Abstract
Ecosystem variability must be assessed over a range of timescales in order to fully understand natural ecosystem processes. Long-term climate change, at millennial and centennial scales, is a major driver of natural ecosystem variability, but identifying evidence of past climate change is frequently confounded by human-induced impacts on the ecosystem. Iceland is a location where it is possible to separate natural from anthropogenic change in environmental archives, as the date of settlement is accepted to be around AD 874, prior to which the island was free from proven human impacts. We used a lake sediment core from Breiðavatn, near Reykholt, a major farm of the Norse period in western Iceland, to examine landscape development. A change in pollen concentration in the sediments, especially the decline in Betula, indicated initial landscape degradation immediately post-settlement, whereas the chironomid fauna and reconstructed temperatures were relatively complacent during this period. The pollen evidence is corroborated by 14C analyses, which indicate an increase in older carbon entering the lake, inferred to have been caused by increased erosion following settlement. Further decreases in Betula pollen occurred around AD 1300, pre-dating a drop in chironomid-inferred temperatures (CI-T) of ~1°C over 100–200 years. The CI-T reconstruction also shows a significant cooling after ~AD 1800, likely indicative of the coldest phase of the Little Ice Age. The evidence suggests that the chironomid record was relatively unaffected by the increased landscape degradation and hence reveals a temperature reconstruction independent of human impact.
Similar content being viewed by others
Explore related subjects
Discover the latest articles, news and stories from top researchers in related subjects.Avoid common mistakes on your manuscript.
Introduction
Iceland is an island in the North Atlantic, and its climate is influenced by changes in both atmospheric and oceanic circulation patterns (Andrews and Giraudeau 2003). It is a key locality for identifying past variations in Holocene climate change (Jiang et al. 2002, 2005; Caseldine et al. 2003, 2006; Dugmore et al. 2005, 2007). If the detection of past climate changes can be clearly distinguished from human impacts, then their differential effects on the landscape can be determined (Ólafsdóttir et al. 2001; Ólafsdóttir and Guðmundsson 2002; Haraldsson and Ólafsdóttir 2003). Humans only arrived in Iceland relatively recently—approximately AD 874. The Norse colonisation (known as landnám or land take) caused major impacts on the natural environment. Human land-use practices and introductions of grazing animals, particularly sheep, precipitated catastrophic environmental change (Buckland et al. 1990; Edwards et al. 2005), including widespread, irreversible erosion (Ólafsdóttir and Guðmundsson 2002). Vegetation simulations suggest that total vegetation cover fell from 52% of Iceland’s surface area at landnám to 28% in 1990, and forest cover (Betula pubescens >2 m tall) fell at an even greater rate, from ~7 to <1% of Iceland’s surface area for the same period (Ólafsdóttir et al. 2001). This rate may have been even greater, as landnám-time vegetation cover has been estimated elsewhere at ~68%, with that of birch woodland alone at around 27% of the total land area (Arnalds 2005).
A core was taken from the small, upland lake of Breiðavatn, and the subfossil chironomids and pollen were examined (Erlendsson 2007) to investigate climate change during the past two millennia, as well as to examine the effects of human colonisation near the historically important settlement of Reykholt in the Borgarfjörður region of West Iceland. The chitinous head capsules of chironomid larvae are well preserved in lacustrine sediments, and identification is usually possible at least to genus and sometimes to species group and/or morphotype (Brooks et al. 2007). A range of studies on contemporary chironomid communities has shown that temperature is often the primary driver of chironomid distribution in the absence of other significant environmental factors that are more often associated with human impact, such as increased eutrophication and hypoxia (Brodersen and Quinlan 2006; Walker and Cywnar 2006; Brooks 2006). Development of chironomid-temperature transfer functions (e.g. Larocque et al. 2001; Brooks and Birks 2004) provides good estimates of the temperature optima for most taxa, facilitating temperature reconstructions from fossil assemblages. This approach has been shown to give good estimates of recent climate, particularly mean July air temperatures (Larocque and Hall 2003). Improvements in taxonomic resolution (Rieradevall and Brooks 2001; Brooks et al. 2007), larger training sets (e.g. Barley et al. 2006) and a better understanding of chironomid ecologies, are also improving the precision of Holocene temperature reconstructions (but see Velle et al. 2005a).
A multi-proxy approach was employed here. Pollen spectra from Breiðavatn were used to examine the vegetation changes since the early settlement period. This was coupled with loss-on-ignition (LOI), sediment magnetic profiles and 14C dating as aids to indicate erosion that was a consequence of human landscape impacts within the catchment area. Analyses of the fossil chironomid sequence allowed us to test whether landnám or any subsequent human impacts had a direct effect on chironomid communities (Edwards et al. 2007; Lawson et al. 2007). Given that anthropogenic effects appeared to be negligible, we were able to utilise a recently developed Icelandic chironomid-temperature transfer function (Langdon et al. 2008) to produce a temperature reconstruction for the last two millennia.
Methods
Study site
Breiðavatn (64°40′415″N 21°15′043″W) is a small oval lake, ca. 2 km northeast of the hamlet of Reykholt, approximately 100 m above sea level and situated on a low ridge just beyond the Reykholtsdalur watershed (Fig. 1). The surrounding land is used as summer pasture, and the ground surface is much disturbed by þúfur (frost hummocks). The lake has a surface area of ~3.5 ha and a fairly flat bottom with a maximum water depth of 80 cm. There are no visible inflow or outflow streams and the lake does not dry out in summer (local sources). The nearest weather station in current use, Stafholtsey (14 m a.s.l.; ~16 km west of Breiðavatn) has a mean July temperature (1989–2001) of 11°C and receives mean precipitation of at least 768 mm per annum, although Einarsson (1992) states that because of the poor design of Icelandic rain gauges, the actual figure is likely to be around 25% higher.
