Stable isotope compositions of brachiopod shells from the Alamogordo Member of the Lake Valley Formation (south-central New Mexico) provide information about Early Mississippian paleoclimate. The Alamogordo Member was deposited on a ramp at the southwestern edge of the North American craton, at lat 20°S. It consists of cherty lime-mudstone to lime-packstone (predominantly mudstone and wackestone) strata and scattered mud-cement (Waulsortian) mounds. The brachiopods analyzed lived at water depths estimated to have been between 100 and 300 m.
The δ18O values of well-preserved shells of a single species of brachiopod average −2.3‰ relative to PDB (the Peedee belemnite isotope standard). This average represents a temperature of 21–26 °C if the isotopic composition of the seawater (δ18Osw) was between 0‰ and −1‰, as inferred from Pleistocene and Holocene δ18Osw and other considerations. Sea-surface temperature would have been 26–31 °C (after we applied a +5 °C correction for water depth). This tropical climate for an Early Mississippian low-latitude site agrees with previous isotopic and climate-modeling results, but contrasts with a temperate climate inferred from comparison of the lithology and biota of the Alamogordo Member with those of modern temperate carbonates.