Modulation of the bipolar seesaw in the Southeast Pacific during Termination 1

https://doi.org/10.1016/j.epsl.2007.04.040Get rights and content

Abstract

The termination of the last ice age (Termination 1; T1) is crucial for our understanding of global climate change and for the validation of climate models. There are still a number of open questions regarding for example the exact timing and the mechanisms involved in the initiation of deglaciation and the subsequent interhemispheric pattern of the warming. Our study is based on a well-dated and high-resolution alkenone-based sea surface temperature (SST) record from the SE-Pacific off southern Chile (Ocean Drilling Project Site 1233) showing that deglacial warming at the northern margin of the Antarctic Circumpolar Current system (ACC) began shortly after 19,000 years BP (19 kyr BP). The timing is largely consistent with Antarctic ice-core records but the initial warming in the SE-Pacific is more abrupt suggesting a direct and immediate response to the slowdown of the Atlantic thermohaline circulation through the bipolar seesaw mechanism. This response requires a rapid transfer of the Atlantic signal to the SE-Pacific without involving the thermal inertia of the Southern Ocean that may contribute to the substantially more gradual deglacial temperature rise seen in Antarctic ice-cores. A very plausible mechanism for this rapid transfer is a seesaw-induced change of the coupled ocean–atmosphere system of the ACC and the southern westerly wind belt. In addition, modelling results suggest that insolation changes and the deglacial CO2 rise induced a substantial SST increase at our site location but with a gradual warming structure. The similarity of the two-step rise in our proxy SSTs and CO2 over T1 strongly demands for a forcing mechanism influencing both, temperature and CO2. As SSTs at our coring site are particularly sensitive to latitudinal shifts of the ACC/southern westerly wind belt system, we conclude that such latitudinal shifts may substantially affect the upwelling of deepwater masses in the Southern Ocean and thus the release of CO2 to the atmosphere as suggested by the conceptual model of [Toggweiler, J.R., Rusell, J.L., Carson, S.R., 2006. Midlatitude westerlies, atmospheric CO2, and climate change during ice ages. Paleoceanography 21. doi:10.1029/2005PA001154].

Introduction

The termination of the last ice age (Termination 1; T1) is the last major climate transition of the Earth's recent geological history and is thus crucial for our understanding of recent climate processes and the validation of climate models. Though T1 is accordingly very well studied involving numerous proxy records from both marine and terrestrial archives (e.g., Alley and Clark, 1999, Clark et al., 1999, Clark et al., 2004, Rinterknecht et al., 2006) as well as modelling studies (e.g., Knorr and Lohmann, 2003, Weaver et al., 2003), there are still a number of open questions regarding for example the exact timing and the mechanisms involved in the initiation of deglaciation and the subsequent interhemispheric pattern of the warming. Based on the Milankovitch concept, the ultimate drivers for the glacial termination are the increase in Northern Hemisphere (NH) summer insolation and non-linear responses from continental ice-sheets and particularly atmospheric greenhouse gases such as CO2 that transfer the northern signal globally (e.g. Clark et al., 1999). However, it has also been repeatedly suggested that the Southern Hemisphere (SH) leads the deglaciation and warming in the NH (e.g., Bard et al., 1997), whereas a re-evaluation of available ice-core and marine records covering T1 (Alley et al., 2002) suggests a northern temperature lead on orbital time-scales.

Part of the divergent views on possible interhemispheric leads or lags during T1 arise from the pronounced millennial-scale variations that are superimposed on primarily insolation-driven orbital-scale changes and are markedly different between the NH and SH. The general warming trend that may start as early as 23,000 years before present (23 kyr BP), based on Greenland and Antarctic ice-core records (e.g., Alley and Clark, 1999, Blunier and Brook, 2001), is further accentuated between ∼ 17 and 19 kyr BP in the south, whereas NH records show a return to cold conditions that culminate at the time of Heinrich event (HE) 1 (e.g., Alley and Clark, 1999). Thereafter, NH temperature abruptly increased into the Bølling/Allerød (B/A) warm period. In Antarctica, the deglacial warming trend was partly interrupted by a millennial-scale cooling event (Antarctic Cold Reversal, ACR) that began around the time of the B/A warming and ended close to the beginning of the Younger Dryas (YD) cold phase observed in the NH (e.g., Blunier and Brook, 2001, Morgan et al., 2002). The present picture of climate pattern during T1 is thus largely focussed on high latitude records in particular from Greenland and Antarctic ice-cores that have been synchronized by correlating globally recordable methane fluctuations (e.g., Blunier and Brook, 2001, Morgan et al., 2002, Epica Community Members, 2006). However, this correlation reveals ambiguities over the interval of the beginning deglacial warming in the SH making the analysis of interhemispheric climate pattern over this important interval more difficult.

