My research aims to enhance our ability to understand past climate change by strengthening and expanding our toolkit of paleoclimate proxies. I do this by developing, testing, and applying new organic geochemical proxies to reconstruct past climate and environmental conditions, with a particular emphasis on the distribution and isotopic composition of lipids. Lipids are useful because they are resistant to degradation compared to other organic compounds, and thus persist in the sedimentary record. My work weaves together three main threads: empirical calibrations of proxy responses along modern environmental gradients, experiments designed to test the mechanistic underpinnings of proxies, and applications to sedimentary records.
ADAPT: Algal Dynamics and Productivity through Time
The occurrence and species composition of algae in lakes influence water quality, food webs and the carbon cycle. One of the best ways to understand algae in the present and how they will react to future changes is to study their past occurrence by analyzing their remains in lake sediments. New methods based on chemical fossils and sedimentary DNA can help to reconstruct a more complete picture of the past. The overall goal of my Swiss National Science Foundation project, ADAPT, is to understand how algae in lakes have reacted over time in response to human and natural changes. The project begins by assessing how well chemical fossils in water and surface sediments represent current species composition of algae. These measurements are then applied to sediments to study how algae in Swiss lakes (1) reacted to rising and falling phosphorus concentrations in the 20th century, (2) how they reacted to changes in water quality caused by ancient Romans, and (3) to what extent they have changed in the last 10,000 years independently of human influence.
B2 WALD: Biosphere 2 Water Atmosphere and Life Dynamics
During 2019, I co-organized an ambitious interdisciplinary project in the Biosphere 2 (B2) research facility in Arizona to track how drought affects water and carbon cycling from molecular to ecosystem scales. The core B2-WALD campaign took place in the Autumn of 2019 and involved over 70 scientists, students, and technicians from 15 institutions in five countries. I’ve collected a large sample set to assess how drought impacts leaf lipid distributions, production, and stable isotopic composition — stay tuned for results! See campaign updates by searching #B2WALD, or read 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 (Ladd et al., 2020), 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; He et al., 2020)
Nemiah Ladd, PhD 