Earth and Planetary Science Letters 505 (2019) 110–117

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Earth and Planetary Science Letters

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Roman-driven cultural eutrophication of Lake Murten, ∗ Mischa Haas a,b, , Franziska Baumann b, Daniel Castella c, Negar Haghipour b,d, Anna Reusch e, Michael Strasser f, Timothy Ian Eglinton b, Nathalie Dubois a,b a Surface Waters – Research and Management, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland b Department of Earth Sciences, ETH Zürich, Sonneggstrasse 5, CH-8006 Zürich, Switzerland c Roman Site and Museum , CH-1580 Avenches, Switzerland d Laboratory of Ion Beam Physics, Otto-Stern-Weg 5, ETH Zürich, CH-8006 Zürich, Switzerland e Faculty of Geosciences, University of Bremen, Klagenfurter Straße, DE-28359 Bremen, Germany f Institute of Geology, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria a r t i c l e i n f o a b s t r a c t

Article history: Land cover transformations have accompanied the rise and fall of civilizations for thousands of years, Received 14 February 2018 exerting strong influence on the surrounding environment. Soil erosion and the associated outwash of Received in revised form 21 May 2018 nutrients are a main cause of eutrophication of aquatic ecosystems. Despite the great challenges of water Accepted 18 October 2018 protection in the face of climate change, large uncertainties remain concerning the timescales for recovery Available online xxxx of aquatic ecosystems impacted by hypoxia. This study seeks to address this issue by investigating the Editor: J. Adkins sedimentary record of Lake Murten (Switzerland), which witnessed several phases of intensive human Keywords: land-use over the past 2000 years. lake sediments Application of geophysical and geochemical methods to a 10 m-long sediment core revealed that soil early human land-use erosion increased drastically with the rise of the Roman City of Aventicum (30 CE). During this period, cultural eutrophication the radiocarbon age of the bulk sedimentary organic carbon (OC) increasingly deviated from the modeled varves deposition age, indicating rapid flushing of old soil OC from the surrounding catchment driven by radiocarbon anomalies intensive land-use. Enhanced nutrient delivery resulted in an episode of cultural eutrophication, as aquatic ecosystem recovery shown by the deposition of varved sediments. Human activity drastically decreased towards the end of the Roman period (3rd century CE), resulting in land abandonment and renaturation. Recovery of the lake ecosystem from bottom-water hypoxia after the peak in human activity took around 50 years, while approximately 300 years passed until sediment accumulation reached steady state conditions on the surrounding landscape. These findings suggest that the legacy of anthropogenic perturbation to watersheds may persist for centuries. © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

1. Introduction Neolithic Revolution and the first development of agricultural and pastoral activity (Turner, 1990). Human activity has become an important geological agent, in- During deforestation and land cultivation, nutrients of the fluencing key biogeochemical cycles by modifying the Earth’s sur- catchment soils, such as phosphorus (P), nitrogen (N) and iron face (Olofsson and Hickler, 2008). Although agricultural and urban oxides (Fe oxides), are washed out following hydrological path- areas have increased dramatically in the last decades to ensure ways, exerting strong influence on key geochemical cycles in food security and living space, land cover transformations are not aquatic ecosystems within corresponding watersheds (Ekholm and Lehtoranta, 2012). Lake ecosystems are sensitive to environmental a recent phenomenon, and are intimately linked with human his- changes of both climatic and anthropogenic origin. Increased nutri- tory. The balance between natural and cultivated land has followed ent export from catchment soils due to urbanization is nowadays the rise and fall of civilizations for several thousand years since the – and was in the past – a key cause of eutrophication, acidifi- cation, and bottom-water hypoxia of lakes (Jenny et al., 2016). Especially the transport and bioavailability of P is a limiting fac- Corresponding author at: Surface Waters – Research and Management, Eawag, * tor controlling the primary productivity and subsequently the Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland. autochthonous OC flux to the sediment. In the upper millime- E-mail address: [email protected] (M. Haas). ters of the lacustrine sediments organic matter is oxidized laying https://doi.org/10.1016/j.epsl.2018.10.027 0012-821X/© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). M. Haas et al. / Earth and Planetary Science Letters 505 (2019) 110–117 111 the way for nutrient recycling by anaerobic microbial communi- ties. Most of the