Pliocene to Earliest Pleistocene (5–2.5 Ma) Reconstruction of the Kuroshio Current Extension Reveals a Dynamic Current

Abstract

Western boundary currents flow along the western edge of ocean basins, transporting vast amounts of heat and moisture from equatorial regions to higher latitudes. The Kuroshio Current (KC) and the Kuroshio Current Extension (KCE) is the western boundary current system of the North Pacific Subtropical Gyre (Figure 1). The KC originates from the westward-flowing North Equatorial Current and has an estimated volume transport of 23.7-25.0 Sverdrup (Sv; 1 Sv = 1 million cubic meters per second) (Ichikawa & Beardsley, 1993). The current flows past the Izu Ridge until it separates from the Japan coast at approximately 36°N, 141°E flowing east into the Pacific Ocean (Imawaki et al., 2013) where it becomes the KCE. Maximum transport volume of the KCE has been estimated to reach up to 130 Sv (Wijffels et al., 1998). Thus, the KCE jet alone provides massive amounts of heat and moisture for North Pacific midlatitude cyclones. By modifying the path and intensity of storm tracks, changes to the dynamic state of the KCE can alter the stability and pressure gradient within the local atmospheric layers and basin-scale wind stress patterns (Frankignoul & Sennechael, 2007;Kwon et al., 2010) which, in turn, influences atmospheric processes affecting precipitation patterns over Japan and the west coast of North America (Latif & Barnett, 1994).The KCE region is one of the most dynamic and important regions of the world's ocean. Meeting of subtropical water masses brought north by the KC with southward flowing subpolar waters by the Oyashio Current (Figure 1) creates a relatively steep temperature gradient across these currents in the northwest Pacific. This condition leads to annual average water temperatures differing by about 8°C from 35°N to 40°N across the KCE and Oyashio Current (Locarnini et al., 2013). Measurements of air-sea CO 2 fluxes over

Expanded North Pacific Subtropical Gyre and Heterodyne Expression During the Mid‐Pleistocene

“…Throughout this interval, low ln Zr/Sr, MS, and terrigenous element (Zr, Ti, and Si) counts suggests an overall relative decrease in westerly wind strength over Hole 1207A, whereas, the ln Ca/Ti proxy indicates a period of enhanced burial of calcium carbonate (Figures 5 and 6). Similarly, high‐resolution dust proxies across the north Pacific during the mid‐Pliocene (i.e., ODP 885/886 and 1208), when global mean temperatures were 2–3°C warmer than preindustrial values, imply a decrease in dust flux and wind strength (Abell et al., 2021) and potential northward shift of the KCE and expansion of the North Pacific Subtropical Gyre (Lam et al., 2021). Results from the XRF analyses at Hole 1207A suggest that similar processes during Pleistocene interglacials, when global mean temperatures were only 1–2°C warmer (i.e., MIS 15, 11‐9; Lang & Wolff, 2011).…”

“…For example, a southward shift of the Aleutian Low (low‐pressure system) and Oyashio Current would displace subarctic waters southward during colder‐than‐present intervals of the Pleistocene, thus making it likely Site 1207 was positioned below the southern edge of the subarctic front. During warmer‐than‐present times, expansion of the North Pacific Subtropical Gyre would extend beyond Site 1207 (e.g., Fedorov et al., 2013; Ford et al., 2016; Lam & Leckie, 2020a; Lam et al., 2021).…”

“…These lithological changes most likely reflect variability in surface ocean circulation as a result of expansion and contraction of the subtropical gyre (Bralower et al., 2002; Lam & Leckie, 2020a) rather than dissolution (Lam & Leckie, 2020a). While it has been argued that this frontal system is stabilized via the bathymetric high of Shatsky Rise (e.g., Jacobi & Hayes, 1989; Venti et al., 2013), initial results from Leg 198 (Bralower et al., 2002) and subsequent work (Lam & Leckie, 2020a; Lam et al., 2020, 2021) demonstrate that through the Neogene and Quaternary, latitudinal shifts of the KCE and water masses may have been much more significant compared to those that have occurred within the last 600 kyr (e.g., Kawahata & Ohshima, 2002; Ujiié, 2003; Yamane, 2003).…”

“…While it has been argued that this frontal system is stabilized via the bathymetric high of Shatsky Rise (e.g., Jacobi & Hayes, 1989;, initial results from Leg 198 (Bralower et al, 2002) and subsequent work (Lam & Leckie, 2020a;Lam et al, 2020Lam et al, , 2021 demonstrate that through the Neogene and Quaternary, latitudinal shifts of the KCE and water masses may have been much more significant compared to those that have occurred within the last 600 kyr (e.g., Kawahata & Ohshima, 2002;Ujiié, 2003;Yamane, 2003).…”

“…Thus, it is imperative to understand how the Kuroshio Current system will behave due to increased anthropogenic warming under a range of warming scenarios using an array of methods. One of the most advantageous methods to observe long‐term climate trends of the current is through the use of the sedimentary record (e.g., Abell et al., 2021; Lam et al., 2021; Venti & Billups, 2013).…”