Report

Deepwater Formation in the North Pacific During the Last Glacial Termination

See allHide authors and affiliations

Science  09 Jul 2010:
Vol. 329, Issue 5988, pp. 200-204
DOI: 10.1126/science.1190612

Switching Basins

Most of the densest, deepest water at the bottom of the oceans comes from two regions, the North Atlantic and the circum-Antarctic. Have other regions been able to produce significant quantities of deep water in the past? For decades, researchers have looked, with limited success, for evidence of deepwater formation in the North Pacific since the time of the Last Glacial Maximum, about 23,000 years ago. Okazaki et al. (p. 200) combine published observational evidence from the North Pacific with model simulations to suggest that deep water did form in the North Pacific during the early part of the Last Glacial Termination, between about 17,500 and 15,000 years ago. The switch between deep-water formation in the North Atlantic and the North Pacific is likely to have had an important effect on heat transport and climate.

Abstract

Between ~17,500 and 15,000 years ago, the Atlantic meridional overturning circulation weakened substantially in response to meltwater discharges from disintegrating Northern Hemispheric glacial ice sheets. The global effects of this reorganization of poleward heat flow in the North Atlantic extended to Antarctica and the North Pacific. Here we present evidence from North Pacific paleo surface proxy data, a compilation of marine radiocarbon age ventilation records, and global climate model simulations to suggest that during the early stages of the Last Glacial Termination, deep water extending to a depth of ~2500 to 3000 meters was formed in the North Pacific. A switch of deepwater formation between the North Atlantic and the North Pacific played a key role in regulating poleward oceanic heat transport during the Last Glacial Termination.

Massive meltwater discharges during the Last Glacial Termination [~19 to 10 thousand years ago (ka)], such as Heinrich event 1 (H1: ~17.5 to 15 ka) and the Younger Dryas event (YD: ~13 to 11.5 ka), reduced North Atlantic surface density conditions and interrupted the steady flow of the Atlantic meridional overturning circulation (AMOC) (1). Through atmospheric and oceanic teleconnections, climate conditions changed substantially in the North Pacific, as documented by numerical climate modeling studies (24) and paleo-proxy data (5, 6).

Recent studies (711) using data obtained from single-sediment cores in the North Pacific reported local evidence for major reorganizations of the North Pacific Ocean circulation during H1, the Bølling-Allerød (BA: ~14.5 to 13 ka) warm period, and the YD. The interpretation of radiocarbon data in these cores reveals that during H1, very old water masses seeped into intermediate layers of the eastern tropical Pacific, whereas the western North Pacific sites near Japan show relatively young radiocarbon ventilation ages at intermediate depths (1000 to 2000 m). Understanding the mechanisms by which old carbon-rich water from the deep North Pacific was vented to the surface and replaced by younger water masses may also hold important clues toward resolving the puzzle of the carbon sources that caused the rise of atmospheric CO2 during the last deglaciation.

The goal of this study is to provide a more comprehensive view of what triggered ventilation changes in the North Pacific during the early deglaciation and to reconstruct spatial patterns and pathways of intermediate-deepwater circulation. For these purposes, we compiled a comprehensive data set of published radiocarbon sediment core data from the North Pacific with temporal coverage large enough to resolve the H1/BA transition (12). Together with a climate model simulation that mimics H1, we used these data to determine the effects of AMOC changes on the large-scale conveyor-belt circulation, with an emphasis on the North Pacific.

To better understand the driving mechanisms of North Pacific circulation changes, we provide an overview of North Pacific climate conditions during the Last Glacial Termination (Fig. 1). During H1, a time of weak AMOC (Fig. 1A) (1), the eastern subarctic Pacific was warm and relatively saline, as documented by dinocyst assemblages in core PAR87A-10 [54.36°N, 148.47°W, 3664-m water depth (13)] (Fig. 1, B and C). Planktonic foraminiferal Mg/Ca and δ18O analyses in core GH02-1030 [42.23°N, 144.21°E, 1212-m water depth (14)] in the western North Pacific reveal the presence of cold and saline surface waters during the early deglaciation (Fig. 1, D and E).

