Solar modulation of flood frequency in central Europe during spring and summer on interannual to multi-centennial timescales

Solar influences on climate variability are one of the most controversially discussed topics in climate research. We analyze solar forcing of flood frequency in central Europe during spring and summer on interannual to multi-centennial timescales, integrating daily discharge data of the River Ammer (southern Germany) back to AD 1926 (∼  solar cycles 16–23) and the 5500-year flood layer record from varved sediments of the downstream Ammersee. Flood frequency in the River Ammer discharge record is significantly correlated to changes in solar activity when the flood record lags the solar signal by 2–3 years (2-year lag: <i>r</i> = −0.375, <i>p</i> = 0.01; 3-year lag: <i>r</i> = −0.371, <i>p</i> = 0.03). Flood layer frequency in the Ammersee sediment record depicts distinct multi-decadal variations and significant correlations to a total solar irradiance reconstruction (<i>r</i> = −0.4, <i>p</i> &lt;  0.0001) and ¹⁴C production rates (<i>r</i> = 0.37, <i>p</i> &lt;  0.0001), reflecting changes in solar activity. On all timescales, flood frequency is higher when solar activity is reduced. In addition, the configuration of atmospheric circulation associated with periods of increased River Ammer flood frequency broadly resembles that during intervals of reduced solar activity, as expected to be induced by the so-called solar top-down mechanism by model studies. Both atmospheric patterns are characterized by an increase in meridional airflow associated with enhanced atmospheric blocking over central Europe. Therefore, the significant correlations as well as similar atmospheric circulation patterns might provide empirical support for a solar influence on hydroclimate extremes in central Europe during spring and summer by the so-called solar top-down mechanism

Synchronizing ¹⁰Be in two varved lake sediment records to IntCal13 ¹⁴C during three grand solar minima

Timescale uncertainties between paleoclimate reconstructions often inhibit studying the exact timing, spatial expression and driving mechanisms of climate variations. Detecting and aligning the globally common cosmogenic radionuclide production signal via a curve fitting method provides a tool for the quasi-continuous synchronization of paleoclimate archives. In this study, we apply this approach to synchronize 10Be records from varved sediments of Tiefer See and Lake Czechowskie covering the Maunder, Homeric and 5500 a BP grand solar minima with 14C production rates inferred from the IntCal13 calibration curve. Our analyses indicate best fits with 14C production rates when the 10Be records from Tiefer See were shifted for 8 (−12∕ + 4) (Maunder Minimum), 31 (−16∕ + 12) (Homeric Minimum) and 86 (−22∕ + 18) years (5500 a BP grand solar minimum) towards the past. The best fit between the Lake Czechowskie 10Be record for the 5500 a BP grand solar minimum and 14C production was obtained when the 10Be time series was shifted 29 (−8∕ + 7) years towards present. No significant fits were detected between the Lake Czechowskie 10Be records for the Maunder and Homeric minima and 14C production, likely due to intensified in-lake sediment resuspension since about 2800 a BP, transporting old 10Be to the coring location. Our results provide a proof of concept for facilitating 10Be in varved lake sediments as a novel synchronization tool required for investigating leads and lags of proxy responses to climate variability. However, they also point to some limitations of 10Be in these archives, mainly connected to in-lake sediment resuspension processes

Solar modulation of Mid- to Late Holocene flood frequency in detrital layers from varved sediments of Lake Ammersee (southern Germany)

