Benthic foraminifera as proxies for reconstructing past seawater oxygenation in the Southeast Pacific

Abstract

In the current ocean, deoxygenation significantly threatens marine life and ecosystems, particularly in regions with oxygen depleted zones like the Southeast Pacific (SEP). While the response of benthic foraminifera to low oxygen environments has been studied, it remains poorly understood how effectively they can be used to quantitatively reconstruct past oxygen levels, especially in the dynamic SEP with steep oxygen gradients. This thesis addresses this gap by calibrating and applying two benthic foraminifera-based proxies to reconstruct bottom water dissolved oxygen concentrations (BWDO) quantitatively: (i) the test porosity of epifaunal species, mainly Cibicidoides wuellerstorfi, and (ii) the carbon isotope gradient Δδ13C between epifaunal (C. wuellerstorfi) and infaunal (Globobulimina spp.) species, using specimens from surface sediments along the Chilean and Peruvian coasts. A taxonomic revision of the key species used in the calibrations is presented, refining their identification and enabling accurate proxy calibration. Age evaluation of specimens is applied to ensure that specimens accurately reflect modern conditions. Results indicate that both porosity and Δδ13C are reliable proxies for reconstructing BWDO, each with limitations and strengths. The porosity proxy is reliable for BWDO values less than 100 µmol kg-1, and its accuracy in estimating BWDO depends on the number of specimens analyzed and their standard deviation. The Δδ13C proxy is contingent on the availability of Globobulimina species in the samples. It can be applied in waters deeper than 500 m; in shallower low oxygen settings, denitrification may influence the δ13C values of Globobulimina tests. Both proxies were applied to assess the influence of warmer/cooler climates during the Marine Isotope Stage (MIS) 9 and 13 on deep waters in the SEP. Compared with the Holocene, the results show deoxygenation during the warm interglacial period of the Marine Isotope Stage (MIS) 9 and likely higher oxygen levels during the cooler MIS 13. This research refines the application of benthic foraminifera as paleoxygenation proxies, outlining their advantages and disadvantages. Both proxies offer an insightful tool to understand past deoxygenation and to provide potential analogues for future scenarios in a warming world