4.5 Å is greater than that in any other tephra and appears to represent smectite.
Coring at ODP Site 1253 during Leg 205 was directed at completing recovery of the sediment section overlying the igneous oceanic crust initially cored during ODP Leg 170 at Site 1039. Consequently, we choose to assign the sediments and sedimentary rocks recovered at Site 1253 to the existing stratigraphic framework. All the sediment recovered above the gabbro sill at 400 mbsf was assigned to Subunit U3C (Fig. F7). Below this level there were a number of thin (<6 cm) baked crystalline sediment layers between igneous rock in the lower igneous unit (Subunit 4B), as well as between this complex and the upper gabbro sill (Sections 205-1253A-10R-2 to 12R-1, 26 cm; 431-442 mbsf) (see "Petrology"). In zones of low core recovery, the thickness of sediment within Subunit 4B is not clear. Zones of faster than normal drilling penetration rate within the lower unit may represent additional sedimentary intervals, but could also be simply more fractured igneous rock. Baked sedimentary rocks were recovered from within Subunit 4B in intervals 205-1253A-25R-1, 0-1 cm (513 mbsf), 26R-1, 17-18 cm (516 mbsf), and 27R-1, 1-6 cm (519 mbsf). These intervals form part of Unit U4, although the lithologies are the same as those seen in Subunit U3C.
Sediments cored in Hole 1253A are dominated by a mixture of nannofossil chalks and varying but subordinate proportions of clay. The facies are typically fine grained and massive with few identifiable sedimentary structures apart from bioturbation (Fig. F8). The light color reflects the dominant biogenic carbonate origin of the sediments. Prominent dark green clay layers constitute <5% of the section and are massive, commonly burrowed, and usually <1 cm thick. They are interpreted as primary sedimentary deposits. Going downsection from Cores 205-1253A-1R to 4R, there is an increase in the proportion of clay in the sediment and in the degree of lithification, changing from a firm chalk in Core 1R (Fig. F8) to a hard mudstone in Core 4R (Fig. F9). At the same time, the facies become more laminated downsection, suggesting less bioturbation, although this is occasionally observed over narrow intervals between laminated intervals. Although these changes occur as the contact with the gabbro sill is approached, there is no indication that the presence of the sill has caused these changes in facies. Rather, it is likely that the sill exploited a change in sediment composition as a place for intrusion.
Below the gabbro sill (Subunit 4A), in Section 205-1253A-10R-2 (430.72 mbsf), the sediment returns to softer carbonate lithologies. However, unlike the nannofossil chalks seen higher in the section within Subunit U3C, this core is dominated by a clastic granular limestone, defined as packstone with clay. The sediment shows an original sedimentary fabric that is interpreted to reflect lamination during deposition under the influence of moderate current activity. A minor amount of micritic nannofossil chalk is interbedded and represents sedimentation during lower-energy episodes. Downsection in Core 205-1253A-11R (436 mbsf), chalk dominates and appears to be more similar to those lithologies recovered from the top of Hole 1253A (<396 mbsf). There is no convincing evidence to indicate that the sediment between Subunits 4A and 4B differs in a significant fashion from that above the sill.
Although the facies are typically pelagic, there are a number of minor redeposited intervals that are still carbonate dominated but contain elevated clay concentrations. An example is seen in interval 205-1253A-1R-1, 78-104 cm (Fig. F10), where a brownish gray bed shows parallel laminations and a graded base, suggestive of redeposition. However, the graded layer accounts for only 2-3 cm of the total bed thickness of 26 cm. Additional clastic sections are seen within the bed, indicating that the flow was not a single pulse but instead involved a series of closely spaced events or, perhaps, pulses within a single mass flow event.
Volcanic ash layers represent an important minor lithology but account for <1% of the total stratigraphic thickness. They are typically thin (<5 cm) and structureless, with grading only rarely apparent. Mafic layers account for >70% of the layers identified, but there is no variation with depth in the bulk composition. Lighter-colored siliceous ashes are seen as far downsection as Core 205-1253A-4R. Alteration is variable, with some shards found in some intervals appearing to be clear and very fresh under microscopic examination (Fig. F11C), whereas others are almost entirely converted to zeolites. Individual tephra shards are commonly found in minor quantities throughout the section (Fig. F12), with the exception of Subunit U3B, suggesting that originally there may have been many more thin tephra layers that have been disrupted by bioturbation after deposition. Indeed, smear slide analysis shows that volcanic glass shards are scattered through the background pelagic sediments.
