University of Massachusetts Amherst
2022-2023 Distinguished Lecturer
Terrestrial molecules in ocean sediments: Lipids of soil bacteria and land plants reveal past continental climate variability
Reconstructing the climate of Earth’s arid continental regions can be challenging due to a lack of continuous archives. Dust blown from the continents to the oceans carries molecules of land plants and soil bacteria with it, thereby allowing past conditions on land to be studied from ocean sediments. Two regions that receive large quantities of dust are the Atlantic Ocean offshore northern Africa and the eastern Indian Ocean offshore northwest Australia. Scientific ocean drilling has provided sedimentary archives from both regions, allowing for past climates to be studied from the adjacent arid continents. Understanding how Earth’s climate has varied in the past, prior to the influence of human activities, is important for understanding how the climate system works and can provide insights into future climate variability. A time interval of interest is the mid-Pliocene warm period (~3.3 to 3.0 million years ago), when Earth likely had similar atmospheric carbon dioxide concentrations to today.
In 2015, IODP Expedition 356 recovered Plio-Pleistocene sediments from the northwest Australian Shelf. Site U1463 is situated directly underneath the northwest dust pathway, which transports material from central and northwestern Australia offshore. In 1986, ODP Leg 108 recovered Plio-Pleistocene sediments from the tropical eastern Atlantic Ocean. Situated beneath the path of the African Easterly Jet, Site 660 receives dust from the Sahara/Sahel region of central North Africa. At both Sites, branched glycerol dialkyl glycerol tetraethers (GDGTs) produced by soil bacteria are used to reconstruct continental temperature variability. At ODP Site 660, plant wax deuterium and carbon isotopes are also used to examine hydroclimate fluctuations and past shifts in the dominant vegetation type (C3 versus C4 plants), respectively. Plant waxes are also present at Site U1463, but concentrations are generally too low to allow for isotopic measurements. At both Sites U1463 and 660, relationships between continental conditions and sea surface temperature are made by examining alkenone (Uk’37 index) and TEX86 (isoprenoid GDGT) temperature reconstructions from the same samples. These paired continental and marine observations provide insight into the role of ocean warming and cooling in influencing continental climates over the past 4 million years.
Isla Castañeda is an Associate Professor at the University of Massachusetts Amherst with a joint appointment between the Department of Geosciences and Commonwealth Honors College. She received her bachelor’s degree from Syracuse University, her master’s degree from the University of Colorado at Boulder, and her doctorate from the University of Minnesota. As a postdoctoral research associate at the Royal Netherlands Institute for Sea Research, she used organic geochemical and isotopic proxies to investigate the paleoceanographic and climatic history of the Eastern Mediterranean Sea and the southwestern Indian Ocean. In 2015, Isla sailed on IODP Expedition 356; she and her students are currently working on several ODP/IODP Sites including Site U1463 (NW Australia), ODP Site 647 (south of Greenland), Site U1540 (central South Pacific), and ODP 660 (NW Africa). Isla also uses organic geochemical techniques to reconstruct past climate from lakes and is currently has ongoing research on tropical Lake Malawi, artic Lake El’gygytgyn, and several small lakes in southern Greenland.