Reconstructing the biological carbon pump with ancient plankton DNA

Our international team aims to reconstruct the past functioning of the biological carbon pump, a marine ecosystem process involved in the regulation of the Earth climate. The biological carbon pump refers to the series of processes by which atmospheric CO2 is transformed into organic carbon by phytoplankton, consumed by pelagic organisms, exported through sinking particles (i.e. the marine snow) and sequestered in deep waters for centuries or buried in sediments for thousands of years. Despite its importance, little is known on the potential response of the biological carbon pump to climate change. Knowing how this process has responded to past climate transitions will inform us on its potential response to the rapidly warming climate. We now have access to a unique set of modern plankton and seafloor sediment samples, as well as three sediment cores from the Nordic Seas and Arctic Ocean, spanning the last ca 130,000 years. The sediment cores contain information about past marine biodiversity, that can be compared to modern biological communities. To generate biodiversity data, we will sequence massively (high-throughput DNA sequencing) the environmental DNA extracted from the modern and ancient samples. We will then use the empirical links between modern biological communities and the current functioning of the biological carbon pump to produce ancient plankton DNA-based reconstructions of the biological carbon pump back to ca. 130,000 years ago. This time interval, known as the Last Interglacial, is highly relevant to do this research because Earth’s climate was warmer than today. Here, we will compare our DNA-based reconstructions with the ones obtained from simulations using the Norwegian Earth System Model.

The biological carbon pump is a key ecosystem process regulating the ocean carbon cycle and the climate system. Pelagic and benthic biodiversity shape the strength of this process by governing the amount of organic carbon transferred from the surface to the seafloor sediment, where it may be respired or buried for geological timescales. Despite its importance, it remains unclear whether climate change will weaken or strengthen the biological carbon pump. Filling this knowledge gap is crucial as its weakening would amplify global warming and narrow our chances to meet the objectives set by the Paris Agreement. Yet so far, there is no proxy to reconstruct the biological pump from the sediment record, limiting our capacity to assess its responses to past climate changes and consequently informing us on its potential functioning under future warmer-than-present climate. Here, we will implement a cross-disciplinary approach to develop ancient plankton DNA as a proxy for reconstructing the past functioning of the biological carbon pump. We will utilise a novel observation methodology, based on ancient sedimentary DNA, to recover ancient ocean biodiversity beyond the fossil record. We will combine this new data with relevant existing geochemical, climate and DNA data to study the response of the biological carbon pump to past climate changes. We focus our approach on a unique set of surface sediment samples and three Late Quaternary sediment cores in the North Atlantic and Nordic Seas, where carbon sequestration processes via the biological pump are prominent. We will produce the first reconstruction of the biological pump back to the Last Interglacial, a period considered as a future climate analogue. Its past functioning will inform on its potential response to global warming. BIOCAP is hence highly relevant to better understand the effects of climate change on ecosystems and on the processes they support, with direct consequences on the pace of global warming.