Ulrich von Grafenstein

Quantification of biotic responses to rapid climatic changes around the Younger Dryas — a synthesis

To assess the presence or absence of lags in biotic responses to rapid climatic changes, we: (1) assume that the δ18O in biogenically precipitated carbonates record global or hemispheric climatic change at the beginning and at the end of the Younger Dryas without any lag at our two study sites of Gerzensee and Leysin, Switzerland; (2) derive a time scale by correlating the δ18O record from these two sites with the δ18O record of the GRIP ice core; (3) measure δ18O records in ostracods and molluscs to check the record in the bulk samples and to detect possible hydrological changes; (4) analyse at Gerzensee and Leysin as well as at two additional sites (that lack carbonates and hence a δ18O record) pollen, plant macrofossils, chironomids, beetles and other insects, and Cladocera; (5) estimate our sampling resolution using the GRIP time scale for the isotope stratigraphies and the biostratigraphies; and (6) summarise the major patterns of compositional change in the biostratigraphies by principal component analysis or correspondence analysis. We conclude that, at the major climatic shifts at the beginning and end of the Younger Dryas, hardly any biotic lags occur (within the sampling resolution of 8–30 years) and that upland vegetation responded as fast as aquatic invertebrates. We suggest that the minor climatic changes associated with the Gerzensee and Preboreal oscillations were weakly recorded in the biostratigraphies at the lowland site, but were more distinct at higher altitudes. Individualistic responses of plant and animal species to climatic change may reflect processes in individuals (e.g. productivity and phenology), in populations (e.g. population dynamics), in spatial distributions (e.g. migrations), and in ecosystems (e.g. trophic state). We suggest that biotic responses may be telescoped together into relatively short periods (50 to 150 years), perhaps disrupting functional interactions among species and thus destabilising ecosystems.

Isotope signature of the Younger Dryas and two minor oscillations at Gerzensee (Switzerland): palaeoclimatic and palaeolimnologic interpretation based on bulk and biogenic carbonates

Oxygen- and carbon-isotope ratios in the carbonate of benthic ostracodes (Pseudocandona marchica) and molluscs (Pisidium ssp.) were measured across the transitions bordering the Younger Dryas chronozone in littoral lacustrine cores from Gerzensee (Switzerland). The specific biogenic carbonate records confirm the major shifts already visible in the continuous bulk-carbonate oxygen-isotope record (δ18OCc). If corrected for their vital offsets, oxygen-isotope ratios of Pisidium and juvenile P. marchica, both formed in summer, are almost identical to δ18OCc. This bulk carbonate is mainly composed of encrustations of benthic macrophythes (Chara ssp.), also mainly produced during summer. Adult P. marchica, which calcify in winter, show consistently higher δ18O, larger shifts across both transitions, and short positive excursions compared with the summer forms, especially during early Preboreal. Despite such complexity, the δ18O of adult P. marchica probably reflects more accurately the variations of the δ18O of former lake water because, during winter, calcification temperatures are less variable and the water column isotopically uniform. The difference between normalised δ18O of calcite precipitated in winter to that formed in summer can be used to estimate the minimum difference between summer and winter water temperatures. In general, the results indicate warmer summers during the late Allerod and early Preboreal compared with the Younger Dryas. Altogether, the isotopic composition of lake water (δ18OL) and of the dissolved inorganic carbonate (δ13CDIC) reconstructed from adult Pseudocandona marchica, as well as the seasonal water temperature contrasts, indicate that the major shifts in the δ18O of local precipitation at Gerzensee were augmented by changes of the lakes water balance, with relatively higher evaporative loss occurring during the Allerod compared with the Younger Dryas. It is possible that during the early Preboreal the lake might even have been hydrologically closed for a short period. We speculate that such hydrologic changes reflect a combination of varying evapotranspiration and a rearrangement of groundwater recharge during those climatic shifts.

