Observed increases in strontium concentration with depth can be explained by either release of strontium from carbonate recrystallization (closed system) or transport of strontium into the strata from a region of higher concentration (open system), or a combination of both. The systematic increase in the strontium concentration with depth and distance from the Blake Escarpment (Fig. F2A) suggests that pore-water strontium concentration is increasingly dominated by local recrystallization and/or transport of high-strontium fluids from interior portions of the platform. An increase in pore-water strontium concentration caused by carbonate recrystallization should be a function of sediment age or burial depth. However, neither factor provides a simple explanation for the concentration data. Concentration profiles in Figure F2 show that pore-water strontium is independent of burial depth and sediment age. Sediments exposed at the seafloor are approximately late Eocene to early Oligocene in age and have strontium concentrations and isotopic compositions (Fig. F3) nearly the same as modern seawater. This indicates that the modern seafloor is an erosional feature that has been exposed for a sufficient amount of time to allow extensive chemical exchange in the upper sediment column. Thus, transport of strontium either by diffusion or advection must supplement local diagenetic release of strontium to the pore water.
The strontium isotopic composition of the acid-leachable sedimentary carbonate was measured on all squeeze cakes from Site 1050 to determine whether the biostratigraphically determined strontium isotopic value is an accurate predictor for the actual isotopic value of the sedimentary carbonate. The results for Site 1050 (Fig. F3) show a close agreement between predicted and measured values. Apparently, clay minerals have not released measurable amounts of exotic strontium to the pore water. Thus, we interpret the differences between measured pore-water strontium isotopic compositions and those predicted for the host sediment as being caused by transport of strontium.
The largest differences in the measured 87Sr/86Sr values of the pore water and the predicted 87Sr/86Sr composition of the host sediment generally occur at the seafloor, and the difference diminishes with depth (Fig. F3). However, the patterns are slightly different among the sites. At Site 1049 the largest difference in 87Sr/86Sr is seen at the bottom of the hole. In the three deeper holes (Sites 1050, 1051, and 1052), the pore-water and predicted host sediment 87Sr/86Sr values converge or cross (Fig. F3). Site 1053 values are similar to Site 1052, but the depth at Site 1053 is significantly less.
At Site 1050, both the measured pore-water 87Sr/86Sr values and the predicted 87Sr/86Sr composition of the host sediment converge within a zone between 300 and 500 mbsf that comprises Eocene-age sediment (Fig. F3). Below 500 mbsf the predicted host sediment 87Sr/86Sr values decrease abruptly below a 24-m.y. hiatus that separates Eocene-age from Cretaceous-age sediments. In this interval the measured pore water contains strontium values of ~0.707775. The zone below 300 mbsf also experiences a decrease in strontium concentration (Fig. F2). The three deepest pore-water samples at Site 1050 have 87Sr/86Sr values that are clearly distinct from the host sediment but similar to pore waters in overlying sections. This suggests that a major portion of the strontium has been derived from Eocene-age sediments that overlie the unconformity, rather than from older sediments below. The seawater strontium isotope curve (Fig. F3) shows that deeper sediments contain strontium of lower isotopic values and are unlikely sources for the strontium found in the deepest sediments drilled at Site 1050.
Site 1051 shows a simple profile (Fig. F3). The measured pore-water 87Sr/86Sr values converge with the nearly constant predicted 87Sr/86Sr values of the Eocene-age host sediments at ~400 mbsf.
Patterns of differences in the measured 87Sr/86Sr values of the pore water and the predicted 87Sr/86Sr composition of the host sediment at Site 1052 show variations with depth. From the seafloor to 160 mbsf pore-water 87Sr/86Sr values converge with the host sediment 87Sr/86Sr values. Between 160 and 460 mbsf the pore-water values are distinctly below those of their Eocene-age host sediments. However, in the four deepest samples (between 580 and 650 mbsf) the pore waters and their Cretaceous-age sediments have similar 87Sr/86Sr compositions. The strontium in the pore waters between 160 and 460 mbsf is apparently derived from stratigraphically Early Cretaceous-age strata.
The commonly observed discrepancies (Table T1) between the 87Sr/86Sr values measured in the pore waters and predicted for the host sediments from all sites along the transect argues against the Blake Spur sediments being a simple closed system with respect to strontium over geologic time.
A composite isotope-mixing diagram (1/Sr concentration vs. isotopic composition; Faure, 1977) for the pore-water strontium measurements shows a linear array of points for each of the sites (Fig. F4). This linear relationship indicates that simple two-component mixing can explain the range of pore-water strontium isotope values and concentrations spanning the drilling transect. The low concentration strontium end-member resembles modern seawater concentration and isotopic composition; however, individual trends are slightly different, reflecting differences in the source of the high concentration strontium end-member. At Site 1050 the values at the highest concentration (~37 ppm) indicate an Eocene-age end-member (87Sr/86Sr = 0.707750). Regression lines for Sites 1051 and 1052 (Fig. F4) indicate that at source concentrations equal to the highest concentrations measured, the potential end-member strontium isotopic values are 0.707735 and 0.707340, respectively. The minimum ages of these presumed sources, based on the strontium isotope seawater curve of Howarth and McArthur (1997), are late Eocene (Sites 1050 and 1051) and mid-Cretaceous (Site 1052).