10
Introduction Biotic responses to rapid warming about 14,685 yr BP: Introduction to a case study at Gerzensee (Switzerland) Brigitta Ammann a, , Ulrich von Grafenstein b , Ulrike J. van Raden c a Oeschger Centre for Climate Change Research, University of Bern, Zaehringerstrasse 25, 3012 Bern, Switzerland b Laboratoire des Sciences du Climat et de l'Environnement (CNRS, CEA, UVSQ), Gif-sur-Yvette, France c Geological Institute, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland abstract article info Available online 22 November 2013 Keywords: Late-Glacial Biotic responses to rapid warming Bølling warming Internal structure of the Bølling/Allerød Late-glacial timescale by correlation to NGRIP Southern Central Europe The late-glacial climatic warming indicated in the Greenland ice-core record about 14,685 years before 1950 AD belongs to a type of very rapid high-amplitude warming similar the onsets of DansgaardOeschger events during marine isotope stage 3 (MIS 3). In order to estimate the nature and rates of change of biotic responses to such a major climatic shift we need a reliable time scale as well as climatic indicators independent of the biota. Both are provided by a high-resolution oxygen-isotope record from precipitated carbonates of lake marl in Gerzensee, Switzerland (van Raden et al., 2013this issue). On the basis of the assumption of synchronous climatic changes between Greenland and Gerzensee, the close correlation of the oxygen-isotope changes at the two sites allows the use of the NGRIP GICC05-timescale at Gerzensee (with the zero point at 1950 AD to maintain comparability with the numerous radiocarbon dates in Europe). The δ 18 O-record measured in precipitated carbonates is checked and rened by the δ 18 O measured in mono-specic ostracod samples (Von Grafenstein et al., 2013in this issue). The shift of 3.6δ 18 O PDB in only about 112 years at the end of the GS-2 represents a very rapid tem- perature increase of at least 6.2 °C. This increase is conrmed by reconstructions based on transfer functions for pollen and chironomids by Lotter et al. (2012) (possibly 47 °C in the annual mean and 25 °C in summer temperatures). After this major shift the Greenland late-glacial interstadial GI-1 (corresponding to the regional biozones Bølling and Allerød) δ 18 O-records of both Greenland and Gerzensee exhibit four minor uctuations (about 1.01.2δ 18 O), of which the second and the fourth are especially clearly correlated with several of terrestrial records in the northern hemisphere. The oxygen-isotope record is used as a template for all these sediment- and bio-stratigraphies. The biotic responses may include at least ve types of process: (1) changes in productivity of individuals (within a year or two), (2) changes in populations (usually somewhat slower, depending on life cycles and environmental constraints), (3) changes due to migration (often rather slow, depending again on life cycles and environmental constraints), (4) changes in the terrestrial and aquatic communities and (5) changes on the level of ecosystems (including pedogenesis, nutrient cycling, and species interactions). These biotic responses to the early rapid warming about 14.685 ka BP are elaborated in subsequent papers for plants, chironomids, ostracods, and Cladocera (Ammann et al., 2013in this issue-a; Brooks and Heiri, 2013this issue; Von Grafenstein et al., 2013in this issue; Nováková et al., 2013-this issue). In addition, lake levels are reconstructed by Magny (2013this issue) and vegeta- tion dynamics and N 2 O-emissions are modelled (Lischke et al., 2013this issue; Pfeiffer et al., 2013this issue). The changes in the mammal fauna of the Swiss Plateau are summarized by Nielsen (2013this issue). © 2013 Elsevier B.V. All rights reserved. 1. Introduction A better understanding of biotic responses to climatic changes in the past is crucial for assessing the effects of present and future global warming on ecosystems. The rapid warming around 14.685 yr BP is not a direct analogue to the recent and future global warming because of many differences in the geophysical, climatologic, biogeographic, and ecologic conditions. However, studying the past may lead to a better understanding of the biological processes and their velocities and ampli- tudes that are involved during very rapid warming phases (Solomon et al., 2007; Gosling and Bunting, 2008). Qualitative and quantitative observations of climatic and biological changes in the past are of partic- ular importance when temperature changes had higher amplitudes and rates than those registered in the meteorological record (i.e. the last 100150 years). The two most dramatic warming periods at the end of the last glacial were at the beginning of the Holocene (11,650 cal yr BP) and the beginning of the late-glacial interstadial (the Bølling/Allerød about 14,685 cal yr BP, e.g. Dansgaard et al. (1993); Lowe et al. (1994); Lowe et al. (2008);Vandenberghe et al. (2001); Walker (2001); Hoek Palaeogeography, Palaeoclimatology, Palaeoecology 391 (2013) 312 Corresponding author. E-mail addresses: [email protected] (B. Ammann), [email protected] (U. von Grafenstein), [email protected] (U.J. van Raden). 0031-0182/$ see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.11.006 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo

Biotic responses to rapid warming about 14,685yr BP: Introduction to a case study at Gerzensee (Switzerland)

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Page 1: Biotic responses to rapid warming about 14,685yr BP: Introduction to a case study at Gerzensee (Switzerland)

Palaeogeography, Palaeoclimatology, Palaeoecology 391 (2013) 3–12

Contents lists available at ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology

j ourna l homepage: www.e lsev ie r .com/ locate /pa laeo

Introduction

Biotic responses to rapid warming about 14,685 yr BP: Introduction to acase study at Gerzensee (Switzerland)

Brigitta Ammann a,⁎, Ulrich von Grafenstein b, Ulrike J. van Raden c

a Oeschger Centre for Climate Change Research, University of Bern, Zaehringerstrasse 25, 3012 Bern, Switzerlandb Laboratoire des Sciences du Climat et de l'Environnement (CNRS, CEA, UVSQ), Gif-sur-Yvette, Francec Geological Institute, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland

⁎ Corresponding author.E-mail addresses: [email protected] (B. A

[email protected] (U. von Grafenstein), u(U.J. van Raden).

