Reconstructing Western Boundary Current Stability in the North Atlantic Ocean for the Past 700 Kyr From Globorotalia truncatulinoides Coiling Ratios

Abstract

Down-core changes in the coiling direction of Globorotalia truncatulinoides in the northwestern subtropical Atlantic (KNR140-37PC and Ocean Drilling Program Sites 1063, 1059, 1056, 1058) provide a tracer for the hydrographic conditions in the western boundary current over the past 700 kyr (Marine Isotope Stage, MIS, 1-17). A consistent association between percent G. truncatulinoides (sinistral) abundances, total test counts, and bulk sediment CaCO 3 content is established by MIS 11 suggesting a response to ocean-atmosphere interactions during the mid Brunhes event. Commencing with MIS 11, interglacial maxima are associated with high total test counts and either distinct sinistral test minima (MIS 9e, 11c) or maxima (MIS 1, 5e, 7a). High sinistral test abundances with relatively high test counts is similar to the late Holocene relationship at the study sites. Low sinistral test abundances despite high test counts means that coiling ratios are dominated by dextral forms. We interpret this pattern to indicate more intense flow in the subtropical gyre either via the western boundary current drawing toward the gyre center, or a more northern influence of the North Equatorial and Antilles Currents. This suggests that the western boundary current may have been more intense during MIS 11c and MIS 9e then during MIS 7a, MIS 5e, and MIS 1 consistent with climate warm anomalies in northern Europe at these times. Regardless of the mechanism, the observation that minima and maxima in sinistral test abundances are prolonged at these times indicates that the western gyre boundary remained stable during relative warm intervals.

Micropaleontological and Isotopic Perspective of Surface Water Mass Variability in the NE Atlantic from MIS 6 to 5e (188 to 115 ka)

“…The near absence of G. inflata, a temperate water mass indicator (~8-18 • C) associated with the NAC in the NE Atlantic [64,65], at sites U1304 [25] and U1314 and core M23414, further corroborates the dominance by the polar and subpolar water masses at these sites during MIS 6. Billups et al [66] suggested compression of the subtropical gyre due to the southward advancement of the Polar and Arctic fronts during glacial times; however, the extent to which such a physical mechanism operated at Site U1313 cannot be assessed due simply to the lack of Globorotalia truncatulinoides (sinistral) data for MIS 6 period.…”