Potential impact of the 74 ka Toba eruption on the Balkan region , SE Europe

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Abstract
The 74 ka Toba eruption in Sumatra, Indonesia, is considered to be one of the largest volcanic events during the Quaternary.Tephra from the Toba eruption has been found in many terrestrial and marine sedimentary deposits, and acidity peaks related to the eruption have been used to synchronize ice core records from Greenland and Antarctica.Seismic profiles from Lake Prespa on the Balkan Peninsula, SE Europe, indicate a lake level lowstand, which is confirmed by the occurrence of an exceptional bivalve shell layer of Dreissena sp. in the sediment record, the isotope composition of the shells, as well as pollen data.ESR dating and extrapolation of the tephra and radiocarbon chronologies indicate that this lowstand occurred at 73.6 ± 7.7 ka.Our data, showing a short term shift to aridity superimposed on the general cooling trend at the end of MIS 5 and coupled with the chronological correspondence to the Toba eruption imply a distinct impact of the Toba eruption on environmental conditions in the northern Mediterranean region.The recovery from this dramatic environmental condition, in turn, may have triggered spatial expansion events in one of the lake's most abundant benthic species, the carino mussel Dreissena presbensis.

Introduction
The Toba Caldera Complex in Northern Sumatra, Indonesia, formed due to number of volcanic eruptions, of which the most recent one at 74 ka is considered to be one of the largest volcanic eruptions on Earth during the Quaternary (Chesner, 2012).The tephra products from this eruption can be found in many records from the Arabian Sea in the west to the South China Sea in the east (e.g.Williams, 2012a and references therein).Though 73 and 74 ka can be found in the literature, the most precise dating of the Toba eruption is using Ar/Ar of the tephra found in Malaysia and indicates an age of 73.88 ± 0.3 (Storey et al., 2012).The impact of the Toba eruption on regional climatic and hydrologic conditions, and on regional human dispersal and activity are relatively Introduction

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Full well known (e.g.Blinkhorn et al., 2012;Jones, 2012;Petraglia et al., 2012).Some authors suggest that the Toba eruption was followed by a volcanic winter, i.e. a decade or two of drastic cooling (compiled in Williams, 2012a, b).On a regional scale, this could have affected plant and animal as well as human populations (e.g.Jones, 2012;Louys, 2012).The global impact of the Toba eruption is more under debate (Williams, 2012b).Acidity peaks occurring at the transition from Greenland Interstadial 20 (GI-20) into Greenland Stadial 20 (GS-20) in the NGRIP ice core record from Greenland and during a prominent isotope minima between Antarctic Isotope Maxima (AIM) 20 and 19 in the EDML and EDC ice core records from Antarctica are correlated with the Toba eruption (Svensson et al., 2013).However, Svensson et al. (2013) point out that it is difficult to explain sustained cold temperatures over more than a millennium as a result of the Toba event.Climate models suggest both a strong and long lasting cooling and devastating impacts on both ecosystems and humans (e.g.Ambrose, 1998;Robock et al., 2009), while others suggest the contrary, with only moderate changes in temperature in S and E Africa and in the Indian subcontinent, but with significant precipitation anomalies (Timmreck et al., 2012).The potential global climate impact of the Toba eruption could have also affected the dispersal of modern humans from Africa to Europe (Oppenheimer, 2012;Petraglia et al., 2012).More records from distal sites would help to better understand the environmental impact of the Toba eruption on a global scale.Here, we provide seismic and sedimentological data from Lake Prespa, which is situated in SE Europe between Albania, Macedonia and Greece and located on the pathway of human dispersal from Africa to Europe (Oppenheimer, 2012).Data for the last 92 ka indicate that the sedimentary record is very sensitive to environmental change, which is partly due to a relatively low volume (3.6 km 3 ), a large surface area (254 km 2 ), and a mean water depth of only ca.Introduction

