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Modern Approaches to Temperature Reconstructions in polar Ice Cores

Final Report Summary - MATRICS (Modern Approaches to Temperature Reconstructions in polar Ice Cores)

Novel quantitative climate parameters from polar ice cores are needed that go beyond previous studies in terms of temporal resolution, spatial representativeness and robustness of the paleoclimate information. The objective of MATRICs was to develop new methods to provide such quantitative novel climate information from different regions of the Earth consistently from ice core samples, thereby avoiding crucial cross-dating issues that arise when different climate archives are compared. This comprised (i) continuous quantitative reconstruction of local temperature changes on polar ice sheets using diffusion thermometry, (ii) a new physical gas thermometer for mean global ocean temperature based on precise noble gas analyses on ice cores, and (iii) estimates of climate and environmental changes in continental regions based on changes in the methane cycle.

All three of these objectives could be achieved in MATRICs. To accomplish (i) a new online method for continuous stable water isotope measurements was established using a laser spectrometer coupled to a special evaporation unit developed in our lab. Moreover, the theoretical background and evaluation routine for diffusion thermometry was established; a novel approach promoted by the late Sigfus Johnsen only in recent years that uses the temperature dependent diffusion of stable water isotopes in the firn to reconstruct in situ temperatures. For the first time we performed a rigorous calibration of this method using synthetic ice core data and developed a new numerical forward diffusion method to evaluate the ice core data. Application of this method to high-resolution data from an Antarctic ice core showed that previous temperature estimates significantly underestimate the cooling during the Last Glacial Maximum.

In order to realize objective (ii), we established a new quantitative gas extraction and mass spectrometric technique that allows high precision measurements of the ratios of noble gases (Ar, Kr, Xe) and nitrogen enclosed in ice core air bubbles as well as of the isotopic composition of all of these gases. The amount of these gases dissolved in the ocean is temperature dependent and, therefore, changes slightly during past climatic changes. Accordingly, measuring these gas ratios in ice core samples using our new method allows us to reconstruct the mean global ocean temperature on a single ice core sample with a precision of 0.5°C after correction of fractionation effects occurring during the gas transport in the firn column. Within MATRICs this method was applied to samples from several Greenland and Antarctic ice cores, showing that the late glacial ocean was on average about 2.5 °C colder than today in line with paleoceanographic evidence. Our measurements, however, also point to significantly warmer mean ocean temperatures during the last interglacial compared to today, questioning previous estimates of climate conditions and ocean heat uptake during the last interglacial.

Finally, novel gas chromatography-mass spectrometry techniques were developed to realize objective (iii). These allow us to precisely determine the hydrogen and carbon isotopic signatures of methane in small air samples extracted from bubbles entrapped in polar ice cores. These techniques were used to identify the methane sources responsible for rapid CH4 changes in parallel to abrupt warming events during the last ice age and for the long-term changes in the methane cycle over the entire last glacial/interglacial cycle. The hydrogen isotopic studies showed that a potential release of CH4 from marine gas hydrates in ocean sediments was not responsible for the CH4 increases during these warming events. The long-term changes in the carbon isotopic signature of CH4 indicate that climate affects not only the amount of CH4 emitted from individual sources but that varying climate and CO2 levels substantially changed ecosystem structure and plant composition leading to strong changes in the carbon isotopic composition of CH4 released.

The MATRICs results provide important knowledge about the response of the Earth system to global warming, such as heat uptake of the ocean, changes in ecosystems and greenhouse gas emissions in response to global warming or the regional expression of climate conditions to global climate forcing through CO2 and insolation changes. In this respect the MATRICs data serve also as important benchmarks for climate and biogeochemical models used for future climate predictions.