| Co-Chiefs: Mark Leckie and Haraldur Sigurdsson | Staff Scientist: Gary Acton |
| Cruise Dates: 24 December 1995-18 February 1996 | Operations Superintendent: Mike Storms |
Leg 165 investigated a wide array of scientific problems through drilling at five sites that spanned the Caribbean Ocean from the Cayman Rise (Site 998), which lies to the southeast of the Yucatan Peninsula, to the Cariaco Basin (Site 1002), just north of Venezuela. Drilling was highlighted by the recovery of K/T boundary clays and ejecta deposits at Sites 999 and 1001; recovery of igneous basement from the Caribbean Oceanic Plateau at Site 1001; and recovery of a 200,000-year-long, ultra-high-resolution sedimentary record of tropical climate change at Site 1002 in the Cariaco Basin.
Beyond any of the pre-cruise goals or expectations, a spectacular record of Eocene and Miocene explosive volcanism, unmatched in its magnitude and chronostratigraphic resolution, was discovered at Sites 998, 999, 1000, and 1001. This includes the first documentation of arc volcanism along the Cayman Ridge. The temporal distribution of the megascopic ash layers recovered, over 2000 layers in all, include particularly vigorous volcanic episodes in the mid- to late Eocene and in the early to mid-Miocene, with eruption frequencies on the order of 40 events/m.y. Also recorded above the excellently preserved basement/sediment contact in the two holes at Site 1001 is a mid-Campanian volcanic episode, probably the waning stages in the formation of the basaltic plateau that is the foundation of the Caribbean Plate.
Objective: To recover the K/T boundary impact event at locations proximal and distal to the proposed Chicxulub meteorite impact site in order to ascertain the mechanisms of ejecta dispersal and the environmental consequences from aerosols and fallout of ejecta.
Conclusion: The K/T boundary interval was recovered in three holes (999B, 1001A, and 1001B), though it was not reached at Site 998, the site most proximal to Chicxulub, because of time constraints and a thicker than expected Tertiary sedimentary section. All three boundary intervals recovered contain an interesting limestone layer, directly overlying clays and claystone, that comprises the middle and lower parts of the interval. The limestone, though only a few centimeters thick, is anomalous in that it appears more massive, more indurated, and lighter in color (white to very light gray) than any other limestone recovered during the leg. Thus, Caribbean sedimentation immediately following the K/T boundary impact appears to be characterized by a brief period of nearly pure carbonate deposition and a very low input of terrigenous clay.
The sedimentary section recovered and logged through the K/T boundary interval at Site 1001 was about three times thicker than that at Site 999, even though the sites are similar distances from the impact. These thicknesses provide important observational constraints on the impact trajectory and angle, and tend to support the hypothesis that the bolide collided with the Yucatan peninsula from the southeast at an oblique angle. Shipboard identification of shocked quartz crystals, with characteristic planar deformation features, in the uppermost claystone unit of the boundary deposit, and the recovery of a smectite layer consisting of altered impact glass spherules or tektites are important contributions to the study of this catastrophic event. This new material will help to clarify the sedimentation and dispersal processes associated with impact as well as its environmental effects.
Objective: To study catastrophic extinction events and biotic recovery.
Conclusion: Two major extinction events were recovered: the K/T boundary event (described above) and the "late Paleocene thermal maximum" (LPTM) event. The LPTM was recovered in Holes 999B, 1001A, and 1001B, where it is characterized by a faintly laminated claystone unit, less than one meter thick, with significantly lower carbonate content than surrounding chalks and limestones. This event, previously observed in only five deep-sea cores, corresponds to a period of rapid warming coincident with the massive extinction of microscopic organisms living on the seafloor and the most devastating event to strike these micro-organisms in the past 100 million years. Paleontological and isotopic studies are planned for both extinction events following careful and detailed shore-based sampling of the cores.
Objective: To analyze the nature of climate forcing in the pre-Neogene world and to test climate models with boundary conditions very different from those of today.
Conclusion: Magnetic susceptibility, geochemical data, and downhole logs from Site 999 and 1001, in addition to shore-based isotopic and rock magnetic analyses, are being used to examine the nature of orbital forcing during the Maastrichtian and Paleocene. The sections recovered appeared to have lithologic (clay and limestone alternations) and magnetic susceptibility variations that are periodic, particularly in the Late Cretaceous. The relationship between these variations, ocean temperatures, Milanovitch cycles, and paleoceanographic conditions will be the subject of several post-cruise studies.
Objective: To study the paleoceanographic and paleoclimatic conditions in the Caribbean region, including (1) the conditions during several episodes of moderate to extreme climatic warmth, (2) the evolution of tropical sea-surface temperatures and changes in meridional temperature gradients, (3) the changes in oceanic circulation and in sources of deep- and intermediate-water masses through Late Cretaceous and Cenozoic time, (4) the opening of a major intra-Caribbean gateway during the middle to late Miocene and the closing of the Central American Seaway during the late Miocene and Pliocene, and (5) the role of tectonics in the opening and closing of seaways.
Conclusion: These studies required successful recovery of sedimentary sections deposited in fairly continuous, undisturbed conditions over long time intervals. Several such Neogene sections, particularly those in the Pedro Channel (Site 1000) and the Colombian Basin (Site 999), provide not only continuous depositional sequences, but also sample a range of water depths from less than 1 km to over 3 km. Thus, these sequences offer an opportunity to study Neogene water-mass history and circulation across the basin. Most of these studies are being conducted post-cruise, although several interesting results were obtained during Leg 165.