International audienceClimate is the principal driving force of hydrological extremes like floods and attributing generating mechanisms is an essential prerequisite for predicting future flood risk. Instrumental flood records, however, rarely exceed more than the last century and are too short for satisfactory assessing flood responses to changing climate boundary conditions. Intercalated into annually laminated (varved) lake sediments, flood triggered layers of detrital catchment material provide a natural archive of flood frequency with the potential to extend instrumental runoff data for millennia even down to seasonal precision. Lake Ammersee in the pre-alpine region of southern Germany is an ideal site to build up a long time series of flood triggered sedimentation because the varved sedimentary record enables both accurate detection and precise dating of detrital flood layers. Extensive late moraine formations in the lake catchment provide abundant easy erodible detrital material transported downstream into the gully shaped lake by only one main tributary, River Ammer. A 5500-year flood layer time series was established at seasonal resolution back to 400 yr BP and at annual resolution from 400 yr BP to 5500 yr BP conducting high-precision sediment microfacies analyses and geochemical X-ray fluorescence scanning (µ-XRF). Flood layer frequency is in agreement with reconstructed total solar irradiance back to 5500 yr BP, indicating more frequent flood layers during periods of reduced solar irradiance. Further, comparison of the seasonal flood layer time series with instrumental data reveals that most spring and summer flood layers are triggered by cyclonic weather regimes from the North Atlantic. Since spring and summer flood layers comprise 89 % of all flood layers in the seasonally resolved part of the record, we assume for the complete 5500 years an intensified cyclonic activity in spring and summer for the Ammersee region during intervals of reduced solar irradiance

Interannual to millennial‐scale variability of River Ammer floods and its relationship with solar forcing

The relationship between River Ammer flood frequency variability, extreme summer climate over Europe, and solar forcing is investigated. First, we used observational data to evaluate extreme weather and climate anomaly patterns associated with flood and solar forcing as well as the possible dynamical mechanisms behind them. Then, the annual resolution flood layer record from the Lake Ammer sediments is analysed to evaluate millennial‐scale variability of floods and possible related extreme climate patterns back to 5,500 years BP. A composite analysis reveals that observed River Ammer flood frequency variability at interannual to multidecadal time scales is connected to large‐scale extreme precipitation and temperature patterns. From a synoptic‐scale perspective, the extreme precipitation pattern associated with floods is related to an increase in the frequency of high upper‐level potential vorticity (PV) events over western Europe and a decrease over eastern Europe and western Russia. Increased (decreased) frequency of upper‐level high PV events is related to more (less) surface extreme precipitation occurrence. Furthermore, we show that increased frequency of upper‐level high PV events over western Europe is associated with enhanced blocking activity over eastern Europe. Therefore, the out of phase interannual to millennial‐scale variations of River Ammer flood frequency and solar irradiance, as presented in previous studies, can be explained by a solar modulation of eastern European‐western Russia summer blocking and associated upstream upper‐level wave breaking activity. In addition, we identify two distinct quasi‐periodic signals in both frequency of Lake Ammer flood layer and solar irradiance records with periods of ~900 years and ~2,300 years. We argue that similar cycles should dominate millennial‐scale variations of blocking activity in eastern Europe‐western Russia as well as extreme precipitation and flood frequency variability over central and western Europe during the last ~5,500 years

A 450 year history of extreme floods in annually laminated sediment from pre-alpine Lake Ammersee (Southern Germany)

International audienceForecasting of extreme events and their impacts on the human habitat requires comprehensive understanding of the underlying physical processes and recurrence intervals. Since instrumental time series rarely exceed a century, geo-archives are adequate tools to examine such events on longer time scales. In particular, lakes with annually laminated (varved) sediments provide continuous high-resolution records of climate and environmental variability. Flood-triggered sediment fluxes of detrital catchment material into these lakes provide long flood time series that can be precisely dated through counting of annual layers. The pre-alpine Lake Ammersee (Southern Germany) is an ideal site for reconstructing long time series of flood frequencies because its annually laminated sediment profile allows precise dating and reliable detection of even microscopic layers by their sedimentological and geochemical characteristics. Furthermore, instrumental time series of local precipitation and runoff can be used for calibrating the palaeo-record. The existing high-resolution Holocene palaeotemperature reconstruction derived from ostracods in Lake Ammersee sediments facilitates the discussion of changes in flood frequency patterns in relation to changes in larger scale climate boundary conditions. A novel methodological approach combining micro-facies analyses, high-resolution element scanning and stable isotope measurements allowed to reconstruct a 450 year time series of detrital layers in two varved sediment cores from Lake Ammersee located 1.8 km apart from each other. The seasonal occurrence of each layer was determined by its micro-stratigraphic position within a varve. The comparison of our record with measured runoff data from the main tributary River Ammer for the last 73 years and the proximal-distal pattern of detrital layer thickness towards the Ammer river mouth confirm the interpretation of these layers as triggered by floods. To better understand the effects of precipitation characteristics on major runoff events we compared our flood layer record with continuous daily precipitation data from the Meteorological Observatory Hohenpeiβenberg back to AD 1880. For investigating the role of atmospheric circulation patterns on flood frequencies we compared our record with high-resolution atmospheric pressure field reconstructions of the last 450 years

