165 Preliminary Report

Site 998 Results

Site 998 is located on the Cayman Rise, between the Yucatan Basin to the north and the Cayman Ridge and Cayman Trough to the south. This site was targeted for recovery of a continuous Cenozoic section to record the evolution of Caribbean ocean circulation, a Cretaceous/Tertiary boundary section relatively proximal to the Chicxulub impact site, and basement. Owing to the thickness of the sedimentary section and the very slow rate of penetration, the two deeper objectives could not be met within the time constraints of the leg.

A largely complete sedimentary section spanning the lower Eocene (nannofossil Zone CP10) to Pleistocene was cored in two holes at Site 998. Hole 998A was APC cored to a depth of 160.8 mbsf (upper Miocene) with 103.1% recovery, and then XCB cored to a depth of 637.6 mbsf with 57.7% recovery. Hole 998A terminated in upper Eocene nannofossil chalk. A free-fall funnel was deployed but was too deeply buried in soft sediment to successfully reenter with the RCB, so a second hole was drilled. Hole 998B was drilled ahead to 567.9 mbsf before resuming coring operations to a total depth of 904.8 mbsf. Recovery with the RCB was very good, with 83.1% of the drilled interval recovered. Hole 998B terminated in lower Eocene calcareous volcaniclastic mixed sedimentary rock.

Four lithologic units were recognized at Site 998 (Fig. 2). Unit I (0.0-94.3 mbsf; Pleistocene-lower Pliocene) consists of nannofossil ooze with foraminifers and clays interbedded with graded foraminiferal ooze and ash layers. Unit II (94.3-161.0 mbsf; basal Pliocene-uppermost middle Miocene) is subdivided into three subunits, all of which have interbedded turbidites and ash layers. The dominant lithologies in Unit II are clayey nannofossil mixed sediment, nannofossil ooze with foraminifers and clays, and clay with nannofossils. Unit III (161.0-766.0 mbsf; middle Miocene-middle Eocene) consists of nannofossil chalks that grade with depth into limestone with clay. The dominant lithologies of Unit IV (766.0-904.8 mbsf; middle Eocene-lower Eocene) are limestone with clay and calcareous volcaniclastic mixed sedimentary rock, with interbedded altered volcanic ash and volcaniclastic turbidites.

A seismic reflection at 5.15 s two-way traveltime is interpreted as acoustic basement, which we suggest is the top of a volcanic arc of unknown age. Based on logging and physical property velocities obtained at the site, the depth to basement is estimated to be 1100-1130 mbsf at the location of Site 998 (Fig. 2). Thus, about 210 m of the sedimentary section above basement was not cored.

Shipboard biostratigraphy and magnetostratigraphy suggest that the cored section is virtually complete. Sedimentation rates averaged 15-24 m/m.y. in the lower Eocene to middle Miocene interval, 8 m/m.y. in the upper Miocene, and nearly 19 m/m.y. for the Pliocene-Pleistocene; the highest rates are recorded for the Oligocene interval of nannofossil chalk where there are fewer turbidites and discrete ash layers. A shipboard magnetostratigraphy was established for the uppermost 200 m of Hole 998A (Pleistocene-middle Miocene) and all of Hole 998B (basal Oligocene-lower Eocene). A strong drill string overprint and the nature of core quality in the middle Miocene-Oligocene XCB cores of Hole 998A contributed to an absence of interpretable paleomagnetic data through the middle part of the cored section at Site 998.

The sedimentary section at Site 998 yielded several unexpected results, including numerous pelagic turbidites (Fig. 3) and thick ash fall deposits in the Miocene, and thinner but much more frequent volcaniclastic turbidites in the lower to middle Eocene (Fig. 4). The bases of these older turbidites consist of foraminifers in a matrix of redeposited volcanic ash. The Miocene ashes are rhyolitic fallout layers, which were probably derived from distant silicic volcanoes in Central America, while the Eocene volcaniclastic turbidites and associated ash falls were derived from a proximal island arc source, probably the nearby Cayman Ridge. More than 500 ash layers were described at Site 998. Turbidite frequency at this site may have been linked closely with spreading activity in the Cayman Trough and strike-slip motion along the fault zone that defines the northern boundary of the Caribbean Plate.

The concentrations of total organic carbon are extremely low in sediments recovered from Holes 998A and 998B, and most measurements were below analytical resolution. Similarly, methane was found to occur at only trace levels, and additional hydrocarbon gases were not observed. The chemical composition of pore waters in the sediments displays a large range that varies systematically with depth and lithology. A downcore decrease in sulfate in the uppermost 50 m of the sediment and increase in Fe are associated with redox conditions typical for deep-marine suboxic diagenesis.

Marked increases in pore-water Ca and Sr with depth most likely are related to dissolution of carbonate and weathering reactions of the volcanic ash and basement. The decrease in Mg with depth likely reflects the alteration of volcanic ashes to tri-octahedral smectite, which creates a sink for Mg, as well as dolomite precipitation. A number of pore water constituents, notably silica, Rb, K, and alkalinity show downcore variations that strongly parallel the abundance trends for the Miocene silicic volcanic ash layers in the sediments, and the peak abundance in these dissolved components is thought to reflect their release during the initial stages of alteration of volcanic glass in the ash layers. The solid phase geochemistry of the sediments reflects the deposition of three principal components at Site 998: biogenic calcium carbonate, terrigenous detrital material, and dispersed volcanic ash. Using refractory elemental ratios such as Ti/Zr as discriminants of terrigenous and volcanic ash sources, we find that terrigenous material represents about 9% to 10% of total sediment, with a marked peak abundance in the top 120 m of the core, where it approaches concentrations of about 15% to 20% (Fig. 5). The total dispersed ash abundance is about 7% in the cored sediment, but shows a peak up to 30% at the 150-m level, where ash fall layers are also most common.

An important discovery at Site 998 is a marked reduction in pelagic carbonate deposition that occurred during the time of the middle/late Miocene boundary interval about 10-12 Ma. This oceanographic event is well developed in upper Miocene sequences of the central and eastern equatorial Pacific but has not been previously documented in the tropical Atlantic or Caribbean Sea (Fig. 6). Tectonic activity associated with the closing of the Central American Seaway during the late Neogene is suspected to have been an important variable in the evolution of Northern Hemisphere climate and ocean circulation. The signature of strong carbonate dissolution on both sides of the present-day Isthmus of Panama has important tectonic and paleoceanographic implications for the cause and extent of the late Miocene "carbonate crash."

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