Paleogene and Cretaceous Intermediate Water History
Staff Scientist: Adam Klaus
Co-Chief Scientists: Richard D. Norris & Dick Kroon
The proposed transect of cores will be used to: (1) interpret the vertical structure of the Paleogene and Cretaceous oceans and test the 'warm saline deep water' hypothesis near the proposed source areas; (2) provide critically needed low latitude sediments for interpreting tropical SST and climate cyclicity in the Cretaceous and Paleogene; (3) provide well preserved planktic microfossils for refinement of low latitude Paleogene and Cretaceous chronologies and evolutionary dynamics; (4) recover a complete Cretaceous/Paleogene boundary along a depth transect to describe the events surrounding the boundary and water depth-related changes in sedimentation of the boundary beds; (5) recover sections suitable for magnetic stratigraphy so that low latitude biochronologies may be tied directly to the magnetic reversal record; (6) interpret the thermocline and intermediate water structure of low latitude, Lower Cretaceous oceans and refine the biochronology of this period.
Leg 171C will drill a transect on the Blake Plateau (Fig. 1) to test current models for Paleogene and Cretaceous history of intermediate and deep waters in the Atlantic Ocean and Tethys Seaway. Presently, deep waters are formed in the North Atlantic and Southern Ocean, and it is the mixture and aging of these water masses that produces the characteristic chemistries of the deep Indian and Pacific oceans. Maps of 13C in Paleogene benthic foraminifera suggest that most deep waters of this era have a southern source, but periods of weak latitudinal gradients and short episodes of anomalously-warm deep water indicate that deep or intermediate waters may have formed near the equator or in a northern source area (Miller et al., 1987; Barrera and Huber, 1990; Stott and Kennett, 1990; Pak and Miller, 1992). Another theory is that intermittent production of warm saline deep waters may have continued in the Oligocene to middle Miocene in the remnants of the Tethys Seaway (Woodruff and Savin, 1989). Alternatively, northern component waters may have formed throughout this time, most probably in the North Atlantic (Wright et al., 1992). The absence of a depth transect in the North Atlantic Paleogene prevents resolution of this debate. The northern subtropical location of the Blake Plateau and its position adjacent to the western opening of the Tethys Seaway would place it in the mixing zone between water masses of different origins during the Paleogene and Late Cretaceous.
Presently, most reconstructions of deep-water geometry have focused on the Late Neogene to Holocene record. Paleogene sequences have generally been too deeply buried to be recovered either completely or consistently along depth transects. Yet, the three dimensional structure of Mesozoic and early Cenozoic oceans is of great interest since these oceans record climates and patterns of water mass development under conditions very different from those of modern seas. As such, an understanding of Paleogene and Cretaceous deep-water structure is necessary to provide boundary conditions on global climate models (GCMs) and to test the assumptions employed in models of the Quaternary oceans.
One of the best existing depth transects through pre-Neogene sections was drilled on Maud Rise during Leg 113. Two sites (Sites 690 and 689) found intriguing evidence for deep water formation at low latitudes (the 'warm, saline deep water hypothesis'--Kennett and Stott, 1990). However, these sites were located within the region of southern source water formation and so were not well located to detect the chemistry or history of northern source waters, should they exist. A depth transect in the North Atlantic would be well placed to identify northern component water masses. Patterns of mixing between water masses from different sources could be used to reconstruct their three dimensional structure and origins (Corfield and Norris, 1996).
Paleogene Climate History and Paleoceanography
High global temperatures in the early Eocene promise a means to test global climate models run under conditions of increased atmospheric CO2 (Popp et al., 1989; Berner, 1990). Knowledge of low latitude temperatures provides a major constraint on GCMs since equatorial seas play a major role in regulating heat exchange with the atmosphere (Barron and Washington, 1982).
The Eocene had the most equitable climate of the Cenozoic (Dawson et al. 1976; Shackleton and Boersma, 1981; Axelrod, 1984; Rea et al., 1990), and it is possible that these relatively equitable climates were promoted by higher heat transport between latitudes (Covey and Barron, 1988). Isotopic data suggest both surface waters and deep waters reached their maximum temperatures in the early Eocene (Savin, 1977; Miller et al., 1987; Stott and Kennett, 1990). High latitudes were substantially warmer than at present and reached temperatures of 15-17°C (Zachos et al., 1994). This warm interval lasted 3-4 m.y. and marked fundamentally different climate conditions than were present at any other time in the Cenozoic. Latitudinal thermal gradients were probably less than 6-8°C during the Eocene, about half the modern pole-to-equator gradient (Zachos et al., 1994). Yet the interval is poorly known both because low-latitude records are rare and those that do exist are either spot cored or disturbed by drilling through Eocene cherts (Stott and Zachos, 1991). Hence, low latitude temperature data are needed to constrain model runs of Paleocene climate.
