Magnetic proxy for the deep (Pacific) western boundary current variability across the mid-Pleistocene climate transition

Abstract

The Deep Western Boundary Current (DWBC) inflow to the SW Pacific is one of the largest, transporting ∼ 40% of the total input of deep water to the world's oceans. Here we use a sedimentary record from the giant piston core MD97-2114 collected on the northern flank of the Chatham Rise located at 1935 m water depth, east of New Zealand, to investigate DWBC variability during the Pleistocene epoch when the period of glacial cycles changed progressively from a 41 kyr to 100 kyr rhythm. Magnetic grain-size may be directly related to orbitally forced fluctuations in the strength of the upper circumpolar deep water (UCDW) through its interaction with terrigenous sediments supplied from the south and west. The long-term trends in magnetic properties are characterized by two main perturbations centered at 870 ka (Marine Isotope Stage, MIS 22) 450 ka (MIS 12), which is broadly consistent with the inferred perturbation during the mid-Pleistocene climate transition based on sedimentological paleocurrent reconstruction from Ocean Drilling Program Site 1123 located at 3290 m water depth in the main core of the DWBC flow on the North Chatham Drift. This similarity suggests that both the upper and middle CDW are modulated by similar processes and fluctuations of Antarctic Bottom Water production could be directly responsible for this deep Pacific Ocean inflow variability over the past 1.2 Ma.

Introduction

The sedimentary system off the eastern margin of New Zealand (Eastern New Zealand Oceanic Sedimentary System) (e.g., Carter et al., 1996, Carter et al., 2004a), extending from the Solander Channel north to the Kermadec Trench (Fig. 1), is considered a key area to study the interactions between the largely wind-driven Antarctic Circumpolar Current (ACC) and the west Pacific Ocean circulation. Today, the supply of deep water to the South Pacific Ocean is dominated by the thermohaline Deep Western Boundary Current (DWBC) east of New Zealand, which was established since at least the latest Oligocene (Carter et al., 1996, Carter et al., 2004a). In terms of flux, with an average volume transport of 16 ± 11.9 Sverdrups (1 Sv = 10⁶ m³ s^− 1) at 32°30′S, this is one of the largest DWBC in the world ocean (Whitworth et al., 1999).

The DWBC, in concert with the deep-reaching ACC, flows northeastward along the eastern edge of the Campbell Plateau until about 50°S latitude. Here, the DWBC and ACC decouple, with the ACC turning eastward, continuing its journey around Antarctica, and the DWBC passing the entrance to Bounty Channel, and then beneath the productive surface waters associated with the Subtropical Front (Fig. 1) (Murphy et al., 2001). The DWBC intensifies as it veers around the eastern tip of the Chatham Rise, being constrained by the steep sides (Carter and McCave, 1994, McCave and Carter, 1997, Warren, 1981), and then decelerates after passing through Valerie Passage, which separates the Chatham Rise from the NW–SE trending Louisville Seamount Chain (Hall et al., 2003).

The DWBC principally transports Circumpolar Deep Water (CDW) containing three major components (Fig. 1b) (McCave and Carter, 1997, Warren, 1973, Warren, 1981, Whitworth et al., 1999): (i) Upper CDW, a strongly nutrient-enriched and oxygen-depleted layer located between 1400 and 2800 m water depth, (ii) middle CDW, a distinct higher salinity (S = 34.72-34.73 practical salinity units, PSU) layer between 2800 and 3800 m, including the zone of maximum influence of North Atlantic Deep Water (2800 and 3400 m), and (iii) lower CDW, a cold and lower salinity (S = 34.68 PSU) layer > 3800 m water depth. The latter waters are largely generated around Antarctica, in particular within the Weddell Sea, Ross Sea, and along the Adelie Coast (Rintoul et al., 2001) (i.e., sensu lato Antarctic Bottom Water). Hall et al. (2001) using sedimentological and geochemical paleohydrographic proxies from Ocean Drilling Program (ODP) Site 1123 (3290 m water depth), reported evidence for intensified DWBC inflow and Pacific Ocean ventilation during glacial periods over the past 1.2 Ma. This is believed to be directly related to increased glacial production of Antarctic Bottom Water. This time series records the middle CDW component of the DWBC flow, mainly derived from Indian Ocean outflow added to Pacific outflow returning through the Drake Passage. Three longer-term periods of differing mean flow speeds are identified within the past 1.2 Ma, with the interval related to the mid-Pleistocene climate transition between 870–450 ka the period of change in the dominant response of the Earth's climate from orbital obliquity to eccentricity forcing (e.g., EPICA Community Members, 2004, Schmieder et al., 2000) characterized by generally weaker DWBC flow.

Here we present marine sediment records of magnetic properties and planktonic foraminifera δ¹⁸O isotope measurements from a middle/upper Pleistocene depositional sequence from the northern flank of the Chatham Rise, east of New Zealand (Fig. 1). The site is well placed to monitor the variability of the upper CDW component of the deep Pacific inflow. This study examines whether (1) the “mid-Pleistocene climate transition” in ice volume and its frequency of variation was associated with changes in the upper DWBC circulation, and (2) whether these changes are recorded by sedimentary magnetic properties.