Precessional forcing in the Western Equatorial Atlantic during the mid-Pleistocene Transition

We are using down-core changes in the coiling direction (sinistral versus dextral) of planktonic foraminifer Globorotalia truncatulinoides from the northwestern tropical Atlantic Ocean (Ocean Drilling Program Site 925) to reconstruct upper ocean hydrography during the mid-Pleistocene Transition. In the Southern Hemisphere subtropical regions, G. truncatulinoides morphotype I (sinistral tests) are associated with the North Brazil Current (NBC) (Renauld and Schmidt, 2003). Thus we interpret variations in sinistral versus dextral tests at Site 925 as indicative of variations in the strength of the NBC flowing across the equator toward the study site. Our results indicate that prior to the mid-point of the mid Pleistocene transition (MPT) ( 1,200-920 Ka), G. truncatulinoides sinistral abundances display a series of well-defined cycles spaced about 23 kyr. These cycles closely follow variations in orbital precession, which is confirmed by cross spectral analyses. We interpret these results to indicate precession forced variations in the strength of the NBC, and hence cross-equatorial heat transport. These results contribute unequivocal evidence for the importance of precession forcing on tropical ocean hydrography, which differs from results in previous studies spanning this interval of time (Bassinot et al., 1997; Tiedemann et al., 1994). At 920 Ka, G. truncatulinoides sinistral tests disappear, and the assemblages are dominated by dextral tests until around 600 Ka. However, disappearance of sinistral types at around this time has been documented at other sites in the Atlantic Ocean (Lohmann, 1992; Ufkes and Kroon, 2012; Kaiser et al., 2018), and therefore we cannot ascribe the pattern to a unique change in the regional hydrography at this time. Expansion of Northern Hemisphere glaciers during the MPT may have affected upper ocean hydrography basin-wide in the Atlantic Ocean in a way that temporarily limited the proliferation of sinistral types of G. truncatulinoides.