Whereas most volcanic ashes alter to a dark gray color, the most lithified Core 205-1253A-4R contains a bright white altered ash layer (Fig. F13) that contains fine sand- and silt-sized particles of cloudy, partially palagonitized glass. Identified tephras appear to be totally dominated by volcanic glass and degraded glass shards, with no feldspar, hornblende, or pyroxene grains identified. The evidence strongly points to these layers being the product of primary air fall events followed by settling through water. There is no support for redeposition as volcaniclastic turbidites or as hyaloclastites.
Minor breccia layers, up to 3 cm thick, are observed, especially in the more carbonate-rich intervals in Cores 205-1253A-1R and 2R, although these always represent <1% of the total stratigraphy. We interpret these as being related to neptunian diking, because they crosscut horizontal bedding markers in the core and do not show a simple flat-bedded geometry. The clasts comprise pieces of broken nannofossil chalk, indicating intraformational reworking. These breccia could have formed far from the trench as a result of intraplate stress, although the stresses acting on the sequences would have increased as the site approached the deformation front. We suggest that these breccia are probably linked to deformation in front of the forearc wedge, although the exact timing of their emplacement cannot be definitively constrained from this work.
Trace fossils are developed through much of the cored interval, testifying to well-oxygenated bottom water conditions. Trace fossils of the Zoophycos ichnofacies were identified, including Zoophycos, Planolites, and Chondrites, consistent with deposition in lower bathyal to abyssal water depths (>2000 m) (Ekdale et al., 1984). The degree of bioturbation is strongest in the lighter carbonate-rich intervals and becomes less common in the clay-rich laminated sedimentary rocks of Core 205-1253A-4R (395-400 mbsf). Trace fossils are especially apparent where they disrupt the occasional dark green clay-rich layers (Fig. F14).
Diagenesis has resulted in a moderate lithification to a chalky strength, except in Core 205-1253A-4R, lying immediately above a gabbro sill where the sediment is much more lithified. This section is also much less carbonate rich than other cores examined, with clays and zeolites forming increasing large volumes of the sediment in the 3 m above the upper gabbro sill. Thin chert layers are also seen in interval 205-1253A-4R-1, 53-61 cm, indicating mobilization and precipitation of silica in the sediment, likely derived from the abundant sponge spicules. As noted at Site 1039, the carbonate sediments are occasionally stained by a fine dark purple lamination that may cut across bedding fabrics and is often developed as a halo around larger burrows. Termed Liesegang rings, these features reflect mobilization of metals in the sediments during early diagenesis. It should, however, be emphasized that the overall composition of the sediment stained in this fashion does not appear to be significantly changed, and they cannot be considered as metalliferous sediments in the normal usage of the term.
Smear slide analysis allows the variation in major sedimentary components to be analyzed and their variation with depth to be assessed (see "Site 1253 Smear Slides") (Fig. F12). Apart from nannofossils and clay minerals, other grain types that contribute significantly to the sediments include siliceous sponge spicules, foraminifer test fragments, diatoms, opaque minerals, volcanic glass shards, and zeolites derived from the degradation of volcanic glass shards (Fig. F11). Minor, albeit common, components include radiolarian tests and fragments of tests, grains of crystalline calcite, and occasional silicoflagellates. There is no evidence of terrestrial grain types, such as quartz, feldspars, or micas.