Marine records from the SH have been involved to a much lesser extent. The available data from the Southern Ocean (e.g., Bianchi and Gersonde, 2004, Shemesh et al., 2002) and southern mid-latitudes (e.g., Pahnke et al., 2003) are generally consistent with the Antarctic records but dating uncertainties are high due to scarce datable material and/or large and potentially variable 14C reservoir ages. In addition, an increasing number of high-resolution records from the tropics have recently become available (e.g., Lea et al., 2006, Visser et al., 2003). As deglacial warming in some of these records occurred largely in phase with the CO2 increase as observed in Antarctic ice-cores, they have been interpreted in support for a tropical “trigger” for the deglaciation (Visser et al., 2003).

In this paper, we attempt to better understand the sequence of events over the last termination, on an absolute time-scale, based on a new sea surface temperature (SST) record from the SE-Pacific with exceptional time-resolution and dating accuracy over T1 (i.e., 10–25 kyr BP). Our SST data are from Ocean Drilling Project (ODP) Site 1233 located at the southern Chilean continental margin at 41°S within the northernmost reach of the Antarctic Circumpolar Current (ACC) and the southern westerly wind belt (Fig. 1). In previous works, we showed that the complete ∼ 70-kyr-long alkenone SST record at Site 1233 closely follows millennial-scale temperature fluctuations as observed in Antarctic ice cores (Kaiser et al., 2005, Lamy et al., 2004). However, the absolute age-scale over the earlier part of the last glacial, when large amplitude methane fluctuations allow a detailed inter-correlation of Greenland and Antarctic ice-cores (Blunier and Brook, 2001, Epica Community Members, 2006), is less well defined in marine sediments due to increasing uncertainties in radiocarbon dating and calendar year conversion. Therefore, we now substantially increased the time resolution around T1, an interval that spans ∼ 27 m composite core depth at Site 1233, and added a number of new 14C AMS dates, now with an average spacing of ∼ 1200 years.

Section snippets

Investigation area

Site 1233 (41°00′S; 74°27′W) is located 38 km offshore (20 km off the continental shelf) at 838 m water depth in a small forearc basin on the upper continental slope off Southern Chile (Fig. 1) away from the pathway of major turbidity currents (Mix et al., 2003). The region is located within the northernmost reach of the Antarctic Circumpolar Current (ACC) at the origin of the Peru–Chile Current (PCC) (Fig. 1). The ACC brings cold, relatively fresh, nutrient-rich, Subantarctic Surface Water

Sampling

Five Advanced Piston Corer holes were drilled at Site 1233 to ensure a complete stratigraphic overlap between cores from different holes. Detailed comparisons between high-resolution core logging data performed shipboard demonstrated that the complete sedimentary sequence down to 116.4 meters below surface (mbsf) was recovered. Based on these data, a composite sequence (the so-called splice) was constructed representing 135.65 meters composite depth (mcd). Discrete samples for alkenone analyses

Sea surface temperatures off Chile compared to Antarctic ice-core records

Deglacial warming in our alkenone SST record starts at ∼ 18.8 kyr BP with a ∼ 2-kyr-long increase of nearly 5 °C until ∼ 16.7 kyr BP (Fig. 4A). Thereafter, temperatures remain comparatively stable until the beginning of a second warming step of ∼ 2 °C between ∼ 12.7 and ∼ 12.1 kyr BP. A comparison of our SST record to different Antarctic ice-core records suggests a general correspondence in the major temperature trends, particularly the two-step warming over T1. As in our SST record, in the Pacific

