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Quaternary Science Reviews 25 (2006) 2314–2331

Early Holocene climate variability and the timing and extent of theHolocene thermal maximum (HTM) in northern Iceland

Chris Caseldine�, Peter Langdon, Naomi Holmes

Department of Geography, School of Geography, Archaeology and Earth Resources, University of Exeter, Exeter EX4 4RJ, UK

Received 7 July 2005; accepted 2 February 2006

Abstract

The magnitude and timing of Holocene maximum warmth in the Arctic and sub-Arctic has been the subject of considerable recent

interest, particularly in the context of future climate change. Although lying at a crucial location in the North Atlantic close to significant

atmospheric and oceanic boundaries, terrestrial Holocene climatic data from Iceland are few and predominantly derive from glacial and

palaeoecological evidence. Here we present new datasets from Trollaskagi, based on chironomid-inferred temperatures (CI-T), using sub-

fossil chironomids from the same lake sediments supplemented by pollen data. July air temperatures have been derived using an Icelandic

training set, and the data suggest optimal temperatures at sea level up to 1.5 1C above current levels around 8 k cal. yr BP, a time when

birch woodland was well developed in Trollaskagi, but when woodland had still not fully developed in the more isolated NW peninsula.

Our data thus suggest that optimal summer warmth did not occur in Iceland until 8 kcal. yr BP at the earliest, possibly lasting until

6.7 kcal. yr BP. The amount of warming for July was therefore at least 1.5 1C, but possibly up to 2–3 1C higher than the 1961–1990

average on the basis of the tree-line data. Comparison with data from elsewhere in adjacent Arctic regions, Greenland and Eastern Arctic

Canada show peak warmth to be later in Iceland, and less pronounced. It also appears that there were enhanced temperature gradients

during the first half of the Holocene between the two study areas Trollaskagi and the NW Peninsula and that they influenced patterns of

vegetation colonisation, with current spatial temperature patterns only developing as Holocene climate deteriorated after around

6 kcal. yr BP.

r 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Understanding the nature of climatic variability duringthe first half of the Holocene, and in particular the timingand extent of the Holocene thermal maximum (HTM), is ofcritical importance for understanding the nature oforbitally-forced interglacial climate variability (Ruddiman,2003) and for testing models of the linkages between theocean–atmosphere–land systems and possible cyclicities inNorth Atlantic climate (e.g. O’Brien et al., 1995; Alleyet al., 1997; Bianchi and McCave, 1999; McDermott et al.,2001; Langdon et al., 2003; Turney et al., 2005). The NorthAtlantic region as a whole has been a focus for such studieswith particular emphasis on the marine record (e.g. Bondet al., 1997, 2001; Eirıksson et al., 2000a; Andrews et al.,2001b; Bauch et al., 2001; Jiang et al., 2001; Andersen

e front matter r 2006 Elsevier Ltd. All rights reserved.

ascirev.2006.02.003

ing author. Fax: +44 139 226 3342.

ess: C.J.Caseldine@exeter.ac.uk (C. Caseldine).

et al., 2004; Eynaud et al., 2004) but as yet there isrelatively little terrestrial evidence for locations such asIceland that are heavily influenced by North Atlanticoceanic and atmospheric circulation. There has also been atendency in recent years to focus on the impact of eventssuch as the ‘8.2 ka’ event (Alley and Agustdottir, 2005)rather than trying to establish longer-term climaticvariability from terrestrial evidence.Although lying close to sharply defined atmospheric and

oceanic boundaries, terrestrial evidence for climate changefrom Iceland has been extremely sparse. Studies of glacialchange have concentrated on the Later Holocene due to alack of evidence from the earlier period (Stotter et al., 1999;Mackintosh et al., 2002) and biological evidence hasrelied on a rather generalised view of vegetation changefrom pollen and macrofossil evidence (Hallsdottir andCaseldine, 2005) with a limited range of detailed site studiesfor the Early Holocene (Hallsdottir, 1991; Rundgren, 1995,1998, 1999; Rundgren and Ingolfsson, 1999). This was

ARTICLE IN PRESSC. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–2331 2315

improved with the first chironomid-based tempera-ture reconstruction from Efstadalsvatn in NW Iceland(Caseldine et al., 2003), but this work was based on the useof a Norwegian training set for deriving mean Julytemperatures. There has, however, been a significantexpansion in evidence from marine sediments off northernand western Iceland over recent years (Eiriksson et al.,2000a, b; Andrews et al., 2001a, b, 2003; Jiang et al., 2001,2002; Knudsen and Eirıksson, 2002; Andrews and Gir-audeau, 2003; Castaneda et al., 2004; Giraudeau et al.,2004; Knudsen et al., 2004; Andresen et al., 2005; Smithet al., 2005).

In this paper, we present pollen, chironomid andsedimentary data from two neighbouring sites on theTrollaskagi peninsula covering the period ca 10.2–7.0 kcal.yr BP and provide the first quantitative estimate oftemperature change for terrestrial sites based on anIcelandic chironomid training set (Langdon, unpublisheddata).

2. Study area, field sampling and stratigraphy

The Trollaskagi peninsula lies between Eyjafjorjur andSkagafjorjur just below the Arctic Circle in northernIceland (Fig. 1). Palaeoecological data have previouslybeen presented from Vesturardalur at around 400m (Wastl

NO

R

TH

WE

S

Snæfellsnes Peninsula

Efstadalsvatn

H ú n a f

Í s a f j a r ð a r d j u p B997-332

B997-324

B997-32

B997-33

MD99-2269

KN158-4-72GCC

Other pollen sites / palaeo sites

Marine cores

Analysed sites (this paper)

66°

24° 22°

24°

65°

22°

Fig. 1. Map of N and NW Iceland showing location of the sample sites, terrest

KN158-4-72GCC (Andresen et al., 2005); MD997-324/332/329/330 (summarise

2269 (Andersen et al., 2004).

et al., 2001) but here data are presented for the EarlyHolocene from two small basin sites at lower altitudes:Hamundarstajahals—(651580N 181270W) a small,

90� 60m diameter kettle hole now infilled with peat lyingat an altitude of 100m. The analysed core, HH8, was takenfrom the centre of the basin.Vatnamyri—(651540N 181180W) a more complex larger

basin almost 2 km long by 0.5 km wide at 220m, currentlylargely infilled but with occasional open water. The selectedcore was obtained in two parts, Vatnamyri 2 and 3 (VAT2and VAT3), above and below the almost impenetrableSaksunarvatn tephra layer (Gronvold et al., 1995; Bjorcket al., 2001), from the western and deepest part of thebasin. Current mean July temperature at sea level based on1961–1990 data is 10.5 1C.In both cases, upper sediments were cored by hand using

Russian corers with power-assisted coring required torecover lower sediments penetrating the Saksunarvatntephra layer. Cores were sub-sampled in the field and keptat 4 1C prior to later laboratory analyses. Materialpresented here was taken from the lowermost lacustrinesediments, principally silty clays with gyttjas underlyingterrestrial peat (Carex–Equisetum) that dominated theupper 3m at HH8 and 4.5m at Vatnamyri. The followingstratigraphical characteristics are important for the deriva-tion of age–depth models and are highlighted in thepercentage total carbon (%C) curve (Fig. 2).