Plotting the new Site 1253 smear slide data with that collected at Site 1039 during Leg 170 allows total sedimentary variations with depth to be assessed. The microscopic analysis shows that grain sizes are fine throughout the core, with minor amounts of sandy material preferentially preserved in uppermost lithostratigraphic Units U1 and U2 (Fig. F12). This is consistent with a model in which the oceanic crust into which Site 1253 penetrates originated in a pelagic environment at a ridge crest and then approached the trench through time. Only when the site came close to the active continental margin was it in a position to receive coarser clastic material. Clay minerals are rare in the carbonate-rich Subunits U3B and U3C and are especially common in Unit 2 and Subunit U3A. Of the biogenic components, diatoms are seen in significant volumes throughout the section, but are most numerous in Unit U1, possibly reflecting the arrival of the drill site within the high-productivity upwelling zone along the Costa Rica margin. Abundance of diatoms is typically considered to indicate high biogenic productivity in nutrient-rich conditions. Nannofossils conversely show the opposite trend to decreased relative concentrations in Units U2 and U1, while being very abundant lower in the core. The trend in nannofossil abundance may partly reflect dilution by diatoms in these upper units and also corrosion and dissolution by colder, nutrient-rich waters in the coastal upwelling zone.
X-ray diffraction (XRD) analyses were performed routinely. The results of XRD analyses of randomly oriented bulk sediment powders are summarized in Tables T2 and T3. Site 1253 peak intensity and peak area data are listed in Table T2, and peak ratio data are listed in Table T3.
The sample suite considered here includes dominant lithology carbonate sediments, minor lithology clay-rich sediments, and several tephras. A representative carbonate Sample is 205-1253A-4R-1, 14-15 cm, for which the calcite to quartz peak area ratio is 265, the clay to quartz peak area ratio is 10, and the plagioclase to quartz ratio could not be determined because of the absence of the plagioclase peak. Cristobalite/quartz peak area ratio data are typically used to monitor opal diagenesis. The cool temperatures (<10°C) encountered at this site render such considerations moot.
As expected, investigation of the peak area data shows that calcite peaks dominate the diffractogram data for samples of the major lithology (Subunit U3C). Three samples of carbonate sediments at baked contacts near and within gabbro sills (intervals 205-1253A-5R-1, 0-1 cm; 25R-1, 10-12 cm; and 27R-1, 4 cm) and seven tephras (intervals 205-1253A-2R-1, 49-50 cm; 2R-3, 101-102 cm; 4R-1, 86-87 cm; 4R-3, 87-88 cm; 4R-3, 90-92 cm; 10R-2, 85-87 cm; and 11R-2, 42-44 cm) were also analyzed to constrain finer-scale mineralogic variation at this site.
The baked sediments in contact with the gabbro intrusions are not identical. At interval 205-1253A-5R-1, 0-1 cm, at the top of the gabbro sill, bulk XRD data show a clay-rich mineralogy with abundant amorphous glass and suggest that illite is present. Sample 205-1253A-25R-1, 10-12 cm, is nearly pure calcite. At Section 205-1253A-27R-1, 4 cm, XRD data show a clay-rich mineralogy with abundant amorphous glass, and smectite and mixed layer smectite-illite clays dominate the clay component.
Tephra mineralogy also varies with stratigraphic depth as a function of initial composition, alteration, and extent of mixing with background sediment. Samples 205-1253A-2R-1, 49-50 cm, and 2R-3, 101-102 cm, show very little clay, abundant calcite, and minor plagioclase and quartz presence. Samples 205-1253A-10R-2, 85-87 cm, and 11R-2, 42-44 cm, are similar to those tephras just described, but also contain a smectite clay, likely saponite. Smectite may be present as a volcanic glass alteration product. Samples 205-1253A-4R-1, 86-87 cm, and 4R-3, 90-92 cm, have weak calcite peaks; these tephras consist mainly of plagioclase, quartz, and detrital hornblende. Compared to the dark, mafic tephras that dominate the section, the tephra at 205-1253A-4R-3, 87-88 cm, is in its own category, being a white siliceous ash. Mineralogically, the composite clay peak area at d 4.5 Å is greater than that in any other tephra and appears to represent smectite.
Major and trace element analyses of 20 sediments, volcanic ashes, and baked sediments found intercalated between the divisions of the igneous subunits of Hole 1253A were conducted shipboard following the methods described in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. Major, minor, and trace element analyses are presented in Table T4. The tephras and baked sediments are analyzed by the ICP-AES separately from the sediments. Cerium, copper, and nickel values are near detection limits and are not reliable in those sediments overlying Subunit 4B that were analyzed. Copper, however, was above the detection limit in the tephras and baked sediments analyzed. Analyses were made on freeze-dried bulk samples that are a mixture of sediment particles and sea salts (see "Inorganic Geochemistry" in the "Explanatory Notes" chapter). The data have not yet been corrected for porosity and pore water contribution to the bulk sediment composition.
