Comparisons of the ecology and stable isotopic compositions of living (stained) benthic foraminifera from the Sulu and South China Seas

Abstract

Significant differences are observed between living (Rose Bengal stained) deep-sea benthic foraminifera found in 14 box cores (510–4515 m) from the thermospheric (> 10°C) environments of the Sulu Sea and the psychrospheric (<10°C) conditions in the South Ch ina Sea. Gavelinopsis, Bolivinopsis, Astrononion, Osangularia and Ceratobulimina are common taxa in the South China Sea, but are rare to absent in the Sulu Sea; Siphonina and Valvulineria are dominant genera at certain depths in the Sulu Sea, but are rare to absent in the South China Sea. Faunal differences appear to result from large differences of the bottom-water temperatures (differences from about 6 to 8°C) between these basins. Faunal abundance patterns within each basin are suggested to be related to the organic carbon contents in the sediments, since temperatures, salinities and dissolved oxygen levels of the bottom-waters are relatively uniform. The δ 18O values show a > 2‰ range and are similar to those presented by previous workers, but have no consistent relationship with microhabitat preferences. Vertical distribution patterns and carbon isotope compositions of species, however, reflect microhabitat preferences and are consistent with previous observations from other regions. Epifaunal species (0–1 cm interval) such as Cibicidoides pachyderma, Cibicidoides wuellerstorfi, Hoeglundina elegans and Anomalinoides colligera, have higher δ 13C values than taxa which have the ability to live deeper within the sediments. Infaunal taxa that live in the upper 2–3 cm, including Uvigerina peregrina, Uvigerina proboscidea, and Bulimina mexicana, have lower δ 13C values than epifaunal species, and the deep infaunal species, Chilostomella oolina, has the lowest δ 13C. Cibicidoides bradyi and Oridorsalis umbonatus are found between 0 and ∼ 4 cm and have lower carbon isotope values (by > 1.4‰ in some cores) than epifaunal Cibicidoides species. Exceptions to this pattern include the aragonitic species, Gavelinopsis lobatulus, (0–4 cm) which produces significantly lower δ 13C values than deep infaunal taxa, and the shallow infaunal species, Ceratobulimina pacifica (also aragonitic) and Bolivinopsis cubensis (deep infaunal), which yield higher carbon isotopic values than epifaunal taxa. These exceptions are found primarily in only one core, and additional samples are needed to confirm the relationship between their distribution patterns and isotopic compositions. Each of the species examined has a relatively consistent δ 13C value throughout its distribution within the sediments that may result from heterogeneity of microhabitats within the intervals sampled. Intrageneric differences in δ 13C of Cibicidoides, and possibly Uvigerina and Bulimina, are evident. The isotopic differences between C. bradyi and many other Cibicidoides species are related to differences in microhabitat preferences between species. The δ 13C results confirm the influence of microhabitat preferences on the carbon isotopic composition of deep-sea benthic foraminifera and reaffirm the importance of assessing the microhabitat preferences of species used for isotopic analyses.