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

Since the Industrial Revolution, humans have been altering the global nitrogen cycle through fossil fuel burning and fertilizer usage. Today, the rate of human generation of fixed 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 the anthropogenic nitrogen would end up in the environments, causing severe consequences such as eutrophication, water hypoxia, and harmful algae blooms. However, the footprint of anthropogenic nitrogen in the ocean is complex and not well understood, mainly because of limited continuous oceanic and atmospheric measurements. For example, it is unclear how much of the anthropogenic nitrogen has reached the open ocean. Fast-growing, long-live corals in the surface ocean are an excellent archive for tackling this problem. In our study, we use coral-bound nitrogen isotope records to assess the impacts of anthropogenic nitrogen on 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. The history of ocean deoxygenation can provide important insights into their sensitivities to climate change and anthropogenic nutrient input. 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

On glacial-interglacial timescales, the atmospheric CO2 concentration has varied between 180 and 280 parts per million (ppm), with lower values during glacial periods. Since the first measurement of ice age CO2 four decades ago, much efforts have been devoted to understand why CO2 was lower during ice ages. The answer has important implications for the fate of anthropogenic CO2 since the Industrial Revolution. However, to this day it has remained a conundrum in the field of paleoceanography and paleoclimatology. One of the leading hypotheses pointed to changes in the ocean’s biological pump, especially in the Southern Ocean. The surface nutrients are not fully consumed by phytoplankton in the modern Southern Ocean, resulting in a leakage of deeply sequestered CO2 to the atmosphere. It was proposed that more complete nutrient consumption in the Southern Ocean would have caused the lower pCO2 during the ice ages. By measuring the nitrogen isotopic composition of >250 well-dated deep-sea fossil corals collected from the Southern Ocean, we showed that the entire region was more nutrient-deplete during the last ice age.

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.

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