Jérôme Kaiser

Holocene changes in the position and intensity of the southern westerly wind belt

The position and intensity of the southern westerly wind belt varies seasonally as a consequence of changes in sea surface temperature. During the austral winter, the belt expands northward and the wind intensity in the core decreases. Conversely, during the summer, the belt contracts, and the intensity within the core is strengthened. Reconstructions of the westerly winds since the last glacial maximum, however, have suggested that changes at a single site reflected shifts throughout the entire southern wind belt 1‐4 . Here we use sedimentological and pollen records to reconstruct precipitation patterns over the past 12,500 yr from sites along the windward side of the Andes. Precipitation at the sites, located in the present core and northern margin of the westerlies, is driven almost entirely by the wind belt 5 , and can be used to reconstruct its intensity. Rather than varying coherently throughout the Holocene epoch, we find a distinct anti-phasing of wind strength between the core and northern margin over multi-millennial timescales. During the early Holocene, the core westerlies were strong whereas the northern margin westerlies were weak. We observe the opposite pattern in the late Holocene. As this variation resembles modern seasonal variability, we suggest that our observed changes in westerly wind strength can best be explained by variations in sea surface temperature in the eastern South Pacific Ocean. Chile is ideally located to reconstruct past variability of the southern westerly wind belt (SWW) as the SWW almost entirely controls precipitation on the western side of the Andes in southern South America with an extreme northsouth

Melting of the Patagonian Ice Sheet and deglacial perturbations of the nitrogen cycle in the Eastern South Pacific

local 230 Th-normalized biogenic vertical fluxes from the Chilean continental margin. They document in detail the sharp transition from relatively low WCD rates during the Last Glacial Maximum (LGM) to high ones during the deglaciation and Holocene, but give no evidence that the glacial-interglacial difference of WCD could have been caused by changes in local primary productivity. Furthermore, we found no evidence that changes in ventilation due to SAMW formation could explain the nitrogen isotope record. We present evidence for an alternative mechanism related to the melting of the Patagonian Ice Sheet (PIS), which is consistent with recent published proxy data and the regional physical oceanography.

Glacial reduction and millennial-scale variations in Drake Passage throughflow

Significance The Drake Passage (DP) represents the most important oceanic gateway along the pathway of the world’s largest current: the Antarctic Circumpolar Current (ACC). Resolving changes in the flow of circumpolar water masses through the DP is crucial for advancing our understanding of the Southern Ocean’s role in affecting ocean and climate change on a global scale. We reconstruct current intensity from marine sediment records around the southern tip of South America with unprecedented millennial-scale resolution covering the past ∼65,000 y. For the last glacial period, we infer intervals of strong weakening of the ACC entering the DP, implying an enhanced export of northern ACC surface and intermediate waters into the South Pacific Gyre and reduced Pacific–Atlantic exchange through the cold water route. The Drake Passage (DP) is the major geographic constriction for the Antarctic Circumpolar Current (ACC) and exerts a strong control on the exchange of physical, chemical, and biological properties between the Atlantic, Pacific, and Indian Ocean basins. Resolving changes in the flow of circumpolar water masses through this gateway is, therefore, crucial for advancing our understanding of the Southern Ocean’s role in global ocean and climate variability. Here, we reconstruct changes in DP throughflow dynamics over the past 65,000 y based on grain size and geochemical properties of sediment records from the southernmost continental margin of South America. Combined with published sediment records from the Scotia Sea, we argue for a considerable total reduction of DP transport and reveal an up to ∼40% decrease in flow speed along the northernmost ACC pathway entering the DP during glacial times. Superimposed on this long-term decrease are high-amplitude, millennial-scale variations, which parallel Southern Ocean and Antarctic temperature patterns. The glacial intervals of strong weakening of the ACC entering the DP imply an enhanced export of northern ACC surface and intermediate waters into the South Pacific Gyre and reduced Pacific–Atlantic exchange through the DP (“cold water route”). We conclude that changes in DP throughflow play a critical role for the global meridional overturning circulation and interbasin exchange in the Southern Ocean, most likely regulated by variations in the westerly wind field and changes in Antarctic sea ice extent.

Glacial to Holocene Paleoceanographic and Continental Paleoclimate Reconstructions Based on ODP Site 1233/GeoB 3313 Off Southern Chile

ODP Site /GeoB 3313 located at the upper continental slope off southern Chile (41°S) is ideally located to study latitudinal shifts of atmospheric and oceanographic circulation off southwestern South America. Extraordinarily high sedimentation-rates allow for high resolution reconstructions and detailed comparisons of various continental climate and paleoceanographic proxy records within the same archive avoiding problems linked to age model uncertainties. We discuss the major paleoclimatic findings of Site 1233/GeoB 3313 in chronological order from the last glacial to the Holocene within the regional context and explore links to tropical and high southern latitude records. During the last glacial, sea surface temperatures (SSTs) off southern Chile were about 4.5°C colder than today and ∼6–7°C colder than during the early Holocene maximum. Deglacial warming started at ∼18.8 kyr BP with a ∼2-kyr-long increase of nearly 5°C and was followed by relatively stable SSTs until the beginning of a second warming step of ∼2°C during the early halve of the Northern Hemisphere Younger Dryas cold period. Maximum warm conditions in the early Holocene (∼12–9 kyr BP) were followed by a gradual decline towards modern SSTs in the Late Holocene. The paleoceanographic changes and related regional continental climate variations since the last glacial are primarily controlled by latitudinal shift of both the oceanographic and the atmospheric circulation systems in the southeast Pacific. In general, they appear to be closely linked to changes in the Southern Ocean and Antarctica.

Black Sea temperature response to glacial millennial-scale climate variability

The Eurasian inland propagation of temperature anomalies during glacial millennial-scale climate variability is poorly understood, but this knowledge is crucial to understanding hemisphere-wide atmospheric teleconnection patterns and climate mechanisms. Based on biomarkers and geochemical paleothermometers, a pronounced continental temperature variability between 64,000 and 20,000 years ago, coinciding with the Greenland Dansgaard-Oeschger cycles, was determined in a well-dated sediment record from the formerly enclosed Black Sea. Cooling during Heinrich events was not stronger than during other stadials in the Black Sea. This is corroborated by modeling results showing that regular Dansgaard-Oeschger cycles penetrated deeper into the Eurasian continent than Heinrich events. The pattern of coastal ice-rafted detritus suggests a strong dependence on the climate background state, with significantly milder winters during periods of reduced Eurasian ice sheets and an intensified meridional atmospheric circulation.

The Southern Westerlies During the Holocene: Paleoenvironmental Reconstructions from Chilean Lake, Fjord, and Ocean Margin Sediments Combined with Climate Modeling

This project aimed at investigating centennial to millennial-scale changes of the strength and position of the southern westerly wind belt (SWW) using multi-proxy paleoprecipitation and paleoceanographic records combined with transient model runs. The proxy data records reveal a distinct latitudinal anti-phasing of wind changes between the core and northern margin of the SWW over the Holocene. During the early Holocene, the SWW core was enhanced and the northern margin was reduced, whereas the opposite pattern is observed in the late Holocene. These Holocene changes resemble modern seasonal wind belt variations and can be best explained by varying sea-surface temperature fields in the eastern South Pacific. Transient modeling experiments from the mid- to late Holocene are not yet consistent with these proxy results. However, a good data-model agreement exists when investigating the potential impact of solar variability on the SWW at centennial time-scales during the latest Holocene with periods of lower (higher) solar activity causing equatorward (southward) shifts of the SWW.