Strontium Isotopes in Pore Waters of East Equatorial Pacific Sediments: Indicators of Seawater Advection through Oceanic Crust and Sediments

Abstract

Pore-water samples from the equatorial sedimentary bulge area show reversals in depth profiles of 87 Sr/ 86 Sr ratios at the sedimentfàasement interface. Results of this work support inferences made from previous pore-water data (from DSDP drilling in the area) that large-scale horizontal advection of seawater has occurred through the basement underlying the thick sedimentary sequence in this region. The area of apparent advection includes the eastern part of the equatorial high-productivity zone and part of the Guatemala Basin. We attempted to find links between the observed near-basement reversals in pore-water chemistry and sedimentary thickness, age, and topography of the area. Most of the sites that show horizontal advection have disturbed basement topography or outcrops within 10 to 20 km, suggesting that the cooling effects of outcrops may extend for at least 20 km horizontally. Heat-flow data from the area were compared to determine whether sites showing near-bottom chemistry reversals were consistent with areas of low conductive heat flow. This was generally true for the area of the sedimentary bulge and Guatemala Basin. Not enough pore-water data from the Nazca Plate were available to establish any reliable systematics. Because the high-productivity area is well-sealed from hydrothermal circulation, the missing heat must be lost by horizontal advective heat transport. From profiles of strontium isotopes and other elements that show departure from seawater values with increasing depth in the sediments, but return to seawater values near the basement, it appears that water flows relatively freely through much of the oceanic crust, even when sealed by considerable sedimentary cover.

Dissolution of biogenic ooze over basement edifices in the equatorial Pacific with implications for hydrothermal ventilation of the oceanic crust

“…4C), but the return to seawater values near basement is evidence of effi cient ventilation (Oyun et al, 1995). Site 846/1226 pore waters at the base of the sediments had detectable nitrate and also had a reversal of Sr, but concentrations differed from seawater values (Oyun et al, 1995). Overall these data indicate ventilated conditions at both sites, Site 1225 being better ventilated than 1226.…”

“…In the center of the sediment column, Sr increases due to recrystallization of biogenic carbonate sediments (Fig. 4C), but the return to seawater values near basement is evidence of effi cient ventilation (Oyun et al, 1995). Site 846/1226 pore waters at the base of the sediments had detectable nitrate and also had a reversal of Sr, but concentrations differed from seawater values (Oyun et al, 1995).…”

“…The area is considered somewhat paradoxical because effi cient ventilation by hydrothermal circulation must occur despite thick sediment deposits. Basement pore-water compositions similar to modern seawater (Baker et al, 1991;Oyun et al, 1995;You et al, 2003) and microbial populations nourished by seawater-derived oxygen and nitrate at the sediment-basement contact (D'Hondt et al, 2004) also suggest extensive ventilation in this region. Given typical sediment thicknesses of 300-500 m, the well-ventilated conditions must be maintained by recharge and discharge through base-ment outcrops.…”

Biogenic silica and chert in the Pacific Ocean

“…As modeled by Yool and Tyrrell (2005), the oceanic response to an increased dissolved silica supply would have been much faster than that proposed by McGowran (1989). Moore (2008) proposed that the explanation for the widespread abundance of cherts in pelagic sections of the Pacifi c-especially at relatively shallow subbottom depths-lies in the circulation of hydrothermal waters in the upper oceanic crust (e.g., Baker et al, 1991;Oyun, et al, 1995;Fisher et al, 2003;Davis and Becker, 2004;Fisher, 2004;Bekins et al, 2007;Moore et al, 2007). Such waters can dissolve biogenic silica in the basal pelagic sections that form an imperfect aquitard over the crust and reprecipitate the silica in these sections as the waters conductively cool (Moore, 2008).…”

Seawater intrusion through the oceanic crust and carbonate sediment in the Equatorial Pacific: Lithium abundance and isotopic evidence

“…In the eastern Equatorial Pacific, it has long been recognized that surface conductive heat flow is significantly lower than theoretically predicted [ Langseth et al , 1965; Sclater et al , 1980]. Recent geochemical studies in pore waters have concluded that large quantities of seawater may have intruded below thick sediments thereby accounting for the anomalously low heat flow in this area [ Baker et al , 1991; Oyun et al , 1995; Spivack and You , 1997]. The chemical compositions of the deepest pore waters are similar to the present‐day seawater in Ca, Mg, SO 4 −2 , Sr and 87 Sr/ 86 Sr, suggestive of a short water residence time and minimum water‐rock interaction within the basement.…”