P is transported and buried in particulate form, bound to Fe-oxides. Once deposited, these Fe-oxides gets micro- bially reduced to Fe-sulfides, releasing the previously bound P to the water in bioavailable form, further driving eutrophication and oxygen depletion of bottom waters (Ekholm and Lehtoranta, 2012; Rekolainen et al., 2006). Lake eutrophication resulting from land- use change has emerged as an increasingly important issue in the context of protection of water resources for future generations (IPCC, 2014). Significant progress has been made in ameliorating past detrimental changes in lake trophic status since the 1960s. Besides temporary chemical treatments, biomanipulation or oxy- genation, the regulation of nutrient point sources from agriculture, forestry and urban development is one of the most effective miti- gation methods (Carpenter et al., 1999). Despite these measures, and accompanying drastic reductions in water column nutrient loadings, most lakes worldwide are yet to fully recover (Gächter and Wehrli, 1998). With few exceptions, e.g. Lake Lucerne (BAFU, 2016), lakes worldwide that have been affected by a period of cul- tural (i.e. man-made) eutrophication still experience deep-water hypoxia supporting e.g. the growth of toxic algae (Jenny et al., 2016). Despite their great societal importance as water resources, large uncertainties remain regarding the timescales over which lakes currently impacted by hypoxia will improve their trophic sta- tus. In this context, lacustrine sedimentary sequences can provide temporal constraints on past changes in trophic status that may inform about the response of aquatic ecosystems to present and future perturbations. Acting as natural repositories, lakes produce continuous sed- iment archives recording past environmental change (Zolitschka, 1998). The close correlation between human activity, such as land clearance and farming, and lake sedimentation has been described Fig. 1. Bathymetric map of Lake Murten. The sampling location is shown with a red dot. Also indicated are the outline of the ancient city Aventicum, Ro- in several paleolimnological studies that reveal diverse and com- man roads, a Roman canal as well as a shore line estimation of 0 CE (Castella plex lake system responses (Bradshaw et al., 2006; Dreibrodt and et al., 2015). Data source: swisstopo (Art. 30 GeoIV): 5704 000 000 / swiss- Wiethold, 2015). Land-use results in the input of large amounts TLM3D@2011, DHM25@2003, Vector200©2011, reproduced with permission of of minerogenic and chemical erosional products from catchment swisstopo / JA100119. Bottom left: Catchment of Lake Murten and main geologi- cal units. Bottom right: Location of Lake Murten in Switzerland, central Europe. (For soils, which can be traced by a variety of geophysical, geochemi- interpretation of the colors in the figure(s), the reader is referred to the web version cal and biological methods. Common indicators of human activities of this article.) include changes in magnetic susceptibility (MS) (Huang and O’- Connell, 2000; Simonneau et al., 2013) and elemental composition (Lavrieux et al., 2017), as well as changes in the abundance of ar- utary feeding the lake, providing runoff from seasonal rainfall and boreal vs. non-arboreal pollen that document deforestation or field snow melt from the surrounding catchment (Magny et al., 2005). cultivation (Bichet et al., 2013; Tinner et al., 2003). In many cases Most of the detrital sediment originates from conglomerates, sand- the geochemical characteristics of deposited organic matter (OM), stones, clays and marls, deposited during the late Alpine orogeny such as the total organic carbon or nitrogen content (TOC, TN), (Pfiffner, 2014). The climate can be described as temperate, with differs significantly from periods prior to human influence (Enters deciduous forests as the dominant natural vegetation today (Magny et al., 2006). The stable isotopic signature (δ13C and δ15N) have et al., 2005). The region has yielded a wealth of archeological find- also proven to be a useful source of information on lacustrine sed- ings, enabling human activities over the historical past to be well imentary OM (Meyers, 2003). The distribution and stable isotopic documented, and offering ideal preconditions for coupling natural composition of the sedimentary OM hold information not only on (sedimentary) with historical archives. past natural environmental changes but also those arising from From the Neolithic to the Bronze Age, 6500–800 before the prior human activity (Dubois and Jacob, 2016). Common Era (BCE), the Seeland region was populated by Pile The 14C content of different organic components (e.g., macro- Dweller communities (Menotti, 2004). Preserved organic remains fossils, TOC, lignin or leaf waxes) has emerged as a powerful tracer from Neolithic lake shore settlements in and around Lake Murten of carbon storage and mobilization (Feng et al., 2013), and