0 50km

T I C

E L A N D

Hámundarstaðahals

Hafratjörn

Krosshólsmýri

Hella Árskogsströnd

Vatnskotsvatn Vesturárdalur

Vatnamýri

Eyjaf jö

rðu

r

Skagi Trö l laskag i

Torfadalsvatn

l ó i

9

0

HM107-04

HM107-05

66°

20° 18°

North Alantic Ocean

NO

RW

AY

NO

RW

AY

ICELANDG R E E N L A N D

G R E E N L A N D

rial sites and selected marine cores discussed in the text. For marine cores:

d in Castaneda et al., 2004); HM107-04/05 (Knudsen et al., 2004); MD99-

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S a k s u n a r v a t n t e p h r a

HH8 age model (weighted)

S a k s u n a r v a t n t e p h r a

VAT2/3 age model (weighted)

(a)

(b)

Fig. 2. Age–depth models and %C (a) HH8 and (b) VAT2/3.

C. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–23312316

HH8 (Fig. 2a)—the sediments below Saksunarvatn aresilty clays with very low organic content (o5%C). TheSaksunarvatn tephra is almost 80 cm thick with asignificant component of reworked tephra. The remainderof the core is an organic silty clay with %C rising fromo5% to just under 20% before a relatively sharp transitionto sedge peat at 292 cm, when %C rises to over 50%.

Several thin but discrete tephra layers occur between316–473 cm.VAT 2 and 3 (Fig. 2b)—the sediments below Saksunar-

vatn (VAT3) are very similar to HH8 comprising silty claywith extremely low values for %C. The Saksunarvatntephra spans 20 cm and is overlain by a complex series ofdeposits as seen in the %C curve. These sediments are

ARTICLE IN PRESSC. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–2331 2317

primarily organic silty clay with %C values generally less than10% below 550 cm; between 550–490 cm they are below 20%,between 490–450 cm below 30%, with sedge peat formingabove 450 cm (%C450%). These sediments are also,however, punctuated by numerous primary and secondarythin (usually 273mm) tephra horizons. In VAT2 there are,however, also short sections of much higher organiccomposition, especially between 620–610 and 520–510 cm.

2.1. Dating and development of age–depth models

Both profiles contained several tephra layers, the resultsof which are discussed by Wastl (2000). Apart from thethick deposit of Saksunarvatn tephra (10180760 cal. yr BP,Gronvold et al., 1995), the identification of later Heklatephras in the upper peats, and a possible Snæfell light-coloured tephra at 476 cm in HH8, dated to87707110 14C yr BP (Table 1) in an adjacent core (Wastl,2000), no other layers that were identified or geochemicallyanalysed could be used as marker horizons for the coresanalysed. The tephrochronology of the region (Wastl,2000; Wastl et al., 2001) has proved difficult to establish asthere are variable numbers of very similar tephras at closely

Table 1

Radiocarbon dates and tephra dates from Hamundarstajahals (HH8) and Va

Depth (cm) 14C date BP Error71 S.E. Lab. code R

2s

Radiocarbon

Hamundarstajahals

HH8

589a 8673 56 AA-44909 98

439 8371 56 AA-44910 95

414 8162 69 AA-44911 93

371 7663 70 AA-44912 85

311 6742 69 AA-44913 76

294 6289 57 AA-44914 74

Vatnamyri

VAT3

734.5 9460 120 AA-44915 11

VAT2

645.5a 8969 66 AA-44916 10

606a 8608 86 AA-44917 98

575a 7680 61 AA-44918 85

550a 7108 52 AA-44919 80

485.5a 6168 57 AA-44920 72

450.5a 5668 59 AA-44921 65

Tephras

Depth (cm) Tephra 14C date

BP

Error

71 S.E.

R

2s

Hamundarstajahals (HH8)

476 Light tephra (Snaefell) 8770 110 99

Vatnamyri (VAT2)

361.5 Hekla 5 6065 55 71

236.5 Hekla 4 3935 50 45

157.5 Hekla 3 2840 45 31

aDates not used in the age–depth model.

located sites. Thus, for example at Vatnamyri, below theSaksunarvatn tephra, the type profile examined and datedby Wastl (2000) had 11 geochemically analysed tephras in70 cm of sediment, many of which were inseparablegeochemically; the parallel core analysed here (VAT3) onlyhad pollen in the top 30 cm below Saksunarvatn with amore limited number (seven) of generally thin tephras,none of which could be assigned to a reliable age estimate.Samples for radiocarbon dating were taken from all

three cores and the results are summarised in Table 1. Inthe absence of sufficient plant macrofossils for dating alldeterminations are on bulk C and it is clear that some dateshave significant errors. For Hamundarstajahals, the datedtephras and radiocarbon dates, except for a single datebelow Saksunarvatn, are considered reliable and anage–depth curve has been constructed for the period10.2 k–7.5 kcal. yr BP (Fig. 2a). At Vatnamyri, the majorityof radiocarbon dates have significant errors and are allthought to be too young. The approach adopted has beento construct an age–depth profile by establishing a smallnumber of fixed points using the dated tephras and thencarry out curve matching based on the pollen andchironomid results, assuming that where patterns are

tnamyri (VAT2/3)

ange (cal. yr BP)

error

Age (cal. yr BP)

used in model

d13C

88–9533 9670 �23.5

23–9151 9380 �25.0

99–8992 9170 �25.8

89–8355 8470 �28.0

91–7440 7580 �28.4

07–7013 7170 �27.9

159–10406 10800 �21.4

237–9873 10080 �21.1

89–9437 9640 �22.3

88–8380 8480 �21.5

22–7791 7900 �24.0

50–6943 7100 �29.1

68–6308 6440 �29.5

ange (cal. yr BP)

error

Age (cal. yr BP)

used in model

Reference: Lab code

10–9540 9700 Wastl (2000) AA-23876

56–6747 6900 Wastl et al. (2001) AA-28985

21–4186 4370 Wastl et al. (2001) AA-28982

55–2793 2960 Wastl et al. (2001) AA-30906

ARTICLE IN PRESSC. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–23312318

identical given the proximity of the sites they should reflectlocal changes occurring more or less synchronously giventhe proximity of the sites. In more detail:

Hamundarstajahals: This has the simplest lithostratigra-phy and the most robust age determinations. It is notpossible to date sediments below Saksunarvatn because theradiocarbon date is clearly too young (i.e. younger thenthat of the tephra above). Above this level, it is possible toconstruct a model based on the radiocarbon and tephradates that gives a linear fit (Fig. 2a). This is only valid forthe predominantly inorganic sediments, hence the age ofthe uppermost peat is less certain. Although Hekla 5 (H5)was not found in this particular profile it was widely foundin adjacent cores occurring between 25–32 cm above thegyttja/peat transition. Extrapolation of the age model tothe surface through an estimated H5 level at 28 cm abovethe transition implies a change of accumulation rate from15 yr cm�1 in the gyttja to 25 yr cm�1 in the peat. Estimateddates below Saksunarvatn are discussed below.

