Description: 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-O2), 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-O2. 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-O2. This relation is, however, species-specific. Overall, our data suggest that this morphological response could provide the basis for an independent proxy for BW-O2 in future studies.

Description: Under modern conditions only North Pacific Intermediate Water is formed in the northwest Pacific Ocean. This situation might have changed in the past. Recent studies with general circulation models indicate a switch to deep-water formation in the northwest Pacific during Heinrich Stadial 1 (17.5-15.0 ka) of the last glacial termination. Reconstructions of past ventilation changes based on paleoceanographic proxy records are still insufficient to test whether a deglacial mode of deep-water formation in the North Pacific Ocean existed. Here we present deglacial ventilation records based on radiocarbon-derived ventilation ages in combination with epibenthic stable carbon isotopes from the northwest Pacific including the Okhotsk Sea and Bering Sea, the two potential source regions for past North Pacific ventilation changes. Evidence for most rigorous ventilation of the intermediate-depth North Pacific occurred during Heinrich Stadial 1 and the Younger Dryas, simultaneous to significant reductions in Atlantic Meridional Overturning Circulation. Concurrent changes in d13C and ventilation ages point to the Okhotsk Sea as driver of millennial-scale changes in North Pacific Intermediate Water ventilation during the last deglaciation. Our records additionally indicate that changes in the d13C intermediate-water (700-1750 m water depth) signature and radiocarbon-derived ventilation ages are in antiphase to those of the deep North Pacific Ocean (〉2100 m water depth) during the last glacial termination. Thus, intermediate- and deep-water masses of the northwest Pacific have a differing ventilation history during the last deglaciation.

Description: Temperature reconstructions indicate that the Pliocene was ~3 °C warmer globally than today, and several recent reconstructions of Pliocene atmospheric CO2 indicate that it was above pre-industrial levels and similar to those likely to be seen this century. However, many of these reconstructions have been of relatively low temporal resolution, meaning that these records may have failed to capture variations associated with the 41 Kyr glacial-interglacial cycles thought to operate in the Pliocene. Here we present a new, high temporal resolution alkenone carbon isotope based record of pCO2 spanning 2.8 to 3.3 million years ago from ODP Site 999. Our record is of high enough resolution (~19 Kyrs) to resolve glacial-interglacial changes beyond the intrinsic uncertainty of the proxy method. The record suggests that Pliocene CO2 levels were relatively stable, exhibiting variation less than 55 ppm. We perform sensitivity studies to investigate the possible effect of changing sea surface temperature, which highlights the importance of accurate and precise SST reconstructions for alkenone palaeobarometry, but demonstrate that these uncertainties do not affect our conclusions of relatively stable pCO2 levels during this interval.