0031-0182/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.palaeo.2013.11.006

a b s t r a c t

a r t i c l e i n f o

Available online 22 November 2013

Keywords:Late-GlacialBiotic responses to rapid warmingBølling warmingInternal structure of the Bølling/AllerødLate-glacial timescale by correlation to NGRIPSouthern Central Europe

The late-glacial climatic warming indicated in the Greenland ice-core record about 14,685 years before 1950 ADbelongs to a type of very rapid high-amplitudewarming similar the onsets of Dansgaard–Oeschger events duringmarine isotope stage 3 (MIS 3). In order to estimate the nature and rates of change of biotic responses to such amajor climatic shift we need a reliable time scale as well as climatic indicators independent of the biota. Both areprovided by a high-resolution oxygen-isotope record from precipitated carbonates of lake marl in Gerzensee,Switzerland (van Raden et al., 2013–this issue). On the basis of the assumption of synchronous climatic changesbetween Greenland and Gerzensee, the close correlation of the oxygen-isotope changes at the two sites allowsthe use of the NGRIP GICC05-timescale at Gerzensee (with the zero point at 1950 AD to maintain comparabilitywith the numerous radiocarbon dates in Europe). The δ18O-record measured in precipitated carbonates ischecked and refined by the δ18O measured in mono-specific ostracod samples (Von Grafenstein et al., 2013–inthis issue). The shift of 3.6‰ δ18O PDB in only about 112 years at the end of the GS-2 represents a very rapid tem-perature increase of at least 6.2 °C. This increase is confirmed by reconstructions based on transfer functions forpollen and chironomids by Lotter et al. (2012) (possibly 4–7 °C in the annual mean and 2–5 °C in summertemperatures). After this major shift the Greenland late-glacial interstadial GI-1 (corresponding to the regionalbiozones Bølling and Allerød) δ18O-records of both Greenland and Gerzensee exhibit four minor fluctuations(about 1.0–1.2‰ δ18O), of which the second and the fourth are especially clearly correlated with several ofterrestrial records in the northern hemisphere.The oxygen-isotope record is used as a template for all these sediment- and bio-stratigraphies.The biotic responses may include at least five types of process: (1) changes in productivity of individuals (withina year or two), (2) changes in populations (usually somewhat slower, depending on life cycles and environmentalconstraints), (3) changes due to migration (often rather slow, depending again on life cycles and environmentalconstraints), (4) changes in the terrestrial and aquatic communities and (5) changes on the level of ecosystems(including pedogenesis, nutrient cycling, and species interactions). These biotic responses to the early rapidwarming about 14.685 ka BP are elaborated in subsequent papers for plants, chironomids, ostracods, and Cladocera(Ammann et al., 2013–in this issue-a; Brooks and Heiri, 2013–this issue; Von Grafenstein et al., 2013–in this issue;Nováková et al., 2013-this issue). In addition, lake levels are reconstructed byMagny (2013–this issue) and vegeta-tion dynamics and N2O-emissions are modelled (Lischke et al., 2013–this issue; Pfeiffer et al., 2013–this issue). Thechanges in the mammal fauna of the Swiss Plateau are summarized by Nielsen (2013–this issue).

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

A better understanding of biotic responses to climatic changes inthe past is crucial for assessing the effects of present and future globalwarming on ecosystems. The rapid warming around 14.685 yr BP isnot a direct analogue to the recent and future global warming becauseof many differences in the geophysical, climatologic, biogeographic,

mmann),[email protected]

ghts reserved.

and ecologic conditions. However, studying the pastmay lead to a betterunderstanding of the biological processes and their velocities and ampli-tudes that are involved during very rapid warming phases (Solomonet al., 2007; Gosling and Bunting, 2008). Qualitative and quantitativeobservations of climatic and biological changes in the past are of partic-ular importance when temperature changes had higher amplitudes andrates than those registered in the meteorological record (i.e. the last100–150 years). The two most dramatic warming periods at the endof the last glacial were at the beginning of the Holocene (11,650 cal yrBP) and the beginning of the late-glacial interstadial (the Bølling/Allerødabout 14,685 cal yr BP, e.g. Dansgaard et al. (1993); Lowe et al. (1994);Lowe et al. (2008);Vandenberghe et al. (2001); Walker (2001); Hoek

Page 2: Biotic responses to rapid warming about 14,685yr BP: Introduction to a case study at Gerzensee (Switzerland)

4 B. Ammann et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 391 (2013) 3–12

(2001)). To estimate the impacts of such a rapid warming on biota weneed data of three types: (1) a non-biological record of temperaturechange, (2) several groups of organisms with different lifecycles, and(3) a good chronology.

Gerzensee was one of the first sites studied for stable isotopeson bulk carbonates in Europe (Eicher and Siegenthaler, 1976;Siegenthaler and Eicher, 1986). The lake provided biogenically pre-cipitated carbonates even before the onset of the late-glacial inter-stadial. Due to the small catchment and thus minor surface-waterinput, combined with high productivity on the littoral terrace, theinput of detrital carbonates was small, so the locally precipitatedcarbonates dominated. Early work by Siegenthaler et al. (1984)and Oeschger et al. (1984) correlated the oxygen-isotope recordof Gerzensee with the Greenland ice core of Dye 3.

2. The concept

In the subsequent set of papers we present sedimentologic, isotopic,and biostratigraphic results from a single sediment core GEJK fromGerzensee, covering the period from about 15,675 to 13,000 yr BPon the GICC-05 time scale. As the record of past temperatures we usethe δ18O values measured on both precipitated freshwater carbonates(van Raden et al., 2013–in this issue) and on monospecific samples ofostracods (Von Grafenstein et al., 2013–in this issue). We use as bioticrecords the stratigraphies of pollen, chironomids (non-biting midges),and the crustaceous groups of Cladocera and Ostracoda (Ammannet al., 2013-in this issue-a; Brooks and Heiri, 2013–in this issue;Nováková, 2013-in this issue; Von Grafenstein et al., 2013–in thisissue). Because the species composition of the aquatic invertebratescan be influenced by factors other than temperature, we estimatewater-level changes from the various morphotypes of precipitated cal-cite that reflect the water depth (Magny, 2013–in this issue).

To establish a chronology we rely on the correlation of theoxygen isotopes of the lake marl with those of the North-GRIP ice core((NGRIP-members, 2004; Rasmussen et al., 2006) as worked out byvan Raden et al. (2013–in this issue)). The basic assumption here isthat the signals in the oxygen-isotope ratios were synchronous onGreenland and in Europe (Haflidason et al., 1995; Schwander et al.,2000; Von Grafenstein et al., 2000; Hendy et al., 2002; Lea et al., 2003;Thornalley et al., 2011). The oxygen-isotope record thus provides twoessential cornerstones of our concept, namely the mean air tempera-tures and the timescale (van Raden et al., 2013–in this issue).

For all subsequent papers in this issue the oxygen-isotope recordserves as the template of climatic change. The biogenically precipi-tated fresh-water carbonates of lake marl can be a good archive ofchanges in the oxygen-isotope ratio of past atmospheric precipita-tion, provided that erosional input can be excluded (Eicher andSiegenthaler, 1976 Lotter et al. 1992; Yu and Eicher, 1998;Schwander et al., 2000; Von Grafenstein et al., 2000; van Radenet al., 2013–in this issue). However, compared to the Greenland in-land ice, where the isotopic composition of past atmospheric pre-cipitation (δ18OP) is preserved close to original value, lake carbonatesintegrate in their isotopic signal (δ18OC)

• hydrological effects, which might alter the link between δ18OP andthe oxygen isotope ratio of the lake water (δ18OL)

• water-temperature effects and vital effects, which may alter therelation between δ18OL and δ18OC.

