The stable-isotope composition of early ground-water cements from sandstone in paleoecological reconstruction

The carbon-isotope composition of soil carbonates in paleosols records the global expansion of C ₄ biomass in the late Miocene. However, soil carbonate is not present in many geological sections, or if present, is often difficult to distinguish from other carbonate cements. Here we report on the δ ¹³C values of sparry, early ground-water cements in sandstones from the Siwalik Group in Pakistan that record, like the soil carbonates from the same sections, the dramatic late Miocene expansion of C ₄ vegetation. The early cements are confined to densely cemented nodules in the sandstones, but can also be found reworked as nodules into adjacent paleo-channels, demonstrating their formation shortly after burial. The early cements are isotopically distinct from later, higher-temperature calcite cements. Our results demonstrate that some early ground-water cements can be used in a manner similar to soil carbonates to reconstruct vegetation and perhaps pCO ₂ changes in the past.