Vatnamyri: The date below Saksunarvatn is consideredto be reliable by comparison with similar Icelandicsequences (Rundgren, 1999; Hallsdottir and Caseldine,2005). Above the Saksunarvatn tephra the radiocarbondates appear to be increasingly too young (Table 1), anassumption confirmed by the presence of Hekla 5 at361.5 cm in the overlying peat (Maria Wastl, pers. comm.).Although the two lowest dates appear to be possibly valid(see Fig. 2b) there are insufficient reliable data to develop asound age–depth model using them. Any model is likely tobe complex because of the changes in sediment type as seenmost clearly in the %C data, with the more organic layersrepresenting drying out and presumably a slower sedimen-tation rate, and the numerous, albeit thin, tephra layersaccelerating sedimentation. An age–depth model wasdeveloped for VAT2 using the following criteria: (i)Saksunarvatn at 685 cm; (ii) Hekla 5 at 361.5 cm (notedin field); (iii) +47 yr at surface; (iv) aligning peaks in thepalaeoecological record (CI-T; Betula % curve; Juniperus

% curve) assumed likely to be local features found both atVatnamyri and Hamundarstajahals in order to accommo-date changing sedimentation rates (see above and discus-sion below). This produced an age–depth model aboveSaksunarvatn with a third order polynomial fit (Fig. 2b).

Because it has not been possible to establish reliable basaldates for either site, the oldest radiocarbon date found ateither of the two sites, 96607100 14CyrBP (AA-23883;10,970–10,560 cal. yrBP, Wastl, 2000) has been taken as areasonable estimate of the oldest biostratigraphic data (thepoint at which pollen is preserved in sufficient quantities toanalyse). At Hamundarstajahals, linear sedimentation to theSaksunarvatn tephra was assumed, as the only radiocarbondate from this part of the core is clearly too young. While theuniform accumulation may not be likely, as seen in thevarying%C record, this is the only possible solution at presentand hence all dates are treated with caution. The age profilecreated for Vatnamyri incorporates the radiocarbon date, andtwo equations are used to create the VAT3 age model

(Fig. 2b). These reflect assumed changes in sedimentation ratefollowing deglaciation and development and stabilisation ofsoils and vegetation within the catchment area of the lake.Support for these assumptions comes from comparing thepalaeoecological data with those from other early Holocenesites (Rundgren, 1999) where very similar radiocarbon agesare found and similar ecosystem developments identified.

3. Laboratory and statistical analyses

A range of palaeoecological and sedimentologicalanalyses were performed on the sediments. Total carbon(C) and total nitrogen (N) were measured using a CarloErba Elemental Analyser model NA2500. Pollen analysisutilised subsamples of up to 1 cm3, which were processedusing standard procedures (Moore et al., 1999) at varyingintervals through the cores, usually 2 cm but at 1 cm belowSaksunarvatn, and tablets of Lycopodium spores wereadded to facilitate calculation of pollen concentrations.Local pollen assemblage zones have been defined usingCONISS (Fig. 3a and b). Morphometric data werecollected for all measureable Betula grains and results forHamundarstajahals have been published (Caseldine,2001). On the diagrams there is a single curve includingall Betula with inferences concerning the relative impor-tance of tree/shrub/dwarf birch made in the text.Chironomid analysis followed standard preparation

methods by deflocculating sediments in 10% KOH andmounting head capsules in Hydromatrixs (Brooks andLangdon, in prep.). Sample sizes of up to 1 cm3 were used,with a minimum of 50 head capsules being counted persample, enough for robust statistical inferences (Heiri andLotter, 2001; Quinlan and Smol, 2001). Taxonomicidentification followed Brooks and Langdon (in prep.).Local chironomid assemblage zones were defined and arepresented with the chironomid diagrams (Fig. 4a and b).Mean July air temperature estimates were reconstructed

from the chironomid data using a transfer functiondeveloped from a contemporary training set of 52 lakesin NW Iceland (Langdon, unpublished data). A weightedaverage partial least squares (WA-PLS) model was used, ofwhich the second component performed best, withr2jack ¼ 0.66 and RMSEPjack ¼ 1.10 1C. Detrended corre-spondence analysis (DCA) was undertaken on bothchironomid datasets in order to assess whether patternsof taxa change were similar to reconstructed temperatures.DCA was performed using CANOCO 4.5.2 (ter Braak andSmilauer, 1998) and used square-root transformation ofspecies data and down-weighting of rare taxa.

4. General biostratigraphic patterns and trends

4.1. Pollen

Both the taxa represented and the general patterns ofchange are very similar to those found at other sites in the

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540

560

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600

620

2020

4020

4060

8020

4060

8020

4020

4060

8020

2040

6080

100

200

300

Betula

Depth

Age cal. years

BP

Sorbus type

Salix

Juniperus communis

Poaceae undiff.

Cyperaceae undiff.Empetru

mEric

ales Angelica sylve

stris

LactuceaeAsteroideae

Armeria

mariti

ma

Caltha palustris

Caryophyllaceae

Campanulaceae

Chenopodiaceae

Brassicaceae

Filipendula

Mentha type

Koenigia islandica

Oxyria Plantago mariti

ma

Potentilla ty

pe

Polygonum aviculare

Polygonum bistorta

Rhinanthus type

Ranunculaceae

RosaceaeRubiaceaeRumex a

cetosa

Rumex acetosella

Saxifragaceae

Sedum Succisa Thalictru

mUrtic

a type

ApiaceaeVicia undiff.

Botrychium lu

naria

Equisetum

Lycopodium annotin

um

Huperzia selago

Lycopodium undiff.

PolypodiumDryopteris

felix

mas type

Selaginella selaginoides

Pteropsida (monolete) u

ndet.

Myriophyllu

m alterniflo

rum

Potamogeton natans type

Typha angustifolia

/Sparganium

Hippuris vulgaris

Isoëtes echinospora

SphagnumPinus s

ylvestris

Ulmus Alnus g

lutinosa

Corylus ave

llana

Pollen concentra

tion

Zon

e

HH

8 i

HH

8 ii

HH

8 iii

HH

8 iv

HH

8 v

HH

8 vi

HH

8 vi

i

AB

CL

E

8000

8500

9000

9500

1000

0

1050

0

Sa

ks

un

arv

atn

te

ph

raS

ak

su

na

rva

tn t

ep

hra

(a)

Fig.3.Pollen

diagramsfor(a)HH8and(b)VAT.Arrangem

entoftaxafollowsHallsdottir(1987):A—

dry

landtaxa;B—

sporesofpteridophytes;C—

aquaticplants;E—

Sp

ha

gn

um;L—exotictaxanot

foundin

indigenousIcelandic

flora.Taxaare

calculatedas

A¼

%S

A;

B¼

%S

A+

B;

C¼

SA+

B+

C;

E¼

SA+

E;

L¼

SA+

L.Concentrationisin

grains�103cm�3.