The quantification of δ18OP from the Gerzensee marl record there-fore relies on an estimation of these secondary effects. This estimationis examined by Von Grafenstein et al. (2013–in this issue) by usingthe oxygen-isotopes of different benthic taxa to derive water tempera-tures and then comparing the results to published δ18OP records fromEurope to narrow down the hydrological effects. This paper alsodiscusses the interpretation of the resulting δ18OP record in termsof air-temperature changes.

In the subsequent papers the biotic responses to rapid warming areestimated with use of the following tools:

• Statistically significant zone boundaries in the four biostratigraphiesand the carbonate lithostratigraphy: How close together are they?

• Scores on thefirst (and for chironomids on second) axis of ordinations(CCA for chironomids, PCA for pollen, and Detrended CorrespondenceAnalysis (DCCA) on a common time scale for all four biostratigraphies).

Van Raden et al. (2013–in this issue) refine the Greenland terminol-ogy (Björck et al., 1998; Lowe et al., 2008) by separating in their isotopestratigraphy the two strong and rapid shifts – the onset of the Bølling(from GS-2 to GI-1) and the onset of the Younger Dryas (from GI-1 toGS-1) – as local transitional isotope zones (GRZibulk-2 and GRZibulk-13). This separation is helpful in addressing the question about biotic re-sponses to climatic shifts, because the onset and the duration of the iso-tope shift are individually specified. To understand the potentialprocesses involved in biotic responses, the onset and duration of isoto-pic shifts may be more important than their point of inflection.

3. Introductory remarks on stratigraphies

The last Late-Glacial period interests scientists of various disciplinesbecause rapid, high-amplitude environmental changes challenge ourunderstanding of climatic and biological processes. Biological responsesto global or hemispheric climatic change may be time-transgressive,e.g. tree migration or afforestation after ice retreat across latitudes(Firbas, 1949; Firbas, 1952). An impressive example was that presentedby de Beaulieu (1977) (his Fig. 25), illustrating the time-transgressiveoccurrence and abundance of juniper across Europe. A useful toolfor communication and correlation is the different but related conceptsof biostratigraphy and chronostratigraphy as outlined in stratigraphicguides (Hedberg, 1972; Birks, 1973; Delaygue et al., 2003; Finsingeret al., 2007; Kamenik et al., 2009; Salvador, 1994b) and for northernEurope inMangerud et al. (1974), or for thewhole North Atlantic regionin Björck et al. (1998) andWalker et al. (1999). The problems of formalchronostratigraphy based on time-transgressive environmental changeswere already discussed by Watson and Wright (1980). Walker (2001)demonstrated how the rapid changes during the last Late-Glacial eludethe conventional stratigraphic procedures and how spatial correlationscan be based on pollen or chironomids or Coleoptera or stable isotopes.

Historically, variations in plant macrofossils and/or the pollenrecorded at sites in Denmark revealed the first changes in the Late-Glacial towardswarmerphases, and thiswork gave thenames to intersta-dial periods: Original type sites were Allerød (just north of Copenhagen)by Hartz andMilthers (1901), and Bølling (on Jutland) by Iversen (1954).The cool phases (Oldest Dryas, Older Dryas, andYoungerDryas, or EarliestDryas, Earlier Dryas, Late Dryas) in Hoek et al. (1999) were named afterDryas octopetala, a dwarf shrub abundant as a pioneer on gravelly soils,because its leaves had been recognized early (Nathorst, 1870; Birks andSeppä, 2010). The characteristic leaves of this arctic-alpine plant (Dryasmeaning “small oak”), as preserved in silt and clay at sites occupiedtoday by deciduous forests, provided remarkable ecological informationand hints of major climatic changes.

The term Bølling was originally coined as a biostratigraphic unit, butat that time could not be directly radiocarbon-dated (Iversen, J., 1942;Iversen, 1954). Later it was also used as a chronozone for the Nordiccountries (Mangerud et al., 1974), lasting in radiocarbon years from13,000 to 12,000 BP (see Table 1 and in van Raden et al., 2013–in thisissue their Table 3). On biostratigraphic grounds Menke (1968), Bocket al. (1985), Usinger (1985), and others questioned the Bølling as thefirst warming period and introduced a possible earlier warming phasetermed Meiendorf (Menke, 1968). Brauer et al. (2001), and Litt et al.(2001) used annually laminated sediments to propose a nomenclaturein which the term Bølling would be used only for a much shorter (andyounger) interval; the earlier warmer phase would then be namedafter the site Meiendorf of Menke (1968) and Bock et al. (1985).

Page 3: Biotic responses to rapid warming about 14,685yr BP: Introduction to a case study at Gerzensee (Switzerland)

Table1

Threelate-glacialev

entsda

tedby

four

metho

ds.Three

distinctev

entsin

southe

rnCe

ntralE

urop

edu

ring

theW

ürmianLate-G

lacialaretheon

seto

fthe

bioz

oneof

Bölling

,the

Laache

rsee

teph

ra,and

theon

seto

fthe

Youn

gerD

ryas.Fou

rdatingtoolsare

used

:the

correlationof

theox

ygen

-isotope

ratios

betw

eenGerzens

eean

dNGRIP,radiocarbo

nda

ting

,varve

coun

ting

,and

234U/2

30Th

(for

thestalag

miteCh

au-stm

6at

Chau

vet,SW

—Fran

ce).Th

eag

e-diffe

renc

ebe

twee

ntheLSTan

dtheon

seto

fthe

Youn

gerDryas

isab

out2

00ye

ars,inde

pend

ento

fthe

arch

ivean

dtheda

ting

metho

d.Th

eag

ediffe

renc

ebe

twee

ntheLSTan

dtheon

seto

fBöllin

gisab

out1

630ye

arsat

Gerzens

ee;the

agediffe

renc

ebe

twee

ntheLSTan

dtheon

seto

fMeien

dorfat

Mee

rfelde

rMaarisbe

twee

n12

80an

d15

70ye

ars,thus

quitecompa

rable,seen

theva

riou

sprob

lemsof

dating

earlylate-glacialleve

ls.The

question

markne

ar“LST

inNGRIP”

refers

tothefact

that

thean

desiticashsh

ards

associated

withthesu

lpha

tepe

akin

NGRIPareve

rydiffe

rent

from

theph

onolithicLST(M

ortens

enet

al.2

005).

δ18Oin

icean

dCa

CO3

Radioc

arbo

nag

esVarve

s234U/2

30Th

Even

tNGRIP

Rasm

ussenet

al.(20

06)

Gerzens

eeva

nRa

denet

al.

