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LEG 202 SITE SUMMARIES (continued)

Site 1241


Site 1241 (proposed Site COC-2A) is located on a gently sloping sediment-covered ramp on the north flank of Cocos Ridge at 2027 m water depth in the Guatemala Basin. The crust underlying the site formed at the Galapagos hotspot ~11–13 m.y. ago. A tectonic backtrack path on the Cocos plate moves Site 1241 southward and slightly to the west relative to South America (Fig. F6). The site was probably located close to the equator and at shallower depths early in its history.

The pelagic sediment drape is ~400 m thick (Fig. F60). Dominant sediments are nannofossil ooze, lithifying at depth, and occasional ash layers that may record the history of volcanism in Central America, northern South America, and the Galapagos Island chain.

Site 1241 is under the warm, relatively low-salinity waters of the Panama Basin. Nutrients at the sea surface are low, and biological productivity is substantially lower than at the equator.

Site 1241 is likely to record changes in upwelling and biological productivity, from higher values early in its history (when it was close to the equator) to relatively low values at present. Furthermore, the site is expected to provide a record of surface-water salinity reduction that occurred in response to the closure of the Isthmus of Panama during the late Miocene, along with a record of volcanism in Central America based on the accumulation rates of ash. On shorter timescales within the Pleistocene, Site 1241 has the potential to record oscillations of ice age climate in changing sea-surface salinity and pycnocline depth, both of which are associated with heavy rainfall under the Intertropical Convergence Zone.

Deep waters of the Guatemala Basin derive from middepth waters of the North Pacific that enter through fracture zones in the East Pacific Rise. Waters of North Pacific origin are relatively depleted in oxygen and enriched in nutrients. The region is an important site of denitrification. The relatively shallow depth of Site 1241 will facilitate the study of changes in the upper reaches of the lysocline in the Guatemala Basin.

The primary objective at Site 1241 is to provide a continuous sedimentary sequence over the Miocene–Pliocene to assess variability of upper ocean processes, including the development of Atlantic to Pacific salinity associated with the closure of the Isthmus of Panama and other late Neogene climate changes. In its recent history, the site is well positioned to monitor the location of the intertropical convergence in the eastern Pacific. Its water depth is suitable for monitoring Pleistocene changes in carbonate dissolution within the Guatemala Basin. Given the tectonic backtrack to the south and to shallower water depths in the past, it also is likely to be suitable for monitoring intermediate waters during late Miocene time.


At Site 1241, three holes offset by 20 m, were cored with the APC to 314.2, 259.4, and 143.5 mbsf. Holes 1241A and 1241B were advanced with the XCB to 394.4 and 307.6 mbsf, respectively. Several short intervals were drilled without coring in Holes 1241B and 1241C to adjust the coring depth and optimize stratigraphic overlap.

A total of 20 core barrels that could not be pulled with a force of 60 kilopounds were drilled over. The nonmagnetic core barrel was deployed on even- or odd-numbered cores until the first barrel had to be drilled over in each hole. The piston cores in Hole 1241A were oriented starting with the fourth core. Cores from the subsequent holes were not oriented because one Tensor tool failed and the remaining one was spared so that it could be used at the last site.

Seven downhole and one bottom-water temperature measurements were taken with the APCT, and the results indicated a low thermal gradient of ~2.6°C/100 m.

After the completion of coring operations in Hole 1241B, the hole was flushed and displaced with sepiolite mud. The bit was placed at depths between 97.5 and 82 mbsf for downhole logging. The triple combo tool string with the MGT on top was deployed first, followed by the FMS-sonic tool string. One pass with the triple combo was conducted from total depth (395 mbsf) to the mudline, followed by one full pass from 395 mbsf to the bit with the MGT. One subsequent pass with the FMS-sonic tool string also reached the bottom of the hole. The diameter of the hole ranged from 10 –14 log quality.

A supply vessel from Panama (Viveros) arrived at the JOIDES Resolution in the early morning hours of 25 May, just when operations at Site 1241 were completed. Both vessels waited for daylight for the Viveros to come alongside the JOIDES Resolution and transfer 150 boxes of D-tubes to replenish the nearly depleted stock. The Viveros departed at 0730 hr that morning.

Scientific Results

A 446-m-thick (mcd) sediment sequence was recovered at Site 1241 that spans the interval from the late Miocene (~11.2–11.6 Ma.) to the Pleistocene (Fig. F61). Magnetic susceptibility and GRA bulk density data were the primary parameters used to determine the overlaps of cores between three holes and to construct a composite depth. Complete recovery was documented for the upper 303.14 mcd. XCB cores from Hole 1241A and 1241B were appended onto the mcd scale using a 13% growth rate of mcd relative to mbsf, resulting in a composite section of 446.46 mcd. The splice was not continued below 303.14 m due to a lack of representative signals from these cores. However, density and NGR records from borehole and core logging data (XCB-cored interval of Hole 1241A) exhibit strong correlation of meter-scale variability and may help to generate a spliced section for the XCB-cored intervals of Holes 1241A and 1241B.

