Climate-biogeochemistry interactions in the tropical ocean of the SE-American oxygen minimum zone – Cruise No. M77 – October 22, 2008 – February 18, 2009 – Talcahuano (Chile) – Colon (Panama)

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    163 pages
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    Elektronische Ressource
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    DDC:    550 Earth sciences and geology


Meteor cruise M77 comprised of four legs to investigate the water column and sediments of the oxygen minimum zone (OMZ) in the coastal upwelling areas off Peru and to a lesser extend off Ecuador (Fig 1.). The research was carried out in the context Sonderforschungsbereich 754 at the University of Kiel, “Climate – Biogeochemistry Interactions in the Tropical Ocean” funded by the German Research Council (DFG). Oceanic oxygen levels are controlled by the interplay of physics and biology. Circulation and mixing transport oxygen into the ocean interior from the near-surface where it is produced by photosynthesis and exchanged with the atmosphere. Oxygen consumption occurs throughout the ocean and is fuelled by organic matter sinking out of surface waters into the depths. Both the supply and consumption of oxygen are sensitive to climate change in ways that are not fully understood. Major changes to marine sources and sinks of important nutrient elements such as nitrogen, phosphorus and iron occur when oceanic oxygen concentrations decrease below threshold levels. On crossing the threshold, radically different microbial and chemical processes start to operate. Oxygen levels can therefore be viewed as a “switch” or “tipping point” for nutrient cycling. The Oxygen Minimum Zones (OMZs) of the tropics are the key regions of low oxygen in today’s ocean. The effects of oxygen-dependent nutrient cycling in these relatively small regions are carried into the rest of the ocean by the circulation. Hence “small” OMZs can impact nutrient budgets, biological productivity and CO2-fixation of the global ocean. Paleo-records from the late Permian and Cretaceous give evidence for periods of dramatically reduced oceanic oxygen levels that had major consequences for marine ecosystems (including mass extinctions). Major low oxygen events, including Cretaceous Ocean Anoxic Events, were associated with warmer climates and higher atmospheric CO2 levels. Recent modelling results suggest that oceanic oxygen levels will decrease significantly over the next decades in response to high atmospheric CO2, climate change, and altered ocean circulation. Hence the future ocean may experience major shifts in nutrient cycling triggered by expansion and intensification of tropical OMZs. There are numerous feedbacks between oxygen levels, nutrient cycling and biological productivity. Positive biogeochemical feedbacks would accelerate climate-initiated oxygen depletion and the spreading of the oxygen minimum zones. Such changes would have profound global consequences for the future ocean, as they have had in the past. However, our existing knowledge is insufficient to understand past interactions or to adequately assess the potential for future change. The SFB 754 addresses what we consider to be a newly recognised ‘tipping point’ of the global climate-biogeochemistry system. Specifically, the following key questions will be addressed: How does subsurface dissolved oxygen in the tropical ocean respond to changes in ocean circulation and ventilation? What are the sensitivities and feedbacks linking low oxygen levels and key nutrient source and sink mechanisms? What are the magnitudes, timescales and controlling factors of past, present and likely future variations in oceanic oxygen and nutrient levels? The overall goal is to improve understanding of the coupling of tropical climate variability and circulation with the ocean’s oxygen and nutrient balance, to quantitatively evaluate the nature of oxygen-sensitive tipping points, as well as to assess consequences for the Ocean’s future. To address these questions we study interactions, tracers, mechanisms and thresholds operating in the present-day tropical ocean as well as examine new records of past changes. The SFB will link experimental studies with the development of improved models of redoxsensitive processes involving multiple bio-reactive elements: the biogeochemical models will be integrated with state-of-the-art models of climate change and ocean circulation. Addressing the SFB goals requires multi-disciplinary study. The SFB builds upon wide-ranging expertise available in Kiel, including chemical and physical oceanography, sediment biogeochemistry, marine ecology, molecular microbiology, paleoceanography, geology, as well as climate and biogeochemical modelling.