can for were previously excavated and have been precisely dated using example be used to identify periods of accelerated erosion and dendrochronology (Schibler and Jacomet, 2010). Archeological find- export of “pre-aged” soil organic carbon (SOC) from surrounding ings reveal that Pile Dweller communities of the Bronze Age to catchments to lake sediments (Edwards and Whittington, 2001; the Early Iron Age cultivated fields in close proximity to the lake Gierga et al., 2016). and even used ox-pulled ploughs and other bronze tools (Jacomet, In this article, we present paleolimnological data from the sedi- 1998). mentary record of Lake Murten, Switzerland, which witnessed sev- Around 400 BCE, Celtic tribes started to migrate into the re- eral phases of human occupation since the Neolithic period. Lake gion and established new settlements (La Tène, Müller, 1999a). Murten (aka ) is located in the Seeland region (Land Around 15 BCE, after the Gallic Wars, large parts of the Swiss Low- of Lakes) between the Alps and the in the low- lands were integrated into the Roman Empire. A time of peace lands of Western Switzerland (Morphometric and limnologic data and prosperity followed, often referred as Pax Romana (Im Hof, in Fig. 1). The Broye River from the South represents the main trib- 2007). South of Lake Murten, the Roman city Aventicum was built 112 M. Haas et al. / Earth and Planetary Science Letters 505 (2019) 110–117 and many farms and villas were distributed over the rural area (Castella et al., 2015). Large-scale land clearance and intensive agri- culture followed, completely modifying the landscape (Tinner et al., 2003). In its heyday, during the 1st and 2nd century of the Com- mon Era (CE), Aventicum had around 20000 inhabitants (Castella et al., 2015). Towards the end of the 3rd century CE a combination of political instability, economic crises and repeated incursions of Germanic tribes resulted in the demise of the city. The Roman oc- cupation ended rapidly when the last troops were withdrawn in the 5th century CE (Im Hof, 2007). During the ensuing Migration Period, the population declined and agricultural activity ceased. It also marked one of the last major phases of forest regrowth in Switzerland (Tinner et al., 2003). Starting with the rise of Frankish kingdoms, a slow and gradual intensification of agriculture can be observed in Switzerland through the Middle Ages (Im Hof, 2007). The medieval city of Murten was built in the 9th century CE di- rectly at the lakeshore, and remains the largest conurbation in the catchment today. The mechanization of agriculture in the course of industrialization started around 1950 in the Seeland region (Nast, 2006). Agricultural societies were, and are still, dependent on favor- able environmental conditions. Decade- to century-scale climate deteriorations are known to be a causal factor in harvest fail- ures, famines and migration (Büntgen et al., 2016; Tinner et al., 2003), leading to changes in land-use and soil stability. Tinner et al. (2003)found a good correlation between warm periods and cultural indicators (CI) from pollen assemblages recovered from sediment cores of 4 lakes in Switzerland. For instance, the period 50 BCE–100 CE falls into a warm climate period and correlates well with the phase of enhanced Roman land-use. Furthermore, enhanced soil erosion can also be caused by changes in precipita- tion or a loss of stabilizing plants through droughts or flood events Fig. 2. Age–depth model and the subdivision of the sediment core into six main (Arnaud et al., 2016). It is assumed that Late Holocene cold phases lithostratigraphic units and several subunits. Additionally, bulk sediment radiocar- in Switzerland were characterized by enhanced precipitation (Haas bon dates are indicated with a black dashed line. et al., 1998;Magny, 1993; Tinner et al., 2003). The Roman period of highest land-use, however, falls into a warm period and thus should not be characterized by enhanced precipitation (Holzhauser Gamma spectrometer) (Appleby, 2008). Radiocarbon dating of ter- et al., 2005). Still a multi-proxy approach combining different in- restrial macrofossil remains, such as seeds, twigs, wood or leaf dicators for human land-use was chosen in this study in order to fragments, was applied for estimating the deposition age of the obviate climatic effects. sediments. Fourteen specific sediment intervals were wet-sieved to A comparison between the sedimentary archive and the histor- obtain sufficient macrofossil material for the measurement. Prior ical development of agriculture in the Lake Murten region allows to the dating, the samples underwent an Acid–Base–Acid clean- for an investigation into how the lake system responded to phases ing procedure to remove contamination of carbonates and humic 14 of intensive human land-use in the catchment, as well as phases substances, following the method of Hajdas et al. (1995). C- of land abandonment, with