ALBERT

Description: The Mid-Pleistocene transition (MPT) of the global climate system, initiated by a shift towards much larger northern hemisphere ice shields at around 920 ka and ending with predominance of 100 kyr ice age cyclicity since about 640 ka, is one of the fundamental enigmas in Quaternary climate evolution. Climate proxy records not exclusively linked to global ice volume are necessary to advance understanding of the MPT. Here we present a high-resolution Pleistocene magnetic susceptibility time series of 12 sediment cores from the subtropical South Atlantic essentially reflecting dissolution driven variations in carbonate accumulation controlled by changes in deep water circulation. In addition to characteristics known from delta18O records, the data sets reveal three remarkable features intimately related to the MPT: (1) an all-Pleistocene minimum of carbonate accumulation in the South Atlantic at 920 ka, (2) a MPT interim state of reduced carbonate deposition, indicating that the MPT period may have been a discrete state of the Pleistocene deep water circulation and climate system and (3) a terminal MPT event at around 540-530 ka documented in several peculiarities such as thick laminated layers of the giant diatom Ethmodiscus rex.

Description: Sr and Nd isotopic compositions of Late Quaternary surface sediment and sediment cores from the south Atlantic and southeast Pacific sectors of the Southern Ocean are used to constrain the provenance and transport mechanisms of their terrigenous component. We report isotopic and mineralogical data for core samples from three localities, the Mid-Atlantic Ridge at 41°S and the northern and southern Scotia Sea. In addition, data for surface sediment samples from the south Atlantic and southeast Pacific sectors of the Southern Ocean are presented. The variations of Sr and Nd isotopic compositions of the bulk sediment samples in all cores were correlated with the magnetic susceptibility of the sediment and with the inferred glacial-interglacial stages. The isotopic data indicate that, during glacial periods, sediment was delivered from continental crust with a shorter residence time than that supplying material during interglacial periods. At the core site near the Mid-Atlantic Ridge, Nd isotopic, combined with mineralogical evidence indicates interglacial period deposition of a relatively high amount of kaolinite and silt with low epsilon-Nd values 〈 -8. The material was probably supplied by North Atlantic Deep Water from low latitudes. For glacial periods, a high contribution of silt and clay with epsilon-Nd 〉 -4.5, probably derived from southern South America, was indicated. The glacial-interglacial shift in sources may be due to either a decreasing influence of North Atlantic Deep Water during glacial times or by a larger contribution of glaciogenic detritus from southern South America. At the core site in the northern Scotia Sea, sediment of interglacial periods is dominated by smectite with epsilon-Nd 〈 - 6 and silt with epsilon-Nd 〉 -4. We suggest that smectite was derived from the Falkland shelf and silt was derived from the Argentinian shelf. During glacial periods, the Argentinian shelf was an important source for silt and chlorite with epsilon-Nd 〉 -4. The contribution from the Falkland shelf seems to have remained similar during glacial and interglacial periods. Hydrographic transport by bottom currents and turbidites could account for the high glacial detrital flux. An evaluation of the significance of an aeolian contribution to deep sea sediment suggests that it plays only a minor role. In the southern Scotia Sea, the Antarctic Peninsula is considered an important source for young material with epsilon-Nd 〉 -4, in particular during glacial periods. During interglacial periods, sediment supply from the Antarctic Peninsula was lower than during glacial times, resulting in a relatively high contribution of old material (epsilon-Nd 〈 -8) from East Antarctica. Deep water currents and icebergs could account for the transport of the old component to the southern Scotia Sea. The accumulation rates of material from the various source regions for glacial times are in agreement with an increase in the strength of the Antarctic Circumpolar Current. The production rate and the circulation pattern of bottom water in the Weddell Sea appear to have remained similar over most of the last 150 kyr.