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0

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740

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2040

6020

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8020

4060

8010

020

2020

2020

2020

4020

2020

4060

8020

4060

8010

020

2020

100

300

500

Vat

iV

at ii

Vat

iii

Vat

iv

Vat

v

Vat

vi

Vat

vii

Vat

viii

Vat

ix

Betula

Juniperus communis

Salix

Sorbus typePoaceae undiff.

Cyperaceae undiff.

Calluna vulgaris

Empetrum Pyrola Ericales Apiaceae

AsteroideaeBrassicaceae

Campanulaceae

Caryophyllaceae

Centaurea type

Chenopodiaceae

LactuceaeLamiaceaeRanunculaceae

RosaceaeRubiaceaeSaxifra

gaceae

Armeria

maritima

Artemisia

\Caltha palustris type

Cirsium type

Dryas octopetala

Gentianella campestris type

Filipendula

Helianthemum

Hydrocharis

Koenigia islandica

Lotus Lychnis flos-cuculi

Mentha type

Oxyria Parnassia palustris

Plantago maritima

Plantago lanceolata

Polygonum bistorta

Polygonum viviparum

Potentilla type

Rhinanthus type

Rubus typeRumex acetosa

Rumex acetosella

Rumex/Oxyria

Rumex undiff.

Sagina Saxifraga nivalis

Saxifraga stellaris type

Saxifraga oppositifo

lia type

Sedum Thalictrum

Trifolium type

Urtica dioica

Viola palustris type

Botrychium lunaria

EquisetumHuperzia selago

Lycopodium annotinum

Lycopodium undiff.

Cryptogramma crispa

PolypodiumPteropsida (m

onolete) undet.

Selaginella selaginoides

Thelypteris palustris

Isoëtes echinospora

Littorella uniflo

ra

Myriophyllum alterniflo

rumMyrio

phyllum spicatum

Myriophyllum undiff.

Potamogeton natans type Hippuris vulgarisMenyanthes SphagnumUnidentifie

d

Alnus Corlyus Hedera PinusQuercus Pollen concentra

tion

Zon

e

Vat

nam

ýri

AB

CL

E

Sparganium / Typha angustifo

lia

Depth

Age cal. years

BP

7500

8000

8500

9000

9500

1000

0

1050

0

Sa

ks

un

arv

atn

te

ph

raS

ak

su

na

rva

tn t

ep

hra

(b)

Fig.3.(C

on

tin

ued)

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ropelo

pia

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Procla

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Tany

podin

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Coryn

oneur

a sc

utel

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Cricot

opus

pul

chrip

es ty

pe

Cricot

opus

sylve

stris

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Cricot

opus

trem

ulus

type

Lim

nophy

es

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Orthoc

ladius

cons

obrin

us typ

e

2040

Psect

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dellu

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osm

ittia

Smitt

ia Orthoc

ladiin

ae u

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.

20

Chiron

omus

ant

hrac

inus t

ype

Chirono

min

i larv

ula

2040

Dicrot

endi

pes

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60

Tany

tars

us lu

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type

20

Parat

anyt

arsu

s pen

icilla

tus t

ype

2040

Micr

opse

ctra

spp

Tany

tars

ini u

ndiff

.

100

200

300

400

500

Conce

ntra

tion

Zon

e

HH

8-5

HH

8-4

HH

8-3

HH

8-2

HH

8-1

Chi

rono

mid

ae

8000

8500

9000

9500

1000

0

1050

0

Depth

(cm

)

Age ca

l. yea

rs B

P

Sa

ks

un

arv

atn

te

ph

raS

ak

su

na

rva

tn t

ep

hra

(a)

Fig.4.Chironomid

assem

blages

(a)HH8and(b)VAT2/3.Percentages

are

oftotalnumber

ofheadcapsulesidentified.

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Parochlus kieffe

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AblabesmyiaMacro

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Tanypodinae undiff.

Corynoneura lobata

Corynoneura scutellata Cric

otopus pulchrip

es type

Cricotopus s

ylvestris

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Krenosmittia

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ARTICLE IN PRESSC. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–2331 2323

region; and nomenclature and diagram constructionfollowed Hallsdottir (1987).

Below the Saksunarvatn tephra, pollen concentrationsare generally very low (generally o500 grains cm�3), and arange of open ground taxa predominate, such as Poaceae,Cyperaceae and Salix (probably Salix herbacea). Empe-

trum and Ericales undiff., probably mainly Vaccinium sp.,appear before the deposition of the tephra but all tree taxarepresented almost certainly derived from outside Iceland.Overall the patterns are very similar between the sites.Pollen concentrations gradually increase after Saksunar-vatn (up to 10,000–16,000 grains cm�3 in HH8, generallynearer 10,000 grains cm�3 at Vatnamyri; 500–800 and1000 grains cm�2 yr�1, respectively), although there is quitea lot of variability with occasional much higher valuestending to correlate with periods of higher %C and short-term slowing of the sedimentation rate. Nevertheless, theseinflux rates compare well with the limited data oncontemporary deposition rates in comparable environ-ments (Hicks, 2001). Low pollen concentrations are usuallydue to samples including quantities of tephra. As values forBetula and other shrub taxa rise there is a reduction inspecies diversity, especially at Vatnamyri, with the smallerbasin at Hamundarstajahals reflecting the local persistenceof a greater range of low-growing taxa as seen in almostcontinuous curves for Brassicaceae, Ranunculaceae andRubiaceae. Because both sites represent open-water en-vironments there are no problems with over-representationof local terrestrial taxa, but aquatic taxa are wellrepresented, especially Isoetes echinospora at HH8 andMyriophyllum and Potamogeton at Vatnamyri, the formerpredominantly Myriophyllum alterniflorum.

4.2. Chironomids

Both chironomid profiles (Fig. 4a and 4b) show cleartrends from being dominated by cold stenotherms at thebase, such as Tanytarsus lugens-type, Orthocladius conso-

brinus-type and Micropsectra spp., to a dominance ofrelatively more thermophilic taxa towards the top, such asParatanytarsus penicillatus-type, Dicrotendipes, and Chir-

onomus anthracinus-type. It is also clear from bothchironomid diagrams that there are rapid fluctuations inrelative percentage taxa abundance superimposed on thesegeneral trends, as indicated in the mean July temperaturereconstructions (Fig. 5) and DCA axis 1 scores (Fig. 6).The reconstructions and DCA results (Fig. 6) agreeremarkably well, indicating that early Holocene fluctua-tions in chironomid assemblages were most likely driven bytemperature, or another environmental factor that co-varied with temperature during this period.