Conclusions

Our SE-Pacific SST record provides a unique opportunity to discuss globally relevant processes over Termination 1 on an absolute radiocarbon-based time-scale. This point is particularly important as the lack of reliable dating accuracy often hampered the exact dating of the onset of deglacial warming in the Southern Ocean (due to large and variable reservoir ages). Furthermore, Antarctic ice core records cannot be unambiguously synchronized to the Northern Hemisphere because of only minor

Acknowledgments

We thank P. Clark, A. Ganopolski, A. Mix, A. Schmittner, B. Stenni, T. Stocker, J. Stoner, B. Weninger, and R. Tiedemann for comments and suggestions as well as T. Blunier, H. Fischer, and J. McManus for data. The constructive reviews by M. Siddall and two anonymous reviewers improved this paper. Financial support was made available through the Deutsche Forschungsgemeinschaft (DFG). This research used samples provided by the Ocean Drilling Program (ODP). The ODP is sponsored by the U.S.

References (63)

  • P.J. Müller et al.

    Calibration of the alkenone paleotemperature index UK'37 based on core-tops from the eastern South Atlantic and the global ocean (60(N-60(S)

    Geochim. Cosmochim. Acta

    (1998)
  • F.G. Prahl et al.

    Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions

    Geochim. Cosmochim. Acta

    (1988)
  • N.J. Shackleton et al.

    Absolute calibration of the Greenland time scale: implications for Antarctic time scales and for [Delta]14C

    Quat. Sci. Rev.

    (2004)
  • G. Shaffer et al.

    Circulation and variability in the Chile Basin

    Deep-Sea Res., Part I, Oceanogr. Res. Pap.

    (2004)
  • M. Siddall et al.

    Using a maximum simplicity paleoclimate model to simulate millennial variability during the last four glacial cycles

    Quat. Sci. Rev.

    (2006)
  • R.B. Alley et al.

    The deglaciation of the Northern Hemisphere

    Annu. Rev. Earth Planet. Sci.

    (1999)
  • E. Bard et al.

    Interhemispheric sychrony of the last deglaciation inferred from alkenone palaeothermometry

    Nature

    (1997)
  • T. Blunier et al.

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

    Science

    (2001)
  • T. Blunier et al.

    Synchronization of ice core records via atmospheric gases

    Clim. Past Discuss.

    (2007)
  • P.U. Clark et al.

    Northern Hemisphere ice-sheet influences on global climate change

    Science

    (1999)
  • P.U. Clark et al.

    The role of the thermohaline circulation in abrupt climate change

    Nature

    (2002)
  • P.U. Clark et al.

    Rapid rise of sea level 19,000 years ago and its global implications

    Science

    (2004)
  • B. Delmonte et al.

    Glacial to Holocene implications of the new 27000-year dust record from the EPICA Dome C (East Antarctica) ice core

    Clim. Dyn.

    (2002)
  • Epica Community Members

    Eight glacial cycles from an Antarctic ice core

    Nature

    (2004)
  • Epica Community Members

    One-to-one coupling of glacial climate variability in Greenland and Antarctica

    Nature

    (2006)
  • P.M. Grootes et al.

    Comparision of oxygen isotope records from the GISP2 and GRIP Greenland ice cores

    Nature

    (1993)
  • K. Hughen et al.

    C-14 activity and global carbon cycle changes over the past 50,000 years

    Science

    (2004)
  • A. Indermühle et al.

    Atmospheric CO2 concentration from 60 to 20 kyr BP from the Taylor Dome ice core, Antarctica

    Geophys. Res. Lett.

    (2000)
  • J. Kaiser et al.

    A 70-kyr sea surface temperature record off southern Chile (ODP Site 1233)

    Paleoceanography

    (2005)
  • G. Knorr et al.

    Southern Ocean origin for the resumption of Atlantic thermohaline circulation during deglaciation

    Nature

    (2003)
  • R. Knutti et al.

    Strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation

    Nature

    (2004)
  • Cited by (0)

    View full text