Rolf Kilian

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

Weather Observations Across the Southern Andes at 53°S

Regional variations of weather pattern were analyzed along a west-to-east profile across the Southern Andes (53°S), one of the most pronounced climate-divides in the world. For the first time we present a meteorological record from an array of three automatic weather stations (AWS), operated by the authors, for the central part of the climate divide which, together with previously existing Chilean weather stations, complete the transect. These data cover a time period of 3 yr. from October 1999 until September 2002. Air temperatures along the profile are highly correlated. Annual precipitation drops from between 6000 mm and 7000 mm at sea level along the main divide of the mountains to only about 1000 mm at the eastern slopes of the Andes and to as little as 430 mm at Punta Arenas. The variations of rainfall with wind direction and synoptic weather types are markedly different between the central part of the Andes and Punta Arenas. At the center of the climate divide precipitation correlates positively with wind speed from the west, whereas at Punta Arenas, east of the Andes, higher rainfall rates occur with easterly air flow. It is assumed that this reflects the barrier effect of the mountain range of the Andes. The results indicate that in order to make references about present or past climatic variations in Patagonia, it is essential to consider the effect of changes in circulation patterns.

Increased Dust Deposition in the Pacific Southern Ocean During Glacial Periods

Dust deposition in the Southern Ocean constitutes a critical modulator of past global climate variability, but how it has varied temporally and geographically is underdetermined. Here, we present data sets of glacial-interglacial dust-supply cycles from the largest Southern Ocean sector, the polar South Pacific, indicating three times higher dust deposition during glacial periods than during interglacials for the past million years. Although the most likely dust source for the South Pacific is Australia and New Zealand, the glacial-interglacial pattern and timing of lithogenic sediment deposition is similar to dust records from Antarctica and the South Atlantic dominated by Patagonian sources. These similarities imply large-scale common climate forcings, such as latitudinal shifts of the southern westerlies and regionally enhanced glaciogenic dust mobilization in New Zealand and Patagonia. A million-year-long marine sedimentary record of dust supply to the Pacific Southern Ocean reflects global climate. Dust in the Sea The effect of windblown dust on marine productivity in the Southern Ocean is thought to be a key determinant of atmospheric CO2 concentrations. Lamy et al. (p. 403) present a record of dust supply to the Pacific sector of the Southern Ocean for the past one million years, derived from a suite of deep-sea sediment cores. Dust deposition during glacial periods was 3 times greater than during interglacials, and its major source region was probably Australia or New Zealand.

Constraints on the interaction between slab melts and the mantle wedge from adakitic glass in peridotite xenoliths

Peridotite xenoliths from the Quaternary Cerro del Fraile basalts, southernmost South America, sample the mantle less than 25 km east of the Andean Austral Volcanic Zone (AVZ), an arc segment characterized by melting of a young, ‘hot’, subducted slab and the eruption of adakites. Many of these peridotite xenoliths are modified by either modal and/or cryptic Na-rich metasomatism, which produced elevated Sr/Y, La/Yb and La/Nb ratios typical of slab melts. Some of the metasomatized xenoliths, derived from a relatively deep and hot portion of the mantle, contain an interconnected network along mineral grain boundaries of high-Mg#, low-Y andesitic glass with major and trace element composition similar to the high-Mg adakites erupted in the AVZ. We interpret this adakitic glass to be a quenched slab melt that has infiltrated the mantle wedge from below. The texture and chemistry of this quenched melt and surrounding mantle minerals suggest that selective assimilation of predominately mantle clinopyroxene, some spinel and minor olivine is an important process in producing high-Mg adakites from primary low-Mg slab melts.

Elevated mercury accumulation in a peat bog of the Magellanic Moorlands, Chile (53‡S) ^ an anthropogenic signal from the Southern Hemisphere

Increasing mercury deposition rates in the Northern Hemisphere recorded in natural archives such as peat bogs or lake sediments have been documented in numerous studies. However, data on atmospheric Hg deposition in the Southern Hemisphere dating back to pre-industrial times are rare. Here, we provide a continuous record of atmospheric Hg deposition in the Southern Hemisphere recorded by an ombrotrophic peat bog of the Magellanic Moorlands, Chile (53‡S), extending back 3000 yr. Pre-industrial mercury accumulation rates range between 2.5 and 3.9 Wg/m 2 /yr. In the past 100 yr, Hg accumulation rates increased 18-fold from about 3 Wg/m 2 /yr to a maximum of 62.5 Wg/m 2 /yr. If Hg accumulation rates were normalized to peat accumulation rates, maximum rates were 7.9 Wg/m 2 / yr, which is only 2.5 times the pre-industrial rates. Thus, Hg accumulation rates normalized to peat accumulation rates are more comparable to the three-fold net increase in atmospheric Hg concentrations estimated for the same period. We suggest that the increase in Hg accumulation rates in the Magellanic Moorlands within the past 100 yr is at least partly attributed to global dispersion of Hg derived from anthropogenic sources in the Northern Hemisphere. The finding that no increase of atmospheric deposition of Pb could be observed in the bog indicates the extraordinary long-range transport and ubiquitous dispersion of anthropogenic derived gaseous Hg compared to other metals. < 2002 Elsevier Science B.V. All rights reserved.