There are some interesting faunal differences betweenthese sites and the only other early Holocene chiron-omid stratigraphy from Iceland, that from Efstadalsvatn(Caseldine et al., 2003). The most noticeable difference isthe lack of Heterotrissocladius grimshawi at both Hamun-darstajahals and Vatnamyri (there are very small amounts,

o3%, at 2 levels from Vatnamyri), whereas at Efstadals-vatn this taxon formed a large part of the fauna post-Saksunarvatn. This could be due to colonisation routes,which may also explain the later arrival of some Cricotopus

taxa, (cf. Efstadalsvatn—Caseldine et al., 2003), althoughthis could also be related to temperature. H. grimshawi iscommon in contemporary Icelandic lakes (Hrafnsdottiret al., 2000; Hrafnsdottir, 2005; Langdon, unpublisheddata) and thus there might be other reasons why it is notpresent in HH8 and Vatnamyri. One other point of notefrom the chironomid stratigraphies is an unusual faunalcomposition at 610 cm (ca 9.6 kcal. yr BP) from Vatnamyri.This level is dominated by Paraphaenocladius and Limno-

phyes, and also includes other semi-terrestrial taxa such asKrenosmittia and Parasmittia. The associated rapid rise in%C and increase in pollen concentration (Fig. 5) indicatethat the site may have dried out, or water depth and/or lakehydrology was severely altered at this time, as indicated bythe increased presence of the semi-terrestrial chironomidtaxa. This leads to what is likely to be an anomalously highchironomid-inferred temperature (CI-T) for this level(Fig. 5). The following level shows chironomid assemblages,%C and pollen concentrations all returning to previousvalues, suggesting that the perturbation was probably shortlived and did not affect the lake development significantly.

5. Climatic and environmental change in north Iceland

10.8–7 kcal. yr BP

5.1. Ca 10.8–10.2 kcal. yr BP (pre-Saksunarvatn)

Both sites show very similar patterns in the pollen record(Figs. 3a, b and 5) from the earliest period of organicdeposition following local deglaciation to the deposition ofthe thick Saksunarvatn tephra. The first record for anyvegetation cover reveals a very open grass-dominatedlandscape with a range of open ground taxa characteristicof disturbed environments: Caryophyllaceae (over 20% atVatnamyri), Rumex/Oxyria, Apiaceae, Saxifragaceae, Hu-

perzia selago, Brassicaceae and occasional grains ofKoenigia islandica as examples. It is possible that theVatnamyri record is slightly earlier, for at Hamundarsta-jahals Salix was already well established by the start of therecord whilst absent at the former. This could however alsorepresent the influence of proximity to wasting ice marginswith ice persisting in the higher basin. The presence ofpollen from ‘exotic’ trees including Pinus, Betula and Alnus

in the very lowest sediments is taken as indicative of verylow local pollen production (influx estimates are almostcertainly o500 grains cm�2 yr�1 despite dating uncertain-ties) with significant representation of pollen from extremelong distance sources outside Iceland. Both the dating andthe nature of the vegetation support the Preboreal ice limitsproposed by Norjdahl and Petursson (2005), whosereconstruction of ice margins in the area indicate that icemust have covered or lain very close to both sites duringthe preceding Younger Dryas.

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Fig. 5. Summary diagrams for principal pollen taxa (SBetula%, Juniperus% and TLP concentration in grains cm�3), %C, C/N, CI-T): (a) HH8 and (b)

VAT2/3. Note that CI-T values are for the site and have not been adjusted to sea level.

C. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–23312324

In Vat (ii) the gradual expansion of Salix occurs inparallel with sedges and ericaceous taxa, a pattern virtuallyidentical to that at Hamundarstajahals, such that by thetime of the deposition of the Saksunarvatn tephra there is asimilar more stable landscape around both sites, with dry

areas dominated by dwarf shrubs, sedges in extensivedamper habitats and a much reduced presence of openground herb taxa. The shift in vegetation reflectingassumed warming conditions with less evidence forperiglacial ground disturbance can also be seen at other

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Fig. 6. Summary curves for HH8 and VAT2/3 (SBetula, DCA axis 1 scores and CI-T).

C. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–2331 2325

Icelandic sites in the same period with the main shift tomore stable conditions occurring around 10.8 kcal. yr BP(Rundgren, 1999).

Apart from the very lowest sample from Vatnamyriwhere temperature estimation should be treated withcaution as it is based on the presence of only two taxa,the chironomid records estimate a July temperature of6–7 1C at both sites during the earliest phase of organicdeposition (Figs. 5 and 6), although at Vatnamyri the moredetailed record suggests an initial phase of quite rapidtemperature rise from below this figure. Overall though,temperatures only rose slowly with some evidence,particularly from Hamundarstajahals, for lower tempera-tures around 10.3 kcal. yr BP, dipping below 7 1C. By thetime of the deposition of the Saksunarvatn tephra at10.2 kcal. yr BP, July temperatures of 7 1C are indicated atboth sites. This results from a gradual reduction in relativeabundance of the cold stenotherms, notably T. lugens-typeand O. consobrinus-type, even though the overall warmingis represented by slightly different fauna at each site.

Neither site has very high %C in the sediments duringthis period, although there was higher local productivity atVatnamyri (up to 5%) with the presence of Potamogeton

and sporadic M. alterniflorum, the latter reinforcing thechironomid interpretation of July temperatures at least ashigh as 7.5 1C (Aalbersberg and Litt, 1998).

5.2. 10.2—9 kcal. yr BP

Both sites show quite complex changes in the localvegetation, and by inference from both this and the

chironomid record imply a period of significant climaticchange and instability. It is difficult to interpret the degreeto which the deposition of the tephra affected the localvegetation. Both profiles have thick tephra deposits, almost1m at Hamundarstajahals and up to 30 cm in cores fromVatnamyri, and there has been reworking in the upperparts of the units making recovery of primary post-eruption pollen assemblages difficult. Apart from somereduction in the principal taxa and enhanced occurrencesof open ground elements in Vat (iv) that may representpollen within the reworked tephra layer, there is really verylittle evidence to suggest that tephra deposition had anylasting radical effect. The flora characteristic of the pre-eruption landscape quickly recovered. Pollen concentra-tions similarly quickly return to pre-eruption levels.The main feature of the pollen record is a period of

dwarf-shrub (Salix-Empetrum) and Poaceae/Cyperaceaedominance at both sites that predates the deposition ofthe light tephra at around 9.7 kcal. yr BP at Hamundar-stajahals. This tephra is not found in the core examined forVatnamyri but has been discovered in other cores from thebasin, usually about 15–20 cm above the Saksunarvatntephra (Wastl, 2000). At both sites (HH8 (iv); Vat (vi))frequencies for Betula then rise, either with or followed byJuniperus, representing the spread of dwarf birch-domi-nated heath (as seen in the Betula morphometric data(Caseldine, 2001)), although pollen concentrations only riseslowly. Both Betula and Juniperus peak at ca 9.7 kcal.yr BP. The chironomid temperature reconstructions sup-port this trend of warming climate reaching 8–9 1C at thesame time, a gradual rather than abrupt change inferred

ARTICLE IN PRESSC. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–23312326

mainly on the basis of the replacement of T. lugens-type byrelatively more thermophilic taxa, such as Psectrocladius atboth sites. At Vatnamyri there is also a local expansion ofthe aquatic flora with a shift from Potamogeton to M.

alterniflorum.This is followed by a very similar sequence in the pollen

records at both sites with the dwarf shrubs almostdisappearing and sedges expanding. At Vatnamyri it isclear that the site dries out as %C values rise to 33% andthere is a change in the chironomid fauna to semi-terrestrial taxa. This change leads to an apparent rise inthe temperature reconstruction but as the transfer functionis based on lake data this can be seen as an artefact of theprocedure, not a genuine rise in temperature. At Hamun-darstajahals where water depths are not obviously affectedthere is a drop in temperature of almost 1.5 1C by9.5 kcal. yr BP. This period of retrogressive vegetationdevelopment, reduced temperatures and increased drynessprobably lasted only 100–200 yr at most, the brief record atVatnamyri reflecting very slow sedimentation due to dryingout. By just after 9.5 kcal. yr BP both Betula (predomi-nantly B. nana) and Juniperus had recovered. CI-T stillonly rose erratically and appeared highly unstable atVatnamyri in particular until around 9 kcal. yr BP whentemperatures possibly reached between 9–10 1C.

