Date of Award

Summer 8-21-2020

Level of Access Assigned by Author

Open-Access Thesis

Language

English

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Brenda Hall

Second Committee Member

George Denton

Third Committee Member

Jacquelyn Gill

Additional Committee Members

Aaron Putnam

Katherine Allen

Abstract

The Southern Hemisphere Westerlies (SHW) are an important driver of climate in the mid-latitudes of the Southern Hemisphere. Abrupt latitudinal migration of this coupled atmospheric-oceanic system is thought to be linked to the onset of the Termination at the end of the last ice age and to subsequent climatic variation through the late-glacial period and Holocene. However, the timing and spatial extent of these shifts, as well as variations in wind intensity, are poorly constrained, hindering our understanding of abrupt climate change in the Southern Hemisphere. In addition, future changes in the position and intensity of the SHW are a critical part of model projections, because the SHW affect Southern Ocean upwelling and CO2 sequestration. Insight into the future behavior of the SHW can come from examination of past fluctuations. My focus is the South Atlantic region, thought to be a key area for interactions between the SHW and other components of the climate system. However, there are few terrestrial datasets constraining past variations in the SHW in this region and many of these appear contradictory.

This study is comprised of two alpine lake sediment cores extracted from tarns occupied by alpine glaciers during the last ice age on Mount Usborne of East Falkland (51oS). This terrestrial record, which spans the last 23 ka, uses stratigraphy, organic content, biomarkers (with a focus on plant wax), isotopic composition of plant waxes, and a preliminary pollen record to identify relative wind intensity, wetness, precipitation source, and temperature of the site. Moisture source is particularly useful as it can be tied to the average position of the SHW over time, with enriched precipitation reflecting a southerly location and depleted precipitation indicating a northerly shifted wind belt.

My data suggest climate at Mount Usborne was cold and windy until 16.4 ka, when the SHW moved south and the area may have warmed. This shift represents the local expression of the onset of the Termination. Following a brief period at 13.6-14 ka when the SHW returned to a northern position during the Antarctic Cold Reversal, climate became wetter at 12.5-13.6 ka, associated with a southward migration of the SHW. At 11.2-12.5 ka, the westerlies again moved north and climate in the Falkland Islands became more humid. The start of the Holocene was marked by increasingly warmer and wetter conditions, with the SHW migrating south between 9-11 ka. Southward migration from 6-9 ka resulted in drier, windier conditions over the Falkland Islands. A brief event in the mid-Holocene (5.5-6 ka) is wet and less windy. A distinct reversal in the southward trend occurred at 5.5 ka. During the rest of the Holocene, the SHW have slowly migrated to the north. Climate was windy and dry from 3-5.5 ka and less windy and wet from 0-3 ka.

My dataset suggests a highly variable position of the SHW over the past 23 ka, with multiple north-south migrations, including 1) southward migration during the Termination 2) periodic northward shifts in the late-glacial period, 3) a southerly position during the early Holocene, and 4) northward movement in the mid to late Holocene, particularly after 5.5 ka. The relative position of the SHW calculated in this study combined with other climate records at 51-54oS suggest that variations in the position of the SHW correspond closely with temperature, wind intensity, and precipitation variations, as well as to well-known climate events, regionally. The correspondence between changes in the SHW and periods of abrupt climate change support the hypothesis that the SHW are linked to much of the climatic variation in the South Atlantic region.

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