The ICP-AES geochemical data at this reference site can be useful for (1) tracking the lithologic and sedimentologic variations within the sedimentary section, (2) determining the average composition of the sediment column for key elements entering the subduction zone for further recycling studies, and (3) helping to constrain estimates of tectonic underplating under the forearc.
Figure F15 shows variations in TiO2, Al2O3, SiO2, and Ti/Al with depth below seafloor. Titanium and aluminum show similar trends in the sediments overlying igneous Subunit 4B at Site 1253, and concentrations remain fairly constant with depth. The baked sediments within Subunit 4B have similar aluminum and titanium concentrations to the sediments above it. Ash layers are common at Site 1253, and volcanic detritus is nearly ubiquitous at low concentrations in much of the sediment recovered. The tephras analyzed from Site 1253 have consistently higher aluminum and titanium concentrations than the surrounding pelagic sediments. Consequently, the proportion of volcanic material in the sediments will tend to control the bulk sediment Ti/Al ratios. The small range in Ti/Al ratios (0.1-0.01) in the sediments overlying igneous Subunit 4B suggests that the percentage of volcanic ash in the sediments analyzed is fairly constant.
Silica concentrations within the section at Site 1253 reflect the lithologic change from a carbonate-rich nannofossil chalk down into a calcareous claystone immediately above the gabbro sill. Silica concentrations increase with depth as a result of the formation of clay minerals and zeolites associated with sill emplacement and subsequent sediment diagenesis. Furthermore, XRD data indicate diatom and radiolarian recrystallization to quartz in the sediments above the sill. The authigenic formation of clay minerals and zeolites is also responsible for the uptake of dissolved potassium in the sediments immediately above the sill (Fig. F72) (see "Inorganic Geochemistry"). Below the sill, the sediments are composed mainly of nannofossil chalk, resulting in the lower silica concentrations observed (Fig. F15).
In Figure F16, CaO, MgO, strontium, and barium are plotted against depth for the sediments, volcanic ash, and baked sediments within Subunit 4B at Site 1253. Decreases in calcium and strontium with depth in the sediments above the sill reflect the reduction in carbonate content. In general, the calcium and strontium concentrations in the tephras are less than those of the background sediments overlying Subunit 4B. The baked sediments have concentrations similar to the sediments above the sill. Calcium and strontium concentrations increase between the sill and Subunit 4B because of the dominant nannofossil chalk lithology. Magnesium concentrations, like silica, increase with depth and reflect decreasing carbonate content immediately above the sill. Formation of in situ magnesium-bearing clays is also responsible for the increase in magnesium concentrations and the decrease in dissolved magnesium immediately adjacent to the sill. The incompatible element barium is highly enriched in these sediments (Fig. F16D), but concentrations decrease with depth to ~200 µg/g above the sill. Interestingly, dissolved barium concentrations are highest in this same region of only moderate barium concentration in the sediment. The maximum value observed in the basal sediment section is 3136 µg/g, whereas the maximum value observed in the volcanic ash samples is 1254 µg/g. This pattern suggests that the barium enrichment in the sediment is not entirely characteristic of the volcanic detritus and is largely the result of barium accumulation in barite.
Figure F17 shows variations in Na2O and K2O with depth. Sodium concentrations in the sediments above the sill are fairly constant with depth and range between 1.06 and 1.97 wt%. The baked sediments analyzed have similar concentrations to the sediments recovered above the sill; however, there is a marked increase in sodium concentrations with depth in the tephras above the sill, which corresponds to an increase in dissolved sodium concentrations within the same interval (see "Pore Water Results" in "Inorganic Geochemistry"). Potassium concentrations decrease with depth from 0.511 to 0.133 wt% between the sill and Subunit 4B (Fig. F17B). The baked sediments and the sediments above the sill have similar potassium concentrations, likely reflecting mobilization of this element during diagenesis, synchronous with and after sill emplacement.
