Description: The distribution of dissolved oxygen in the world oceans mirrors deep circulation commencing where oxygen is exchanged with the atmosphere. Consumption by bacterial decay of organic matter reduces oxygenation of intermediate and deep waters along their pathways. Supply and consumption of oxygen at depth depend on circulation vigour and surface ocean primary productivity, both are sensitive to climate change. Reconstructions of past ocean ventilation from geological archives relied ratios of redox-sensitive elements and biotic indicators. Dissolved oxygen is a predominant environmental factor controlling the abundance and distribution of benthic organisms. In particular benthic foraminifera are sensitive to oxygenation changes but show a mutual response to both, an increase in particulate organic matter flux or a decrease in dissolved oxygen in near-bottom and pore waters. Reconstructions of ancient deep-water oxygen concentrations by using the ratio of species with oxic vs. suboxic or dysoxic environmental preferences showed a sufficient accuracy only at oxygen levels 〈65 µmol kg-1. Multivariate analyses and transfer functions improved the accuracy and robustness of the benthic foraminiferal proxy against changes in organic matter flux but they are applicable only between 180 and 270 µmol kg-1 and mainly rely on oxyphylic species. Recent approaches focussed on the pore density (PD) in foraminiferal tests, which covaries with the oxygen availability in the ambient seawater ([O2]). Calibrations of PDs versus [O2] for shallow endobenthic Bolivina spissa, B. pacifica, and Fursenkoina mexicana are constrained between 1 to 37, 4 to 130, and 50 to 200 µmol kg-1 repectively. The epibenthic Planulina limbata depicted a higher accuracy though only between 2 and 13 µmol kg-1. For the deep endobenthic Globobulimina turgida exists a calibration between 50 and 200 µmol kg-1, while Chilostomella oolina shows no significant relationship between PD and [O2].

Description: Lateglacial and Holocene faunal and stable-isotope records from benthic foraminifers in the eastern Mediterranean Sea (EMS) suggest a high spatiotemporal variability of deep-water oxygenation and biogeochemical processes at the sea floor during that time. Changes in the oxygenation and food availability of the deep-sea ecosystems are closely linked to the hydrology of the EMS borderlands; they reflect orbital and suborbital climate variations of the high northern latitudes and the African monsoon system. During the last glacial maximum, cool surface waters and high evaporation resulted in maximum convection and oxic deep-waters in all sub-basins. Strong wind-induced mixing fostered surface-water production with seasonal phytodetritus fluxes. During the glacial termination and the Holocene, oxygenation and food availability of deep-sea benthic ecosystems were characterized by a pronounced regional differentiation. Local deep-water formation and trophic conditions were particularly variable in the northern Aegean Sea as a response to changes in riverine runoff and Black Sea outflow. During the interval of sapropel S1 formation in the early Holocene, average oxygen levels decreased exponentially with increasing water depth, suggesting a basin-wide shallowing of vertical convection superimposed by local signals. In the northernmost Aegean Sea, deep-water ventilation persisted during the early period of S1 formation, owing to temperature-driven local convection and the absence of low-salinity Black Sea outflow. At the same time, severe temporary anoxia occurred in the eastern Levantine basin at water depths as shallow as 900 m. This area was likely influenced by enhanced nutrient input of the Nile river that resulted in high organic matter fluxes and related high oxygen-consumption rates in the water column. In the southern Aegean and Levantine Seas, we observe a gradual increase in deep-water residence times, preceding S1 formation by approximately 1–1.5 kyr. Once oxygen levels fell below a critical threshold, the benthic ecosystems collapsed almost synchronously with the onset of S1 deposition. The recovery of benthic ecosystems during the terminal phase of S1 formation is controlled by subsequently deeper convection and re-ventilation over a period of approximately 1500 years. After the re-ventilation of the various sub-basins had been completed during the middle and late Holocene, deep-water renewal was more or less similar to recent rates. During that time, deep-sea ecosystem variability was driven by short-term changes in food quantity and quality as well as in seasonality, all of which are linked to millennial-scale changes in riverine runoff and associated nutrient input.