A sedimentary record of Holocene surface runoff events and earthquake activity from Lake Iseo (Southern Alps, Italy):

This study presents a record of Holocene surface runoff events and several large earthquakes, preserved in the sediments of pre-Alpine Lake Iseo, northern Italy. A combination of high-resolution seismic surveying, detailed sediment microfacies analysis, non-destructive core-scanning techniques and AMS 14C dating of terrestrial macrofossils was used to detect and date these events. Based on this approach, our data shed light on past seismic activity in the vicinity of Lake Iseo and the influence of climate variability and human impact on allochthonous detrital matter flux into the lake. The 19 m long investigated sediment sequence of faintly layered lake marl contains frequent centimetre- to decimetre-scale sandy-silty detrital layers. During the early to mid Holocene, these small-scale detrital layers, reflecting sediment supply by extreme surface runoff events, reveal a distinct centennial-scale recurrence pattern. This is in accordance with regional lake-level highstands and minima in solar activity and thus apparently mainly climate-controlled. After c. 4200 cal. yr BP, intervals of high detrital flux occasionally also correlate with periods of enhanced human settlement activity. In consequence, deposition of small-scale detrital layers during the late Holocene apparently reflects a rather complex interplay between climatic and anthropogenic influences on catchment erosion processes. Besides the small-scale detrital layers, five up to 2.40 m thick large-scale detrital event layers, composed of basal mass-wasting deposits overlain by large-scale turbidites, were identified, which are supposed to be triggered by strong earthquakes. The uppermost large-scale event layer can be correlated to a documented Mw=6.0 earthquake in ad 1222 in Brescia. The four other large-scale event layers are supposed to correspond to previously undocumented local earthquakes. These occurred around 350 bc, 570 bc, 2540 bc and 6210 bc and most probably also reached magnitudes in the order of Mw = 5.0–6.5.

A 1600 yr seasonally resolved record of decadal-scale flood variability from the Austrian Pre-Alps

We present a record of extreme spring–summer runoff events for the past 1600 yr preserved in the varved sediments of Lake Mondsee (Austrian Pre-Alps). Combined sediment microfacies analyses and high-resolution micro-X-ray fluorescence element scanning allow us to identify 157 detrital event layers deposited in spring–summer and to discriminate between regional flood and local debris flow deposits. Higher spring–summer flood activity with a mean event recurrence of 3–5 yr occurred in several well-confined multidecadal episodes during the Dark Ages Cold Period and Medieval time (A.D. 450–480, 590–640, 700–750, and 1140–1170) as well as during the early Little Ice Age (LIA; A.D. 1300–1330 and 1480–1520). In contrast, lowest spring–summer flood activity with an event recurrence of only 30–100 yr is observed during the Medieval Climate Anomaly (A.D. 1180–1300) and the coldest interval of the LIA (A.D. 1600–1700). These findings indicate a complex relationship between temperature conditions and extreme hydro-meteorological events and suggest that enhanced summer Mediterranean cyclogenesis triggers large-scale floods in the northeast Alps during climatic transitions. The Lake Mondsee data demonstrate the climatic sensitivity of spring–summer floods and prove the potential of varved sediment records to investigate the impact of changing climate boundary conditions on seasonal flood activity for pre-instrumental time.

Oxygen-Isotope Studies of Ostracods from Deep Lakes

Oxygen-isotope records measured on valves of benthic ostracods from deep lakes provide a quantitative record of the oxygen-isotope composition of former lake water (δ 18 O L ), because bottom water temperatures in such lakes are nearly constant. This chapter gives an overview of existing ostracod-based deep-lake records, followed by a more specific discussion of the different hydrological and climatic processes controlling δ 18 O L , and of the specific effects influencing the quantitative link between δ 18 O L and the isotopic composition of the valve calcite. Based on well-studied examples from southern Germany, it will be shown that for lakes characterized by a narrow range of hydrological conditions, δ 18 O L is closely linked to the oxygen-isotope composition of the atmospheric precipitation (δ 18 O P ). Thus, ostracod-based oxygen-isotope records from those lakes provide quantitative estimates of the δ 18 O P , which, at least in Europe, is a valuable proxy for air temperature.

