Evidence of Southern Ocean influence into the far Northwest Pacific (Northern Emperor Rise) since the Bølling–Allerød warming

Highlights

• Stable isotope, lithological and micropaleontological proxies of 6 cores from the Western Subarctic Gyre (WSG) were studied.

• Age models of cores over the last 25kyr were modified by radiocarbon data calibrated by 14C atmospheric plateau tuning.

• Multiple lines of evidences support abrupt flushing of Southern Ocean origin water into the deep WSG since 14.5 ± 0.2 ka.

Abstract

The role of the Southern Ocean in releasing CO₂ (sequestered in the global ocean during the Last Glacial Maximum) into the atmosphere during deglaciation is an important topic for investigation of Earth's climate. Changes in global deep water circulation associated with upwelling of Circumpolar Deep Water (CDW) around Antarctica may have played a part in the CO₂ release, but remain poorly studied. The potential response of the Pacific Ocean, including the North Pacific, to upwelling of CDW with its vast reservoir of CO₂ remains unresolved. Here we combine productivity proxies, oxygen and carbon isotope values in benthic and planktic foraminifera, data on the occurrence and abundance of ice-rafted debris, and benthic foraminiferal species composition in three sediment cores with published data for three cores from the Northern Emperor Rise for the Last Glacial Maximum, deglaciation and Holocene in order to elucidate the North Pacific role in CO₂ redistribution in the past. Age models of the cores are based on radiocarbon data calibrated by ¹⁴C atmospheric plateau tuning. The calcium carbonate content in all cores increased abruptly around 14.5 ka, indicating an influx of relatively young water enriched in carbonate ion, oxygen and nutrients, and sourced in the Southern Oceans. A decrease in the extent of sea ice at the NER area during early deglaciation is reflected in sharp increases in the productivity of siliceous phytoplankton near the onset of Bølling/Allerød warming, possibly facilitated by the influx of Southern Ocean-sourced nutrient-rich waters.

Introduction

The concentration of carbon dioxide (CO₂) in the atmosphere is largely determined by the interplay between primary productivity, which regulates the uptake of CO₂ into the deep oceanic carbon reservoir (biological carbon pump), and ocean circulation, which determines organic respiration in water and escape of CO₂ into the atmosphere (Sigman and Boyle, 2000) The atmospheric CO₂ content during the Last Glacial Maximum (LGM, ~23–19 ka) was ~80 ppmv lower than its preindustrial level, but the exact mechanisms responsible for this change remain elusive. During Heinrich Stadial 1 (HS1, 17.5–14.7 ka), a millennial-scale cold event in the Northern Hemisphere, there was a first pulse in the rise in atmospheric CO₂, a drop in atmospheric ¹⁴C accompanied by rapid deposition of opal in the Southern Ocean, and a dramatic decrease in the Atlantic Meridional Overturning Circulation (AMOC) (Anderson et al., 2009; Marchitto et al., 2007; McManus et al., 2004; Monnin et al., 2001). The Southern Ocean may have been a main driver in controlling atmospheric CO₂ levels, by accumulating CO₂ in the deep oceans during glacials, then releasing it into the atmosphere through upwelling of deep waters around Antarctica during deglaciation (Anderson et al., 2009; Burke and Robinson, 2012; Jansen and Nadeau, 2016; Marshall and Speer, 2012; Sigman et al., 2010). However, the potential mechanisms by which changes in meridional overturning circulation may control ocean–atmosphere CO₂ exchange are not clear, and especially the role of the Pacific Ocean, the largest ocean and thus the main reservoir of CO₂, is not known (Marshall and Speer, 2012). Paleoceanographers have been studying the subarctic North Pacific extensively during the last decades, because this region is the last link of the global meridional overturning circulation (Du et al., 2018; Galbraith et al., 2007; Gebhardt et al., 2008; Gong et al., 2019; Jaccard et al., 2005; Jaccard and Galbraith, 2013; Keigwin, 1998; Okazaki et al., 2010). Radiocarbon data of paired benthic (BF) and planktic foraminifera (PF), the δ¹³C of BF and other proxies suggest that North Pacific Intermediate Water (NPIW) was well ventilated during HS1 (Ahagon et al., 2003; Max et al., 2014; Okazaki et al., 2012), while deep oceans remained poorly ventilated (Galbraith et al., 2007; Jaccard and Galbraith, 2013; Max et al., 2014). Deep water ventilation of the western and eastern subarctic Pacific may have been significantly different (Okazaki et al., 2012, Okazaki et al., 2010). There is considerable evidence for increased deep water ventilation in the northeastern (NE) Pacific during the last deglaciation (Du et al., 2018; Marchitto et al., 2007; Okazaki et al., 2010). A large drop in radiocarbon activity in intermediate waters during HS1 and Younger Dryas (YD, 12.9–11.8 ka), as measured in BF in a core from the continental margin of southern Baja California, requires an injection of old waters from a previously isolated deep-ocean carbon reservoir (Marchitto et al., 2007). Based on changes in neodymium isotope values in a core from the NE Pacific (Gulf of Alaska), Du et al. (2018) inferred that acceleration of NE Pacific abyssal circulation during HS1 was mostly controlled by flushing of Antarctic Bottom Water, triggered by Southern Hemisphere warming and sea ice retreat in the Southern Ocean.

