Using Portuguese stalagmites to constrain Holocene arid and humid intervals and drivers of regional hydroclimate

• Main Author: Thatcher, Diana Lynn
• Format: Text
• Language: English
• Published: Iowa State University Digital Repository 2020
• Subjects: carbon; hydroclimate; isotopes; oxygen; Portugal; stalagmite; North Atlantic
• Online Access: https://lib.dr.iastate.edu/etd/18236; https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=9243&context=etd

Paleoclimate records are important for understanding past climate variability and also to provide a means for testing our understanding of potential drivers of Earth’s past, current, and future climate. Of utmost importance is having paleoclimate records that place current and future changes into context as well as provide robust input for global climate models. The Iberian Peninsula, including Portugal and the focus of this dissertation, is predicted to become substantially drier under future warming scenarios, which will inevitably cause more water scarcity issues. However, instrumental records of precipitation and temperature in many regions span only the last 100-150 years. Thus, in order to fully explore the natural range in climate variability proxy records must be developed and used from natural archives. Cave deposits, such as speleothems, and, more specifically, the geochemical signature in stalagmites, can be used to extend our understanding of past hydroclimate and environmental variability. However, for stalagmites, a detailed cave monitoring program is crucial for understanding the main controls on stalagmite isotopic variability. At Buraca Gloriosa (BG) cave, a small cave in western Portugal, internal and external monitoring has been ongoing for more than six years. From these monitoring data, paired with local precipitation and cave drip water isotopes, back trajectory modeling of airmasses, and an assessment of vegetation density from satellite data, a more complete understanding of the main controls on stalagmite geochemistry is emerging. This dissertation uses stable isotope proxy records that are sensitive to changes in hydroclimate from seven BG cave stalagmites that grew over various portions of the Holocene. Stable carbon and oxygen isotopes records from these stalagmites are used to assess past hydroclimate variability over much of the Holocene with higher values of both isotopes indicating drier conditions in the region. Over the Holocene, conditions in western Portugal have been studied using a composite carbon and oxygen isotope record developed from six overlapping stalagmites from BG cave. From this record, the climate of western Portugal has become increasingly arid, tracking the decreasing trend in insolation due to Earth’s orbital changes and trends in North Atlantic sea surface temperature over the Holocene. Additionally, centennial-scale variability in the isotope records tracks variability in total solar irradiance, especially in the Late Holocene. One of the driest times in the record is centered at 4.2 kyr BP, a well-documented arid interval across the region which had substantial cultural impacts. One of the wettest intervals over the last 9.0 kyr, according to the BG isotope records, is associated with a climate anomaly called the Little Ice Age. In order to investigate regional coherence in hydroclimate during the last 1200 years, three stalagmites were sampled at a finer scale than the Holocene study. From this composite stable carbon isotope record, drivers of climate over the last 1200 years were investigated by comparison with other regional proxy records and Last Millennium Ensemble climate modeling to specifically assess the behavior of the Azores High during this time period. The Azores High is a semi-permanent high pressure system centered near the Azores Islands and has a dominant influence on weather and climate in Iberia and northwest Africa. Several regional records, including the stalagmite carbon isotope record from BG, point to a drier Medieval Climate Anomaly (CE 850-1250), a transition to a wetter Little Ice Age (CE 1450-1850), and drying conditions towards the Modern era (CE 1850-present). Changes in the intensity, size, and/or location of the Azores High are likely impacting all of these regional records. The current drying trajectory in Portugal is reason for concern, and this trend is likely to worsen with future regional warming. Together the work in this dissertation advances the knowledge of climate in this region and is the first Holocene stalagmite proxy record from Portugal. These records are highly sensitive to changes in hydroclimate in the region. Additionally, the monitoring efforts and characterization of dominant factors impacting stalagmite isotopes will allow for additional climate proxies to be developed from this and nearby caves.