It has been hypothesized that global temperature trends are tightly linked
to tropical thermocline depth, and that thermocline shoaling played a
crucial role in the intensification of late Pliocene Northern Hemisphere
glaciation. The Pliocene thermocline evolution in the Pacific Ocean is well
documented and supports this hypothesis, but thermocline records from the
tropical Atlantic Ocean are limited. We present new planktonic foraminiferal
The Pliocene (5.33–2.58 Ma) is the most recent geological epoch with substantially higher greenhouse gas concentrations (de la Vega et al., 2020; Martínez-Botí et al., 2015; Stap et al., 2016) and elevated global surface temperatures (Brierley et al., 2009; Dowsett et al., 2016; McClymont et al., 2020) compared to preindustrial times. This makes it an interesting interval to study for its potential analogies with our future climate (Burke et al., 2018). Typical features of the warm Pliocene ocean are the poleward expansion of the tropical warm pools and a reduction of the zonal sea surface temperature (SST) gradient in the Pacific (Brierley et al., 2009; Dekens et al., 2008; Fedorov et al., 2013). Another key feature of the Pliocene ocean is the deep tropical Pacific thermocline that gradually shoaled towards the end of the Pliocene (Dekens et al., 2007; Ford et al., 2012, 2015; Steph et al., 2010), while the meridional and zonal SST gradient steepened (Fedorov et al., 2015). It has been suggested that this shoaling reached a critical threshold around 3 Ma and played an important role in the onset of Northern Hemisphere glaciation (Fedorov et al., 2006; Philander and Fedorov, 2003; Steph et al., 2010). Moreover, because there is general coherence between Pliocene changes in meridional, zonal, and vertical (i.e., thermocline depth) temperature gradients in the Pacific, Fedorov et al. (2015) suggested that these temperature gradients are somehow mechanistically linked.
Interestingly, while the thermocline shoaled across the tropical Pacific,
We present the first records of late Pliocene thermocline depth variability
in the EEA. We generated
We used sediments recovered at ODP Site 959 during Leg 159 in the Gulf of
Guinea at
General surface currents and upwelling areas in the Eastern Equatorial Atlantic. Blue shading: coastal upwelling in boreal summer; pink shading: equatorial upwelling; orange shading: permanent coastal upwelling. NEC: North Equatorial Current; NECC: North Equatorial Counter Current; CC: Canary Current; GC: Guinea Current; SEC: South Equatorial Current; EUC: Equatorial Undercurrent; SECC: South Equatorial Counter Current; BCC: Benguela Coastal Current. Green stippled lines mark the annual range of the Intertropical Convergence Zone (ITCZ). Figure after Norris (1998), Wagner (1998), and Wiafe and Nyadjro (2015). Map generated with Ocean Data View (Schlitzer, 2019).
The thermocline is typically defined as the depth at which the vertical
temperature change is at its steepest. This depth can be approximated with
the 20
Conceptual manifestation of the thermocline depth as a
Samples were taken at 5–20 cm intervals from cores 959C-5H and 959C-6H
between 35.77 and 48.73 revised meters composite depth (rMCD; splice by
Vallé et al., 2016). The studied interval of nannofossil or foraminifer
ooze (Mascle et al., 1996) was dated using oxygen isotope stratigraphy (van
der Weijst et al., 2020) and spans 3.5–2.8 Ma. The preservation of
planktonic foraminifera in the Pliocene of 959 is generally very good; a
large proportion of the tests have a “glassy” appearance, and
For each measurement, 50–60 foraminiferal tests (
Sediment samples were wet-sieved with tap water and dried overnight at a low
temperature. Per sample, between 20 and 60 specimens of the surface-dwelling
Ambient seawater temperature is typically the dominant control on the ratio
of magnesium to calcium in foraminiferal shells (e.g., Hönisch et al., 2013; Tierney and Malevich, 2019). At present, there is no standard approach
to calibrate
Here, we apply a constant Pliocene
Late Pliocene
The
The decreasing surface–subsurface
Besides analytical and calibration errors, several confounding factors limit
the use of
In addition to ambient temperature, calcite
The magnitude of variability in the
In the absence of major circulation changes, the
Our results show that the Eastern Equatorial Atlantic thermocline and
nutricline deepened following the warmest mPWP interglacials, while SST and
deep ocean records show global cooling (Herbert et al., 2016; Lisiecki and
Raymo, 2005). Late Pliocene thermocline and nutricline changes are similar
at Site 959 in the Eastern Equatorial Atlantic and Site 1000 in the western
tropical Atlantic (Fig. 5), indicating that tropical thermocline deepening
occurred across the entire basin. To further explore global patterns of
Pliocene tropical thermocline movements, we connect our new Site 959
Compilation of tropical
It was suggested by Fedorov et al. (2015) that global temperature trends are tightly linked to tropical thermocline depth and that thermocline shoaling played a crucial role in the onset of Northern Hemisphere glaciations around 3 Ma (Fedorov et al., 2006). Heat that is gained in the tropics must be balanced by heat loss at high latitudes, and because more heat can be gained by an ocean with a shallow thermocline, tropical thermocline shoaling should lead to surface ocean cooling at high latitudes (Boccaletti et al., 2004; Fedorov et al., 2006; Philander and Fedorov, 2003). While this theory is supported by the majority of the Pacific thermocline records (Ford et al., 2012, 2015; LaRiviere et al., 2012; Steph et al., 2006b, 2010), it appears to be inconsistent with basin-wide deepening of the tropical Atlantic thermocline (Fig. 6). The link between ocean SST gradients, tropical thermocline depth, and ocean heat transport may be different in the Atlantic and Pacific oceans, because the asymmetric geometry of the Atlantic basin leads to a different pattern of tropical thermocline ventilation (Harper, 2000). Also, Atlantic Meridional Overturning Circulation (AMOC) leads to northward ocean heat transport in both hemispheres, in contrast to the Pacific Ocean (Forget and Ferreira, 2019). However, because the tropical Pacific is larger than the tropical Atlantic, Pacific thermocline shoaling may have been sufficient to balance heat loss at high latitudes, even if partly counteracted by Atlantic thermocline deepening.
