*5.1. Foraminifera Shell Weights Are Tracers of Past Oceanic Circulation*

The sieved-based weight analysis of *G. bulloides* shells from the South Atlantic core GeoB 1710-3 during the last 200 kyr was combined with two additional eastern Atlantic shell weight records and revealed reorganizations in the meridional circulation of the Atlantic Ocean. The measured foraminifera shell masses were converted to seawater density values using a lineal relationship that was calibrated for the Atlantic Ocean using geochemical data [17]. The mass-based reconstructed seawater densities from the three different eastern Atlantic localities describe their hydrography and unveil two occasions, SMCE I and SMCE II, when the seawater densities between these regions appear similar (Figure 3). Interhemispheric convergence of surface Atlantic densities denotes the absence of seawater density gradients across the basins and thus momentary cessation of the Atlantic Meridional Overturning Circulation (AMOC), which implies an increase in surface ocean stratification [49] and abrupt, large changes in climate [50].

The basin-scale reconstructions of seawater densities using planktonic foraminifera shell mass measurements have enabled the consideration of several aspects of the Atlantic hydrography. Figure 3 showed that planktonic foraminifera can alter their shell mass considerably and verifies that the degree of this alteration in time is a function of latitude [51] with no overall response to atmospheric *p*CO2. The water from the S. Atlantic site is generally found to be denser than the other two eastern areas. This illustrates the northward water movement towards less dense regions, which is known to feed the northern Atlantic latitudes with southern-sourced waters [52]. The high S. Atlantic densities of enhanced variability may be the result of the site's location offshore the Kalahari Desert, which as a hyper-arid area would favor the development of high salinity in the sea water, while pulses of leaking warmer and thus lighter Agulhas waters may contribute to abrupt density variabilities (see Section 5.2). Seawater densities at the tropical Atlantic site GeoB 8502-2 are stable throughout the studied interval and *G. bulloides* specimens were the lightest between records. As discussed in Zarkogiannis*,* et al. [40], this is a tropical region of insolation and climatic stability, under the influence of the ITCZ, which is a zone of enhanced precipitation and of low-density surface waters [53].

Planktonic foraminifera shell weights start recording the glacial/interglacial cyclicity signal at mid-latitudes or at latitudes that are more sensitive to insolation changes. That seawater densities are here reconstructed higher for the site at 23◦ S (GeoB 1710-3) than that of 57◦ N (ODP 982A) might imply that the overturning circulation and oceanic density gradients may not respond directly to the amount of summer insolation falling across northern high-latitude regions. They are possibly dictated by moisture fluxes from Hadley cells, driven by the Earth's latitudinal insolation gradient (LIG) [54] and the latitudinal temperature gradient (LTG) that it creates, which drives the poleward energy flux via the atmospheric and ocean circulation [55]. Site GeoB 1710-3, where the highest shell weights were recorded, is located directly at the descending limb of the Hadley cell. Furthermore, since the LGM is an arid period [56,57] and may have been more arid that the penultimate glacial maximum [58], the highest-density waters (of increased salinity and decreased

temperature) were to be expected in the Atlantic basins of the mid-latitudes (Figure 5a). The response of planktonic calcification to high-density waters is notably manifested in Figure 4i,j.

Furthermore, Figure 3 suggests that the Atlantic Ocean was slightly lighter during MIS 6 than the subsequent glaciation as it can been seen in all three records. The penultimate glacial, MIS 6, appears to have been approximately as extreme as the last ice age in terms of global ice volume and sea level [59,60]. Yet, unlike the last glacial interval, no major ice-rafted debris (IRD) deposits, known as Heinrich events [61,62] that suggest increased iceberg formation, were recorded prior to the penultimate deglaciation in the Atlantic [63,64], while on the Iberian margin, MIS 6 has been described as a warmer glacial interval compared with the last ice age [63,65]. Thus, for occupying the same volume at higher temperatures, its density should have been reduced. Except for this glacial interval, which raises arguments that the interocean salt leakage is not as straightforward as previously suggested [66], the South Atlantic has most of the time been the densest of the three basins as it is today [67]. Furthermore, its record appears to be "nervous" with increased variability.

The most important outcome of Figure 3 in terms of abrupt Atlantic hydroclimate changes may however be the identification of intervals of surface seawater density convergence. Independent indications of a collapse of the Atlantic meridional circulation during SMCE I and SMCE II are provided by geochemical evidence of <sup>231</sup>Pa/230Th records from N. Atlantic cores. <sup>231</sup>Pa/230Th is a kinematic proxy for the meridional overturning circulation. For a given scavenging rate, lower rates of AMOC in the past would result in comparatively less <sup>231</sup>Pa export from the Atlantic and in higher sedimentary <sup>231</sup>Pa/230Th, reaching a maximum of 0.093 for a total cessation [68]. In the present study, the first instance of surface ocean density gradient attenuation is recorded at ~122.4 ka (SMCE II), shortly after the peak in the last interglacial (MIS 5.5) within a cold climatic phase [69,70]. During the same interval, the sedimentary record of core MD95-2037 from the north central Atlantic shows increased <sup>231</sup>Pa/230Th values that also indicate cessation of the overturning circulation in the region (Figure 2 in [71]).

