LITHIUM ISOTOPE GEOCHEMISTRY OF PORE WATERS FROM OCEAN DRILLING PROGRAM SITE-918 AND SITE-919, IRMINGER BASIN

The distribution of Li isotopes in pore waters to a depth of 1157 m be low seafloor is presented for ODP Sites 918 and 919 in the Irminger Ba sin, offshore Greenland. Lithium isotope data are accompanied by stron tium isotope ratios to decipher diagenetic reactions in the sediments which are characterized by the pervasive presence of volcanic material , as well as by very high accumulation rates in the upper section. The lowering of the Sr-87/Sr-86 ratio below contemporaneous seawater valu es indicates several zones of volcanic material alteration. The Li iso tope profiles are complex suggesting a variety of exchange reactions w ith the solid phases. These include cation exchange with NH4+ and mobi lization from sediments at depth, in addition to the alteration of vol canic matter. Lithium isotopes are, therefore, a sensitive indicator o f sediment-water interaction. delta(6)Li values of pore waters at thes e two sites vary between -42 and -25 parts per thousand. At shallow de pths (<100 mbsf), rapid decreases in the Li concentration, accompanied by a shift to heavier isotopic compositions, indicate uptake of Li in to alteration products. A positive anomaly of delta(6)Li observed at b oth sites is coincident with the NH4+ maximum produced by organic matt er decomposition and may be related to ion exchange of Li from the sed iments by NH4+. Tn the lower sediment column at Site 918, dissolved Li increases with depth and is characterized by enrichment of Li-6. The Li isotopic compositions of both the waters and the solid phase sugges t that the enrichment of Li in deep interstitial waters is a result of release from pelagic sediments. The significance of sediment diagenes is and adsorption as sinks of oceanic Li is evaluated. The maximum dif fusive flux into the sediment due to volcanic matter alteration can be no more than 5% of the combined; inputs from rivers and submarine hyd rothermal solutions. Adsorption on to sediments can only account for 5 -10% of the total inputs from rivers and submarine hot springs. Copyri ght (C) 1998 Elsevier Science Ltd.