| Co-Chiefs: Richard Norris and Dick Kroon | Cruise Dates: 9 January-14 February, 1997 |
| Staff Scientist: Adam Klaus | Operations Superintendent: Roland Lawrence |
The origin of deep waters is a fundamental control on biogeochemical cycles and global climate. Analysis of time periods in which deep-water formation and global temperature gradients may have been much different from well-known Pleistocene variability offers a test of the models developed to explain climate change.
Oceanic circulation in the Paleogene and Cretaceous was quite different from that of the modern ocean in part because of the greenhouse conditions that existed during part of this time and the apparent absence of major centers for deep-water formation in the northern basins. Many authors have suggested that deep-water circulation was enhanced in the Cretaceous and Paleogene by the production of warm, saline waters by evaporation in marginal seas such as the basins of the Tethys, but there are few observational data to provide unequivocal support for this "Warm Saline Deep Water" hypothesis.
The sediments on the Blake Plateau and Blake Nose in the Western North Atlantic offer an ideal record for reconstructing water-mass chemistry and circulation in the Cretaceous and early Cenozoic. The plateau's location in the Northern Hemisphere, proximal to the western end of the Tethys seaway, makes the deposited sediments ideal for determining northern sources of deep and intermediate waters. Leg 171B will drill five sites in a transect from the margin of the Blake Plateau to the edge of the Blake Escarpment. Paleogene and Barremian-Maastrichtian strata crop out or are present at shallow burial depths in present water depths of 1200 m to more than 2700 m across the plateau. Today this depth range spans deep thermocline water to upper North Atlantic Deep Water. The plateau spanned a similar range of depths in the early Cenozoic because margin subsidence was largely complete by the Early Cretaceous, and minor subsidence since then is partly offset by reduced sea level after the Eocene.
The deep waters of the world represent one of the largest reservoirs of nutrients and CO2 in the biosphere. The history of deep ocean circulation is integrally tied to the CO2 storage capacity of the oceans and the preservation of carbonate sediments in the deep sea. Aging of deep water by remineralization of sinking organic material makes these waters extremely corrosive and gives them a major role in the inorganic carbon cycle by remineralizing carbonates that would otherwise be stored in sedimentary sequences. Hence, changes in the age and sources of deep waters regulate the alkalinity and CO2 content of the deep sea.
Studies of Paleogene and Cretaceous deep-water circulation and its impact on low-latitude climate are needed to understand the mechanisms that regulate the formation and geographic distribution of nutrient-rich deep waters in the modern oceans. Examining deep-water history during periods with different boundary conditions will allow us to better appreciate the mechanisms that drive biogeochemical cycles. Leg 171B will drill a transect on the Blake Nose to test current models for the Paleogene and Cretaceous history of intermediate and deep waters in the Atlantic and Tethys. Recovery of well-preserved pelagic microfossils and a detailed history of sediment sources will be used to determine the linkages between climatic and biological evolution and changes in deep-water circulation during the Paleogene and Cretaceous.
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. The distribution of d13C in Paleogene benthic foraminifers 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. Another theory is that intermittent production of Warm Saline Deep Water may have continued in the Oligocene to middle Miocene in the remnants of the Tethys seaway. Alternatively, northern component waters may have formed throughout this time, most probably in the North Atlantic. The absence of a Paleogene depth transect in the North Atlantic 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.
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 because these oceans record climates and patterns of water-mass development under conditions 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 and test the assumptions employed in models of the Quaternary oceans. Likewise, records of surface-water temperatures and variations in biotic assemblages are needed to constrain reconstructions of latitudinal thermal gradients.
The objectives of Leg 171B will be to drill five shallow sites (170-600 m penetration) in a transect from the margin of the Blake Plateau to the edge of the Blake Escarpment. The proposed transect of cores will be used to:
[ Contents of the Semiannual Report, No. 2, June-November 1996 |
| Program Updates | New Initiatives | Project Summaries | Laboratory Working Groups |
| Panel Recommendations | Appendixes |
| Semiannual Report, No. 1, December-May 1996 ]