Toward a global calibration for quantifying past oxygenation in oxygen minimum zones using benthic Foraminifera

Abstract

Abstract. Oxygen minimum zones (OMZs) are oceanic areas largely depleted in dissolved oxygen, nowadays considered in expansion in the face of global warming. To investigate the relationship between OMZ expansion and global climate changes during the late Quaternary, quantitative oxygen reconstructions are needed but are still in their early development. Here, past bottom water oxygenation (BWO) was quantitatively assessed through a new, fast, semi-automated, and taxon-independent morphometric analysis of benthic foraminiferal tests, developed and calibrated using WNP (western North Pacific, including its marginal seas), ENP (eastern North Pacific), and ESP (eastern South Pacific) OMZ samples. This new approach is based on an average size and shape index for each sample. This method, as well as two already published micropalaeontological techniques based on benthic foraminiferal assemblages' variability and porosity investigation of a single species, was calibrated here based on availability of new data from 45 core tops recovered along an oxygen gradient (from 0.03 to 2.88 mL L−1) from the WNP, ENP, EEP (eastern Equatorial Pacific), ESP, SWACM (southwest African continental margin), and AS (Arabian Sea) OMZs. Global calibrated transfer functions are herein proposed for these methods. These micropalaeontological reconstruction approaches were then applied to a palaeorecord from the ENP OMZ to examine the consistency and limits of these methods, as well as the relative influence of bottom and pore waters on these micropalaeontological tools. Both the assemblage and morphometric approaches (which are also ultimately based on the ecological response of the complete assemblage and faunal succession according to BWO) gave similar and consistent past BWO reconstructions, while the porosity approach (based on a single species and its unique response to a mixed signal of bottom and pore waters) showed ambiguous estimations.

Intermediate- and Deep-Water Oxygenation History in the Subarctic North Pacific During the Last Deglacial Period

“…Here, we study BF assemblages in comparison with previously obtained results from two sediment cores retrieved in 2009 during the SO201-KALMAR Leg 2 cruise of the R/V "Sonne" (Max et al, 2012;Ovsepyan et al, 2013;Max et al, 2014;Ovsepyan et al, 2017;Riethdorf et al, 2013a, Riethdorf et al, 2013b. Based on our results, available published data, and transfer functions (Tetard et al, 2017;Tetard et al, 2021a), we estimate millennial-scale variations in oxygen concentrations at present-day water depths (w.d.) of 963 and 2,163 m on the Shirshov Ridge in the western Bering Sea, from 22 to 9 ka BP and from 21 to 1 ka BP, respectively.…”

“…Color bars mark warm (light orange) and cold (light blue) intervals, according to the North Atlantic climatostratigraphy (AICC2012 timescale, Veres et al, 2013). The red shades represent the 23% uncertainty of the estimates for cores SO201-2-85KL and SO201-2-77KL (Tetard et al, 2021a). Acronyms as in Figure 3.…”

“…To assess the past dissolved oxygen concentrations, we used the transfer function technique developed by Tetard and co-authors (2017), which is based on the BF data from core MD02-2508 retrieved off Baja California (Tetard et al, 2017) and later recalibrated using 45 worldwide core-tops of which the modern [O 2 ] is known (Tetard et al, 2021a). This approach can be divided into three main steps.…”

“…The age models for Cores MD07-3088 and TN057-13PC are based on recalculated AMS 14 C ages using SHCal20 (Hogg et al, 2020) and consistently estimated reservoir ages (For details, see Supplementary Information). The red shades represent the 23% uncertainty of the estimates for cores SO201-2-85KL and SO201-2-77KL (Tetard et al, 2021a). Acronyms as in Figure 3.…”

“…In these areas, a slight enhancement of bottom-water oxygenation was reported for the Younger Dryas (YD, 12.9-11.7 ka BP, Veres et al, 2013). The records of benthic foraminiferal (BF) assemblages and geochemical data have been discussed as proxies to quantitatively assess the past oxygen variations at the intermediate and deep water levels (e.g., Gooday, 2003;Jorissen et al, 2007;Tetard et al, 2017;Tetard et al, 2021a, Tetard et al, 2021bHoogakker et al, 2015;Hoogakker et al, 2018;Erdem et al, 2020).…”