Lithology as well as fossil assemblages and abundances at Site 1241 reflect the influence of the equatorial high productivity belt, the Miocene to Holocene path of the site away from the equator toward its modern more northeasterly position at ~6°N, and the vicinity of volcanic islands in the early history of the site. The recovered pelagic sediments are dominated by nannofossil ooze with varying amounts of foraminifers, diatoms, clay, micrite, and volcanic glass. Increases in biogenic silica are reflected by decreases in bulk density. Color reflectance data (lightness L*) correlate well with changes in carbonate contents (R2 = 0.8), and increases in magnetic susceptibility data are associated with increases in clay. Almost the entire sequence is characterized by meter-scale cyclic changes in color reflectance (particularly L*) and bulk density that are interpreted to reflect orbital-scale changes in carbonate vs. biogenic opal. One lithologic unit is defined and divided into three subunits on the basis of changes in sediment composition and associated changes in core logging data (color reflectance, NGR, GRA bulk density, magnetic susceptibility).

The Pleistocene interval of Subunit 1A (0 to ~52 mcd) is marked by moderate carbonate concentrations of 55–75 wt%, including high amounts of foraminifers. Low amounts of biogenic opal and a low mean sedimentation rate of 26 m/m.y. suggest a low productive environment outside of the equatorial upwelling zone. Clay content and magnetic susceptibility values are higher in the upper 15 mcd, probably as a result of less carbonate rather than enhanced eolian supply.

The Pliocene interval from ~2 to 4.5 Ma is characterized by an increase in sedimentation rates from ~36 to ~67 m/m.y and an increase in carbonate contents to ~85 wt% associated with a decrease in the abundance of foraminifers. Considering the backtracking of Site 1241 toward the equator, this may reflect the enhanced influence of the equatorial high-productivity belt. Magnetic susceptibility decreases slightly, probably in response to an increasing carbonate/clay ratio.

The upper Miocene to lower Pliocene sequence (~4.5–9 Ma) of Unit 1B is remarkably homogenous and characterized by high carbonate concentrations ranging between ~70 and 90 wt%, low amounts of foraminifers, moderate amounts of biogenic opal, and high amounts of calcareous nannofossils. Overall, siliciclastics are a minor component. The mean sedimentation rate is high (67 m/m.y) in the interval between ~4.5 and 6.5 Ma, suggesting a period of relatively high productivity. Although changes in lithology are small over the entire upper Miocene sequence, sedimentation rates decrease to a lower level (~34 m/m.y) in the early lower upper Miocene interval (6.5 to 9 Ma).

The period of high carbonate accumulation reflects the vicinity of Site 1241 to the equatorial upwelling/high productivity zone during the late Miocene to early Pliocene interval. A similar interval of rapid accumulation (~4.5–6.5 Ma) was found at the more southern Sites 1236 (~21°S) and 1237 (16°S), as well as in the equatorial Pacific at Sites 846–850 (3°S–1°N, 90°–115°W) and is often referred to as the late Miocene to early Pliocene biogenic bloom. Whether this bloom is related to the influence of the Peru-Chile Current and its westward extension into the South Equatorial Current, to the closure history of the Isthmus of Panama, and/or to changes in deep-sea circulation, remains a question for postcruise research.

The early late Miocene interval (~9 to ~11.2–11.6 Ma; Subunit IC) is marked by low carbonate concentrations averaging ~40 wt% and relatively high mean sedimentation rates of ~50 m/m.y. Biogenic opal, organic carbon, and siliciclastics become significant contributors to the sediment composition. This sequence corresponds to the interval of the carbonate crash (~8.6–11.2 Ma), characterized by low carbonate accumulation rates and poor preservation of calcareous microfossils. The carbonate crash has been documented in sediment records from the eastern equatorial Pacific and the Caribbean and was more recently interpreted to reflect periods of enhanced carbonate dissolution in response to changes in global thermohaline circulation associated with an intensified influx of corrosive southern-source intermediate waters. Although Site 1241 was located well above the lysocline during the middle Miocene, the preservation of calcareous nannofossils, planktonic foraminifers, and even benthic foraminifers was strongly affected by carbonate dissolution. The increase in carbonate dissolution at Site 1241 may, however, result from the degradation of organic carbon. The overall increase in biogenic opal (including laminated diatom oozes), organic carbon, and sedimentation rates is indicative of high surface productivity and enhanced organic carbon rain. To what extent the decrease in carbonate concentration is due to carbonate dissolution or dilution caused by enhanced flux of biosiliceous material and siliciclastics remains a question for postcruise studies.

