Lessons from the Deep

Paleobiologists from HKU’s School of Biological Sciences recently completed a study on the sub-Antarctic zone in the Southern Ocean which revealed that temperature changes and food input have played distinct roles in shaping deep-sea ecosystems. The study was published in the journal Current Biology.

The research was a collaboration between HKU and Princeton University in the US. Professor Moriaki Yasuhara and Ms Raine Chong from HKU’s School of Biological Sciences, the Swire Institute of Marine Science, and the Institute for Climate and Carbon Neutrality, joined forces with Dr May Huang from Princeton’s Department of Geosciences.

“We showed that the Southern Ocean deep-sea ecosystem we see today was established following the Mid-Brunhes Event – an important climatic transition which occurred around 430,000 years ago – and that the abundance of individual species waxed and waned, depending on sediment input from land, as well as dust concentrations,” said Professor Yasuhara.

“However, prior to the Brunhes Event, the species present and how they responded to environmental variability were different. The event thus marks a substantial turnover from colder-water to warmer-water species.”

This result suggests that if future human-induced climatic warming weakens global deep-water circulation from the Atlantic through the Southern Ocean to the Pacific, a deep-sea biodiversity hotspot in the Southern Ocean may diminish or even vanish.

Extreme environment

Asked about the methodology used for the study, Professor Yasuhara explained that the understanding of long-term history such as climatic impact in a remote and extreme environment is not an easy job. However, one way of getting around this problem is to study fossil organisms, a key tool for understanding the complex biological history of Antarctica and obtaining glimpses of entire ancient ecosystems.

Professor Yasuhara and colleagues leveraged the fossil record of ostracods – microscopic crustaceans whose shells are preserved in abundance, making them ideal for studying deep-sea ecosystems through deep time – that is, dating back tens of millions of years and beyond.

They obtained the fossils from sediment cores taken from the Tasman Sea and documented the ecosystem response of these fossilised organisms to the most recent major shift in these astronomical perturbations and their accompanying climatic changes.

“Components of fossil fauna showed substantially similar change – one to deep-water temperature and the other to dust flux changes,” said Professor Yasuhara. “This indicates Southern Ocean deep-sea fauna responded both to climatic change and to iron fertilisation in different ways, and this could have important implications for ocean-based climatic interventions as it is possible that they could have negative impacts on the deep-sea ecosystem itself.”

The interventions the team are concerned about are geoengineering technologies, referred to as ocean-based climate intervention, which includes proposals such as the removal of marine carbon dioxide with the aim of reducing future warming by putting and storing carbon or carbon dioxide in deep-sea sediment under the ocean bed, where in theory it will remain stable because of the low-temperature and high-pressure environment.

Professor Yasuhara said: “The intention basically is to enhance the surface primary production, for example through iron fertilisation, in order to increase carbon flux to the deep sea – or to put more organic matters such as crops and macroalgae into the deep sea.

“But our research indicates we should be cautious about taking such actions, especially if we are not very sure about their effectiveness and how long such mitigation will actually work sustainably and/or continuously. For example, if carbon will go back to the atmosphere in just 10 years or even 100 years or so, it won’t be worth doing.”

Researchers acknowledge that the deep-time development of the Southern Ocean’s deep-sea ecosystem remains poorly understood, despite being a key region in global ecological, climatological and oceanographic systems, where deep water forms and biodiversity are unexpectedly high.

Carbon dioxide modulators

The Southern Ocean accounts for as much as 75 per cent and 40 per cent of global ocean heat and carbon uptake, respectively, and its currents are key modulators of global carbon dioxide flux. A breakdown of this current system will have significant negative effects on marine ecosystems from the surface to the deep seafloor.

But, while acknowledging that concerns are growing for how climate change and the rise in ocean temperatures will affect even these, the deepest areas of the oceans, Professor Yasuhara pointed out that it’s crucial to have foundational knowledge of the deep-sea ecosystem and biodiversity, particularly if the intention is to alter it.

“Even if the intention is good the outcome might not be,” he said. “Before taking any action of this kind, we need to have a fuller picture of what the consequences may be.”

The team now plan to conduct high-time resolution research. “We want to ascertain if the results will be similar at, for example, the centennial/millennial time scale. That is the time scale we are worrying about for the future,” said Professor Yasuhara.