Millennial Scale Cycles

Antarctic ice cores record frequent abrupt changes in temperature at millennial scales that are difficult to explain by variations in orbital parameters. Over the last 90 kyr, Antarctica has been subject to seven prominent warm periods designated A1-A7 (Blunier and Brook, 2001, and discussed above). With the possible exception of A5 and A6, the others are well expressed in SST records from southern mid-latitudes (Fig. 11.6; Barrows et al., 2007). But perhaps the best-documented abrupt change in terms of identifying oceanic responses is the Antarctic Cold Reversal (ACR), which is timed at around ~ 14.1-12.4 ka according to the EPICA deuterium record (Blunier et al., 1997; EPICA Community Members, 2004). An abrupt cooling of ~2°C appears to have been accompanied by an expansion of sea ice (Shemesh et al., 2002) and a modest intensification of winds as identified from sea salt and dust proxies in ice cores (Stenni et al., 2001; Rothlisberger et al., 2002). As noted previously, an expanded Antarctic cryosphere can invigorate the THC (e.g. Hall et al., 2001), and judging by the modern oceanography, such a change would be translated rapidly through the Southern Ocean (e.g. Haine et al., 1998). Thus, off eastern New Zealand, the Pacific gateway for the THC, benthic 818O becomes heavier directly in phase with the ACR (Carter et al., 2008). In contrast, change in the surface ocean about the time of the ACR is variable. South of the Subtropical Front, SSTs cool in phase with the ACR, as recorded in the SW Pacific (Pahnke et al., 2003), SE Pacific (Lamy et al., 2004) and the south Atlantic (Kanfoush et al., 2000; Sachs et al., 2001; Shemesh et al., 2002; Gersonde et al., 2003). These sites have direct atmospheric and oceanic links to Antarctica, especially during glacial periods when polar effects are enhanced through the northward migration of westerly winds and surface ocean waters. By comparison, mid-latitude records have a more delayed and muted response to the ACR. Off eastern New Zealand, for example, there was no obvious response until ~ 13.5 ka when the ACR was at its coldest (Carter et al., 2008). Then, SSTs became cooler, marine fertility dropped and the uppermost ocean either became more mixed or the thermocline more shallow. Onshore, pollen and speleothem data reveal cooler, windier conditions that were accompanied by an expansion of glaciers and, at the coast, by a likely stillstand in sea level (Carter et al., 1986; Turney et al., 2003; Williams et al., 2004). The delayed reaction to the ACR is likely connected to a contemporaneous re-establishment of the Subtropical Inflow as it migrated south during the deglacial phase (e.g. Martinez, 1994). Like the southwest Pacific, the south Indian Ocean also cooled out of phase. SSTs reduced by 0.8°C between 13.2 and 12 ka, ~ 1,000 years after commencement of the ACR demonstrating the regionality of abrupt climate change effects (Stenni et al., 2001).

As core chronologies improve (see Steig, 2001), it is becoming apparent that some millennial-scale variability in the Southern Ocean is linked with abrupt changes documented in the NH. Barrows et al. (2007) draw attention to the timing of Southern Ocean warm phases, A1-A4, with warm phases of D-O cycles, 8, 12, 14 and 17 identified in the Greenland ice core, GISP 2. Likewise, Sachs and Anderson (2005) observe phases of marked algal productivity off eastern New Zealand that occurred within 1-2 ka of massive iceberg influxes associated with Heinrich Events 1-6 in the North Atlantic. Another northern cold event that has received much attention in the Southern Ocean is the Younger Dryas (YD) of 13-11.5 kyr (e.g. Morigi et al., 2003; Turney et al., 2003; Bianchi and Gersonde, 2004; Sachs and Anderson, 2005; amongst others). Because of the YD's partial overlap with the ACR (14.1-12.4kyr) and the Oceanic Cold Reversal (13.2-12kyr) in the Indian Ocean (Stenni et al., 2001) conclusive evidence for a YD effect is still under debate, the resolution of which hinges on the quality of palaeoenvironmental chronologies. Once resolved, the mechanisms that translate YD and other abrupt NH perturbations to the Southern Ocean (and vice versa) can be better evaluated to allow a more informed assessment of potential change in the future.

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