Salinization processes in paleowaters in coastal sediments of Suriname: evidence from delta Cl-37 analysis and diffusion modelling

In the coastal plain of Suriname a stagnant body of fresh (<250 mg/l Cl) an d moderately brackish (250-1000 mg/l Cl) groundwater of meteoric origin is found in permeable Tertiary formations. This groundwater body, formed durin g the last Wisconsin regression, extends offshore into deposits on the cont inental shelf. The authors found that this paleowater is slowly being salin ized by downward solute transport from overlying Holocene marine clays and upward transport from saline Cretaceous sediments. The downward transport h as been studied at two sites by relating analyses of chloride contents and Cl-37/Cl-35 isotope ratios in pore water with computer simulations of diffu sive transport. Optimization of process parameters yielded an effective dif fusion coefficient of 7 x 10(-10) m(2)/s and a fractionation factor of 1.00 27, which agrees well with earlier studies. With the computer model the fol lowing periods of marine influence were found: 6000-4,100 yr BP and 10,000- 500 yr BP at the sites located 20 and 3 km from the coastline, respectively . According to the diffusion model the downward flux of salts has not penet rated deeper than 100 m. Salts below this depth are thought to originate fr om the Cretaceous sediments. However, groundwater chloride concentration an d chlorine isotope data are scarce below this depth, while uncertainty rega rding the various regressions and transgressions precludes solute transport modelling. Clay layers at the base of the Tertiary appear to inhibit the u pward transport by lateral dispersive mixing at the Tertiary/Cretaceous bou ndary. Oil reservoirs are also related to these clay seals. The presence of the seal, and perhaps the oil below, can therefore be predicted by the chl oride profile in the upper 250 m. (C) 2000 Elsevier Science B.V. All rights reserved.