Oxygen isotopes in meteoric calcite cements as indicators of continental paleoclimate

Abstract

Because meteoric water δ 18 O values decrease with decreasing ambient temperature and increasing latitude, δ 18 O values of meteoric calcite cement should exhibit a similar relation with paleolatitude and be an indicator of continental paleotemperatures. To test this, we compiled isotopic and paleolatitude data for 20 meteoric cements and nine speleothems ranging in age from Devonian to modern and in paleolatitude from 3.5° to 83°. Mean δ 18 O values for meteoric cements and speleothems both show the same negative correlation with paleolatitude. The δ 18 O vs. latitude trend for these carbonates is almost identical to that predicted for modern inland environments, but differs from the trend for coastal environments. This suggests that the ground water controlling the ultimate composition of meteoric cement is derived predominantly from inland recharge. If it is assumed that the modern meteoric water δ 18 O vs. temperature relation is valid for the past and that insignificant evaporation occurred prior to carbonate precipitation, then coastal and inland paleotemperatures can be calculated from the δ 18 O values of meteoric calcite (δ 18 O mcl ) and seawater (δ 18 O sw ) by using the equations T coastal =13.3 ±32.6[-0.231- 0.0613(δ 18 O mcl + δ 18 O sw ) ½ and T inland =17.8 ±16.2[-0.572 - 0.1233(δ 18 O mcl + δ 18 O sw ) ½ , where T is temperature in °C. Calcite precipitated from coastal meteoric water at temperatures between 0 and 25 °C will exhibit a narrow range in δ 18 O(-6‰ to -4‰, where δ 18 O sw = 0‰). The δ 18 O of calcite precipitated from inland meteoric water will be sensitive to paleotemperature, ranging from -14‰ to -5‰ (where δ 18 O sw = 0‰) for temperatures of 0 to 25 °C.