Overall the period between 9.5–9 kcal. yr BP saw vegeta-tion recover and stabilise with a period of maximumdevelopment for juniper growing in an upright form abovethe dwarf birch allowing for greater production anddispersal of pollen. At Vatnamyri, frequencies reached20% suggesting increased importance with altitude, achiev-ing levels of pollen representation not seen in Icelandduring the rest of the Holocene. Whether it was favouredby the particular climatic regime of the time, with enhancedseasonality-driven continentality allowing it to outcompeteother dwarf shrubs in the absence of taller tree birches, isuncertain, but this phase can be seen at a range of sitesacross Iceland. At Vatnskotsvatn to the west (Hallsdottir,1995) there is a comparable Juniperus phase, and on thenorth of the Skagi peninsula at Torfadalsvatn Rundgren(1999) identified a phase with persistent high Juniperus

values after 9 kcal. yr BP, although there are datingproblems in this part of the core. In NW Iceland atEfstadalsvatn juniper also peaks between 9.5–9 kcal. yr BP,although in this area it also persists for much longer in theabsence of competition from tree birches (Caseldine et al.,2003). In both areas the CI-T data support the idea thatJuly temperatures were not yet high enough for tree birchesto successfully spread (see below).

At around 9 kcal. yr BP the chironomid data show anisolated peak of higher temperatures, 10 1C at Hamundar-stajahals and 11 1C at Vatnamyri, although the latterreconstruction may have been influenced by the shallowernature of the site. The similarity between the sites (Fig. 6) isbelieved to reflect a genuine climatic effect. Examination ofthe single assemblage on which the reconstruction is basedat Vatnamyri shows that the sample is composed of several

semi-terrestrial taxa (although in relatively small abun-dances), which produced an artificially high temperatureearlier around 9.6 kcal. BP. The basins themselves showdifferent characteristics during this period with Hamun-darstajahals dominated by Isoetes suggesting a very clearsmall lake, with a depth of around 2m (Hannon andGaillard, 1997), possibly slightly deeper (Schierup et al.,2002), producing sediments with o10%C, and Vatnamyrishowing further shallowing with Potamogeton becomingdominant and %C values steadily rising to 20% by9 kcal. yr BP.

5.3. 9–8 kcal. yr BP

By 9 kcal. yr BP Juniperus had disappeared from Vatna-myri and had almost gone from Hamundarstajahals.Pollen concentrations at both sites, although showing alot of inter-sample variability, were relatively stable andhigher than in the previous millennium. At the lower siteSalix remained an important part of the local flora inassociation with Betula, grasses and sedges whilst atVatnamyri the pollen assemblages are dominated by Betula

and Cyperaceae. What this represents in terms of thespread of birch woodland is open to question and haspreviously been discussed for Hamundarstajahals byCaseldine (2001). Measurement of pollen grains of Betula

in these and subsequent levels, and comparison with datafrom current Icelandic birch woodland and from Scandi-navian sites purportedly having comparable tree species(Betula pubescens ssp. tortuosa), show very clearly thatduring this period, full birch woodland dominated by treesthe same as those comprising the present Icelandic birchwoodland had not yet occurred. Diameters reflect probableshrub birch (i.e. diameters had not reached valuescomparable with tree birches today although there was ashift to larger diameters at 9 kcal. yr BP (Caseldine, 2001)).It is though noticeable that by this date CI-T for July hadreached a value considered suitable for tree growth (seebelow). Thus the record from 9kcal. yr BP to at least8.4 kcal. yr BP appears to represent a time when birch wasspreading as a tree, possibly hybridising leading toextensive lower-growing shrub birches and eventuallybecoming dominant within the landscape. Because of thelack of any direct size equivalence in the pollen of treebirches between the Early and Middle Holocene andcurrent trees, it is not known where, and when, the originaltree forms arrived in Iceland. It is possible, althoughconsidered unlikely, that tree birches did arrive before theYounger Dryas and survived in very favourable locations(Rundgren and Ingolfsson, 1999) but there is no directevidence for this from the few sites covering this period.Similarly there is no direct evidence at any site for any earlypost-Younger Dryas arrival of trees. Until more sites havebeen examined for both pollen and macrofossils thisproblem will remain unsolved.At 8.4 kcal. yr BP at Hamundarstajahals, there is a sharp

rise in percentages for Betula (up to 80%), with more

ARTICLE IN PRESSC. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–2331 2327

consistent and larger mean grain diameters (although still3-4 mm lower than in current tree populations (Caseldine,2001)), as woodland completely dominated the immediatearea. At Vatnamyri, birch percentages drop due to theexpansion of sedge-dominated peat across the basinisolating the coring site further and further away fromthe nearest woodland, hence determining the actual localimportance of woodland is difficult. On the western slopesof Eyjafjorjur it appears that just after 9 kcal. yr BP ashrub (tree?) birch community became established, but stillwithin a mosaic of developing low-lying bogs (the originalhallamyri) in the valley floors, and other dwarf-shrubcommunities on higher valley sides, presumably havingout-competed the previously significant juniper.

Temperatures between 9–8.5 kcal. yr BP based on thechironomid data show a consistent pattern with the initialpeak preceding a long period of reduced temperatures,down as low as 8.5 1C by 8.5 kcal. yr BP, only returning tolevels as high as 11 1C by 8.1–8.2 kcal. yr BP, althoughreconstructions from Vatnamyri are complicated by thisdate due to terrestrialisation of the site and the spread ofpeat. The consistency of the pattern from the chironomidfauna, however, underlines the duration of the period ofthe suppressed temperatures over several centuries and,notwithstanding any concerns over the validity of thechronology, it is apparent that reduced temperaturesoccurred well before any ‘8.2k event’. It should perhapsbe noted that at both sites there is a very small dip inreconstructed temperatures close to 8.2 kcal. yr BP but thisis more of a retardation of a rising temperature signal byonly 0.5 1C rather than any major temperature decline andin neither site is it possible to identify any short-termvegetation response. Such variations are also within theerror terms of the transfer function.

