**4. Results**

The record of *G. bulloides* shell mass attained from GeoB 1710-3 for the last 200 kyr shows enhanced climatic cyclicity (Figure 2a) and variability between mass values of more than 100%, but shell weights were consistent between replicates. During the cold MIS 6 interval, shell weights were generally increased but with considerable variation, of up to 80% since some of the lowest mass values in the record are at 148 and 180 ka. During MIS 5e, shell masses are low and they gradually increase after MIS 5d. The lowest shell mass values followed MIS 5b, after which shell weights increase until they reach their maximum value at 20 ka, which possibly marks the LGM interval for this core. During the last deglaciation, mass values drop abruptly and they remained low during the Holocene.

In contrast to GeoB 1710-3, the shell weight record of the lower latitude core GeoB 8502- 2 shows no cyclicity between glacial and interglacial periods the last 200 kyr. *G. bulloides* shell masses are stable, fluctuating only on a small scale ( ± 1.1 µg, 1σ) around an average of 13.4 µg. Lower shell mass weights are found within MIS 6, while values almost increase progressively after MIS 5b. The broad maximum in shell weight centered at the MIS 5/6 boundary of Termination II recorded for approximately 2300 yr, during which shell masses increase by 30% above average, was attributed to contamination clay infillings. In core ODP 982A, shell weights show again a clearer glacial/interglacial pattern, which is less "nervous" than that of GeoB 1710-3. Here, *G. bulloides* shell masses consistently increased at around 17.3 ( ± 1.3) µg during the cold MIS 6 interval. Low mass values are recorded during the warmest MIS 5e interval, after which weights increase gradually during the course of the last glaciation until around 21 ka. At this age, the highest mass values are recorded, denoting the LGM period in this core, after which values drop and decrease even further during the late Holocene.

By using Equation (1), the planktonic foraminifera shell weights were converted to ocean density values and the results for the three records are summarized in Figure 3. The superposition of the three records reveals a convergence in Atlantic Ocean densities for two instances during the last 200 kyr. The first convergence event (SMCE I) takes place after the onset of the last deglaciation around 18.4 ka and the second (SMCE II) around 122.4 ka within the peak of the penultimate deglaciation. During both of these instances, the water densities convergence to the same value of ~1026.82 kg/m<sup>3</sup> and this value is similar to seawater densities reconstructed for the modern core-top samples. It also appears that most of the time the South Atlantic is densest. Prior to SMCE II, density gradients are more or less constant between the different Atlantic localities, while after this convergence event densities start to diverge between the tropical site and the sites at higher latitudes until the LGM when the divergence becomes the maximum between the sites. After SMCE I,

seawater density gradients between the different eastern margins of the ocean alleviate considerably until today.

**Figure 2.** Shell weight records of the last 200 kyr (before present, BP) for: (**a**) core GeoB 1710-3 from the eastern South Atlantic; (**b**) core GeoB 8502-2 from the eastern Equatorial Atlantic [40]; and (**c**) core ODP 982 from the eastern North Atlantic. The 1σ confidence interval for each shell mass plot is shown as a color-shaded area. Numbers refer to Marine Isotopic Stages (MIS) or substages and grey-shaded areas interglacial periods. Data are available in Supplementary Table S1.

The results from the intervals that surround the two SMCE events are summarized in Table 2. The average *G. bulloides* shell masses during SMCE I across the different sites are 13.9 ( ± 0.5) µg and 13.7 ( ± 0.2) µg during SMCE II, which equal to almost identical Atlantic seawater densities (1026.86 and 1026.79 kg/m<sup>3</sup> , respectively) also between the two intervals. The density value of ~1026.8 kg/m<sup>3</sup> to which Atlantic seawater densities converged on average during the two SMCEs resembles that of the modern ocean. The weight-derived Atlantic seawater densities are comparable within error to geochemically reconstructed seawater density values from combined Mg/Ca and δ <sup>18</sup>O measurements on the weighed *G. bulloides* specimens from core GoeB 8502-2 for this interval.

**Figure 3.** Atlantic Ocean density reconstructions for the last 200 kyr based on planktonic foraminifera shell weights from three different sites. Note the convergence in seawater density/planktonic foraminifera shell mass values for two instances in the records and how these values match the modern seawater densities.



The results from the CT analyses are summarized in Table 3. The specimens that were available for CT scanning are from the GeoB cores. The analysis mainly focused on the specimens from SMCE I and II intervals and reveal other physical characteristics of the specimens with the same mass across the Atlantic basins and time intervals. In addition to these intervals, LGM specimens from core GeoB 1710-3 displayed the highest shell mass in the studied records. The CT analyses showed that contamination by sediment infilling of specimens is minimal (only ~5% by volume) and thus the reported shell mass values are

not artifacts. Furthermore, the inspection of the tomographs (found in the data availability statement section below) verified the good preservation of the specimens and thus the shell weight variation due to dissolution can be considered minimal too.

**Table 3.** Biometric data of foraminifera from weighing and XµCT analysis. Test thickness is the ratio of calcite volume to calcite surface, and shell and test density are the ratio of the average shell weight to test and shell volume, respectively. Sediment infilling is the specimen's internal volume percentage occupied by sediment impurities. The µCT analysis results for individual specimens are given in Supplementary Table S2.


The merging of shell masses is also reflected in a convergence in the mean wall thickness of the shells at ~5 µm (Figure 4a–h). Although the shell thickness is similar, during SMCE I shell masses are slightly more elevated than during SMCE II but the shells are found to be more voluminous. Thus, the shell thickness remains the same because mass changes are compensated for by changes in total volume, and this is reflected in the shell density values of Table 3. Furthermore, during convergence events the foraminifera shell comprise 23% of the total cell, while the overall shell density (i.e., volume-normalized weight) varies only by a little and is on average 0.62 g/cm<sup>3</sup> . The test density (an indication of test porosity) of SMCE I individuals is slightly increased compared with that of SMCE II. Test density values vary around the values of the calcite's density.

**Figure 4.** X-ray tomographs of *G. bulloides* specimens of cores GeoB 8502-2 and GeoB 1710-3; (**a–d**) specimens from shell mass convergence event SMCE I; (**e–h**) specimens from SMCE II; and (**i–j**) Last Glacial Maximum specimens from core GeoB 1710-3.

The LGM specimens of GeoB 17010-3 exhibit different characteristics to those of the SMCEs (Table 3). Their mass almost doubles and so does their shell wall thickness. This is also visually evident in their tomographs of Figure 4i,j, where the specimens are found to be heavily encrusted. Internal chamber walls are very delicate and thin (Figure 4j), while in most of the cases these chamber walls are completely dissolved and are thus not evident within the shell (Figure 4i). LGM shells are ~20% more voluminous that the others and this extra volume is due to the increase in their shell calcite, which now comprises the 42% of the total cell volume (Table 3), while the total volume of the cavities that are filled with protoplasm is similar to the SMCE specimens (Supplementary Table S2). The overall shell density during the LGM is increased by more than 50%, while the test itself is found to be less dense and thus more porous (Table 3).