the rise and demise of the Roman City measurements were carried out on an Accelerator Mass Spectrom- Aventicum providing an ideal test bed. In this context, we recon- eter (AMS) equipped with and Elemental Analyzer unit (EA), using struct and examine the impact of large-scale deforestation, Roman the MIni CArbon DAting System (MICADAS) at the radiocarbon fa- land-use practices, as well as recovery following reduced human cility of the Eidgenössische Technische Hochschule ETH in Zurich. activity and land abandonment, on Lake Murten. In particular, we All radiocarbon ages were calibrated using the IntCal13 calibration assess timescales for recovery of the lake system and attainment curve (Reimer et al., 2013). A linear interpolation and a smooth- of steady state conditions following human perturbation. The sed- ing spline was applied, including the results of all dating meth- imentary time series is used to detect periods of mobilization of ods, radionuclide and radiocarbon dating and varve counting, in old soil organic carbon, evident by increasing 14C age offsets be- order to develop an age-depth model using the CLAM R-code of tween the bulk sediment OC and the corresponding depositional Blaauw (2010). In total 4 macrofossil dates were rejected for the age, and are further constrained by determination of mass accu- age model, as they would lead to age-reversals. The sedimentation mulation rates of specific components and the 13C and 15N isotopic rates from the varve counts in 5 m depth were integrated into the signature of the OM. age-depth model as “floating varves”, using the calibrated age of macrofossil Macro3 as an anchor point (Fig. 2). 2. Material and methods 2.2. XRF elemental composition and magnetic susceptibility 2.1. Chronology X-ray fluorescence (XRF) measurements were performed on an The chronology of the upper 70 cm of the sediment core is AVAATECH core scanner (5 mm resolution). The magnetic suscep- based on varve counting and 24 137Cs radionuclide measurements tibility and gamma-density was determined using a Geotek Multi- (Fig. 2) using a high-purity Germanium Well Detector (HPGe, sensor Core Logger (5 mm resolution). M. Haas et al. / Earth and Planetary Science Letters 505 (2019) 110–117 113

2.3. Bulk geochemical analyses Pile Dweller communities were the first to populate the See- land region from the Neolithic to the Bronze Age (5000–800 BCE). Prior to geochemical analyses, 1 cm slices of the working half of The 14C age offset is relatively low at 200–500 years and also the the core were freeze-dried and homogenized in a mortar. In order SAR, including autochthonous and allochthonous fluxes, remains to identify the sources of OM, we quantify the total organic carbon stable at approximately 1.0 kg/m2 y during this period (see also (TOC), total inorganic carbon (TIC) and total nitrogen (TN) con- Fig. 3). During this time period the C/N ratio has a mean of 12 13 14 13 15 tent, as well as the C and N isotopic signature of the OM and and the δ COC and δ Nisotopic signature of the organic mat- the biogenic Silica (bSi) content. TC and TN content of 67 samples ter remains relatively stable around −30h and 5h respectively. 13 were measured with a EURO EA 3000 and the TIC with a titration δ COC values are in the range of that of terrestrial plants and soils Coulometer (CM5015). TOC was calculated using the equation TOC (Stuiver, 1975), pointing towards predominantly terrestrial input of 15 = TC – TIC, and the CaCO3 content using CaCO3 = TIC × 8.33. The OM. The relatively enriched δ Nvalues and the C/N ratio con- − atomic TOC to TN ratio (C/N, mol mol 1) was used to infer sources firm this assumption (Enters et al., 2006). The MS and Ti records − of OM. δ15Nof 64 bulk sediment samples were measured on an show only minor fluctuations, and remain relatively low (5 × 10 5 EA-IRMS (EA Vario Pyro Cube by Elementar and IRMS by GV In- SI and 1900 XRF counts, respectively). We attribute the Neolithic 13 struments, Isoprime). Prior to δ COC measurement, bulk sediment to Bronze Age variations (e.g. Ti and MS) as reflecting a climatic aliquots were decarbonized with HCl (16%) for 12 h. Acetanilide signal because they are not accompanied by shifts in any of the #1 was used as a reference standard for quantification of the sta- fluxes or the 14C age offset. Despite innovations in agriculture, the ble isotopic composition. Biogenic silica (biSi) concentrations of 53 land-use impact of the relatively modest Pile Dweller community samples were measured according the method described in full de- seems to be too small to be detected in the sedimentary record. tail in Ohlendorf and Sturm (2008). Since 450 BCE Celtic tribes (La Tène) started to settle in the For radiocarbon dating of the bulk sediment fraction, an Acid– Seeland. Their arrival correlates roughly with the first increase of Base pretreatment with HCl (37%) and NaOH (Pellets) was chosen soil erosion indicators in Unit 4a and 3c. Starting from 400 BCE, for carbonate removal. 