Holocene climate variability
The Holocene (a term first coined in the early 19th century) is the most recent and current interglacial. The start of the Holocene record is one of the most easily identified features in palaeoclimatic records, although accurately dating this start has proved challenging. The start of the Holocene was commonly placed at 10 ka radiocarbon (14C) yrs BP. However, developments in more accurate chronologies, especially with regard to calibration of radiocarbon dates with annually resolved archives (e.g. tree rings, lake sediments and ice cores) to compensate for the prominent 14C plateaux which span this time period, have revealed the start of the Holocene to be transgressive (i.e. it occurs at different times in different regions of the world). The Holocene is generally taken as starting between c. 11.0-11.8 ka cal yrs BP (calibrated years before present; see Wolff 2007).
Previous papers on Holocene climate variability in Lake Baikal tended to use what I would now consider inappropriate climate classification schemes (e.g. Blytt-Sernander) which were based on observations made from northern European peat bogs. Moreover, early radiocarbon chronologies from Lake Baikal proved difficult to calibrate. Both these factors meant that robust reconstructions of Holocene climate variability, together with associated ecosystem impacts, were not as advanced as in other regions of the world.
However, three detailed papers were published last year on Holocene climate impacts in the Lake Baikal region which considerably advance our understanding on the extent of climate variability experienced in this region: Mackay 2007; Tarasov et al. 2007; Prokopenko et al. 2007. Each of the papers has taken a very different approach to the problem.
The review by Mackay 2007 sought to provide a synthesis of all the papers published (up to 2006) on Lake Baikal diatoms (including biogenic silica and oxygen isotope analysis of diatom silica) spanning the Quaternary period. Relevant to this entry is the mounting empirical evidence for millennial-scale cycles and centennial cooling events determined from sites around the world. Mackay 2007 therefore re-interpreted diatom and oxygen isotope records from biogenic silica in the context of centennial-scale cooling events during the early to mid-Holocene from well dated profiles. He then compared these cooling events to other regions around the northern hemisphere by reference to several recent Holocene studies (e.g. Mayewski et al. 2004) (Fig 1).
Fig 1: Synthesis of Holocene paleoclimate records from Lake Baikal, and other records from across central Asia, northwest Europe and the North Atlantic ocean. All profiles are scaled to calendar years BP (Mackay 2007).
Central to this synthesis was a study done by Morley et al. 2005, who undertook the first oxygen isotope analysis of Baikal silica. Despite problems with clay contamination, there were several notable episodes of lower δ18O values, which correspond to prevailing cooler temperatures and shifts in river input into Lake Baikal.
The first episode occurred between 10.2 and 10.4 ka cal yr BP, coincident with the prominent cooling event seen at c. 10.35–10.15 ka cal yr BP in several archives from the North Atlantic and Greenland ice cores (see Björck et al., 2001 for a synthesis). It has been suggested that this cooling event was caused by a weakening of the North Atlantic thermohaline circulation (THC) and the southwards extension of polar waters. Moreover, potassium records from GISP2 (a Greenland ice core) indicate at this time a significant strengthening of the Siberian High (Mayewski et al., 2004). The evidence here suggests therefore that weakening of the THC in the North Atlantic at this time has also had substantial and perhaps almost simultaneous impact on the Lake Baikal ecosystem through intensification of the Siberian High.
A second prominent episode of lower δ18O values in the Lake Baikal isotope record can be observed during the mid-Holocene, between ca. 7 ka – 6.5 ka BP. Corrected values (Brewer et al. 2008) show this minimum to have occurred ca. 6.8 ka BP. Nevertheless, these findings are coincident with increasing GISP2 Na+ (ppb) (indicative of a deepening Icelandic Low and GISP2 K+ (ppb) (indicative of a more intense Siberian High pattern (Mayewski et al., 1997)) (Fig 1). Mayewski et al. (2004) suggest that this episode may have been caused by solar forcing, as there is no evidence of other major forcing factors especially freshwater input into the North Atlantic. Low values for δ18O in diatom silica suggests that maximum cooling was caused by an intensification of the Siberian High, altering fluvial input into the lake (Morley et al. 2005). As well as shifts in fluvial sources in the Lake Baikal region, this cooling phase may also have had an impact on diatom production in the lake, as there is evidence of a decline in BioSi values (Colman et al., 1999).
Two further identified cool periods occurred at approximately 3 – 2.5 ka BP (based on diatom abundances only) and at c. 0.5 ka BP (based on diatoms and oxygen isotope values of diatom silica), coincident with the period know as the Little Ice Age (see Mackay et al. 2005) and Post “Recent climate impacts…” (Fig 1). These cool periods are coincident with an increase in steppe landscape identified by Tarasov et al. 2007 below, and with shifts to possible modelled cooler conditions identified by Bush (2005).
