Benthic foraminiferal response to orbital-scale variability in primary productivity off the Portuguese margin over the last 1.3 Myr
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.
Section snippets
Regional oceanographic setting off the Portuguese margin
IODP Site U1391 (37°21.532′ N, 9°24.656′ W; Fig. 1) is situated in the Northeast Atlantic Ocean off the Portuguese margin at a water depth of 1085 m in the pathway of the northward MOW current (Fig. 1; Expedition 339 Scientists, 2012). MOW derives from the outflow of Mediterranean intermediate and deep waters through the Strait of Gibraltar. Dense, saline Mediterranean waters entrain the overlying less dense North Atlantic Central Water (NACW) when descending from the 300-m deep Strait of
Material and methodology
For micropaleontological studies 549 samples (about 20 cm3) were taken from the shipboard splice generated by Holes U1391A and U1391B (Expedition 339 Scientists, 2013) at ~80 cm intervals from the top of the sediment sequence down to 415 m composite depth (mcd). Samples were dried at 70 °C and weighed. Dried samples which varied in weight from 15 g to 50 g were washed through a 63 μm sieve. The residues were dried at 70 °C and weighed again. In order to compare with previous studies in adjacent
General characteristic of benthic foraminiferal fauna off the Portuguese margin
In total 288 benthic foraminiferal species identified at Site U1391, 37 species are arenaceous, 38 species are porcelaneous and 213 species are hyaline. The arenaceous and the porcelaneous benthic foraminifera account for an average of 5.0% and 12.6%, respectively. The hyaline species constitute the majority (82.4%) of the benthic foraminiferal population, and the most abundant species together accounting on average for >60% are: Cibicidoides pachyderma (8.2%), Bulimina mexicana (6.8%),
New age model of Site U1391
The age model of the upper 415 mcd of Site U1391 had been established by Guo et al., 2017, Guo et al., 2020. However, major overlaps in two depth intervals between 160 and 180 mcd and from 230 to 250 mcd were detected based on the log for Ca/Ti and Ca/Fe measured along the cores of Hole U1391A and Hole U1391B (Abrantes et al., 2017b), and they were neglected by the previous age model. This study considered the overlaps and developed a new age model of upper 415 mcd, combined with the chronology
Conclusions
Variations in primary productivity off the Portuguese margin were reconstructed based on benthic foraminiferal fauna of the last 1.3 Myr at IODP Site U1391. The benthic foraminiferal record shows distinct faunal turnover which is primarily controlled by the delivery of organic matter to the seafloor deriving from surface water productivity. High productivity characterized by the dominance of the S. bulloides assemblage with subsidiary P. ovata assemblage, mainly occurred during the interglacial
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Samples and part of data used in this research were provided by the Integrated Ocean Drilling Program (IODP). The authors thank the RV JOIDES Resolution crew and the IODP technical staff. This work was supported by the Chinese Academy of Sciences (CAS) Strategic Priority Project (XDB26000000), National Natural Science Foundation of China (41276044, 41776073 and 41806069), the State Key Laboratory of Palaeobiology and Stratigraphy (20182104) and the Ministry of Ocean and Fisheries Korea
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Intermediate water variability of the subtropical Northeastern Atlantic during 490–424 ka (late MIS 13 and MIS 12)
2022, Marine GeologyCitation Excerpt :G. inflata and G. bulloides have a similar dwelling depth off the western Iberian margin (Salgueiro et al., 2008), so their stable isotope records are comparable. In our previous studies (Guo et al., 2020, 2021), planktonic δ18O curve of Site U1385 was used as a tuning target, but in this study we switched the tuning target to core MD01–2446 because core MD01–2446 had a higher-resolution planktonic δ18O record during MIS 12 relative to Site U1385, which allowed the establishment of a high-resolution stratigraphic framework. Benthic foraminifera were picked and counted from the >125 μm size fraction according to the universal protocol for benthic foraminiferal studies proposed by Schönfeld et al. (2012).