According to the Book of Settlements (Benediktsson 1968), the Reykholtsdalur area was settled extensively. Although not noted as a major early farm, Reykholt had become an important ecclesiastical and political centre by ~AD 1000 and it maintained its status through many centuries (Sveinbjarnardóttir et al. 2007). The environmental record from the farmed areas close to Reykholt also contains clear evidence for agricultural impact in the early settlement (Norse) period (Erlendsson 2007). Farm prosperity was maintained by extensive and diverse utilisation of natural resources. This is indicated by mixed livestock and numerous shielings, summer pastures, and woodland exploitation far beyond the homefields (Þorláksson 2000; Eyþórsson 2007). Arable activity and iron-smelting appear to have been practised at least during the medieval period (Erlendsson 2007; Sveinbjarnardóttir et al. 2007). It is likely that the concentration of settlement in the area and the apparent intensity of agriculture would have put severe pressure on vegetation and soils over the wider area.
Field and laboratory methods
Three overlapping core segments were taken from a boat near the middle of the lake using a Russian-type corer (Jowsey 1966) 50 × 8 cm in size, in a water depth of 80 cm, down to 120 cm below the mud/water interface. In addition, the uppermost sediments were sampled with a gravity corer, extruded on site (although the almost liquid unconsolidated uppermost 3 cm of sediment could not be sampled), and stored at 4°C. Core sections were X-rayed and core segments were aligned using stratigraphic changes and tephra bands. All depths presented are from the top of the solid sediment.
Sediments were described using a modification of the Troels-Smith system (Aaby and Berglund 1986). Samples of measured volume were analysed for bulk dry density and LOI (Dean 1974). These samples were generally of 1 cm3 at 1 cm contiguous intervals. Relative changes in magnetic susceptibility were measured at low frequency (0.46 kHz) using a Bartington Instruments MS-2 metre and MS-2b probe, with measurements at 1 cm contiguous intervals.
Core chronology
A combination of tephrochronology and 14C dating was used to build a chronology. Too few terrestrial plant macrofossils were found in the core for dating purposes and seven samples of bulk sediment (1 cm3 gyttja per sample) were used (Table 1). The 14C dates were calibrated using OxCal, version 3.10 (Bronk-Ramsey 2005). Of the eight tephra samples that underwent geochemical analysis, only two provided sufficiently secure tephra dates. The lower (121–119 cm) is by far the most prominent tephra layer in the core. Geochemical analysis confirmed its Katla origin (Erlendsson 2007). Judging by the known tephra stratigraphy for the area, this is the Katla-E tephra, the younger of two prominent, closely-spaced Katla tephras commonly found in western and southwestern Iceland. It is chronologically constrained between the Hekla-C (~870 cal BC [calibrated 14C date from Kjartansson et al. 1964]) and the Hekla-3 (~1050 cal BC [calibrated 14C date from Dugmore et al. 1995]) tephras (Róbertsdóttir 1992). In the absence of a more rigorous chronological precision for this tephra date, it seems reasonable to round the age for Katla-E to 1000 cal BC. The tephra layer at 41–40.5 cm has also been analysed for geochemistry (Erlendsson 2007). It is the so-called Landnám (Veiðivötn/Torfajökull) tephra couplet, dated on Greenland ice core evidence to AD 871 ± 2 (Grönvold et al. 1995). Interpolation between the tephra-derived dates suggests that the results from 14C dating consistently show ages that are too old, even before landnám and its associated environmental disruption. Immediately after landnám, the 14C dates become inverted, probably due to soil erosion (either from water-borne or aeolian inputs) that allowed older carbon to enter the lake (cf. Edwards and Whittington 2001). As a consequence, reliance was placed on the two identified tephras for the construction of an age-depth model. The uppermost recovered sample is estimated to date to ~AD 1900 based on linear interpolation between the tephras and extrapolation to the surface sediments (Table 1, Fig. 2). Whilst two dates for the past three millennia may not provide the most robust chronology, we can at least be reasonably confident in these two tephra dates and the date of AD 1900 seems sensible. Interpolated dates will have some associated uncertainty, as sedimentation rates are unlikely to have been linear throughout this period. Other studies on Holocene Icelandic lake sequences (e.g. Axford et al. 2007; Lawson et al. 2007), however, have demonstrated linear sedimentation rates throughout this period.
