My research focuses on developing and applying new organic geochemical tools to reconstruct past environmental conditions. My work contains many facets, including empirical calibrations, experiments designed to test the mechanistic underpinnings of my proxies, and applications to sediment cores. I have made particular use of compound specific hydrogen isotopes of lipid biomarkers, including triterpenoids and sterols.

B2 WALD: Biosphere 2 Water Atmosphere and Life Dynamics

I am co-organizing an ambitious project in the Biosphere 2 (B2) research facility in Arizona to track how drought affects water and carbon cycling on an ecosystem scale, and how these changes impact the organic geochemical proxies we use to reconstruct past drought. The core campaign is taking place in the autumn of 2019 — stay tuned for updates! More about B2 WALD

Interactions between tropical Pacific hydroclimate and human activities during the Common Era

The tropical Pacific is a dynamically important part of Earth’s climate system, but remains poorly understood relative to other regions due to a short instrumental record and limited high resolution, continuous paleoclimate archives. The Pacific also contains remote islands that are some of the sites most recently populated by humans. Reconstructing the ways that early settlers influenced their environment, and the climatic and environmental conditions that drove them to seek out new homes offer a unique opportunity to understand interactions between people and the Earth system. I am working to develop new organic geochemical proxies suitable for this task (Krentscher et al., 2019), test the applicability of existing proxies in tropical Pacific island lakes, and using these to reconstruct the early environmental impacts of initial human settlers and the influence of climate on their activities.

Hydrogen isotope response of algal lipids to variable nutrient concentrations in Swiss lakes

Eutrophication (nutrient pollution of lakes and other aquatic ecosystems) is commonly caused by fertilizers and detergents. It can result in harmful algal blooms and widespread fish death, and reduces the economic and aesthetic value of water bodies. The Swiss government has been especially proactive about reducing the causes of eutrophication and remediating polluted lakes. However, despite concerted efforts over the past thirty-five years, a range of nutrient concentrations persists among lakes in the central Swiss plateau. During my NSF-funded postdoctoral fellowship, I studied the impact of elevated phosphorus availability on lipid distributions and isotope composition in algae growing in Swiss lakes by analyzing surface sediment, sediment traps, and suspended particles from ten lakes in central Switzerland with different histories of nutrient loading (Ladd et al., 2017; Ladd et al., 2018) . I also collaborated with aquatic ecologists at Eawag to study how changes in phytoplankton communities in response to nutrient loading affects lipid biomarkers, making use of mesocosms in Eawag’s unique experimental pond facility. A follow-up to this project was funded last year through ETH’s postdoc career seed grant program, and applied the results from our Swiss lakes study to a sedimentary record from Lake Greifen, spanning a well-documented period of eutrophication and partial recovery.

Quantitative reconstructions of salinity & water isotopes from Paired H & C Isotopes in Mangrove Lipids

Mangroves are woody trees that have adapted to live in brackish and saline water. They are prominent on coastlines throughout the tropics and subtropics and form extremely productive ecosystems. Although they cover only a small portion of the globe, mangroves play an outsized role in the global carbon cycle, and are responsible for up to 15% of the organic carbon stored in marine sediment. This means that organic biomarkers from mangroves are abundant in tropical and subtropical coastal sediments, and that lipids from these plants can provide important insights on environmental change at low latitudes.

My PhD research established a new tool for reconstructing past salinity and rainfall rates using the stable hydrogen and carbon isotope ratios of mangrove lipid biomarkers (Ladd & Sachs, 2012; 2013; 2015a; 2015b). This proxy is applicable to coastal lakes, swamps, and near shore sedimentary deposits in the tropics, and is based on systematic changes in the isotopic ratios of mangrove biomarkers with salinity. I have continued to investigate the mechanisms responsible for these relationships through a combination of field and greenhouse based studies (Ladd & Sachs, 2017; He et al., 2017;  Park et al., 2019)

The National Science Foundation (NSF) has funded much of my research and I am very grateful for their support. However, any opinions, findings, and conclusions or recommendations expressed in this material are mine and do not necessarily reflect the views of the National Science Foundation.