22 samples were weighted into cleaned Ag- SAR (including detrital input, but also CaCO3, and TOC flux) begins boats and placed together with smoking HCl, and NaOH, respec- to increase continuously showing a temporal maximum at 3.8 kg/ ◦ 2 13 tively, in a desiccator for 24 hat 60 C. The Ag-boats containing the m y, 90 BCE (Unit 3c, Fig. 3). In the same time range δ COC is samples were folded into cleaned Sn-boats. The 14C-measurements peaking at −29h, 130 BCE, indicating input of organic carbon were carried out on the EA-AMS (MICADAS) at ETH Zürich. Oxalic from a more 13C-enriched source. δ15N remains relatively stable 13 acid II NIST was used as reference material and phthalic anhydride and shows only a small increasing trend. The rise in δ COC to- as fossil blank. A coal process blank was included in the sequence gether with the decrease in the C/N ratio which started in Unit for monitoring the 14C background. 4a can be assigned to enhanced soil input into the lake system To test which phase of human activity affected soil carbon dy- (Meyers and Teranes, 2002). Simultaneously, the 14C age offset namics the most, we reconstruct a time series of radiocarbon age starts to rise as well in Unit 4a, suggesting mobilization of pre- offsets between the pre-aged (soil-derived) bulk sediment fraction aged SOC. Later in Unit 3c, corresponding to the late La Tène and terrestrial macrofossils, whose ages correspond to the sedi- period, a decrease the soil erosion proxies (SAR, MS, XRF Ti) can be ment layers in which they were deposited. The time series is used observed, probably indicating a small relaxation of human pressure to detect periods of mobilization of old soil organic carbon, visible at the end of the La Tène culture. In the pollen record of Lake Lob- by increasing age offsets between the bulk sediment fraction (Me- sigen sediments (∼20 km away), a strong decrease in tree pollen dian) and the corresponding sediment age (referred to as 14C age and an increase in cultural indicators (CI, including Cerealia pollen) offset in years). A table containing the radiocarbon measurements was detected during the period of La Tène (Tinner et al., 2003), can be found in the supplementary information (S1). indicating deforestation and field cultivation. Similar evidence for intensive land-use during La Tène and early Roman period was 2.4. Calculation of mass accumulation rates and fluxes found in Lake Holzmaar, Western Germany (Zolitschka et al., 2003). It was a time when agricultural innovations were adopted, such In order to calculate sediment mass accumulation rates (SAR) as the use of dung as fertilizer, but also the use of iron ploughs and yearly input of TOC, TN biSi, but also flux of CaCO3 and silici- and scythes (Müller, 1999b). Forest top soils were also sometimes clastics, several estimations were necessary. For the calculations a used to manure fields (“Celtic Fields”, Behre, 2006), potentially ex- resolution of 0.5 cm was chosen. plaining the geochemical evidence for an early enhancement in soil The dry bulk density was estimated from the TOC content erosion. following the calculation of Müller et al. (2005). Estimations for The beginning of the Roman period, Pax Romana (15 BCE–3rd the water content and porosity were obtained using the running century CE), correlates roughly with Unit 3b and the onset of mean of the gamma-density from the measurements of the Geotek varved (i.e., annually laminated) sediments that occur from 50 CE Multisensor Core Logger. Including the sedimentation rates from and remain until 170 CE. This interval records the second and the age-depth model, mass accumulation rates but also individ- sharpest increase of SAR (starting ca. 30 CE and peaking 120 CE 2 14 ual fluxes, such as the yearly TOC, TN, biSi, CaCO3 and detrital at 6.5 kg/m y in Unit 3b). The C age offset reaches a maxi- input, were calculated when subtracted from the total mass, fol- mum of 1580 years by 80 CE and is sinking slowly during the lowing Khalil et al. (2007). For estimating standard errors, a full period of Unit 3b and 3a. We attribute the 14C age offset mostly error propagation was conducted. to input of old soil, as the age offset increases simultaneous with TOC flux, SAR, XRF Ti and MS. Sedimentary OC is comprised of 3. Results and discussion various fractions of different origin and retention times and com- prises besides soil organic carbon also fresh aquatic and terrestrial Based on visual description, elemental compositions (K and Ti OM. Taking into account this heterogeneity, we assume that the profiles), and CaCO3 and TOC contents, the 10 m sediment core fraction of soil organic carbon is certainly older than indicated in was subdivided into 6 lithostratigraphic main units. Furthermore the 14C age offset due to the presence of fresh OM from other Unit 1, 3 and 4 were divided into Subunits based on their lithology sources. Rejected macrofossils dates, which were several hundreds (Fig. 2; see supplementary information S2 and S3 for a detailed to thousands of years older than expected, support the hypothe- sedimentary description). sis of an OC fraction substantially older than the bulk sediment 114 M. Haas et al. / Earth and Planetary Science Letters 505 (2019) 110–117