Chironomids
Chironomids were extracted from the sediment at 4–8 cm intervals, using the methods proposed by Lang et al. (2003). The 23 samples were deflocculated for 15 min in 10% KOH solution at 85°C, and sieved through a 90 μm mesh. The residue was put into 100 ml water in a sonic bath for 10 seconds, re-sieved, and the residue sorted using a grooved perspex sorting tray under a binocular microscope at 35× magnification. Chironomid heads were mounted using Hydro-Matrix®. They were identified using Hofmann (1971), Wiederholm (1983), Rieradevall and Brooks (2001) and Cranston (1982), and related to the Norwegian and Icelandic training sets at The Natural History Museum, London and the University of Southampton. Tanytarsini were identified using an unpublished key (Brooks unpubl.) and chironomid taxonomy was synthesised with Brooks et al. (2007). Chironomid distribution was expressed as percentages of the total head count and was plotted using PSIMPOLL (Bennett 2008a). Principal references for ecological information were Wiederholm (1983), Cranston (1982), Moller-Pillot and Buskens (1990) and Brooks et al. (2007). A mean July air temperature reconstruction was carried out using a two-component WA-PLS model with r 2jack = 0.66 and RMSEP = 1.095°C from an Icelandic training set of 52 lakes from northwestern Iceland (Langdon et al. 2008). All fossil taxa were present in the model. In order to investigate the relationships between the chironomid data and temperature, detrended correspondence analysis (DCA) was carried out using the CANOCO program version 4.51 (ter Braak and Smilauer 1998) with rare species downweighted.
Pollen analysis
Sub-samples for pollen analysis were treated by standard chemical (10% HCl, 10% NaOH, 40% HF, acetolysis) and sieving (180 μm sieves) methods (Fægri and Iversen 1975; Moore et al. 1991). Tablets of Lycopodium clavatum spores were added to each sample (Stockmarr 1971) enabling the calculation of palynomorph concentrations. The residue was mounted in silicone oil of 12,500 cSt viscosity. At least 300 indigenous terrestrial pollen grains were counted for each sample with primary identification based on Moore et al. (1991) and the University of Aberdeen pollen type slide collection. Pollen of Betula pubescens is separated from B. nana by size, where pollen ≥20 μm is attributed to B. pubescens and smaller grains to B. nana (Mäkelä 1996; Caseldine 2001; Karlsdóttir et al. 2007). The construction of pollen diagrams was undertaken using TILIA (Grimm 1991) and TGView (Grimm 2004) with palynomorphs expressed as percentages of the total land pollen (TLP) sum. Pollen and spore taxonomy follows Bennett (2008b) with a few amendments (Erlendsson 2007). Diagram zonation, with three local pollen assemblage zones (BR1-I to BR1-III) was aided by the use of the CONISS routine within TILIA.
Results
Chironomids
A total of 19 chironomid morphotypes were found throughout the profile (Fig. 3), which was dominated by Psectrocladius sordidellus type, Chironomus, Dicrodentipes and Tanytarsus lugens type. There are relative peaks in Dicrotendipes and Ablabesmyia from 32–24 cm (c. AD 1100–1300), which are among the more thermophilous Icelandic taxa according to Langdon et al. (2008), although abundances of Dicrodentipes and Psectrocladius sordidellus type fall from 16 cm (c. AD 1500), with Dicrodentipes having disappeared from the assemblage in the topmost sample. Meanwhile, taxa that are more commonly found in relatively cooler Icelandic lakes such as Micropsectra, Tanytarsus lugens type and Heterotrissocladius grimshawi type (ibid.) all increase after this point. The shallow water or semi-terrestrial Limnophyes also becomes more abundant in the top 8 cm.
Much of the core is characterised by relatively low concentrations of chironomid head capsules, with the majority of samples from 0–50 cm depth containing ~5–15 head capsules per cm3 (Fig. 3). Because of this, several samples from different depths have been amalgamated to bring the number of head capsules per sample up to ≥50, recognised as being the minimum number required for a reliable temperature reconstruction (Heiri and Lotter 2001; Larocque 2001; Quinlan and Smol 2001). Head capsule concentration is highest at the bottom of the core, and declines with time (Fig. 3). The total number of chironomid taxa found per sample changes little throughout the core.
Temperature reconstruction
The temperature reconstruction (Fig. 5) suggests that mean July temperatures were fairly stable at around 9°C for most of the period covered by this core. Some increases in temperature (~0.5°C) are evident at 72 cm and also between 39 and 32 cm. Temperature declines at 16 cm (c. AD 1500), where the mean July air temperature reconstruction falls by about a degree to 8°C. There is a short period where conditions were warmer at 3 and 4 cm (AD 1825 and 1800), and then they decline again, to the coldest point in the core, at the top (7.2°C).
The DCA results show that axis 1 explains 49% of the variance of the species data, with an eigenvalue of 0.227 (Table 2). The temperature reconstruction and DCA axis 1 scores show a similar pattern, indicating that temperature was most likely the dominant environmental variable affecting the chironomid assemblages in this core.
Pollen
The pre-landnám part of the core (zone BR1-I) is characterised by high relative abundances of Betula pubescens pollen (Fig. 4), with some B. nana and lesser quantities of Salix. Among herbaceous taxa, Cyperaceae and Poaceae are well represented. B. pubescens declines in relative abundance at 92 cm, and Thalictrum alpinum makes its first appearance at this time. By 60 cm, the relative frequency of B. pubescens is back to previous levels. Cyperaceae were relatively abundant in the pre-landnám era, as was Sphagnum. After landnám, values for well-preserved pollen of B. pubescens show a sudden decline (zone BR1-II), and between 26 and 23 cm they fall to very low levels (zone BR1-III), which persist to the top of the profile. The apparent birch decline follows a rise in values for microscopic charcoal. Also around 24 cm, Thalictrum alpinum, Galium and Plantago maritima increase in relative abundance, as do the number of Pteropsida (monolete) indet. spores and, strikingly, the representation for degraded tree birch pollen. Pollen concentrations increase above 20 cm, suggesting that the rising values for these taxa are not an artifact of the percentage calculation.