Fig. 1

Sediment proxy records in the North Pacific from 23 to 10 ka on their re-established time scales (table S2) with (A) Pa/Th ratio as a proxy for the AMOC strength in the North Atlantic (1); sea-surface temperature and salinity changes in (B and C) the eastern North Pacific based on dinocyst assemblages (13) and in (D and E) the western North Pacific based on planktonic foraminiferal Mg/Ca and δ18O (14); (F) raw benthic foraminiferal δ13C data in the western North Pacific (15) (1366 m, gray squares), the Bering Sea (2391 m, open circles), and spline-interpolated and averaged benthic δ13C values (green circles); and (G) percentage of dysaerobic benthic foraminifera species in the Bering Sea (2391 m) (16).

As documented by these analyses, salinity dropped in the eastern and western North Pacific after ~16 thousand years, attaining minimum values during BA. By accounting for dating uncertainties among the different cores, we found that the drop of surface salinity during the H1/BA transition coincided with an increase in near-bottom phosphate contents inferred from benthic foraminiferal δ13C values in cores MR01K03-PC4 from the western North Pacific [41.12°N, 142.40°E, 1366-m water depth (15)] and BOW9A from the Bering Sea (54.04°N, 178.68°E, 2391-m water depth) (Fig. 1F). This change in the western North Pacific indicates a transition from nutrient-depleted and well-ventilated intermediate-deep waters during H1 to nutrient-enriched less-ventilated waters during BA.

The reduced abundance of dysaerobic benthic foraminiferal taxa in the Bering Sea core BOW9A during H1 compared with that in the BA period (16) is also consistent with the notion of enhanced deepwater ventilation extending to a depth of ~2500 m (Fig. 1G). Oxygen-rich intermediate-deep waters during H1 were also reconstructed from benthic foraminiferal assemblages (17) and redox-sensitive metals (18) in records from the western North Pacific. Because the millennial-scale variations in oxygen concentrations found in these cores are out of phase with those obtained from deeper sites in the North Pacific (ODP882 at 3200 m, and ODP887 at 3600 m) (9), we estimate the maximum vertical extent of North Pacific intermediate to deep water spreading during H1 to be ~3000 m. When this value is compared with present-day values of about 800 m (19), it becomes evident that such changes would have required a major reorganization of the entire North Pacific ocean circulation.

To develop a better understanding of the nature of ocean circulation changes during the deglaciation, we compiled published radiocarbon data from 29 intermediate-deepwater cores in the North Pacific (table S1). 14C ages were converted to calendar ages on planktonic foraminifera samples from these cores using the Calib 6.0 routine with the Marine09 calibration data set (20). Ventilation ages estimated from 14C age differences between coexisting benthic and planktonic foraminifera shells (BF−PF age), projection ages (21) (table S3), and reconstructed bottom water Δ14C activities (9) (table S4) were then calculated for all cores.

The compilation of western North Pacific radiocarbon ages (Fig. 2, for depth range 900 to 2800 m) reveals notable changes in deepwater ventilation during the last deglaciation. By averaging radiocarbon age estimates derived from the projection method and the BF−PF method, we obtained ventilation age estimates for the Last Glacial Maximum (LGM: ~23 to 19 ka), H1 (~17.5 to 15 ka), and BA (14.5 to 13 ka) periods of ~1500 years, ~950 years, and ~1550 years, respectively. The ~600-year drop in ventilation age between the LGM and H1 is significantly larger than the typical one-sided uncertainties in the projection method during this time (385 years), which supports the notion of a major reorganization of North Pacific intermediate-deepwater flow during H1.

