*5.2. Assessing Atlantic Planktonic Foraminifera Shell Mass Records as Seawater Density and AMOC Proxies*

The use of planktonic foraminifera shell mass as a seawater paleodensity proxy has revealed changes in the circulation mode of the Atlantic Ocean and has provided estimates of absolute past density values of the seawater column at the depths where *G. bulloides* live when they attain a cell size of 300–355 µm. The average Atlantic Ocean density during the periods of quasi-total cessation of the AMOC was found to be ~1026.82 kg/m<sup>3</sup> . This value is similar to 1026.72 kg/m<sup>3</sup> , which is the average density reconstructed for the modern ocean by the three core top samples and refers to a depth of 130–170 m in the study area today [94]. Such depths are in the range of the calcification depths reported for *G. bulloides* for the southern Atlantic [95], the Indian [96] and Southern Ocean [97], and the Atlantic [98,99], although for the N. Atlantic shallower depths have also been reported [100]. Since calcification depths change with ontogeny and thus with size [99,101], it is reasonable to assume that specimens as large as 355 µm calcify at greater depths and so the proposed reconstructions are plausible. However, like many applications of transfer functions on fossil foraminifera to reconstruct past environments, the shell-weightbased seawater density reconstructions fundamentally rely on the assumption of stable environmental niches in both space and time. Niche stability in environmental space is found to be greatest for subpolar species such as *G. bulloides* across glacial and interglacial intervals [102].

Planktonic foraminifera shell mass biomineralization responds very differently to atmospheric *p*CO<sup>2</sup> with latitude (Figure 2a,c) if at all (Figure 2b). Shell weight variability is greater at intermediate latitudes and especially at those related to the descending limb of the Hadley circulation, such as the region of core GeoB 1710-3. The increased shell masses recorded during the last glacial maximum could also have been attributed to enhanced upwelling, a wind-driven oceanic circulation, of denser water masses during the LGM but this is definitely not the case for core GeoB 8502-2, which is beneath one of the major upwelling areas in the Atlantic Ocean [30]. Furthermore, the intervals of shell weight convergence between the records would suggest an instantaneous decline in the wind-driven circulation and its attenuation of 18 ka for which no evidence exists. Alternatively, the shell weight variations are overall better explained by changes in the thermohaline circulation that match the <sup>231</sup>Pa/230Th geochemical indications of AMOC cessation and thus increased shell masses are related to densification of the Atlantic waters. The geochemically reconstructed density estimates for SMCE II from GeoB 8502-2 in Table 2 agree within error (which is ± 1.73 kg/m<sup>3</sup> [17]) with the weight-based density estimates;

however, more geochemical analyses of the present records are required to confirm the accuracy of the results.

#### **6. Conclusions**

Species-specific planktonic foraminifera shell weights have the potential to be a valuable tool for the determination of surface ocean paleoseawater circulation and thus a powerful proxy for physical paleoceanographic applications in paleoclimatology. The use of planktonic foraminifera shell weight as a direct seawater paleodensity proxy has revealed two intervals in the Atlantic Ocean during the past 200 kyr, when the meridional circulation may have been disrupted momentarily due to the absence of interhemispheric seawater density gradients in the Atlantic. Shell-weight-based seawater density values not only converge in the three studied cores within these extraordinary intervals but they also converge between both of these intervals to the same value, which also characterizes the modern Atlantic Ocean density at the same depth horizons. After the convergence at 18 ka, the Atlantic seawater density gradients alleviate and this weakening suggests a decline in the AMOC strength with only small variability thereafter.

Furthermore, it confirms that the surface South Atlantic has always been denser [67] and that, after the last convergence (at ~18.4 Ka) towards the Holocene, the interhemispheric surface Atlantic density gradients alleviate, possibly suggesting an attenuated AMOC thereafter.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/jmse9050519/s1, Table S1: *G. bulloides* sieve (300-355 µm) based shell weights from core GeoB 1710-3, Table S2: Biometric analyses of XµCT scanned *G. bulloides* specimens.

**Funding:** This research was published during my research fellowship at the University of Oxford funded by the Royal Society.

**Data Availability Statement:** The raw tomographic data from the XµCT scanning of *G. bulloides* specimens from core GeoB 8502-2 can be found at 10.6084/m9.figshare.14370959 and for core GeoB 1710-3 at 10.6084/m9.figshare.14397884.

**Acknowledgments:** I would like to thank the University of Bremen in Germany and the University of Cambridge in the United Kingdom for providing sample materials and resources. I would also like to thank the Maxwell Institute at the University of Cambridge for providing the X-ray facilities. This research was completed during my research fellowship in the University of Oxford, funded by the Royal Society under the Newton International Fellowships scheme.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **References**


*Article*