Sediment
Magnetic susceptibility and LOI co-vary, largely inversely, through the pre-landnám section of the core (Fig. 4). There is a single spike in magnetic susceptibility before landnám at 92 cm. At 24 cm, magnetic susceptibility rises, and this increase is maintained (apart from a brief drop in the thick, fibrous, organic layer at 8 cm) to the top of the core. LOI oscillates around ~25% for the majority of the sequence, with brief declines (not associated with tephras) around 107, 92, 33 and 24 cm, and a significant peak associated with the thick, fibrous, organic layer at 8 cm.
Discussion
Results from the lake sediment sequence from Breiðavatn suggest that the landscape was subject to human impact, notably just after landnám when there was a significant decline in local woodland and a commensurate expansion of sedges. The 14C dating provides further evidence for post-settlement catchment changes. Until landnám, the 14C bulk sediment dates were all offset by ~300 years compared to the tephra dates (Fig. 2). This may be due to a natural reservoir effect in the lake, and/or inputs of older carbon to the lake. The fact that the offset is relatively consistent throughout ~2000 years (although only three dates are involved) may indicate that the processes controlling it probably changed little over this period. This relationship alters at landnám, where the 14C date from immediately below the landnám tephra is ~400 years older than the tephra, and the disparity between dates increases through time (Fig. 2). The increase in the proportion of older carbon entering the lake suggests that the surrounding organic soil/peat was being eroded, and older material was exposed and washed or blown into the lake (cf. Edwards and Whittington 2001). The evidence from the lake sediments thus suggests that considerable changes to the environment, particularly increased erosion, followed the arrival of humans at the end of the 9th century AD. Introduction of grazing animals (especially sheep), and clearance of woodland are thought to be the major causes of this environmental change (cf. Hallsdóttir 1987).
The chironomid fauna show little variation over this period, with an increase in the relatively thermophilous Dicrotendipes only occurring long after both landnám and the change in vegetation, suggesting that the chironomid fauna was not affected directly by the initial impacts of settlement. There was, however, an increase in relative abundance of Pediastrum (Fig. 4), a green alga used as an indicator of nutrient levels (Lawson et al. 2005, 2007; Woodward and Shulmeister 2005) just after landnám from the start of zone BR1-II, but it did not rise above pre-landnám levels. This is quite unlike the situation in Helluvaðstjörn (also a small, spring-fed lake, in the north of Iceland, near Mývatn), where erosion of the landscape led to increased deposition of nutrient-rich matter into the lake and a locally unprecedented rise in Pediastrum (Lawson et al. 2007). In Helluvaðstjörn, although there was no change to the chironomid assemblage, increases in nutrients reaching the lake allowed total chironomid abundance, previously nutrient-limited, to increase dramatically (ibid.). The chironomid assemblage from Breiðavatn shows no sign of any changes associated with increased nutrient levels—indeed, taxa that are associated with increased macrophyte abundance (a potential result of increased nutrients in an oligotrophic system) such as Dicrotendipes, Ablabesmyia and Cricotopus (Brodersen et al. 2001; Langdon et al. in press) are all reduced throughout this phase (39-37 cm). The chironomid DCA axis 1 scores closely follow the temperature reconstruction throughout the core (Fig. 5), and the other axes explain very little of the variation in the assemblage, indicating that the assemblage composition is most likely driven largely by temperature. Hence this temperature reconstruction can be considered independent of human-induced landscape change. It appears that the erosional areas are too distant for significant nutrients to be added to the waters of Breiðavatn. Damaged pollen and spores, minerogenic soil, and deposits containing old carbon have been blown and/or washed into the lake, but not sufficiently to cause any noticeable changes to the lake biota investigated.
The chironomid-inferred temperature reconstruction shows that from c. 800 BC until c. AD 1000 (32 cm), mean July air temperatures around Breiðavatn were about 9°C, with only small (~0.5°C) fluctuations and a very slight decreasing trend. The 0.5°C fluctuations are within the boundaries of the transfer function model error, and hence must be treated with some caution, especially as there is little change in the DCA axis 1 scores over this period. More significantly though, there is an increase in relatively thermophilous taxa between 32 and 24 cm (c. AD 1100–1300), which corresponds with a rise in temperature of just <1°C. The magnitude and timing of this phase is very similar to other records from the region (Andresen and Björck 2005; Jiang et al. 2002, 2005), reflecting conditions analogous to the Medieval Warm Period (MWP). Additionally, cooler temperatures around AD 1000 (34 cm) agree with the reconstruction of Jiang et al. (2005) that show this period, though generally warmer than average, underwent several rapid cooling events.
From around AD 1200 (28 cm) the relative abundance of Betula pubescens pollen began to fall, and by AD 1300 (24 cm), it was at very low levels (<20%). The birch was replaced by open environment taxa (e.g. Thalictrum alpinum, Galium, Plantago maritima). Relative abundances of degraded birch pollen and Pteropsida (monolete) indet. spores also increase, as does magnetic susceptibility, again indicating increased erosion. The relatively low resolution of the climate reconstruction over this period means it is difficult to invoke a purely climatic cause for this second, stepped decline in birch woodland (the first being just after landnám), although there was perhaps an increase in summer air temperature variability throughout these centuries (Fig. 5). The decline in woodland does not appear to be associated directly with climate cooling.
