The late Quaternary palaeoenvironmental changes along the western South-American continental slope: A reconstruction based on dinoflagellate cysts and TEX₈₆

The necessity to predict future climate change has never been greater than today. However, in order to validate climate models and to make accurate predictions, a good comprehension of past climate dynamics is a prerequisite. Since the basic patterns of past climate change are yet not fully understood, fundamental research remains a necessity to elucidate the timing and the extension of pronounced climatic events. Until recently, most palaeoclimatological studies concentrated on the northern hemisphere since the North Atlantic deep water formation was considered as the main mechanism regulating millennial-scale climate variability. In 2003, several authors however demonstrated that the Southern Ocean could have played a prominent role in global climate regulation. This statement led to a gradual increase in the number of palaeoclimatological studies in the southern hemisphere. Other scientists pointed to the importance of tropical circulations, such as the El Niño Southern Oscillation, in controlling glacial/interglacial transitions. There is still controversy about the impact and the extent of major high-latitude climate reversals such as the northern hemisphere Younger Dryas and the southern hemisphere Antarctic Cold Reversal. Particularly, the extent to which the southern hemisphere high-latitude ocean-atmosphere dynamics determine the southern South American climate, caused by shifts of the Antarctic Circumpolar Current (ACC) and southern westerly wind (SWW) belt, is still a matter of debate. A late Quaternary palaeoenvironmental reconstruction using dinoflagellate cysts and organic geochemical proxies was carried out at ODP Site 1233 (41°0’S, 74°27’W) in the Southeast Pacific, and allowed a better insight into the late Quaternary climate dynamics, i.e., temperature variations, latitudinal shifts of the ACC/SWW-coupled system, changes in the supply of nutrients, etc. Additionally, studies were carried out to improve and to refine environmental proxies such as the process length variation of Operculodinium centrocarpum as a density proxy, the knowledge of ecological preferences of certain dinoflagellate cyst species and the TEX86 index as a temperature proxy. These proxies subsequently allowed a more detailed reconstruction of the palaeoenvironment at Site 1233 during the last 25 kyr. Our study demonstrates that dinoflagellate cyst assemblages cannot be used unambiguously to quantify past variations in a particular environmental parameter, such as sea surface salinity and sea surface temperature. Assemblage compositions are controlled by an interplay between multiple environmental variables, which render it difficult to separate the unique effects of diverse environmental factors in altering the cyst compositions. However, the presence of particular species may point to specific oceanographic dynamics, such as the presence or absence of upwelling. In contrast, the process length variability of the dinoflagellate cyst Operculodinium centrocarpum can be used to quantify past changes in sea surface density, as long as the average process lengths do not exceed 10.5 µm. The reason for this limitation is the absence of modern analogues in high density environments of more than 1,026 kg m-3. The productivity variations of dinoflagellates, which are dominated by heterotrophic species, at Site 1233 are regulated by their prey availability, mainly diatoms, which in turn are dependent on nutrient availability. Our data suggests that nitrate availability is the limiting factor regulating productivity variations offshore South Chile, while iron fertilisation negatively affects the silica/nitrate consumption rates of diatoms leading to a decrease in productivity because of nitrate depletion. The TEX86 palaeothermometer down-core ODP 1233 is often interrupted by the enrichment of 13C-depleted isoprenoidal glycerol dialkyl glycerol tetraether (GDGT)-1 and GDGT-2, mainly produced by methane-consuming Archaea during anaerobic oxidation of methane. Other deviations with respect to the alkenone-based sea surface temperature record are the result of variable growing seasons of pelagic Thaumarchaeota, caused by variations in primary productivity. The terrestrial supply of isoprenoidal GDGTs in turn was too low to bias the TEX86 signal as indicated by the BIT index. The latter demonstrates that the variable supply of soil organic