Efstadalsvatn – a multi-proxy study of a Holocene lacustrine sequence from NW Iceland

Multi-proxy data, both lithostratigraphic and biostratigraphic, are presented from Efstadalsvatn, a lake in NW Iceland. The sequence covers the period 10,000 to 3500 ¹⁴C yr B.P. The biostratgraphic data include the first Icelandic chironomid-based reconstruction of Holocene mean July air temperatures, using a Norwegian training set in the absence of modern Icelandic data. The results show that deglaciation and ecosystem development probably began before 10,000 ¹⁴C yr B.P. and that July temperatures were around 4°C at ca. 9500 ¹⁴C yr B.P. Temperatures then rose to ca. 8°C at the time of the deposition of the Saksunarvatn tephra (9100 ¹⁴C yr B.P.), reaching ca. 10°C by 8500 ¹⁴C yr B.P., high enough for the growth of tree birch, although successful birch colonisation did not take place until 6750 ¹⁴C yr B.P. There is some evidence for cooling immediately preceding 9100 ¹⁴C yr B.P. There is little firm biostratigraphic evidence for the 8200 cal. B.P. event, although this may be due to a relatively low resolution pollen sampling interval, but there are changes at this time in the total carbon (TC) and mass susceptibility (MS) data. Optimal temperatures and relative vegetation stability may have occurred between 8000–6100 ¹⁴C yr B.P. but the chironomid assemblages indicate higher temperatures after 5000 ¹⁴C yr B.P. This latter interpretation may, however, reflect delayed colonisation of thermophilous taxa and requires further investigation. There is evidence in the lithostratigraphy for greater local terrestrial instability after 6100 ¹⁴C yr B.P. but it seems unlikely that this led to the redevelopment of ice in the catchment. The biostratigraphic records appear to show a degree of resistence to climate forcing throughout the early and middle Holocene. The new chironomid-based temperature reconstruction needs to be refined by further studies in Iceland, particularly the development of an Icelandic training set, but has already demonstrated the problems of paleoclimatic interpretations based on pollen and/or macrofossil evidence alone.     

Late Glacial and Holocene Palaeoenvironmental Changes in the Rostov-Yaroslavl’ Area, West Central Russia

Three lake sediment sequences (lakes Nero, Chashnitsy, Zaozer’e) from the Rostov-Jaroslavl’ region north of Moscow were studied to provide information on palaeoclimatic and palaeoenvironmental changes during the past 15,000 cal yr. The multi-proxy study (i.e., pollen, macrofossils, mineral magnetic measurements, total carbon, nitrogen and sulphur) is chronologically constrained by AMS ¹⁴C measurements. Lake Nero provided the longest sedimentary record back to ca. 15,000 cal yr BP, while sediment accumulation began around ca. 11,000 cal yr BP in the two other lakes, possibly due to melting of permafrost. Limnic plant macrofossil remains suggest increased lake productivity and higher mean summer temperatures after 14,500 cal yr BP. While the late glacial vegetation was dominated by Betula and Salix shrubs and various herbs, it appears that Betula sect. Albae became established as early as 14,000 cal yr BP. Major hydrological changes in the region led to distinctly lower lake levels, starting 13,000 cal yr BP in Lake Nero and ca. 9000 cal yr BP in lakes Chashnitsy and Zaozer’e, which are situated at higher elevations. These changes resulted in sedimentary hiatuses in all three lakes that lasted 3500–4500 cal yr. Mixed broad-leaved – coniferous forests were widespread in the area between 8200 and 6100 cal yr BP and developed into dense, species-rich forests between 6100 and 2500 cal yr BP, during what was likely the warmest interval of the studied sequences. Agricultural activity is documented since 500 cal yr BP, but probably began earlier, since Rostov was a major capital by 862 A.D. This apparent gap may be caused by additional sedimentary hiatuses around 2500 and 500 cal yr BP.     

Palaeolimnological Development of Lake Njargajavri, Northern Finnish Lapland, in a Changing Holocene Climate and Environment

This study used palaeolimnological approaches to determine how Holocene climatic and environmental changes affected aquatic assemblages in a subarctic lake. Sediments of the small Lake Njargajavri, in northern Finnish Lapland above the present treeline, were studied using multi-proxy methods. The palaeolimnological development of the lake was assessed by analyses of chironomids, Cladocera and diatoms. The lake was formed in the early Holocene and was characterized by prominent erosion and leaching from poorly developed soils before the establishment of birch forests, resulting in a high pH and trophic state. The lake level started to lower as early as ca. 10,200 cal. BP. In the resulting shallow basin, rich in aquatic mosses, pH decreased and a diverse cladoceran and chironomid assemblage developed. It is likely that there was a slight rise in the water level ca. 8000 cal. BP. Later, during the mid-Holocene characterized by low effective moisture detected elsewhere in Fennoscandia, the lake probably completely dried out; this is manifest as a hiatus in the stratigraphy. The sediment record continues from ca. 5000 cal. BP onwards as the lake formed again due to increased effective moisture. The new lake was characterized by very low pH. The possible spread of pine to the catchment and the development of heath community may have contributed to the unusually steep (for northern Fennoscandia) decline in pH via change in soils, together with the natural decrease in leaching of base cations. Furthermore, the change in pH may have been driven by cooling climate, affecting the balance of dissolved inorganic carbon in the lake.     

