HKU Bulletin November 2020 (Vol. 22 No.1)

Abrupt sea-level events caused by ice sheets melting are crucial to our understanding of Earth’s climate system and how it is influenced by glacial conditions. A new discovery that eustatic sea-level rises can be discontinuous and sudden, has big implications, especially for low-elevation cities like Hong Kong. Mysteries of the deep Our study showed that sea-level change and warming are not linear in relationship. Future warming may not mean a gradual global sea-level rise, but may result in some sudden sea-level jumps at unpredictable times. Dr Moriaki Yasuhara A great mystery in palaeoclimatology is the timing and magnitude of the second largest meltwater pulse (MWP-1B). A meltwater pulse is an abrupt rise in sea levels caused by a sudden influx of meltwater. The first MWP, known as 1A, is well documented but until now the exact timing and magnitude of MWP- 1B have remained under debate. As often happens at such moments, the MWP- 1B was not actually the subject under study when Ms Skye Tian Yunshu (who was doing a Major in Ecology and Biodiversity at the time, and is now a PhD student) made the discovery. She was focussing on reconstructing past oceanographic change (palaeoceanography) of temperature and salinity using small fossils called Ostracoda as a proxy. Skye led the study under the supervision of Dr Moriaki Yasuhara and Dr Yuanyuan Hong, both from the School of Biological Sciences and Swire Institute of Marine Science, and in collaboration with Professor Tine Rasmussen of UiT The Arctic University of Norway, who is a top specialist of Arctic palaeoceanography. Faunal shift Dr Yasuhara said: “We were not expecting to see a sea-level change recording a meltwater pulse from the data. But during the study, Skye discovered a strong faunal shift and we gradually realised it must reflect the meltwater pulse.” Skye said: “Based on other studies of the last deglacial-Holocene history of the Svalbard region, we initially hypothesised the salinity changes may have contributed to ostracod faunal turnovers in our two cores. Only after we had the ostracod census data and investigated the ecology of every species in the references, did we realise the ostracod faunal turnover at 11,300 years BP [Before Present] reflected the abrupt sea-level changes of the MWP-1B because of the melting of large ice sheets. “The MWP-1B was first discovered from coral cores in low-latitude regions [known as far- field – that is, places far enough from the polar ice sheets, like low latitude areas such as Hong Kong]. But the MWP-1B as an abrupt sea-level event remains controversial because its timing, rate, and magnitude are not well constrained under the background of continuous deglacial sea-level changes. Unlike elusive far-field records, our study indicates abrupt sea-level changes of 40 to 80 metres in approximately 300 years in Svalbard, a high-latitude region near the polar ice sheets and source of meltwater [known as near-field, where ice sheet melt causes large sea-level change]. We think it’s clear evidence of MWP-1B.” The research group used fossil Ostracoda preserved in two marine sediment cores as an indicator to quantitatively reconstruct the water depth changes in Svalbard in the past 13,000 years. More than 5,000 specimens and 50 species were recorded in two sediment cores from Storfjorden in Svalbard. “Ostracods are a group of small (usually <1mm) aquatic crustaceans which are very sensitive to water conditions,” said Dr Yasuhara. “They have calcareous shells that are very well preserved as fossils and their fossil shells have a variety of morphological characters that allow precise species identification of specimens. In addition, they are abundant in a small amount of sediment and so it’s reasonably possible to obtain enough numbers of specimens needed for robust statistical analyses, even from a small amount of sediment typically available from sediment cores that are usually <10cm in diameter. There are very few such organisms and fossils ostracods are an ideal proxy to reconstruct palaeoenvironmental changes.“ Much of the hard work was down to the efforts of Skye, who embarked on this research as her Final Year Project with Dr Yasuhara. Dr Yasuhara said: “Skye did an outstanding job and proved that an undergraduate student can do first-class scientific discovery important for the global research community and publish the result in a very top journal like Quaternary Science Reviews .” Global sea-level rise While their studies focussed on a period more than 10,000 years ago, the implications of the discovery in connection with today’s rising sea levels are important. Dr Yasuhara said: “Our study showed that sea-level change and warming are not linear in relationship. Future warming may not mean a gradual global sea- level rise, but may result in some sudden sea- level jumps at unpredictable times, which has An international team of marine biologists have been looking at another aspect of temperature change in oceans – the impact of the 2016 heatwave on the Great Barrier Reef in Australia and how it affected five species of reef fishes. The marine heatwave (MHW) was the longest and hottest thermal anomaly on record and killed a third of the reef’s corals. The team, led by Dr Celia Schunter, Assistant Professor in the School of Biological Sciences and Swire Institute of Marine Science, measured RNA in fish livers and discovered many genes changed expression levels across different time points of the MHW, revealing important functions such as cellular stress response and changes in metabolic function. Given similar heatwaves in recent years in the South China Sea, Dr Schunter is calling for more research to be done on the marine waters of Hong Kong. Dr Yasuhara (right) and Professor Tine Rasmussen of UiT The Arctic University of Norway (left). Scanning Electron Microscopy image of typical shallow-marine (neritic) ostracod species from the study sites. With elevated temperatures during a marine heatwave this cardinalfish species (Cheilodipterus quinquelineatus) shows the least changes in gene expression and appears to be more tolerant. huge implications for our society, especially cities on coastal plains of low elevation, like Hong Kong.” The Yasuhara Lab broadly works on past ecosystems, biodiversity, environments, and their interactions mainly via ostracod micropalaeontology. “We continue our research in the Svalbard region with Tine,” said Dr Yasuhara. “We are studying methane seep activity and ecosystem changes using sediment cores in the region. “Closer to home, we are also working on reconstructing Holocene sea-level change and historical anthropogenic impacts [for example, pollution and eutrophication] on marine ecosystems in Hong Kong using sediment cores and other samples. The result will give a nice far field sea-level record and a robust reconstruction of the history of human-induced marine ecosystem degradation.” FEELING THE HEAT COVER STORY 07 06 The University of Hong Kong Bulletin | November 2020