The history of western subarctic Pacific deep ventilation during the last deglaciation is controversial (Galbraith et al., 2007; Gebhardt et al., 2008; Keigwin, 1998; Max et al., 2014; Okazaki et al., 2010). Based on BF stable isotope data from more than 30 cores from the Northern Emperor Rise (NER) and the Okhotsk Sea from depths of 1000–4000 m, Keigwin (1998) suggested that an intermediate water mass in the far northwestern (NW) Pacific was better ventilated during the LGM than during the Holocene, but deep waters were as nutrient-rich as today. Compiling radiocarbon ages from the NW Pacific, Okazaki et al. (2010) concluded that the average ventilation rate of intermediate-to-deep waters (900 to 2800 m) during the LGM was ~1500 years, then decreased to nearly 950 years during HS1, and increased again to ~1550 years during the Bølling/Allerød (B/A) warming (14.7–12.9 ka). Galbraith et al. (2007) concluded that abrupt and strong NW Pacific deep water ventilation since 14.6 ka was caused by an increase in deep water formation in the North Atlantic, but Jaccard and Galbraith (2013) combined redox-sensitive trace metal data from core PC13 (2393 m depth, NW Pacific; Fig. 1) with previously published data and did not find changes in ventilation at intermediate water depths during the early deglaciation. They suggested that the deepest core showing evidence for enhanced ventilation during HS1 was from 1366 m water depth, reflecting intensified intermediate water formation. Simulation of North Pacific ventilation by Earth System Modeling shows enhanced intermediate-to-deep ocean stratification due to intensified NPIW formation during HS1, relative to the LGM period (Gong et al., 2019). Increased input of nutrient- and CO₂-rich water into this region, caused by wind-driven upwelling within the subpolar North Pacific and the collapse of NPIW formation, may have led to enhanced productivity during the B/A warming, based on boron isotope data of PF in a NW Pacific core and climate model simulations (Gray et al., 2018).

Here we present records of several productivity proxies, sea ice influence, δ¹⁸O and δ¹³C in BF and PF, redox-sensitive trace metals from six sediment cores from the NER (new data from three cores and previously published data from other three cores) and BF abundance and species composition for core MD-16 over the last 20 kyr. Age models of the sediment cores were based on accelerator mass spectrometry (AMS) ¹⁴C dating and robust regional indicators for correlation with the well-dated core MD-16 by atmospheric ¹⁴C plateau tuning (Sarnthein et al., 2015; Sarnthein, personal communication, 2019). In the results we provide new evidence of environmental evolution and deep water ventilation in the NER area since the LGM with abrupt input of Southern Ocean origin water into the deep water of NER at 14.5 ka, near the onset of B/A warming.