Changes in Eastern Pacific ocean ventilation at intermediate depth over the last 150 kyr BP

doi:10.1016/j.epsl.2010.08.002

Earth and Planetary Science Letters

Volume 298, Issues 1–2, 15 September 2010, Pages 217-228

https://doi.org/10.1016/j.epsl.2010.08.002 Get rights and content

Abstract

The circulation patterns of the deep glacial Pacific Ocean are still debated. Difficulties arise due to the scarcity of reliable paleoceanographic records that can document the past movements and properties of Pacific Ocean water masses. Here, we jointly use δ¹³C and δ¹⁸O measured on the epibenthic foraminifer Cibicidoides wuellerstorfi, from the MD02-2529 sediment core collected at 1619 m water depth in the eastern equatorial Pacific, to monitor changes in water mass circulation spanning the past 150 kyr BP. After the extraction of short-term (centennial to millennial-scale) δ¹³C and δ¹⁸O changes, which were ~ 1.0 and 0.5‰, respectively, we observed that these rapid δ¹³C and δ¹⁸O shifts were closely interrelated during the last 150 kyr BP. A comparison of MD02-2529 with other benthic δ¹³C records localized to the north and south of the core location revealed that MD02-2529 was alternately bathed by a northern nutrient-rich and a southern nutrient-poor water mass. The comparison provided a diagnostic for the latitudinal movements of a sharp water mass front that was particularly evident during marine isotope stages 4 and 3 on the millennial timescale. By considering that δ¹³C is an indicator of the northern vs. southern origin of the water that bathed the MD02-2529 coring site in the past, we found that a North Pacific water mass, that occasionally spreads to the eastern Pacific Ocean as deep as 1600 m and as far south as 8°N, was responsible for shifts toward the positive δ¹⁸O we observed in the past. We then used the δ¹³C/δ¹⁸O relationship to reconstruct latitudinal temperature and/or salinity gradients of the water mass that were linked to changes in the northern and/or the southern water mass end-members. Evolution of the δ¹³C/δ¹⁸O relationship spanning the past 150 kyr BP has shed light on how hydrological processes occurring at northern and southern high latitudes are transmitted to the ocean's interior through water mass advection.

Introduction

Modern deep-water formation sites are localized in the northern North Atlantic and around Antarctica in the Weddell and Ross Seas (Kuhlbrodt et al., 2007), while there is no deep-water formation in the North Pacific. North Atlantic Deep Water (NADW) ventilates most of the present-day deep Atlantic Ocean. The fact that there is no NADW equivalent for the North Pacific makes the present-day Thermohaline Circulation (THC) asymmetric. The absence of North Pacific deep-water formation leads to oxygen depletion between ~ 500 and ~ 2500 m in the entire present-day North Pacific Ocean (Helly and Levin, 2004) (Fig. 1a).

Drastic changes in Pacific Ocean circulation triggered by high-latitude hydrological processes are candidates for explaining part of the observed changes in atmospheric carbon dioxide during the Pleistocene (Haug and Sigman, 2009, Sigman et al., 2010). However, up until now there have not been any simple schemes for how past circulation in the Pacific could have differed from modern circulation. On one hand, an increase in the nutrient content of the deep glacial ocean could suggest that deep Pacific Ocean circulation was reduced (Sigman et al., 2010). On the other hand, Wunsch (2003) argued that reduced circulation during glacial periods is unlikely since increased wind-induced mixing may have favoured enhanced overturning.