Aquatic Biota and the Detection of Climate Change: Are there Consistent Aquatic Ecotones?

In this study, we analyse the cumulative rate of compositional change along an altitudinal gradient in the Swiss Alps for three different groups of aquatic organisms – Cladocera, chironomids, and diatoms. In particular, we are interested in the magnitude of unusually large changes in species composition that allows the detection of critical ecotones for each of these three organism groups. The estimated rate-of-change is the distance in ordination space using principal coordinate analysis based on chord distance and chi-square distance. These analyses highlight the cumulative rate-of-change and the cumulative relative rate-of-change, as the chi-square distance is relative to the total species composition. We found that the major changes in taxonomic composition for the three organism groups and therefore also the major ecotones are just below the modern tree-line (1900–2000 m a.s.l.), which may indirectly be an effect of the tree-line. For diatoms and Cladocera (only chi-square distance) there is also an ecotone at 2055 m a.s.l., which may be a direct or indirect response to climate. Further, the ecotone region below the modern tree-line is much wider for chironomids, with an extension downwards due to a shift in relative abundance patterns. For diatoms there is a stronger rate-of-change above 1650 m a.s.l. when chi-square distance is used. Coupled with the even distribution of diatom richness, this suggests that at higher altitudes the change is more strongly associated with a few species becoming dominant compared to lower elevations. Hence, there are considerable differences among the three organism groups, suggesting that different environmental factors may influence the rates of compositional change within and among groups. This supports the general usefulness of multi-proxy studies, namely the study of several independent groups of organisms to reconstruct past environmental conditions but also points to the importance of careful site selection in such studies.     

Multi-proxy approach to long- and short-term Holocene climate-change: evidence from eastern Lake Ontario

We use a multi-proxy (n = 11) paleolimnological approach on deep-water sediment from eastern Lake Ontario to characterize both long- and short-term regional climate change over the past ~10,000 calendar years. Proxies included % total organic matter, % total carbonate, magnetic susceptibility, C/N ratios, % organic carbon, % total nitrogen, % biogenic silica and ¹⁸O and ¹³C of carbonate, as well as ¹³C and ¹⁵N of bulk organic matter. There is a marked shift in most proxies at ~9.4 ka which defines the start of Holocene warmth in this region. Prior to this, the area was influenced by the post-Younger Dryas cold/wet interval, controlled by a southward displacement of the polar front jet stream, when many proxies were at their minimum. The Hypsithermal interval (~9.4–5.3 ka) was the warmest and wettest of the Holocene due to a long-term increase in summer insolation. The Hypsithermal, however, was interrupted by two cold climates; the 8.2 ka event (~8.4–8.0 ka) and the Nipissing Rise (~6.8–5.0 ka), both of which are linked to a reduction in thermohaline circulation and northward oceanic heat transport. The Neoglacial interval (~5.3 ka to ~1850 AD), driven by a long-term decrease in summer insolation, was cooler and dryer, but more stable, than the Hypsithermal. The short Historic interval (post ~1850 AD) was characterized by some of the largest amplitude and most abrupt anomalies of the past 10,000 years, due to intense anthropogenic activity, when a number of proxies reached unprecedented values.     

Impacts of climate and river flooding on the hydro-ecology of a floodplain basin, Peace-Athabasca Delta, Canada since A.D. 1700

Multi-proxy paleolimnological analyses on lake sediment cores from “Spruce Island Lake” (58° 50.82′ N, 111° 28.84′ W), a perched basin in the northern Peace sector of the Peace-Athabasca Delta (PAD), Canada, give insights into the relative roles of flow regulation of the Peace River and climatic variability on the basin hydro-ecology. Results indicate substantial variability in basin hydro-ecology over the past 300 years ranging from seasonal to periodic desiccation in the 1700s to markedly wetter conditions during the early 1800s to early 1900s. The reconstruction is consistent with (1) dry climatic conditions that defined the peak of the Little Ice Age and subsequent amelioration evident in conventional ring-width and isotopic analyses of tree-ring records located hydrologically and climatically upstream of the PAD, and (2) Peace River flood history inferred from sub-annual magnetic susceptibility measurements from another lake sediment record in the Peace sector of the PAD. Although regulation of the Peace River for hydroelectric power generation since 1968 has long been considered a major stressor of the PAD ecosystem leading to reduced frequency of ice-jam and open-water flooding and an extended period of drying, our results show that current hydro-ecological status is not unprecedented as both wetter and drier conditions have persisted for decades in the recent past under natural climatic variability. Furthermore, paleolimnological evidence from Spruce Island Lake indicates that recently observed dryness is part of a longer trend which began some 20-40 years prior to Peace River regulation.