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