The Blake Plateau was located on the western gateway from the relatively restricted Tethys/Atlantic basins to the open Pacific during the early Paleogene. Comparisons between Caribbean and existing equatorial Pacific sites could show whether deep waters aged as they entered the Pacific as expected, if the Tethys was a major source of saline bottom waters. Existing records from the Caribbean (DSDP Site 152) and Central Pacific (DSDP Site 577; ODP Site 865) provide some of the few low latitude temperature estimates for the Eocene. However, more complete temperature records are needed to document surface-water temperatures in the Eocene tropics and subtropics. In addition, the Eocene warm interval provides a test for climate models that integrate latitudinal thermal gradients, atmospheric CO2 levels, and ocean-atmosphere heat transports.
The Cretaceous-Paleocene Meteorite Event
Recent evidence of the impact of a bolide on the Yucatan Platform has focused debate over the history and consequences of this Cretaceous-Paleogene event. Evidence of an impact in the Caribbean includes discoveries of glass spherules and shocked quartz in boundary sections at Haiti and Mimbral, as well as gravity measurements and drill core data that imply the existence of a 180 km diameter structure of Maastrichtian-earliest Paleocene age beneath the Yucatan (Sigurdsson et al., 1991b; Margolis et al., 1991; Alvarez et al., 1991; Hildebrand and Boynton, 1990). Re-analysis of DSDP Sites 536 and 540 in the Gulf of Mexico has led to the discovery of thick deposits of reworked carbonates of diverse ages. These deposits contain uppermost Maastrichtian nannofossils and occur immediately below lowermost Paleocene sediments. Alvarez et al. (1991) have interpreted these K/P boundary deposits as part of the ejecta blanket. Size distributions of spherules found in boundary beds on Haiti and in the DSDP sites indicate that the impact occurred within the Caribbean (Sigurdsson et al., 1991ba). Finally, geochemical analysis of K/P glasses suggests that the impact occurred in carbonates rich in evaporites-a chemistry consistent with a source similar to rocks beneath the Yucatan structure (Sigurdsson et al., 1991a).
A depth transect in Cretaceous strata offers an unparalleled opportunity to study the hydrographic structure of the low latitude Cretaceous oceans. There are five principal issues that may be addressed by recovery of a depth transect of cores from this region:
Present knowledge of the plateau is based on (1) a series of JOIDES and United States Geological Survey (USGS) boreholes drilled on the shallow parts of the plateau, (2) a COST well drilled on the continental shelf, (3) one rotary cut Deep Sea Drilling Project (DSDP) hole (Site 390) drilled at the edge of the Blake Nose, (4) regional geologic maps of the inner Blake Plateau, (5) low resolution Gloria side scan sonar maps of the exclusive economic zone (EEZ), including the Blake Plateau, (6) regional geologic syntheses of multichannel and single channel seismic lines that focus nearly exclusively on the tectonic development of the Plateau and Early Cretaceous reef sedimentation, (7) manned submersible surveys of the Cretaceous reef deposits along the Blake Escarpment and phosphorite pavements on the inner Blake Plateau. Figure 2 presents some of the existing geophysical survey tracks and core locations for the Blake Plateau. The present core archives are inadequate to construct a depth transect through Paleogene or Cretaceous deposits since all existing cores have low recovery rates or were studied with well cuttings. None of the JOIDES (Holes 1-6) or the USGS Atlantic Slope Project (ASP) holes were drilled at water depths as great as those proposed here although ASP 3 was drilled just updip of the proposed drilling transect. However, the dense grid of available multichannel lines, single channel lines, and 3.5-kHz echo soundings are ideal for locating future holes in the area.
Cretaceous-Paleogene boundary beds are typically thin in the deep sea yet they contain our best evidence for the geographic distribution and magnitude of the extinctions and the oceanographic history of the earliest Paleocene. There are three major objectives to drilling on the Blake Plateau:
(1) Reconstruct the Paleogene climate history and paleoceanography of this area. Paleogene strata are known to exist at shallow burial depth at all sites along the proposed drilling transect. There are three principle issues that could be addressed by recovery of cores from these areas: a) low latitude surface temperature history during the Paleocene and Eocene, b) nature of the Paleocene-Eocene event at low latitudes, and c) sources of deep waters during the Paleocene and Eocene.