After AD 1500 (16 cm), tree birch levels had declined to negligible levels (<5% TLP) and remained so throughout the rest of the profile. At this time there is a significant decline in mean summer temperatures and (with the exception of two data points at 4 and 3 cm) the cooler climate remains until the top of the core. This is inferred to reflect the Little Ice Age (LIA). This temperature decline is the most severe in the whole Breiðavatn core, and evidence in the Denmark Strait area indicates that it was the most severe cooling period during the whole of the latter part of the Holocene (Andresen and Björck 2005). The Breiðavatn sequence is one of the few chironomid-inferred reconstructions that show an unambiguous LIA record (Velle et al. 2005a, 2005b; Brooks and Birks 2004). This is likely a result of the fact that the lake’s topographic position means that there was no significant change in productivity after landnám. As the effects of changes in lake productivity and temperature on chironomid assemblages are often autocorrelated (Brodersen and Quinlan 2006), it is likely that any increase in productivity would have obscured the apparent effects of the LIA temperature decline. In fact, the generally unchanging nature of this lake’s waters as well as the close correlation between DCA axis 1 scores and the temperature reconstruction, indicate that temperature is probably the key factor affecting the chironomid species assemblage, and that the reconstructed temperatures (or the trends) are reliable.
In common with other records (e.g. McKinzey et al. 2005), it appears that around Breiðavatn, there were periods of climate amelioration during the LIA, principally around the first half of the 19th century. This has been found in other studies, with Cook et al. (2002) reporting a generally positive trend in temperatures associated with changes in the North Atlantic Oscillation during the first part of the century, glaciers undergoing a retreat in the southeast (McKinzey et al. 2005), and the earliest instrumental records of temperature at Stykkishólmur showing a higher than average July temperature from about 1835–55 (Icelandic Meteorological Office). The second half of the nineteenth century, however, showed a marked temperature decline (McKinzey et al. 2005; Icelandic Meteorological Office), and the top sample of the Breiðavatn core (around AD 1900) shows the lowest reconstructed temperature at 7.2°C.
Although widespread erosion has been attributed to the LIA (Ólafsdóttir and Guðmundsson 2002; McKinzey et al. 2005), it appears that most of the changes that occurred in the Breiðavatn catchment (increased erosion and the reduction of forest cover) were set in motion before the LIA and some had occurred during the latter stages of the inferred MWP. Although generally warmer than average, the MWP can actually be characterised as a period of quite unstable climate, with frequent colder spells, as illustrated by the finer-resolution climate reconstruction in Jiang et al. (2005). The relatively low-resolution independent climate record from the chironomids does suggest more unstable climate in the last millennium compared with the previous two millennia. It is difficult, however, to resolve from these data what impact climate change may have had on promoting landscape erosion directly, as the catchment had been subjected to human impact. Most likely there is an interaction between humans and climate with respect to the magnitude of landscape erosion. For example, farmers tend to optimise stocking densities commensurate with minimal environmental degradation. With sudden cooling after a period of relative warmth, it is probable that stocking levels would not be reduced immediately to a sustainable level, allowing overgrazing to facilitate erosion. Of course, the extent of erosion around Breiðavatn was likely to have been compounded by other anthropogenic factors, such as over-exploitation of remaining woodland for fuel and charcoal production (Smith 1995; cf. Church et al. 2007).
Increases in magnetic susceptibility and the proportion of degraded Betula pollen throughout the uppermost core samples indicate that erosion was even more severe during the LIA. The LIA temperature decline probably compounded the negative effect of soil loss due to erosion on the agricultural economy around Breiðavatn. Mean July temperatures were at least 1°C below the pre-LIA average for most of this period (Fig. 5). This temperature drop would have generated a decline in primary productivity equivalent to a ~180 m increase in altitude, thus reducing the carrying capacity in the rangeland surrounding the lake by about 10–20% (Haraldsson and Ólafsdóttir 2003). Grazing pressure is unlikely to have dropped during this period of declining rangeland area, and this would have resulted in overgrazing and increased erosion. Although short-lived warmer episodes (e.g. around AD 1800) allowed re-expansion of farms to previously abandoned areas, these were often abandoned again as the climate worsened (McKinzey et al. 2005). Since the beginning of the 20th century, the climate has been warming. This warming and the projected 21st century increase in temperature, if coupled with cautious management of livestock densities, could allow re-vegetation of eroded land (Haraldsson and Ólafsdóttir 2003).
Conclusion
Sediment variables investigated in this lake core responded very differently to the environmental changes of the past two and a half millennia. Palynological evidence shows that the largest change to the landscape around Breiðavatn was anthropogenic, associated with post-landnám woodland clearance. The 14C evidence indicates increased post-landnám erosion, leading to the degraded environment found today. The chironomids, however, seem to have been unaffected by human-induced environmental changes, and their assemblages were influenced primarily by climate. The ability to separate climate-driven from human-induced impacts on the aquatic and terrestrial environments allowed the development of a chironomid-inferred temperature reconstruction that was relatively uncomplicated by anthropogenically-modified environmental variables. Both climate variability and human impact are likely to have contributed to the landscape erosion around Breiðavatn, with a ~1°C decline in July temperature during the LIA, which likely reduced primary productivity and would have had a profound effect on the region’s farming community.
