**1. Introduction**

The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents that has an essential role in Earth's climate, redistributing heat and influencing the carbon cycle [1,2]. It is a basin-scale baroclinic ocean circulation with a northward flow of warm water and a cold return flow at depth [3]. During its northward travel, the surface water exchanges heat with the atmosphere, modifying the climate of the Northern Atlantic region and contributing to the relatively mild climate in Europe. This overturning circulation is a meridional plane portrait of a much more complex three-dimensional circulation in the Atlantic, which can be conditionally split into wind-driven and thermohaline circulations [4]. This latter circulation depends in part on oceanic density gradients and hence on temperature and salinity gradients controlled by warming/cooling and evaporation/precipitation at the surface of the ocean.

Progress in the reconstruction of past Atlantic circulation changes has revealed that AMOC reductions coincided with colder episodes within the Last Glacial, especially Heinrich Events [5,6]. Additionally, a prominent chemocline has been identified in the North Atlantic during the Last Glacial Maximum (LGM) and Heinrich Event 1 (H1) [6], which suggests an altered deep-water circulation state. However, so far hardly anything is known about the past subsurface density structure in the North Atlantic [7], while it is still

**Citation:** Zarkogiannis, S.D. Disruption of the Atlantic Meridional Circulation during Deglacial Climates Inferred from Planktonic Foraminiferal Shell Weights. *J. Mar. Sci. Eng.* **2021**, *9*, 519. https:// doi.org/10.3390/jmse9050519

Academic Editor: Christos Stefanakos

Received: 16 April 2021 Accepted: 7 May 2021 Published: 11 May 2021

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**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

poorly constrained beyond the LGM. As this structure is fundamental for understanding deep-water circulation [8], it is critically important that new means are established for assessing changes in oceanic vertical density structures. I present new insight into this structure in the eastern Atlantic from a novel approach that centers on the physical controls on planktonic foraminifera biomineralization through time.

Although the effects of ocean chemistry on plankton are being extensively studied, there is a lack in the literature about the effects of physical oceanic properties such as buoyancy or pressure, which very likely affect foraminifera physiology and morphology [9]. Different foraminifera species have different optimum living depth habitats [10,11], to which they adapt according to the oceanic inhomogeneity. These organisms are able to biosynthesize out of equilibrium with their ambient environment by maintaining chemical gradients [12–14]; however, as plankton they must thus always retain equilibrium with the seawater to remain floating. It can thus be argued that plankton physiology is more sensitive to the physical rather than the chemical characteristics of seawater. In order to inhabit certain depths, planktonic foraminifera should regulate their (cell) density to match that of the surrounding liquid in which they are immersed. Should this not be the case, then the organisms must relocate until they reach a particular density horizon to equilibrate.

Foraminifera may have different strategies (e.g., storage of metabolic gases) for shortterm displacement or micro-positioning in the water column, such as diurnal migrations, but compared to gasses and lipids the most inert way for non-motile plankton to regulate buoyancy in the long term is by biomineralization [15,16]. Based on the foraminiferal need for certain habitat acquisition and recent findings on the influence of surface ocean density on their calcification [17], the application of foraminifera shell weights as a (paleo)seawater density proxy is introduced here as a means to reconstruct paleoseawater density and stratification of the surface Atlantic Ocean and thus the rigorousness of its meridional overturning circulation. For this purpose, synchronous sieve-based shell weights of the planktonic foraminifera species *G. bulloides* are compared across three Atlantic locations. *G. bulloides* is a subsurface, cosmopolitan foraminifera species with a wide use in paleoceanographic studies that thus allowed for the comparison of the results of a new S. Atlantic core with two more shell weight records from the bibliography.