(201

3–in

thisissue)

Chrono

-zon

esNordiccoun

tries

Man

gerudet

al.(19

74)

Bioz

ones

atBö

lling

sjö,

Benn

ikeet

al.(20

04)

OntheSw

issPlatea

uAmman

nan

dLo

tter

(198

9);

Lotter

etal.(19

92),

*Blockley,

Bron

kRa

msey;

Lane

,Lotter(200

8)

Eifel,

Brau

eret

al.(19

99),

Littet

al.(20

01)

Chau

vet

stalag

mite

Chau

-stm

6Gen

tyet

al.

(200

6)δ1

8O

GICC-05

BPZe

roat

1950

AD

Unc

alBP

CalB

PUnc

alBP

CalB

PUnc

alBP

Cal.BP

1σCa

l.BP

2σIm

prov

edat

Sopp

ensee*

234U/2

30Th

Ons

etof

YD12

,693

12,877

to12

,710

onseta

nden

dof

tran

sition

11,000

Somew

hatyo

unge

rthan

11,430

Somew

hatyo

unge

rthan

13,180

–13

,765

10,700

to12

,721

–12

,818

12,656

–12

,843

1245

0–12

750

12,680

12,700

±35

0(2σ)

10,800

12,804

–12

,856

12,750

–12

,891

LST

12,800

??13

,035

Outside

theplum

esof

LST

Outside

theplum

esof

LST

1100

0to

12,874

–12

,966

12,858

–13

,055

12,743

–12

,975

12,880

11,230

13,085

–13

,193

12,992

–13

,239

Ons

etof

Bølling

14,553

14,665

13,000

15,193

–15

,491

,1σ

Or

15,077

–15

,683

,2σ

Centre

at 12,500

14,425

–14

,850

(1σ)or

14,237

–14

,957

(2σ)

12,600

to14

,690

–14

,995

14,473

–15

,126

Not

varved

attheon

seto

fMeien

dorf,

butex

trap

olated

to14

,158

–14

,450

15,160

±25

0(2σ)

12,700

14,886

–15

,127

14,724

–15

,225

5B. Ammann et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 391 (2013) 3–12

Unfortunately “Meiendorf” is also not well dated at its type site. Thestarting date for the Bølling proposed by Menke (1968) is 10,700 BC,or 12,650 uncal. yr BP, based on other spatially and biostratigraphicallycorrelated dates; after calibration this corresponds to 14,788–15,060 yrBP with one sigma (or 14,651–15,177 yr BP with two sigma). Later,radiocarbon datings at Meiendorf were made on reindeer remains in anarchaeological context (Fischer and Tauber, 1986). At Bølling Sø new bio-stratigraphieswere combined, and four radiocarbon dates for the onset ofthe Bølling biozone cluster around 12,500 uncal. yr BP (Bennike et. al,2004). This date corresponds to a calibrated age of 14,425–14,850 yr BP(with 1 sigma) or 14,237–14,957 yr BP (with 2 sigma). This coincidesboth with the (global) plateau of constant radiocarbon age andthe (regional) onset of the Bølling biozone on the Swiss Plateau (seeAmmann and Lotter (1989) and Table 1 in Ammann et al. (2013-inthis issue-a)). Mortensen et al. (2011) provide new evidence from thesite Slotseng near Bølling Sjø, where the record of plant macrofossilsclarifies the distinction between dwarf and tree birches as well as thepresence/absence problem caused by long-distance transport of pollen.

On the Swiss Plateau and in the Alps the term Bølling has been usedas defined byWelten (1982a, 1982b), in accordancewithmost of south-ern Central Europe. Bølling sensuWeltenwas both a biozone (dominat-ed first by Juniperus and Hippophaë and then by tree birches) and achronozone, for which Welten defined the ages from 13,000 to 12,000radiocarbon years BP (in accordance with the scheme of Mangerudet al. (1974)). With the advent of AMS dating the hard-watererrors of bulk samples (measured by decay counting) could be avoided,and Ammann and Lotter (1989) separate the Bølling chronozone ofWelten (1982a, 1982b) (starting at 13,000 radiocarbon yr BP) from theBølling biozone (starting at about 12,600 radiocarbon yr BP). The age ofthe latter after calibration is thus between 15,235 and 14,314 yr BP. Italso became clear that environmental processes around the beginningof the Bølling are very difficult to date because of a plateau of constant ra-diocarbon ages (Andrée et al., 1986; Ammann and Lotter, 1989; Lotter,1991; Blockley et al., 2008). Rates of changewouldbebest estimated if an-nually laminated sedimentswere available for this time interval, but suchhave not been found in Europe so far for the period in question (glacigenicvarves are found before and biogenic varves after the transition from theOldest Dryas to the Bølling, see belowSection 6.1). Besides being compat-ible with the bio- and chronostratigraphies of many sites in Europe, theuse of the term Bølling is also in harmony with large parts of the litera-ture on Greenland ice records (e.g. Severinghaus and Brook, 1999). VanRaden et al. (2013–in this issue) provide a useful overview of the termi-nology for these intervals for the northern hemisphere in their Table 3.

The bio- and chronostratigraphic meaning of the term “OldestDryas” as the cool phase before the Bølling was of course also affectedby the definition of the term Bølling. For the Swiss Plateau we followWelten (1982a, 1982b), Ammann and Lotter (1989), and Lotter et al.(1992). The Oldest-Dryas section between the youngest till layers andthe Bølling is commonly quite long and shows 2–8 different pollenassemblage zones in the Alpine foreland. They can usually be groupedin at least three distinct bio-zones (Lang, 1952; Müller, 1962; Welten,1972; Welten, 1982a; Gaillard, 1984; Küttel, 1974; Rösch, 1983; andAmmann and Tobolski, 1983; Lotter, 1988; Vescovi et al., 2007). Thefirst phase above the till is characterized by low pollen concentrations,high percentages of reworked pollen (including Tertiary types),and high percentages of Pinus (probably representing partlyreworked, partly long-distance transported pollen). The secondphase contains a high diversity of non-arboreal pollen (NAP), lessPinus, and hardly any reworked pollen (a combination attributed to“steppe-tundra”). The third phase is similar but in addition showssubstantial percentages of Salix, Juniperus, and especially Betulanana (indicating “shrub-tundra”). It is only in this third phase thatenough biogenically precipitated carbonate allows an oxygen-isotope analysis of the shallow-water sediments of some CentralEuropean hard water lakes such as Gerzensee, Faulensee, andAegelsee, (Eicher and Siegenthaler, 1976; Eicher, 1987; Lotter et al.,

Page 4: Biotic responses to rapid warming about 14,685yr BP: Introduction to a case study at Gerzensee (Switzerland)

Fig. 1. Maps: Top: Map of Central Europe indicating the position of sites mentionedin the text: Gościąż (wnw of Warsaw), Meerfelder Maar (near Bonn), Ammersee(near München), Mondsee (near Salzburg), and Schleinsee (ne of Lake Constance).Middle: Map of Switzerland, Soppensee and Rotsee (near Lucern), Faulensee (near LakeThoun), and Aegelsee (Bernese Oberland). Bottom: Local map of Gerzensee situated onthe interfluve between two rivers. Both valleyswere under the ice of the Aare glacier duringthe LGM of the Würm glaciation.

