171B Scientific Prospectus


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 (Farrell and Prell, 1991). 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. Ocean Drilling Program (ODP) Leg 171B will drill a transect on the Blake Nose (Figs. 1, 2) 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 13C 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 (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 Water 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 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 (GCMs) 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.


Depth-Transect Strategy
Recently, much attention has been focused on reconstructing the Cenozoic history of deep-water chemistry and carbonate dissolution by drilling depth transects in the equatorial oceans. Transects have been drilled on the Walvis Ridge in the South Atlantic, Ontong-Java Plateau in the Pacific, Madinley Rise and Oman margin in the Indian Ocean, Maud Rise in the Southern Ocean, and, most recently, the Ceara Rise in the equatorial Atlantic. The strategy behind these transects is to determine bathymetric changes in carbonate preservation. These data provide a history of changes in the lysocline depth, oceanic alkalinity, and surface ocean carbonate production (e.g., Curry et al., 1990; Peterson et al., 1992; Berger et al., 1993). Transects that include sites at shallow depths also can be used to reconstruct the changes in sources of intermediate waters and detect patterns of water-mass circulation (Curry and Lohmann, 1986; Slowey and Curry, 1987; Kennett and Stott, 1990). The origins of a water mass can be inferred both by measuring the characteristic 13 C and Cd chemistry preserved in benthic foraminifers and by analyzing the faunal composition (Streeter and Shackleton, 1979; Boyle and Keigwin, 1982; Curry et al., 1988; Boyle, 1990; Charles and Fairbanks, 1992; Wright et al., 1992; Oppo and Rosenthal, 1994).

The best existing depth transect through pre-Neogene sections was drilled on Maud Rise during ODP Leg 113. The transect of two sites found intriguing evidence for deep-water formation at low latitudes (the Warm Saline Deep Water hypothesis; Kennett and Stott, 1990). However, these sites were within the region of formation of southern source waters and are 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). The depth transect will aid in documentation of patterns of sedimentation across the slope and the history of ocean circulation.

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