#### **2. Oceanographic Setting of the Core Locations**

The present study involves the analyses of three sediment cores from the eastern margins of the Atlantic Ocean (Figure 1). The southernmost one is GeoB 1710-3 from the northern Cape Basin and was taken from the southwestern African lower continental slope (2987 m). The Cape Basin, located in the subtropical eastern South Atlantic Ocean, is bordered to the east by the African continent, to the north and west by the Walvis Ridge and the Mid-Atlantic Ridge, respectively, and to the south by the Agulhas Ridge (Figure 1). The wind system is almost entirely dominated by the southeast trade winds [18]. Surface waters in the Cape basin may be derived from three different regions: the Indian Ocean's Agulhas current, the South Atlantic current, and the Subantarctic Surface Waters [19–21]. Below the surface currents lies the Antarctic Intermediate Water (AAIW), which spreads to the north between 500 and 1000 m water depth. The dominant modem deep water mass, in the Cape Basin, is a mixture of ~60% Circumpolar Deep Water (CPDW) and ~40% North Atlantic Deep Water (NADW) from the western South Atlantic [22,23]. The relatively warm, saline, southward-flowing NADW is injected in the equatorward-flowing CPDW and extends between about 1700 and 3900 m water depth [24]. Between 1000 and 1700 m, the NADW is overlain by Upper Circumpolar Deep Water (UCPDW) and underlain by Lower Circumpolar Deep Water (LCPDW) [25]. The LCPDW is formed when Antarctic Bottom Water (AABW) is mixed with the slightly less dense overlying water [26]. The extremely cold, oxygen-depleted, nutrient-enriched, of low CO<sup>3</sup> <sup>2</sup><sup>−</sup> and high CO<sup>2</sup> content AABW is encountered below 4000 m [27]. During glacial times, the conveyor circulation was weak, and the abyssal Cape Basin was filled with less corrosive and aged

deep waters [28]. The core location is currently bathed in NADW [27] and must have been so for the last 245 Kyrs [29].

**Figure 1.** Location of the sediment cores and schematic of the Atlantic Meridional Overturning Circulation. Red is the surface flow, blue the deep one, and yellow and green represent transition flows between depths. Terrain after the general bathymetric chart of the Ocean.

GeoB 8502 (19◦13.27' N, 18◦56.04' W) is a Tropical North Atlantic pelagic site at the lower reaches of the Cap Timiris Canyon, approximately 250 km offshore the Mauritanian coast (Figure 1), and was retrieved from 2956 m water depth on the lower Northwestern African continental rise and consists of levee sediments that are predominantly hemipelagic deposits. As part of the Eastern Boundary Current system, the Mauritanian upwelling region is one of the major upwelling areas in the Atlantic Ocean [30]. Along the NW African margin, the temporal dynamics of the coastal upwelling are driven basically by the intensity of the northeast trade-winds, itself dependent on the seasonal Intertropical Convergence Zone (ITCZ) migration [31,32] on a perennial basis, producing cold nutrientrich surface waters with modern sea surface temperatures (SSTs) as low as 16◦C. The main water masses encountered in the upwelling region are the Tropical Surface Water (TSW), the North and South Atlantic Central Waters (NACW and SACW), and AAIW. Both central water masses appear in the permanent pycnocline between depths of 150 m and 600 m at temperatures greater than about 8◦C, below which lies the AAIW [32]. At greater depths, the core sediments are currently bathed in the carbonate-saturated NADW and may have remained so during the glacials [33].

Core ODP 982 was retrieved from the Rockall Plateau, which is an extensive shallow water (~2000 m) area located south of Iceland and west of the British Isles (Figure 1). The surface circulation in the Rockall area is characterized by warm, highly saline water of the North Atlantic Drift Current (NADC), which forms the continuation of the Gulf Stream heading to the Nordic Seas [34]. The NADC is the major surface water component of the AMOC, which is one driving factors behind the global conveyor belt and NADW formation [35,36]. The NADW may be divided into two main components: the upper

NADW, in the intermediate depths, and the lower NADW (deeper than 2000 m). Intermediate depths in the North Atlantic, near the Rockall Plateau, contain three principal water masses between the AAIW, Mediterranean Overflow Water (MOW), and Labrador Sea Water (LSW). The upper NADW consists of a mixture of LSW, MOW, and overflows from the Nordic Seas and is the densest of the intermediate water masses, occupying depths between 1500 and 1600 m in the interior ocean. The lower NADW is composed of a mixture of the dense overflows from the Nordic Seas and LSW [37]. The deepest water mass in the Rockall area (≥ 3500 m) consists of modified AABW, which is characterized by lower salinity than the waters above [38].
