Abstract
Abstract. Deciphering the dynamics of dissolved oxygen in the mid-depth ocean during the last deglaciation is essential to understand the influence of climate change on modern oxygen minimum zones (OMZs). Many paleo-proxy records from the eastern Pacific Ocean indicate an extension of oxygen-depleted conditions during the deglaciation, but the degree of deoxygenation has not been quantified to date. The Peruvian OMZ, one of the largest OMZs in the world, is a key area to monitor such changes in near-bottom-water oxygenation in relation to changing climatic conditions. Here, we analysed the potential to use the composition of foraminiferal assemblages from the Peruvian OMZ as a quantitative redox proxy. A multiple regression analysis was applied to a joint dataset of living (rose-bengal-stained, fossilizable calcareous species) benthic foraminiferal distributions from the Peruvian continental margin. Bottom-water oxygen concentrations ([O2]BW) during sampling were used as the dependant variable. The correlation was significant (R2=0.82; p<0.05), indicating that the foraminiferal assemblages are rather governed by oxygen availability than by the deposition of particulate organic matter (R2=0.53; p=0.31). We applied the regression formula to three sediment cores from the northern part of the Peruvian OMZ between 3 and 8∘ S and 997 and 1250 m water depth, thereby recording oxygenation changes at the lower boundary of the Peruvian OMZ. Each core displayed a similar trend of decreasing oxygen levels since the Last Glacial Maximum (LGM). The overall [O2]BW change from the LGM and the Holocene was constrained to 30 µmol kg−1 at the lower boundary of the OMZ.
Highlights
Oxygen minimum zones (OMZs) occur where intense upwelling and high primary productivity result in elevated oxygen consumption within the water column in combination with sluggish ventilation (Wyrtki, 1962; Helly and Levin, 2004; Fuenzalida et al, 2009)
(1) Did the Peruvian oxygen minimum zones (OMZs) structure show differences in terms of vertical and horizontal extension since the Last Glacial Maximum (LGM)? (2) If there are such differences, can we quantify these changes in bottom-water oxygen concentrations ([O2]BW) by reverting to the oxygen demands of today’s living faunas? (3) If so, how much did the [O2]BW levels change since the LGM?
We focused on the following time intervals with 300- to 500-year resolution at each core: the late Holocene (LH; 3–5 cal ka), the early Holocene (EH; 8–10 cal ka), the Bølling Allerød/Antarctic Cold Reversal (BA/ACR; 13–14.5 cal ka), the Heinrich Stadial-1 (HS1; 15–17.5 cal ka), and the Last Glacial Maximum (LGM; 20– 22 cal ka)
Summary
Oxygen minimum zones (OMZs) occur where intense upwelling and high primary productivity result in elevated oxygen consumption within the water column in combination with sluggish ventilation (Wyrtki, 1962; Helly and Levin, 2004; Fuenzalida et al, 2009). Only a few studies attempted bottom-water oxygen reconstructions of the Peruvian margin, and they used records from sediment cores recovered from depths shallower than 400 m (Oberhänsli et al, 1990; Heinze and Wefer, 1992; Scholz et al, 2014; Moffitt et al, 2015; Salvatteci et al, 2016). The present study aimed to reconstruct paleo-oxygen conditions since the Last Glacial Maximum (LGM) by using benthic foraminiferal records from sediment cores from the Peruvian OMZ between 900 and 1250 m water depth. We compiled all available information on living (rose-bengal-stained) benthic foraminiferal faunas from the Peruvian margin as a calibration dataset to investigate the following questions. (1) Did the Peruvian OMZ structure show differences in terms of vertical and horizontal extension since the LGM? (2) If there are such differences, can we quantify these changes in bottom-water oxygen concentrations ([O2]BW) by reverting to the oxygen demands of today’s living faunas? (3) If so, how much did the [O2]BW levels change since the LGM?
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