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1992). A precondition is that neither detrital carbonates brought inby rivers nor the ratio of evaporation to water-input is too high.The transition from the second to the third phase is not well dated,but the available radiocarbon dates at Schleinsee (13,325 ± 120),Lobsigensee (13,360 ± 280), and Rotsee (13,600 ± 220, Ammannand Lotter, 1989) translate after calibration into an age band of15,100–16,800 yr BP. This “warmer period before the Bølling withshrub-tundra” is therefore neither bio- nor chronostratigraphicallycomparable to the much younger Meiendorf period of Menke (1968)or Litt et. al (2001), which started at ca. 14 600 yr BP (resemblingthus the 14,650 yr BP for the start of Bølling at Gerzensee, see Table 1).

The whole history of various stratigraphic concepts of the EuropeanLate Glacial (such as biostratigraphy, and chronostratigraphy), partly inharmony and partly in conflict with the stratigraphic code (Salvador,1994))was tackled by de Klerk (2004), who describeswheremisunder-standings or erroneous correlations were made. He comes to the con-clusions that (1) terminologies are based on incompatible methodsand concepts, (2) identical or similar stratigraphical names are usedwith slightly or totally different meanings and/or contexts, (3) thetype locality Allerød has no palynostratigraphic value because no pollenanalysis was carried out, (4) Bølling Sø is unsuitable as a type locality fora number of reasons, (5) the Meiendorf period is inconsistently and in-appropriately defined, (6) the various “Dryas” terms lost their value,and (7) the onlyway out of this problem is the development of schemesbased on various regional records. We follow de Klerk (2004) in his lastpoint by using the terms as defined regionally (here for the Swiss Pla-teau), first by Welten (1982a, 1982b) and then by Ammann and Lotter(1989) and Lotter et al. (1992). Interestingly enough in studies faraway from pollen stratigraphy, namely in geophysical publications,the term Bølling-warming is as well established as the term YoungerDryas (Kienast et al., 2003; Menviel et al., 2011; Stanford et al., 2011).

4. Materials and methods

Lake Gerzensee is a small lake on the Swiss Plateau at 603 m asl(46°49′56.95″ N, 7°33′00.63″E, Fig. 1). The Miocene Molasse sandstoneis covered byWürmian till of the Aare glacier. The area became ice-freeabout 17–18 ka BP (ca. 14.5–15.5 ka BP in radiocarbon years (Preusserand Schluechter, 2004); Ivy-Ochs et al., 2008). The lake originated as akettle-hole on the interfluve of the valleys of the Aare and the Gürberivers and was thus protected from subsequent erosion. Today itssurface is 25.16 ha, and its maximum depth is 10.7 m. The hydrologicalcatchment is small (2.6 km2), and the lake has no major inlets. Duringthe Würmian late-glacial the water level was higher, and Eicher(1979) estimated that the original surface area during the late-glacialmay have been double that of today. The late-glacial lake marl formedthe littoral sub-aquatic terrace, covered today by riparian woodlandwith Salix alba and Salix cinerea and by reeds of Phragmites australis.

In September 2000 two cores of 8 cm diameter named GEJ andGEK were recovered with a Streif-modified Livingstone corer (Merktand Streif, 1970) in the Phragmites-belt near the site of earlier corings(Eicher and Siegenthaler, 1976; Schwander et al., 2000). These twincores were taken only 40 cm apart and had an overlap of 50 cm(Wright, 1991). The recovered 1 m-sections were correlated by over-lapping δ18O measurements, resulting in a composite core labelledGEJK (from GEJ the levels 414–330 cm, from GEK the levels 329–272 cm). An earlier study of the Younger Dryas was made on thecores GEA and GEB combined as GEAB. The cores GEAB and GEJK canbe linked at themarker horizon of the Laacher See Tephra (LST, referencelevel of 272 cm core depth (Ammann, 2000); Ammann et al., 2000;Schwander et al., 2000; Wick, 2000; Hofmann, 2000; van Raden et al.,2013–in this issue).

Core GEJK was sampled at identical levels for the various geochem-ical and biological analyses. Chronological consistency between the bio-stratigraphies is thereby granted. Sampling resolution was adapted tothe question of biotic response to rapidwarming, i.e. at a 0.5 cm intervalwhere the isotopic shift was expected and 1–2 cm intervals elsewhere.

The minor isotope fluctuations during the Bølling/Allerød differedsomewhat between the core GEJK and the older core GEAB, possiblybecause either of breaks between the 1-metre sections of the cores orspatially variable sedimentation rates. Therefore an additional 3 m longcore GEM in one piece was taken in September 2008 with a 63 mm-Niederreiter corer. Results of correlation and stacking of all the availableisotope records of Gerzensee (including GE III, by Eicher (1987)) andtheir correlation to NGRIP resulted in the GICC-05-chronology given invan Raden et al. (2013–in this issue).

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Local biozones and isotope zones in all subsequent papers arelabelled with GRZ for Gerzensee and a subscript for the type of stratig-raphy, e.g. ibulk for isotopes measured on precipitated carbonates, iosfor isotopes measured on ostracods, po for pollen, cla for Cladocera,and ch for chironomids. Biotic and isotopic zones are numbered frombottomup, as is common for biostratigraphic zones in terrestrial recordssuch as pollen diagrams (Birks, 1986). This contrasts with the traditionamong oceanographers, where numbering runs top-down (e.g. for ma-rine isotope stages or for the GRIP-events). This can even lead to seem-ing contradictions when zones are labelled with letters (e.g. at HawesWater, Jones et al., 2002) where, compared to GRIP-events, B corre-sponds to d and D corresponds to b). Nonetheless, for the minor eventsin the Greenland sequence, which are so convincingly matched atGerzensee, we use the top-down numerical subdivisions of the glacialstadial and interstadial intervals (such as GS1, GS2, GI1, GI2) and thetop-down subdivisions of GI-1c and GI1e of van Raden et al. (2013–inthis issue).

