Large-scale lateral advection of seawater within the oceanic crust is thought to be the cause of low heat flow in the Equatorial Pacific. Until now supportive evidence is limited to reversals of B and δ11B, Ca, Mg, SO4-2, Sr and 87Sr/86Sr in sediment pore waters. Here we report new evidence from a detailed study of Li and its isotopes in sediment cores from ODP Sites 844 and 851. Carbonates at these sites were significantly recrystallized leading to large variations in Li and δ7Li in the sediments and associated pore waters. In addition to diagenetic effects, distinct reversals in lithium concentration and isotopic ratio toward modern seawater composition are observed in waters near the basaltic basement, lending further support to the seawater intrusion hypothesis. Three endmembers are identified in the pore waters: seawater, a diagenetically altered component, and evolved crustal fluid. The new lithium isotopic results underscore the importance of diagenetic artifacts in carbonate sediments.