ALBERT

Description: The chemical structure of refractory marine dissolved organic matter (DOM) is still largely unknown. Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS) was used to resolve the complex mixtures of DOM and provide valuable information on elemental compositions on a molecular scale. We characterized and compared DOM from two sharply contrasting aquatic environments, algal-derived DOM from the Weddell Sea (Antarctica) and terrigenous DOM from pore water of a tropical mangrove area in northern Brazil. Several thousand molecular formulas in the mass range of 300-600 Da were identified and reproduced in element ratio plots. On the basis of molecular elemental composition and double-bond equivalents (DBE) we calculated an average composition for marine DOM. O/C ratios in the marine samples were lower (0.36 ± 0.01) than in the mangrove pore-water sample (0.42). A small proportion of chemical formulas with higher molecular mass in the marine samples were characterized by very low O/C and H/C ratios probably reflecting amphiphilic properties. The average number of unsaturations in the marine samples was surprisingly high (DBE = 9.9; mangrove pore water: DBE = 9.4) most likely due to a significant contribution of carbonyl carbon. There was no significant difference in elemental composition between surface and deep-water DOM in the Weddell Sea. Although there were some molecules with unique marine elemental composition, there was a conspicuous degree of similarity between the terrigenous and algal-derived end members. Approximately one third of the molecular formulas were present in all marine as well as in the mangrove samples. We infer that different forms of microbial degradation ultimately lead to similar structural features that are intrinsically refractory, independent of the source of the organic matter and the environmental conditions where degradation took place.

Description: Varve micro-facies analyses have been carried out on a 405-cm-thick annually laminated sediment sequence from the Piànico palaeolake in the southern Alps considered to correlate with the MIS 11 interglacial. This varve counted interval represents the uppermost 5900 yr of a ca 400 kyr old interglacial lacustrine deposit with a minimum length of 16,000 varve years. Intercalated in the varve succession are: (i) detrital layers, (ii) debris flows (slump deposits), and (iii) micritic calcite layers. The annual couplets are composed of a dominant light summer layer of biochemically precipitated calcite and a thin dark winter layer consisting of organic and minerogenic detritus. Microscopic correlation of individual couplets in two outcrops 130 m apart demonstrated basin-wide continuity of varve deposition. Summer layers have been found to be composed of up to four sub-layers differing in grain size, most likely reflecting discrete pulses of calcite precipitation. Two long periods of pronounced decreases in varve thickness occur within the interglacial (1100 varve years) and during the decline of full interglacial conditions (480 varve years) suggesting colder climatic conditions. The decreases in varve thickness are further accompanied by changes in the pattern of summer sub-layering. The first cold interval reflects an exceptionally long intra–interglacial climatic fluctuation indicating a mode of climatic variability different from the present interglacial. Wavelet analyses of seasonal layer thickness data revealed that periodic variations of solar activity (88-yr and 208-yr cycles) and probably of thermohaline circulation (512-yr cycle) impact on varve formation at certain intervals of the studied time window.

Description: Sediment cores retrieved from 6 locations in Lake Baikal were subjected to a paleomagnetic study in order to establish detailed age models based on correlations of relative paleointensity records. Additional data were provided by calibrated accelerator mass spectrometry (AMS) 14C dating, as well as by documentation of geomagnetic excursions like Laschamp at ~ 42 ka and Iceland Basin at ~ 185 ka. Few intervals were affected by diagenetic features like selective reductive dissolution of magnetite and greigite mineralization (Demory et al., 2005-this issue), and those that were left out of paleointensity records. These records were tuned to the well-dated paleomagnetic record from ODP Site 984 Channell [Channell, J.E.T., 1999. Geomagnetic paleointensity and directional secular variation at Ocean Drilling Program (ODP) site 984 (Bjorn Drift) since 500 ka: comparisons with ODP site 983 (Gardar drift). J. Geophys. Res., B: [Solid Earth], 104 (10):22, 937-22, 951]. The complex shape of the resulting depth/age curves highlights the need for a high-resolution age model. We focused on the climatic boundary between marine isotopic stage (MIS) 7 and 6 where the Iceland Basin paleomagnetic excursion is clearly documented in the North Atlantic Channell et al.[Channell, J.E.T., Hodell, D.A., Lehman, B., 1997. Relative geomagnetic paleointensity and d18O at ODP Site 983 (Gardar Drift, North Atlantic) since 350 ka. Earth Planet. Sci. Lett., 153 (1–2), 103-118] and in Lake Baikal Oda et al. [Oda, H., Nakamura, K., Ikehara, K., Nakano, T., Nishimura, M., Khlystov, O., 2002. Paleomagnetic record from Academician Ridge, Lake Baikal: a reversal excursion at the base of marine oxygen isotope stage 6. Earth Planet. Sci. Lett., 202 (1), 117-132] ; present study). During this period, we provide evidence for a return to cold conditions in the Lake Baikal region simultaneous to the sea surface cooling, but earlier than the global ice volume change observed in North Atlantic planktonic and benthic δ18O records, respectively. The classical strategy of age model reconstruction, based on direct correlation of the climatic record from Lake Baikal sediments with the marine δ18O reference curves is shown here to be unreliable. Moreover, this strategy does not consider (i) the nonlinearity of the age model in Lake Baikal sediments and (ii) the time lags between the global ice volume change and sea surface cooling observed in δ18O marine records. Finally, the “Baikal 200” compilation of the paleointensity records established in this study provides a 200-ka-long synthetic paleomagnetic record for Central Eurasia.

Description: Recent studies have drawn attention to differences in the seasonal impact of the 8.2 ka event, with longer cooler summers and shorter cooler/drier winters. However, there are no data available on the simultaneity or the rate of onset of the seasonal changes in Europe. Based on the microfacies and geochemical analyses of seasonally laminated varved sediments from Holzmaar, we present evidence of differences in duration and onset time of changes in summer temperature and winter rainfall during the 8.2 ka event. Since both summer and winter climate signals are co-registered within a single varve, there can be no ambiguity about the phasing and duration of the signals. Our data show that the onset and withdrawal of the 8.2 ka summer cooling occurred within a year, and that summer rains were reduced or absent during the investigated period. The onset of cooler summers preceded the onset of winter dryness by ca. 28 years. In view of the differences in nature and duration of the impact of the 8.2 ka event we suggest that a clearer definition of the 8.2 ka event (summer cooling or winter cooling/dryness) needs to be developed. Based on regional comparison and available modelling studies we also discuss the roles of solar variability, changes in North Atlantic Thermohaline circulation, and North Atlantic Circulation (NAO) during the period under consideration. Wavelet analyses of seasonal laminae indicates that the longer NAO cycles, linked to changes in the N. Atlantic temperatures, were more frequent during the drier periods.