Isotopic Variations of Oxygen (δ¹⁸O) in Benthic Foraminifera under Antiestuarine Conditions in the Colorado River Delta

Abstract

Benthic foraminifera are excellent environmental indicators; CaCO 3 test records the isotopic concentration of their surroundings and can be used to analyze environmental changes that occur during a certain time period. Stable isotopes, particularly those of oxygen ( δ 18 O), are useful for interpreting ancient environments, given that they are used as “proxy” environmental variables (temperature and salinity). In this study, we provide ranges of isotopic variation in benthic foraminifera communities from the Colorado River delta. Four sampling campaigns were conducted in one year (2009-2010) in the adjacent subtidal zone of Baja California. Four transects with a total of sixteen sample station were drawn perpendicular to the coastline. Here, we recorded the following in situ : sand-type, salinity, and temperature. In the laboratory, 300 individuals per sample were separated, and sub-samples were taken for isotopic analysis. Data was processed using the software’s R 2.12.2, PAST 1.81 and Arc Map 9.3. Forty species were identified in the dead assemblages (Thanatocoenosis), whereas thirteen species were found in the living assemblages (Biocoenosis). The most abundant species in both communities belong to the genera Ammonia and Cribroelphidium . In the living assemblages, isotopic variation − 2.15% to 5.94% within a temperature interval of 11 ° C, indicated antiestuarine conditions. In the dead assemblages, isotopic composition − 3.04 to − 0.74 served as a sign of estuarine conditions prior to damming.