matter towards Site 1233 is related to Patagonian ice sheet dynamics and not to variations in onshore precipitation. Our findings indicate a 6 to 7° northward shift of the ACC/SWW-coupled system during the Last Glacial Maximum (25-18.6 cal ka BP). Upwelling was prevented by the onshore blowing westerlies, and macro-nutrients were therefore supplied from the Southern Ocean by cross-frontal northward advection of Subantarctic Surface Water. A slight poleward shift of the ACC/SWW occurred around 21.3 cal ka BP, followed by a partial return between 20 and 18.6 cal ka BP. At the same time, the Patagonian ice sheet gradually extended towards the end of the Last Glacial Maximum. A two step warming phase during the last deglaciation has been observed. At 18.6 cal ka BP, the ACC/SWW started to migrate towards Antarctica as the result of a global reorganisation of atmospheric circulations related to a northern hemisphere cooling event. Together with a weakening of the Atlantic Meridional Overturning Circulation (AMOC), this resulted in a fast rise in SST (4 °C) in the SE-Pacific mid-latitudes. A southward shift of the ACC caused a decrease in nutrient availability at Site 1233, which subsequently became even more diluted after 17.8 cal ka BP by a large fresh water input related with a first melting phase of the Patagonian ice sheet. The Antarctic Cold Reversal period (14.4-12.9 cal ka BP) is characterised by unstable conditions and/or extreme seasonality caused by the vicinity of the Subtropical Front. The ACC/SWW did not considerably shift equatorward in response to a northern hemisphere warming and a stronger AMOC. Deep mixing (=100 m) may have occurred, associated with a strengthening of the westerlies at 41°S. At the same time, the Patagonian glaciers stabilised or slightly readvanced. The second warming phase of ~2 °C between 12.9-11.1 cal ka BP coincides with the northern hemisphere Younger Dryas and with a weakening of the AMOC. The latter induced a global atmospheric reorganisation, and caused a southward shift of the ACC/SWW. The Subtropical Front and the northern margin of the SWW during summer were now located southward of the study area. Upwelling of nutrient-rich subsurface water occurred during austral summer, but nutrients were diluted by a second fresh water input associated with melting glaciers onshore and by a decrease of the silica:nitrate uptake ratio by diatoms caused by iron fertilisation. The Holocene climatic optimum is observed between 11.6 and 9.8 cal ka BP, and is characterised by the most southward position of the ACC/SWW. The upwelling continued, and the nutrient availability in the surface waters increased caused by a decrease in fresh water supply. At the same time, nitrate was less intensively consumed because of a decline in iron input. The intensification of the AMOC resulted in a cooling of the southern hemisphere and a northward shift of the ACC/SWW between 9.8 and 7 cal ka BP. No upwelling occurred during this period and the westerlies were probably more intense because of the Antarctic sea ice extension and the occurrence of La Niña-like conditions. During the midto late Holocene (7 cal ka BP to present), the AMOC remained fairly constant, such that latitudinal shifts of the ACC/ SWW are mainly regulated by the Antarctic sea ice extension and the variability of tropical circulations, such as the El Niño Southern Oscillation and Hadley Cell. The effects of those tropical circulations on the strength and position of the southeastern Pacific anticyclone and the SWW lead to a variable sea surface density at Site 1233. The latitudinal shifts of the SWW furthermore controlled upwelling intensity at 41°S; seasonal upwelling occurred during dry periods while no indications for upwelling are observed during wet periods. The northward shift of the ACC during the Holocene made that the Subtropical Front was again located equatorward of 41°S after 5.4 cal ka BP. A fast northward shift of the ACC/SWW occurred between 0.8 cal ka BP and present, and was most likely related to a cooling on Antarctica. Our results demonstrate that climate variability in the Southeast Pacific mid-latitudes during the last 25 kyr is closely coupled to global atmospheric and oceanographic reorganisations. Both the northern and southern hemisphere high-latitudes play a crucial role in regulating millennial-scale climate variability, while the effects of variable tropical circulations seem to superimpose on the large scale fluctuations controlled by (sub)polar dynamics