Fig. 3. Section of the last 4000 years of the Lake Murten sediments. Shown are time series of the 14C age offset, magnetic susceptibility, titanium (XRF Ti) counts, organic 13 15 carbon to nitrogen ratio (C/N), δ COC and δ Nisotopic composition of the organic matter, the sediment mass accumulation rate (SAR) including its subdivisions such as detrital input, CaCO3 flux, total organic carbon (TOC) flux, biogenic silica (bSi) flux. age (Fig. 2). Interestingly these “aged” macrofossils were found in This intense soil erosion in the catchment brought high intervals characterized by large 14C age offsets. amounts of nutrients to the lake, leading to an early eutrophi- A change in 14C content of the dissolved inorganic carbon pool cation phase. Hypoxic conditions in the bottom water are clearly (DIC) could possibly lead to a similar ageing effect and superim- recorded by the deposition of varves (Unit 3b). The present-day pose the soil signal. However we dismiss this possibility as the re-occurrence of varves in Lake Murten sediments begins around carbonate flux is at a minimum during the interval of highest 14C 1950, concomitant with the onset of deep-water hypoxia (Figs. 2 offset (Subunit 3c and beginning of 3b). In addition, the XRF Ca and 3). With ongoing eutrophication in Unit 3b, rising contribu- values, representing the carbonate flux, do not co-vary with detri- tions from young and autochthonous OC visible in peaking bSi tal input proxies such as Ti, K, Fe or the MS signal (PCA in the flux would have diluted the sedimentary OM leading to the ob- 14 13 supplementary information S3) and therefore seem not to be re- served decrease in the C age offset. δ COC shows a minimum of −31h at 80 CE and increases slightly along with C/N, reflect- sponsible for the aging effect of the sedimentary OC. ing enhanced input of light terrestrial OC. δ15Nis peaking around The growing influence of the city Aventicum, especially land 7.5h at 80 CE, recording input of an isotopically heavier nitrogen clearing and large-scale farming required to sustain its 20000 in- source. The increase could be triggered by sewage and manuring habitants, led to increased soil erosion. The demand for wood for practices (δ15N = 10 to 25h), which probably contributed to some construction, heating and craftsmanship must have been high for a extent to the early eutrophication of Lake Murten. However, deni- city of this size. We estimate that around 100000 tree trunks were trification processes might have occurred in the water column due used to build the northern city wall. Although we assume that the to anoxia, and would lead to a similar enrichment (Hodell and population around the lake was constantly growing from the La Schelske, 1998; Teranes and Bernasconi, 2000). Tène period until the Migration period, we observe a decline in The varves begin to fade out around 170 CE (Start of Unit 3a), the soil erosion indicators after the peak around 120 CE (Fig. 3). implying a change in the redox state of the lake floor to aerobic Anselmetti et al. (2007)observed a similar pattern in the sedi- conditions. Simultaneously bSi flux decreases rapidly. Under the as- ments of Lake Salpetén, Guatemala, where relationships between sumption that SAR and 14C age offset are closely linked to Roman SAR and the population density revealed that highest soil loss oc- land-use, we can conclude that ∼50-yr delay in the disappearance curred during the first land opening, despite continuous growth in of varves following the peak in soil erosion (120 CE) corresponds to the population. This would imply that La Tène and early Roman the reaction time of the lake system. Aventicum reached its heyday land-use had a great impact on the lake system by triggering the in the years 70–200 CE, after the city had been declared a Ro- first phase of soil erosion. man colony and became a capital of the region (Civitas Helvetiorum) M. Haas et al. / Earth and Planetary Science Letters 505 (2019) 110–117 115