The second recent high-resolution paper to investigate Holocene palaeoclimate in Lake Baikal is based on pollen evidence from a core taken on the Buguldieka Saddle, separating the south and central basins (Tarasov et al. 2007). This study is novel for several reasons. It is the first to provide empirically based quantitative estimates of past temperatures and precipitation from the pollen record. Moreover, for the Holocene period, it makes a serious attempt to relate significant periods of prehistoric settlement close to Lake Baikal, with possible impacts on the landscape and extent of deforestation. Finally, the authors also compare environmental reconstructions for the Holocene period with those from the last interglacial between 128 ka – 117 ka years ago (in southern Siberia, this period is commonly known as the Kazantsevo (see Rioual & Mackay 2005).
Pollen in each Holocene core sample was classified into a particular biome, e.g. taiga forest, steppe, tundra etc so that changing landscape patterns could be determined at high resolution. Quantitative reconstructions of mean July temperature, mean January temperature, mean annual precipitation and a moisture index were also determined using a technique called best modern analogue approach (BMA) (Fig 2). Tarasov et al. were able to determine that in the south of Lake Baikal at least, taiga landscape was significant after 13.3 ka, associated with a relatively warm and wet climate. By 9 ka- 7 ka taiga was dominant with fir and spruce at their highest levels for the whole Holocene period. Interspersed into this record, steppe landscapes expanded at last 4 times between 7.5 ka, 5.5 ka, 3 ka and 1-0.5 ka ago, generally coincident with declines in reconstructed temperatures. The authors were also able to determine that human settlements effected little if any impact on deforestation in the region, although climate may have played a role in the occurrence of a well-define cultural hiatus between c. 6.8 – 6.1 ka BP (Weber 2002).
Fig 2 The charts of the climate variables reconstructed from the Buguldeika (VER93-2 st.24GC) pollen record using the BMA approach: annual precipitation (A); the mean temperature of the warmest month (B); the mean temperature of the coldest month (C); and the moisture index (D) (Tarasov et al. 2007).
Prokopenko et al. 2007 provided a new interpretation of Holocene climate change in the Lake Baikal region, based on diatom and pollen records from Lake Baikal itself, and Lake Hovsgol. Lake Hovsogol is also a tectonic rift lake, but is situated at 1645 m a.s.l. to the south-west of Baikal (Fig 3). Its outlet, the Egiin-Gol river, feeds its way into the Selenga River, Baikal’s largest tributary. Prokopenko et al. 2007 provide the first ever data-model comparison for the Holocene in Lake Baikal. Stemming from this, two important points are raised in this paper, which Prokopenko believes challenges conventional wisdom about palaeoclimate during the Holocene in this region of central Asia.
Fig 3 Lake Baikal watershed and bathymetric maps of Lake Hovsgol and Lake Baikal (Prokopenko et al. 2007)
Firstly, over the Holocene period, the role of global CO2 / water vapour feedback was more important than direct insolation forcing in driving regional mid-Holocene temp maximum. This is based on the observation that insolation peaked at 11 ka in the continental interior of Asia, yet diatom and pollen proxy records for Lakes Baikal and Hovsgol suggest significant temperature increases after ca. 7 ka BP. For example, between 4-2.5 ka BP, there is proxy evidence for elevated summer temperatures in Transbaikalia and increased aridity in northern Mongolia, which agree well with regional GCM predictions undertaken by Bush (2005). Note that direct temperature reconstructions by Tarasov et al. 2007 however suggest that for the early to mid Holocene period, mean July temperatures were highest at c. 7 ka BP, while estimates of mean January temperature were highest between c. 8.5 – 8 ka BP.
Secondly, according to Prokopenko et al. 2007 there is no evidence contained in either the Lake Baikal or Lake Hovsgol records for mid-Holocene humidity or precipitation/evaporation (P/E) maxima that can possibly be associated with peak Asian summer monsoon intensity. Instead, and to the contrary there is a consistent decline in P/E since the early Holocene which accelerated at c. 7.5-8 ka, and ended in a prolonged minimum in P/E balance between 6-4 ka. This increase in aridity at this time can be linked to rising temperatures causing an increase in P/E in northern Mongolia.
Clearly there is still some uncertainty in trying to tie all these records together, especially with regard to discrepancies in radiocarbon dating of bulk sediments in Baikal: for example ‘reservoir’ ages applied by Tarasov and Prokopenko for the same core material are different. Nevertheless, consensus is building that the ecosystem of Lake Baikal is being impacted by e.g. centennial cool events, and that terms such as ‘holocene optimum’ originally derived for northern European climates are not applicable to the records determined here.
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