(2) Examine the effects of the Cretaceous-Paleogene meteorite impact on the biostratigraphy and sedimentology of the Atlantic Ocean. Drilling the K/P boundary on the Blake Plateau, will:
Triple APC and XCB coring will be done on five shallow-water (170-450 m deep) sites in a transect from the margin of the Blake Plateau to the edge of the Blake Escarpment. Boreholes will be located along existing, high quality multichannel lines that are crossed by a dense web of single channel and 3.5-kHz lines. Seismic interpretations are supported by a series of JOIDES, USGS, and DSDP boreholes as well as observations from submersibles. The boreholes are intended to penetrate between 170 and 600 m of nannofossil ooze of Eocene, Paleocene, Maastrichtian, Campanian and Aptian-Albian age. All holes extend below Reflector Purple, which is identified as basal Campanian in Hole 390A. All holes should penetrate middle to lower Eocene oozes, Paleocene, and Maastrichtian-Albian strata. Sites BN-2A and BN-3A have a high probability of recovering a complete K/T boundary sequence because these boreholes will penetrate the most stratigraphically complete sequences where the upper Paleocene to Maastrichtian section is about 300 m thick.
The drilling strategy is intended to recover a depth transect in pre-Eocene strata that approaches the depth-resolution possible in upper Pleistocene piston core transects (e.g, Slowey and Curry, 1987; 1992; Lynch-Stieglitz et al., 1994). This will ensure a complete >1300 m depth transect from 1200 mbsl to 2500 mbsl in the Paleocene-middle Eocene and bathyal Barremian basal Albian. The K/P boundary will be recovered in at least four of the five sites and will permit studies of sedimentation across a depth transect of the boundary beds. Triple coring will ensure nearly 100% recovery of the sedimentary section at each site. Logging will enhance site to-site correlation, which is critical for reconstructing hydrography.
There is no evidence from previous drilling or from existing seismic records for appreciable hydrocarbons in the slope sediments proposed for drilling. It is unlikely that there are mobile hydrocarbons of Early Cretaceous or younger age on the Blake Nose. The sediments have never been deeply buried to depths sufficient for hydrocarbon maturation. Hydrocarbons may exist in the limestones below the pelagic drape of Barremian to Eocene sediments, but the proposed drilling will not penetrate these limestones except during redrilling of Site 390 (Proposed Site BN-1A). Truncation of the Albian-Barremian clinoforms by the middle Cretaceous unconformity (Reflector Purple) could represent a stratigraphic trap. However, there is no evidence of hydrocarbons below this unconformity in DSDP 390. In addition, there are no obvious cross-strata reflectors that could represent trapped hydrocarbons within the sedimentary section proposed for drilling.
A complication to drilling in this area is the existence of cherts in the lower Eocene and upper Paleocene. Deep Sea Drilling Project Hole (DSDP) 390A encountered seven partly lithified layers of limestone and chert near the edge of the Blake Plateau. The layers are all about 5 cm thick or less and consist of irregularly replaced carbonates in a 30 m thick interval of nannofossil ooze. Once through this section, the lower Paleocene through upper Aptian section is entirely ooze at Site 390.
The proposed five-site depth transect will span paleowater depths ranging from ~1 to ~3 km across the Blake Plateau. Logging data will be very important to achieve the objectives of this proposed leg. Only holes deeper than 400 m will be logged, and these holes will be logged with standard tools to aid inter-site correlation. Logging would enhance site-to-site correlation, which is critical for reconstructing hydrography. We propose that all but one of the holes be logged with Quad combo, geochemical, Formation MicroScanner (FMS), and Geological High-resolution Magnetometer (GHMT) tool strings. The exception is proposed hole BN-1A where there is too little penetration.
DSDP Site 390 (location of BN-1A) penetrated five distinctive reflectors (Fig. 3) that are color coded as:
1) Middle Eocene nannofossil ooze (Reflector Orange),
2) Paleocene/lower Eocene nannofossil ooze and chert (Reflector Red),
3) Lower/upper Paleocene nannofossil ooze (Reflector Green),
4) Campanian nannofossil ooze (Reflector Purple), and
5) Barremian strata (Reflector Blue-between Barremian clayey nannofossil ooze and Barremian periplatform limestone).
