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Description
The proposed project ‘CHARME’ seeks to document the physical, biological, and chemical signature of present day glacial retreat recorded and preserved in the seabed. Although the influence of ice sheets on the ocean in the geological past has been studied widely, few studies exist to provide a real-time perspective and “check” on interpretations of ancient sediment records. Such oversight is largely due to the lack of available modern environments suitable for such studies; the proposed work in Antarctica thus represents an unprecedented and unique scientific opportunity. This project will significantly advance our understanding of the effect of retreating ice sheets on ecosystems, revealing their signature in the geological record, and the potential of using specific organism groups to indicate such environmental shifts. Furthermore, the project will also reveal how new, deglaciated territory is colonised by a succession of organisms. Results generated in CHARME will serve as an exceptional blueprint for the interpretation of deglacial to ice-free sediment records of past environments both in polar regions and in temperate areas previously occupied by ice sheets. In this way, this study of present environments will serve as a key to understand the past, to better (and correctly) interpret past glacial dynamics and ice sheet behaviour. In turn, this information will be utilized for the modelling of future ice behaviour and its effect on ecosystem composition and dynamics. The latter aspect of this project is especially timely given the abrupt, greenhouse gas driven climate warming seen since AD 1850 caused by human activity.
Summary of project results
CHARME (CHanging AntaRctic Marine Environments) aimed to provide insights into the physical, biological, and chemical signature of modern-day glacial retreat in the fjords of Antarctica. Although the influence of ice sheets on the ocean in the geological past has been studied widely, few studies exist to provide a real-time perspective and “check” on interpretations of ancient sediment records, especially in the climatically-sensitive but understudied region of the Antarctic Peninsula. This project aimed to advance our understanding of the effect of retreating ice sheets on ecosystems, revealing their signature in the geological record, and the potential of using specific organism groups to indicate such environmental shifts in associated environmental factors such as sea ice cover. Furthermore, the project also aimed to reveal how new, deglaciated territory is colonised by a succession of organisms.
The overarching aim of CHARME was to fingerprint the biological and biogeochemical signature of modern marine-based ice retreat recorded in Antarctic seabed sediments. CHARME examined three fjords along the Antarctic Peninsula which had been sampled in a North to South transect, span the gamut from subpolar to polar fjords, and have actively retreating ice margins that have been documented by air photo and satellite analyses. The project focused on identifying the biogenic preserved climate indicators (proxies) in seabed sediments most useful for reconstructing key environmental gradients in deglaciating environments: 1. distance from ice front, 2. sea ice cover, 3. ocean temperature. CHARME examined both modern (seabed surface; ~45 samples total) and ancient (sediment core; 3 cores in total) samples, employing a variety of analyses, including a suite of microfossils (foraminifera, dinoflagellate cysts, diatoms), biogeochemistry (organic carbon, biogenic silica, sea-ice and open-water biomarkers), and DNA preserved in the seabed (sedaDNA). To provide a timeline to environmental reconstructions, sediment cores were dated by radiocarbon and lead dating.
CHARME revealed the suitability of specific microfossils (benthic foraminifera) to indicate the modern distance to the marine-terminating glacier and vicinity to the open ocean. Chemical fossils (HBI biomarkers) produced by sea-ice diatoms and the preserved DNA of sea-ice organisms in the modern fjord samples can be used to detect sea-ice fluctuations. This combined biomarker-DNA technique was used back in time by sediment cores. Sea ice was reconstructed over the past 130 years in the southernmost fjord, showing severe sea ice changing to thinner sea ice starting in 1940, and subsequent more open water in the last two decades, in agreement with observational data. DNA in cores revealed that biodiversity (number of species and their distribution), a measure of ecosystem health, has increased in the last few decades, hand-in-hand with climate warming and sea-ice decline, as proposed by other studies. However, recent increases are dwarfed when examining the longest record in the southernmost fjord where biodiversity was much higher in the late 19th until the mid 20th century and included different species. These results imply that polar ecosystems are adversely, and perhaps irrevocably, affected by recent climate warming. These results will help inform the use of climate proxies for accurate environmental reconstructions. Further, environmental reconstructions past the brief observational period generated in the project elucidate the progression of polar fjords and their ecosystems under climate warming.
CHARME has highlighted the valuable insights provided by palaeoclimate proxies whilst critically examining the use of available palaeoenvironmental indicators and tools. It generated detailed and nuanced time series of sea ice and environmental histories (biological productivity, open water) that extend beyond the short observational period and can be tied to available observational data such as glacial retreat patterns and air temperature measurements. CHARME has advanced our understanding of applying these novel tools in Southern polar settings, creating an impact on the disciplines of high-latitude marine geology, palaeoceanography, palaeoecology, and biogeochemistry. By examining a wide spectrum of available climate proxies and their indicator potential in polar fjord environments, the project also advanced the disciplines of micropalaeontology and biogeochemistry. In light of the topical nature of CHARME, emerging results will no doubt play an important role in our understanding of the consequences of human-induced climate change on polar ecosystems and their fate in an increasingly warmer World. Such information will be of high value to forthcoming assessments by bodies such as the Intergovernmental Panel on Climate Change. Whilst CHARME has officially ended as a project, its result will continue to be relevant to both scientists and policy makers in the near future.