A fossil diatom perspective from Lago Cipreses (51°s, southwestern Patagonia) on the Holocene history of the southern westerly winds.

Abstract

A global climate component we faintly understand is the Southern Westerly Winds (SWW). Long-term SWW trends play a major role in determining Southern South American precipitation and fire regimes, deep-sea CO2 ventilation, Antarctic sea-ice extent, along with energy and moisture transfer between high and low latitudes. SWW behavior is determined by large-scale pressure gradients related to orbital configuration and Earth system circulation patterns. However, on millennial to centennial time scales, key coupled Southern Ocean-atmosphere dynamics are currently blurred by contrasting interpretations regarding SWW evolution during the early Holocene, that ultimately influence how we interpret subsequent periods. Here I present the first continuous diatom record from Southwestern Patagonia (SWP) that spans the entire Holocene, with the intention of contesting contingent hypothesis regarding SWW evolution since the glacial-interglacial transition. The most salient diatom assemblages from Lago Cipreses (51°S) suggest changes in lake freezing and lake mixing regimes, i.e., long and moderate freeze-over seasons occur at times of increased small fragilarioid and Aulacoseira abundance, respectively, and turbulent and stratified early-ice off periods occur at times of increased Aulacoseira and Non-small fragilarioid benthos abundance, respectively. The results I obtained show the recurrence of environmental conditions reflected in the evolution of the diatom assemblages. In sum with the results of a previously published elemental analysis, pollen, and charcoal record from the same lake, I infer conspicuous warm/dry conditions between ~9-7.4 ka, ~6.1-5.2 ka, and ~3.8-3.1 ka (ka=calibrated kiloyears before 1950 CE), interpreted as weak SWW influence over SWP, and prominent cold/wet conditions between ~14.2–12 ka and ~3.1–0 ka, interpreted as strong SWW influence over SWP. Enhanced hydroclimatic variability is noted to have developed A global climate component we faintly understand is the Southern Westerly Winds (SWW). Long-term SWW trends play a major role in determining Southern South American precipitation and fire regimes, deep-sea CO2 ventilation, Antarctic sea-ice extent, along with energy and moisture transfer between high and low latitudes. SWW behavior is determined by large-scale pressure gradients related to orbital configuration and Earth system circulation patterns. However, on millennial to centennial time scales, key coupled Southern Ocean-atmosphere dynamics are currently blurred by contrasting interpretations regarding SWW evolution during the early Holocene, that ultimately influence how we interpret subsequent periods. Here I present the first continuous diatom record from Southwestern Patagonia (SWP) that spans the entire Holocene, with the intention of contesting contingent hypothesis regarding SWW evolution since the glacial-interglacial transition. The most salient diatom assemblages from Lago Cipreses (51°S) suggest changes in lake freezing and lake mixing regimes, i.e., long and moderate freeze-over seasons occur at times of increased small fragilarioid and Aulacoseira abundance, respectively, and turbulent and stratified early-ice off periods occur at times of increased Aulacoseira and Non-small fragilarioid benthos abundance, respectively. The results I obtained show the recurrence of environmental conditions reflected in the evolution of the diatom assemblages. In sum with the results of a previously published elemental analysis, pollen, and charcoal record from the same lake, I infer conspicuous warm/dry conditions between ~9-7.4 ka, ~6.1-5.2 ka, and ~3.8-3.1 ka (ka=calibrated kiloyears before 1950 CE), interpreted as weak SWW influence over SWP, and prominent cold/wet conditions between ~14.2–12 ka and ~3.1–0 ka, interpreted as strong SWW influence over SWP. Enhanced hydroclimatic variability is noted to have developed since ~4 ka. I posit that tropical and extra-tropical climate covariability has evolved throughout the Holocene with variable SWW expressions and suggest that the SWW attained its current position and behavior at ~2.7 ka.

ix RESUMEN ESPAÑOL El Cinturón de Vientos del Oeste (CVO) es un componente climático que entendemos vagamente. Tendencias de largo plazo del CVO juegan un rol principal al determinar regímenes de precipitación y de fuego en el Sur de Sudamérica, la ventilación de CO2 del mar profundo, la extensión de la capa de hielo marino Antárctico y la transferencia de energía y humedad entre latitudes altas y bajas. La naturaleza del CVO está determinada por gradientes de presión de gran escala relacionados con la configuración orbital y patrones de circulación del sistema Terrestre. No obstante, no tenemos un entendimiento acabado de las dinámicas claves del sistema acoplado del Océano Austral-atmósfera debido a interpretaciones contrastantes acerca de la evolución de CVO durante el Holoceno temprano, que afectan la interpretación de periodos subsiguientes. En esta ocasión, presento el primer registro continuo de diatomeas de Patagonia Suroriental (PSO) que abarca todo el Holoceno, con la intención de examinar las hipótesis vigentes sobre la evolución del CVO desde la transición glacial-interglacial. Los ensambles de diatomeas más conspicuos de Lago Cipreses (51°S) sugieren cambios en los regímenes de congelamiento superficial y de mezcla, tales que, periodos largos y moderados de congelamiento superficial ocurren cuando abundan small fragilarioides y Aulacoseira, respectivamente, y periodos de turbulencia y estratificación con fecha temprana de derretimiento ocurren cuando abundan Aulacoseira y bentos no-small fragilarioide, respectivamente. Los resultados que obtuve muestran la recurrencia de condiciones ambientales que se reflejan en la evolución de los ensambles de diatomeas. En suma, con los resultados del registro de análisis elemental, polen y carbón previamente publicados del mismo lago, infiero condiciones anómalas cálidas/secas entre ~9-7.4 ka, ~6.1-5.2 ka y ~3.8-3.1 (ka=kilo años calibrados antes 1950 e.c.), interpretado como influencia débil del CVO sobre PSO, y condiciones anómalas frías/húmedas entre ~14.2-12 ka y entre ~3.1-0 ka, atribuible a una fuerte influencia del CVO en la región. Se observa mayor variabilidad hidroclimática desde ~4 ka. Discuto que la covariabilidad entre el clima tropical y extra-tropical ha evolucionado a través del Holoceno con expresiones variadas del CVO y sugiero que la posición y conducta actual del CVO se alcanzó luego de ~2.7 ka.