According to the density differences between the Atlantic regions of Figure 3 and in line with previous findings, the overturning rate at the ocean surface was weak during MIS 5e, and a change to a more vigorous circulation pattern occurred mostly during the glacial inception, i.e., the transition from MIS 5.5 to MIS 5.4 [71–73]. Hence, during the warm optimum of MIS 5.5, the structure of the AMOC was similar to the modern one (Figure 5b). The records are also in agreement for a sluggish Atlantic circulation at ~100 ka, since the weak density gradients that appear in Figure 3 are synchronous with an increase in the <sup>231</sup>Pa/230Th records [71]. Nevertheless, the weight-based reconstructed density gradients show that the glacial Atlantic circulation mode started after 120,000 years ago with an increase in the overturning rate [71] and thus a more vigorous behavior of the AMOC [74,75] following the decrease in Northern Hemisphere summer insolation, which favored the initiation of ice-sheet growth [76].

**Figure 5.** The hydrographic properties of water parcels are strongly influenced by the atmosphere through air–sea interaction and once subducted from the surface layer the density (temperature and salinity) of a water parcel is conserved and remains constant since mixing across isopycnal surfaces is generally much weaker than mixing on isopycnal surfaces [77]. In oceanic regions of normal salinity conditions, the latitudinal temperature changes determine the density of the subducted waters. (**a**) During extreme glacial conditions, such as the LGM, enhanced atmospheric thermal gradients increase the number of seawater subduction zones that lead to enhanced oceanic density gradients (Figure 3) and thus more vigorous oceanic circulation; (**b**) during deglacial times of relaxed thermal gradients, surface ocean stratification prevails that can lead to the cessation of the oceanic circulation (SMCEs). Note the density difference between the glacial and interglacial ocean, which can be up to ~3.5 kg/m<sup>3</sup> [78] due to ice cap buildup. However, the seawater density of tropical regions under the influence of the ITCZ is expected to be stable with perhaps only small glacial/interglacial variations.

The surface ocean density values during MIS 5e (~1026.8 kg/m<sup>3</sup> ) were found to be remarkably similar to the values recorded for SMCE I during Termination I (~18.4 ka) and for most of the Holocene. <sup>231</sup>Pa/230Th radiochemical data from sediment core OCE326- GGC5 from the western subtropical Atlantic also provide independent evidence of a collapse and rapid resumption of the AMOC [5]. In that <sup>231</sup>Pa/230Th record, the circulation is found to momentarily cease at 17.5 kyr ago, during the H1 stadial. This is a few hundred years later than suggested from the convergence of the planktonic foraminifera shell weight records. This discrepancy may be due to the difference in the resolution of the records and/or because the response time of sediment <sup>231</sup>Pa/230Th to changes in circulation is ∼500 yr [68,79]. Thus, the <sup>231</sup>Pa/230Th profiles lag the information of surface ocean circulation changes that is provided by the foraminifera proxy.

The two records also agree in the duration of the shutdown of the AMOC, which appears to last 2000 yr by the <sup>231</sup>Pa/230Th signal and 1600 to 2000 years in the foraminiferal records. The synchrony between changes in <sup>231</sup>Pa/230Th and foraminifera weights suggests a connection between the AMOC and local surface hydrography, and the inferred nearcessation of AMOC during early deglaciation appears to be directly linked to the freshening and increased buoyancy of Northern Atlantic surface water during the H1 [80–82]. During this event, decreasing *G. bulloides* shell weights were recorded in three additional cores from

the N. Atlantic, all converging towards the beginning of the Holocene [43]. These findings thus support earlier suggestions that melt water associated with catastrophic iceberg discharge freshened the high-latitude surface ocean, stabilized the water column, and weakened the AMOC [83,84]. The results indicate that the effect was dramatic, resulting in a near-total collapse of the AMOC possibly during periods of substantial regional cooling.

Present planktonic foraminifera shell weight records reveal an attenuation of surface Atlantic density gradients after SMCE 1 (~18.4 ka), and this is supported by investigations that provide ample evidence for large-scale glacial and deglacial AMOC reorganizations [85–87]. While discrepancies remain between the different studies regarding the timing, overall tendency, and amplitude of circulation variations [88], there is a general consensus that AMOC variability was subdued during the Holocene compared with the last glacial termination [89,90]. This is also supported by the present record, where similar shell weight values are shown to characterize entire Holocene sections (14.1 ± 0.6 µg), suggesting no major changes in AMOC during at least the past 18 kyr (Figure 3). The slightly steeper gradients during the early Holocene imply a weakly increased rate of AMOC, supporting previous findings [5], but the convergence of shell mass values in the most recent sediment samples supports recent studies that suggest a decline in the AMOC strength during the past centuries [91–93]. However, these results must be confirmed with additional high-resolution analyses.