(Castella et al., 2015), and agricultural activity must have increased trital and biogenic (CaCO3, TOC and bSi) fluxes. Dilution by the in parallel with population growth after 70 CE. This would imply latter phases is likely responsible for the decrease in MS and Ti 13 that land clearance for fields and pasture, thus triggering soil ero- counts (Fig. 3). δ COC starts to decrease from 1850 CE to a current sion, had a greater influence on the lake system than the following minimum of −32h, whereas δ15Nshows a mirrored behavior and field cultivation period. is increasing to a maximum of 7h. Together, these data are con- Towards the end of the Roman period, Unit 3a, SAR remains sistent with gradual intensification of agriculture and soil erosion, high but shows a decreasing trend, which is also reflected in the with 14C age offsets rising to a maximum of 2300 years (1910 CE) diminishing 14C age offset that reaches a minimum of 280 years over the same time period. Compared to the Roman period, this at 770 CE, comparable to the values recorded during the Neolithic second pulse of pre-aged soil carbon is even older. Further soil to the Bronze Age. Over this time period the C/N ratio increases excavation due to agricultural innovations leading to mobilization to a maximum value of 13 at 240 CE, whereas MS and Ti records of deeper horizons could be a possible explanation. Support for show a temporal minimum around 200 CE (although 2 nearby core this hypothesis is provided by the two rejected macrofossils dates transitions might have distorted their precise position). Soil sta- (Fig. 2) of Unit 1b, which reveal ages that are between 2000 and bilization processes, as a result of declining human pressure and 4000 years older than the sediment layers in which they were de- renaturation led to a decrease in detrital input (Fig. 3). The shift posited. to aerobic lake bottom waters is documented with the fade out Fossil fuel derived organic carbon might also be responsible for of varves around 170 CE, while bSi and TOC fluxes start to rise this last increase in the 14C age offset. The δ13C signal, would sup- slightly later, around 200 CE, potentially indicating a delayed re- port a “Suess effect” in the 20th century, as it decreases rapidly sponse to changes in lake redox state and nutrient availability. The by 2h in the modern varves interval. The decreasing trend of the rapid increase in the OC burial rate may be explained by vege- last three 14C age offset values, however, would contradict this as- tation regrowth and soil build-up in the catchment (Meyer-Jacob sumption. Alternatively, it could also be the result of peat drainage et al., 2015), with higher C/N ratio indicative of supply of fresh, and mobilization of old OC. Draining of surrounding swamps and terrestrial plant-derived OM to the lake (Enters et al., 2006). This redirection of the Aare River took place at the end of the 19th is consistent with the lower 14C age offset, which implies ex- century during the first Jura Water Correction. The mechanization port of mostly modern OC via surface run-off during renaturation. of agriculture started 1950 CE in the Seeland region (Nast, 2006), Our observations are consistent with archeological evidence. In the driving further soil degradation and eutrophication. This timing second half of the 3rd century CE, a combination of political insta- coincides with the onset of varved sediment deposition that con- bility, economic crises and repeated incursions of Germanic tribes tinues today. resulted in the demise of Aventicum. Roman artefacts become in- creasingly rare in the 3rd century CE, and during the following 4. Conclusions migration period almost no archeological evidence of human ac- tivity can be found in the Seeland region (Delbarre-Bärtschi and The historical legacy of past land-use is well preserved in the Hathaway, 2013). Lake Murten sediments. Although Pile Dwellers populated the re- After the collapse of the Roman Empire, 5th century CE, large gion for several thousand years, it was the celtic La Tène people parts of the Swiss lowlands region were abandoned, leading to for- and the Romans who left the first traces in the sediment record, est regrowth and pedogenesis (Tinner et al., 2003). The sediments starting 400 BCE. Simultaneous increases in soil erosion indica- from this migration period (Unit 2, 380–760 CE) are characterized tors, such as SAR, 14C age offsets, MS and XRF Ti provide coher- by evidence of increased autochthonous input, such as CaCO3, TOC ent sedimentary evidence for changes in land-use, with this first 13 and bSi indicative of primary production. δ COC values are rel- phase of land-clearing exerting substantial impact on the soil car- atively low and reach a minimum of −32h at 480 CE, whereas bon dynamics as indicated by mobilization and export of pre-aged 15 13 δ N remains relatively stable around 6h. The low δ COC isotope SOC to the lake. During Roman times (15 BCE–3rd century CE) progression might show a more allochthonous OM source from for- rapid agricultural expansion and the growing influence of the an- est regrowth and soil build up. The SAR remains consistently low cient city Aventicum led to increased runoff of nutrient-rich soil as well (around 3.0 kg/m2 y) indicating that the autochthonous materials, resulting in eutrophication of Lake Murten and the ac- fractions are less strongly diluted by allochthonous detritus, as cumulation of varved sediments. Although the Roman occupation also indicated by the brighter sediment color (Fig. 2) and lower ended