THE EFFECT OF SIZE VARIATION ON PERCEIVED FORAMINIFERAL ABUNDANCE DURING OCEANIC ANOXIC EVENTS

It is frequently reported that the average test size of foraminiferal specimens is smaller in low-oxygen environments, and their size reduction is potentially due to the lack of oxygen availability. A decrease in test size can potentially influence foraminiferal abundance when using a standard 63-µm sieve size to remove muds. Sediments cored in the western North Atlantic (IODP Site U1407) from Oceanic Anoxic Event 2 (OAE2, ~94 Ma) were investigated to determine how much lower oxygen affected foraminiferal abundance between sieve fractions of >63 µm and 38-63 µm. Site U1407’s OAE2 sediments contain both organic-rich and organic-poor intervals representing separate levels of oxygenation since the organic-rich layers accumulated under low-oxygen conditions. Both benthic foraminifera (useful in assessing paleoproductivity) and planktic biserial heterohelicids (indicative of certain trophic conditions and low oxygen tolerant) were counted in sediments characterized as high or low oxygen by the concentration of total organic carbon (TOC) and redox-sensitive elements (RSTEs). In low-oxygen settings (TOC >6% and/or high RSTEs), benthic foraminiferal abundances were similar in both fractions with the fine fraction yielding 0-15/g and the finest 0-20/g. In higher-oxygen settings (TOC <2% lower RSTEs) benthic abundances were variable with the fine fraction yielding 300-2500/g and the finest 570-1100/g. For biserial heterohelicids, abundances were much higher in the finest fraction below and within the initial stage of OAE2; one sample had 2,200/g in the >63 µm fraction and 41,600/g in the 38-63 µm. Within OAE2, heterohelicid abundances were only slightly higher in the finest fraction than in >63 µm. In general, both benthic foraminifera and biserial heterohelicids (with few exceptions) display the same pattern in both fractions. While examining the finest fraction may yield higher counts in some instances the process can be time-consuming and using a standard 63-µm sieve for low-oxygen studies is suitable for accurate assessments of foraminiferal abundances and paleoceanographic interpretations based on them.