Reconstructing the variability of the ocean’s largest oxygen-deficient zone
Some areas of the Tropical Ocean harbor very productive fisheries that feed millions of people and thus contributes 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. Many 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. However, the impact of global warming on the ocean’s oxygen-deficient zones is uncertain, partly because of a lack of data on past changes.
Coral cores can be used to reconstruct past changes in ocean biogeochemistry and climate at very high temporal resolution. The picture illustrate annual changes in coral skeletal density using X-ray imaging
The AMG lab has pioneered the use of coral cores to reconstruct the variability of ODZs across the Antropocene. In our first publication on this topic, we report monthly resolved records of coral skeleton–bound nitrogen isotopes (CS-δ15N) to reconstruct denitrification in the Eastern Tropical North Pacific (ETNP) ODZ over the last 80 years (Duprey et al. 2024). The data indicate strong decadal variation in ETNP denitrification, with maxima during the cool North Pacific phase of Pacific Decadal Variability. The maxima in denitrification (and thus oxygen deficiency) were likely due to stronger upwelling that enhanced productivity leading to greater oxygen demand in the thermocline. These results indicate that the largest oxygen-poor region of the ocean is more variable than previously thought. Moreover, our findings imply that ODZ evolution over the next century will depend on how global warming interacts with the decadal oscillations.
Decadal changes in ODZ variability reconstructed using coral-bound Nitrogen isotopes (Duprey et al. 2024)
Anthropocene
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The AMG lab studies climate, oceanographic and biogeochemical changes during the Anthropocene. We primarily use corals as high-resolution archives to reconstruct past changes in ocean propertiesover the past centuries. In recent years, we have focussed on the study of the variability of the Oxygen Deficient Zones of the ocean, the extend and frequency of mesophotic coral bleaching events and the impacts of anthropogenic activities on the ocean's Nitrogen cycle.
Bleaching and mortality of Mesophotic coral reefs in the Eastern Tropical Pacific
Coral bleaching and associated mortality, which occur when stressed corals expel their photosymbionts, are viewed as a major ongoing threat to coral reef ecosystem functionality that will be exacerbated under future climate change scenarios. Early studies of Mesophotic Coral Ecosystems (MCEs) highlighted their potential as thermal refuges for shallow-water coral species in the face of predicted 21st century warming. However, recent genetic evidence implies that limited ecological connectivity between shallow- and deep-water coral communities inhibits their effectiveness as refugia; instead MCEs host distinct endemic communities that are ecologically significant in and of themselves. In either scenario, understanding the response of MCEs to climate change is critical given their ecological significance and widespread global distribution. Such an understanding has so far eluded the community, however, because of the challenges associated with long-term field monitoring, the stochastic nature of climatic events that drive bleaching, and the paucity of deep-water observations.
Expedition diver Rose Dodwell documenting the extent of coral bleaching along a transect at 32 meters depth at Clipperton Atoll
In a recent study, we have documented the first observed cold-water bleaching of a mesophotic coral reef at Clipperton Atoll, a remote Eastern Tropical Pacific (ETP) atoll with high coral cover and a well-developed MCE (Foreman et al. 2024). The severe bleaching (>70 % partially or fully bleached coral cover at 32 m depth) was driven by an anomalously shallow thermocline, and highlights a significant and previously unreported challenge for MCEs. Prompted by these observations, we compiled published cold-water bleaching events for the ETP, and demonstrate that the timing of past cold-water bleaching events in the ETP coincides with decadal oscillations in mean zonal wind strength and thermocline depth. The latter observation suggests any future intensification of easterly winds in the Pacific could be a significant concern for its MCEs. Our observations, in combination with recent reports of warm-water bleaching of Red Sea and Indian Ocean MCEs, highlight that 21st century MCEs in the Eastern Pacific face a two-pronged challenge: warm-water bleaching from above, and cold-water bleaching from below.
Illustration of the pressures on mid-depth and deep coral ecosystems as a result of consecutive extreme warm- and cold-water bleaching events (Foreman et al. 2024).