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 properties over the past centuries. In recent years, we have focussed on the study of the variability of the Oxygen Deficient Zones of the ocean, the analysis of the drivers of the Great Atlantic Sargassum Belt, the extend and frequency of mesophotic coral bleaching events and the impacts of anthropogenic activities on the ocean's Nitrogen cycle.
Identifying the drivers of the Great Atlantic Sargassum Belt
The Great Atlantic Sargassum Belt (GASB) acts as a floating ecosystem for marine life, providing essential food and shelter for important species, such as tuna, marlin, turtles and birds. However, at highest abundances, its stranding events burden coastal ecosystems and impair the well-being of coastal communities. During major GASB years, tons of Sargassum wash ashore, affect community health and tourism, and require costly management and removal efforts. Thus, the GASB has strong and complex effects on wildlife and coastal populations.
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The magnitude of Sargassum blooms is modulated interannually by various physical oceanic processes, with record-high biomasses observed in 2015, 2018, 2021 and 2022. There is a general consensus that shifts in nutrient availability govern the recent surge in Sargassum blooms. However, the origin and dynamics of the nutrient sources that drive the episodic nature of the GASB and the relative importance of the different nutrients are yet to be resolved.
Playa del Carmen, a popular vacation destination in Mexico, faces significant Sargassum strandings during summer months, as do other Caribbean coastlines.
© Arkadij Schell
In a recent study, we used coral-bound N isotopes to reconstruct N2 fixation, the ultimate source of the ocean’s bioavailable N, across the Caribbean over the past 120 years (Jung et al. 2024). Our data indicate that changes in N2 fixation were primarily controlled by multidecadal and interannual changes in equatorial Atlantic upwelling of ‘excess P’, that is, P in stoichiometric excess relative to fixed N. We show that the supply of excess P from equatorial upwelling and N from the N2 fixation response can account for the majority of Sargassum variability since 2011. Sargassum dynamics are best explained by their symbiosis with N2-fixing epiphytes, which render the macroalgae highly competitive during strong equatorial upwelling of excess P. Thus, the future of Sargassum in the tropical Atlantic will depend on how global warming affects equatorial Atlantic upwelling and the climatic modes that control it.
Seasonal CB-δ15N values (driven by N2 fixation changes) from Martinique from 2000 to 2021 compared with Sargassum biomass and the Atlantic Multi-decadal Mode (AMM) index that modulates the supply of excess P via equatorial upwelling
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).