Forcing of western tropical South Atlantic sea surface temperature across three glacial-interglacial cycles
Introduction
The western tropical Atlantic (WTA) is the major source of heat and salinity for the northward-flowing surface branch of the Atlantic Meridional Overturning Circulation (AMOC). A number of modelling studies have demonstrated that a sluggish northward heat transport during phases of weak AMOC causes warm water masses to accumulate in the WTA (particularly south of the equator), resulting in anomalous SST warming (Chang et al., 1997; Dahl et al., 2005; Vellinga and Wood, 2002; Yang, 1999). This suggests that the surface layer of the WTA may function as an important reservoir for excess heat and salinity when northward heat transport is reduced. Paleoclimate reconstructions of the last deglaciation reveal that when the AMOC was in an “off” mode (e.g., Heinrich Stadial 1) (Böhm et al., 2015), millennial-scale warming events occurred in the tropical and subtropical latitudes of the South Atlantic western boundary (Chiessi et al., 2015; Rühlemann et al., 1999; Weldeab et al., 2006), while concurrent cooling occurred in the midlatitude North Atlantic (Bard et al., 2000). These proxy studies thus confirm the ability of the western tropical and subtropical South Atlantic to store surplus heat during abrupt phases of severe AMOC weakening (Chiessi et al., 2015; Weldeab et al., 2006).
While the ability of the WTA to respond to AMOC disruptions by storing excess heat is well- documented for Heinrich Stadial 1 (Arz et al., 1999; Chiessi et al., 2015; Rühlemann et al., 1999; Weldeab et al., 2006), there is a shortage of long-term records encompassing several glacial/interglacial cycles. Hence, it is uncertain whether glacial-interglacial variations in the strength of the AMOC (i.e., heat export to the North Atlantic) played an important role in the long-term thermal evolution of the southern WTA or if other climatic processes were more influential at these timescales. There is documented evidence of prolonged SST warming in the western subtropical South Atlantic during the penultimate and last glacials in response to a glacial-weakening of the AMOC (Santos et al., 2017). However, this SST record is likely not representative for the tropics as it may be influenced by the northward intrusion of water masses from the mid-latitude Brazil-Malvinas Confluence (Campos et al., 1996; Santos et al., 2017). Additionally, given that past upwelling at this study site has been of comparable intensity to the most important upwelling zone along the coast of southeastern Brazil, SST variations observed at this locality may reflect a local signal (Lessa et al., 2017, Lessa et al., 2019). As such, it remains unknown whether a glacial reduction of the overturning circulation can likewise produce significant surface changes in the tropical latitudes of the western boundary. Previous SST records from the tropical oceans have shown a dominance of 100-kyr cycles attributed to glacial-interglacial variations in atmospheric CO2 concentration (Herbert et al., 2010; Lea, 2004). Therefore, we would expect direct radiative forcing by CO2 to have a similarly strong influence on SST in the southern WTA. In addition, glacial reductions in atmospheric pCO2 have been shown to enhance the intra-hemispheric thermal gradient (Broccoli and Manabe, 1987), which in turn influence the intensity of tropical trade winds (Broccoli and Manabe, 1987; Mix et al., 1986; Rind, 1998). Glacial-interglacial variations in the strength of the southern hemisphere trade wind system are well-documented particularly from eastern boundary upwelling regions along the coast of Africa (Little et al., 1997a, Little et al., 1997b; Shi et al., 2001; Stuut et al., 2002). While a glacial intensification of the southern trades has been shown to produce cool SST anomalies along the eastern boundary, it is rather unclear if and how glacial-interglacial changes in their strength influences SST in the southern WTA.
An important consequence of surface hydrographic changes in the WTA are potential changes in the terrestrial hydroclimate of South America. Reconstructions of continental precipitation indicate that the build-up of excess heat in the southern WTA produced millennial-scale wet periods over the otherwise semi-arid northeastern Brazil via a southward displacement of the Intertropical Convergence Zone (ITCZ) (Mulitza et al., 2017; Wang et al., 2004). Hence, future SST changes in the WTA in response to global climate change may impact human societies and natural ecosystems in regions prone to drought, such as northeastern Brazil (Marengo and Bernasconi, 2015). In order to mitigate the potential negative consequences that may arise from WTA SST anomalies, it is of utmost societal and ecological importance to accurately predict how anthropogenic forcing (e.g. higher greenhouse gas concentrations) will affect WTA SST. Investigating the long-term thermal evolution of this region will provide information regarding the sensitivity of WTA SST to climatic perturbations, its modes of variability, as well as the major climatic drivers of WTA SST evolution. The lack of SST reconstructions of sufficient temporal length and resolution, particularly from the southern WTA sector, however, has thus far prevented a thorough investigation of the long-term thermal evolution of this region. Therefore, additional SST records spanning multiple glacial-interglacial intervals from the southern WTA are urgently needed.
