Fusing pollen-stratigraphic and dendroclimatic proxy data to reconstruct summer temperature variability during the past 7.5 ka in subarctic Fennoscandia

Abstract

A new palaeoclimatic reconstruction of midsummer (July) temperatures for the last 7.5 ka in northern Fennoscandia is presented. It is based on two botanical proxies: spectra of fossil pollen and tree rings of Scots pine logs recovered from lacustrine sediments in the Arctic tree-line region. A newly developed method of proxy fusion is used to integrate the proxy-specific reconstructions of past summer temperature variability based on the pollen-stratigraphic and dendroclimatic data. The rationale behind the method is that the two proxies are likely to be connected to climate variability in a timescale-dependent fashion and, accordingly, the new reconstruction makes use of the low-and highfrequencies from pollen-stratigraphic and tree-ring data, respectively. The most prominent features of the new reconstruction are: (1) the long-term decline of temperatures by 2.0°C over the past 7.5 ka, (2) the midHolocene warmth culminating between 5 and 4 ka as a deviation from the cooling trend, (3) the Little Ice Age cool phase between 0.7 and 0.1 ka, and (4) the subsequent warming during the past century. These periods are superimposed on year-to-year variations in climate as dated to calendar-year accuracy by dendrochronology. Within the modern period, the years 1934 and 1937 are among the warmest, and the years 1903 and 1910 are among the coldest summers in the context of the past 7.5 ka. On average, the reconstructed Holocene climate was approximately 0.85°C warmer than the twentieth century.

A palaeotemperature record for the Finnish Lakeland based on microdensitometric variations in tree rings

“…The wamth around the turn of the first millennium AD was seen closely synchronous with the MXD and TRW evidence from northern Fennoscandia (Grudd, 2008;Esper et al, 2012) and northern Finland (Helama et al, 2009a(Helama et al, , 2009b(Helama et al, , 2010a(Helama et al, , 2010b(Helama et al, , 2012b. The northern MXD (Esper et al, 2012) and TRW (Helama et al, 2010a) reconstructions showed their warmest 250-year periods in AD 816-1065 and AD 932-1181, respectively.…”

“…These chronologies originate from regions near the timberline where tree growth is predominantly governed by summer temperature. In consequence of their cold habitat, the tree-ring width chronologies provide excellent proxy records of past temperature variability (Grudd et al, 2002;Helama et al, 2002Helama et al, , 2009aHelama et al, , 2009bHelama et al, , 2010aHelama et al, , 2010bHelama et al, , 2012bLinderholm and Gunnarson, 2005). In many regions, however, an improved dendroclimatic signal of summer temperature can be attained using X-ray based microdensitometric series of maximum latewood densities (MXD), rather than using annual widths of the rings (Briffa et al, , 2002a(Briffa et al, , 2002b.…”

Introduction to the JoPL special issue, “Holocene paleoenvironmental records from Arctic lake sediment”

“…At the latter two lakes in Alaska, the interpretation of higher temperatures is supported by midge assemblages that also indicate higher summer temperatures during intervals of higher productivity. Intervals of relative warmth during the first part of the Holocene were reported by other authors using other proxies, such as the pollen-dendro fusion from northern Fennoscandia (Helama et al 2012). Their reconstruction shows that summer temperatures were 2°C above modern (1950( -2000 for several centuries during the first millennium of the 7.5-kalong record.…”

“…The papers in this Special Issue explore a large variety of paleoenvironmental indicators, including some emerging techniques, such as analyzing the isotopic composition of specific compounds (Thomas et al 2012;Wooller et al 2012a), measuring particle-size distribution from thin-section imagery (Lapointe et al 2012), and fusing data from different proxy types (Helama et al 2012). Many studies are notable for their multi-proxy approach.…”

“…Twothirds of the records (18) analyzed the floral and faunal assemblages of pollen (seven studies), chironomids (four studies), or diatoms (three studies). These analyses were typically conducted at multi-centennial resolution, with the notable exception of the study by Helama et al (2012) who fused pollen with tree-ring data to generate an annual-scale summer temperature reconstruction for the last 7,500 years in northern Fennoscandia.…”