Si cycling in transition zones: a study of Si isotopes and biogenic silica accumulation in the Chesapeake Bay through the Holocene

Si fluxes from the continents to the ocean are a key element of the global Si cycle. Due to the ability of coastal ecosystems to process and retain Si, the ‘coastal filter’ has the potential to alter Si fluxes at a global scale. Coastal zones are diverse systems, sensitive to local environmental changes, where Si cycling is currently poorly understood. Here, we present the first palaeoenvironmental study of estuarine biogenic silica (BSi) fluxes and silicon isotope ratios in diatoms (δ30Sidiatom) using hand-picked diatom frustules in two sediment cores (CBdist and CBprox) from the Chesapeake Bay covering the last 12000 and 8000 years, respectively. Constrained by the well-understood Holocene evolution of the Chesapeake Bay, we interpret variations in Si cycling in the context of local climate, vegetation and land use changes. δ30Sidiatom varies between + 0.8 and + 1.7‰ in both sediment cores. A Si mass balance for the Chesapeake Bay suggests much higher rates of Si retention (~ 90%) within the system than seen in other coastal systems. BSi fluxes for both sediment cores co-vary with periods of sea level rise (between 9500 and 7500 a BP) and enhanced erosion due to deforestation (between 250 and 50 a BP). However, differences in δ30Sidiatom and BSi flux between the sites emphasize the importance of the seawater/freshwater mixing ratios and locally variable Si inputs from the catchment. Further, we interpret variations in δ30Sidiatom and the increase in BSi fluxes observed since European settlement (~ 250 a BP) to reflect a growing human influence on the Si cycle in the Chesapeake Bay. Thereby, land use change, especially deforestation, in the catchment is likely the major mechanism

Seasonal flood layer distribution of the last 450 years in annually laminated sediment from Lake Ammersee (Southern Germany)

International audienceLakes can be utilized as long-term natural observatories of environmental and climate change in the human habitat because they act as ideal sediment traps accumulating continuous sediment records reaching far back in time. Especially from annually laminated lake sediment records detailed seasonal information can be obtained. Such long time series of high-resolution data ideally complement multi-scale observational data in order to achieve a comprehensive mechanistic understanding of climate and environmental variability. For this case study two short annually laminated sediment cores from Lake Ammersee (Southern Germany) have been studied using a combination of micro-facies analyses, high resolution element scanning ( -XRF) and stable isotope (^13C, ^18O) data. Our results provide a precise and independent chronology established by counting of calcite varves using a petrographic microscope and the identification of short-term fluxes of detrital matter into the lake. The seasonal occurrence of these detrital layers was determined by their micro-stratigraphic position within a varve. The record of detrital layers within the last 73 years is in good agreement with observed runoff data from the main tributary river (Ammer) and local precipitation data. This leads in combination with a proximal-distal pattern of detrital layer thicknesses to an interpretation as flood layers. Our data indicate maxima of spring and summer flood layers during solar minima in the Little Ice Age triggered by intensified snowmelt events and atmospheric circulation changes