Antarctic Circumpolar Current Dynamics at the Pacific Entrance to the Drake Passage Over the Past 1.3 Million Years

Abstract

The Antarctic Circumpolar Current (ACC) is the world's largest current system connecting all three major basins of the global ocean. Our knowledge of glacial‐interglacial changes in ACC dynamics in the southeast Pacific is not well constrained and presently only based on reconstructions covering the last glacial cycle. Here we use a combination of mean sortable silt grain size of the terrigenous sediment fraction (10–63 μm, ) and X‐ray fluorescence scanner‐derived Zr/Rb ratios as flow strength proxies to examine ACC variations at the Pacific entrance to the Drake Passage (DP) in the vicinity of the Subantarctic Front. Our results indicate that at the DP entrance, ACC strength varied by ~6–16% on glacial‐interglacial time scales, yielding higher current speeds during interglacial times and reduced current speeds during glacials. We provide evidence that previous observations of a reduction in DP throughflow during the last glacial period are part of a consistent pattern extending for at least the last 1.3 Ma. The orbital‐scale cyclicity follows well‐known global climate changes from prevailing ca. 41‐kyr cycles in the early part of the record (1.3 Ma to 850 ka; marine isotope stage 21) across the mid‐Pleistocene transition into the middle and late Pleistocene 100‐kyr world. A comparison to a bottom water flow record from the deep western boundary current off New Zealand (Ocean Drilling Program Site 1123) reveals anti‐phased changes between the two sites. The enhanced supply of deep water along the DP and into the Atlantic Ocean during interglacials corresponds to a weakened flow of the SW Pacific deep western boundary current.

Orbital- and millennial-scale variability of the Antarctic Circumpolar Current over the past 140,000 years

“…2E). This suggests that the ACC flow speed during MIS 5e was slightly higher than during the present interglacial, supported by corresponding changes in Zr/Rb ratios ( occurred in deep water (~3781 m) near the SAF 8 . In the central Drake Passage ~20 nm north of the PF, the ACC throughflow exhibits a ~28-43% glacial reduction at ~3090 m water depth, which is considerably greater than variations along the Scotia Sea transect south of the PF (≤ 10%, Scotia Sea transect cores, 2000-4300 m water depth) 6 .…”

“…Differences between these results are likely due to the geographical settings within latitudinal subdivisions of the ACC and the related fronts. Therefore, we suggest an enhanced sensitivity of the ACC to glacial-interglacial climate changes around the PF and the SAF 5,7,8 , in contrast to minimal changes south of the PF along the Scotia Sea transect 6 . Such presumption would be consistent with modern ACC transport and highest current velocities prevailing in the vicinity of the SAF and PF in the Drake Passage region 26,36 .…”

“…Therefore, the Zr/Rb ratios ( Fig. 2C) can reflect changes in sediment fractions by current transport 5,8 , or aeolian input 29 . Major aeolian input can be ruled out in our study area by mineralogical and geochemical properties 25,31,32 .…”

“…Several processes might cause these changes in the ACC strength at site PS97/085, including changes in the strength and latitudinal position of SWW 9 , oceanic fronts shift 37 and buoyancy forcing 10 . Although the intensity and position of the glacial SWW remain uncertain, a northward displacement of the SWW is widely assumed during the LGM 38,39 , with reduced impact on the ACC in the Drake Passage sector 5,6,8 . The glacial oceanic fronts were likewise suggested to have shifted equatorward 37 , thus the South American continent would have obstructed the ACC flow through the Drake Passage 7 .…”

“…S1). Yellow dots mark sediment cores in north of the Subantarctic Front 5,7,8 . Grey dots indicate Scotia Sea transect cores south of the Polar Front 6 .…”