The proposed sites (Figs. 2 and 3) are located at 1410, 1972, 2264, and 2586 mbsl and are projected to penetrate between 170 and 600 m of nannofossil ooze of Eocene, Paleocene, Maastrichtian, Campanian, and Aptian-Albian age. All holes will extend below Reflector Purple, which is identified as basal Campanian in Hole 390A. All holes should penetrate middle to lower Eocene oozes, and Paleocene and Maastrichtian-Albian strata. By far the most stratigraphically complete sequences should be present at sites BN-2A and BN-3A where the upper Paleocene to Maastrichtian section is about 300 m thick. These sites have a high probability of recovering a complete K/P Boundary sequence, because at least the lower Danian was biostratigraphically complete in the much thinner (20 m) section recovered in Hole 390A.
This site is proposed as a reoccupation of Site DSDP 390. The principal objective is to recover a sequence of Eocene though Campanian strata from deep water that can be compared with age-equivalent, but more expanded, sections at shallower water depths. In particular, the middle Eocene, and lower Paleocene-Maastrichtian are thick enough to permit high-resolution studies of Eocene paleoceanography, and the nature of the K/P boundary event.
DSDP Site 390 recovered about 200 m of sediment including 160 m of pelagic sediment that ranges in age from middle Eocene (?) to Barremian, and 40 m of Barremian oolitic limestone. The pelagic sediments were entirely stiff to soupy nannofossil ooze with the exception of seven 5-cm-thick limestone and chert stringers encountered in the 30-m-thick lower Eocene and upper Paleocene sections. Eocene sediments compose about 75 m of the cored section, whereas the Paleocene (35 m), Maastrichtian (28 m), Campanian (2 m), and Barremian to lower Albian pelagic sediments (20 m) make up the remainder. During RCB drilling of DSDP 390A, coring penetrated about 50 m/hour except during drilling through the cherts, where the drilling rate dropped to about 20 m/hour.
Site BN-1A is included mainly to increase the total depth range of the transect (to about 1371 m total water depth range) and recover sediments deposited well within the Paleogene equivalent of modern upper deep water. Further, since DSDP Site 390 was rotary cored, the existing core is useless for paleoceanography. This is a good site for recovering a moderately thick and well-preserved Eocene-Paleocene section and for estimating the chemistry of upper deep water. However, the Cretaceous sequence is too condensed to be of much use for paleoceanography. The Mesozoic section should be of interest to sedimentologists studying the control of water depth on sedimentation at the K/P boundary, and processes of sedimentation (e.g., tying expanded sections (BN-3A) into more condensed intervals (BN-1A)).
Site BN-1B would recover a thicker section than Site BN-1A (400 m compared to ~170 m) at a slightly shallower depth (2480 mbsl). Nearly all the increased thickness would be in a more expanded Paleocene-Eocene section. The site is located at shotpoint 2241 on MCS Line TD-5. This site is proposed as an alternate to Site BN-1A.
Site BN-2A was previously located in 2265 m water depth at the intersection of SCS line 18 of the R/V Gloria Farnella and the profile made by the Glomar Challenger during the survey for DSDP 390. This site has been relocated to shotpoint 1291 on Gillis Line 26 at essentially the same water depth as before and with nearly the same stratigraphy and thickness of the stratigraphic section. This move was made entirely because of uncertainties about the correct navigation of the Glomar Challenger line.
The site is located immediately NW of MCS line TD-5 to take advantage of an unusually thick sequence of upper Paleocene (?) strata. At this location the depth to Reflector Purple is about 400 m, due primarily to an unusually expanded section above Reflector Green (lower/upper Paleocene) and below Reflector Red (Paleocene/Eocene Boundary). The upper Paleocene is probably about 200 m thick at this site and the lower Paleocene Campanian section (between reflectors Green and Purple) is about 75 m thick. The sediment packages between the other reflectors have similar thicknesses to their equivalents at Site DSDP 390.
This is a high priority site because it represents an expanded section of Paleocene Cretaceous sediments that are ideal as a deep-water end member of the depth transect. The expanded section should help considerably in refining Paleocene and Eocene stratigraphy and paleoceanography, since the best available low latitude sites (such as DSDP 577, DSDP 384; and ODP 758) are comparatively condensed. The Aptian and older Cretaceous section is also a good end member for a thermocline-intermediate water depth transect in the Early Cretaceous. In addition, the site is likely to recover a complete K/P boundary with well-preserved microfossils. Drilling is proposed to about 600 m to recover 400 m of middle Cretaceous-Eocene and 200 m of Lower Cretaceous sediments.
