Diagenesis and Interstitial-Water Chemistry at the Peruvian Continental Margin—Major Constituents and Strontium Isotopes

Abstract

Two distinct hydrogeochemical regimes currently dominate the Peruvian continental margin. One, in shallower water (150-450 m) shelf to upper-slope regions, is characterized by interstitial waters with strong positive chloride gradients with depth. The maximum measured value of 1043 mM chloride at Site 680 at ITS corresponds to a degree of seawater evaporation of ~2 times. Major ion chemistry and strontioum isotopic composition of the interstitial waters suggest that a subsurface brine that has a marine origin and is of pre-early Miocene "age," profoundly influences the chemistry and diagenesis of this shelf environment. Site 684 at ~9°S must be closest to the source of this brine, which becomes diluted with seawater and/or interstitial water as it flows southward toward Site 686 at ~13°S (and probably beyond) at a rate of approximately 3 to 4 cm/yr, since early Miocene time. The other regime, in deep water (3000-5000 m) middle to lower-slope regions, is characterized by interstitial waters with steep negative and nonsteady-state chloride gradients with depth. The minimum measured value of 454 mM chloride, at Site 683 at ITS, corresponds to-20% dilution of seawater chloride The most probably sources of these low-chloride fluids are gas hydrate dissociation and mineral (particularly clay) dehydration reactions. Fluid advection is consistent with (1) the extent of dilution shown in the chloride profiles, (2) the striking nonsteady-state depth profiles of chlorides at Sites 683 and 688 and of 87 Sr/ 86 Sr ratios at Site 685, and (3) the temperatures resulting from an average geothermal gradient of 50°C/km and required for clay mineral dehydration reactions. Strontium isotope data reveal two separate fluid regimes in this slope region: a more northerly one at Sites 683 and 685 that is influenced by fluids with a radiogenic continental strontium signature, and a southerly one at Sites 682 and 688 that is influenced by fluids with a nonradiogenic oceanic signatures. Stratigraphically controlled fluid migration seems to prevail in this margin. Because of its special tectonic setting, Site 679 at ITS is geochemically distinct. The interstitial waters are characterized by seawater chloride concentrations to-200 mbsf and deeper by a significantly lower chloride concentration of about two-thirds of the value in seawater, suggesting mixing with a meteoric water source. Regardless of the hydrogeochemical regime, the chemistry and isotopic compositions of the interstitial waters at all sites are markedly modified by diagenesis, particularly by calcite and dolomite crystallization.

A hydrothermal seep on the Costa Rica margin: middle ground in a continuum of reducing ecosystems

“…These include clay mineral dehydration reactions, particularly the smectite-to-illite transition that occurs at approximately 1.5 km depth [24], differential-stress-induced smectite dehydration [25], and the opal-A to opal-CT transformation [26]. Evidence for migration of low salinity water is found along the Peruvian margin [27], along the décollement at the Costa Rica margin [28,29], and the Barbados margin [30].…”

Site 827

“…45). Such high concentrations of chloride (630 mM) are unusual; in other forearcs chloride concentrations decrease below seawater values (Gieskes et al, 1990a;Kastner et al, 1990). In addition, sodium and potassium concentrations decrease rapidly below depths of 75 and 125 mbsf, suggesting that igneous rocks at the base of the site are a sink for these elements.…”

“…The maximum chloride concentration is -14% greater than seawater at only -175 mbsf. The cause of this high chloride concentration is unknown; other convergent margin pore fluids are usually characterized by chloride concentrations less than seawater values (Gieskes et al, 1990a;Kastner et al, 1990;Mottl et al, in press). At least two processes may increase chloride concentrations: (1) hydration of basement rocks and sedimentary minerals, particularly volcanic ash, and (2) ion exclusion as water flows through semipermeable clay 400 0.0 0.5 1.0 Total organic carbon (%) Figure 14.…”

The Strontium Isotopic Composition of Pore Waters from Leg 133: Northeast Australian Margin

“…However, this limits the end-member 87 Sr/ 86 Sr ratio only to -0.70875 . Work during ODP Leg 112 (Peru margin), where high chlorinities at depth were also observed (Kastner et al, 1990;Elderfield et al, 1991), showed that profiles were consistent with the mixing of an evolved brine associated with evaporite formation with the seawater as it was transported at depth within the marginal sediments. Thus, the high-chloride end-member at a particular site may be itself a mixture of brine and seawater that is in diffusive communication with the overlying pore-water system.…”

“…However, evidence for large-scale fluid flow was not found at previous ODP sites adjacent to carbonate platforms, such as in the Bahamas (Swart and Guzikowski, 1988) and the Maldives (Swart and Burns, 1990). Another topic of interest was that previous work on hemipelagic sediments drilled by ODP has shown evidence for the presence of evaporites at depth Kastner et al, 1990) that can influence diagenetic processes.…”