abruptly, the lake system and its catchment required ap- − MS and Ti values (10 × 10 5 SI and 1200 Ti counts, respectively). proximately 300 years to recover and return to steady state con- The absence of varves points towards oxygenated bottom waters, ditions after the peak of Roman land-use. Land abandonment and and together with the decreased detrital input and 14C age offsets, forest regrowth led to soil stabilization and nutrient retention, sig- these characteristics imply catchment stabilization lasting for more nificantly improving the redox state of the lake. We estimate that it than 300 years. Notably, however, the SAR never returns to Ne- took approximately 50 years after the peak of human influence be- olithic baseline conditions, which prevailed at a level of 1.0 kg/m2 y fore aerobic conditions were reestablished. Large-scale deforesta- for almost 1000 years (Fig. 3). tion that removes stabilizing vegetation thus seems to have had Coinciding with the rise of the Frankish kingdoms and the es- the largest effect on soil erosion and nutrient mobilization. The tablishment of the medieval city of Murten in the 9th century lake system has been unable to return to conditions prior to this CE, detrital inputs (higher MS and Ti values) and 14C, age offsets early human influence, although relatively stable conditions per- 13 and δ COC begin to increase, indicating re-intensification of land- sisted for almost 1000 years during the Middle Ages. The most re- use and associated soil degradation in the catchment, while SAR cent dramatic increase in soil degradation (as evidenced by marked remains near-constant (∼3 kg/m2 y) but elevated relative to pre- increases in SAR and 14C age offsets) following industrialization Roman times. The elevated MS and Ti values around 1500 CE might lends support to the notion of a “Great Acceleration”, inducing reflect allochthonous input originating from phases of higher river ecosystem shifts that exceed that witnessed during the Holocene activity due to increased precipitation. Historical data of the 16th (Steffen et al., 2015). Our data shows that cultural eutrophication century describe numerous and regular floods in the alluvial plain is not exclusively a modern phenomenon but already took place of La Broye river (Weidmann, 2006). 2000 years ago during the Roman period (Dubois et al., 2018; During the last 500 years, SAR increases steadily to a current Mensing et al., 2018). Whilst, smaller in extent, this prior anthro- value of ∼15.0 kg/m2 y, with this increase manifested in both de- pogenic activity led to changes that took more than a century for 116 M. Haas et al. / Earth and Planetary Science Letters 505 (2019) 110–117 the lake system to recover from hypoxia, even though the accu- Gächter, R., Wehrli, B., 1998. Ten years of artificial mixing and oxygenation: no ef- mulation rates never returned to base conditions. Given current fect on the internal phosphorus loading of two eutrophic lakes. Environ. Sci. SAR values and age offsets recovery rates for recent eutrophication Technol. 32, 3659–3665. Gierga, M., Hajdas, I., van Raden, U.J., Gilli, A., Wacker, L., Sturm, M., Bernasconi, problems are likely to be equal or even slower. S.M., Smittenberg, R.H., 2016. Long-stored soil carbon released by prehistoric land use: evidence from compound-specific radiocarbon analysis on Soppensee Acknowledgements lake sediments. Quat. Sci. Rev. 144, 123–131. Haas, J.N., Richoz, I., Tinner, W., Wick, L., 1998. Synchronous Holocene climatic oscil- We would like to thank the Roman Site and Museum Avenches lations recorded on the and at timberline in the Alps. Holocene 8, 301–309. for discussing the wealth of archeological information. We would Hajdas, I., Zolitschka, B., Ivy-Ochs, S.D., Beer, J., Bonani, G., Leroy, S.A., Negendank, also like to thank Silvia Bollhalder, Irene Brunner, Alfred Lück, Beat J.W., Ramrath, M., Suter, M., 1995. AMS radiocarbon dating of annually lami- Müller, Marvin Rabold and Serge Robert from the Eawag for their nated sediments from Lake Holzmaar, Germany. Quat. Sci. Rev. 14, 137–143. support during sample preparation, measurement and interpreta- Hodell, D.A., Schelske, C.L., 1998. Production, sedimentation, and isotopic composi- tion. Special thanks goes to the anonymous reviewers who greatly tion of organic matter in Lake Ontario. Limnol. Oceanogr. 43, 200–214. Holzhauser, H., Magny, M., Zumbuühl, H.J., 2005. Glacier and lake-level variations in helped improving the manuscript. This research would not have west-central Europe over the last 3500 years. Holocene 15, 789–801. been possible without the funding of the Swiss National Science Huang, C., O’Connell, M., 2000. Recent land-use and soil-erosion history within a Foundation (SNSF): Grant Nr. 200021_160066/1. small catchment in Connemara, western Ireland: evidence from lake sediments and documentary sources. Catena 41, 293–335. 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