Fig. 2

Compilation of ventilation age changes based on published radiocarbon data in the western North Pacific (8, 14, 3032). BF−PF ages (open diamonds), projection ages (gray circles), and smoothed spline interpolation of averaged BF−PF and projection ages (blue line). Uncertainty of calendar age and ventilation age is 1σ; Pa/Th ratio (orange squares) as a proxy for the AMOC strength in the North Atlantic (1).

No coherent averaged projection and BF−PF age ventilation changes were observed between the LGM (~1540 years), H1 (~1450 years), and BA (~1600 years) in the eastern North Pacific (900 to 2800 m) (table S5 and fig. S6). These findings for the western and eastern North Pacific are consistent with reconstructed bottom water Δ14C activities (table S5). The presence of a well-ventilated water mass above 2000 m, the so-called glacial North Pacific intermediate water (GNPIW), has been suggested previously for LGM conditions (22, 23), with the Bering Sea being the source of this water mass (24). Our reconstructed ventilation changes during H1 show a much more pronounced reorganization of deep western North Pacific flow.

To explore how this reorganization of deep North Pacific flow in response to changes of the AMOC came about, freshwater perturbation experiments were conducted with the Earth system model of intermediate complexity LOVECLIM (version 1.1) (25, 26) under both preindustrial and LGM background conditions. These experiments enabled us to compare simulated changes in radiocarbon with those reconstructed by the sediment core compilation (table S3). The freshwater perturbation experiments simulate the climate response to an idealized glacial meltwater pulse that mimics H1. By linearly increasing the freshwater forcing to 2 Sv in the northern North Atlantic for 100 years, the water becomes less dense, leading to a reduction of deep convection in the North Atlantic and a subsequent collapse of the AMOC. An associated reduction of poleward heat transport leads to large-scale cooling in the Northern Hemisphere, as in other climate models (24). Air-sea interactions in the North Atlantic and emanating atmospheric teleconnections are vital for cooling the western North Pacific (4). Furthermore, in response to the AMOC weakening and the associated North Atlantic cooling, the Atlantic-Pacific moisture transport weakens (25) and the Pacific Intertropical Convergence Zone shifts south, leading to a persistent reduction of precipitation and hence an overall increase of surface salinity in the North Pacific. Once the Pacific meridional overturning cell (PMOC) establishes, the anomalous poleward surface currents (fig. S3) transport more saline subtropical waters into the North Pacific, thereby providing a positive-salinity feedback. A closed Bering Strait facilitates the build-up of such North Pacific salinity anomalies (27). The resulting changes in surface density trigger deep oceanic convection in the subarctic Pacific. The three-dimensional adjustment to the resulting North Pacific density anomalies in the preindustrial and LGM LOVECLIM1.1 freshwater perturbation experiment leads to the establishment of a deep PMOC (Fig. 3B and fig. S2), which distributes young and oxygen-rich deep water throughout the entire western North Pacific down to ~2500 m (Fig. 3B), in accordance with reconstructions of oxygen conditions in BOW9A (Fig. 1G).

Fig. 3

(A) The shading indicates the sections of zonally averaged simulated radiocarbon age anomalies (in years) in the North Pacific between a collapsed AMOC state (years 400 to 500 of the freshwater experiment) and the control simulation using preindustrial background conditions. The squares indicate the reconstructed projection age anomaly (in years) (colored shading) of western North Pacific cores (8, 9, 14, 31, 33) (numbers of cores refer to table S1). (B) Shading indicates simulated zonally averaged Pacific oxygen anomalies (in ml oxygen per l of water) between a collapsed AMOC state (years 400 to 500 of the freshwater experiment) and the preindustrial control simulation. The contours indicate the simulated Pacific mean meridional stream function (106 m3/s) (years 400 to 500 of the freshwater experiment). (C) Simulated upper ocean (0 to 65 m) temperature (shading) (°C) and velocity (vectors) (m/s) anomalies between collapsed AMOC state (years 400 to 500 of the freshwater perturbation experiment) and control simulation, using pre-industrial background conditions. (D) Colored symbols indicate the projection age anomalies (in years) between H1 and the LGM (811, 14, 31, 3335) of radiocarbon data used in this analysis (numbers in symbols refer to table S1). The shading indicates simulated deep (1225 to 2963 m) minus near-surface (0 to 20 m) radiocarbon age anomalies (in years) between collapsed AMOC state (years 400 to 500 of the freshwater perturbation experiment) and control simulation. The arrows indicate the anomalous deep ocean currents averaged between 1225 and 2963 m.