This site is located at shotpoint 1821 on MCS Line TD-5 and is very similar to BN-2A in terms of the thickness of the sediment packages between all the reflectors. As at Site BN 2A, the middle Eocene is about 75 m thick and overlies a 50-m section of lower Eocene to uppermost Paleocene (Reflector Orange to Reflector Red). The middle Paleocene is nearly 200 m thick (above Reflector Green) and overlies a 100-m-thick drape of lower Paleocene to Campanian strata (above Reflector Purple). The site is located at the intersection of the Glomar Challenger SCS line and MCS line TD-5 at about 1972 mbsl.
The section below Reflector Purple consists of 370 m of Lower Cretaceous (?) deposits. The middle Cretaceous represents the distal upper package of clinoforms that overlaps the pre-Barremian (?) reef about 40 km to the southwest and can be traced updip to deposits below Sites BN-4A and BN-5A. Coring is proposed to a depth of about 600 mbsf to recover the Paleogene and part of the Lower Cretaceous sections.
Site BN-3A is about 300 m shallower than Site BN-2A so both add to the resolution of the depth transect. Further, both Sites BN-2A and BN-3A penetrate a thick section of Paleocene sediments, which should provide a fairly high-resolution sequence in well preserved sediments that is singularly lacking in existing ODP holes. Site BN-3A should recover a well-preserved K/P boundary interval and expanded Aptian to older Cretaceous sequence. The site could be used as an alternate for Site BN-2A, but it would also serve as a site at intermediate depth between Sites BN-4A and BN-2A where it improves the likelihood of correlating accurately between shallower and deeper sites, and increases the probability of recovering a complete section of the Lower Cretaceous strata.
This site lies at a water depth of 1410 m and appears to preserve about 200 m of upper Paleocene to Eocene strata over about 100 m of Campanian to lower Paleocene deposits. Reflector Purple is underlain by about 250 m of deposits that appear to correlate with the seismic package that lies between reflectors Blue and Purple at Site BN-3A. These are likely to be Barremian to pre-lower Albian nannofossil ooze and clay similar to sediments recovered from the equivalent interval at DSDP 390. Coring is propose to about 250 m below Reflector Purple for a total recovery of about 550 m of section.
Site BN-4A is ~560 m shallower than BN-3A and over 800 m shallower than Site BN-2A and should contain a record of the Paleogene equivalent of modern, lower intermediate water. This is a critical site primarily because it provides the best shallow-water end member for the depth transect through Paleocene and Lower Cretaceous strata. In addition, the site should recover a K/P boundary section.
Site BN-4B Site BN-4B is proposed as an alternate to Site BN-4A. The site is located on shotpoint 1461 at a water depth of 1652 m. The objectives are similar to those described above, but the site would recover a slightly thicker section (~600 m) and so be at a deeper water depth than Site BN-4A. Site BN-4B would recover a more expanded section above Reflector Purple than Site BN-4A but at the expense of about 100 m of section below this reflector.
The objectives are to recover the middle Eocene-Maastrichtian (?) and underlying Aptian Barremian section for correlation with equivalent strata at Sites BN-4A, BN-3A, and BN 1A. Correlation to Site BN-2A will permit a depth transect of about 900 m between Sites BN-5A and BN-2A where the sections are expanded compared to that at Site BN-1A. A total of about 450 m of section will be cored: 200 m of Oligocene-Upper Cretaceous (?) and 250 m of Lower Cretaceous deposits.
Site BN-5A is included since it is the shallowest site that is likely to have a good Cenozoic section. At 1200 m depth, Site BN-5A is well within modern intermediate water and hence is a good end member for paleoceanography. The site probably lacks a K/P boundary section, or it is very condensed relative to Sites BN-2A, 3A, and 4A. However, it should recover an Eocene to upper Paleocene section similar to the deeper sites as well as a shallow-water, end-member Lower Cretaceous section. Hence, this is a desirable site primarily because it expands the total depth range of the transect and increases the probability of being able to reconstruct vertical hydrographic structure during the late Paleocene to middle Eocene, and in the Early Cretaceous.
Site BN-5B is proposed as an alternate to Site BN-5A. The site is located on shotpoint 1241 on MCS Line TD-5 at a water depth of 1293 m. The objectives are similar to those described above but the site would recover a more expanded section above Reflector Purple and do so at a deeper water depth than Site BN-5A. Site BN-5B would core about 100 m of Oligocene and younger strata, 200 m of Eocene to Upper Cretaceous (?), and 150 m of Lower Cretaceous deposits for a total penetration of about 450 m.
To Leg 171C Proposed Site Information
To Leg 172
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