References
Aaby B, Berglund BE (1986) Characterization of peat and lake deposits. In: Berglund BE (ed) Handbook of Holocene Palaeoecology and Palaeohydrology. John Wiley and Sons, Chichester, pp 231–246
Andresen CS, Björck S (2005) Holocene climate variability in the Denmark Strait region. Geogr Ann 87:159–174. doi:10.1111/j.0435-3676.2005.00250.x
Andrews JT, Giraudeau J (2003) Multi-proxy records showing significant Holocene environmental variability: the inner N. Iceland shelf (Hunafloi). Quat Sci Rev 22:175–193
Arnalds A (2005) Approaches to landcare–a century of soil conservation in Iceland. Land Degrad Dev 16:113–125
Axford Y, Miller GH, Geirsdóttir Á, Langdon PG (2007) Holocene temperature history of northern Iceland inferred from subfossil midges. Quat Sci Rev 26:3344–3358. doi:10.1016/j.quascirev.2007.09.003
Barley EM, Walker IR, Kurek J, Cwynar LC, Mathewes RW, Gajewski K, Finney BP (2006) A northwest north American training set: distribution of freshwater midges in relation to air temperature and lake depth. J Paleolimnol 36:295–314. doi:10.1007/s10933-006-0014-6
Benediktsson J (ed) (1968) Íslensk fornrit I–Landnámabók. Hið íslenska fornritafélag, Reykjavík
Bennett KD (2008a) Psimpoll and pscomb programs for plotting and analysis. http://www.chrono.qub.ac.uk/psimpoll/psimpoll.html (last accessed 16 May 2008)
Bennett K (2008b) Catalogue of pollen types. http://www.chrono.qub.ac.uk/pollen/pc-intro.html (last accessed: 16 May 2008)
Brodersen KP, Odgaard B, Vestergaard O, Anderson NJ (2001) Chironomid stratigraphy in the shallow and eutrophic Lake Søbygaard, Denmark: chironomid-macrophyte co-occurrence. Freshw Biol 46:253–267. doi:10.1046/j.1365-2427.2001.00652.x
Brodersen KP, Quinlan R (2006) Midges as palaeoindicators of lake productivity, eutrophication and hypolimnetic oxygen. Quat Sci Rev 25:1195–2012. doi:10.1016/j.quascirev.2005.03.020
Brooks SJ (2006) Fossil midges (Diptera: Chironomidae) as palaeoclimatic indicators for the Eurasian region. Quat Sci Rev 25:1894–1910. doi:10.1016/j.quascirev.2005.03.021
Brooks SJ, Birks HJB (2004) The dynamics of Chironomidae (Insecta: Diptera) assemblages in response to environmental change during the past 700 years on Svalbard. J Paleolimnol 31:483–498. doi:10.1023/B:JOPL.0000022547.98465.d3
Brooks SJ, Langdon PG, Heiri O (2007) The identification and use of Palaearctic Chironomidae larvae in palaeoecology. QRA Technical Guide No. 10. Quaternary Research Association, London
Bronk-Ramsey C (2005) OxCal Version 3.10 http://www.rlaha.ox.ac.uk/O/oxcal.php
Buckland PC, Dugmore AJ, Perry DW, Savory D, Sveinbjarnardóttir G (1990) Holt in Eyjafjallasveit, Iceland. A paleoecological study of the impact of Landnám. Acta Archaeol 61:252–271
Caseldine C (2001) Changes in the Holocene record from Iceland–a palaeoclimatic record or evidence for early Holocene hybridisation? Rev Pal Pal 117:139–152
Caseldine CJ, Geirsdóttir Á, Langdon PG (2003) Efstadalsvatn–a multi-proxy study of a Holocene lacustrine sequence from NW Iceland. J Paleolimnol 30:55–73. doi:10.1023/A:1024781918181
Caseldine CJ, Langdon PG, Holmes N (2006) Early Holocene climate variability and the timing and extent of the Holocene Thermal Maximum (HTM) in Northern Iceland. Quat Sci Rev 25:2314–2331. doi:10.1016/j.quascirev.2006.02.003
Church MJ, Dugmore AJ, Mairs KA, Millard AR, Cook GT, Sveinbjarnardóttir G, Ascough PA, Roucoux KH (2007) Charcoal production during the norse and early medieval periods in Eyjafjallahreppur, southern Iceland. Radiocarbon 49:659–672
Cook ER, D’Arrigo RD, Mann ME (2002) A well-verified, multiproxy reconstruction of the winter North Atlantic oscillation index since A.D. 1400. J Clim 15:1754–1764. doi:10.1175/1520-0442(2002)015<1754:AWVMRO>2.0.CO;2
Cranston PS (1982) A key to the larvae of the British Orthocladinae (Chironomidae). Scientific publication 45, Windermere, Freshwater Biological Association
Dean WE (1974) Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: comparison with other methods. J Sediment Petrol 44:242–248
Dugmore AJ, Cook GT, Shore JS, Newton AJ, Edwards KJ, Larsen G (1995) Radiocarbon dating tephra layers in Britain and Iceland. Radiocarbon 37:379–388
Dugmore AJ, Buckland PC, Church M, Edwards KJ, Lawson I, McGovern TH, Panagiotakopulu E, Simpson IA, Skidmore P, Sveinbjarnardóttir G (2005) The Norse landnám on the North Atlantic islands: an environmental impact assessment. Polar Rec (Gr Brit) 41:21–37. doi:10.1017/S0032247404003985