Information regarding oceanic geochemistry and the temperature and/or the salinity of past water masses can be extracted from stable isotopes measured on benthic foraminifera tests that are contained in marine sediment cores. The distribution of the δ¹³C of ΣCO₂ in the modern ocean is tightly linked to the oxygen and nutrient content, which are shaped by large-scale oceanic water mass movements (Kroopnick, 1985). Since the δ¹³C of benthic foraminifera reflects the δ¹³C of the Dissolved Inorganic Carbon (DIC) in seawater, past changes in the seawater oxygen and/or nutrient content can be reconstructed using the δ¹³C obtained from benthic foraminifera tests in deep-sea sediments (Duplessy et al., 1984). On the other hand, the δ¹⁸O of seawater (δ¹⁸O_sw) is related to salinity (Delaygue et al., 2000), and past changes in δ¹⁸O_sw can be monitored using benthic foraminifera δ¹⁸O (Zahn et al., 1991, Lynch-Stieglitz et al., 1999). However, the temperature fractionation that modulates the difference between the δ¹⁸O_sw and the δ¹⁸O of foraminifer tests, makes foraminifera δ¹⁸O values a mixed signal for both temperature and δ¹⁸O_sw (salinity).

The work described here makes a first attempt to jointly use benthic foraminifera δ¹³C and δ¹⁸O signatures in order to combine the information provided by each independently. We present a new high-resolution record of stable isotope measurements performed on epibenthic foraminifera that were obtained from a marine sediment core located in the Panama Basin (core MD02-2529, 08°12.33′N; 84°07.32′W; 1619 m water depth) that spans the past 150 kyr BP. The eastern equatorial Pacific (EEP) is situated at the confluence of northern suboxic and southern oxic waters at intermediate depths (Fig. 1). Therefore, studying past changes in water mass properties from this region will provide insights regarding how the North Pacific shadow zone has evolved in the past.

Section snippets

Modern-day Pacific Ocean circulation

In the modern North Pacific Ocean, relatively low sea surface salinities restrict North Pacific Intermediate Water (NPIW) formation to the upper 500 m of water depth (Warren, 1983, Talley, 1993; Reid, 1997; Emile-Geay et al., 2003, Fig. 1). Waters flowing below this depth have a remote southern component composed of a mixture of both NADW and waters originating from the Southern Ocean, at both intermediate (the Antarctic Intermediate Water, AAIW) and abyssal depths (the Antarctic Bottom Water,

Pacific Ocean paleoceanography deduced from benthic δ¹³C and δ¹⁸O

For the Last Glacial Maximum (LGM) time slice a deconvolution of temperature and salinity can be performed using an analysis of sedimentary pore water, for which salinity (chlorinity) together with benthic foraminifera stable isotopes and pore water δ¹⁸O_sw measurements provide temperature and salinity estimations (Adkins et al., 2002). Such an analysis was performed on sediments collected from different oceanic basins and indicated that the deep ocean was uniformly cold, but that sharp salinity

Methods

We measured stable carbon and oxygen isotopes on the benthic foraminifer species Cibicidoides wuellerstorfi, and on the planktonic foraminifera species Globigerinoides ruber and Neogloboquadrina dutertrei in the > 250 μm size fraction in MD02-2529 core. Samples containing one to four benthic foraminifera and two to six planktonic foraminifera were treated with H₃PO₄ at 70 °C. The resulting CO₂ was analysed using a Finnigan Delta Advantage mass spectrometer at CEREGE. We performed ~ 20 replicates on

Patterns and significance of the C. wuellerstorfi isotopic signatures

With epibenthic foraminifera considered as oxic taxa (see e.g. Jorissen, 1999), the presence of C. wuellerstorfi throughout the sedimentary sequence indicates that bottom water oxygen concentrations never reached dysoxic values over the time interval studied (Fig. 2). The δ¹⁸O of C. wuellerstorfi (δ¹⁸O_Cw) for the last 10 kyr BP was ~ 2.6 ± 0.1‰ (Fig. 2). Between 10 and 20 kyr BP, the last glacial termination was marked by a δ¹⁸O_Cw increase of ~ 2‰ (Fig. 2). MIS2 values of benthic δ¹⁸O remained rather

The origin and extent of glacial EEP water masses at mid-depth

One valid explanation for long-term and rapid variations in δ¹³C_Cw recorded during the last glacial period in core MD02-2529 potentially lies in long-term and rapid movements of a sharp water mass front located in the vicinity of the coring site at approximately 1600 m water depth. To test this hypothesis we first compared epibenthic foraminiferal δ¹³C records available from the north and the south of the MD02-2529 core location in the 1400 to 1800 m water depth range (Fig. 1).