One important discovery revealed by shipboard analysis of the carbonate content on the Neogene sequences is that a marked reduction in pelagic carbonate deposition occurs near the middle/late Miocene boundary interval about 10-12 Ma at Sites 998, 999, 1000, and 1001. A similar regional event observed during Leg 138 in the central and eastern equatorial Pacific is referred to as the "carbonate crash." This event previously had not been documented elsewhere, but drilling and research on Leg 165 revealed that the "carbonate crash" occurred widely across the Caribbean and extends into the equatorial Atlantic Ocean, as indicated by Leg 154 results from the Ceara Rise. At the Caribbean sites, the noncarbonate accumulation rates peak at the end of the "carbonate crash" at 10.6 to 10.8 Ma. A major fall in global sea level at ~10.5-11.0 Ma appears to be synchronous with the end of the "carbonate crash" and could explain the high accumulation rates of noncarbonate components at that time. This increase of noncarbonate input into the ocean due to exposure of continental shelves would have enhanced the dilution effect of the "carbonate crash." Post-cruise study will investigate the link between this event and the formation of Miocene gateways and sills, changes in oceanic circulation, and variations in water-mass sources and chemistry.
In addition, uppermost Paleocene sequences recovered during Leg 165 record the effect of the late Paleocene thermal maximum (LPTM) on the surface and deep waters of the Caribbean. Because the paleodepths of Sites 999 and 1001 are deeper than most other LPTM sections, these records add important constraints to our knowledge of deep-water circulation and chemistry during the event. Diminished carbonate contents in claystones are thought to reflect shoaling of the lysocline and CCD in the LPTM interval, among the first evidence for changes in the corrosiveness of deep waters at this time. Evidence for dysoxia suggests that only the deepest part of the water column was truly oxygen-deficient, that the Caribbean deep waters were very old, or, alternatively, that a source of warm, saline deep waters was close by. Alternatively, reduced carbonate flux, rather than dissolution, may be the principal reason for the reduced carbonate content. This is supported by the lack of measurable organic carbon in the claystone at either site, and is compatible with other lines of evidence for surface-water oligotrophy during the LPTM interval.
Sites 998, 999, and 1000 show a decline in carbonate mass accumulation rates (MARs) from late Pliocene through Pleistocene time. This trend is particularly pronounced at Site 1000 in the Pedro Channel and may reflect, in part, increased strength (and variability?) of the Caribbean Current following the onset of Northern Hemisphere glaciation (i.e., significant changes in the flow rate with rising and falling sea level). Another important feature of the Pliocene record at Site 1000 is the sharp increase in carbonate MARs in the mid-Pliocene. The closing of the Central American Seaway at this time may have had a downstream influence on the Caribbean Current by enhancing the flow rate and inducing topographic upwelling along the northern Nicaraguan Rise.
Objective: To determine the tropical climate variability during the late Quaternary and the environmental conditions of anoxic basin development.
Conclusion: The multiple piston cores recovered at Site 1002 in the Cariaco Basin, the world's second largest anoxic basin, provide an unprecedented opportunity to study (1) large and abrupt climate changes in a unique tropical locality that is the counterpart to high-latitude ice cores, and (2) redox chemistry at extremely high temporal resolution. The sedimentary sequence at Site 1002 essentially duplicates the upper Quaternary section recovered at DSDP Site 147 at virtually the same location. However, in terms of recovery and core quality, almost no comparison can be made between the two drilling efforts. Perhaps the best indication of the difference in quality is the fact that APC coring on Leg 165 has recovered a sequence that is laminated over much of its length, whereas Site 147 cores were so badly disturbed that preserved laminae were found only rarely. The bulk of the sequence consists of clayey nannofossil mixed sediments, olive gray to greenish gray in color, which appear to have been deposited under both anoxic (laminated) and oxic (massive) conditions. These sediments are punctuated periodically by episodes of bluish gray and yellowish brown clay deposition, laid down under clearly oxic conditions. Generally following this, deposition of diatom-rich, distinctly laminated sediment indicates strong upwelling. Earlier periods of clay deposition, followed by the accumulation of diatom-rich sediments, may similarly signal earlier periods of deglaciation. Shore-based efforts are underway to develop a composite stratigraphy for this site from the shipboard magnetic susceptibility data and core descriptions from Hole 1002C, which was the only core split from Site 1002 during Leg 165. In May, all cores will be split for high-resolution sampling, and detailed studies of this ultra-high-resolution climate record will begin.
Objective: To determine the nature and origin of Caribbean crust.
Conclusion: The age, physical characteristics, and geochemistry of the basalts recovered at Site 1001 will bear importantly on the tectonic history of the Caribbean. By drilling into the Caribbean Oceanic Plateau, the foundation of the Caribbean Plate, Leg 165 scientists have discovered that the upper part of the plateau consists of a thick pile of submarine basaltic lava flows that were erupted onto the ocean floor about 77 million years ago. These eruptions produced great sheet-flows of hot magma, which spread rapidly along the Caribbean ocean floor over great distances. They have furthermore discovered that these volcanoes, which had been constructed on top of the plate, and are currently 3.5 kilometers below sea level, reached up into shallow water when they were active. Evidence provided by 75-million-year-old volcanic sand layers containing fossils of relatively shallow-water origin indicates that some of these volcanoes may have formed islands. A study of the magnetic properties of the basalt lavas indicates that the Caribbean Oceanic Plateau may have been located at a more southern latitude when the submarine volcanoes were active. These findings are consistent with the idea that this crustal plate was originally formed in the Pacific and that it has subsequently drifted northeastward to its current position between North and South America.