Mid-depth oxygen drawdown during Heinrich events: evidence from benthic foraminiferal community structure, trace-fossil tiering, and benthic δ13C at the Portuguese Margin

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The distribution of trace fossils and benthic foraminiferal assemblages in two sediment cores from 2160 m water depth at the northern and 1100 m at the southern Portuguese continental margin was used to reconstruct the bottom-water oxygenation during the last 40 ka. Emphasis was given to the Heinrich events, during which enhanced meltwater flux associated with the iceberg surges reduced or even halted deep water formation in the North Atlantic. Condensed trace-fossil tiering structures and high proportions of low-oxygen-tolerant benthic foraminifera suggest a major drawdown of bottom-water oxygenation during Heinrich events. Using the benthic foraminiferal oxygenation index (BFOI) of Kaiho (1991, 1994)that links benthic foraminiferal communities to ambient bottom-water oxygenation, we extracted apparent oxygen concentrations from benthic assemblages. BFOI values suggest low-oxic conditions during Heinrich events 1 and 4 with mean oxygen levels of 2.6 to 2.8 ml l −1 and high-oxic conditions with 4.1 ml l −1 during Heinrich event 2 at the shallower site. High proportions of low-oxygen-tolerant foraminiferal species and BFOI minima are recorded a few centimetres below the maximum concentrations of ice-rafted debris, indicating that the original microhabitat depth of dysoxic species is preserved beneath the Heinrich layers even though bioturbation continued during the events. Trace-fossil tiering calibrated with the redox boundary in low-oxic environments and dysoxic ichnotaxa commonly associated with black shales indicate oxygen levels lower than those implied by the BFOI. Oxygenation estimates derived from trace-fossil tiering denote low-oxic conditions during H1 (13.3 ka) with oxygen concentrations slightly above 1 ml l −1, and dysoxic conditions during H2 (20.6 ka), H3 (27 ka) and H4 (33.5 ka) with oxygen concentrations between 0.1 and 1 ml l −1. During the Younger Dryas, bottom waters likely were low oxic. Negative excursions of benthic carbon isotope values provide independent control on the oxygen drawdown during Heinrich events. Considering a bottom-water oxygenation of 6.1 to 6.5 ml l −1 during the last glacial, the benthic δ 13C minima reveal an oxygen depletion to values between 3.4 and 5.0 ml l −1 during H1 through H4 at the shallower core site. Differences between estimates from benthic δ 13C, BFOI and trace-fossil tiering suggest that benthic foraminiferal communities are rather robust to oxygen depletion if the level does not fall below 3 ml l −1, whereas burrowing organisms react sensitively to depth variations of the redox boundary in near-surface sediments due to even small changes of bottom-water oxygenation.