The apparently limited effect of the ‘8.2 k event’ in anarea close to one of the strongest signals (the GISP andGRIP/NGRIP records from the Greenland ice cap)appears surprising, but the chironomid data emphasisethat 8.2 k was embedded in a longer period of lowerJuly temperatures. This longer phase of reduced tempera-tures, albeit with some variability, is now well establishedfrom a range of records summarised in Rohling and Palike(2005). Furthermore if recent interpretations of the 8.2 ksignal are correct (Alley and Agustdottir, 2005; Denton etal., 2005) then the impact was predominantly on wintertemperatures. It would be possible therefore for recon-structed July temperatures not to reflect any significantchange, especially if the event was of relatively shortduration, and for the local vegetation cover to barelyregister any change, apart from an inability for trees toexpand further.

5.4. Post 8 kcal. yr BP

There are only a few levels representative of the periodafter 8 kcal. yr BP. At Hamundarstajahals up to the changeto peat after 7.4 kcal. yr BP birch levels remain high, and at

Vatnamyri despite the difficulties in interpretation due topeat expansion birch is still important at this time. CI-T areat their highest at both sites around 8 kcal. yr BP at 11 1C,reflecting a dominance of Dicrotendipes and Paratanytarsus

penicillatus type at Hamundarstajahals and increases inParatanytarsus at Vatnamyri. There is some slight indica-tion of temperature lowering by 7.6 kcal. yr BP with a dip inbirch but the evidence for this is slight. On the basis of allthe evidence from the two sites it is argued thattemperatures were at their highest in the early to middleHolocene by 8 kcal. yr BP with a relatively stable vegetationcover of birch woodland dominating the drier areas,representing both the HTM and the nearest to thermalmaximum equilibrium vegetation conditions that probablyoccurred during the Holocene.The results from Eyjafjorjur are supplemented at this

time by tree-line studies from 450m at Vesturardalur at thehead of Skıjadalur (Wastl et al., 2001). Although anisolated find of macrofossils of tree birch occurred before8 kcal. yr BP the real development of woodland at thisaltitude, as represented both by pollen and macrofossils,only occurred after 7.6 kcal. yr BP, and lasted until around6.8 kcal. yr BP, with a short dip around 7 kcal. yr BP, thedating of this being based both on radiocarbon and thepresence of the Hekla 5 tephra. Rather than assume thatthere are problems with the dating at one of the locations itis argued that woodland spread to higher altitudes afterstabilisation at the lower sites and that it is possible that thereal HTM occurred slightly after the chironomid recordterminates at Hamundarstajahals and Vatnamyri, i.e.between 7.6–6.8 kcal. yr BP.Support for this hypothesis also comes from using the

birch pollen/macrofossil data from Vesturardalur toreconstruct temperatures. Birch is strongly correlated withsummer temperatures at the tree line in Scandinavia(Odland, 1996; Mook and Vorren, 1996), whether Julytemperature alone or tri- or tetratherms. Even assumingthat Icelandic tree birch differed to some extent fromScandinavian species it is probable that its behaviour at thetree line was affected by similar thermal factors. In simplestterms, with a 10 1C July isotherm as the limiting factor anda lapse rate of 0.65 1C100m�1, the presence of trees at450m would mean a July temperature of 12.3 1C at thelowest of the sites, Hamundarstajahals, about a degreeabove the optimum reconstructed temperature. Althoughwithin the error of the CI-T reconstruction, the delay inmigration to the higher site is taken as representing aslightly higher HTM value for the area than the youngestchironomid-reconstructed figure, with a sea-level Julytemperature of 13 1C between 7.5–6.8 kcal. yr BP, 2.5 1Chigher than the 1961–90 mean for the area (Bjornsson,2003). The similarity between the birch-derived estimateand the inferred chironomid-reconstructed data addsweight to the overall interpretation providing the firstquantitative estimates of Early and Middle Holocenetemperature for Iceland, incorporating what is believed tobe the HTM.

ARTICLE IN PRESSC. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–23312328

To the west of Trollaskagi at Vatnskotsvatn (Fig. 1)Betula pollen concentrations peaked around 8 kcal. yr BPbefore falling off towards low later Holocene levels after7 kcal. yr BP (Hallsdottir, 1995), causing Hallsdottir toremark that ‘‘during the time interval of 7200 (8000) to6000 (7000) yr BP [14C yr] the landscape of northernIceland was as close to being forested as it ever became’’(p. 22). Further to the west a similar pattern is seen atHafratjorn (Vasari and Vasari, 1990) although the spreadof tree birches was later, and on the NW Peninsula birchonly really peaked at 7 kcal. yr BP (Caseldine et al., 2003).This delay and the relatively low values for Betula pollen incomparison with the north, albeit from a much larger lakesystem, suggest either that birch had physical problemsmigrating into the northwest, or that temperatures werelower than in the Skagi-Trollaskagi area. This may haveprecluded successful tree birch colonisation, perhapsleading to enhanced hybridisation with dwarf birch, aphenomenon still observable at present in the area(Elkington, 1968). Differences of up to 3 1C in CI-T areapparent between Efstadalsvatn and Trollaskagi at thistime, the latter being warmer, although the 1961–1990observed temperatures overlap. It seems increasingly likelyfrom the palaeoecological evidence that there wereenhanced temperature gradients during the first half ofthe Holocene between the two areas and that theyinfluenced patterns of vegetation colonisation, with currentspatial temperature patterns only developing as Holoceneclimate deteriorated after around 6 kcal. yr BP. The limiteddensity of pollen sites does, however, also show differencesin birch pollen representation within regions, as seen forinstance in the pollen diagram for Hella Arskogsstrond, apeat site that lies just above sea level on the western edge ofEyjafjorjur (Hallsdottir and Caseldine, 2005), whereBetula pollen percentages were very low for much of theearly Holocene despite the clear presence of dense wood-land nearby at Hamundarstajahals.

6. Early Holocene climate change and links with offshore

and wider records

The climate of Iceland has been closely linked to thenature and relative strength of ocean currents derivingfrom the south west, the Irminger current that is a warmsaline branch of the North Atlantic Current, and from thenorth, the East Greenland Current originating in the ArcticOcean that brings much colder and fresher water from thenorth, particularly affecting north and north west Iceland(Jonsson and Valdimarsson, 2005). Over the historicalperiod, the contrast has been manifested by varyingamounts of sea-ice impacting on the Icelandic coast leadingto severe climate, especially in summer, and hardship forhuman settlement (Ogilvie, 1984, 1997). Considerableattention has been paid to the Weichselian and Holoceneevolution of marine conditions to the west and north ofIceland following several successful coring cruises,although Andrews and Giraudeau (2003) have pointed

out the paradox of a lack of agreement between differentproxies, in their case for productivity and sea surfacetemperature (SST). Climate on Iceland might be expectedto respond principally to SST and the interplay between thedominance of northern and southern (western) currents,especially in terms of responses at the timescales identifiedin the pollen and chironomid record rather than longermillennial periodicities.The initial warming seen in both records on Trollaskagi,