Both model results (Fig. 3C) and paleo reconstructions (Fig. 1, B and D) suggest the existence of an east-west sea surface temperature gradient in the North Pacific during H1. The simulated cooling in the western North Pacific can be partly explained in terms of an intensification of the Aleutian Low (4) and cold-air advection. Simulated warming in the eastern North Pacific results from an enhanced poleward advection of heat, which is associated with the northeastward-flowing surface branch of the PMOC (Fig. 3C). Moreover, a very pronounced east-west gradient in intermediate-deepwater ventilation (Fig. 3D) is found, both in the paleo-proxy reconstructions and the modeling experiments. The main simulated pathway of deepwater circulation is along the western margin of the North Pacific, in a deep western boundary current that is analogous to the one currently in the North Atlantic. The model results demonstrate that the western boundary flow is the principal factor in establishing the east-west gradient of intermediate-deep Pacific ventilation, which is also evident in the compilation of radiocarbon-based ventilation data (Fig. 3, A and D).

However, simulated radiocarbon age anomalies associated with the onset of the PMOC are too weak to explain the data from recent eastern Pacific intermediate-depth sediment cores (10, 11) that reveal the presence of extremely old intermediate water. To explain the very old eastern tropical Pacific intermediate waters during H1, a process unrelated to the onset of a PMOC has to be invoked (11).

The match between the simulated ventilation age anomalies in the western North Pacific and those reconstructed from a compilation of radiocarbon records (Fig. 3, A and D) makes a compelling case for the existence of a deep PMOC during H1 and an associated see-saw in deepwater formation between the North Atlantic and Pacific.

In our preindustrial and LGM freshwater perturbation experiments, the PMOC is maintained by the poleward advection of salinity from the subtropical regions (fig. S3) (3, 28), a southward shifted Intertropical Convergence Zone, and reduced atmospheric export of fresh water from the Atlantic to the Pacific (25). As a result of the reorganization of the global conveyor belt circulation, the simulated Indonesian Throughflow weakens by 30 to 50% (25), which leads to a strengthening of the Kuroshio and the intrusion of North Pacific Intermediate Water into the tropical Pacific intermediate layers (29).

The shutdown of the AMOC in our model reduces the Atlantic heat transport at 40°N by ~0.6 PW (1 PW = 1015 W). At the same time, and in response to the establishment of a PMOC, poleward heat transport in the Pacific at 40°N increases by 0.4 PW. The PMOC hence played an important role in buffering the decrease in the poleward global oceanic heat transport caused by the shutdown of the AMOC. The evidence presented here demonstrates that the North Pacific was a very active region of thermohaline flow during the deglaciation and operated out of phase with the North Atlantic (Fig. 2). Viewing the North Pacific as participating in the ventilation of the global oceans could also help to explain the release of old carbon during the Last Glacial Termination.

Supporting Online Material

www.sciencemag.org/cgi/content/full/329/5988/200/DC1

Materials and Methods

Figs. S1 to S6

Tables S1 to S5

References

References and Notes

  1. Materials and methods are available as supporting material on Science Online.
  2. This research was supported by NSF grant number ATM-0712690. Additional support was provided by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), by NASA through grant number NNX07AG53G, and by the National Oceanic and Atmospheric Administration through grant number NA09OAR4320075, all of which sponsor research at the International Pacific Research Center. A.M. acknowledges funding from the Belgian Federal Science Policy (contract SD/CS/01).
View Abstract

Navigate This Article