Koralline Algen als neues Archiv zur Entschlüsselung vergangener Meereisbedeckung und Wassertemperaturdynamik im Nordatlantik/Arktis: Klimatrends und dekadische Variabilität
Zusammenfassung der Projektergebnisse
The fast decline of Arctic sea-ice is one of the most striking examples of ongoing global climate change and is receiving substantial public and scientific attention. Recent dramatic changes in the Arctic have profound environmental, societal, and ecological impacts, and have led to an increased demand for reliable sea-ice predictions. A better understanding of the internal processes driving natural variability in Arctic sea-ice characteristics, freshwater flux, and the overprint of anthropogenic warming are essential to improving the prediction of future evolution of Arctic climate. It is necessary to resolve year-to-year variability of important climate parameters prior to the industrial revolution, when humans started impacting climate and ocean chemistry. However, in high Arctic latitudes data coverage is highly restricted, with observation-based data generally not extending before the 1950s, too short to accurately calibrate climate models. Thus, longer seasonally-resolved reconstructions of past Arctic climate and ocean variability based on paleoclimate proxies are highly needed. In the Arctic, however, the availability of seasonal-resolution proxy data is extremely limited. Most time series utilised to this date come from indirect land-based archives, while seasonal-resolution marine proxy archives are missing. Long-lived coralline algae have recently emerged as novel seasonal-resolution paleoclimate archives for the high-latitudes. The overarching goal of this project was to build the first multicentury subannually-resolved coralline algal based paleo-records of surface ocean and climate variability for the high-Arctic Svalbard Archipelago, where algal samples were collected in summer 2016. At northern Svalbard warming has accelerated since the 1990s, having wide-reaching consequences for land and ocean ecosystems, e.g. decrease in seasonal sea-ice cover, reduction of glaciated areas possibly leading to increased sediment transport to the ocean and contributing to global sea-level rise. Multiple algal specimens were analyzed from northern Spitsbergen using high-resolution imaging techniques, sclerochronological methods, and laser ablation ICP-MS to resolve seasonal-resolution geochemical variability in the skeletons of C. compactum. The oldest specimen covers the time period 1813-2015, spanning a continuous growth record of more than 200 years and making it the longest annualresolution record from the high-Arctic surface ocean to date. Several samples have yielded more than 100 years of record length, which allowed to check interspecimen signal reproducibility and generate multisample averages. The records have allowed comparison and calibration of geochemical proxies and extension rates with remotely-sensed data (e.g. sea-ice, seawater temperature, air temperature). Results were presented in two separate studies. In the first study coralline algal Mg/Ca and annual extension rates were used to build an algal sea-ice proxy. Data from three samples were employed, facilitating calibration of a seasonal-resolution proxy-based marine sea-ice record with satellite data and historic observations and assessment of decadal-scale sea-ice variability and trends. We show that lowest sea-ice values within the past 200 years occurred from the 1980s to the early 2000s in this region. Our record is significantly correlated to sea-ice reconstructions from other regions in the Arctic. Surprisingly, the onset of the 20th century sea-ice decline has started earlier at northern Svalbard than in other Arctic regions, and earlier than suggested by land-based reconstructions. The reconstruction points to significant underlying decadal- to multidecadal-scale variability in sea-ice, as well as interannual to decadal-scale variability linked to large-scale Arctic atmospheric climate modes. Although the proxy provides evidence for brief periods of higher sea-ice cover as part of low-frequency variations, the long-term declining trend in sea-ice, similar to the observed Arctic-wide decline, is also present at northern Svalbard and is expected to continue with rapid future Arctic warming. The Svalbard sea-ice reconstruction has delivered critical new information on the evolution and start of the decreasing trend in high-Arctic Svalbard sea-ice related to anthropogenic forcing. In the second paper, we focus on algal Ba/Ca as new proxy for land runoff input into the surface ocean. We show that amplified anthropogenic warming already strongly affects Svalbard hydrology and surface ocean variability. Long-term records of continuous runoff monitoring do not exist. Thus, estimation of runoff development is mainly based on model calculations and reanalysis data. Annual-resolution algal Ba/Ca ratios at Mosselbukta provide a continuous and highlyresolved record of runoff, allowing to study past changes in regional glacier-derived meltwater input. We show that a significant increasing trend starting in the late-1980s, evident in all samples, matches regional model data of land-derived runoff and has occurred in concert with accelerating land and ocean temperature trends. Our record for the first time allows to assess runoff variability before the mid-20th century, showing that the rate of increase in northern Svalbard coastal runoff is unprecedented over the last 200 years. The algal Ba/Ca time series is the first annually-resolved record of past runoff variability from the high-Arctic. In summary, both reconstructions represent important steps towards establishing a baseline of preindustrial natural low-frequency climate and ocean variability, extending before the period of available observations and the start of human-related climate trends. Thus, our results may help reduce the large uncertainties that exist among ocean model simulations.
Projektbezogene Publikationen (Auswahl)
- 2017, Multicentennial record of Labrador Sea primary productivity and sea-ice variability archived in coralline algal barium, Nature Communications, 8, 15543
Chan, P., Halfar, J., Adey, W., Hetzinger, S., Zack, T., Moore, G.W.K., Wortmann, U.G., Williams, B., Hou, A.
(Siehe online unter https://doi.org/10.1038/ncomms15543) - 2018, Comparison of climate signals obtained from encrusting and free-living rhodolith coralline algae, Chemical Geology, v. 476, pp. 418-428
Williams, S., Halfar, J., Zack, T., Hetzinger, S., Blicher, M., Juul-Pedersen, T.
(Siehe online unter https://doi.org/10.1016/j.chemgeo.2017.11.038) - 2018, Coralline algae archive Fjord surface water temperatures in Southwest Greenland, Journal of Geophysical Research: Biogeosciences, v. 123
Williams, S., Halfar, J., Zack, T., Hetzinger, S., Blicher, M., Juul-Pedersen, T., Kronz, A., Noël, B., Broeke, M., Berg, W.J.
(Siehe online unter https://dx.doi.org/10.1029/2018JG004385) - 2018, Reproducibility of Clathromorphum compactum coralline algal Mg/Ca ratios and comparison to high-resolution sea surface temperature data, Geochimica et Cosmochimica Acta, v. 220, pp. 96-109
Hetzinger, S., Halfar, J., Kronz, A., Simon, A., Adey, W.H., Steneck, R.S.
(Siehe online unter https://doi.org/10.1016/j.gca.2017.09.044) - 2019. Early start of 20th-century Arctic sea-ice decline recorded in Svalbard coralline algae. Geology, v. 47, pp. 963-967
Hetzinger, S., Halfar, J., Zajacz, Z., Wisshak, M.
(Siehe online unter https://doi.org/10.1130/G46507.1)
