2. Pore-Water Strontium Isotopes from the Leg 171B Drilling Transect down the Blake Spur¹

William Ussler III,² Charles K. Paull,² and Paul D. Fullagar³

ABSTRACT

Strontium concentrations and ⁸⁷Sr/⁸⁶Sr values were measured on pore-water and sedimentary carbonate samples from sediments recovered at Sites 1049-1053 on the Blake Spur during Ocean Drilling Program Leg 171B. These sites form a 40-km-long depth transect extending along the crest of the Blake Spur from near the upper edge of the Blake Escarpment (a steep cliff composed of Mesozoic carbonates) westward toward the interior of the Blake-Bahama Platform. Although these sites were selected for paleoceanographic purposes, they also form a hydrologic transect across the upper eastern flank of the Blake-Bahama Platform. Here, we use pore-water strontium concentrations and isotopes as a proxy to define patterns of fluid movement through the flanks of this platform.

Pore-water strontium concentration increases with depth at all sites implying that strontium has been added during sediment burial and diagenesis. The isotopic values decrease from seawater-like values in the shallow samples (~0.70913) to values as low as 0.707342 in one of the deepest samples (~625 meters below seafloor). The change in pore-water strontium isotopic values is independent of the strontium isotopic compositions predicted from the host sediment age and measured on bulk carbonate in some samples. In most cases the difference between predicted sediment strontium isotopic composition and measured value is less than ±2 about the mean of the measured strontium value. Both the increase in concentration and the decrease in the strontium isotope values with increasing depth indicate that strontium was expelled from older carbonates. The strontium concentration and isotope profiles vary between sites according to their proximity to the Blake-Bahama Platform edge. Profiles from Site 1049 (nearest the platform edge) show the greatest amount of mixing with modern seawater, whereas the site most distal to the platform edge (Site 1052) shows the most significant influence of older, deeper carbonates on the pore-water strontium isotopic composition.

¹Ussler, W. III, Paull, C.K., and Fullagar, P.D., 2000. Pore-water strontium isotopes from the Leg 171B drilling transect down the Blake Spur. In Kroon, D., Norris, R.D., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 171B [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/171B_SR/chap_02/chap_02.htm>. [Cited YYYY-MM-DD]

²Department of Geological Sciences, University of North Carolina, Chapel Hill NC 27599-3315, USA.

Present address: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing CA 95039-0628, USA. Correspondence author (Ussler): methane@mbari.org

³Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill NC 27599-3315, USA.

Initial receipt: 16 June 1999
Acceptance: 12 July 1999
Web publication: 11 August 2000
Ms 171BSR-118