The largest oxygen-poor region of the ocean is more variable than previously thought

Some areas of the Tropical Ocean harbor very productive fisheries that feed millions of people and thus contribute to the socio-economic stability of many countries around the world. Paradoxically, however, this natural bounty can also create conditions adverse to marine life. This is because the abundant biomass that is produced in these areas sinks to depth and is degraded by bacteria. During this process, oxygen is consumed through respiration, which leads to the formation of vast areas deprived of dissolved oxygen, a few hundred meters below the surface. These areas with little, or no oxygen, are referred to as Oxygen Deficient Zones, or ODZs. The largest ODZ on the planet is located in the Eastern Tropical Pacific, where it stretches from the Central American coastline westwards to the Central Pacific.

Given the juxtaposition of this large oxygen-deprived water mass and the highly-productive marine ecosystems above it, it has long been a research priority to understand the natural rhythms and variations in the size and position of this ODZ. In particular, many modelling studies have expressed concern that ongoing global warming could cause the volume of low oxygen waters in the world’s oceans to expand because of changes in ocean currents and oxygen solubility. As measured time series of dissolved oxygen concentrations are too sparse in this remote area, it has not yet been possible to determine whether this process is already underway. An international research team led by the Max Planck Institute of Chemistry (Mainz, Germany) in collaboration with scientists from the Universidad Autónoma de Baja California (Mexico), the Scripps Institution for Oceanography (USA), the Senckenberg Natural History Museum (Germany), and Princeton University (USA) now reconstructed the evolution of the oxygen levels in the eastern tropical Pacific over the past 80 years.

The Oxygen-deficient zone expands and contracts more often than assumed

“With an ensemble of coral cores we were able to generate multiple time series of stable nitrogen isotopes covering the past 80 years. This data revealed that the ODZ expands and contracts every ten years or so, much more often than was previously understood to be the case”, says Alan Foreman, a post-doctoral researcher at the Max Planck Institute for Chemistry.

The researchers documented the natural waxing and waning of the ODZ in the eastern Tropical North Pacific with this ensemble of observations. Their results, published this week in the journal Science, demonstrated that Pacific decadal climate variability is controlling the size of this ODZ via changes in the strength of easterly winds. The close coupling of the Tropical Pacific wind strength and the extent of the oxygen-poor waters demonstrates that ODZs are more dynamic than previously thought and could respond quickly to climate changes in the coming decades, with important consequences for marine ecosystems.

Coral skeleton records the oxygen concentration of the ocean

The team made this discovery using hard-coral geochemical records. More precisely, they measured the nitrogen isotope ratio of the tiny amount of organic matter locked in the coral skeleton as a novel tracer of oxygen concentration in the ocean. In oxygen-poor waters, complex bacterial biological processes alter the isotopic composition of nitrate, leaving behind a geochemical 'fingerprint' on this vital nutrient for the coral reef ecosystem located above. Nitrate originating from the ODZ is brought to the surface by mixing and diffusion, where it helps corals sustain their growth. In doing so, the isotopic fingerprint of ODZ is propagated to the coral skeleton, where it is recorded and preserved within the skeletal matter of the colony. “In our lab, we are able to analyze the extremely minute amounts of nitrogen locked in the calcium carbonate crystals of coral skeletons, allowing us to infer how the Oxygen Deficient Zone (ODZ) varied in the past”, says Alfredo Martínez-García, the group leader of the Organic Isotope Geochemistry research group at the Max Planck Institute.

The coral skeletons analyzed in this study were collected last year during an expedition to the isolated islands of the Revillagigedo Archipelago in the Pacific Ocean. The archipelago is geographically located next to the world´s largest ODZ. This expedition, led by Alan Foreman, in collaboration with the Sailing Yacht Acadia (Mark and Rachel Rohr Foundation), retrieved several coral cores of the stony coral Porites growing on the rocky seafloor of San Benedicto and Soccoro Islands. The surface of these corals is composed of thousands of tiny animals, so-called polyps. These polyps secrete calcium carbonate to form the coral's skeleton. As the density of the coral skeleton changes over the course of the year, the deposits are robust chronological markers. In the X-ray image, the light and dark bands of high and low density resemble the annual rings of a tree. Based on the alternating banding of the coral skeleton, the researchers were able to create detailed time series of the variability of the ODZ. The new set of coral cores was complemented with additional coral cores previously collected by José Carriquiry (Universidad Autónoma de Baja California), Sara Sanchez (University of Boulder, Colorado), and Christopher Charles (Scripps Institution for Oceanography).

Corals are history books of the ocean

“Our coral collection allows us to reconstruct the variability of the oxygen-poor waters during the 20^th Century over an area spanning thousands of miles, with great accuracy. I often consider this collection like a library where each new core is a book that tells a story about our ocean. Ultimately, our work is to decipher each of these books to understand the impact of human activities on the ocean and raise awareness about this pressing issue”, says Nicolas Duprey, the lead author of the study and post-doctoral researcher at the Max Planck Institute for Chemistry.