Sea-surface temperature reconstruction in the Southwest Pacific

Uniquely in the Southern Hemisphere the New Zealand micro-continent spans the interface between a subtropical gyre and the Subantarctic Circumpolar Current. Its 20° latitudinal extent includes a complex of submerged plateaux, ridges, saddles and basins which, in the present interglacial, are partial barriers to circulation and steer the Subtropical (STF) and Subantarctic (SAF) fronts. This configuration offers a singular opportunity to assess the influence of bottom topography on oceanic circulation through Pleistocene glacial - interglacial (G/I) cycles, its effect on the location and strength of the fronts, and its ability to generate significant differences in mixed layer thermal history over short distances. For this study we use new planktic foraminiferal based sea-surface temperature (SST) estimates spanning the past 1 million years from a latitudinal transect of four deep ocean drilling sites. We conclude that: 1. the effect of the New Zealand landmass was to deflect the water masses south around the bathymetric impediments; 2. the effect of a shallow submerged ridge on the down-current side (Chatham Rise), was to dynamically trap the STF along its crest, in stark contrast to the usual glacial-interglacial (G-I) meridional migration that occurs in the open ocean; 3. the effect of more deeply submerged, downstream plateaux (Campbell, Bounty) was to dynamically trap the SAF along its steep southeastern margin; 4. the effects of saddles across the submarine plateaux was to facilitate the development of jets of subtropical and subantarctic surface water through the fronts, forming localized downstream gyres or eddies during different phases in the G-I climate cycles; 5. the deep Pukaki Saddle across the Campbell-Bounty Plateaux guided a branch of the SAF to flow northwards during each glacial, to form a strong gyre of circumpolar surface water in the Bounty Trough, especially during the mid-Pleistocene Climate Transition (MIS 22-16) when exceptionally high SST gradients existed across the STF; 6. the shallower Mernoo Saddle, at the western end of the Chatham Rise, provided a conduit for subtropical water to jet southwards across the STF in the warmest interglacial peaks (MIS 11, 5.5) and for subantarctic water to flow northwards during glacials; 7. although subtropical or subantarctic drivers can prevail at a particular phase of a G-I cycles, it appears that the Antarctic Circumpolar Current is the main influence on the regional hydrography. Thus complex submarine topography can affect distinct differences in the climate records over short distances with implications for using such records in interpreting global or regional trends. Conversely, the local topography can amplify the paleoclimate record in different ways in different places, thus enhancing its value for the study of more minor paleoceanographic influences that elsewhere are more difficult to detect. Such sites include DSDP 594, which like some other Southern Ocean sites, has the typical late Pleistocene asymmetrical saw-tooth G-I climate pattern transformed to a gap-tooth pattern of quasi-symmetrical interglacial spikes that interrupt extended periods of minimum glacial temperatures.

(Table 2) Age determination of sediment cores from the Gulf of Cadiz

We present micropalaeontological and grain-size records for a set of sediment cores from the Gulf of Cadiz (southwest Spain) that reflect changes in the position and strength of the Mediterranean Outflow (MO) current. The cores sample a sediment drift (the Gil Eanes Drift) that is positioned lower on the slope in the Gulf of Cadiz than the position of the main current today. The data indicate that the drift is of glacial age and that the glacial MO current was positioned lower on the slope than today but also that it was active over a considerably reduced area of the slope. We argue that this observation is consistent with physical constraints on the Gibraltar Exchange and on the likely settling and spreading behavior of the MO plume along the Iberian Margin under glacial environmental and sea level conditions. The deeper settling of the MO is likely to have influenced the formation of glacial North Atlantic Intermediate Water and also may have exerted indirect influence on the formation of glacial North Atlantic Deep Water.

Benthic foraminifera assemblages of the Gulf of Aden

Benthic and selected planktic foraminifera and stable isotope records were determined in a piston core from the Gulf of Aden, NW Arabian Sea that spans the last 530 ka. The benthic foraminifera were grouped into four principal assemblages using Q-mode Principal Component Analyses. Comparison of each of these assemblages with the fauna of the nearby regions enabled us to identify their specific environmental requirements as a function of variability in food supply and strength of the oxygen minimum zone and by that to use them as indicators of surface water productivity. The benthic foraminiferal productivity indicators coupled with the record of Globigerina bulloides, a planktic foraminifer known to be sensitive to productivity changes in the region, all indicate higher productivity during glacial intervals and productivity similar to present or even reduced during interglacial stages. This trend is opposite to the productivity pattern related to the SW summer monsoon of the Arabian Sea and indicates the role of the NE winter monsoon on the productivity of the Gulf of Aden. A period of exceptionally enhanced productivity is recognized in the Gulf of Aden region between ~60 and 13 kyr indicating the intensification of the NE winter monsoon to its maximal activity. Contemporaneous indication of increased productivity in other parts of the Arabian Sea, unexplained so far by the SW summer monsoon variability, might be related to the intensification of the NE winter monsoon. Another prominent event of high productivity, second in its extent to the last glacial productivity event is recognized between 430 and 460 kyr. These two events seem to correspond to periods of similar orbital positioning of rather low precession (and eccentricity) amplitude for a relatively long period. Glacial boundary conditions seem to control to a large extent the NE winter monsoon variability as also indicated by the dominance of the 100 ka cycle in the investigated time series. Secondary in their importance are the 23 and 41 ka cycles which seem also to contribute to the NE monsoonal variability. Following the identification of productivity events related to the NE winter monsoon in the Gulf of Aden, it is possible now to extend this observation to other parts of the Arabian Sea and consider the contribution of this monsoonal system to the productivity fluctuations preserved in the sedimentary records.

The Atlantic Meridional Overturning Circulation during the last glacial

We reconstruct the geometry and strength of the Atlantic Meridional Overturning Circulation during Heinrich Stadial 2 and three Greenland interstadials of the 20-50 ka period based on the comparison of new and published sedimentary 231Pa/230Th data with simulated sedimentary 231Pa/230Th. We show that the deep Atlantic circulation during these interstadials was very different from that of the Holocene. Northern-sourced waters likely circulated above 2500 m depth, with a flow rate lower than that of the present day North Atlantic Deep Water (NADW). Southern-sourced deep waters most probably flowed northwards below 4000 m depth into the North Atlantic basin, and then southwards as a return flow between 2500 and 4000 m depth. The flow rate of this southern-sourced deep water was likely larger than that of the modern Antarctic Bottom Water (AABW). Our results further show that during Heinrich Stadial 2, the deep Atlantic was probably directly affected by a southern-sourced water mass below 2500 m depth, while a slow southward flowing water mass originating from the North Atlantic likely influenced depths between 1500 and 2500 m down to the equator.