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Dataset and discussion
Most of the seismic and sedimentological data presented here from Lake Prespa have been previously published (Wagner et al., 2010(Wagner et al., , 2012;;Leng et al., 2013).Radiocarbon dates, tephrostratigraphy, ESR dating and cross correlation with the NGRIP ice core indicate that the sediment core Co1215 spans the last ca.92 ka (Damaschke et al., 2012; Fig. 2).Total inorganic carbon (TIC) and total organic carbon (TOC) from the sediments show both Glacial/Interglacial periods, as well as short-term climate events, including Heinrich and the 8.2 ka events (e.g.Wagner et al., 2010Wagner et al., , 2012;;Aufgebauer et al., 2012; Fig. 2).The stable isotope data also show both long-term and short-term changes in the lakes hydrology (Leng et al., 2013).
The most exceptional lithological horizon in the core is a horizon between 14.63-14.58m depth, where shells and shell fragments of the carino mussel (probably Dreissena presbensis) occur (Fig. 1).Most likely this horizon represents a distinct lake level lowstand when near-shore habitats of D. presbensis became subaerial and eroded, leading to the redistribution of shell fragments into the deeper parts of the lake.A lake level lowstand is also indicated by an undulated reflector in the seismic data (see Wagner et al., 2012, Fig. 1) and by high δ 18 O from the shell fragments (see Leng et al., 2013, Fig. 2).A concurrent maximum of non-arboreal pollen percentages suggests dry conditions at the catchment.A conspicuous Poaceae peak (Fig. 2), which includes Phragmites sp.pollen grains, points to an expansion of the littoral zone and a decrease in the distance from the coring location.This is supported by a peak in the TOC/N ratio indicating higher allochthonous organic input.These data are very different from other abrupt events seen throughout the core (Fig. 2).It is likely that this lowstand also affected the lake level of neighbouring Lake Ohrid, which is partly fed by Lake Prespa via karst aquifers (Matzinger et al., 2006b).There is evidence for significant lake level lowering at the end of Marine Isotope Stage (MIS) 5 as indicated by subaquatic terrace at ca. 30 m water depth (Lindhorst et al., 2010).In core Co1202 from the northern part of Lake Ohrid (Fig. 1), a hiatus was thought to span the period ca.98-82 ka (Vogel

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Full  , 2010).However, the exact timing of this hiatus is weakly constrained and it could have been generated much later by a significant lake level lowstand due to less water input from Lake Prespa.The lake level lowering could then have led to destabilisation of the slopes and a mass wasting event.
The ESR dates of the Dreissena shells and shell fragments in the Lake Prespa core provide a mean age for the deposition of the shell horizon and the reconstructed lake level lowstand of 73.6 ± 7.7 ka (Damaschke et al., 2012).Despite a relatively large uncertainty on this age, the mean age is supported by tephrostratigraphic markers above the shell horizon and by chronological tie points below and above the shell horizon (Damaschke et al., 2012).The chronological tie points were obtained by cross correlation of mainly TOC, which is known to be a very sensitive proxy of climatic changes in the Lake Prespa sediments (Wagner et al., 2010), with the NGRIP ice core record.Moreover, extrapolation of tephrostratigraphic ages and radiocarbon ages as well as the lithology of the core imply that there is no significant change in sedimentation conditions above and below the shell horizon.We can exclude that the lake level lowstand is related to a long-term change from interglacial to glacial conditions at the MIS 5 to MIS 4 boundary.We can also exclude that the lowstand is triggered by a short-term change correlated with a Heinrich event (cf.Wagner et al., 2010), as potential Heinrich events around 74 ka are not reported.Moreover, the more pronounced Heinrich events, which occurred later between MIS 4 and 2 and which are characterized by distinct peaks in TIC (siderite), have unusual δ 13 C values related to their mode of formation (Fig. 2) and are not seen in the seismic data.Other short-lived aridity events, such as the 8.2 ka event, are also not known to have occurred around 74 ka from the region.