It has been hypothesized that Central American Seaway (CAS) closure played a major role in the global tropical thermocline depth (Steph et al., 2010; Zhang et al., 2012). In this scenario, salt transport to the North Atlantic increased as a consequence of reduced inflow of relatively fresh Pacific surface waters, which in model simulations promotes the production of NADW (Lunt et al., 2007; Sepulchre et al., 2014; Steph et al., 2010; Zhang et al., 2012). Steph et al. (2010) reasoned that an increased volume of NADW was associated with a greater volume of the “cold water sphere”, which raised the tropical thermocline everywhere except for the Caribbean region. There, reduced inflow of cold Pacific waters caused the thermocline to deepen locally. However, our study shows that tropical thermocline deepening occurred across the basin (Fig. 6). Even if CAS closure had promoted NADW production and/or AMOC strength, it is not clear how this, in turn, would have affected tropical thermocline depth, as both negative (Lopes dos Santos et al., 2010) and positive (Venancio et al., 2018) relationships between thermocline depth and AMOC strength have been inferred from proxy data. Moreover, it is unclear if CAS closure could have had an opposite effect on tropical Atlantic and Pacific thermocline depths, as this conflicts with modeling studies (Steph et al., 2010). A dominant role of CAS closure in shaping the tropical thermocline evolution in the Atlantic and Pacific basins is not consistently supported by data and models. Future work could be aimed at modeling the spatial extent of the region where the inflow of cold Pacific waters is reduced in response to CAS closure. Moreover, it is vital to constrain the chronology of CAS closure (Montes et al., 2015; O'Dea et al., 2016) and determine during which intervals this process might have had extensive and long-lasting oceanographic effects (Molnar, 2008; Sepulchre et al., 2014).
Regionally sorted midlatitude temperature records.
Tropical thermocline waters are sourced from midlatitude surface waters
(Harper, 2000). It has therefore been suggested that extratropical cooling
contributed to the Pliocene shoaling of the tropical Pacific thermocline
(Ford et al., 2012, 2015). During the early Pliocene, midlatitude SSTs were
indeed generally higher than preindustrial (Brierley et al., 2009). However,
Pliocene midlatitude temperature evolutions were regionally variable
(Herbert et al., 2016; Karas et al., 2017), with a notable asymmetry between
the Northern and Southern Hemisphere (Pontes et al., 2020). At present,
Pacific thermocline waters are sourced from midlatitude surface waters in
both the Northern and Southern Hemisphere, whereas in the Atlantic, the
thermocline is predominantly sourced from the Southern Hemisphere as a
consequence of the asymmetric basin geometry (Harper, 2000). Midlatitude
SST records from potential Pacific tropical thermocline source regions
predominantly register cooling between 5.5 and 2.8 Ma. In contrast, South
Atlantic Site 1088 registers warming during the latest Miocene–early
Pliocene, and during the mPWP (Fig. 7), in tandem with tropical Atlantic
thermocline deepening. However, South Atlantic cooling between
Tropical thermocline depth is also affected by cyclone activity, which, in turn, is linked to the latitudinal SST gradient in a positive feedback mechanism (Fedorov et al., 2010). In other words, source region SSTs are potentially relevant to tropical thermocline dynamics in terms of both stand-alone trends, and in the context of distant SST trends. Tropical cyclones force vertical mixing in the upper ocean, which deepens the tropical thermocline (Bueti et al., 2014; Jansen et al., 2010). Tropical cyclone activity was higher in the early Pliocene than at present, especially in the Pacific (Fedorov et al., 2010). Gradually increasing zonal and meridional SST gradients (Fedorov et al., 2015) would have promoted stronger Walker and Hadley circulation, thereby reducing tropical cyclone activity and raising the tropical thermocline (Brierley et al., 2009; Fedorov et al., 2010). The Pacific underwent a stronger reduction in tropical cyclone activity than the Atlantic (Fedorov et al., 2010), which may be linked to the contrasting evolution of the Pacific and Atlantic tropical thermocline depth. This feedback mechanism between midlatitude temperatures and tropical cyclone activity could further be explored in relationship to tropical thermocline depth. The hypothesized link should be tested in model simulations and could additionally be supported or refuted by better proxy records from midlatitudes.
Our new
New Site 959 data are available as a supplement to this paper and will be uploaded to the PANGAEA online data repository upon publication.
The supplement related to this article is available online at:
The study was designed and the paper drafted by CMHvdW. Data was generated by CMHvdW, JW and WdN. AvdH, GJR, FS and AS contributed to data interpretation, scientific discussion and writing the final paper.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We thank the International Ocean Discovery Program and the predecessors for samples and data as well as Arnold van Dijk (UU), Cindy Remijnse-Schrader (UU), Wim Boer (NIOZ), and Geert-Jan Brummer (NIOZ) for technical support and advice. We are also grateful to editor Luc Beaufort (Cerege) and two anonymous reviewers for constructive criticism on an earlier draft of this work.
This work was carried out under the program of the Netherlands Earth System Science Centre (NESSC; grant no. 024.002.001), financially supported by the Ministry of Education, Culture and Science (OCW). Appy Sluijs has been supported by the European Research Council for consolidator grant no. 771497.
This paper was edited by Luc Beaufort and reviewed by two anonymous referees.