How does modern temperature in the northern Gulf of California compare to the temperature during the last interglacial? Oxytgen isotope (d18O) profiles from Pleistocene (oxygen isotope substage 5e) bivalves provide a unique perspective. Isotope ratios in biogenic carbonates are useful because the d18O is directly related to the temperature and isotopic composition of the water from which they precipitated. Additionally, intertidal marine mollusks are useful because they grow throughout the year, providing a complete record of their environment. Recent and Pleistocene Chione californiensis, a common intertidal bivalve mollusk, were collected from Bahia la Cholla in the northern Gulf of California. d18O values form unaltered Pleistocene C. californiensis range from +0.2 (winter) to -2.6 (summer) per mil. These values are similar to modern C. californiensis from the Gulf, which range form +0.8 (winter) to -2.2 (summer) per mil. However, Pleistocene shells consistently have approximately 0.5 per mil more negative winter and summer values.
Previous investigators suggest a -0.1 per mil decrease for each 10m increase of sea-level. During Pleistovene isotope substage 5e, sea-level is thougth to hgave been 5-10m higher than today; this represents an isotopic shift of -0.05 to -0.1 per mil. This correction results in a -0.45 to -0.4 per mil difference between Pleistocene and Recent C. californiensis. Today, river water input to Bahia la Cholla is minimal and salinity is normal marine or higher. If similar conditions existed during isotope substage 5e, a -4.34 ° C/1.0 per mil offset would represent a temperature difference of 2.0 to 1.7 ° C. These data suggest that Pleistocene winter and summer temperatures in the northen Gulf of California could have been as much as 2 ° C warmer than today.