Interpretation and conclusions
Overall, we conclude that the lake level lowstand in Lake Prespa (and potentially Lake Ohrid) was probably triggered by an extraordinary event, which was too short lived to Figures

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Full be clearly visible in the pollen data, particularly given the relatively low resolution of this dataset.Though tectonic activity in the Balkan region is common (e.g.Reicherter et al., 2011) and could have affected the karst aquifer system connecting lakes Prespa and Ohrid, a significant tectonic event at the end of MIS 5 has not been described thus far.Lake Prespa is mainly fed today by river runoff from numerous small streams (56 %) and by direct precipitation (35 %; Matzinger et al., 2006a), which peaks in winter when snowfalls are frequent (Hollis and Stevenson, 1997).We cannot completely exclude that the lake level lowstand in Lake Prespa was due to a period of higher aridity related to the cooling at the transition from GI-20 to GS-20 or to a Heinrich event.However, there is a remarkable correlation between the age of a lake level lowstand (73.6±7.7 ka based on ESR dating) and the Toba eruption (73.9 ± 0.3 ka).This leads us to the assumption that the severity of the lake level lowstand and its short-lived character at Lake Prespa may be an expression of a volcanically triggered aridity event superimposed on the overall cooling trend between GI-20 and GS-20.Tephra particles from the Toba eruption have not so far been found in Lake Prespa (or Ohrid).Hence, the lake level lowstand can only be correlated to the Toba eruption by a significant change in climate conditions with reduced precipitation in SE Europe.A potential recovery from the 74 ka old Prespa lake level lowstand and/or with the end of a potential volcanic winter is probably correlated with an important spatial expansion event of D. presbensis from Lake Prespa to neighbouring water bodies.According to independent mitochondrial DNA data, this event took place about 72 ka ago, however, the confidence interval for this estimate is very high (95 % CI: 0-153 ka; Wilke et al., 2010).
As water loss from Lake Prespa is mainly through evaporation (52 %) and outflow through karst aquifers (46 %) into the larger Lake Ohrid located 150 m below and ca. 10 km to the west (Fig. 1), the hydrologic changes at Lake Prespa likely led to a significantly reduced water input in Lake Ohrid.This could have triggered mass movement events in Lake Ohrid, which are supported by the steep slopes in this lake.A lake level lowstand around 74 ka in Lake Prespa could have led to the formation of the terrace at ca. 30 m water depth as it is described by Lindhorst et al. (2010) and it could also

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Full have led to the hiatus between ca.98-82 ka in core Co1202 (Vogel et al., 2010) as described before assuming slightly different sedimentation rates.
As the lake level lowstand in Lake Prespa (and Ohrid?) apparently is close in age to the 74 ka Toba eruption, there is a potential correlation between the eruption and the regional climate on the Balkans.If there was a so-called volcanic winter, it could have been correlated with drier conditions in the lake catchment.We did not find independent evidence for a correlation of the lake level lowstand with the Toba eruption, i.e. glass shards of calc-alkaline composition, but the eruption of the Tambora volcano in Indonesia in 1815 AD has demonstrated the impact of volcanic eruptions from the equator belt to the climate of the Northern Hemisphere (i.e.being responsible of the world-famous "year without summer"; e.g., Oppenheimer, 2003).
Several records from the northern Mediterranean region show cold and dry conditions, which are probably related to the GS-20 (e.g., Tzedakis et al., 2004;Brauer et al., 2007), but chronological uncertainties hamper the correlation between the individual records and a potential link to the Toba eruption.Moreover, these records do not indicate the exceptionality as it is expressed in the Prespa record for the period discussed.Our evidence from Lake Prespa is therefore key to contributing to the ongoing discussion related to the possible global impact of the Toba eruption (Williams, 2012b).
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Fig. 1 .
Fig. 1.Map showing the location of Lake Prespa (and Lake Ohrid) in the northeastern Mediterranean region.The red line indicates the hydro-acoustic profile taken in 2007 with an INNOMAR 2000 compact transducer and using the ISE 2.9.2 software.The coring location Co1215 is indicated by a yellow dot.The depth in the seismic profile is indicated below lake surface and based on a P wave velocity of 1450 m s −1 .The black bar in the seismic profile indicates the location and the recovery depth of sediment core Co1215.The reflectors related to the Y-5 tephra and the Pleistocene/Holocene transition (P/H T) are marked by the black lines (modified from Wagner et al., 2011).