Description: To test the pore density in benthic foraminifera as a potential proxy for bottom-water oxygenation, pore density analyses were carried out on tests of living (rose Bengal-stained) specimens of the deep-infaunal and anoxia-tolerant foraminiferal species Globobulimina turgida. Three stations within and two stations below the oxygen minimum zone (OMZ) off Namibia were investigated and compared to in situ-measured bottom-water oxygen content (BW-O2). Pore density was first conventionally assessed by rather time-consuming manual pore counting on SEM photographs and measurement of the analyzed test areas. To significantly shorten the measurement time we tested and evaluated an automation of the pore density measurement using the image analysis software package analySIS (version 5.0, Olympus Soft Imaging Solutions). Pore density data from automated analyses are compared to manually acquired data from G. turgida. Our study shows almost identical results for both manually and automatically acquired data. Consequently, we assume that the new technique provides an alternative and more rapid method to analyze the pore density of foraminifera. For both methods, our results show a distinct negative linear correlation (automatically analyzed pore density: τ = −0.50, p 〈 0.001; manually analyzed pore density: τ = −0.49, p 〈 0.001) between pore density and BW-O2, suggesting that G. turgida increases its pore density in response to decreasing oxygen. Thus, we suggest that, similar to other recently described low-oxygen-tolerant benthic foraminiferal species, G. turgida may improve its O2 uptake by increasing pore density to survive in low-oxic environments. This morphological adaption might be useful for future studies to establish an independent proxy for BW-O2. In addition, pore density has been compared to in situ-measured bottom-water nitrate concentration (BW-NO3−). Our investigation of the pore density-to-BW-NO3− relationship for G. turgida suggests that nitrate seems to be a minor factor influencing pore density in this species compared to BW-O2. Add to CiteULike

Description: The δ13C value measured on benthic foraminiferal tests is widely used by palaeoceanographers to reconstruct the distribution of past water masses. The biogeochemical processes involved in forming the benthic foraminiferal δ13C signal (δ13Cforam), however, are not fully understood and a sound mechanistic description is still lacking. We use a reaction–diffusion model for calcification developed by Wolf-Gladrow et al. (1999) and Zeebe et al. (1999) in order to quantify the effects of different physical, chemical, and biological processes on δ13Cforam of an idealised benthic foraminiferal shell. Changes in the δ13C value of dissolved inorganic carbon (δ13CDIC) cause equal changes in δ13Cforam in the model. The results further indicate that temperature, respiration rate, and pH have a significant impact on δ13Cforam. In contrast, salinity, pressure, the δ13C value of particulate organic carbon (δ13CPOC), total alkalinity, and calcification rate show only a limited influence. In sensitivity experiments we assess how combining these effects can influence δ13Cforam. We can potentially explain 33 to 47% of the interglacial-to-glacial decrease in δ13Cforam by changes in temperature and pH, without invoking changes in δ13CDIC. Furthermore, about a quarter of the − 0.4‰ change in δ13Cforam observed in phytodetritus layers can be accounted for by an increase in respiration rate and a reduction in pH.