and echoed elsewhere in northern Iceland, was clearly aresponse to warmer SSTs around Iceland identified at ca11 kcal. yr BP in various records (Knudsen et al., 2004;Andersen et al., 2004), representing the appearance ofwarm Atlantic water in the Irminger current and probablypreventing the formation of annual sea ice. The slight dropin temperatures around 10.3 kcal. yr BP, which althoughwithin the error terms of the reconstruction is in both theTrollaskagi and Efstadalsvatn chironomid records, corre-lates well with IRD event 7 (VM 29-1910 (Bond et al.,1997)), with a drop in temperature seen in the marinerecord north of Iceland (Castaneda et al., 2004), withincreased basalt/plagioclase indicative of enhanced IRD offNW Iceland (Andresen et al., 2005) (Fig. 7), with coolingidentified in the Faroes and other locations by Bjorck et al.(2001) and Hannon et al. (2003), as well as in the GISP andGRIP ice cores where there are lower accumulation rates atthe same time (Fig. 7) (Grootes et al., 1993). Despite theproximity to the potential source of cold water flowingdown through the Denmark Strait and the widespreadsignature of this event, it should be noted that Icelandicterrestrial ecosystems barely registered any change; therewas for instance only a slight drop in %C values in thelakes, and July temperatures fell by less than a degree, i.e.conditions tended to stabilise rather than continue toimprove.Looking at marine records for the period between

10–7.5 kcal. yr BP, the general signal is one of a persistentadvection of warm Atlantic water modulated by fluctua-tions of up to 2–3 1C in SST (Andrews and Giraudeau,2003; Knudsen et al., 2004; Smith et al., 2005), the latterassumed to represent inflow of cold arctic water due toenhanced melt events, supplemented by IRD evidence(Andresen et al., 2005). The period of drier conditions withlower temperatures seen on land centred on 9.6 kcal. yr BPwas possibly triggered by ocean cooling dated to 9.8 kcal.yr BP (Andersen et al., 2004) with the persistence ofnortherly airstreams significantly reducing precipitationover Iceland. This period is particularly evident in veryhigh plagioclase/basalt ratios off NW Iceland centred on9.6–9.4 cal. yr BP (Fig. 7), interpreted as indicating highIRD rates bringing material from eastern Greenland due toincreased northwesterly/northerly wind frequency. Thesubsequent recovery of SST is matched by a recovery interrestrial temperatures and the short duration temperature‘spike’ seen in the chironomids has a parallel in the NorthAtlantic deepwater (NAD) coccolith record from Hunafloi(Andrews and Giraudeau, 2003), reconstructed August

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5

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Fig. 7. Comparison of CI-T records from HH8 and VAT2/3 with: d18O in

foraminifera from B997-329 (Castaneda et al., 2004) (Fig. 1); GISP2

accumulation rate (Grootes et al., 1993); basalt/plagioclase ratios in the

63–100mm fraction off NW Iceland (Andresen et al., 2005).

C. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–2331 2329

SSTs from the North Iceland shelf (Andersen et al., 2004)and in the Greenland ice core temperature record (Alley,2000).

After 9 kcal. yr BP a continuation of warm, but notoptimal marine conditions on the N. Iceland shelf, is seenin all records; conditions were warmer than before with theconsistent presence of N. Atlantic water, but not at HTMlevels. Castaneda et al. (2004) point out that although the‘8.2 k event’ registers as slightly heavier d18O values on theN. Iceland shelf the change is relatively slight, becomingmore noticeable with distance offshore and in sites furtherto the east (Eirıksson et al., 2000a, b; Knudsen et al, 2004)and south (Andersen et al., 2004). Andersen et al. (2004)similarly noted little change in surface ocean temperaturesbut a clear signal in oxygen isotope values and radiolarian-based records indicative of changes in deeper ocean waterand the degree of stratification, with a decoupling ofsurface and sub-surface temperatures. As it is likely that

terrestrial climate is closely linked to surface oceanconditions then it is perhaps not surprising that the lackof an oceanic surface response leads to little terrestrialvariation. The relationship of these marine records to theterrestrial evidence reinforces the view that in general theHTM (at least as reflected in July temperature) had notbeen reached in Northern and NW Iceland during thisperiod and that temperatures were still not stable with thebrief effect of the ‘8.2 k event’ barely impacting onecosystem development as conditions were still only justsuitable for a slow successional change allowing woodland,or at best shrubby woodland, to develop.Castaneda et al. (2004) argue that the marine HTM on

the N. Iceland shelf occurred between 7.8–6.2 kcal. yr BP,dates that agree well with the terrestrial evidence presentedhere for the onshore record, although Andersen et al.(2004) argue that the Irminger Current as measured on theReykjanes Ridge south of Iceland was strongest only after7 kcal. yr BP. Thus the HTM appears to have developedvirtually immediately after any ‘8.2 k’ impact and generallypersisted for between 1–1.5 ka, albeit with possible furtherminor coolings over this period. This was therefore a timewhen temperatures were not only warmer than the presentboth offshore and on land, by between 2–3 1C, but alsorelatively stable and by the end of this period woodlanddeveloped to its maximum extent across northern Icelandmarking optimal sustained Holocene vegetation develop-ment. These results therefore provide a much clearerdefinition of the timing and nature of the HTM in Icelandthan previously published (Kaufman et al., 2004).

7. Conclusions

In comparing the terrestrial and offshore reconstructedtemperature conditions for the Early to Middle Holocene,it is possible to determine close links between SSTs andJuly air temperatures reflecting the changing directinfluences of warm Atlantic and cold Arctic currents onterrestrial climate in Iceland. The relative importance ofthese currents was to some extent modified by orbitally-enhanced seasonality, but it is difficult to infer annual andwinter temperatures for the land because the availablebiological proxies are mainly influenced by summerconditions. Nevertheless, all records clearly demonstratesome temperature variability in the Early Holocene, withina generally rising trend, which eventually leads to HTM ofconditions warmer than today after 8–7.8 kcal. yr BP thatlasted for about 1–1.5 ka, at least 2 ka later than HTMconditions at higher Arctic latitudes (Kaufman et al.,2004).

Acknowledgements

We thank Hans Stotter and Maria Wastl from theUniversity of Innsbruck for coring the site with studentsfrom the University of Munich and for all their assistanceand helpful discussions; the NERC RCLSC for awarding

ARTICLE IN PRESSC. Caseldine et al. / Quaternary Science Reviews 25 (2006) 2314–23312330

the 14C determinations and Charlotte Bryant and the RCLin East Kilbride for their help and support; AslaugGeirsdottir from the University of Iceland, Giff Millerfrom INSTAAR and RANNIS for field support indeveloping the chironomid-based transfer function; ArtAmes for the laboratory work in Exeter; Jackie Hatton forsome of the pollen analyses and Sue Rouillard forpreparing the diagrams. Matts Rundgren and Les Cwynarprovided valuable comments on an earlier version of themanuscript. Naomi Holmes was supported by NERC(Award No. NER/S/A/2002/10368A).

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