5. Results

5.1. The time scale for Gerzensee

Under certain conditions the ratio of oxygen isotopes is a temperatureproxy both for precipitation and for the calcite of lacustrine sediments(Von Grafenstein et al., 1999b; Von Grafenstein et al., 2000; VonGrafenstein et al., 2013–in this issue). The very similar trends of theoxygen-isotope records fromGreenland ice cores and European lake sed-iments during the Last Glacial Termination suggest that they werecaused by the same drastic climatic change and thus occurredquasi-simultaneously on an extra-regional, probably hemisphericscale (Siegenthaler et al. (1984)).

This circumstance was used in the earlier Gerzensee study aboutthe Allerød to Younger Dryas to create an age scale for lake sedi-ments based on the chronology of Greenland isotope records (NGRIP-members, 2004; Rasmussen et al., 2006). A wiggle-matching techniquewas applied to find an age scale providing the best correlation betweentwo stable-isotope records (Schwander et al., 2000). In order to applythis technique, the records to be matched must have unequivocally dis-tinct patterns. For the present set of studies van Raden et al. (2013–inthis issue) developed the time scale on the basis of an improvedGerzensee record that stacked core GEJK with the new core GEM andthe old cores GEAB and GE III.

Under the assumption of the synchroneity between Greenland andEuropean climatic events (Haflidason et al., 1995; Hendy et al., 2002;Lea et al., 2003; Schwander et al., 2000; Von Grafenstein et al., 2000;Thornalley et al., 2011) we can adopt the NGRIP-time scale GICC05(Rasmussen et al., 2006) for the record at Gerzensee, and this is usedthroughout the subsequent papers of this issue. We can also estimatethat the temporal resolution of the samples is about 17 years in thecore sections where sampling distance was 1 cm, and about 8.5 yearswhere sampling distance was 0.5 cm. In the latter higher-resolutioncase, the isotope measurements range between 15,675 and 13,000 yearsBP, i.e. over the period of the most rapid warming.

Althoughwe are aware that the ice-core records are now commonlypresented in years AD 2000 (b2k), we present our data in BP (before AD1950) to be able to compare the Gerzensee chronologywith those of themany terrestrial, marine, and archaeological records dated by radio-carbon. Speleothem records dated by Uranium–Thorium isotopes arealso reported in years before AD 1950 (Genty et al., 2006). Even forthe ice-core andmarine records the terrestrial terms Bølling and Allerødare commonly applied instead of the numerical scheme, becausethey are well known vegetational/climatic phases of the Europeanlate-glacial (e.g. Rasmussen et al., 2006; Stanford et al., 2011).

We should add that whenwe use ages such as “14,685 yr BP”we donot imply that the last two digits are correct in absolute ages andwe areaware that also the NGRIP-chronology will get improved. But what is

important for estimates of biotic responses and their rates are the differ-ences between two dated events, rather than their absolute ages. There-fore, we do not round the ages.

5.2. Major climate shifts: a rapid warming and a rapid cooling

The oxygen-isotope record of the core GEJK displays twomajor shiftsthat can easily be correlated with the palynological changes at thebeginning of the Bølling/Allerød interstadial and at the beginning ofthe Younger Dryas (i.e. the beginning of Greenland Interstadial 1 andStadial 1), respectively, in the event stratigraphy of the INTIMATEgroup (Björck et al., 1998; Lowe et al., 2008). The first rapid shiftfrom about −9.08‰ PDB to −5.46‰ occurs between about 14,685and 14,573 GICC-05-years BP (between 374 and 369.5 cm in GEJ),i.e. 3.6‰ in about 112 years.

The second rapid shift, from about−7.2 to−9.6‰ PDB in about 94–100 years at the beginning of the Younger Dryas, is not the concernof the present set of papers but was described for Gerzensee bySchwander et al. (2000) and von Grafenstein et al. (2000) and the bioticresponses to it by Ammann et al. (2000), Brooks (2000), Hofmann(2000), Lemdahl (2000), Tobolski and Ammann (2000), and Wick(2000).

5.3. Minor climatic changes: the internal structure of the oxygen-isotoperecord in the Bølling/Allerød

Between these shifts of large amplitudes (2.5 to 3.25‰ in theoxygen-isotope record) at the beginning and end of the late-glacialinterstadial of the Bølling and Allerød we observe at least four minoroscillations of about half to a third of this amplitude of the major shifts(i.e. changes of about 1 per mille δ18O, Figs. 3 and 4, light blue bandsin Fig. 4 and Table 2 in van Raden et al. (2013–in this issue)). They arecomparable to the threeminor fluctuations in the δ18O recordmeasuredon ostracods atMondsee/Austria (Lauterbach et al., 2011). At Gerzenseethese four minor cool phases appear on a general slope between −6‰and −8‰. For a while it was thought that the Laachersee Tephra LST(deposited during the Gerzensee oscillation) can be correlated to asulphur peak in the Greenland ice cores. But Mortensen et al. (2005)showed that this peak must have an origin different from Laachersee.

5.4. Biotic responses to the early rapid warming

Biotic responses to climatic change can be assessed from the lengthsof time for a statistically significant response visible in the differentgroups of organisms, and from their biological functions, such as

• productivity (fast response, e.g. pollen productivity, tree-ring width),• population density (medium response, e.g. increasing or decreasingpopulation by changes in survival and mortality, depending on theduration of a generation),

• migration (relatively slow response, e.g. range expansions or contrac-tions, i.e. a biogeographical response)

• species composition of communities depend on species pools(e.g. of terrestrial plants or of aquatic invertebrates)

• species interactions, ecosystem processes on several time scales.

In our earlier study of the rapid warming at the end of the YoungerDryas (Ammann et al., 2000) we found primarily the first two typesof responses. The third and forth types of responses were very minor,probably because we included a very limited time period after therapid warming (transition from the Younger Dryas to the Holocene).If the early Holocene would have been included certainly processes ofmigration and ecosystem interactions would play a major role.

As in the earlier study, the relationship of optima and tolerances ofbiota to temperature can also be used in the opposite direction, i.e. notto estimate biotic responses to climatic change but to reconstruct thesummer temperatures by applying transfer functions (i.e. inference

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models, Lotter et al., 2000). For the record presented here theserelationships have been analysed by Lotter et al. (2012).