Dugmore AJ, Borthwick DM, Buckland PC, Church MJ, Dawson AG, Edwards KJ, Keller C, Mayewski P, McGovern TH, Mairs K-A, Sveinbjarnardóttir G (2007) The role of climate in settlement and landscape change in the North Atlantic islands: an assessment of cumulative deviations in high resolution proxy climate records. Hum Ecol 35:169–178. doi:10.1007/s10745-006-9051-z
Edwards KJ, Lawson IT, Erlendsson E, Dugmore AJ (2005) Landscapes of contrast in Viking Age Iceland and the Faroe Islands. Landscapes 6:63–81
Edwards KJ, Langdon PG, Sugden H (2007) Separating climatic and possible human impacts in the early Holocene: biotic response around the time of the 8200 cal. yr BP event. J Quat Sci 22:77–84. doi:10.1002/jqs.1018
Edwards KJ, Whittington G (2001) Lake sediments, erosion and landscape change during the Holocene in Britain and Ireland. Catena 42:143–173. doi:10.1016/S0341-8162(00)00136-3
Einarsson MA (1992) Climatic conditions of the Thingvallavatn area. Oikos 64:96–104. doi:10.2307/3545045
Erlendsson E (2007) Environmental change around the time of the Norse settlement of Iceland. Unpublished PhD thesis, University of Aberdeen
Eyþórsson B (2007) Búskapur og rekstur staðar í Reykholti. Unpublished MA thesis, University of Iceland
Fægri K, Iversen J (1975) Textbook of Pollen Analysis, 3rd edn. Blackwell Scientific Publications, Oxford
Grimm EC (1991) TILIA and TILIA GRAPH. Illinois State Museum, Springfield
Grimm EC (2004) TGView. Illinois State Museum, Springfield
Grönvold K, Óskarsson N, Johnsen J, Clausen HB, Hammer CU, Bond G, Bard E (1995) Ash layers from Iceland in the Greenland GRIP ice core correlated with oceanic and land sediments. Earth Planet Sci Lett 135:149–155. doi:10.1016/0012-821X(95)00145-3
Hallsdóttir M (1987) Pollen analytical studies of human influence on vegetation in relation to the Landnám Tephra layer in Southwest Iceland. Lundqua Thesis, Lund University, Lund
Haraldsson HV, Ólafsdóttir R (2003) Simulating vegetation cover dynamics with regards to long-term climatic variations in sub-Arctic landscapes. Global Planet Change 38:313–325
Heiri O, Lotter AF (2001) Effects of low count sums on quantitative environmental reconstructions: an example using subfossil chironomids. J Paleolimnol 26:343–350
Hofmann W (1971) Zur Taxonomie und Palökologie subfossiler Chironomiden (Dipt.) in Seesedimenten. Arch Hydrobiol 6:1–50
Icelandic Meteorological Office http://www.vedur.is/ (last accessed 30 May 2008)
Jiang H, Seidenkrantz M-S, Knudsen KL, Eiríksson J (2002) Late-Holocene summer sea-surface temperatures based on a diatom record from the north Icelandic shelf. Holocene 12:137–147
Jiang H, Eiríksson J, Schulz M, Knudsen KL, Seidenkrantz M-S (2005) Evidence for solar forcing of sea-surface temperature on the North Icelandic Shelf during the late Holocene. Geology 33:73–76. doi:10.1130/G21130.1
Jowsey PC (1966) An improved peat sampler. New Phytol 65:245–248. doi:10.1111/j.1469-8137.1966.tb06356.x
Karlsdóttir L, Thórsson ÆT, Hallsdóttir M, Sigurgeirsson A, Eysteinsson T, Anamthwhat-Jónsson K (2007) Differentiating pollen of Betula species from Iceland. Grana 46:78–84. doi:10.1080/00173130701237832
Kjartansson G, Þórarinsson S, Einarsson Þ (1964) C14-datings on samples regarding Icelandic Quaternary geology. Náttúrufræðingurinn 34:98–145
Lang B, Bedford AP, Richardson N, Brooks SJ (2003) The use of ultrasound in the preparation of carbonate and clay sediments for chironomid analysis. J Paleolimnol 30:451–460
Langdon PG, Holmes N, Caseldine CJ (2008) Environmental controls on modern chironomid faunas from NW Iceland and implications for reconstructing climate change. J Paleolimnol 40:273–293. doi:10.1007/s10933-007-9157-3
Langdon PG, Ruiz Z, Wynne S, Sayer CD, Davidson TA Ecological influences on larval chironomid communities in shallow lakes: implications for palaeolimnological interpretations. Freshw Biol (in press)
Larocque I (2001) How many chironomid head capsules are enough? A statistical approach to determine sample size for palaeoclimatic reconstructions. Palaeogeogr Palaeoclimatol Palaeoecol 172:133–142. doi:10.1016/S0031-0182(01)00278-4
Larocque I, Hall RI, Grahn E (2001) Chironomids as indicators of climate change: a 100-lake training set from a subarctic region of northern Sweden (Lapland). J Paleolimnol 26:307–322. doi:10.1023/A:1017524101783
Larocque I, Hall RI (2003) Chironomids as quantitative indicators of mean July air temperature: validation by comparison with century-long meteorological records from northern Sweden. J Paleolimnol 29:475–493. doi:10.1023/A:1024423813384
Lawson IT, Church MJ, McGovern TH, Arge S, Woollet J, Edwards KJ, Gathorne-Hardy FJ, Dugmore AJ, Cook G, Buckland PC, Mairs K-A, Thomson AM, Sveinbjarnardóttir G (2005) Mutual adjustment of environment and society on Sandoy, Faroe Islands: palaeoenvironmental and archaeological perspectives. Hum Ecol 33:651–684. doi:10.1007/s10745-005-7681-1
