Holocene climate in the Atlantic sector of the Southern Ocean: Controlled by insolation or oceanic circulation?

ABSTRACTThe Holocene climate of the Southern Ocean is not well un-derstood, mainly because of the lack of high-resolution reconstruc-tions of ocean surface properties. Here we present a 12,500-yr-long,decadal-scale record of Holocene sea-surface temperatures and sea-ice presence from the Polar Front of the East Atlantic SouthernOcean. The record shows gradual climate change, with no abruptNeoglacial cooling, and an unprecedented late Holocene warming.The dominant forcing factor appears to be precessional insolation;Northern Hemisphere summer insolation correlates to at least theearly to middle Holocene climate trend. Spectral analysis revealscentennial-scale cyclic climate changes with periods of 1220, 1070,400, and 150 yr. The record shows good correlation to East Ant-arctic ice cores and to climate records from South Georgia andBunger Oasis. However, the record shows out-of-phase behaviorwith regard to climate records from the western Antarctic Penin-sula and the Peru-Chile Current; such behavior hints at a climaticdivide through Patagonia, the Drake Passage, and between Westand East Antarctica.Keywords: Southern Ocean, paleoclimate, diatoms, modern analogtechnique, Holocene, insolation, Antarctic Polar Front.INTRODUCTIONThe Holocene climate variability has received less attention thanthe more dramatic changes during the last glacial period. The Holoceneis generally assumed to be a stable interglacial period when climatechanges in the North Atlantic are transmitted to the rest of the worldthrough a strong thermohaline circulation (Imbrie et al., 1992). Thelarge ﬂuctuations of the last glacial period appear to continue into thisinterglacial, although in a subdued manner (Bond et al., 2001). The Southern Ocean has received even less attention than the highnorthern latitudes; oceanic records are few and coarse. They generallydisplay a warm early Holocene followed by cooling (Labracherie etal., 1989; Pichon et al., 1992; Hodell et al., 2001), similar to NorthAtlantic climate (e.g., Koc¸ and Jansen, 1994). High-resolution recordsare restricted to Antarctic ice cores, which mostly display little climatevariability during the Holocene (Jouzel et al., 2001). An exception isthe Taylor Dome deuterium record, in which the Holocene appears tobe split by an abrupt cooling event ca. 5.5–5 ka (Steig et al., 1998). This cooling appears to ﬁt the late Holocene global cooling trend (Por-ter, 2000) as a nonlinear response from the climate system to insolationchanges (deMenocal et al., 2000; Hodell et al., 2001). High-resolution Holocene records available for Southern Oceanclimate reconstruction are rarely from the Antarctic Circumpolar Cur-rent proper (e.g., Domack et al., 2001; Stenni et al., 2001; Lamy et al.,2002), with the exception of a multiproxy approach to Holocene cli-mate that includes a high-resolution description of lithic fragments(Hodell et al., 2001). We present data from site TN057-17 near the Polar Front in theEast Atlantic Southern Ocean. It is based on quantitative estimates ofsummer sea-surface temperature (SSST) and sea-ice presence (SIP),utilizing the down-core variability of diatom assemblages. The recordshows good correlation to Antarctic ice cores, but a gradual change inphase relationships. During the early Holocene, the record shows anin-phase relationship with the Ross Sea region ice cores; during thelate Holocene, the record shows an in-phase relationship with the EastAntarctic region ice cores.Antarctic Polar FrontThe Antarctic Polar Front (APF, Fig. 1) is an important climateboundary in terms of air-sea ﬂuxes and the heat and salt budgets ofthe oceans. At the surface, the APF is a steep sea-surface temperaturegradient, while the subsurface expression is the southern limit for sub-duction of cold, fresh Antarctic Surface Water below warmer and moresaline Subantarctic Surface Water (Orsi et al., 1995). Globally, the APFcould be the southern limit of synchrony with Northern Hemisphereclimate (Broecker, 1996). The APF is subject to large spring and sum-mer blooms of diatoms (Fischer et al., 2002), and achieves the largestbiogenic sedimentation rates of the diatom ooze belt of the SouthernOcean (Burckle and Cirilli, 1987). It is these high sedimentation ratesof diatomaceous oozes and the sensitivity of diatoms especially to lightand temperature that make sites in the APF region suitable for pro-ducing high-resolution records of climate change.