Age of Himalayan bottom ice cores

Dating of tropical and Equatorial ice cores drilled in high mountain glaciers is difficult because seasonal variations can be traced only in the upper part. Modelling of the ice flow is difficult at depth due to the rapid thinning of the ice layers. Fortunately, atmospheric trace gases whose lifetime exceeds the inter-hemispheric mixing time ( 1year) are tracers on a global scale. By combining several gases, it is possible to attribute the age of anunknown ice layer by comparison with other well-dated ice-core records (Chappellaz and others, 1997a; Landais and others, 2003). Methane and the isotopic composition of atmospheric O2 ( Oatm) are preferentially used for such purposes because of their suitable atmospheric turnover time ( 10 years for CH4 and 1500 years for atmospheric O2). We recovered one ice core to bedrock (117.06m long) in 2001, and twomore, also to bedrock, (108.83 and 95.80m) in 2002 on the col of East Rongbuk Glacier (28‡010N,86‡580 E; 6518ma.s.l.) on the north slope of Qomolangma (Mount Everest) (Fig. 1). Hereafter we refer to the 117.06m core as ‘‘core 2001’’, and the108.83m core as ‘‘core 2002’’. East Rongbuk Glacier covers an area of 48 kmwith a length of 14 km. Borehole temperatures at the 108.83m core site range from ^8.9‡C at 10m to a minimum of ^9.6‡C at 20m, then increase slightly to ^8.9‡C at the bottom. Measurements of Oatm and methane were performed at the Laboratoire des Sciences du Climat et de l’Environnement (LSCE) and the Laboratoire de Glaciologie et Ge¤ ophysique de l’Environnement (LGGE) respectively. Details of the measurements are available elsewhere (Chappellaz and others, 1997b; Landais and others, 2003). The narrow range of our Oatm results (^0.09% to 0.26%) with respect to the Greenland Ice Sheet Project 2 (GISP2) ice core (Fig. 2; note the different vertical scales) suggests no glacial origin of the bottom ice. But we cannot decipher whether the bottom ice originated from the early or late Holocene, because the narrow Oatm band of our cores runs across the GISP2 Oatm profile during these two periods. If early-Holocene, the age of core 2001 at 102.5m depth or of core 2002 at 87.5m depth should be > 8000 years if adopting the GISP2 Oatm time-scale. However, an 80.4m annually dated ice core recovered from the same East Rongbuk Glacier has a maximum age of 154 years (Kang and others, 2002; Qin and others, 2002). This excludes any possibility of early-Holocene origin for the bottom ice of our cores. Both methane profiles show a rapid increase during the industrial period and low methane concentrations (a little below 700 ppbv pre-industrial methane level) in the very bottom sections, but these values are still much higher than the methane concentrations during the middle Holocene ( 570 ppbv) as depicted by the Greenland Ice Core Project (GRIP) ice core (Fig. 2), even after taking into account the interpolar methane gradient (35 ppbv for the period 250^ 1000 years BP: Chappellaz and others, 1997b; Houweling and others, 2000). This further indicates the age of the bottom ice to be late Holocene. To estimate precisely the age of the bottom ice, we prepared a CH4/ Oatm phase plane by using the GRIP CH4 and GISP2 Oatm records, then superimposed our CH4 and Oatm pairs (Fig. 3). All the CH4 and Oatm pairs from the bottom 2m of our cores are situated within BOX1 (in Fig. 3).The ages of the CH4 and Oatm pairs of the GRIP and GISP2 ice cores within BOX 1 are in the range 1498^2055 years BP, confirming the late-Holocene origin of the bottom ice. Furthermore, we exclude any possibility of connecting the CH4/ Oatm pairs within BOX 1 to the early Holocene. Otherwise, some of the CH4/ Oatm pairs within BOX 2 (in Fig. 3; from shallower sections, thus younger than those in BOX 1) would be dated at middle Holocene (Fig. 3), a period that is beyond any possible connection to BOX 2, even after considering the cumulative