Lawson IT, Gathorne-Hardy FJ, Church MJ, Newton AJ, Edwards KJ, Dugmore AJ, Einarsson Á (2007) Environmental impacts of the Norse settlement: palaeoenvironmental data from Mývatnssveit, northern Iceland. Boreas 36:1–19. doi:10.1080/03009480600827298
Mäkelä EM (1996) Size distribution between Betula pollen types–a review. Grana 35:248–256
McKinzey KM, Olafsdottir R, Dugmore AJ (2005) Perception, history, and science: coherence or disparity in the timing of the Little Ice Age maximum in southeast Iceland? Polar Rec (Gr Brit) 41:319–334. doi:10.1017/S0032247405004687
Moller-Pillot HKM, Buskens RFM (1990) De larven der Nederlandse Chironomidae (Diptera). C: Autoekologie en verspreiding. Nederlandse Faunistische Mededelingen 1C:1–277
Moore PD, Webb JA, Collinson ME (1991) Pollen Analysis, 2nd edn. Blackwell Science, Oxford
Ólafsdóttir R, Guðmundsson HJ (2002) Holocene land degradation and climatic change in northeastern Iceland. Holocene 12:159–167. doi:10.1191/0959683602hl531rp
Ólafsdóttir R, Schlyter P, Haraldsson HV (2001) Simulating Icelandic vegetation cover during the Holocene. Implications for long-term land degradation. Geogr Ann 83:203–215. doi:10.1111/j.0435-3676.2001.00155.x
Quinlan R, Smol JP (2001) Setting minimum head capsule abundance and taxa deletion criteria in chironomid-based inference models. J Paleolimnol 26:327–342. doi:10.1023/A:1017546821591
Rieradevall M, Brooks SJ (2001) An identification guide to subfossil Tanypodinae larvae (Insecta: Diptera: Chironomidae) based on cephalic setation. J Paleolimnol 25:81–99. doi:10.1023/A:1008185517959
Róbertsdóttir BG (1992) Gjóskulagatímatal fyrir Suðurlandsundirlendi. Forsöguleg gjóskulög frá Kötlu, áður nefnd Katla 5000. In Veggspjaldaráðstefna Jarðfræðafélags Íslands, 28 Apríl 1992. Jarðfræðafélag Íslands, Reykjavík, pp 8–9
Smith KP (1995) Landnám: the settlement of Iceland in archaeological and historical perspective. World Archaeol 26:319–347
Stockmarr J (1971) Tablets with spores used in absolute pollen analysis. Pollen Spores 13:614–621
Sveinbjarnardóttir G, Erlendsson E, Vickers K, McGovern TH, Milek K, Edwards KJ, Simpson IA, Cook G (2007) The palaeoecology of a high status Icelandic farm. Environ Archaeol 12:197–216
ter Braak CJF, Smilauer P (1998) CANOCO reference manual and user’s guide to Canoco for windows: software for canonical community ordination (version 4.5.1). Microcomputer Power, Ithaca
Velle G, Brooks SJ, Birks HJB, Willassen E (2005a) Chironomids as a tool for inferring Holocene climate: an assessment based on six sites in southern Scandinavia. Quat Sci Rev 24:1429–1462. doi:10.1016/j.quascirev.2004.10.010
Velle G, Larsen J, Eide W, Peglar SM, Birks HJB (2005b) Holocene environmental history and climate of Råtåsjøen, a low-alpine lake in south-central Norway. J Paleolimnol 33:129–153. doi:10.1007/s10933-004-2689-x
Walker IR, Cwynar LC (2006) Midges and palaeotemperature reconstruction–the north American experience. Quat Sci Rev 25:1911–1925. doi:10.1016/j.quascirev.2006.01.014
Wiederholm T (1983) Chironomidae of the Holarctic region, keys and diagnoses. Part 1. Larvae. Entomol Scand 19:1–457
Woodward CA, Shulmeister J (2005) A Holocene record of human induced and natural environmental change from lake Forsyth (Te wairewa), New Zealand. J Paleolimnol 34:481–501. doi:10.1007/s10933-005-5708-7
Þorláksson H (2000) Icelandic society and Reykholt in the 12th and 13th centuries with special reference to Snorri Sturluson. In: Sveinbjarnardóttir G (ed) Reykholt in Borgarfjörður. National Museum of Iceland, Reykjavík, pp 11–20
Acknowledgments
This work was funded by the Leverhulme Trust and the Icelandic Research Fund for Graduate Students. We thank Guðrún Sveinbjarnardóttir for her comments and assistance, Ian Lawson and Mike Church for their help during coring, Andy Dugmore for the geochemical analyses of tephra samples and the owners of the land surrounding Breiðavatn for kindly allowing us access. Bob Smith from the Cartographic unit, University of Southampton helped with drawing the figures. The valuable comments of two anonymous referees helped to improve the manuscript.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Gathorne-Hardy, F.J., Erlendsson, E., Langdon, P.G. et al. Lake sediment evidence for late Holocene climate change and landscape erosion in western Iceland. J Paleolimnol 42, 413–426 (2009). https://doi.org/10.1007/s10933-008-9285-4
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s10933-008-9285-4