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Establishing stable IRON isotopes of laminated LAKE sediments as novel palaeoclimate proxy

Periodic Reporting for period 1 - IRONLAKE (Establishing stable IRON isotopes of laminated LAKE sediments as novel palaeoclimate proxy)

Reporting period: 2016-06-01 to 2018-05-31

To improve climate projections and adaptation strategies to climate change and environmental stress, a better understanding of the past natural climate variability is crucial. In this respect, the IRONLAKE project aimed on developing a novel high-resolution technique; that is using iron isotopes of laminated lake sediments to infer changing redox conditions in lakes in relation to past climatic changes. Therefore, this project addressed two major concerns:
(1) The lack of highly resolved paleoclimate reconstructions: The report of the Intergovernmental Panel on Climate Change (IPCC) stresses the importance of high-resolution palaeoclimate reconstructions that allow reducing the uncertainties of climate projections and advance climate adaptation strategies. Thereby, seasonally resolved climate archive data become more and more important as climate models improve. Annually laminated lake sediments are such invaluable climate archives, as these capture palaeoclimatic and -environmental information over long time-scales with up to seasonal resolution and provide important information about past abrupt climate changes, when human influence was minor or absent.
(2) Climate warming is expected to cause anoxic dead zones in lake ecosystems, leading to severe environmental problems, like the loss of fisheries and biodiversity, lower water quality, or the generation of cyanobacteria blooms. Besides temperature, wind-induced lake mixing as well as man-made eutrophication impact on the reducing-oxidising (redox) conditions in lakes, but disentangling natural and anthropogenic driving factors is difficult. This can only be overcome by searching for analogous situations in the past and deciphering the effects of climate warming on the redox state of lakes. Hence, a better understanding of the temperature-wind-anoxia link in lakes allows developing suitable adaptation strategies to environmental problems.
Only recently, first studies have hinted that variations in stable iron isotopes of marine and lake sediments reflect changing redox conditions in oceans and lakes, possibly linked to variations in past wind and/or temperature. The IRONLAKE project aimed on testing the hypothesis of a redox-climate-relationship mirrored in iron isotopes. Lago Fagnano (55°S Tierra del Fuego, Argentina/Chile) was selected as the ideal test archive for the project due to the remote location of the lake without human interference, its Fe-rich laminated sediments, and its proven sensitivity to record past changes in climate.
Within the IRONLAKE project, three main objectives were aimed to be achieved:
(1) Understanding the formation of Fe minerals and the cyclicity of Fe-rich laminae in lake sediments of Lago Fagnano and the relation to external forcing on changes of redox conditions.
(2) Applying the stable Fe isotope methodology to lake sediments to reconstruct changing redox conditions in Lago Fagnano.
(3) Developing Fe isotopes of laminated lake sediments as a proxy for past redox and climate variations.
The IRONLAKE project enabled a profound understanding of sedimentological processes in lakes and its relation to redox and climate conditions, and paved the way to a novel concept by testing Fe isotopes of lake sediments for climate reconstruction, which is highly relevant to better understand the past and adapt to the recent climate change, and, therewith, relevant for society.
Before the sensitivity of iron isotopes could be tested, the sediment formation processes and the involved climatic drivers had to be understood. Therefore, high-resolution analyses of the Fe-rich laminae and identification of authigenic (i.e. built in the lake) and detrital (i.e. transported to the lake from its watershed) Fe-phases in the sediments have been performed using state-of-the-art sediment-geochemical methods. These techniques have enabled a profound understanding of the formation, preservation and frequency of sediment laminae in Lago Fagnano and their underlying lake-internal (i.e. redox conditions) and external (i.e. climatic) forcing. Fe- and Mn-oxides were identified as generating the lamination on (sub-) decadal timescales. Black and greenish laminae are interpreted as buried palaeo-oxidation fronts that underwent early diagenetic processes. The burial of redox fronts in Lago Fagnano is most likely promoted by cyclic rapid increases of sedimentation due to higher runoff and mass-wasting events. Increased runoff is related to the strength of the Southern Hemisphere Westerlies that, in turn, is modified by climate oscillations. Therefore, it is suggested that the cyclic repetition of the buried palaeo-redox fronts, showing periodicities of ~52 and ~4.5 years in the western and eastern sub-basins of Lago Fagnano, respectively, is forced by climate modes. The most likely candidates are the Antarctic Oscillation (AAO) and the El Niño Southern Oscillation (ENSO), both impacting southernmost South America and showing similar sub-decadal modes and multi-decadal variations. Although the burial of redox fronts in lake or marine sediments is well-known, Lago Fagnano is the first record of cyclic recurrence of these fronts triggered by changing climate. The results of this study have been presented in several scientific conferences, and will be soon publicly available in the form of an article in a high-impact scientific journal.
Based on these high-resolution sediment-geochemical analyses, amorphous Fe-(oxy)hydroxides and Fe-monosulphides (FeS) were identified as authigenic Fe-phases that are mostly responsible for the observed lamination pattern in Lago Fagnano and form in a cyclic pattern due to changed redox conditions. These two phases are hence suitable to test the working hypothesis of the project. In the next step, these Fe-phases were extracted from the sediment by chemical separation, and their fluctuating concentrations along the sediment profile were determined. Finally, iron isotopes will be measured on these phases, and by combining the gained knowledge from all steps, the sensitivity of stable Fe isotopes to capture climatic variations will be inferred. Iron isotope measurements have been delayed due to a number of technical and analytical problems during the project. However, we are certain that these results are paving the way to the establishment of Fe isotopes in Lago Fagnano as a new proxy for climate reconstruction.
The IRONLAKE project adds significantly to our understanding of Fe- and Mn- cycling in lakes and its underlying redox processes under natural conditions, i.e. without human interference. Furthermore, the results from Lago Fagnano are unique providing critical data into the use of iron isotopes to reconstruct former environmental conditions. Therefore, the project provides innovative ideas to climate modellers and environmental policymakers. Furthermore, the IRONLAKE project has significant impact on the competitiveness of the European Research Area, as the highly innovative approach is an important contribution to enhancing EU scientific excellence and keeping Europe’s leading position in high-resolution climate reconstruction to improve climate projections and adaptation strategies to changing climate. Moreover, by addressing environmental concerns, the project focussed on timely and for the societies highly relevant impacts of ongoing climate change.
Laminated sediments from Lago Fagnano are ideal to test Fe isotopes as new climate proxy