6. Discussion

6.1. The time scale

The time scale worked out by van Raden et al. (2013–in this issue) isnot independent but is based on a correlation to NGRIP (NGRIP-members, 2004) and to GICC05 (Rasmussen et al., 2006). This is a disad-vantage; but at present there are no promising alternatives to betterdate the beginning of the Bølling:

• Parts of the period fall in a plateau of constant radiocarbon ages(Andrée et al., 1986; Ammann and Lotter, 1989), and thereforedetailed dating by radiocarbon is not possible. The rapid climaticwarming and the plateau of constant age probably have a commoncause, i.e. the reorganisation of the thermohaline circulation (Broeckeret al., 1988a, 1988b; Stocker, 2000; Stanford et al., 2011). Late-glacial ra-diocarbon datingmay be especially prone to enigmatic differences evenwithin samples of plantmacrofossils (Turney et al., 2000). Palynologicalcorrelation of conspicuous events in vegetation history does notnecessarily provide absolute dating. In a restricted and environmen-tally rather homogenous area such as the Swiss Plateau with severalradiocarbon-dated sites (e.g. Ammann and Tobolski 1983; Gaillard,1984; Ammann and Lotter, 1989; Ammann, 1989a; Ammann,1989b; Ammann et al., 1996; Hajdas and Michczynski, 2010) clearlymarked events such as the afforestation by juniper and tree-birchesat the beginning of the (palynologically defined biozone) Bølling can-not be too time-transgressive; a concentration of radiocarbon ages isfound around 12,600 to 12,700 uncal. years BP (see Table 1). Butthis radiocarbon-age falls into a major plateau of constantradiocarbon-age (Reimer and Reimer, 2007).

• Tephra in Central Europe is not numerous in the period concerned.The only one found at Gerzensee is the Laachersee Tephra (LST),which can be used as a time horizon for many sites on the Swiss Pla-teau (Gaillard, 1984; Ammann and Lotter, 1989; Ammann, 1989a;Lotter et al., 1992; Blockley et al., 2008) and parts of Central Europe.Possibly a record of ashes from Puy de la Nugère is found just belowLST (Walter-Simonnet et al., 2008) although no evidence for theseashes could be detected in the core GEM (analysed by XRF (vanRaden et al., 2013–in this issue)).

• Annual laminations (i.e. varves) could provide a dating alternative tocorrelation to NGRIP or radiocarbon dating. But the sediments ofGerzensee show no laminations, neither in the late-glacial nor in theHolocene. At the Central European sites showing laminations, however,the varves unfortunately donot reach down into the transition from theOldest Dryas to the Bølling around 14,685 cal yr BP, but only startsomewhere within the Bølling: For example, at Soppensee (50 kmnortheast of Gerzensee (Lotter, 1991; Lotter, 1999)) varves start duringthe Betula-phase of the Bølling. At Meerfelder Maar (nearly 400 kmnorth of Gerzensee, Brauer et al., 1999) they start at 14,020 varve-years BP, and in Lake Gościąż in central Poland the laminations startduring the Allerød (Goslar et al., 1995; Goslar et al., 1998; Goslaret al., 1999; for sites see Fig. 1). An exception are the nearly 300 clay-varve sequences in southern Sweden cross-correlated by Ringberget al. (2003) that cover the 1040 years long period from the end ofthe Oldest Dryas to the early Allerød.

Possible causes for the absence of varves during the late-glacialinclude (1) the loss of contact with the glaciers for the formation ofclastic varves in formerly glaciated areas (e.g. at Gerzensee, Soppenseeand Lake Gościąż, but still in function in southern Sweden), (2) toolow a productivity in the lake for the formation of biogenic varves beforethe Bølling (in all lakes indicated on Fig. 1), and (3) too shallow a basinor a littoral position of the coring site as for GEJK and thus no anoxia (beit during summers only or throughout the year) and thus too much

bioturbation to preserve the seasonal input to the sediments in formof laminations. Also at the site of Laghi di Monticchio in southern Italythe clastic varves are formed earlier, namely in the full glacial period,and the organic varves have formed later, namely in the Holocene(Zolitschka and N., 1996).

In contrast, the dating of the onset of the Younger Dryas can bechecked with the laminated records of Soppensee, Meerfelder Maar,and Lake Gościąż, see Table 1. The correlation with NGRIP puts thebeginning of the decline in the oxygen-isotopes at about 12,690GICC05-yr BP (Rasmussen et al., 2006, see also van Raden et al., 2013–in this issue). However, in the present sequence of papers we concen-trate on the beginning of the late-glacial interstadial of the Bølling–Allerød and its internal structure. The chronology of the early rapidwarming that we apply to Gerzensee uses in a way a “tripod”, i.e. pri-marily the correlation to NGRIP on the GICC05 age scale expressed inyr BP AD1950 (van Raden et al., 2013–in this issue) which is thenchecked by the tephra of LST and by the regional palynology (with itsset of radiocarbon dates, see Table 1 that also includes U/Th datesfrom the cave of Chauvet by Genty et al. (2006)). The timing of theminor fluctuations within the late-glacial interstadial Bølling–Allerødis more difficult and is discussed in Section 6.3.

6.2. The two major climatic shifts

The two major changes of the Würmian late-glacial, i.e. the begin-ning of the Bølling–Allerød interstadial (onset of GI-1) and the begin-ning of the Younger Dryas (onset of GS-1) are very sharp andtherefore well known from many sites in the northern hemisphere(and possibly elsewhere, Benson et al., 1997; Lea et al., 2003). Thehigh velocity of these changes is probably due to their trigger in theocean circulation (Broecker et al., 1988a, 1988b; Broecker et al., 1992;Stocker, 2000; Clark et al., 2001; Clark et al., 2002). At Gerzensee the be-ginning of GI-1 is reflected in a rise of δ18O of about 3.7‰ in about112 years, representing an increase of the mean air temperatures of atleast 6.2 °C (VonGrafenstein et al., 2013–in this issue). The reconstruct-ed warming, with respect to the overall change and its internal struc-ture is remarkably similar to that reconstructed from Ammersee((Von Grafenstein et al., 1999a, 1999b)). In both records, the rate ofchange is highest in the first half of the transition with 4.6 °C in about50 years and a considerable slowing to 1.6 °C over the 62 followingyears.

For the present core GEJK from Gerzensee Lotter et al. (2012) pro-pose an estimated increase of the summer temperatures of 3 to 6 °Cbased on the pollen record and 3 °C based on chironomid head cap-sules. Whereas the first agrees very well with the isotope-inferredchange, the second might be biased by the lack of significant changesin the water temperatures at the temporal deposition depth over thetransition (Von Grafenstein et al., 2013–in this issue; Brooks andHeiri, 2013–in this issue).

The beginning of GS-1, i.e. the sudden cooling at the onset ofthe Younger Dryas, shows a shift to more negative values of about2.5‰ in about 100–155 years. Thismay reflect a drop in themean annu-al temperature of up to 5 °C (Von Grafenstein et al., 1999a, 1999b; VonGrafenstein et al., 2000). The cooling of summer temperatures wasweaker at Gerzensee according to Lotter et al. (2000): about 1.5 °Caccording to pollen and about 3.5 °C according to Cladocera records.We estimate the duration of these periods of abrupt environmentalchange as about 112 years for the beginning of the Bølling, about 100–155 years for the beginning of the Younger Dryas and about 50 yearsfor the end of the Younger Dryas (Schwander et al., 2000).