The increase in clay and volcanic glass during the early late Miocene interval points to an enhanced supply of siliciclastics, possibly from a former island of the Cocos Ridge. The early history of Site 1241 is marked by several ash layers enriched in brown glass and mafic minerals. These ashes and the presence of black lapilli-sized scoria may reflect the volcanic activity of the Galapagos hotspot and its vicinity to Site 1241. The deposition of such ash layers decreased during the late Miocene and ceased at ~6 Ma, probably due to the northeastward movement of Site 1241 away from the Galapagos hotspot. The interval of the last 6 m.y. is marked by ash layers enriched in clear glasses, possibly originating from Central America. The increasing number of such ash layers during the last 2.5 m.y may have resulted from enhanced volcanic activity or from the vicinity of Site 1241 to Central America.

Calcareous nannofossils are abundant and generally well to moderately well preserved throughout the sequence. Planktonic foraminifers are abundant to common in the interval from 0 to 217 mcd and generally rare at depths >217 mcd. The percentage of benthic foraminifers relative to total foraminifers is low (~1%) in the upper interval but high (~99%) in the lower interval. Diatoms are rare to few and are poorly preserved in the upper 184 mcd of the sequence. Diatom abundance increases and preservation improves below ~195 mcd.

Of taxonomic interest is the new observation of a transitional calcareous nannofossil form between Discoaster bellus and Discoaster berggrenii at the site. This fills in the missing link between the two species and has implications for biostratigraphy and evolutionary studies. The biostratigraphies of the three planktonic microfossil groups document a continuous sequence of the lower upper Miocene through the upper Pleistocene. The good preservation of all groups here suggests that this site will form an important reference section of the eastern tropical Pacific. Calcareous nannofossils and planktonic foraminifers constrain the basal age of the site at ~11.2–11.6 Ma. The uppermost part of the sequence might be affected by a hiatus spanning the interval from 0 to 0.26–0.46 ma as indicated by the absence of the calcareous nannofossils E. huxleyi and Pseudoemiliania lacunosa. The last occurrence datum of the planktonic foraminifer G. ruber (pink) between 2.35 and 3.86 mcd, however, indicates an age younger than 0.12 Ma for the overlying interval, suggesting a continuous sedimentation at least for the late Pleistocene and possibly to the Holocene.

NRM intensities both before and after demagnetization were relatively strong at the top of Site 1241, preserving a stable remanence with demagnetized inclination values close to those expected. Intensities drop dramatically over the upper 15 mcd and parallel a decrease in magnetic susceptibility. Little magnetic remanence is left below this level, with no interpretable paleomagnetic signal. Magnetic polarity, therefore, provides a magnetic stratigraphy for the upper 15 mcd. Correlation with the geomagnetic polarity timescale indicates that the Brunhes/Matuyama boundary and the upper and lower Jaramillo boundaries are clearly recognizable from both inclination and declination records.

Chemical gradients in interstitial water at Site 1241 reflect the limited influence of organic matter oxidation, the dissolution of biogenic silica driven by the relatively low thermal gradient, and biogenic calcite recrystallization. Many of the profiles are consistent with a flow of relatively unaltered seawater in the underlying oceanic crust, but the lack of major change in composition makes this more difficult to assess than at Site 1240. Sulfate reduction lowers sulfate concentrations by no more than 4–5 mM relative to seawater and is associated with relatively small increases in alkalinity, phosphate, and ammonium. Methanogenesis at this site is limited by the persistence of dissolved sulfate and the low organic carbon contents ranging between 0.1 and 1.4 wt%.

Downhole logging provided excellent NGR, bulk density, porosity, and FMS data. The most striking feature of the logs is the high-frequency variability in density and resistivity throughout the sequence from 82 to 395 mbsf. Meter-scale rhythmic changes in density and resistivity are encountered in the upper Miocene to Pliocene interval and are interpreted to reflect orbital-scale changes in carbonate vs. biogenic opal in the sediment. Color banding on the FMS images occurs on the same scale and clearly documents the potential for developing an orbitally tuned timescale for this site.

Site 1241 met all of its major objectives. We recovered a complete 303-m-long composite section through the APC interval and a double-XCB cored interval that may be correlated and spliced later to a set of excellent logging data. These cores will provide unique opportunities to study changes in biogeochemical cycles and equatorial oceanography associated with the Miocene to Pliocene closure history of the Isthmus of Panama and the Miocene carbonate crash. Moderately high sedimentation rates of 30–50 m/m.y., combined with observations of rhythmic shifts in sediment composition that probably represent orbital-scale variability, offer the opportunity to integrate the excellent framework of biostratigraphy into an orbitally tuned isotope stratigraphy. Volcanic ashes present an opportunity for tephrochronology and the study of linkages between major ash layers from the Caribbean and Pacific to establish the history of major volcanic events in Central America.

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