Benthic foraminiferal response to orbital-scale variability in primary productivity off the Portuguese margin over the last 1.3 Myr

Highlights

• Distinct glacial/interglacial cyclic variations in primary productivity.

• A long-term decreasing trend in primary productivity.

• A shift in cyclicity of primary productivity across the mid-Pleistocene transition.

• Global ice volume as a primary driver of changes in primary productivity.

Abstract

Detailed analyses of benthic foraminiferal fauna and planktonic foraminiferal stable carbon isotopes at IODP Site U1391 were performed to reconstruct orbital-scale variability in primary productivity off the Portuguese margin over the last 1.3 Myr. Q-mode factor analysis of benthic foraminiferal census data from the >250 μm fraction reveals four assemblages, which is mainly controlled by the organic matter flux to the seafloor deriving from surface water productivity. The Sphaeroidina bulloides assemblage accounts for nearly half of the total variance, consisting of S. bulloides, the agglutinated taxa and Lenticulina as major components and Cibicidoides robertsonianus, Planulina ariminensis and Uvigerina peregrina parva as subsidiary species. Cibicidoides wuellerstorfi and Bulimina mexicana are important accessory species of the Cibicidoides pachyderma-dominated assemblage. The miliolids assemblage is dominated by miliolids and B. mexicana with Uvigerina mediterranea as the important associated species. The Praeglobobulimina ovata assemblage is represented by P. ovata along with Globobulimina affinis, Melonis pompiloides, Melonis barleeanum and U. peregrina parva as important accessory species. High productivity often occurs during the interglacial stages and is characterized by the S. bulloides assemblage with subsidiary P. ovata assemblage, while low productivity often occurs during the glacial stages and is marked by the miliolids assemblage and the C. pachyderma assemblage. In addition to the distinct glacial/interglacial variations, there is a long-term decreasing trend in primary productivity recorded by benthic foraminifera, which is supported by a broadly increasing trend in the sediment redness (a*). Continuous wavelet analysis of high productivity-related Factor 1 loading (the S. bulloides assemblage) suggests that variations in primary productivity have a middle Pleistocene transition (MPT)-related shift in cyclicity from the ~41-kyr dominant cycle prior to the MPT to the ~100-kyr dominant cycle after the MPT. Cross-spectral analyses between the Factor 1 loading record and δ¹⁸O of Globigerina bulloides reveal the control of global ice volume changes on primary productivity. The high coherency between the Factor 1 loading record and δ¹³C of G. bulloides on the glacial/interglacial timescale suggests that primary productivity off the Portuguese margin may be largely affected by nutrient availability which was related to the Iberian surface hydrographic changes associated with changes in global ice volume.