6.3. The minor climatic changes within the interstadial

The minor fluctuations of the oxygen isotopes between these tworapid shifts, within the Bølling–Allerød interstadial are more difficultto assess in their magnitude and their timing because the separation

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of signal from noise becomes more delicate. Similarities between theminor fluctuations in a stacked oxygen-isotope record from Gerzenseewith the oxygen-isotope record of NGRIP form the basis of our correla-tion (van Raden et al., 2013–in this issue), and similarities withother oxygen-isotope records such as the ones of Ammersee (VonGrafenstein et al., 1999a, 1999b) and of Crawford Lake in Ontario (Yuand Eicher, 1998) need to be explored quantitatively. Similarities ofthe stacked oxygen isotopes are also found with various biotic records,especially the changes in the Chironomid-inferred mean July tem-peratures at Whitrig Bog in southeastern Scotland (Brooks and Birks,2000): There the two most marked cool phases within the interstadialmay be correlated with the Greenland-episodes GI-1b and GI-1d. Toavoid confusion of chronozone and biozone around the term “OlderDryas”, Lotter et al. (1992) used the local term Aegelsee oscillation asan isotopic and palynological zone. These authors as well as Björck(1984) remind us that periods shorter than about 200 years may bedifficult to trace biostratigraphically and nearly impossible to date byradiocarbon.

6.4. Biotic responses to temperature changes

In the terrestrial ecosystems the period of the rapidly increasingtemperatures at the transition from the Oldest Dryas to the Bøllingcan be considered as an example of the transition from a cryocratic toa protocratic phase sensu Iversen (1958); Birks (1986a), Andersen(1994):

• Vegetation developed from a steppe-tundra (probably with a fewshrubs and trees) to the first forest that was probably semi-openor in a mosaic more wooded and more open patches. Biomass wasincreasing rapidly.

• Light conditions change as a consequence of afforestation from plentyof light to reduced light under the trees, thus heliophilous plant taxaget reduced.

• Soils develop frombase-rich, skeletal, mineral soils after the ice retreatto more fertile soils with increasing organic matter and increasing Nand P concentrations.

In the aquatic ecosystems physical, chemical, and biological chang-es are also linked with each other as indicated e.g. by changes intemperature, nutrient status, species composition, and speciesinteractions.

The rate of the climatic and hence biotic changes points to the non-linearity of some of the processes involved: when certain thresholdswere reached the ecosystems changed very rapidly both in speciescomposition and in processes. The latter include e.g. soil formation,nutrient cycling, competition of terrestrial plants for space and light,or niche availability for aquatic organisms.

Among the biological processes we can largely distinguish fourorganisational levels:

• On the individual level the productivitymay change, e.g. pollen produc-tivity or tree-ringwidth. Such changes can occurwithin the same year(e.g. late-wood density) or in the following year (e.g. pollen produc-tivity depending on the conditions of the previous year) (van derKnaap et al., 2010). For short-lived organisms such as algae or insectsthis level may be identical to the following one.

• On the population level changes depend on the rapidity of the lifecycles: taxa with short generation times may respond faster, treesthat flower for the first timewhen they are 30 or 50 years old respondmore slowly. In addition the availability of resources may be limited(e.g. soil types, nutrient conditions, and water chemistry). Populationprocesses are of intermediate velocity.

• On the biogeographical level range changes bymigration respondwitha range of velocities depending on the duration of life cycles, dispersalvectors and environmental constraints: short-lived organisms, taxausing fast vectors (e.g. water plants and aquatic invertebrates

dispersed by water-fowl), and pioneer species may be fast. Taxawith long generation times, late maturity and reproduction, slow dis-persal, and dependence onmature soilswill be slow in their responsesto a late-glacial warming.

• On a community level the species pools (controlled by the above men-tioned processes), and species composition of habitats (both terrestrialand aquatic) change over time.

• On an ecosystem level we can expect responses to a climatic change ofhigh amplitude and velocity such as the transition from the OldestDryas to the Bølling. The processes involved may include pedogenesis,nutrient cycling in terrestrial and aquatic ecosystems, aswell as speciesinteractions. Major topics will be the questions to what degree aquaticand terrestrial ecosystems are linked and through which processessuch links functioned.

Processes on all five levels can occur without climate change butmay be favoured by increasing temperatures; whereas decreasing tem-peratures may damage thermophilous taxa and favour cryophiloustaxa.

7. Overview over the contributions to the multi-proxy study

The first three subsequent paperswill work out aspects important tothe ecosystem of the catchment as a whole, namely

• the stable isotopesmeasured on precipitated calcite at high resolutionand correlated to NGRIP, thus deriving the timescale (van Raden et al.,2013-this issue).

• The stable isotopes measured on monospecific samples of ostracodsand molluscs, which allow addressing the water temperature effectfor the bulk precipitated calcite, and in consequence a quantificationof δ18OP changes and air temperatures (Von Grafenstein et al., 2013–in this issue).

• The reconstruction of lake-level changes based on forms of calcites(Magny, 2013–in this issue).

The next three papers deal with the terrestrial ecosystems, namely

• Pollen stratigraphy as a record of the primary succession, includingafforestation (Ammann et al., 2013-in this issue-a).

• Vegetation modelling based on arrival times of woody taxa and theirpopulation dynamics (Lischke et al., 2013–in this issue).

• Possiblemeaning of afforestation and the nitrogen-fixerHippophaë forthe biogeochemical cycle of nitrogen and emission of N2O (Pfeifferet al., 2013–in this issue).

Then two papers deal with the dynamics of aquatic invertebrates,namely

• The Cladocera biostratigraphy and its paleoecological meaning(Nováková, 2013-in this issue).

• The chironomid stratigraphy and its paleoecological meaning(Brooks and Heiri, 2013–in this issue).

Because the ecologically significant change starting around14,685 yr BP saw the vegetation shift from a shrub-tundra to a wood-land, this afforestation had a major effect on the mammal fauna. Notfor Gerzensee but for the Swiss Plateau – and beyond – Nielsen(2013–in this issue) summarizes the faunal changes as recorded in nat-ural and archaeological sites.

Finally a synthesis is presented (Ammannet al., 2013-in this issue-b)in which we try to quantify the biotic changes wherever possible, todiscuss climatic and other environmental causes for such changes, aswell as ecological interactions and feedbacks.

Acknowledgements

We are grateful to Willi Tanner, Herbert E. Wright, and Adrian Gillifor coring, to Florencia Oberli for sample preparation, to Peter von

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Ballmoos for help with the figure and to Herbert E. Wright for hispatience and help along the way from coring to writing.

References

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