Here, we present a 322-kyr, high-resolution SST reconstruction of the southern WTA, off the coast of eastern Brazil using planktic foraminiferal Mg/Ca ratios. The study site is ideally suited for reconstructing regional-scale oceanographic processes within the WTA as it is located within the western tropical Atlantic warm pool, away from zones of maximum upwelling on the Brazil margin (Castelao and Barth, 2006). To reconstruct surface ocean conditions in the southern WTA, we used the planktic foraminiferal species Globigerinoides ruber (pink) which lives in the upper mixed layer of the tropical and subtropical waters (Chiessi et al., 2007; Steph et al., 2009; Tolderlund and Bé, 1971). Using our new WTA SST record, we investigate the role of atmospheric pCO2, AMOC strength and Southern Hemisphere trade-wind intensity in WTA SST evolution across multiple glacial-interglacial cycles and propose a mechanism through which WTA SST responds to climatic forcing.
Section snippets
Hydrography of the western tropical South Atlantic
The surface mixed layer of the western tropical South Atlantic is bathed by warm, saline and oligotrophic Tropical Water (T > 20 °C, S > 36.40) (Castro et al., 2006; Stramma and England, 1999) . At the study site, the modern annual mean SST and sea-surface salinity is 26.02 ± 0.51 °C (Locarnini et al., 2013) and 37.11 ± 0.135 (Zweng et al., 2013), respectively. The present-day amplitude of seasonal SST variability is ~3 °C, with the warmest temperatures (27.62 ± 0.30 °C) occurring during the
Piston core M125-55-7 and parallel multicore M125-55-8
Piston core M125-55-7 (1175 cm length) was retrieved from 1960.8 m water depth in March/April 2016 during R/V METEOR cruise M125. The coring site (20°21.807′S, 38°37.387′W) is located off eastern Brazil, close to the Rio Doce debouchment (Bahr et al., 2016) (Fig. 1). Core M125-55-7 consists of clay to silty clay with bioclasts with sediments varying between dark and light grey in colour without visible hiatus. Since the upper ~35 cm of core M125-55-7 was disturbed during coring, the parallel
Results
The mean SSTMg/Ca value over the past 322 kyr is 26.4 °C (Fig. 3c). The coldest recorded SSTMg/Ca value (23.8 °C) occurred at ~9 ka and the warmest SSTMg/Ca value (28.8 °C) occurred at ~107 ka. Our core-top data from multicore M125-55-8 yields an average modern SSTMg/Ca value of 27.2 °C, which is slightly higher but still within error consistent with the present-day mean annual SST of 26.02 ± 0.51 °C at Site M125–55-7. While the M125-55-7 SST record does not display a pronounced
Long-term Site M125-55-7 SST evolution driven by regional processes of the WTA
To investigate whether the variability in our SST record is more sensitive to local processes of the eastern Brazilian margin or to large-scale climatic drivers of the tropical Atlantic, we compared our record to the SST record from core GL-1090 located downstream to our core site within the BC (Santos et al., 2017) and to SST records of similar time span and resolution from the western tropical North Atlantic (Site 999; western Caribbean Sea) (Schmidt et al., 2006) and the equatorial Atlantic
Conclusions
Our SST record from Site M125-55-7 indicates that the surface mixed layer of the southern WTA experienced low-amplitude variations with generally warm tropical temperatures over the past 322 kyr. The similarity between our SST record and the SST record from Site 999 (western Caribbean Sea) indicates that Site M125-55-7 is influenced by regional processes driving the thermal evolution of the WTA. Based on the lack of clear glacial-interglacial variability in our record, we propose that
Data availability
The original data reported in this paper is archived in Pangaea (https://doi.pangaea.de/10.1594/PANGAEA.905301).
Declaration of Competing Interest
None.
Acknowledgements
We kindly acknowledge the support by captain and crew of the R/V METEOR during expedition M125. We thank Bernd Knape and Christian Scholz for assistance with stable isotope and ICP-OES analyses, respectively. Andrea Jaeschke provided important contributions during manuscript preparation. We are grateful to the S. Szidat and the LARA team at the University of Bern for providing the 14C dates. AB was funded by the Deutsche Forschungsgemeinschaft (DFG) via Grant BA 3809/9-1. CMC acknowledges the
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