Holocene peat and lake sediment tephra record from the southernmost Chilean Andes (53-55°S)

Se han estudiado testigos de tefras y sedimentos de la peninsula Munoz Gamero con el seno Skyring y con el proposito de mejorar la tefrocronologia local. Nuevas edades 14C junto con tasas de crecimiento de las turbas y de sedimentacion, proporcionan nuevas marcas para la edad de las tefras. La capa mas gruesa de tefra (5-15 cm) se origino en una erupcion del volcan del monte Burney, a los 4,254±120 anos calibrados AP. Las isopacas de esta erupcion indican una deposicion de ca. 2,5 a 3 km3 de tefra, principalmente en el area andina forestada, al sureste del volcan. El monte Burney tuvo otra gran erupcion pliniana entre 9,009±17 y 9,175±110 anos cal. AP, y cuatro erupciones menores durante el Holoceno. Se observan, tambien, en algunos testigos tefras de las grandes erupciones de los volcanes Reclus (>15,384±578 cal. anos AP), Hudson (entre 7,707±185 y 7,795±131 cal. anos AP) y Aguilera (<3,596±230 cal. anos AP) como lo indican las composiciones caracteristicas de los vidrios de tefra, los cuales difieren entre los diferentes centros volcanicos. Se establecio una perdida significativa de los elementos alcalinos durante la alteracion del vidrio volcanico en las capas de tefra, especialmente en los testigos de suelos acidos. lo que podria ser un factor importante en el suministro de nutrientes para las plantas.

Geochemical constraints on the sources of Southern Chile Trench sediments and their recycling in arc magmas of the Southern Andes

Pliocene and Pleistocene deep-sea trench sediments cored near the Chile Triple Junction (Ocean Drilling Program Leg 141) were analysed for major and trace element concentrations, and for Sr, Nd and Pb isotopic ratios. Comparisons with the potential source rocks of these sediments suggest little alteration during sediment transport and diagenesis. The sediment compositions reflect exposed area fractions and different erosion rates of upper-crustal rock units of the Southern Andes, indicating that denudation, transport and deposition formed an almost closed system since 2.5 Ma. Pelagic sediments cored farther from the continent (>1000 km) on the Antarctic Plate also contain a significant terrigenous component, mixed variably with hydrogenous precipitates (high Fe–Mn–Th), biogenic barite (high Ba) and opal, but little biogenic carbonate (low CaO and Sr). Our sediment data allow us to estimate the subducted sediment input to southern Andean magmas. Mantle sources of basalts from the Andean southernmost Southern Volcanic Zone (41–47°S) were contaminated by 3–5 vol.% of a terrigenous sediment melt with variable amounts of Ba-rich pelagic sediments, but were not contaminated by slab-derived fluids. Adakites of the Andean Austral Volcanic Zone (49–55°S), formed by melting of a relatively hot subducted slab, contain a variable amount of subducted terrigenous sediment (0–20 vol.% sediment melt) and in some cases Ba-rich pelagic sediments.

Glacier change and climate forcing in recent decades at Gran Campo Nevado, southernmost Patagonia

Abstract Digital terrain models of the southern Chilean ice cap Gran Campo Nevado reflecting the terrain situations of the years 1984 and 2000 were compared in order to obtain the volumetric glacier changes that had occurred during this period. The result shows a slightly negative mean glacier change of 3.80 m. The outlet glacier tongues show a massive thinning, whereas the centre of the ice cap is characterized by a moderate thickening. Thus a distinct altitudinal variability of the glacier change is noticed. Hypothetically this could be explained by the combined effects of increased precipitation and increased mean annual air temperature. Both to verify and to quantify this pattern of climatic change, the mean glacier change as well as its hypsometric variation are compared with the results of a degree-day model. The observed volumetric glacier change is traced back to possible climate forcing and can be linked to an underlying climate change that must be comparable with the effects of a precipitation offset of at least 7–8% and a temperature offset of around 0.3 K compared to the steady-state conditions in the period 1984–2000.

Millennium-scale volcanic impact on a superhumid and pristine ecosystem

Ecosystems damaged by distal volcanic ash and sulfur deposition usually recover within decades. However, sediment, stalagmite, and pollen records from the southernmost Andes indicate a 2000 yr impact on forest and aquatic ecosystems after deposition of a thin tephra layer. SO 2 released from altering pumice produced intense soil and lake acidification in a >150,000 km 2 area. Acidification led to nutrient leaching and affected soil microorganisms, causing plant decay and increased soil erosion in an area larger than 8000 km 2 . We conclude that weakly buffered soils in humid environments are extremely vulnerable to volcanic and anthropogenic acidification, causing long-lasting ecosystem damage and perturbations of paleoclimate proxy records.

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.