BODY SIZE AND ABUNDANCE OF BENTHIC FORAMINIFERA AS INDICATORS OF LOW OXYGEN CONDITIONS OF THE GULF OF ALASKA

In deep-sea environments, benthic foraminiferal communities are influenced by many factors, but are primarily affected by organic carbon fluxes and oxygen concentrations in sediments. Similarities between communities in low-oxygen and high-organic carbon settings make it difficult to reconstruct these factors independently using relative abundances alone. Previous workers incorporated body size to reconstruct oxygen concentration with an assumption that size decreases with decreasing oxygen. However, for size to be useful, it must (a) provide information distinct from relative abundances and (b) vary predictably within species with respect to oxygenation.

Here we analyze Holocene to Pleistocene benthic foraminiferal assemblages from IODP Expedition 341 at Site U1419 in the Gulf of Alaska to evaluate the relationship between body size and gradients in community composition. Due to its proximity to regions of seasonally high surface productivity and the modern oxygen minimum zone, U1419 likely experienced changes in productivity and oxygenation over the 60kyr record.

Ordination analyses of the relative abundances of benthic foraminiferal species were used to summarize the faunal variation. Mean body size is strongly correlated with the ordination axis influenced by opportunistic and low-oxygen tolerant species. In a second analysis, species are divided into four size fractions and each species in each size fraction treated as a “taxon” to test whether body size information influences the ordination results. We also examine the size-fraction species scores along axes to determine whether intraspecific body size varies parallel to faunal gradients.

The sample positions between ordinations are strongly correlated, which suggests size is not contributing information beyond relative abundances alone. The correlation strength between the size-fraction species score and the ordered size fractions suggest intraspecific body size is strongly related to faunal gradients, although the direction of the correlation is equivocal. This suggests intraspecific body size varies along faunal gradients individualistically and small size is not necessarily an indication of low oxygen. Future work will use redox sensitive trace elements as an independent proxy for oxygenation to compare with faunal results.