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We report δ13C and δ18O values for Rose Bengal‐stained benthic foraminifera from a set of box cores from continental margin environments in the Atlantic and Pacific oceans. These isotopic results are compared with foraminiferal distribution data and pore water δ13C profiles to evaluate the importance of environmental (microhabitat) effects on the carbon isotopic composition of benthic foraminifera. The δ13C values of infaunal taxa are consistently lower than those of epifaunal taxa, suggesting that microhabitat effects on test composition do exist. The δ13C differences between foraminiferal carbonate and bottom water dissolved inorganic carbon are not correlated with the δ18O differences between benthic foraminifera tests and equilibrium calcite, but they do correlate with variations in the chemistry of sediment pore waters. However, interspecific δ18O differences as well as δ13C differences between species with similar vertical distributions in the sediments indicate that taxon‐specific “vital” effects also influence test composition.

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The distribution of living (rose bengal stained) deep‐sea benthic foraminifera was determined in the upper 20 cm of sediments of eight Soutar box cores taken from two depth transects (510‐4515 m) in the thermospheric (> 10°C) Sulu Sea. Despite the uniformity of bottom water temperatures, salinities, and dissolved oxygen levels below 1000 m, significant faunal differences exist at different depths in the low‐oxygen (∼1.25 mL/L below 1000 m) basin. The shallowest site (510 m) is dominated (> 10% of the calcareous fauna) by Cibicidoides, Uvigerina, (> 150 µm) and Bolivina (> 63 µm), while Siphonina is codominant with Cibicidoides and Uvigerina in the 1005‐m core. The 2000‐m cores are dominated by Cibicidoides, Gyroidinoides, and Oridorsalis, while Cibicidoides bradyi and Oridorsalis umbonatus dominate the 3000‐ and 4000‐m cores. Infaunal assemblages of Valvulineria mexicana are found in the sediments of the 4515‐m core. Relatively low bottom water oxygen values do not necessarily yield "typical low‐oxygen taxa" such as Bolivina, Uvigerina, Chilostomella, Bulimina, and Globobulimina. Changes in the abundances of these taxa in fossil assemblages have been used as indicators of changes in ancient bottom water oxygen levels but may instead reflect organic carbon contents of the sediments. An examination of the vertical distributions of foraminiferal assemblages from the > 63‐µm and > 150‐µm fractions reveals that taxa have microhabitat preferences similar to those observed in other regions. Taxa found in the upper 0‐ to 1‐cm interval (epifaunal) include Cibicidoides wuellerstorfi and Hoeglundina elegans, while taxa such as Chilostomella and Globobulimina reach maximum abundances in subsurface sediments and have infaunal microhabitat preferences. Cibicidoides bradyi and O. umbonatus live in sediment depths from 0‐ to 4‐cm and have transitional preferences with both epifaunal and infaunal occurrences. Intrageneric differences in test morphologies, including pore distribution, rounded peripheries, and variable spire height, are observed in Cibicidoides and Gyroidinoides and are suggested to be related to microhabitat preferences. Vertical distributions of a number of taxa found in both the 63‐ to 150‐µm and > 150‐µm fractions are similar, suggesting that juveniles and adults live under similar microhabitat conditions. Ontogenetic changes in microhabitat preferences of most species are not observed in this study and therefore would not be expected to account for isotopic vital effects reported for some taxa in previous studies.

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