The sediment core

Conclusions

We measured stable isotopes on benthic foraminifera from the MD02-2529 marine sediment core collected in Panama Basin at ~ 1600 m water depth, which spans the last 150 kyr BP. The δ¹³C_Cw record is characterized by relatively stable and high values for the last 25 kyr BP, as compared to earlier time periods when δ¹³C_Cw values were more variable at millennial and longer timescales. We found that millennial-scale δ¹³C_Cw anomalies prior to 25 kyr BP were negatively correlated with millennial-scale δ¹⁸O_Cw

Acknowledgements

We thank Frauke Rostek for measuring alkenones and Corinne Sonzogni for performing the stable isotope analyses. We thank Jess Adkins, Alan Mix, Olivier Marchal, Rainer Zahn, and Cécile Blanchet for the discussions and advices. We thank Yiming Wang for the English language editing. The quality of the manuscript was substantially improved by the constructive comments of Peter deMenocal and two anonymous reviewers.

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Changes in Eastern Pacific ocean ventilation at intermediate depth over the last 150 kyr BP

Abstract

Introduction

Section snippets

Modern-day Pacific Ocean circulation

Pacific Ocean paleoceanography deduced from benthic δ13C and δ18O

Methods

Patterns and significance of the C. wuellerstorfi isotopic signatures

The origin and extent of glacial EEP water masses at mid-depth

Conclusions

Acknowledgements

Geochim. Cosmochim. Acta

Quater. Res.

Quatern. Sci. Rev.

Quatern. Sci. Rev.

Earth Planet. Sci. Lett.

Quatern. Res.

Prog. Oceanogr.

Quatern. Sci. Rev.

Deep Sea Res. I

Quatern. Sci. Rev.

Earth Planet. Sci. Lett.

Deep-Sea Res.

Quatern. Sci. Rev.

Quartern. Sci. Rev.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Palaeogeogr. Palaeclim. Palaeoecol.

Prog. Oceanog.

Quatern. Res.

Quatern. Res.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Quatern. Sci. Rev.

The salinity, temperature and δ18O content of the glacial deep ocean

Science

Brief interstadial events in the Santa Barbara basin, NE Pacific, during the last 60 kyr

Nature

Formation and preservation of greigite (Fe3S4) in sediments from the Santa Barbara Basin: implications for paleoenvironmental changes during the past 35 ka

Paleoceanography

Timing of millennial-scale climate change in Antarctica and Greenland during the last glacial period

Science

World Ocean Atlas 2001: Objective Analyses, Data statistics, and Figures, CD-ROM Documentation

Deep water source variations during the last climatic cycle and their impact on the global deep water circulation

Paleoceanography

Geothermal heating, diapycnal mixing and the abyssal circulation

Ocean Sci. Discuss.

Warren revisited: atmospheric freshwater fluxes and “Why is no deep water formed in the North Pacific”

J. Geophys. Res.

Interdecadal climate fluctuations that depend on exchanges between the tropics and extratropics

Science

Polar twins

Nat. Geosci.

Evidence for a bathyal front at 2000‐M depth in the glacial Pacific, based on a depth transect on Ontong Java Plateau

Paleoceanography

Intermediate water formation in the Bering Sea during glacial periods: evidence from neodymium isotope ratios

Geology

MARINE04 marine radiocarbon age calibration, 0–26 ka BP

Radiocarbon

Benthic foraminiferal microhabitats below the sediment–water interface

Pacific Ocean paleoceanography deduced from benthic δ¹³C and δ¹⁸O

The salinity, temperature and δ¹⁸O content of the glacial deep ocean

Formation and preservation of greigite (Fe₃S₄) in sediments from the Santa Barbara Basin: implications for paleoenvironmental changes during the past 35 ka