaniclastic horizons; 13 of these can be correlated from Hole 1241A to 1241B. Although most of the ash layers contain mainly clear volcanic glass, ash layers that contain predominantly brown glass are more common with increasing depth. Furthermore, only the uppermost five coarse-grained horizons contain pumice, whereas the lowermost four horizons contain scoria grains ranging from sand sized to 3-5 mm in diameter. The clear glass ash also contains plagioclase and biotite, whereas associated minerals are rare within the ash horizons dominated by brown glass.
The lithologic homogeneity of Subunit IB results in less variable physical properties (Fig. F14). Magnetic susceptibility is constantly low, with values ranging between 2 and -2 instrument units. NGR is also low, averaging ~20 cps, with a cyclicity on the order of tens of meters. Lightness (L*) increases near the top of Subunit IB and reaches a plateau at ~120 mcd, remaining at ~70% through the bottom of the subunit. MAD and GRA bulk densities exhibit trends similar to luminance; they continue to increase at the top of the Subunit IB boundary until ~120 mcd, reaching maximum values of ~1.6-1.7 g/cm3.
The base of Subunit IB is defined as the base of a correlatable ash layer at 318.4 mcd. This horizon approximates the transition from the homogeneous nannofossil ooze to more diatom-rich sediment and the associated shift in physical properties.
Subunit IC is composed primarily of clay and diatom-bearing nannofossil ooze with laminated diatom ooze near the base in Cores 202-1241A-40X through 42X. Sediment color ranges from light greenish gray, light olive gray, and pale olive, with olive brown and dark olive gray dominant below Core 202-1241A-38X. Mottles and burrows, including Zoophycos traces, are common throughout, except within the laminated diatom ooze intervals (Fig. F21). The laminae are millimeter scale and irregular across the core. Deformational features, including microfaults and soft-sediment folding, are common within the laminated interval. The basal nannofossil ooze underlying the laminated diatom ooze is partially lithified, close to chalk firmness. Mafic sand-sized grains appear at ~400 mcd and become increasingly more abundant downcore. The base of Subunit IC is defined by the presence of basaltic sands and vesicular basalt fragments that are indicative of the basement.
Nannofossil abundance in Subunit IC declines downcore from 80% to <50% (Fig. F15). A corresponding increase in diatom abundance, from ~12% to a maximum of 50%, is present between the top of the subunit and ~340 mcd. Minor contents of foraminifers, biogenic siliceous components, and micrite exhibit no noticeable systematic variations. The presence of disseminated volcanic glass increases below ~350 mcd.
Twenty-nine volcaniclastic horizons are present in Subunit IC, and four have been correlated between holes (Table T8). Brown glass ash continues to increase in importance relative to clear glass at greater depths, and all the coarse-grained layers are composed of scoria. The clear glass ash layers contain the same associated minerals that are present in the upper two subunits, dominated by plagioclase and biotite. The ashes that are dominated by brown glass contain increased amounts of associated minerals, including plagioclase, amphiboles, and pyroxenes, relative to their counterparts in Subunit IB. However, they still tend to have lower amounts of associated minerals than the clear glass ashes. The ash layer present in the interval 202-1241A-42X, 13-26 cm, overlies ~50 cm of coarse black grains. Visually, these two layers appear to constitute a single fining-upward sequence.
All physical properties display significant changes at the Subunit IB/IC boundary (Fig. F14). Magnetic susceptibility increases to higher average values with significantly greater amplitude variability. The variability in the natural gamma record increases as well, exhibiting slightly elevated levels relative to Subunit IB. A distinct shift in GRA bulk density and lightness (L*) to lower values occurs at the Subunit IB/IC boundary.
Site 1241 is located at 2027 m water depth in the Panama Basin under relatively low salinity and nutrient-poor surface waters. During the last 11 m.y., the largest shift in sediment composition was a decrease in diatom abundance in the early late Miocene. This major compositional change is accompanied by a decrease in organic carbon content (see "Geochemistry") and chlorins. Moreover, diatom and TOC fluxes were elevated for the entire late Miocene but declined toward less variable, low flux values in the Pliocene-Pleistocene (see "Age Model and Mass Accumulation Rates"). These changes are consistent with the tectonic path followed by Site 1241 on the Cocos plate, where the site gradually exited the active equatorial upwelling zone (Pisias et al., 1995).
Nannofossil abundance and fluxes were low for the early part of the late Miocene, increasing significantly after ~7 Ma toward the Pliocene boundary. This pattern could be the result of a change from a diatom-dominated to nannoplankton-dominated ecosystem during the early part of the late Miocene. Alternatively or complementarily, the low carbonate flux between ~11.2 and 7 Ma could be a result of tectonic-related restriction of deepwater exchange between the Atlantic and the Pacific during the Miocene "carbonate crash" (e.g., Lyle et al., 1995), which is known to have affected most of the tropical eastern Pacific. Site 1241 may have been located above the lysocline, at least in the beginning of the "carbonate crash." The subsequent increase in carbonate accumulation rates after ~7 until ~5 Ma (see "Age Model and Mass Accumulation Rates") is possibly related to the "biogenic bloom" observed throughout the tropical Indo-Pacific (e.g., Farrell et al., 1995).
Distinctly high foraminifer contents and correspondingly low diatom and nannofossil abundance characterize the sediment in Subunit IA. The increase in foraminifer abundance continues a rising trend that started around 6 Ma (~200 mcd). Siliciclastics exhibit a consistently high level over the last ~1 m.y. Changes in physical properties reflect these compositional changes. The variability displayed by most of these properties (i.e., magnetic susceptibility, bulk density, and reflectance) is suggestive of orbital cyclicity.
Seventy-two horizons yielding volcaniclastic materials, including patches and layers of ash, pumice, and scoria of lapilli size and an andesite rock fragment were found at Site 1241 (Table T8; Fig. F22). The presence of separate horizons dominated by clear and brown glass suggests that the volcaniclastic material at Site 1241 originated from multiple sources or types of volcanism. Ash layers and patches with clear glass are frequent between sediments dated between 0 and 2.5 Ma (0-55 mcd) and 8 and 9 Ma (300-320 mcd), whereas those with brown glass are increasingly frequent in an interval older than 10 Ma (>320 mcd) (Fig. F23). In addition, pumice and andesite are present above 210 mcd, whereas scoria is present below 240 mcd. The presence of brown glass ash and scoria suggests a more important role for hotspot volcanism during the earlier part of the depositional history of Site 1241, decreasing in significance as the Cocos plate moved the site away from the Galapagos hotspot (see "Introduction"). The early part of the sedimentary sequence is also characterized by elevated levels of siliciclastics, mainly clays, and mafic sand grains that could be the result of physical and chemical weathering of hotspot volcanic rocks.
The presence of pumice and ash with dominant clear glass and minor brown glass suggests increasing influence of a Central American volcanic source as the site drifted to the northwest. The color of volcanic glass described at DSDP Leg 67 sites provides additional support for a Central American source of volcanics to Site 1241 (Cadet et al., 1982). Frequent ash layers between 0 and 2.5 Ma and 8 and 9 Ma (Fig. F23) correspond to high ash accumulation intervals described at ODP sites in the Caribbean Sea (Sigurdsson et al., 2000), supporting interpretations of intense volcanic activity in Central America during these age intervals.
