Relationship between pore density in benthic foraminifera and bottom-water oxygen content

Reliable estimates of bottom-water oxygen contents are crucial to understanding the formation of past oxygen-depleted environments. Here, we investigate the relationship between pore density in calcareous benthic foraminiferal tests and environmental factors like bottom-water oxygen and nitrate concentration, water depth, and temperature in living (Rose Bengal stained) specimens of the shallow-infaunal species Bolivina pacifica, and the two deep-infaunal species Fursenkoina mexicana, and Chilostomella oolina. Used samples span an oxygen-gradient (0.10 to 4.62 ml L-1) across oxygen minimum zones (OMZ) off Namibia and Pakistan. Bolivina pacifica and F. mexicana display an inverse correlation between pore density and in-situ bottom-water oxygen content (BW-O-2), indicating a morphological response of the foraminifers to decreasing oxygenation. Supporting previous results, we suggest that both species may increase their pore numbers to improve the ability of oxygen uptake in low-oxygen environments. Comparison of the calculated pore densities for B. pacifica and F. mexicana with bottom-water nitrate concentration (BW-NO3-) and temperatures, however, illustrates that these factors might also influence the pore density. Our results for the deep-infaunal species C oolina show no significant relationship between pore density and BW-O-2. This suggests that C oolina, rather than increasing its pore density, has another life-strategy to survive sustained low-oxic conditions, possibly nitrate respiration. The non-correlation between pore densities and BW-NO3- however, suggests that pores are not involved in the denitrification process. According to our data we suggest that the pore density of some benthic foraminiferal species is controlled by BW-O-2. This relation is, however, species-specific. Overall, our data suggest that this morphological response could provide the basis for an independent proxy for BW-O-2 in future studies. (C) 2012 Elsevier Ltd. All rights reserved.