North Atlantic Upper‐Ocean Hydrography During the Mid‐Pleistocene Transition Evidenced by Globorotalia truncatulinoides Coiling Ratios

Abstract

We use the coiling direction of planktic foraminifer Globorotalia truncatulinoides from four sites in the North Atlantic Ocean to assess spatial changes in upper‐ocean hydrography during the mid‐Pleistocene climate transition. Core top sediments indicate that the left‐coiling (sinistral) variety dominates regions characterized by a deep permanent thermocline as is characteristic of the subtropical gyre (Ocean Drilling Program Sites 1058 and Deep Sea Drilling Project Site 607) while right‐coiling (dextral) tests dominate the tropical (Ocean Drilling Program Site 925) and subpolar site (Deep Sea Drilling Project Site 552). We observe pronounced changes in the dominant coiling direction at the three subtropical to subpolar sites during the first of the more extreme glacial to interglacial transitions of the mid‐Pleistocene climate transition, Marine Isotope Stages (MISs) 22 to 21. We interpret these observations to reflect an increase in poleward warm water advection. The time interval between MIS 21 and MIS 15 is marked by a pronounced absence of sinistral tests. Because G. truncatulinoides (sinistral) is a deep dwelling species, their absence may be related to a decrease in dissolved oxygen during warm interglacial intervals. At the tropical site, precessional‐scale variations dominate changes in the coiling direction until midpoint interglacial MIS 23. Occurrences in the sinistral variety at this locale must be linked to cross‐equatorial water transport in the North Brazil Current, and precessional‐scale variations provide evidence for the sensitivity of the tropical regions to precessional forcing, at least until MIS 23 when this morphotype disappears in agreement with observation at other sites in the Atlantic Ocean.

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

“…At Sites 1056-1059 G. truncatulinoides coiling ratios were determined from the 355-500 μm size fraction, and at Site 1063, we picked from the >250 μm fraction because abundances were generally lower. As suggested by Feldmeijer et al (2014), and also shown by Kaiser et al (2019), the different size fractions are comparable. Sample intervals were split until 100 tests could be counted.…”

“…Molecular clocks indicate that this type may have split from its ancestor, type I, a sinistral form, as recently as 170 kyr ago (De Vargas et al, 2001). Down-core studies in the western subtropical Atlantic Ocean, however, have distinguished dextral from sinistral tests during the middle Pleistocene (Chaisson et al, 2002;Kaiser et al, 2019) suggesting that branching occurred earlier. The relative abundance of sinistral versus dextral tests in type II G. truncatulinoides was recognized in early paleocenographic studies as linked to specific water masses and environmental parameters (e.g., Ericson et al, 1954;Herman, 1972;Thiede, 1971).…”

“…G. truncaulinoides (dextral) are more abundant toward the southern extent of the gyre where the water column is stratified and nutrients are advected with the North Equatorial Current (Ujiié et al, 2010). This link between dominant coiling direction and water column structure has recently been explored as a proxy for temporal changes in upper ocean hydrography in the North Atlantic Ocean on geologic time scales (Billups et al, 2016;Feldmeijer et al, 2014;Kaiser et al, 2019). Color reflects water temperature at 300-m water depth (WOA09, Locarnini et al, 2010), the depth of maximum Globorotalia truncatulinoides abundances in this region (Ujiié et al, 2010).…”

“…Between 240 and 400 ka (MIS 8 through MIS 11), the record overlaps with the one from Site 1056, and because they are within 1°latitude/longitude, provide a duplication of results. Site 1056 then extends through 435 ka (MIS 12) and connects with the record from Site 1058 (Kaiser et al, 2019). As our focus is on the later Pleistocene, we only show the 411-700 ka portion of this record, which we have augmented with new data points to increase the temporal resolution to that of the other sites (0.5-2 kyr).…”

Meridional Changes in The South Atlantic Subtropical Gyre During Heinrich Stadials

“…Therefore, G. truncatulinoides increases (decreases) in abundance when the thermocline is deeper (shallower) 19,21 . The coiling direction of G. truncatulinoides has been commonly used to reconstruct the upper water column strati cation 21,22 , although surface sediments from the Atlantic Ocean reveal that the distribution of both sinistral and dextral morphotypes of G. truncatulinoides are closely related to the subtropical gyres 23 , showing high abundance inside the gyres, where the thermocline is deeper, and being virtually absent to the north and south of the gyres, where the thermocline is shallower (Fig. 1a-c).…”