Our group uses stable isotopes to better understand the ocean’s biogeochemical cycles, both today and in the past. Specifically, our lab has the capacity to analyze very small amount of nitrogen in various nitrogen-poor samples (e.g., organic nitrogen preserved in fossils and nitrate in seawater), which enables us to address important questions in environmental and climate sciences.
Anthropogenic nitrogen in the ocean
Similar to the global carbon cycle, human activities have been altering the global nitrogen cycle through fossil fuel burning as well as fertilizer usage. Today, the rate of human generation of fixed nitrogen (i.e., biologically available nitrogen) is >200 Million Ton/year, comparable to the rate of natural N2 fixation on the entire planet (200-250 Million Ton/year). More than half of these anthropogenic nitrogen would end up in the environments, causing severe consequences such as eutrophication, harmful algae blooms, water hypoxia, and increased production of greenhouse gases (e.g., N2O). However, the footprint of anthropogenic nitrogen in the ocean is complex and not well understood, mainly because of limited oceanic and atmospheric measurements. In our study, we use nitrogen isotopes in seawater and biological archives (e.g., corals) to track anthropogenic nitrogen in the global ocean.
Ocean deoxygenation: Past, Present, and Future
Oxygen is critical to marine ecosystems. Observations showed that the ocean’s dissolved oxygen content has been decreasing over the past several decades, mainly attributed to climate change and coastal anthropogenic nutrient input. Predicting future changes in ocean deoxygenation are critical for marine ecosystems as well as human societies that rely on fisheries. Our lab uses stable isotopes to study ongoing and past changes in ocean deoxygenation on various timescales, in both the oxygen-deficient zones of the open ocean (e.g., eastern tropical Pacific) and coastal hypoxic zones (e.g., northern Gulf of Mexico).
Atmospheric CO2 variations and climate change
Increasing CO2 concentration in the atmosphere is the main driver of ongoing climate change. However, atmospheric CO2 has also varied in the past before any significant human impacts. A better understanding of natural CO2 changes (e.g., over ice age cycles) has important implications for the fate of anthropogenic CO2 in the future and facilitate the development of nature-based climate solutions. One important driver of atmospheric CO2 variations is the ocean’s biological pump, especially in so-called “high-nutrient, low-chlorophyll” regions. In those regions, the phytoplankton productivity are not limited by the major nutrients (i.e., N and P) but by the micronutrient iron. Variations in the supply of dust-borne iron could have changed the efficiency of the biological pump and thus the storage of CO2 in the deep ocean. In our lab, we use stable isotopes to study the impact of the biological pump on atmospheric CO2. This work can help develop marine Carbon Dioxide Removal (mCDR) technologies such as iron fertilization.
Coral symbiosis under warmer climate
Corals reefs, home to at least a quarter of all marine species, are severely impacted by ongoing climate change. Corals and algae living inside their tissue (zooxanthellae) have a mutually beneficial symbiotic relationship. However, when corals are stressed by environmental changes such as rising water temperature and nutrient pollution, corals expel their algae partner (“coral bleaching”) and their healthy symbiotic relationship starts to break down. This would lead to reduced coral growth rates, higher mortality rates, and eventually coral reef degradation. However, it’s unclear how coral symbiosis has evolved in the past, especially under warmer climate. We use nitrogen isotopes to track the evolution of coral symbiosis since the first appearance of modern stony corals.