165 Preliminary Report


Site 1000 Results

Site 1000 is located in Pedro Channel, one of a series of channels that dissects the carbonate shelves and isolated carbonate banks that define the ENE trending northern Nicaragua Rise (NNR). One of our principal objectives at Site 1000 was to reach the top of a carbonate platform and document the onset of the Caribbean Current. It has been proposed that the NNR was a large carbonate platform during much of the Paleogene that ultimately became segmented due to tectonic activity along the northern Caribbean Plate boundary. Pedro Channel represents one of the largest segments of the once contiguous megabank, originally thought to have foundered during middle Miocene time (Droxler et al., 1992). The carbonate platform proved to be deeper and older than predicted. The thick and continuous 20-m.y. record of pelagic sedimentation cored at Site 1000, however, provides many new insights into the history of Caribbean sub-thermocline (intermediate) water masses, the segmentation and subsidence history of the NNR, the seismic stratigraphy of the region, and a record of explosive volcanism during the Neogene.

A continuous, fairly homogeneous, and apparently complete lower Miocene-Holocene section was cored at Site 1000. Hole 1000A was APC cored to a depth of 312.9 mbsf (middle Miocene) with 103.5% recovery, and then XCB cored to a depth of 553.2 mbsf with 89.2% recovery. Hole 1000A was terminated in middle Miocene calcareous limestone with nannofossils, after coring mainly periplatform oozes and chalks. At Hole 1000B two cores were recovered in oozes above 117 mbsf before the hole was drilled ahead to a depth of 503.5 m with the RCB system. Hole 1000B was terminated in lower Miocene limestone at a depth of 695.9 mbsf with recovery averaging 67.6% over the cored interval.

The 696-m-thick sedimentary sequence recovered in two holes at Site 1000 consists dominantly of periplatform sediments and sedimentary rocks, interbedded with volcanic ash layers and intervals of redeposited periplatform/pelagic and neritic carbonate sediments from the slopes and top of adjacent shallow carbonate banks. Two main lithologic units were recognized based mainly on degree of lithification (Fig. 11); Unit I is a mixture of biogenic and calcareous ooze and chalk, whereas Unit II comprises mixtures of biogenic and calcareous limestone. Unit I is subdivided into four subunits: Subunit IA (0.0-50.8 mbsf; Holocene to upper Pliocene) is characterized by frequent downcore variations in carbonate content, and by a turbidite-free sedimentary sequence with only a few rare and thin volcanic ash layers. Subunit IB (50.8-370.5 mbsf; upper Pliocene - upper Miocene) is a thick interval of relatively uniform micritic biogenic ooze, with volcanic ash layers common throughout, and a few turbidites occurring in the lower half of the subunit. Subunit IC (370.5-486.0 mbsf; upper Miocene - middle Miocene) is defined by an interval of lower carbonate content, where detrital clays and quartz reach a maximum for the site. Subunit ID (486.0-513.4 mbsf; middle Miocene) is similar in lithologic characteristics to Subunit IB, though it is turbidite-free. Unit II is subdivided into two subunits: Subunit IIA (513.4-591.3 mbsf; middle Miocene - lower Miocene) is characterized by relatively high carbonate content. Subunit IIB (591.3-695.9 mbsf; lower Miocene) is characterized by fluctuating carbonate content values and the highest abundance of volcanic ash layers and turbidites of any subunit.

Sedimentation and mass accumulation rates (MARs) decline from values of 37.2 m/m.y. and 3.55.0 g/cm2/k.y. in the upper Miocene to upper Pliocene interval (2.89.6 Ma) to an average of 27.3 m/m.y. and 2.43.4 g/cm2/k.y. in the upper Pliocene-Pleistocene interval (0.02.8 Ma). A similar pattern was observed at Sites 998 and 999. One notable feature of the Pliocene is the abrupt increase in carbonate MARs at about 4.14.2 Ma, which could be the downstream expression of closing of the Central American Seaway.

Lithologic Subunits IC and ID display the highest sedimentation and accumulation rates of the section, averaging 47.0 m/m.y. and 4.5-7.5 g/cm2/k.y., repectively. The most distinctive pattern in this part of the section is the peak in non-carbonate MARs associated with the "carbonate crash," a trend also observed at Sites 998 and 999 (Fig. 9 and Fig. 10). At Site 1000, this interval is characterized by higher magnetic susceptibility, distinctly lower but highly variable carbonate contents, increased non-carbonate MARs, enrichments of up to 0.8% TOC (in the interval ~360510 mbsf), and relatively high concentrations of uranium.

Sulfate reduction is more pronounced at Site 1000 compared to Sites 998 and 999, with sulfate concentrations approaching zero by 350-400 mbsf (top of the carbonate minimum and hydrocarbon enrichment zones). Paleomagnetic intensities indicate, however, that all but the upper 22.5 m of the section have been severely affected by reduction diagenesis. There are also enrichments of volatile hydrocarbons in the interval from ~400 to 520 mbsf (Fig. 12). Migration of hydrocarbons (updip and/or upsection) into the interval of the carbonate minimum ("carbonate crash") is suspected due to the shallow burial and the absence of an anomalously high thermal gradient. No in situ generation of thermogenic gases within the "hydrocarbon zone" is supported, although bacterial methanogenesis may be a factor for the occurrence of C1 in this zone given the absence of sulfate at this depth. Increased alkalinity associated with sulfate reduction in the carbonate minimum and "hydrocarbon zone" may be responsible for the precipitation of carbonate cements along a lithification front represented by the abrupt change from chalk to limestone at about 510 mbsf (lithologic Unit I/II boundary; Fig. 12).

Many of the turbidites occurring in Subunit IIB contain redeposited pelagic sediments, but some also contain material derived from the upper reaches or tops of the adjacent carbonate banks. The lower Miocene-lower middle Miocene interval represented by the limestones and interbedded ash layers and turbidites of lithologic Unit II (13.5-20.0 Ma) had an average sedimentation rate of 27.3 m/m.y. with widely varying bulk mass accumulation rates of 3.5-6.3 g/cm2/k.y. The accumulation rate and number of volcanic ash layers during early to middle Miocene time at Site 1000 exhibit a pattern that is remarkably close to that of Site 999, although the magnitudes of these parameters are somewhat higher at the latter site (Fig. 4). The combined evidence from Sites 998, 999, and 1000 shows that the Miocene explosive volcanic episode generated a principal fallout axis that trends east from the Central American arc, between Sites 999 and 1000, or very close to the latitude of Site 1000. This is consistent with derivation of the tephra from the ignimbrite-forming volcanism in the Tertiary Igneous Province of Central America. The combined thickness of ash layers observed in the Caribbean sites indicates that this represents one of the major explosive volcanic episodes known. The magnitude of individual eruptive events is of the order 102 to 103 km3, while the fallout of the episode as a whole is certainly in excess of 105 and probably greater than 106 km3.


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