Both MnO and Fe2O3 concentrations remain fairly constant with depth (Table T4) and have maximum values of 0.38 and 3.39 wt%, respectively. Iron concentrations in the tephras are typically higher and range between 1.44 and 9.5 wt%. P2O5, vanadium, yttrium, zirconium, and chromium all remain fairly constant with depth within the sediments above the sill and in the baked sediments.
The seven tephra layers chosen for chemical analyses were selected to span a range of colors, from white to dark gray, presumed to reflect significant differences in the major element chemistry. Low analytical totals of 84-97 wt% indicate a significant degree of alteration, consistent with the observation of advanced diagenesis in the drilled section, especially immediately above the gabbro sill. Microscopic analysis shows that although clear fresh-looking glass shards can be found in the ash layers, these are largely outnumbered by glass in moderate to advanced stages of palagonitization (Fig. F18). The range of silica contents (42-65 wt%) shows a wide spread, with a dominance of basaltic tephra, that typically appears dark in the cut core surface. The bright white tephra observed in interval 205-1253A-4R-3, 85-93 cm, is also chemically distinctive in being the second most siliceous layer (62 wt% SiO2) and having a much lower Fe2O3/MgO ratio than the other ashes. Because of the advanced state of alteration, little useful information concerning petrogenesis can be derived from study of the major elements. The tephra in interval 205-1253A-4R-2, 100-122 cm, differs from the nearby tephras in being thick (22 cm), with an anomalous major element chemistry (e.g., low TiO2, Al2O3, and Fe2O3).
This interval is a light gray laminated clay-rich bed that is quite distinct from the laminated dark claystone facies that dominate the sequence from 395 to 399 mbsf. We suggest that this clay unit represents material that was originally mostly volcanic ash. However, the great thickness, darker color, and laminations indicate that this unit has been reworked and is not a primary volcanic sediment. Nonetheless, this deposit further contributes to the idea of explosive Galapagos volcanism, placing significant quantities of volcanic ash into the Pacific Ocean at this time.
The middle Miocene was singled out as a period of strong explosive volcanic activity by Kennett et al. (1977) and Kennett and Thunell (1977) for the Circum-Pacific region. Hein et al. (1978) demonstrated that the middle-late Miocene was a period of powerful volcanism in the Aleutians. In the Americas, Perkins et al. (1998) have shown that ~17 Ma was a time of extensive ignimbrite eruption in the western United States, whereas closest to Site 1253, Sigurdsson et al. (2000) documented a maximum in explosive tephra sedimentation in the Caribbean at 15-20 Ma presumed to be derived from Central America. The tephras recovered in the Miocene section at Site 1253 now show that the Galapagos hotspot was also very active at this time.
It is noteworthy that the white tephra was not recovered at Site 1039; because the relevant Core 170-1039C-4R reached only 18% recovery, it is likely that the tephra was simply not recovered at that locality. In addition, the white and mafic tephras are deposited at Site 1253 within a laminated sequence. The very fact that the tephras lie in such a sequence of laminated low-CaCO3 sediment is itself of note when compared to the bioturbated nannofossil chalks above and below. We speculate that this period of low pelagic biogenic production and cessation of seafloor burrowing may be the product of the regional volcanism in the Galapagos and in Central America, causing widespread collapse of the ecology through the eastern central Pacific Ocean, much as has been recorded from the aftermath of the Toba eruption (Rampino and Ambrose, 2000).
We conclude that the sedimentary section recovered at Site 1253 is dominated by nannofossil chalks with minor amounts of clays and tephras. These were deposited at lower bathyal to abyssal water depths within a pelagic environment, remote from any continental mass. Deposition occurred above the calcite compensation depth in a well-oxygenated environment, interrupted by a relatively brief phase of more reducing bottom water conditions, allowing laminated clay-rich mudstone to form. In the middle Miocene (~17 Ma) the region received tephra deposits from the Galapagos hotspot during part of a major regional phase of tectonic and magmatic activity. Volcanism is also associated with synchronous seafloor anoxia. Shortly afterward, the section was intruded by a gabbro sill and Subunit 4B that caused advanced diagenesis and lithification, extending ~10 m above the top of the sill. Subsequent diagenesis has resulted in the hydration of volcanic glass shards distributed through the section to form zeolites and palagonite.