Introduction

The western Iberian margin has been a key location to study abrupt climate change and high-resolution paleoceanographic evolution because of Nick Shackleon's study. The “Shackleton Site” (Core MD95-2042) drilled off Portugal has provided a high-fidelity record of millennial-scale climate variability for the last glacial cycle that can be confidently correlated to Greenland ice cores, and at the same time the benthic oxygen isotope record can be correlated to the Antarctic climate signal (Shackleton et al., 2000, Shackleton et al., 2004). Prior to Integrated Ocean Drilling Program (IODP) Expedition 339, many cores had been retrieved from the Iberian margin (Abrantes, 1991; de Abreu et al., 2003, de Abreu et al., 2005; Schönfeld et al., 2003; Skinner et al., 2003; Desprat et al., 2005; Martrat et al., 2007; Voelker et al., 2010; Rodrigues et al., 2011), and the oldest core extends to Marine Isotope Stage (MIS) 15 (Rodrigues et al., 2011). During the IODP Expedition 339, seven sites were drilled off the southwestern and western Iberian margin with one of objectives to determine Mediterranean Outflow Water (MOW) paleocirculation and global climate significance (Expedition 339 Scientists, 2012). Mediterranean Outflow Water (MOW) is an important intermediate water mass in the eastern North Atlantic Ocean that acts upon the Iberian continental margin. As the significance of MOW on the North Atlantic circulation and global climate has been broadly recognized, more and more studies focused on the MOW evolution based on seismological, sedimentological, geochemical and micropalaeontological techniques (Schönfeld and Zahn, 2000; Rogerson et al., 2005; Llave et al., 2006; Voelker et al., 2006; Toucanne et al., 2007; Hernández-Molina et al., 2014; Bahr et al., 2015; Lebreiro et al., 2015; Singh et al., 2015a; Voelker et al., 2015; Kaboth et al., 2016, Kaboth et al., 2017a, Kaboth et al., 2017b; Lofi et al., 2016; García-Gallardo et al., 2017; Guo et al., 2017, Guo et al., 2020). The MOW records that have been acquired based on the sediments retrieved during the Expediton 339 date back to Miocene (Hernández-Molina et al., 2014). Benthic foraminifera played a very important role in the MOW reconstruction. Schönfeld, 1997, Schönfeld, 2002 proposed that the relative abundance of the elevated epibenthos group can be used as an indicator of MOW intensity. Singh et al. (2015a) applied benthic foraminiferal records to reveal a stronger MOW current during the stadials within MIS 5 and MIS 7. Guo et al. (2020) reconstructed the MOW variability of the past 1.3 Myr based on the MOW indicator, which experienced a shift in cyclicity around the middle Pleistocene transition (MPT).

The western Iberian margin is situated in the northernmost section of the Canary/Iberian Current upwelling system, and high productivity associated with strong upwelling exerts important influence on atmospheric CO₂ concentration by pumping it into the deep-sea sediment (Berger et al., 1989). On millennium timescale, summer export productivity off the western Iberian margin was found very low during Heinrich events, especially north of 40°N; on the glacial/interglacial timescale, interglacial export productivity was higher than glacial export productivity at mid to high latitudes during the last 150,000 years, with higher glacial than interglacial export productivity at low latitudes (Salgueiro et al., 2010). Upwelling related productivity was higher during the interglacial stages than during the glacial stages (Rodrigues et al., 2011; Singh et al., 2015b). Long and continuous record of Iberian productivity available so far has been limited to the last 900 kyr (Singh et al., 2015b). ~900 ka is an important time during the MPT which is a transition period from the 41-kyr world of the late Pliocene/Pleistocene to the 100-kyr world of late Pleistocene (Clark et al., 2006). As one of the most important climatic events for the late Cenozoic (Zachos et al., 2001), how the MPT occurred is still an enigmas. More paleoceanographic records are needed in order to figure out the climate processes during this time interval. In view of the fact that benthic foraminifera are very sensitive to changes in organic matter flux (Altenbach et al., 1999), and that pelagic input dominates the input of organic matter in the deep sea (Sarnthein et al., 1988), high organic matter-flux species can be a proxy of primary productivity. Especially in areas where organic carbon flux is high, benthic foraminifera are good indicators of surface productivity (Schnitker, 1994). Almogi-Labin et al. (2000) revealed surface productivity changes in the Gulf of Aden, NW Arabian Sea, over the last 530 kyr based on benthic foraminiferal assemblages. Jian et al. (2001) applied the relative abundance of high organic matter-flux species Uvigerina peregrina as an indicator of surface productivity in the South China Sea over the last 220 kyr. Abundant benthic foraminiferal fauna from Site U1391 retrieved during IODP Expedition 339 (Fig. 1) off the Iberian margin provide an opportunity to reconstruct the variability in ocean productivity across the MPT.

In this work, we present 1.3 Myr-long benthic foraminiferal faunal records and planktonic foraminiferal stable carbon isotope record at IODP Site U1391, (1) to reconstruct orbital-scale variability in ocean productivity across the MPT, and (2) explore its possible forcing mechanisms.