What drives global climate? - The role of deep ocean circulation
Ocean water circulation has an important influence on global climate. In a recent study published by Nature Communications, researchers from German and UK institutions (including Royal Holloway) have used Nd-isotope records from the North and South Atlantic to decipher the control of the opening Atlantic Ocean on ocean circulation and its linkages to the evolution of global climate. The marked convergence of Nd-isotope signatures 59 million years ago indicates a major intensification of deep-water exchange between the North and South Atlantic, which coincided with the turning point of deep-water temperatures towards early Paleogene warming. This study proposes that this intensification of Atlantic overturning circulation together with increased atmospheric CO2 from continental rifting marked a climatic tipping point contributing to a more efficient distribution of heat over the planet. With a more even distribution of heat over the Earth, a long-term cooling phase ended and the world headed into a new greenhouse period.
Neodymium (Nd) isotopes are used as a tracer of water masses. Surface waters acquire a Nd-isotope signature from surrounding land masses through rivers and wind-blown dust. When surface waters sink to form a deep-water mass, they carry their specific Nd-isotope signature with them to depth. As a deep-water mass flows through the ocean, its Nd-isotope signature gets incorporated in sediments. This study analysed the neodymium isotope composition of coatings that form around sediment grains as they are lying on the ocean floor.
Schematic diagram showing how neodymium isotopes enter oceanic deep waters
The story revealed by this study of Nd isotope data begins in the latest Cretaceous (ending 66 million years ago), when the world was between two greenhouse states. Climate had been cooling for tens of millions of years since the peak hothouse conditions of the mid-Cretaceous, around 90 million years ago. Despite long-term cooling, temperatures and sea levels at the end of the Cretaceous were higher than the present day.
The Atlantic Ocean was still young, and the North and South Atlantic basins were shallower and narrower than today. The equatorial gateway between South America and Africa allowed only a shallow, surface-water connection for much of the Late Cretaceous period. Active volcanism formed underwater mountains and plateaus that blocked deep-water circulation. In the South Atlantic, the Walvis Ridge barrier formed above an active volcanic hotspot. This ridge was partially above sea level and formed a barrier for the flow of deep-water masses.
3D visualisation of the Walvis Ridge and Rio Grande rise, based on research conducted by Royal Holloway earth scientists.
As the Atlantic Ocean continued to open, the oceanic crust cooled and subsided. Basins became deeper and wider, and submarine plateaus and ridges sank, along with the crust. At some point, deep water from the Southern Ocean was able to pass north across the Walvis Ridge and fill the deeper parts of the Atlantic basins. “This study is the first to establish how and when a deep-water connection formed” says Sietske Batenburg, first author of the study. “At 59 million years ago, the Atlantic Ocean truly became part of the global thermohaline circulation, the flow that connects four of the five main oceans together in a large global cycle”.
From 59 million years ago onwards, Nd-isotope signatures from the North and South Atlantic fell in a narrow range. This is interpreted to indicate that one deep-water mass, likely originating from the south, made its way through the Atlantic Ocean and filled the basin from deep to intermediate depths.
Nd-isotope records compared to atmospheric CO2 levels and deep-sea temperature evolution
The current rate of climate change by CO2 emissions from human activity far surpasses the rate of warming during past greenhouse climates. Studying ocean circulation during the most recent greenhouse interval in the geologic past may provide clues as to how ocean circulation might develop in the future, and how heat will be distributed over the planet by ocean currents.
This research is the result of an international collaboration with the following institutions in Germany: the Goethe-University Frankfurt; the Ruprecht-Karls-University of Heidelberg; the Geomar-Helmholtz Centre for Ocean Research in Kiel; the Federal Institute for Geosciences and Natural Resources in Hannover, and the UK: the Royal Holloway University of London and the University of Oxford.
The sediments for this study all came from ocean drill cores. Layers of sediment on the ocean floor are good archives of past climate, as they generally accumulate slowly for long periods of time. The International Ocean Discovery Program (IODP) coordinates scientific expeditions to drill the ocean floor to recover these sediments, and stores the sediment cores so that they are available to the whole scientific community.
